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Early Cenozoic evolution of the latitudinal diversity gradient

March 2020
Earth-Science Reviews 202(6):103090

March 2020
202(6):103090

DOI:10.1016/j.earscirev.2020.103090

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Authors:

Alistair Crame

British Antarctic Survey

We are beginning to appreciate that the huge radiations of both marine and terrestrial taxa in the aftermath of the K/Pg mass extinction event were concentrated largely, but not exclusively, in the low-latitude and tropical regions. This in turn means that significant latitudinal diversity gradients were developed well before the onset of global cooling at the Eocene/Oligocene boundary. Net rates of evolutionary radiations were significantly higher through the Early Paleocene – Middle Eocene interval (i.e. ~62–42 Ma) in the tropics than at the poles but this may be due as much to their retardation in the latter regions as to their acceleration in the former. At least in the marine realm, polar assemblages are characterised by the phenomenon of high dominance/low evenness, and it is thought likely that this is due to the extreme seasonality of primary production at the base of the food chain. Many modern polar marine organisms are de facto trophic generalists and occupy significantly broader ecological niches than their tropical counterparts. Although we cannot dismiss the roles of both temperature and area in promoting tropical diversity, it could well be that LDGs are just as much the product of a latitudinal gradient in the seasonality of primary productivity. Such a gradient would have operated in both greenhouse and icehouse worlds.

Regional comparison of present day neogastropod faunas between the Arctic, Tropics and Antarctic. The histograms represent the number of species within 18 common neogastropod families. The Arctic fauna is a compilation of all shelf-depth taxa occurring north of 60°N, and the Antarctic of all shelf and bathyal taxa south of the Polar Front. The Tropics fauna is an average of six faunas: two from the Americas and four from the Indo-West Pacific province. Reproduced with slight modifications from Crame, 2013, (Fig. 1), where further details on the construction of the three faunas are given. Neogastropod family classification based on Bouchet and Rocroi (2005) and Bouchet et al. (2011).

…

Relative abundance distributions to show the contrast between presentday polar and tropical neogastropod faunas. Relative abundance distributions (RADs) are used to examine the structure of biotic assemblages by plotting % relative abundance for each taxon (log scale, y axis) against its rank within the assemblage (x axis). They can be constructed using either the number of individuals per species, or, as in this particular instance, the number of species within a higher taxon such as a family. They are based on the principle that the abundance of a particular taxon reflects the size of its realised niche, which in turn is shaped by ecological interactions within a community or assemblage (Magurran, 2004). Sometimes referred to as dominance -diversity plots, they range in form from sub-vertical curves at one extreme (strong dominance) to sub-horizontal, sigmoidal curves at the other (strong evenness). The polar fauna (from Antarctica) fits closest to the former category and a Zipf distribution, but the tropical one is closer to the latter and a broken stick model. Redrawn from Crame et al., 2018, (Fig. 3) where further information on the composition of the respective faunas and the methods of model fitting are contained. As the two polar faunas show very similar Zipf distributions they have been amalgamated here into just one curve. It should be noted that the polar -tropical contrast seen at the present day is almost exactly repeated by a similar comparison of Middle Eocene faunas of approximately 45 Ma age.

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Evolution of latitudinal diversity contrasts through the Cenozoic era. Stage 1 represents the immediate aftermath of the K/Pg mass extinction event when global faunas were still in a recovery phase. In all probability the LDG was greatly reduced at this time. Stage 2 represents the Early Cenozoic greenhouse world when latitudinal temperature gradients were much shallower than at the present day. However, latitudinal diversity gradients were present in the mid-to highlatitudes and the overall form of the high -low latitude diversity contrast was that of a broad, low dome. Stage 3 is set in the Late Cenozoic after the onset of global cooling at the Eocene/Oligocene boundary. The high -low diversity contrast now has the more familiar modern profile of a bellshaped curve with a much narrower peak. It is envisaged that this is achieved by both the intensification of seasonality in the high-latitude and polar regions, and range retraction into lower latitudes. Net rate of species increase (i.e. speciation minus extinction) is significantly higher in the tropics than at the poles throughout the Cenozoic era. Key: N -northern polar regions; S -southern polar regions; Eq -Equator.

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Earth-Science Reviews

journal homepage: www.elsevier.com/locate/earscirev

Early Cenozoic evolution of the latitudinal diversity gradient

J. Alistair Crame

British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK

ARTICLE INFO

Keywords:

Early Cenozoic

Greenhouse world

Latitudinal diversity gradients

Polar generalists

Primary productivity

Evolutionary rates

ABSTRACT

We are beginning to appreciate that the huge radiations of both marine and terrestrial taxa in the aftermath of

the K/Pg mass extinction event were concentrated largely, but not exclusively, in the low-latitude and tropical

regions. This in turn means that signiﬁcant latitudinal diversity gradients were developed well before the onset

of global cooling at the Eocene/Oligocene boundary. Net rates of evolutionary radiations were signiﬁcantly

higher through the Early Paleocene – Middle Eocene interval (i.e. ~62–42 Ma) in the tropics than at the poles

but this may be due as much to their retardation in the latter regions as to their acceleration in the former. At

least in the marine realm, polar assemblages are characterised by the phenomenon of high dominance/low

evenness, and it is thought likely that this is due to the extreme seasonality of primary production at the base of

the food chain. Many modern polar marine organisms are de facto trophic generalists and occupy signiﬁcantly

broader ecological niches than their tropical counterparts. Although we cannot dismiss the roles of both tem-

perature and area in promoting tropical diversity, it could well be that LDGs are just as much the product of a

latitudinal gradient in the seasonality of primary productivity. Such a gradient would have operated in both

greenhouse and icehouse worlds.

1. Introduction

The latitudinal gradient in taxonomic diversity forms one of the

most striking features of life on Earth at the present day. For many

major groups of plants and animals there are simply far more of them in

the low-latitude and tropical regions than there are in the high-latitude

and polar ones; such diﬀerences often amount to at least an order of

magnitude at the species level (Hillebrand, 2004;Lomolino et al., 2010;

Tittensor et al., 2010;Valentine and Jablonski, 2015;Worm and

Tittensor, 2018). Understanding how such large disparities in the lati-

tudinal distribution of taxa may have arisen remains a major topic at

the heart of contemporary biodiversity research (Gaston, 2000;

Mittelbach et al., 2007;Erwin, 2009;Krug et al., 2009;Brown, 2014;

Jablonski et al., 2017;Worm and Tittensor, 2018).

One particular area of uncertainty to date has been the precise time

of origin of the modern latitudinal diversity gradient (LDG). One school

of thought has placed it at the Eocene/Oligocene (E/O) boundary (34

Myr ago), a time of rapid global cooling and pronounced shift in the

Earth's climate state from greenhouse to icehouse (Zachos et al., 2001,

2008). At ﬁrst sight this would seem to be entirely logical as many

warm-temperate and tropical taxa would simply have been unable to

disperse into the newly expanded cold-temperate and polar regions

(Wiens and Donoghue, 2004). A previously diﬀuse boundary at the

edge of the tropics now became much more clearly deﬁned. Support for

an E/O origin of the modern LDG has come largely, but not exclusively,

from the terrestrial realm, and a combination of molecular phylogenetic

and palaeontological studies on various plants, vertebrates and insects

(Hawkins et al., 2006, 2007;Archibald et al., 2010;Condamine et al.,

2012;Mannion et al., 2014). There is also evidence from the deep sea

fossil record that the origin of the psychrosphere (i.e. the circum-Ant-

arctic cold water current) at this time led to a profound alteration of

global circulation patterns and thermal stratiﬁcation of the oceans. This

in turn led to a marked reduction in the taxonomic diversity of polar

deep-sea benthic taxa such as foraminifera, as well as certain phyto-

plankton and zooplankton (Thomas and Gooday, 1996;Wilson, 1998;

Culver and Buzas, 2000;Thomas, 2007;Houben et al., 2013;Fenton

et al., 2016). The E/O boundary hypothesis is linked very ﬁrmly to the

concept of global climate change and thus, ultimately, to the principle

that temperature directly controls standing levels of biodiversity

(Brown, 2014;Clarke, 2017).

But balanced against an E/O origin for the LDG is the fact that many

modern taxa, in both the terrestrial and marine realms, have their

evolutionary roots in an intense Early Cenozoic radiation in the wake of

the Cretaceous/Paleogene (K/Pg) mass extinction event (i.e. in the in-

terval ~62–42 Ma) (Kafanov, 2001;Briggs, 2006;Jaramillo et al.,

2006;McKenna and Farrell, 2006;Bininda-Emonds et al., 2007;

Stanley, 2007;Ramirez et al., 2007;Schultz and Brady, 2008;

Schuettpelz and Pryer, 2009;Meredith et al., 2011;Near et al., 2013;

https://doi.org/10.1016/j.earscirev.2020.103090

Received 1 November 2019; Received in revised form 14 January 2020; Accepted 15 January 2020

E-mail address: jacr@bas.ac.uk.

Earth-Science Reviews 202 (2020) 103090

Available online 17 January 2020

(http://creativecommons.org/licenses/BY-NC-ND/4.0/).

Feng et al., 2017). Although this pronounced radiation, which is clearly

indicated in long, time-series datasets (Flessa and Jablonski, 1996;

Stanley, 2007;Alroy, 2010;Sepkoski, 2002;Huang et al., 2014), might

have been spread more or less evenly across the Earth's surface, various

lines of evidence suggest that it was in fact concentrated in the tropics

(Jablonski, 1993, 2005a). If this is indeed the case then we may have to

accept that steep, regional LDGs can form in a greenhouse world. This

in turn suggests that factors other than temperature may be important

in their formation.

