The Placental Mammal Ancestor and the Post-K-Pg Radiation of Placentals

Abstract

To discover interordinal relationships of living and fossil placental mammals and the time of origin of placentals relative
to the Cretaceous-Paleogene (K-Pg) boundary, we scored 4541 phenomic characters de novo for 86 fossil and living species.
Combining these data with molecular sequences, we obtained a phylogenetic tree that, when calibrated with fossils, shows that
crown clade Placentalia and placental orders originated after the K-Pg boundary. Many nodes discovered using molecular data
are upheld, but phenomic signals overturn molecular signals to show Sundatheria (Dermoptera + Scandentia) as the sister taxon
of Primates, a close link between Proboscidea (elephants) and Sirenia (sea cows), and the monophyly of echolocating Chiroptera
(bats). Our tree suggests that Placentalia first split into Xenarthra and Epitheria; extinct New World species are the oldest
members of Afrotheria.

Full text

DOI: 10.1126/science.1229237
, 662 (2013);339 Science et al.Maureen A. O'Leary
Placentals K-Pg Radiation of−The Placental Mammal Ancestor and the Post
This copy is for your personal, non-commercial use only.
clicking here.colleagues, clients, or customers by , you can order high-quality copies for yourIf you wish to distribute this article to others
here.following the guidelines can be obtained byPermission to republish or repurpose articles or portions of articles
): April 2, 2013 www.sciencemag.org (this information is current as of
The following resources related to this article are available online at
http://www.sciencemag.org/content/339/6120/662.full.html
version of this article at: including high-resolution figures, can be found in the onlineUpdated information and services,
http://www.sciencemag.org/content/suppl/2013/02/07/339.6120.662.DC1.html
can be found at: Supporting Online Material
http://www.sciencemag.org/content/339/6120/662.full.html#related
found at: can berelated to this article A list of selected additional articles on the Science Web sites
http://www.sciencemag.org/content/339/6120/662.full.html#ref-list-1
, 58 of which can be accessed free:cites 250 articlesThis article
http://www.sciencemag.org/content/339/6120/662.full.html#related-urls
1 articles hosted by HighWire Press; see:cited by This article has been
http://www.sciencemag.org/cgi/collection/evolution
Evolution subject collections:This article appears in the following
registered trademark of AAAS. is aScience2013 by the American Association for the Advancement of Science; all rights reserved. The title CopyrightAmerican Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005.
(print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by theScience
on April 2, 2013www.sciencemag.orgDownloaded from

The Placental Mammal Ancestor
and the Post–K-Pg Radiation
of Placentals
Maureen A. O’Leary,
1,3
¶ Jonathan I. Bloch,
2
John J. Flynn,
3
Timothy J. Gaudin,
4
Andres Giallombardo,
3
Norberto P. Giannini,
5
*Suzann L. Goldberg,
3
Brian P. Kraatz,
3,6
Zhe-Xi Luo,
7
†Jin Meng,
3
Xijun Ni,
3
‡Michael J. Novacek,
3
Fernando A. Perini,
3
||
Zachary S. Randall,
2
Guillermo W. Rougier,
8
Eric J. Sargis,
9
Mary T. Silcox,
10
Nancy B. Simmons,
5
Michelle Spaulding,
3,11
Paúl M. Velazco,
5
Marcelo Weksler,
3
§
John R. Wible,
11
Andrea L. Cirranello
1,3
To discover interordinal relationships of living and fossil placental mammals and the time of
origin of placentals relative to the Cretaceous-Paleogene (K-Pg) boundary, we scored 4541
phenomic characters de novo for 86 fossil and living species. Combining these data with molecular
sequences, we obtained a phylogenetic tree that, when calibrated with fossils, shows that
crown clade Placentalia and placental orders originated after the K-Pg boundary. Many nodes
discovered using molecular data are upheld, but phenomic signals overturn molecular signals to
show Sundatheria (Dermoptera + Scandentia) as the sister taxon of Primates, a close link
between Proboscidea (elephants) and Sirenia (sea cows), and the monophyly of echolocating
Chiroptera (bats). Our tree suggests that Placentalia first split into Xenarthra and Epitheria;
extinct New World species are the oldest members of Afrotheria.
