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Ammonoid genus diversity through the Paleozoic. Arrows and expanded spindles show Frasnian/Famennian (367 Ma), Devonian/Mississippian (354 Ma), and Permian/Triassic (250 Ma) mass extinctions (13, 19). Intervals 1 to 14 correspond to chronostratigraphic divisions in Figs. 2 and 3; for abbreviations see (20).
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The septal sutures of 588 genera of Paleozoic ammonoids showed a 1600
percent increase in mean complexity over 140 million years. Within 475 ancestor/
descendant pairs, descendants were more than twice as likely to be more
complex than their ancestors. Twelve subclades (373 genera) averaged 34
percent increased complexity. These patterns are compat...
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Citations
... Given this, we use two different quantitative measures of morphological complexity, and stress that our results do not necessarily apply to complexity in general but to complexity as defined by these measures. There are other possible measures of complexity ( Saunders et al., 1999;McShea, 1996;Fleming and Mcshea, 2013) but they are not as easy to quantify in an objective way as we attempt here or are not easy to apply directly to 3D morphologies. Roughly, our two measures reflect the likelihood of randomly guessing the position of a cell in 3D space knowing the position of its neighbors at different distances but knowing nothing about the developmental mechanism that produced such morphology. ...
Embryonic development involves gene networks, extracellular signaling, cell behaviors (cell division, apoptosis, adhesion, etc.) and mechanical interactions. How should gene networks, extracellular signaling and cell behaviors be coordinated to lead to complex and robust morphologies?
To explore this question, we randomly wired genes and cell behaviors into a huge number of networks in EmbryoMaker. EmbryoMaker is a general mathematical model of animal development that simulates how embryos change, i.e. how the 3D spatial position of cells change, over time due such networks. Real gene networks are not random. Random networks, however, allow an unbiased view on the requirements for complex and robust development.
We found that the mere autonomous activation of cell behaviors, especially cell division and contraction, was able to lead to the development of complex morphologies. We also found that complex morphologies tend to be less robust to noise than simple morphologies. However, we found that morphologies that developed through extracellular signaling and complex gene networks were, on average, more robust to noise. This stabilization occurs when gene networks and extracellular signaling partition the embryo into different regions where cell behaviors are regulated in slightly different ways. Our results are consistent with theories proposing that morphological complexity arose in early metazoan evolution as a consequence of the cell bio-mechanics already present in protozoa and that robustness evolved by the co-option of gene networks and extracellular cell signaling.
... This gap in knowledge does not mean, however, that conch morphology is completely unrelated to environmental constraints. As shown in the sections above, progressing trends in the complexity increase of ammonoid suture lines follow strict morphogenetic rules and appear to be uncoupled from environmental perturbations (SAUNDERS et al. 1999). The general outline of the suture lines can be explained to be supporting buttressing (SEILA- CHER 1975, 1988 ), and increasing crenulation by digitation of the marginal septa can be seen to increase the transportation capacity of cameral liquid (KULICKI & MUTVEI 1988). ...
The anti-Darwinian “Typostrophe Theory” of O.H.Schindewolf can be put to the test by revisiting the ammonoid examples on which this macroevolutionary model was founded. It is shown
that none of the three theoretical elements saltationism, internalism, and cyclism can be supported by empirical data obtained
from ammonoid research. Putative saltations (“Typogenesis”) were feigned because of the lack of knowledge of intermediate
forms. Internalistic and orthogenetic development (“Typostasis”) can only be favoured by neglecting possible functions of
morphological characters. Preprogrammed extinction of “degenerated” clades (“Typolysis”) is unlikely when ruling out anthropocentric
views regarding ammonoid morphology. In terms of evolution of Palaeozoic ammonoids, there is no basis for the preference of
the “Typostrophe Theory” or some of its composing elements, including the “Type Concept” and “Proterogenesis”, over the Darwinian
evolutionary model and the Modern Synthesis.
Die antidarwinistische „Typostrophentheorie“ von O.H.Schindewolf wird mit den Ammonoiden-Beipielen getestet, auf welchen sie begründet worden ist. Es kann gezeigt werden dass keines der
drei theoretischen Elemente der Theorie (Saltationismus, Internalismus und Zyklismus) durch empirische Befunde gestützt werden
kann. Vermeintliche Saltationen („Typogenese“) werden durch das Fehlen von Zwischenformen vorgetäuscht. Internalistische und
orthogenetische Entwicklung („Typostase“) kann nur postuliert werden, wenn mögliche Funktionen abgelehnt werden. Vorprogrammiertes
Aussterben von „degenerierten“ Entwicklungslinien („Typolyse“) kann ausgeschlossen werden, wenn Ammonoideen-Morphologien frei
von anthropozentrischen Ansichten betrachtet werden. Auf Grund der Studie von paläozoischen Ammonoideen gibt es keinen Grund,
die „Typostrophenlehre“ oder einige der sie aufbauenden Elemente, wie das „Typus-Konzept“ und „Proterogenese“, dem darwinistischen
Evolutionsmodell vorzuziehen.
... Many terminal distributions are skewed, even on a log scale, and the assumption of constancy of parameters (at some temporal scale) is often not unreasonable. Also, the tests based on time-slice statistics often give results that are concordant with the ancestor-descendant test (McShea 1994;Maurer 1999;Saunders et al. 1999), suggesting that the time-slice data do in fact provide some access to trend dynamics, however crudely and indirectly. (Such data are also useful, of course, in analyzing temporal relationships among time-slice statistics themselves [e.g., taxonomic diversity and morphological disparity], i.e., where trend mechanism is not at issue [e.g., Foote 1992].) ...