2. Evidence for an Early Cenozoic latitudinal diversity gradient

Observational estimates of the extinction intensity at the K/Pg

boundary at various global localities are generally in the range of

60–70% at the species level and 30–40% at the generic one (Stanley,

2007;Krug and Jablonski, 2012;Condamine et al., 2013;Aberhan and

Kiessling, 2015). Even though there is no obvious evidence of a lati-

tudinal bias in extinction intensity from these localities (Raup and

Jablonski, 1993;Zinsmeister et al., 1989;Tobin et al., 2012;Witts et al.,

2016), it is still possible that the LDG may have been signiﬁcantly re-

duced in scale overall in the immediate aftermath of the extinction

event, and then for several millions of years afterwards (Jablonski,

2005b;Stanley, 2007;Krug et al., 2009). Empirical evidence for such a

reduction is in short supply but in one recent comparison of Early

Cenozoic molluscan faunas between Antarctica (Seymour Island lo-

cality) and two tropical localities (US Gulf Coast and Paris Basin) it was

shown that in the terminal Cretaceous stage (the Maastrichtian) there

was quite a strong diversity contrast between the tropics and the pole

(N.B. the very small number of rudistid bivalves in the Maastrichtian

were excluded from the analyses). Nevertheless, this is reduced sig-

niﬁcantly in the initial Paleogene stage (the Danian), and especially so

in the bivalves (Crame, 2019). This could be taken as a weakening of

the gradient by a stronger reduction of tropical faunas in the extinction

event, but further work is required to verify this pattern on smaller

temporal and taxonomic scales. Work on microbiotas across the K/Pg

boundary suggests that it may have taken as long as 4 Myr for taxo-

nomic diversity levels to be restored to pre-extinction levels, with un-

doubtedly some variation in rates between major clades (D'Hondt,

2005).

The subsequent radiation of both bivalves and gastropods through

the Paleocene and Eocene epochs has been traced between the same

three localities (Beu, 2009;Crame et al., 2014, 2018;Crame, 2019).

When this is done it is apparent that there is a rapid rise through the

later Paleocene stages into the Early Eocene and then Middle Eocene at

all three localities; indeed, this proves to be the Early Cenozoic diversity

maximum in the marine realm (Piccoli et al., 1987;Dockery and

Lozouet, 2003;Crame et al., 2014;Huyghe et al., 2015). This Early

Cenozoic evolutionary radiation is perhaps best documented in the

extensive Neogastropoda clade which is known to have expanded

through the entire Cenozoic era to form one of the largest clades in

shallow seas at the present day (Crame et al., 2018, and references

therein). A latitudinal contrast of Middle Eocene neogastropods gives

totals of approximately 437 species for the US Gulf Coast, 433 for the

Paris Basin, but only 57 for Antarctica (Beu, 2009;Crame, 2019). At

ﬁrst sight this might be taken as simply reﬂecting a strong sample bias,

with the Antarctic locality being much less well known than either of

the two tropical ones. Nevertheless, it should be pointed out that that

the Antarctic Middle Eocene gastropod fauna comprises at least 7500

individuals and of these approximately 3500 (47%) are neogastropods

(Crame, 2019). In addition, of the 57 species recorded from the La

Meseta Formation of Seymour Island, only 12 (21%) have individuals in

excess of 50 mm; small (<20 mm) and tiny (<5 mm) taxa are rea-

sonably well represented in this fauna (Crame et al., 2018, Suppl. Inf.).

The very large Imparidentia bivalve clade (formerly Heterodonta),

which includes common, shallow-water taxa such as the Cardiidae,

Tellinidae, Mactridae and Veneridae, also shows a very strong Middle

Eocene diversity contrast: US Gulf Coast – 158 species; Paris Basin –

273; Antarctica – 25 (Beu, 2009;Crame, 2019).

It should be emphasised that in making these large-scale diversity

comparisons it was not possible to use rarefaction or related techniques;

for many of the tropical faunas accurate counts of numbers of in-

dividuals are unavailable. The US Gulf Coast locality is contained es-

sentially within a central eastern Texas – Alabama region, but in the

Middle Eocene there is a signiﬁcant eastwards extension to include the

upper Santee Limestone of South Carolina. Further details on the

Paleocene, Early Eocene and Middle Eocene Gulf Coast localities, in-

cluding estimates of the stratigraphic thicknesses of the units studied

are contained in the Appendix to Crame et al. (2018) and Crame

(2019). All of the various sites investigated have been comprehensively

sampled, but of course this does not necessarily equate to complete

sample standardisation.

Another prominent feature of the Early Cenozoic evolution of the

Neogastropoda is the rapid increase in the proportion of taxa that can

be assigned to modern genera. Whereas only 7% of the Maastrichtian

species on the US Gulf Coast fall into this category, 44% of the Middle

Eocene ones do; in Antarctica there are no modern Maastrichtian

genera but the ﬁgure rises to 37% in the Middle Eocene (Crame et al.,

2018). In both the tropics and the pole the pronounced evolutionary

radiation is accompanied by a rapid rise of the modern neogastropod

fauna.

As well as these direct latitudinal contrasts it is important to em-

phasise just how diverse the tropical Middle Eocene fauna of the Paris

Basin as a whole is. From the Lutetian (i.e. early Middle Eocene)

Calcaire Grossier Formation alone some 1550 species of gastropods and

540 species of bivalves have been recorded, and at one single outcrop at

Falunière (Grignon, Yvalines) >800 species of molluscs have been

collected from just one 13 m section (Cossmann and Pissarro, 1904–13;

Pacaud and Le Renard, 1995;Gély, 2008;Merle, 2008;Huyghe et al.,

2012). There is no obvious preservational bias between the Early,

Middle and Late Eocene, and the diversity hotspot actually coincides

with an interval of slight global cooling between Early and Middle

Eocene climatic optima (Huyghe et al., 2012, 2015). This huge phase of

Early Cenozoic diversiﬁcation, which is also recorded in other Paris

Basin taxa such as corals, brachiopods, decapod crustaceans and

benthic foraminifera, took place on a series of very shallow-water

carbonate platforms and banks, and laid the foundations for the modern

hyper-diverse tropical molluscan fauna (Vermeij, 2001;Kafanov, 2001;

Briggs, 2006;Harzhauser et al., 2002, 2008).

Similarly, the Bolca Lagerstätten (BL) of northern Italy has yielded a

spectacular ichthyofauna of some 250+ species contained within ap-

proximately 90 families and 17 orders (Bellwood, 1996;Bellwood and

Wainwright, 2006;Carnevale and Pietsch, 2009;Carnevale et al., 2014;

Friedman and Carnevale, 2018;Marramà et al., 2018). Dated by cal-

careous nannoplankton as between 50.5 and 48.96 Ma in age, i.e. late

Early Eocene (Ypresian) (Friedman and Carnevale, 2018), it reveals a

major radiation of acanthomorph (i.e. spiny-rayed) teleosts in general,

and various percomorph families in particular. The latter comprises a

suite of grazers and herbivores, many of which bear a striking resem-

blance to modern coral reef taxa (Bellwood and Wainwright, 2006;

Friedman and Carnevale, 2018). The taxonomic richness shown by the

BL is higher than that of other Ypresian Lagerstätten (Friedman et al.,

2016), and can also be linked to the initial development of modern

coral reef assemblages (Bellwood, 1996;Friedman and Carnevale,

2018). The BL accumulated in a coastal area periodically aﬀected by

anoxia, probably a protected lagoonal system in close proximity to both

coral reefs and seagrass beds (Carnevale and Pietsch, 2009;Carnevale

et al., 2014).

Like the Calcaire Grossier Formation of the Paris Basin, the BL is

also thought to have been associated with an extensive archipelago in

the western Tethyan circum-tropical ocean. Although it is slightly older

than the proliﬁc faunas of the Calcaire Grossier Formation (late Early

Eocene as opposed to early Middle Eocene), both would seem to have

J.A. Crame Earth-Science Reviews 202 (2020) 103090

been part of a distinct biodiversity hotspot that may have been com-

parable in scale to the modern Atlantic – Caribbean – East Paciﬁc

(ACEP) and Indo-West Paciﬁc (IWP) ones (Merle, 2008;Huyghe et al.,

2012;Renema et al., 2008;Friedman and Carnevale, 2018). However, it

has to be stressed that the boundaries of this western Tethyan hotspot

remain poorly deﬁned. What we do know is that the Paleocene – Middle

Eocene tropical belt, in both the marine and terrestrial realms, was

some 40–50% larger than the present day, extending, at least inter-

mittently, to palaeolatitudes of 55°- 65° N and S, and occasionally even

higher (Wolfe, 1985;Adams et al., 1990;Greenwood and Wing, 1995;

Collinson, 2000;Morley, 2007;Pross et al., 2012).

The strong high – low latitude diversity contrast seen in Early

Cenozoic molluscan faunas is very probably also present in the ich-

thyofaunas. In this particular instance the comparisons are not between

precisely contemporaneous units as the BL fauna is late Ypresian in age

and the most proliﬁc Antarctic one is slightly younger at un-

diﬀerentiated Middle Eocene age (Crame et al., 2014;Douglas et al.,

2014;Amenábar et al., 2019). The latter comes from the lithostrati-

graphic unit Telm 4 of the La Meseta Formation, Seymour Island which

is essentially a condensed shell bed from which some 10,000 specimens

of ﬁsh teeth, vertebrae, bone fragments, spines and otoliths have been

recovered by a combination of surface collecting and dry-sieving

(Reguero, 2019;Regeuro et al., 2002;Reguero et al., 2012). Such an

abundant assemblage is of surprisingly low taxonomic diversity, with

only approximately 35 species from 23 families being recorded. And of

these some 71% are chondrichthyans, with sharks in particular being

very well represented (Reguero et al., 2012). At least 21 species of shark

have been identiﬁed and it is clear that taxa from many diﬀerent types

of habitat converged on this one locality (Long, 1992;Kriwet, 2005). In

marked contrast, the BL chondrichthyans comprise only 7% of the total

ichthyofauna (Marramà et al., 2018). The full ecological and evolu-

tionary signiﬁcance of the pronounced shift in the ratio of teleosts to

chondrichthyans with latitude in the Early – Middle Eocene has yet to

be assessed.