It is disputed whether orders of placental
mammals, the very diverse group of spe-
cies that includes humans, evolved before
or after the significant extinction horizon known
as the Cretaceous-Paleogene (K-Pg) event 66
to 65 million years ago (Ma) (1, 2). Different
models have been proposed to describe ordinal-
level diversification either before (short-fuse mod-
el), near (long-fuse model), or after (explosive
model) this boundary (3). The ~5100 living pla-
cental species collectively exhibit extreme size
ranges (1.5-g bumblebee bat to 190,000-kg blue
whale); dramatic locomotor diversity (e.g., run-
ning, flying, and swimming); and diverse de-
grees of encephalization (4). Moreover, extinct
species in the placental fossil record are even
more numerous and exhibit a broader range of
adaptations (5). Given this diversity, it is of in-
terest to determine the phenotype of the ances-
tral placental mammal.
The hypothesis that the oldest members of
crown Placentalia [the clade of all living pla-
cental mammals (6)] were present by ~100 Ma
in the Mesozoic Era has been supported by mo-
lecular clock analyses (7–9), which suggest that
at least 29 mammalian lineages (7), including
the stem lineages of Primates and Rodentia,
appeared in Late Cretaceous ecosystems (8)
and survived the massive K-Pg extinction event.
However, fossil evidence has not corroborated
this hypothesis, despite discovery of abundant,
well-preserved, small vertebrates (10). By con-
trast, phenomic phylogenies incorporating fossils
have placed ordinal and intraordinal specia-
tion of Placentalia after the K-Pg extinction
event (11).
Determining placental origins and relation-
ships has met with the practical challenge of
codifying phenomic data on a scale compara-
ble to that for genomic data to produce a max-
imally informed phylogenetic tree. We built a
phenomic character matrix (4541 characters;
403 constant and 482 parsimony uninformative)
using MorphoBank (12). The matrix contains
newly scored characters for 86 species repre-
senting all living placental orders plus 40 fossil
species, with more than 12,000 annotated im-
ages supporting the phenomic homologies. These
data were examined with molecular sequences
compiled from 27 nuclear genes from GenBank
(table S1).
Placental orders originated after the K-Pg
boundary. A single tree emerged from our com-
bined phenomic-molecular parsimony analy-
sis (Fig. 1; hereafter, “combined tree”); we also
performed extensive sensitivity analyses using
other tree-searching methods (13). We applied
multiple fossil ages for the oldest members of
the clades sampled and ghost lineage analysis
(14) to this tree to determine minimum diver-
gence dates using fossils alone (13). Results sup-
port the monophyly of most traditional orders
originally identified on the basis of phenotypes,
as well as interordinal groupings discovered
using molecular sequence data (Fig. 1 and Table
1). Twenty nodes (over 40%) are congruent in
partitioned molecular and phenomic analyses
(fig. S2).
When time-calibrated, this tree indicates
tha t none of the six, very complete Mesozoic fos-
sil species (e.g., Ukhaatherium,Maelestes,and
Zalambdalestes) sampled falls within crown clade
Placentalia. Instead, these Mesozoic fossils emerge
as nonplacental members of Eutheria or at
lower nodes. This tree suggests that interor-
dinal and ordinal diversification occurred within
the first few hundred thousand years after
the K-Pg event, and the first members of mod-
ern placental orders began appearing 2 to 3 mil-
lion years (My) later during the Paleocene. All
recent clock-based estimates for the ages of key
clades, with few exceptions, are substantially
older than indicated by the fossil record (7, 8, 15).
Ghost lineage estimates are minimum divergence
dates and may underestimate the timing of ac-
tual splits.
We find that only the stem lineage to Plac-
entalia crossed the K-Pg boundary and then spe-
ciated in the early Paleocene. We estimate that
the minimum age of the diversification of crown
Placentalia is just younger than the K-Pg bound-
ary, or ~36 My younger than molecular clock–
based mean estimates derived from supertree
(15) and supermatrix (7) analyses. We do not
find support for the hypothesis that 29 to 39
(7, 15) mammalian lineages, including Afrothe-
ria, Rodentia, Primates, Lipotyphla, Xenarthra,
RESEARCH ARTICLE
1
Department of Anatomical Sciences, School of Medicine, HSC
T-8 (040), Stony Brook University, Stony Brook, NY 11794–
8081, USA.
2
Florida Museum of Natural History, University of
Florida, Gainesville, FL 32611–7800, USA.
3
Division of Paleon-
tology, American Museum of Natural History, 79th Street and
Central Park West, New York, NY 10024–5192, USA.
4
Depart-
ment of Biological and Environmental Sciences, University of
Tennessee at Chattanooga, 615 McCallie Avenue, Chattanooga,
TN 37403–2598, USA.
5
Department of Mammalogy, American
Museum of Natural History, 79th Street and Central Park
West, New York, NY 10024–5192, USA.