It has been said that most scientists would rather use another scientist's toothbrush than his terminology. In this issue's Matters of the Record, John Alroy suggests certain conceptual and methodological changes for the study of mechanisms, or dynamics, of large-scale trends, and also a revision of current trend-mechanism terminology. On the conceptual and methodological matters, Alroy's suggestions are good ones (with one important exception), and here I applaud them and explain further why they ought to be taken seriously. Predictably, however, I do not endorse (some of) his terminological revisions.
The conceptual matters are more interesting, and I turn to them first. In what follows, I first give some background on trend mechanisms, formulating the issues in my own terms (of course), and then explain Alroy's position in those terms.
A large-scale trend is change in some summary statistic for a state variable (in the sense of McKinney 1990), such as an increase in mean size, in a clade, while mechanism refers to the pattern of change among the lineages within the clade that accounts for the trend. As Alroy explains, in recent years the focus has been on two categories of mechanism, what I have elsewhere called “passive” and “driven” (McShea 1994). A passive trend is one in which change among lineages passively follows the “structure” of the state space (Fisher 1986; McShea 1998). Thus, for example, in Stanley's now-classic argument, if selection or developmental constraint in a group imposes a lower limit on size, and if the group originates close to this lower limit, then mean size will increase as diversity increases and the clade diffuses away from the lower limit (Stanley 1973). The state space is “structured” in that parameter values vary across it in some organized way; in this case, the probability of size decrease …
... However, Wagner's later study of early Paleozoic snails did include phylogenetic data, and it did show nonrandom evolution within lineages after controlling for apparent sorting among lineages (Wagner 1996). Likewise, recent results on body mass in mammals (Alroy 1998) and suture complexity in ammonites (Saunders et al. 1999 ) also can only be interpreted as showing nonrandom evolution within lineages. Still, though, these three studies didn't try to quantify the relative importance of withinand among-lineage factors—they merely sought to establish the fact that within-lineage trends are real. ...
... Instead, most paleontological studies plot morphological distributions directly against time, or present a series of histograms or scatter plots depicting the same morphospace in different time slices (for example, all Cretaceous species in one plot and all Paleocene species in another). Many recent papers have employed this approach , including studies of palynomorphs (Lupia 1999), foraminiferans (Norris 1991; Arnold et al. 1995), trilobites (Foote 1991; Sundberg 1996; Smith and Lieberman 1999), crustaceans (Wills 1998), bivalves (Jablonski 1996), rostroconchs (Wagner 1997), gastropods (Roy 1994Roy , 1996 Wagner 1996 ), ammonites (Boyajian and Lutz 1992; Dommergues et al. 1996; Saunders and Work 1996; Saunders et al. 1999), brachiopods (Carlson 1992 ), blastozoans (Foote 1992), crinoids (Foote 1995 ), echinoids (Eble 1998), ungulates (Jernvall et al. 1996), and even theropod dinosaurs (Gatesy and Middleton 1997). The distributions (often derived from multivariate analysis of complex FIGURE 3. Sketched patterns of progressive morphospace occupation implied by distinct evolutionary dynamics . ...
... Despite this, a large body of extremely sophisticated ''empirical morphospace'' literature has placed a strong emphasis on morphology-by-time plots instead of morphologic change-by-ancestralmorphology plots. Most of these studies unfortunately do not have access to the kind of detailed, species-level phylogenetic data one ultimately would need to untangle withinfrom among-lineage dynamics, and to discriminate the wild variety of within-lineage dynamics illustrated in Figures 2 and 3. However , a small but rapidly growing cohort of studies like those of Wagner (1996), Saunders et al. (1999), and Smith and Lieberman (1999) does include the requisite data, suggesting that there are good reasons to be optimistic. ...
The study of evolutionary trends is one of the oldest and most intriguing topics in evolutionary biology and paleobiology (McNamara 1990). Workers since Cuvier, Lyell, and Owen have wanted to know if the fossil record demonstrates “progression” within temporal sequences of related organisms. Regardless of whether changes in the average values of morphological characters are progressive in any meaningful sense, these changes are still of great interest. In practice, questions about trends are most commonly framed by paleontologists in terms of “complexity” (however defined) or body size (McShea 1998a).
Research on evolutionary trends has intensified over the last few years, bringing several fundamental conceptual issues to a head. Here I analyze these conceptual problems, concluding that paleontologists should largely abandon a key method used in most studies: comparing among-species morphological distributions in successive time slices, irrespective of phylogenetic patterns. These data supposedly may distinguish random evolution, constant directional trends, and diffusion away from morphological boundaries. However, many other simple evolutionary dynamics may result in the same nonrandom trends, making it difficult to distinguish qualitatively distinct mechanisms using time-slice data. Thus, I will try to show that comparisons between ancestral and descendant morphologies need to be made instead.
The fundamental problem here is not so much mathematical or statistical as conceptual. Null hypotheses have not always been framed rigorously, and the logical connection between underlying hypotheses and methods for testing these hypotheses has sometimes been weak. Although paleontologists have long since embraced the use of null models, they still do not employ a single, straightforward definition of “randomness” (Eble 1999). As a result, different authors use different null hypotheses, and depending on their conceptual outlooks they may interpret the same kinds of patterns as either confirming or refuting the existence of trends. Considering the volume of literature and the century-long …