The precise status of LDGs in the Early Cenozoic terrestrial realm is

currently uncertain. The presence of large-scale LDGs in the Eocene

ﬂoras of North America (Harrington, 2004) has been challenged by the

subsequent demonstration of mid-latitude diversity peaks in both plants

and insects (Archibald et al., 2010), and a ﬂat-lying LDG in Early Pa-

leocene mammals (Rose et al., 2011;Mannion et al., 2014). Never-

theless, it is apparent from extensive sporomorph (i.e. spores and

pollen) assemblages from central Colombia and western Venezuela that

there was a very marked increase in equatorial diversity values between

the Early Paleocene and Middle Eocene (Jaramillo and Dilcher, 2000;

Jaramillo et al., 2006;Jardine et al., 2018). Indeed, this was such as to

generate peak Middle Eocene values in excess of those in the present-

day paratropical rainforest (Jaramillo et al., 2006). The rate of Paleo-

cene – Eocene diversiﬁcation of this equatorial rainforest exceeds that

of ﬂoras on the US Gulf Coast (30°N), as well as mid-latitude ﬂoras in

both hemispheres (Jardine et al., 2018;Jaramillo and Cárdenas, 2013).

A comparison of Early – Middle Eocene sites between equatorial Co-

lombia/Venezuela and temperate southern Argentina, using both pa-

lynological and megaﬂoral records, established 572 species from the

former but only 93 from the latter (Jaramillo and Cárdenas, 2013).

Although comprehensive LDGs are still at a premium, there is now a

distinct impression that the Early Cenozoic global radiation of both

marine and terrestrial taxa proceeded at a much higher net rate in the

tropics than at the poles. This is particularly so in three major benthic

marine groups that show very strong LDGs at the present day:

Neogastropoda, Imparidentia bivalves, and teleost ﬁshes (Jablonski

et al., 2013;Crame et al., 2018;Rabosky et al., 2018). Each of these

taxa radiated globally in the Paleocene – Middle Eocene, but a series of

tropics – Antarctica comparisons indicate that they did so at a much

higher net rate in the former region than the latter. A very strong la-

titudinal diversity contrast had developed by the Middle Eocene

(~45 Ma), and this has continued through to the present day.

The precise stratigraphic, taxonomic and biogeographic status of a

proliﬁc Early Paleocene molluscan fauna from the Agatdal Formation,

Nuussuaq Group, West Greenalnd (~63°N palaeolatitude) is uncertain

(Kollmann and Peel, 1983;Petersen and Vedelsby, 2000;Dam et al.,

2009;Grimsson et al., 2016). There are some indications that it is

linked to a series of Early Paleocene (i.e. Danian – Selandian) faunas of

moderately high taxonomic diversity from northern and central Europe

that comprise a genuine admixture of warm- and cool-temperate ele-

ments (Schnetler, 2001;Lauridsen and Schnetler, 2014). But what is

certain is that polewards of these Greenland – European faunas there

was a distinct Early Paleocene (Danian) Arctic/boreal fauna in the

highest northern latitudes. This can be traced from Alaska through El-

lesmere Island to Svalbard and is characterised by a sparse molluscan

fauna of temperate aspect (Crame, 2013, and references therein).

3. Structure of Early Cenozoic latitudinal diversity gradients

The Early Cenozoic interval of high – low latitude biotic diﬀer-

entiation is constrained to an approximately 20 Myr interval between

62 and 42 Ma (i.e. late Early Paleocene to late Middle Eocene). As this

was almost exactly coincident with a period of global greenhouse

warmth (Zachos et al., 2008), the question has to be asked whether this

diﬀerentiation can be attributed to the eﬀects of temperature alone?

Early Eocene latitudinal temperature gradients were very much shal-

lower than those of the present day, with one estimate being that there

was almost no latitudinal sea surface temperature (SST) gradient at this

time between subequatorial and subpolar regions (Bijl et al., 2009).

Even when global cooling began in the late Middle Eocene it is still

likely that latitudinal temperature gradients were less than half their

present day value, and quite possibly substantially so (Pross et al.,

2012;Douglas et al., 2014;Crame et al., 2018). Some clues as to what

else might be contributing to the latitudinal diﬀerentiation of biotas

through the Early Cenozoic have recently come to light from a detailed

comparison of the structure of high- and low-latitude marine faunas.

When we look at the latitudinal diversity contrast of the very large

Neogastropoda clade at the present day two things are immediately

apparent: ﬁrstly, there are far fewer species at the poles than in the

tropics, and secondly, those species that are present tend to be con-

centrated in just a few families (Fig. 1). In the Arctic, Buccinidae s.l.

plus Conoidea (minus Conidae and Terebridae) comprises 89% of the

fauna, and in the Antarctic the equivalent ﬁgure is 74%; however, in

comparison, the tropical fauna comprises only 32% of these families/

family groups (Crame, 2013) (Fig. 1). The two polar faunas show pat-

terns of higher dominance/lower evenness than the tropical one and

can perhaps be taken to be end-members of latitudinal gradients in

species evenness in both hemispheres. This pattern is very similar to

that established in the northern hemisphere for predatory gastropods as

a whole (i.e. Neogastropoda + littorinimorph superfamilies Naticoidea

and Tonnoidea) (Taylor and Taylor, 1977).

When relative abundance distributions (RADs) are ﬁtted statistically

to the three faunas shown in Fig. 1, using the R ‘vegan’ package

(Oksanen et al., 2016), and the number of species occurring in each

family, they show two sharply contrasting patterns: whereas the tro-

pical fauna was best ﬁt by the broken stick model, both the Arctic and

Antarctic were closest to a Zipf distribution (Fig. 2) (Crame et al.,

2018). The former of these distributions represents the comparatively

even apportionment of some major environmental resource, and may

perhaps be expected for a large group of species from essentially the

same trophic guild (May, 1975). But the latter has a very diﬀerent

shape and reﬂects the much higher level of dominance in the polar

faunas (Matthews and Whittaker, 2014) (Fig. 2).

Comparisons of polar – tropical RADs were also used by Harnik

et al. (2010) in their investigation of the taxonomic structure of global

marine bivalve faunas. In this they used numbers of species within the

component genera of each fauna and found that the best ﬁt for four out

of ﬁve climatic zones and 26 out of 27 provinces was to the Zipf and

J.A. Crame Earth-Science Reviews 202 (2020) 103090

Zipf-Mandelbrot (Z-M) family of distributions. The Z-M is inﬂuenced by

two component parameter estimates: gamma, which measures the

evenness of species within genera, and beta, which measures evenness

within the most diverse genera; the Zipf uses only gamma, and is

equivalent to a Z-M where beta = zero. Both polar provinces were

found to have higher gamma values than their temperate and tropical

counterparts, and thus a signiﬁcantly more uneven taxonomic structure

(Harnik et al., 2010).

RADs (again using the number of species per family) have also been

used to compare the structure of the Antarctic Middle Eocene neogas-

tropod fauna with those of the two Middle Eocene tropical localities, US

Gulf Coast and Paris Basin (Crame et al., 2018). The result obtained is

almost exactly the same as that found for modern neogastropod faunas,

as the two tropical localities are best ﬁt by a broken stick, and the

Antarctic by a Zipf (Crame et al., 2018) (Fig. 2). An Early Cenozoic

latitudinal gradient in species richness was also matched by a gradient

in species evenness; the level of high – low latitude diﬀerentiation seen

at the present day in neogastropod faunas was already established by

the Middle Eocene (i.e. ~45 Myr ago) (Fig. 2).

Even though we lack further examples of RADs being ﬁtted to re-

gional faunas, there are strong indications that other modern polar

marine faunas are also characterised by the phenomenon of strong

dominance/low evenness; this is particularly so in the Southern Ocean.

4. The phenomenon of higher dominance/lower evenness in

modern polar biotas

An important example of high polar dominance is provided by

benthic (including bentho-pelagic) ﬁsh on the Antarctic continental

shelf and slope (Eastman, 2000). In total, some 213 species are known

from 18 diﬀerent families, but, within these, three groups are particu-

larly prominent: Liparidae (snail ﬁshes; 67 species), Zoarcidae (eelp-

outs; 23 species), and Notothenioidei (ﬁve component families; 96

species). Together, these three groups comprise 88% of the total benthic

ﬁsh fauna of Antarctica, and this ﬁgure rises to 92% in the most

southerly shelf seas (Eastman, 2000). Nevertheless, species richness on

its own fails to reﬂect the overwhelming dominance of notothenioids in

the Antarctic ﬁsh fauna in terms of abundance and biomass. This is

estimated to be as high as 90–95%, and it is apparent that no other

oceanic ecosystem is so dominated by a single taxonomic group of

benthic ﬁsh (Eastman, 2000;Clarke and Johnston, 1996;Briggs, 2003).

Unfortunately, the fossil record of notothenioids is extremely poor.

A partial dorsal cranium from the La Meseta Formation of Seymour

Island has been assigned to the modern non-Antarctic notothenioid

family Eleginopidae, and if this determination is correct then the stem

group of the modern Antarctic clade could be as old as 45 Ma (i.e.

Middle Eocene judging from the position of the locality on the revised

geological map of Seymour Island) (Eastman and Grande, 1991;Near,

Fig. 1. Regional comparison of present day neogastropod faunas between the

Arctic, Tropics and Antarctic. The histograms represent the number of species

within 18 common neogastropod families. The Arctic fauna is a compilation of

all shelf-depth taxa occurring north of 60°N, and the Antarctic of all shelf and

bathyal taxa south of the Polar Front. The Tropics fauna is an average of six

faunas: two from the Americas and four from the Indo-West Paciﬁc province.

Reproduced with slight modiﬁcations from Crame, 2013, (Fig. 1), where further

details on the construction of the three faunas are given. Neogastropod family

classiﬁcation based on Bouchet and Rocroi (2005) and Bouchet et al. (2011).