6
Western University of
Health Sciences, Department of Anatomy, Pomona, CA 91766–
1854, USA.
7
Section of Vertebrate Paleontology, Carnegie
Museum of Natural History, 4400 Forbes Avenue, Pittsburgh,
PA 15213–4080, USA.
8
Department of Anatomical Sciences
and Neurobiology, University of Louisville, Louisville, KY 40292,
USA.
9
Department of Anthropology, Yale University, Post Office
Box 208277, New Haven, CT 06520–8277, USA.
10
Department
of Anthropology, University of Toronto Scarborough, 1265
Military Trail, Scarborough, Ontario M1C 1A4, Canada.
11
Sec-
tion of Mammals, Carnegie Museum of Natural History, 5800
Baum Boulevard, Pittsburgh, PA 15206, USA.
*Present address: Consejo Nacional de Investigaciones
Científicas y Tecnológicas, Facultad de Ciencias Naturales
e Instituto Miguel Lillo, Universidad Nacional de Tucumán,
Miguel Lillo 205, Código Postal 4000, Tucumán, Argentina.
†Present address: Department of Organismal Biology and
Anatomy, University of Chicago, 1027 East 57th Street,
Chicago, IL 60637, USA.
‡Present address: Institute of Vertebrate Paleontology and
Paleoanthropology, Chinese Academy of Sciences, 142 Xi-Zhi-
Men-Wai Street, Beijing, 100044, P. R. China.
§Present address: Department of Vertebrates, Museu Nacional,
Universidade Federal do Rio de Janeiro, Rio de Janeiro, RJ
20940-040, Brazil.
||Present address: Department of Zoology, Instituto de Ciências
Biológicas, Universidade Federal de Minas Gerais, Avenida
Antônio Carlos 6627, Belo Horizonte, Minas Gerais 31270-
901, Brazil.
¶To whom correspondence should be addressed. E-mail:
maureen.oleary@stonybrook.edu
8 FEBRUARY 2013 VOL 339 SCIENCE www.sciencemag.org662
on April 2, 2013www.sciencemag.orgDownloaded from

Fig. 1. Single tree from parsimony analysis of combined molecular and
phenomic data mapped onto the stratigraphic record (tables S2 and S3).
Crown clade Placentalia diversified after the K-Pg boundary with only the stem
lineage to Placentalia crossing the boundary. Black boxes indicate fossil taxa
hypothesized to be on lineages; black lines indicate stratigraphic ranges;
ranges and ghost lineages (orange) provide minimum divergence dates. When
the matrix includes only one terminal taxon of a crown order, two boxes ap-
pear: the oldest hypothesized member of the crown clade (the younger date)
and the oldest hypothesized taxon on the stem to the crown clade (the older
date). Crown clades (except Eutheria and Metatheria) are defined (table S4).
Space immediately younger than 65 Ma not to scale showing early Paleo-
cene interordinal diversification of Placentalia. Crown clades Marsupialia and
Monotremata also diversified post K-Pg boundary. Bremer support (BS)
(tableS8)abovenodes,jackknifevaluesbelownodes.
www.sciencemag.org SCIENCE VOL 339 8 FEBRUARY 2013 663
RESEARCH ARTICLE
on April 2, 2013www.sciencemag.orgDownloaded from

Table 1. Comparison of divergence datesformammalianclades(ta-
bles S2 and S3) between our combined data analysis, a combined data
supertree [Bininda-Emonds et al.(15)], and a molecules-only super-
matrix [Meredith et al.(7)]. Ranges and ghost lineages are illus-
trated in Fig. 1; date calculations are described in (14). Names listed
specify crown clades (table S4); older dates in (7,15)arenegative
differences (bold). –, no data; NA, not applicable because clade not
found in our study.