Fig. 2. Relative abundance distributions to show the contrast between present-

day polar and tropical neogastropod faunas. Relative abundance distributions

(RADs) are used to examine the structure of biotic assemblages by plotting %

relative abundance for each taxon (log scale, y axis) against its rank within the

assemblage (x axis). They can be constructed using either the number of in-

dividuals per species, or, as in this particular instance, the number of species

within a higher taxon such as a family. They are based on the principle that the

abundance of a particular taxon reﬂects the size of its realised niche, which in

turn is shaped by ecological interactions within a community or assemblage

(Magurran, 2004). Sometimes referred to as dominance – diversity plots, they

range in form from sub-vertical curves at one extreme (strong dominance) to

sub-horizontal, sigmoidal curves at the other (strong evenness). The polar fauna

(from Antarctica) ﬁts closest to the former category and a Zipf distribution, but

the tropical one is closer to the latter and a broken stick model. Redrawn from

Crame et al., 2018, (Fig. 3) where further information on the composition of the

respective faunas and the methods of model ﬁtting are contained. As the two

polar faunas show very similar Zipf distributions they have been amalgamated

here into just one curve. It should be noted that the polar – tropical contrast

seen at the present day is almost exactly repeated by a similar comparison of

Middle Eocene faunas of approximately 45 Ma age.

J.A. Crame Earth-Science Reviews 202 (2020) 103090

2004;Eastman, 2005). There is even some phylogenetic evidence to

suggest that it could be considerably older than this (Near, 2004).

Within the marine invertebrates the dominant group on the

Antarctic shelf is the peracarid crustaceans, comprising some 1350

species that are very largely assigned to either the Amphipoda or

Isopoda (Arntz et al., 1997;Clarke and Johnston, 2003;Dayton, 1990;

De Broyer et al., 2003;Brandt et al., 2007a). Two families, Iphime-

diidae and Epimeriidae, predominate within the approximately 470

amphipod shelf species (Lörz and Held, 2004), and consist of a series of

predators, scavengers and necrophagous taxa on largely sedentary

sponges, cnidarians, bryozoans and holothurians (De Broyer et al.,

2003;De Broyer and Jazdzewski, 1996). Continental shelf isopods in

turn constitute approximately 371 species, with 52% of them being

assigned to just four families: Munnopsididae, Paramunnidae, Serolidae

and Arcturidae (Dayton, 1990;Brandt et al., 2007a). As with the pro-

minent amphipods, our knowledge of the ecology of these families is

limited, but whereas the Arcturidae are thought to be mainly passive

ﬁlter feeders, the Serolidae are known to be both active predators and

scavengers (Brandt et al., 2007a, 2007b).

Although there is no Antarctic fossil record for the peracarid

Crustacea, a preliminary phylogenetic analysis links the South

American shelf-depth genera Cristaserolis and Leptoserolis with the

Australian genus Serolina (Brandt, 1991, 1992). It is suggested that

these taxa were only separated at the time of opening of the Tasman

Gateway some 30 Myr ago (Scher et al., 2015), and before this there

was a distinctive Eocene serolid isopod fauna around the shallow-water

southern Gondwana margins (Brandt, 1991, 1992).

5. Signiﬁcance of high dominance/low evenness in polar biotas

There is a general impression that, as high-latitude and polar or-

ganisms have to contend with more extreme and widely ﬂuctuating

environmental conditions, their constituent species have broader tol-

erances and less speciﬁc microhabitat requirements; selection for gen-

eralism and vagility would form an eﬀective barrier to extinction

(Chown and Gaston, 2000;Dynesius and Jansson, 2000). When di-

versity-dependent resources (i.e. those that can potentially be used up)

are considered in the marine realm it is apparent that attenuation of

trophic resources may be paramount, and in particular that caused by

the extreme seasonality of primary production at the base of the food

chain (Valentine, 1973, 1983;Clarke, 1988, 1990;Arntz et al., 1994;

Valentine et al., 2008). It has been estimated that there is over an order

of magnitude variability in primary production between the relatively

stable tropics and ﬂuctuating polar regions (Valentine and Jablonski,

2015;Krug et al., 2009), and within the latter there will be selection for

organisms that can either sustain long periods of starvation, or live on

material other than primary production (Valentine, 1983).

Many of the dominant benthic marine taxa in both polar regions are

clearly trophic generalists (Dayton, 1990;Valentine, 1983;Valentine

et al., 2008;Pearse, 1965;Arnaud, 1977). This is particularly so in the

Antarctic where examples of taxa known to have very broad-based diets

include the common buccinid gastropod, Neobuccinum eatoni, the star-

ﬁsh Diplasterias brucei, various lysaniassid amphipods, Parbolasia cor-

rugatus (common nemertean), at least four species of holothurians, and

three ophiuroids (Arntz et al., 1994;Pearse, 1965;Arnaud, 1970, 1977;

White, 1984;McClintock, 1994). Studies in both Adélie Land and

McMurdo Sound have shown that certain asteroids, echinoids, pycno-

gonids, isopods and amphipods are only necrophagous during the

austral winter when food supplies are particularly low (Arnaud, 1977).

Some benthic and bentho-pelagic notothenioid ﬁsh have been recorded

as feeding on up to 16 diﬀerent types of prey (Eastman, 2000;Brandt,

2000). In marked contrast, many tropical marine invertebrate species

have become specialised on a much narrower range of diversity-de-

pendent resources without the risk of extinction, and thus far more of

them can be packed into any given area (Taylor et al., 1980;Kohn,

1997;Krug et al., 2009).

Latitudinal gradients in the species richness of predatory gastropods

have already been linked to the stability of the production cycle in the

eastern and western Atlantic, and eastern Paciﬁc (Taylor and Taylor,

1977;Taylor et al., 1980;Taylor, 1981;Valentine et al., 2002). Broader

feeding types, and in particular members of the Buccinidae, char-

acterise the highest latitudes, and in the tropics neogastropod families

such as the Muricidae, Mitridae, Conidae and Terebridae have much

more restricted diets and frequently specialise on single prey items. The

prominence of certain conoidean families, such as the Mangeliidae, in

Arctic/sub-Arctic regions is linked to a preference for predation on

polychaetes. These are thought to be predominantly deposit feeders and

thus a predictable food resource not greatly inﬂuenced by seasonal

ﬂuctuations in the way that many suspension feeders would be (Taylor

and Taylor, 1977;Taylor, 1981).

6. The link between dominance, seasonality, and the diversity of

polar marine faunas

Production of diatoms and other phytoplankton at the base of the

polar marine food chain is concentrated within a dense but brief

summer bloom that is matched by an approximately six-fold increase in

herbivorous, carnivorous and omnivorous zooplankton species (Clarke,

1988). As well as its eﬀect on the dietary breadth of predatory taxa, a

short summer season of primary production is also very probably the

reason why many globally common families of suspension-feeding bi-

valves are either poorly represented, or completely absent, from both

polar regions (Nicol, 1967;Dell, 1972;Valentine et al., 2008). Unless

such bivalves have evolved the capacity to lie dormant for long periods,

or utilise an alternative food resource, they would simply not be able to

survive in the polar regions (Arntz et al., 1994, 1997).

When traced from south to north in the North Atlantic Ocean, both

benthic and pelagic groups show a signiﬁcant fall in diversity at ap-

proximately 40°N (Taylor and Taylor, 1977;Taylor et al., 1980;Angel,

1997). Whereas at latitudes <40°N warm surface waters contain low

concentrations of nutrients and the phytoplankton production cycle is

only weakly seasonal, at latitudes >40°N winter cooling of surface

waters results in convective overturning, mixing, and resupply of nu-

trients back into surface waters (Angel, 1997). At 40° - 60°N there is a

bimodal phytoplankton bloom (i.e. spring and autumn), and at >60°N

there is a single summertime bloom. These latitudinal changes in the

production cycle are thought to have a profound inﬂuence on both

pelagic and benthic diversity values (Angel, 1997), and as the 40° dip in

diversity is clearly marked at the family level (Taylor and Taylor,

1977), they may have done so over a considerable period of time.

The concentration of generalist and opportunist taxa in the polar

regions leads us to consider whether polar – tropical diversity contrasts

could be attributable, at least in part, to a latitudinal gradient in niche

width or overlap? This is especially so in terms of the trophic niche

where there is strong circumstantial evidence to indicate that, for es-

sentially predatory groups such as the Neogastropoda, diets are much

broader in the polar regions than the tropics. Such a concept is indeed

appealing but it has to be emphasised that it has not yet been subjected

to any rigorous form of testing. A rare attempt to do this was based very

largely in the terrestrial realm, but could not ﬁnd a positive relation

between latitude and either niche breadth or the number/proportion of

specialist taxa (Vázquez and Stevens, 2004). In the past, niche theory

has focused primarily on the individual niche and the phenomenon of

resource partitioning between potentially overlapping species. Never-

theless, this is only part of the story and we also need to consider the

concept of the population niche, which centres on how populations of

species are distributed within speciﬁc regions (Ricklefs, 2010). The

population niche is inﬂuenced by a series of regional gradients, such as

temperature, that cannot be partitioned directly by diﬀerent species

within a habitat. A mosaic of shifting patterns in the population niche

may allow species within a clade to coexist within otherwise similar

niche space (Ricklefs, 2010). Our concept of niche width is clearly

J.A. Crame Earth-Science Reviews 202 (2020) 103090

changing, and there would seem to be considerable scope to reassess its

applicability in the marine realm using taxa such as the Neogastropoda.

It remains as a potentially key, but largely unproven, mechanism for

generating polar – tropical diversity contrasts.