Taxon Oldest crown clade member
(this study)
Oldest crown clade member—Age midpoint
(range) and difference from our study
Higher clades Orders Taxon Clade age
(range) (Ma)
Bininda-Emonds
et al. (Ma) Difference Meredith
et al. (Ma) Difference
Mammalia
Linnaeus 1758
Dryolestida
[Martin et al.(38)]
166.2
(167.7–164.7)
166.2* 1.5 217.8
(203.3–238.2)
–50.1
Monotremata
Bonaparte 1838
Obdurodon insignis 20.0
(28.4–11.6)
63.6 T11.4 –35.2 36.7
(22.4–103.1)
–8.4
Theria
Parker & Haswell 1897
Sinodelphys szalayi 127.5
(130.0–125.0)
_ _ 190.0
(167.2–215.3)
–60.0
Marsupialia
Illiger 1811
Peradectes minor 64.85†82.5 T11.1 –17.7 81.8
(67.9–97.2)
–17.0
Placentalia
Owen 1837
Protungulatum donnae 64.85†101.3 T7.4 –36.5 101.3
(92.1–116.8)
–36.5
Xenarthra
Cope 1889
Riostegotherium yanei 58.3
(57.5–59.0)
72.5 T5.1 –13.5 65.4
(58.4–71.5)
–6.4
Epitheria
McKenna (21)
Protungulatum donnae 64.85†__NA_
Afrotheria
Stanhope et al.(39)
Prodiacodon crustulum 64.85†93.4 T3.0 –28.6 80.9
(74.4–96.5)
–16.1
Paenungulata
Simpson (40)
Simpsonotus praecursor 61.8
(62.5–61.0)
_ _ 64.3
(56.0–70.6)
–1.8
Hyracoidea
Huxley 1869
Prohyrax hendeyi 17.3
(23.0–11.6)
19.1 T0.8 –3.9 6.1
(3.9–8.3)
16.9
Macroscelidea
Butler 1956
Miorhynchocyon sp. 21.2
(22.4–20.0)
50.7 + 7.6 –28.3 49.1
(37.7–57.2)
–26.7
Tethytheria
McKenna (21)
Eritherium azzouzorum 58.7 _ _ _ _
Proboscidea
Illiger 1811
Primelephas gomphotheroides 14.2
(23.0–5.3)
19.5 T12.1 –3.5 5.3
(1.8–8.0)
17.7
Sirenia
Illiger 1811
Eotheroides aegyptiacum 44.5
(48.6–40.4)
52.2 T14.4 –3.6 31.4
(25.0–34.4)
17.2
Boreoeutheria
Springer & de Jong (41)
Protungulatum donnae 64.85†_ _ 92.0
(82.9–107.6)
–27.2
Laurasiatheria
Waddell et al.(42)
Protungulatum donnae 64.85†91.8 T2.6 –27.0 84.6
(78.5–93.0)
–19.8
Lipotyphla
Haeckel 1866
Litolestes ignotus 58.3
(58.9–57.8)
84.2 T2.1 –22.5 77.3
(70.7–85.8)
–15.6
Chiroptera
Blumenbach 1779
Archaeonycteris praecursor 55.5 74.9 T3.3 –19.4 66.5
(62.3–71.3)
–11.0
Perissodactyla
Owen 1848
Hyracotherium angustidens 52.9
(55.4–50.3)
58.2 T4.9 –2.8 56.8
(55.1–61.0)
–1.4
Pholidota
Weber 1904
Smutsia gigantea 5.0
(7.3–2.6)
19.9 T20.7 –12.6 25.3
(16.9–35.7)
–18.0
Carnivora
Bowditch 1821
Hesperocyon gregarius 43.3
(46.2–40.4)
67.1 T3.8 –20.9 54.7
(47.4–60.6)
–8.5
Artiodactyla
Owen 1848
Cainotherium sp. 44.9
(55.8–33.9)
74.1 T3.1 –18.3 65.4
(62.3–68.5)
–9.6
Euarchontoglires
Murphy et al.(88)
Purgatorius coracis 64.85†94.5 T2.0 –29.7 83.3
(74.1–97.8)
–18.5
Euarchonta
Wadell et al.(42)
Purgatorius coracis 64.85†_ _ 82.0
(73.7–97.4)
–17.2
Primates
Linnaeus 1758
Teilhardina brandti 53.1
(55.8–50.3)
87.7 T2.7 –31.9 71.5
(64.3–78.4)
–15.7
Dermoptera
Illinger 1811
No crown clade fossils No crown
clade fossils
13.0 T5.2 _ 7.4
(4.5–13.2)
_
Scandentia
Wagner 1855
Eodendrogale parvum 42.9
(48.6–37.2)
32.7 T2.6 15.9 55.9
(45.0–63.9)
–7.3
(Continued on next page)
8 FEBRUARY 2013 VOL 339 SCIENCE www.sciencemag.org664
RESEARCH ARTICLE
on April 2, 2013www.sciencemag.orgDownloaded from

Artiodactyla, and Chiroptera, each crossed the
K-Pg boundary.
We recognize Protungulatum donnae as the
oldest undisputed species within crown Plac-
entalia (Fig. 1), and this species dates to the
earliest Paleocene (13)withinanintervalex-
tending from the K-Pg boundary to ~200,000 to
~400,000 years later (16, 17). Integration of
fossils as primary data in the phylogeny indicates
that ~10 interordinal speciation events might
have occurred in as little as 200,000 years. Most
of the fossil species sampled across Placentalia
fall either within ordinal crown clades or on the
immediate stem to ordinal crown clades (excep-
tions are stem taxa to Glires, Tethytheria, and
Euungulata).
Our results also imply that the total clade
Eutheria (all species more closely related to
Placentalia than to any other living species) is
younger than estimated from prior studies. The
Cretaceous fossil Eomaia scansoria (125 Ma)
has previously been called a placental (18)or
eutherian (11, 18, 19); however, we find with
100% jackknife support that Eomaia falls out-
side of Eutheria as a stem taxon to Theria. The
oldest age of Eutheria in our study is con-
strained by taxa such as Maelestes andis91Ma.
The age of Theria is 127.5 Ma, a clade that some
molecule-based estimates previously suggested to
be 190 Ma (7).
Taxon Oldest crown clade member
(this study)
Oldest crown clade member—Age midpoint
(range) and difference from our study
Higher clades Orders Taxon
Clade age
(range) (Ma)
Bininda-
Emonds
et al. (Ma)
Differ-
ence
Meredith
et al. (Ma) Differ-
ence
Glires
Linnaeus 1758
Mimotona wana 63.4
(65.0–61.7)
_ _ 79.5
(71.5–94.1)
–14.5
Rodentia
Bowditch 1821
Sciuravus sp. 56.8 85.3 T3.0 –28.5 69.0
(64.1–74.8)
–12.2
Lagomorpha
Brandt 1885
Leporidae
[Rose et al.(43)]
53.0 66.8 T5.1 –13.8 50.2
(47.4–56.9)
2.8
*Fixed calibration point. †Age between 65.0 and 64.7 Ma, in the Cenozoic portion of Chron C29r, 230 to 420 ky above the K-Pg boundary (1, 2).
Taxon Oldest crown clade member
(this study)
Oldest crown clade member—Age midpoint
(range) and difference from our study
Higher clades Orders Taxon Clade age
(range) (Ma)
Bininda-Emonds
et al. (Ma) Difference Meredith
et al. (Ma) Difference
Fig. 2. Reconstructions of the phenotype of the hypothetical placental
ancestor derived from the combined data matrix optimized onto the tree in
Fig. 1. The mammal is shown in an early Paleocene ecosystem. (A)External
body, posture, and diet of insectivory; asterisk depicts the plant Paranymphaea
crassifola of the early Paleocene. (B) Cranium and dentary bone, (C)skeleton,
(D) brain in left lateral view, (E) ear ossicles and ectotympanic bone, (F)
uterus, and (G) sperm cell. Numbers designate a subset of the numerous
phenomic characters used to build these reconstructions (appendix S1).
www.sciencemag.org SCIENCE VOL 339 8 FEBRUARY 2013 665
RESEARCH ARTICLE
on April 2, 2013www.sciencemag.orgDownloaded from

Phenomic topologies dominate for key clades.
We resolve the basal diversification within Plac-
entalia, a historically unstable node (20), as a
split between Xenarthra and Epitheria (all other
placentals; 89% jackknife and Bremer support
of 14 steps). These clades were previously pre-
dicted to be monophyletic on the basis of phe-
nomic data alone (21). The phenomic data-only
tree (fig. S2A) supports the clade Sundatheria
[Scandentia (treeshrews) + Dermoptera (flying
lemurs)] as the sister taxon of Primates, a topol-
ogy that prevails in the combined analysis, in
contrast to molecules-only trees that favored
Dermoptera in this role (7, 22),
The existence of two clades within bats
(Chiroptera), one that echolocates (Microchi-
roptera) and one that does not (Megachiroptera),
emerges from the phenomic-only analysis (fig.
S2A), in contrast to molecule-based results (fig.
S2B) (7, 23, 24). Microchiroptera in our com-
bined data tree has low support, most likely
because the molecules-only and phenomic-
only trees each had 100% jackknife support
for mutually incompatible hypotheses. In the
combined tree, the phenomic signal is upheld
relative to the molecular signal. The arrange-
ment of two Eocene fossil bats (Onychonycteris +
Icaronycteris) as a sister clade to extant Mi-
crochiroptera also differs from prior results
(25). Molecules alone, here (figs. S2B and S3
to S6) and in prior studies (7), do not support
Tethytheria. Phenomic data alone do support
Tethytheria, and the combined data tree re-
tains Tethytheria with relatively strong support,
which corroborates previous combined data
analyses (26).