7. Synthesis: evolution of Early Cenozoic LDGs

A synopsis of how very strong latitudinal contrasts in marine

benthic diversity may have evolved through the Cenozoic is presented

in Fig. 3. In stage 1, 66 - ~62 Ma, there is very little direct evidence as

to how steep LDGs may have been, but it is suggested that the recovery

took place from a relatively ﬂat, low diversity feature with weaker la-

titudinal contrasts. It is stage 2, ~62–42 Ma, where signiﬁcant diversity

diﬀerences between the tropics and the poles were re-established and

steep LDGs produced (Fig. 3). This is where, at least in Antarctica, the

modern polar fauna ﬁrst becomes recognisable, but there is evidence to

suggest that some of the key component clades radiated at a much

slower rate than tropical counterparts; this is particularly so of very

large cosmopolitan clades such as the Neogastropoda, Imparidentia

bivalves and teleost ﬁshes. On reﬂection, it may well be that the evo-

lution of the modern LDG through the Early Cenozoic was due not so

much to an acceleration of net evolutionary rates in the tropics as to

their retardation in the high-latitude and polar regions. This in turn is

linked to the marked seasonality of primary production which is es-

sentially a time-invariant feature of both greenhouse and icehouse

worlds. Seasonality leads to a preponderance of generalist taxa in the

high-latitude and polar regions, and simply far fewer of these can be

accommodated than in the low-latitude and tropical regions.

Although steep LDGs evolved in the interval ~62–42 Ma, their

overall form was somewhat diﬀerent from that of today (Fig. 3). This

was the acme of the greenhouse world and we know that the tropics

reached into latitudes of 50°+, and occasionally even higher. There-

fore, the overall proﬁle of the LDG in both hemispheres is more likely to

have been that of a broad dome rather than the bell-shaped curve with

narrow peak of the present day (Fig. 3). The latter feature (stage 3) is

seen as at least a partial retraction feature in response to the onset of

global cooling at 34 Ma, but diversity is still expanding at a signiﬁcantly

higher rate in the tropics than in the high-latitude and polar regions.

All the indications are that in the critical greenhouse interval,

~62–42 Ma, evolutionary radiations were at a signiﬁcantly higher net

rate in the tropics than in the high-latitude and polar regions, and this is

the fundamental process driving the evolution of the LDG through the

Early Cenozoic (Jablonski et al., 2006;Mittelbach et al., 2007;Krug

et al., 2009;Gillman and Wright, 2014). However, it should be em-

phasised that such a process is not necessarily detrimental to the long-

term stability of polar biotas. There is in fact strong theoretical evidence

to suggest that broad-niched polar species will have saturated all the

available ecological space and are thus inherently resistant to invasion.

There is very probably a high proportion of stable, incumbent taxa in

the polar marine realm (Turner et al., 1996;Valentine et al., 2008).

The importance of temperature in playing at least a partial role in

the Early Cenozoic evolution of LDGs should not be overlooked.

Temperature can clearly inﬂuence the rate at which energy is trans-

ferred through the ecosystem, and some form of Metabloic Theory of

Biodiveresity, where an increase in temperature speeds up biochemical

kinetics and rates of molecular evolution, may yet be shown to have

widespread applicability (Brown, 2014;Clarke, 2017). In addition, it is

readily apparent that latitudinal range shifts driven by climate change

are a persistent feature through time and some of these clearly have the

ability to mould large-scale patterns (Jablonski et al., 2006;Condamine

et al., 2012;Huang et al., 2014). The concept of tropical niche con-

servatism, where modern taxonomic groups originated in the extensive

Early Cenozoic tropics but subsequently had little or no success in co-

lonising the high-latitude and polar regions, may have signiﬁcantly

enhanced existing LDGs over the last 34 Myr (Hawkins et al., 2006,

2007;Buckley et al., 2010;Jansson et al., 2013). Furthermore, in the

Fig. 3. Evolution of latitudinal diversity contrasts through the

Cenozoic era. Stage 1 represents the immediate aftermath of

the K/Pg mass extinction event when global faunas were still

in a recovery phase. In all probability the LDG was greatly

reduced at this time. Stage 2 represents the Early Cenozoic

greenhouse world when latitudinal temperature gradients

were much shallower than at the present day. However, la-

titudinal diversity gradients were present in the mid- to high-

latitudes and the overall form of the high – low latitude di-

versity contrast was that of a broad, low dome. Stage 3 is set

in the Late Cenozoic after the onset of global cooling at the

Eocene/Oligocene boundary. The high – low diversity con-

trast now has the more familiar modern proﬁle of a bell-

shaped curve with a much narrower peak. It is envisaged that

this is achieved by both the intensiﬁcation of seasonality in

the high-latitude and polar regions, and range retraction into

lower latitudes. Net rate of species increase (i.e. speciation

minus extinction) is signiﬁcantly higher in the tropics than at

the poles throughout the Cenozoic era.

Key: N – northern polar regions; S – southern polar regions; Eq

– Equator.

J.A. Crame Earth-Science Reviews 202 (2020) 103090

very highest levels of the Seymour Island sedimentary section (informal

mapping units Telm 6 & 7, also known as the Submeseta Formation) a

very marked reduction in taxonomic diversity has been linked to the

onset of major global cooling close to the E/O boundary (Ivany et al.,

2008). Durophagous (i.e. shell-breaking) predators such as crabs,

sharks and most teleost ﬁsh are almost completely absent from this

interval and it is thought that this in turn led to the proliferation of

dense, epifaunal, suspension feeding echinoderm communities domi-

nated by ophiuroids and crinoids (Aronson and Blake, 2001;Aronson

et al., 2007).

Nevertheless, the correspondence between temperature and taxo-

nomic diversity change through the studied Early Cenozoic sections is

not exact. At least in the marine realm, it is apparent that the really

steep diversity increase occurs between the Early and Middle Eocene,

just when global temperatures begin to level oﬀ and then gradually

decline (Zachos et al., 2008). This may represent some sort of time-lag

eﬀect in the diversiﬁcation process, or perhaps the intensity of the

Middle Eocene Climatic Optimum at ~40 Ma has been underestimated

(Crame et al., 2018)? And on a grander timescale it is very unlikely that

the long-term decline of global temperatures from the late Middle Eo-

cene onwards (Zachos et al., 2008) is matched by a parallel decline in

global biodiversity. In the marine realm, there is compelling evidence of

major later Cenozoic evolutionary radiations, including those in the

very large groups such as the Neogastropoda, Imparidentia bivalves and

teleost ﬁshes (Briggs, 2003;Stanley, 2007;Norris et al., 2013;Bush and

Bambach, 2015). Even in the Late Cenozoic, which has been regarded

traditionally as a period of low temperatures, ice sheet extension, and

extinction, it is clear that there was a pronounced phase of polar ra-

diations linked to a major expansion of diatom-based ecosystems (at ~

14 Ma) (Crampton et al., 2016;Crame, 2018). The full eﬀect of these

radiations in the benthic realm has yet to be established, but in the

pelagic realm it has clearly led to the development of low diversity –

high abundance assemblages showing strong dominance at each trophic

level (Ducklow et al., 2007;Murphy et al., 2016). In all probability this

is another area where marine taxonomic diversity levels are dictated

more by food supply than temperature. The widespread assumption

that temperature has been the primary driver of the LDG through deep

time (Brown, 2014;Mannion et al., 2014;Worm and Tittensor, 2018)

now needs to be critically re-examined.

We should also consider the eﬀects of area on the evolution of LDGs,

as the large extent of the Early Cenozoic tropics could have promoted

both speciation and reduced extinction relative to smaller biomes

(Rosenzweig, 1995). In a large-scale study of tree species diversity, Fine

and Ree (2006) could ﬁnd no correlation between the estimated area of

the modern tropical forest biome and tree species richness. However,

there was a good correlation with the expanded area of the Eocene

tropics and this has led to the development of a time-integrated, species

– area concept to explain high tropical diversity (Fine and Ree, 2006;

Fine, 2015). Within the marine realm the later increase in Cenozoic

diversity levels was undoubtedly inﬂuenced by the proliferation of coral

reefs (Bellwood and Wainwright, 2006;Harzhauser et al., 2008;

Kiessling et al., 2010;Norris et al., 2013), but this may be due as much

to an increase in three-dimensional habitat complexity as to two-di-

mensional surface area.

8. Conclusions

•A huge global pulse of biological diversiﬁcation in the wake of the

K/Pg mass extinction event was concentrated in the low-latitude

and tropical regions. This in turn led to the development of steep

LDGs well before the onset of global cooling at 34 Ma.

•We are beginning to appreciate that, at least in the marine realm,

high-latitude and polar regions are characterised not only by low

taxonomic diversity but also by high dominance/low evenness. The

taxonomic structure of polar communities is fundamentally diﬀerent

to that of tropical ones.

•The phenomenon of high dominance in polar communities is linked

to the marked seasonality of primary productivity at the base of the

food chain. This leads to available ecospace being dominated by

comparatively few broad-niched generalists.

•The form of the Early Cenozoic LDG was very diﬀerent to that of

today. The tropical zone was much broader and the steep drop-oﬀ in

species numbers occurred at a higher latitude.

•Both elevated temperatures and extended area could have promoted

diversiﬁcation in the Early Cenozoic tropics. But it is likely that the

evolution of steep LDGs was also signiﬁcantly enhanced by the re-

tardation of net evolutionary rates in the high-latitude and polar

regions. The seasonality of primary productivity is a time-invariant

feature that would have aﬀected both greenhouse and icehouse

worlds.

Declaration of Competing Interest

The author declares that he has no known competing ﬁnancial in-

terests or personal relationships that could have appeared to inﬂuence

the work reported in this paper.