Regarding extinct species, many fossil hoofed
mammals are part of Laurasiatheria, and ex-
tinct relatives of Carnivora known as Creo-
donta lack deep linkages to African taxa (27).
Endemic South American ungulates are split be-
tween Pan-Euungulata and Afrotheria (28). The
fossil Rhombomylus has lagomorph (rabbit)
affinities (29), which implies that loss of incisors
occurred independently in Rodentia and Lago-
morpha. Extinct Palaeanodonta (Metacheiromys)
falls closer to Pholidota than to Xenarthra (30),
and Moeritherium is a member of Tethytheria.
Reconstructing the placental ancestor and
its dental formula. Integration of data for both
fossil and living species permits reconstruc-
tion of ancestral nodes across the placental
tree by using optimizations (Fig. 2 and ap-
pendix S1). We reconstructed the hypotheti-
cal placental ancestor using synapomorphic
and symplesiomorphic characters. It weighed
between 6 and 245 g (character 2026), was
insectivorous (characters 4531 and 4532) and
scansorial (character 4538), and single young
were born hairless with their eyes closed (char-
act er 4290). Females had a uterus with two horns
(character 4265) and a placenta with a tropho-
blast (character 4295), and males produced
sperm with a flat head (character 4274) and
had abdominal testes (characters 4228 and
4229) positioned just caudal to the kidneys.
The brain was characterized by the presence
of a corpus callosum (character 4493), an en-
cephalization quotient greater than 0.25 (char-
acter 4460), facial nerve fibers that passed
ventral to the trigeminal sensory column (char-
acter 4492), and a cerebral cortex that was
gyrencephalic (character 4462) with distinctly
separate olfactory bulbs (character 4482). A
hemochorial placenta (character 4313) opti-
mizes unambiguously to the base of Placen-
talia (31). The basal placental also lacked an
endodermal cloaca (character 4226), having
separate anal and urogenital openings.
Osteologically (Fig. 2), the placental an-
cestor had a triangular, perforate stapes (char-
acters 878 and 882) and lacked epipubic bones
(character 3290). Reconstructing soft tissues
not preserved in fossils is best done by op-
timization (32) when both soft tissues and
osteology have built the underlying tree. The
path of the internal carotid artery sometimes
leaves channels on adjacent bones and is fre-
quently reconstructed in fossils (33). We find
that this artery (scored as a soft tissue charac-
ter in living species) optimizes as present in
the ancestor of Placentalia; however, the three
osteological correlates of the artery are each
Fig. 3. (A) Split between Placentalia and Marsupialia and homologies of hypothesized adult
postcanine dental formulae. Gray lines indicate adult teeth compared for cladistic scoring. (B)
Reconstructions of the dentition of the hypothetical placental ancestor based on optimization
(appendix S1, see also figs. S11 to S14). Views are left side. From top to bottom: occlusal upper
teeth, labial upper teeth, labial lower teeth, and occlusal lower teeth. p, premolar; dp, deciduous
premolar; m, molar.
8 FEBRUARY 2013 VOL 339 SCIENCE www.sciencemag.org
666
RESEARCH ARTICLE
on April 2, 2013www.sciencemag.orgDownloaded from

absent (appendix S1). Thus, the conservative
hypothesis is that the artery was present in the
placental ancestor but did not leave an osteo-
logical correlate.
We have implemented a revised postcanine
tooth formula for clades within Theria and re-
constructed the dentition of the hypothetical
placental ancestor (Fig. 3A). Complexities of
homology arise in Theria regarding the num-
ber of premolars [any postcanine tooth with
two generations (deciduous or “baby”teeth and
permanent or “adult”teeth) or a postcanine
tooth anterior to a tooth with two generations
(34)]. Molars, by contrast, have a single gen-
eration and are posterior to teeth that are
replaced. Metatheria (and Marsupialia) prim-
itively have seven postcanine teeth, three of
which are premolars. The common ancestor of
Placentalia also had seven postcanine teeth,
but four of these were premolars, and some
nonplacental eutherians had as many as five
premolars (11).