Acknowledgements

The pioneering studies of both J.D. Taylor and J.W. Valentine into

the possible links between primary productivity and LDGs have greatly

inﬂuenced my thinking, as did the fundamental insights into the sea-

sonality of primary productivity by my BAS colleague, A. Clarke. I am

grateful to all of them for showing me a potential way forward with

their thought-provoking studies. Many BAS, UK, and international

collaborators have contributed to the establishment of the Early

Cenozoic biostratigraphy and palaeontology of Seymour Island; the

work of W.J. Zinsmeister and his associates has been particularly im-

portant in this respect. Financial support from the British Antarctic

Survey, Natural Environment Research Council, and NE/I005803/1 is

gratefully acknowledged. I would also like to thank two reviewers and

A. Negri for their helpful comments on the manuscript.

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Explanations for latitudinal diversity gradients must invoke rate variation

Article

Full-text available

Aug 2023
P NATL ACAD SCI USA

Erin Saupe

The latitudinal diversity gradient (LDG) describes the pattern of increasing numbers of species from the poles to the equator. Although recognized for over 200 years, the mechanisms responsible for the largest-scale and longest-known pattern in macroecology are still actively debated. I argue here that any explanation for the LDG must invoke differential rates of speciation, extinction, extirpation, or dispersal. These processes themselves may be governed by numerous abiotic or biotic factors. Hypotheses that claim not to invoke differential rates, such as 'age and area' or 'time for diversification', eschew focus from rate variation that is assumed by these explanations. There is still significant uncertainty in how rates of speciation, extinction, extirpation, and dispersal have varied regionally over Earth history. However, to better understand the development of LDGs, we need to better constrain this variation. Only then will the drivers of such rate variation - be they abiotic or biotic in nature - become clearer.

Late Cenozoic cooling restructured global marine plankton communities

Article

Full-text available

Feb 2023
NATURE

The geographic ranges of marine organisms, including planktonic foraminifera¹, diatoms, dinoflagellates², copepods³ and fish⁴, are shifting polewards owing to anthropogenic climate change⁵. However, the extent to which species will move and whether these poleward range shifts represent precursor signals that lead to extinction is unclear⁶. Understanding the development of marine biodiversity patterns over geological time and the factors that influence them are key to contextualizing these current trends. The fossil record of the macroperforate planktonic foraminifera provides a rich and phylogenetically resolved dataset that provides unique opportunities for understanding marine biogeography dynamics and how species distributions have responded to ancient climate changes. Here we apply a bipartite network approach to quantify group diversity, latitudinal specialization and latitudinal equitability for planktonic foraminifera over the past eight million years using Triton, a recently developed high-resolution global dataset of planktonic foraminiferal occurrences⁷. The results depict a global, clade-wide shift towards the Equator in ecological and morphological community equitability over the past eight million years in response to temperature changes during the late Cenozoic bipolar ice sheet formation. Collectively, the Triton data indicate the presence of a latitudinal equitability gradient among planktonic foraminiferal functional groups which is coupled to the latitudinal biodiversity gradient only through the geologically recent past (the past two million years). Before this time, latitudinal equitability gradients indicate that higher latitudes promoted community equitability across ecological and morphological groups. Observed range shifts among marine planktonic microorganisms1,2,8 in the recent and geological past suggest substantial poleward expansion of marine communities even under the most conservative future global warming scenarios.

The latitudinal gradient of functional diversity of Miocene marine mollusks from Chile

Article

Full-text available

Dec 2022
ECOGRAPHY

Understanding latitudinal variations in biodiversity is central for biogeography. Along the coasts of the Southeast Pacific, several taxa show inverse latitudinal patterns of biodiversity, i.e. increasing species numbers from lower to higher latitudes. A plausible explanation for these patterns is that fjords, formed during the Pleistocene glaciations, increased the diversity of available biotopes that allowed for higher diversity in high latitudes. Assessing this hypothesis requires us to analyze latitudinal patterns of functional diversity (which is intimately related to niche use) in the absence of fjords, i.e. earlier than their formation. Here we test if the fossil record before the generation of fjords shows higher functional diversity at lower than at higher latitudes (a ‘regular' functional diversity gradient). We analyzed four components of functional diversity (functional richness, functional divergence, functional dispersion and functional evenness) for a fossil marine mollusk fauna from the lower Miocene (~18 million years ago) across four regions spanning more than 10 latitudinal degrees of the Chilean coast (between ~34°S and 45°S). The functional richness of gastropods and bivalves decreased non‐linearly from low to high latitudes. Contrarily, the functional evenness of gastropods remained relatively constant across the region and that of bivalves peaked at 45°S. The functional divergence and functional dispersion of taxonomic groups did not exhibit clear patterns of latitudinal variation. The multivariate analysis uncovered non‐linear latitudinal patterns in the abundance of functional groups (gastropods and bivalves), with the largest abundance towards lower latitudes. These results suggest that the niche breadth of mollusk assemblages might have decreased with latitude in the lower Miocene. This work can shed light on the mechanisms underlying the latitudinal variation of diversity observed today, offering a better understanding of how these patterns changed over time.

NETWORK ANALYSIS OF THE MARINE MICROPALEONTOLOGICAL RECORD (PART 2): LATE CENOZOIC COOLING RESTRUCTURED GLOBAL MARINE PLANKTON COMMUNITIES

Conference Paper

Jan 2022

Patterns and processes in the history of body size in turritelline gastropods, Jurassic to Recent

Article

Full-text available

Mar 2023
PALEOBIOLOGY

Abstract.—Body size is an important trait with implications for energy use and ecology as well as generation time and evolutionary rates. Turritelline gastropods are widely distributed through geologic time and space, making them an excellent group for evaluating macroevolutionary patterns. To evaluate the pattern of body-size change in turritelline gastropods, we compiled a dataset of shell lengths of 316 species of turritelline gastropods spanning the Jurassic to Recent. Type specimens were almost always signifcantly larger than specimen distributions from the same species. We found that turritelline gastropod size was inversely correlated with latitude, a trend likely driven by the Neogene–Recent diversification of small-bodied Southern Hemisphere taxa. A time series model was applied to distinguish among three possible macroevolutionary patterns: unbiased random walk (no directional trend), biased random walk (directional trend), and stasis (no net change). We determined that turritelline gastropods have experienced stasis in body size throughout their evolutionary history, adding to the growing literature documenting directionless body-size trends in marine invertebrate clades. Stasis of geographically widespread clades may be the result of ecological variability across the environmental range occupied by the group or differential diversification into opposing environments. Turritelline life-history patterns, especially their reproductive strategy that combines a short life span and decline in growth rate around 1 year of age to reallocate energy to reproduction, might circumvent selection for longevity and larger size, while further decrease in minimum size is likely limited by feeding efficiency and anti-predatory defense. The expectation that species or clades should continue to evolve to occupy larger size classes conflicts with the evolutionary advantages of small size, which in turritelline gastropods include high generational turnover and larger population sizes that yield opportunities for genetic variance.

Alexander von Humboldt—Dilettante of Natural History or Oracle of Modern Science?

Chapter

Jan 2022

Ali Mehmet Celal Sengor

The fame of Alexander von Humboldt has declined in the measure that observational sciences lost prestige as against theoretical sciences. This state of affairs is a result of the mistaken view that theoretical sciences deal with ‘predictable generalities’ while observational sciences study ‘contingent particulars’, whereas any scientific statement checked against an insufficient number of observations, be it about the entire universe or, say, about biological evolution on one planet, or even about the history of development of one mountain belt, has precisely the same logical structure. The mistaken view has stemmed in part from the inapposite proposal on the ‘hierarchy of sciences’ by Auguste Comte in his Cours de Philosophie Positive. Popular opinion has long contrasted the way Alexander von Humboldt did science in a ‘Baconian’ way with Einstein’s way of doing science in an almost ‘Cartesian’ way. When examined closely, it is seen that this dichotomy is unreal and that both men were trying to solve different problems, both ultimately a part of the larger human project of understanding Nature, and both used essentially the same approach to do it (albeit in different languages: von Humboldt’s natural, Einstein’s formal), namely putting forward hypotheses and testing them with observation statements with a view to formulating ever better and more comprehensive models of the world around us, including ourselves. This unrealistic polarity assumed to be present in the character of science has had a negative influence on the policies of funding agencies and scientific journals, especially since the collapse, in the twentieth century, of the European colonial empires that had immensely furthered the observational sciences, to the detriment of our understanding of the natural world.KeywordsAlexander von HumboldtAlbert EinsteinSouth AmericaAsiaPhilosophy of scienceHistory of science

Humboldt, Biogeography, and the Dimension of Time

Chapter

Jul 2022

In the nineteenth century, Alexander von Humboldt and Alfred Russel Wallace laid out the basis for the field of biogeography, the discipline that studies the relation between organisms and their geographic distribution. Almost in parallel with the birth of biogeography, the foundations of geology were laid out, providing a dimension of time to spatial changes of the landscape. In this paper, we review the historical context of the early biogeographers and explore what the lasting significance of the nineteenth-century holistic research approach is in present research. We also discuss the historical context in which biogeography and geology developed, and how the concept of deep time, plate tectonics, and mountain building provided a broader perspective to biogeography. To illustrate the benefits of integrating geological and biological methods, we focus on the genesis and chronology of the elevational gradient, core to Humboldt’s work. In particular, we use as example the evolution of two mountain systems, the Andes in South America, and the Tibet-Himalaya-Hengduan region in Asia. We conclude that in the nineteenth century, an interdisciplinary approach and progress in the different scientific fields led to a paradigm shift in the understanding of drivers of biogeography. Implementing such integrative vision today, and aided by advances in molecular phylogenetics and geology, has enabled biogeographers to form new, and more accurate, models of mountain building, climate, and species evolution. These models are particularly relevant in view of present scenarios of climate change and conservation strategies.KeywordsBiogeographyEvolutionGeologyHumboldtMountain geobiodiversity hypothesis

Latitudinal diversity gradient dynamics during Carboniferous to Triassic icehouse and greenhouse climates

Article

Full-text available

Jul 2022
GEOLOGY

Understanding the complex patterns of latitudinal diversity gradients (LDGs) in deep time has been hampered by the absence of long-term records of LDGs through multiple climatic changes. We used records of marine invertebrate fossils to generate LDGs in each age bin from the Carboniferous icehouse to the Triassic greenhouse climates. We evaluated LDGs by subsampling, calculated evolutionary rates for different latitudinal zones, and assessed the modularity of the fossil data within latitudinal zones using the Louvain algorithm. Our results suggest that the LDG peaks may be shaped by multiple factors rather than alternating icehouse and greenhouse climates, although icehouse climates usually restrict diversity at high latitudes. In nearly all clades, peak diversity shifted northward during the late Carboniferous and early Permian, reflecting the northward drift of plates and increased habitat area. Changes in the steepness of the LDG was most pronounced at low latitudes and during biotic crises and recovery, while icehouse to greenhouse transitions created more deviation at high latitudes. Our results show a strong historical influence from previous LDG patterns in LDG dynamics, but one that was interrupted by upheavals in composition after dramatic environmental changes.

Perfect storms shape biodiversity in time and space

Article

Full-text available

Aug 2023

Many of the most dramatic patterns in biological diversity are created by “Perfect Storms” —rare combinations of mutually reinforcing factors that push origination, extinction, or diversity accommodation to extremes. These patterns include the strongest diversification events (e.g. the Cambrian Explosion of animal body plans), the proliferation of hyperdiverse clades (e.g. insects, angiosperms), the richest biodiversity hotspots (e.g. the New World Tropical Montane regions and the ocean's greatest diversity pump, the tropical West Pacific), and the most severe extinction events (e.g. the Big Five mass extinctions of the Phanerozoic). Human impacts on the modern biota are also a Perfect Storm, and both mitigation and restoration strategies should be framed accordingly, drawing on biodiversity's responses to multi-driver processes in the geologic past. This approach necessarily weighs contributing factors, identifying their often non-linear and time-dependent interactions, instead of searching for unitary causes.

Late Cenozoic evolution of the latitudinal diversity gradient

Article

Apr 2023

Alistair Crame

Aim The Late Cenozoic flourishing of polar marine ecosystems, just when temperatures were reaching their lowest levels, has always seemed anomalous. Such an observation is coupled with an increasing volume of molecular phylogenetic evidence to indicate that some polar taxa radiated at exceptionally high evolutionary rates. The canonical latitudinal diversity gradient (LDG) may not be underpinned by a parallel gradient in evolutionary rates. This study provides a review of this critical question. Location A variety of polar – tropical comparisons are made, with some emphasis on the Southern Ocean. Taxa Both marine and terrestrial taxa are used, with some emphasis on marine Mollusca. Methods High – low latitude comparisons of evolutionary rates over the last 66 Myr are made using two approaches: rates derived from molecular phylogenies, and rates derived from the fossil record. For the former of these data are taken from various marine and terrestrial taxa, and for the latter use is made of the excellent fossil record of marine Mollusca. Results For approximately the first half of the Cenozoic era (i.e., 66 Ma onwards) there is strong palaeontological evidence to suggest that the global LDG developed in a conventional way, with higher rates of origination in the tropics than at the poles. However, there then appears to have been a marked change in the latter half of the Cenozoic when origination rates increased in the high‐latitude and polar regions. The modern latitudinal diversity gradient may be a fossil feature that formed in the Early Cenozoic. Main Conclusions As the extra‐tropical regions expanded through the Cenozoic they provided increasingly more ecological opportunities for a wide variety of marine and terrestrial taxa. Substantially lower temperatures in the polar regions were clearly not an obstacle to spectacular evolutionary radiations such as that based on the krill ecosystem.

The delayed rise of present-day mammals

Article

Full-text available

Mar 2007

Did the end-Cretaceous mass extinction event, by eliminating non-avian dinosaurs and most of the existing fauna, trigger the evolutionary radiation of present-day mammals? Here we construct, date and analyse a species-level phylogeny of nearly all extant Mammalia to bring a new perspective to this question. Our analyses of how extant lineages accumulated through time show that net per-lineage diversification rates barely changed across the Cretaceous/Tertiary boundary. Instead, these rates spiked significantly with the origins of the currently recognized placental superorders and orders approximately 93 million years ago, before falling and remaining low until accelerating again throughout the Eocene and Oligocene epochs. Our results show that the phylogenetic 'fuses' leading to the explosion of extant placental orders are not only very much longer than suspected previously, but also challenge the hypothesis that the end-Cretaceous mass extinction event had a major, direct influence on the diversification of today's mammals. Molecular data and the fossil record can give conflicting views of the evolutionary past. For instance, empirical palaeontological evidence by itself tends to favour the 'explosive model' of diversification for extant placental mammals 1 , in which the orders with living representatives both originated and rapidly diversified soon after the Cretaceous/Tertiary (K/T) mass extinction event that eliminated non-avian dinosaurs and many other, mostly marine 2 , taxa 65.5 million years (Myr) ago 1,3,4. By contrast, molecular data consistently push most origins of the same orders back into the Late Cretaceous period 5-9 , leading to alternative scenarios in which placental line-ages persist at low diversity for some period of time after their initial origins ('phylogenetic fuses'; see ref. 10) before undergoing evolutionary explosions 1,11. Principal among these scenarios is the 'long-fuse model' 1 , which postulates an extended lag between the Cretaceous origins of the orders and the first split among their living representatives (crown groups) immediately after the K/T boundary 8. Some older molecular studies advocate a 'short-fuse model' of diversification 1 , where even the basal crown-group divergences within some of the larger placental orders occur well within the Cretaceous period 5-7. A partial molecular phylogeny emphasizing divergences among placental orders suggested that over 20 lineages with extant descendants (henceforth, 'extant lineages') survived the K/T boundary 8. However, the total number of extant lineages that pre-date the extinction event and whether or not they radiated immediately after it remain unknown. The fossil record alone does not provide direct answers to these questions. It does reveal a strong pulse of diversification in stem eutherians immediately after the K/T boundary 4,12 , but few of the known Palaeocene taxa can be placed securely within the crown groups of extant orders comprising Placentalia 4. The latter only rise to prominence in fossils known from the Early Eocene epoch onwards (,50 Myr ago) after a major faunal reorganization 4,13,14. The geographical patchiness of the record complicates interpretations of this near-absence of Palaeocene crown-group fossils 14-16 : were these clades radiating throughout the Palaeocene epoch in parts of the world where the fossil record is less well known; had they not yet originated; or did they have very long fuses, remaining at low diversity until the major turnover at the start of the Eocene epoch? The pattern of diversification rates through time, to which little attention has been paid so far, might hold the key to answering these questions. If the Cretaceous fauna inhibited mammalian diversification , as is commonly assumed 1 , and all mammalian lineages were able to radiate after their extinction, then there should be a significant increase in the net per-lineage rate of extant mammalian diversification , r (the difference between the per-lineage speciation and extinction rates), immediately after the K/T mass extinction. This hypothesis, along with the explosive, long-and short-fuse models, can be tested using densely sampled phylogenies of extant species, which contain information about the history of their diversification rates 17-20. Using modern supertree algorithms 21,22 , we construct the first virtually complete species-level phylogeny of extant mammals from over 2,500 partial estimates, and estimate divergence times (with confidence intervals) throughout it using a 66-gene alignment in conjunction with 30 cladistically robust fossil calibration points. Our analyses of the supertree indicate that the principal splits underlying the diversification of the extant lineages occurred (1) from 100-85 Myr ago with the origins of the extant orders, and (2) in or after the Early Eocene (agreeing with the upturn in their diversity known from the fossil record 4,13,14), but not immediately after the K/T boundary, where diversification rates are unchanged. Our findings-that more extant placental lineages survived the K/T boundary than previously recognized and that fewer arose immediately after it than previously suspected-extend the phylogenetic fuses of many extant orders and indicate that the end-Cretaceous mass extinction event had, at best, a minor role in driving the diversification of the present-day mam-malian lineages. A supertree with divergence times for extant mammals The supertree contains 4,510 of the 4,554 extant species recorded in ref. 23, making it 99.0% complete at the species level (Fig. 1; see also

Antarctic Marine Biodiversity

Article

Full-text available

Jan 2018

The Selandian (Paleocene) mollusc fauna from Copenhagen, Denmark: the Poul Harder 1920 collection

Article

Full-text available

Dec 2001

Kai Ingemann Schnetler

NOTE: This monograph was published in a former series of GEUS Bulletin. Please use the original series name when citing this monograph. Schnetler, K. I. (2001). The Selandian (Paleocene) mollusc fauna from Copenhagen, Denmark: the Poul Harder 1920 collection. Geology of Denmark Survey Bulletin, 37, 1-85. https://doi.org/10.34194/dgub.v37.5021 A detailed study has been made of the molluscan fauna in the material collected by Poul Harder in 1920 from the classical Danish early Selandian (Late Paleocene) locality in the Lellinge Greensand at Sundkrogen (the harbour of Copenhagen). A description is also given of the now submerged locality.The Harder collection, which has remained virtually unstudied for more than 75 years, is discussed in the interesting historical context that it was not included in the monograph on the Paleocene of Copenhagen by J.P.J. Ravn in 1939. Ravn's study was based on material collected the same year from Sundkrogen by A. Rosenkrantz, and on material collected in the thirties from other localities in the Copenhagen area. Some material collected by A. Rosenkrantz and others, but not dealt with by J.P.J. Ravn, is also included in the present study. The long-lasting controversy about publication rights relating to the Sundkrogen material is recalled.Twenty-seven new species are introduced, viz. Portlandia (Yoldiella) nielseni n. sp., Plicatula selandica n. sp., Laternula (Laternulina) ravni n. sp., Dentalium (Dentalium) sundkrogensis n. sp., Solariella (Solariella) ravni n. sp., Solariella (Solariella) hauniensis n. sp., Teinostoma (Teinostoma) ledoni n. sp., Entomope kirstineae n. sp., Harrisianella subglabra n. sp., Bittium (Bittium) oedumi n. sp., Cerithiopsidella (Vatopsis) rasmusseni n. sp., Seila (Notoseila) heilmannclauseni n. sp., Seila (Notoseila) anderseni n. sp., Thereitis weinbrechti n. sp., Cirsotrema (Cirsotrema) hauniensis n. sp., Opalia (Pliciscala) thomseni n. sp., Charonia (Sassia) danica n. sp., Siphonalia arlejansseni n. sp., Astyris (Astyris) lappanni n. sp., Streptolathyrus danicus n. sp., Streptolathyrus lemchei n. sp., Cancellaria (sensu lato) jakobseni n. sp., Pseudocochlespira rosenkrantzi gen. et sp. n., Actaeopyramis marcusseni n. sp., Chrysallida (Parthenina) peterseni n. sp., Syrnola (Syrnola) granti n. sp. and Cingulina harderi n. sp. Within the Turridae, Pseudocochlespira n. gen. is established.A total of 182 taxons are listed. Of these, 36 are new for the Lellinge Greensand, and 60 have not previously been recorded from Sundkrogen. The study demonstrates that several genera have their first occurrence datum in the Selandian. The Selandian mollusc fauna from Sundkrogen and elsewhere in the Copenhagen area has no equivalent in the North Sea Basin, but faunas from boulders of Selandian age from the south-eastern part of Denmark and the southern part of Sweden demonstrate affinities with the Sundkrogen fauna, whereas the fauna from the Kerteminde Marl demonstrates a lesser degree of affinity.The palaeoenvironment is interpreted as a transgression of the Selandian Sea with erosion of the underlying Danian sediments. The near-shore environment was followed by gradually increasing water depth, resulting in deposits of fine-grained sand and finally dark clay. The dark clay was probably deposited in a deep inlet from the eastern margin of the Selandian Sea.