The primitive dental formula for Theria op-
timizes on our tree (Fig. 3) to be seven post-
canine teeth: four premolars and three molars
(Fig. 3A). Both Metatheria and Placentalia have
each lost the third premolar (p3/P3) during
evolution. We hypothesize that the first tooth
in the molar series in Metatheria is homolo-
gous to the deciduous p5 (dp5) of eutherians,
including placentals (34, 35). What has been
lost in Metatheria is the p5 locus tooth re-
placement. The implication for phylogenetic
work is that a tooth at the fourth postcanine
locus of an adult metatherian (the retained
dp5) must be compared directly with a tooth
in the dentition of a juvenile placental (the
dp5). Accordingly, we revise the postcanine
dental formula for Theria such that the prim-
itive adult dentition for Placentalia consists
of upper and lower p1, p2, p4, p5, m1, m2,
and m3 [see (33)]. The reconstructed ances-
tral placental dentition lacks lower fourth
and upper fourth and fifth incisors (charac-
ters 1327, 1388, and 1391), lacks canine di-
morphism (characters 1404 and 1428), and
has small stylar shelves on the upper molars
(characters 2330 and 2476) (Fig. 3B and ap-
pendix S1).
Biogeography and placental paleoenvi-
ronments. Our relatively younger age esti-
mate for Placentalia means that there is no
basis for linking placental interordinal diver-
sification to the Mesozoic fragmentation of
Gondwana (8). The most ancient members of
Afrotheria included in our tree are extinct South
American ungulates and the North American
fossil Leptictis dakotensis, which suggests that
Afrotheria did not originate in Africa. The
oldest afrotherian is the North American lep-
tictid Prodiacodon crustulum, whose antiq-
uity constrains Afrotheria’s minimum age and
extends several afrotherian lineages into the
early Paleocene (Fig. 1). Members of Afrotheria
would have been present in two regions of
the New World by the early Paleocene. Given
that afrotheres are not found in the Mesozoic
and that South America was an island con-
tinent for most of the Late Cretaceous and
Cenozoic, a vicariant explanation for this pat-
tern is precluded. Afrotheres would have had
to disperse either from North to South Amer-
ica, or the reverse, in the Paleocene, and then
to Africa.
The early Paleocene diversification of pla-
centals occurred in a radically transformed
terrestrial ecosystem lacking nonavian dino-
saurs and other species terminated at the K-Pg
event (10). Maximum K-Pg extinction estimates
for plants are 57% of megaflora and 30% of
pollen-producing plants from North American
localities (36). In some areas, insects and plants
were substantially affected by the K-Pg event
(37), and such changes may have left availa-
ble to the insectivorous placental ancestor a
different diet than would have existed in the
Mesozoic. This interval of dramatic environ-
mental transformation would have bracketed
several interordinal speciation events within
Placentalia. The incompleteness of the fos-
sil record will always constrain what we can
infer about the past, but integration of phe-
nomic and genomic data have here corrobo-
rated the hypothesis that ordinal and interordinal
diversification of Placentalia most closely
fits the explosive model (3)andthatthere
was no Cretaceous Terrestrial Revolution (7)for
Placentalia.
References and Notes
1. K. F. Kuiper et al., Science 320, 500 (2008).
2. W. A. Berggren, P. N. Pearson, J. Foraminiferal Res. 35,
279 (2005).
3. J. D. Archibald, D. H. Deutschman, J. Mamm. Evol. 8, 107
(2001).
4. R. M. Nowak, Walker’s Mammals of the World
(Johns Hopkins Univ. Press, Baltimore, MD, 1999).
5. M. A. O’Leary, M. Allard, M. J. Novacek, J. Meng,
J. Gatesy, in Assembling the Tree of Life, J. Cracraft,
M. J. Donoghue, Eds. (Oxford Univ. Press, New York,
2004), pp. 490–516.
6. K. de Queiroz, Syst. Biol. 56, 956 (2007).
7. R. W. Meredith et al., Science 334, 521 (2011).
8. W. J. Murphy et al., Nature 409, 614 (2001).
9. M. S. Springer, J. Mamm. Evol. 4, 285 (1997).
10. M. J. Novacek, Ann. Mo. Bot. Gard. 86, 230
(1999).
11. J. R. Wible, G. W. Rougier, M. J. Novacek,
R. J. Asher, Bull. Am. Mus. Nat. Hist. 327,1
(2009).
12. M. A. O’Leary, S. Kaufman, Cladistics 27, 1 (2011).
13. Materials and methods are available as supplementary
materials on Science Online.
14. M. A. Norell, in Extinction and Phylogeny, M. J. Novacek,
Q. D. Wheeler, Eds. (Columbia Univ. Press, New York,
1992), pp. 89–118.
15. O. R. P. Bininda-Emonds et al., Nature 446, 507
(2007).
16. J. J. Eberle, Rocky MT Geol. 38, 143 (2003).
17. D. L. Lofgren, J. A. Lillegraven, W. A. Clemens,
P. D. Gingerich, T. E. Williamson, in Late Cretaceous
and Cenozoic Mammals of North America,
M. O. Woodburne, Ed. (Columbia Univ. Press,
New York, 2004), pp. 43–105.