A catalogue of Danian gastropods from the Baunekule facies, Faxe Formation, Denmark

Article

Full-text available

Apr 2015

This catalogue of 194 gastropod taxa is based on the collection of Danian gastropods from the Baunekule facies, Faxe Formation in eastern Denmark. The gastropod fauna is extremely rich and well preserved. Most of the gastropods (106 species) are referred to genus level only, 9 morphotypes to even higher taxonomical levels and 79 gastropods are referred to species level. The gastropods are classified following Bouchet & Rocroi (2005) as 4 different clades: Vetigastropoda (represented by 26 species and 10 superfamilies), Caenogastropoda (represented by 142 species and 17 superfamilies), Heterobranchia (represented by 23 species and 5 superfamilies) and Opisthobranchia (represented by 1 species and 1 superfamily). The new species Zaclys? nuetzeli n. sp. is introduced. The Faxe Formation is recognised as a cold-water coral ecosystem with interfingering smaller bryozoan mounds. The Baunekule facies is found in the upper part of the coral mound complex of the Faxe Formation, where it forms isolated lensoidal bodies in the flanks of some of the coral mounds. It is characterised by a high diversity invertebrate fauna that occurs in weakly consolidated coraldominated floatstone to rudstone. The diagenesis of the Baunekule facies is of special significance because a high proportion of the originally aragonite-shelled fauna is preserved by recrystallization to calcite during early burial diagenesis. Most of the gastropods are not known from other parts of the Faxe Fm. The fauna is very important for comparative evolutionary studies of fossil and modern gastropods on cold-water coral mounds. Many of the genera have not previously been recorded from Danian strata. None of the gastropod species found in the Baunekule facies are known for certain to range below the Cretaceous–Palaeogene boundary. The fauna is comparable to gastropods found on modern cold-water coral mounds in the North Atlantic. The gastropod fauna from the Baunekule facies is characterised by a very high diversity of rather small millimetre-sized gastropods with a preference for hard substrates; 63.9% of the taxa belong to the browsing carnivore trophic group, feeding mainly on sedentary animals. Surprisingly, the fauna contains some common occurrences of typically warm-water species. The fauna consists mostly of Cenozoic genera and up to 87% of the species may be endemic to the cold-water coral ecosystem of the Faxe Formation. The diverse and rather unusual gastropod fauna from the Baunekule facies is undoubtedly linked to the evolution of cold-water coral ecosystems.

Antarctic Paleontological Heritage: Late Cretaceous–Paleogene vertebrates from Seymour (Marambio) Island, Antarctic Peninsula

Article

Full-text available

Aug 2019

Marcelo A. Reguero

Antarctica has significant environmental, scientific, historic, and intrinsic values, all of which are worth protecting into the future. This continent has a discrete number of places of scientific interest that exhibit great potential as natural heritage sites; its geodiversity is of fundamental importance to scientific values of the continent, and the pursuit of geological and paleontological knowledge has had a strong influence on its historical values. Seymour Island was once called the 'Rosetta Stone' of Southern Hemisphere paleobiology, because this small island provides the most complete and richly fossiliferous Late Cretaceous-Paleogene sequence in Antarctica. In particular, fossil vertebrates form part of the evidence used in reconstructing the history of life on Antarctica. Paleontological heritage is considered a subset of geo-heritage that embodies both natural and historical components which has received only indirect recognition. Seymour Island is an outstanding paleontological area with high heritage value of its Late Cretaceous/Paleogene vertebrates and should be considered for geo-conservation and protection. This paper reviews vertebrate fossil occurrences and outcrops on Seymour Island and discusses some threats to these fossil sites.

Dinoflagellate cysts of the La Meseta Formation (middle to late Eocene), Antarctic Peninsula: implications for biostratigraphy, palaeoceanography and palaeoenvironment

Article

Full-text available

Jul 2019

Dinoflagellate cyst assemblages recovered from the La Meseta Formation cropping out in Seymour Island, Antarctic Peninsula, are studied herein and their distribution is compared with the biostratigraphic scheme available for the Palaeogene of the Southern Ocean and other high-latitude regions. In this way, the La Meseta Formation is dated as middle Lutetian to Priabonian (46.2–36 Ma), which differs from the age provided by other fossils, isotopes and also with the magnetostratigraphic scheme recently performed for the unit. The dinoflagellate cyst data support the proposal of ocean circulation patterns on the South American Shelf prior to the opening of Drake Passage. Assemblages from the La Meseta Formation contain Antarctic-endemic taxa which are also dominant in several circum-Antarctic sites, located south of 45° S. Their distribution reflects an ocean-circulation scheme with wide clockwise gyres surrounding Antarctica that were disrupted as a consequence of the deepening and definitive apertures of the Tasmanian Gateway and Drake Passage towards the Eocene/Oligocene transition. The palaeoenvironmental inference based on the S/D ratio (sporomorphs versus dinoflagellate cysts) and the P/G ratio (peridinioid versus gonyaulacoid dinoflagellate cysts) suggests an overall trend through the section from marine-dominated assemblages with poorly productive waters in the lower part of the section to more terrestrially dominated assemblages with increasing productivity in the upper part of the unit, reflecting a shallowing trend to the top.

An inverse latitudinal gradient in speciation rate for marine fishes

Article

Full-text available

Jul 2018
NATURE

Far more species of organisms are found in the tropics than in temperate and polar regions, but the evolutionary and ecological causes of this pattern remain controversial1,2. Tropical marine fish communities are much more diverse than cold-water fish communities found at higher latitudes3,4, and several explanations for this latitudinal diversity gradient propose that warm reef environments serve as evolutionary ‘hotspots’ for species formation5,6,7,8. Here we test the relationship between latitude, species richness and speciation rate across marine fishes. We assembled a time-calibrated phylogeny of all ray-finned fishes (31,526 tips, of which 11,638 had genetic data) and used this framework to describe the spatial dynamics of speciation in the marine realm. We show that the fastest rates of speciation occur in species-poor regions outside the tropics, and that high-latitude fish lineages form new species at much faster rates than their tropical counterparts. High rates of speciation occur in geographical regions that are characterized by low surface temperatures and high endemism. Our results reject a broad class of mechanisms under which the tropics serve as an evolutionary cradle for marine fish diversity and raise new questions about why the coldest oceans on Earth are present-day hotspots of species formation.

Why are coral reef communities so diverse?

Chapter

Dec 1997

Alan J. Kohn

The biodiversity of many ecosystems is under threat and although seas cover the majority of our planet's surface, far less is known about the biodiversity of marine environments than that of terrestrial systems. It is also not clear whether many of the patterns known to occur on land also occur in the sea. Until we have a firmer idea of the diversity of a wide range of marine habitats and what controls it, we have little hope of conserving biodiversity, or determining the impact of human activities such as mariculture, fishing, dumping of waste and pollution. This book brings together key studies from the deep sea and open ocean, to tropical shores and polar regions to consider how comparable the patterns and processes underlying diversity are in these different ecosystems. Marine Biodiversity will be a major resource for all those interested in biodiversity and its conservation.

A Theory of Global Biodiversity (MPB-60)

Book

Jun 2018

The number of species found at a given point on the planet varies by orders of magnitude, yet large-scale gradients in biodiversity appear to follow some very general patterns. Little mechanistic theory has been formulated to explain the emergence of observed gradients of biodiversity both on land and in the oceans. Based on a comprehensive empirical synthesis of global patterns of species diversity and their drivers, this book develops and applies a new theory that can predict such patterns from few underlying processes. The book shows that global patterns of biodiversity fall into four consistent categories, according to where species live: on land or in coastal, pelagic, and deep ocean habitats. The fact that most species groups, from bacteria to whales, appear to follow similar biogeographic patterns of richness within these habitats points toward some underlying structuring principles. Based on empirical analyses of environmental correlates across these habitats, the book combines aspects of neutral, metabolic, and niche theory into one unifying framework. Applying it to model terrestrial and marine realms, the book demonstrates that a relatively simple theory that incorporates temperature and community size as driving variables is able to explain divergent patterns of species richness at a global scale. Integrating ecological and evolutionary perspectives, the book yields surprising insights into the fundamental mechanisms that shape the distribution of life on our planet.

Fitting and comparing competing models of the species abundance distribution: assessment and prospect

Article

Jun 2014

Early Cenozoic evolution of the latitudinal diversity gradient

Abstract and Figures

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