18. Q. Ji et al., Nature 416, 816 (2002).
19. Z.-X. Luo, C.-X. Yuan, Q.-J. Meng, Q. Ji, Nature 476,
442 (2011).
20. A. B. Prasad, M. W. Allard, E. D. Green; NISC
Comparative Sequencing Program, Mol. Biol. Evol. 25,
1795 (2008).
21. M. C. McKenna, in Phylogeny of the Primates,
W. P. Luckett, F. S. Szalay, Eds. (Plenum, New York,
1975), pp. 21–46.
22. J. E. Janecka et al., Science 318, 792 (2007).
23. C. M. Miller-Butterworth et al., Mol. Biol. Evol. 24, 1553
(2007).
24. E. C. Teeling et al., Science 307, 580 (2005).
25. N.B.Simmons,K.L.Seymour,J.Habersetzer,G.F.Gunnell,
Nature 451, 818 (2008).
26. R. J. Asher, M. J. Novacek, J. H. Geisler, J. Mamm. Evol.
10, 131 (2003).
27. F. Solé, E. Gheerbrant, M. Amaghzaz, B. Bouya, Zool. J.
Linn. Soc. 156, 827 (2009).
28. F. L. Agnolin, N. R. Chimento, Mamm. Biol. 76, 101
(2011).
29. R. J. Asher et al., Science 307, 1091 (2005).
30. T. J. Gaudin, R. J. Emry, J. R. Wible, J. Mamm. Evol. 16,
235 (2009).
31. D. E. Wildman et al., Proc. Natl. Acad. Sci. U.S.A. 103,
3203 (2006).
32. H. N. Bryant, A. P. Russell, Philos. Trans. R. Soc. London
B Biol. Sci. 337, 405 (1992).
33. M. J. Novacek, Bull. Am. Mus. Nat. Hist. 183,1
(1986).
34. W. P. Luckett, in Mammal Phylogeny: Mesozoic
Differentiation, Multituberculates, Monotremes, Early
Therians, and Marsupials, F. S. Szalay, M. J. Novacek,
M. C. McKenna, Eds. (Springer, New York, 1993),
pp. 182–204.
35. R. Owen, The Anatomy of the Vertebrates , vol. 3,
Mammals (Longmans, Green and Co., London,
1868).
36. P. Wilf, K. R. Johnson, Paleobiology 30, 347 (2004).
37. P. Wilf, C. C. Labandeira, K. R. Johnson, B. Ellis, Science
313, 1112 (2006).
38. T. Martin, A. Averianov, A. Lopatin, J. Vert. Paleontol. 31
(Suppl. 2), 153 (2011).
39. M. J. Stanhope et al., Proc. Natl. Acad. Sci. U.S.A.
95, 9967 (1998).
40. G. G. Simpson, Bull. Am. Mus. Nat. Hist. 85,1
(1945).
41. M. S. Springer, W. W. de Jong, Science 291, 1709
(2001).
42. P. J. Waddell, N. Okada, M. Hasegawa, Syst. Biol. 48,
1 (1999).
43. K. D. Rose et al., Proc. Biol. Sci. 275, 1203 (2008).
Acknowledgments: We thank R. Asher, L. Jurgielewicz,
M. Marotta, S. Parent, E. Seiffert, and E. Woodruff for data
collection; K. Johnson for paleobotanical contributions;
K. de Queiroz and A. Turner for discussion; and S. Kaufman,
K. Alphonse, M. Passarotti, and D. Ferguson for software
development. Artist C. Buell drew Fig. 2A and L. Betti-Nash all
other figures. Research assistance came from P. Bowden,
D. Malinzak, S. B. McLaren, N. Milbrodt, R. Morgan, and
J. Morgan Scott. Data are archived in the supplementary
materials and in Project 773 of the public repository
MorphoBank.org. Supported by NSF grants 0743309 and
0827993, and by 0629959, 0629836, and 0629811 from the
“Assembling the Tree of Life”program of the Divisions of
Environmental Biology and Earth Sciences.
Supplementary Materials
www.sciencemag.org/cgi/content/full/339/6120/662/DC1
Materials and Methods
Supplementary Text
Figs. S1 to S15
Tables S1 to S8
References (44–323)
Appendices S1 to S4
23 August 2012; accepted 12 December 2012
10.1126/science.1229237
www.sciencemag.org SCIENCE VOL 339 8 FEBRUARY 2013 667
RESEARCH ARTICLE
on April 2, 2013www.sciencemag.orgDownloaded from