morphological support for a close relationship between hippos and ...

MORPHOLOGICAL SUPPORT FOR A CLOSE RELATIONSHIP BETWEEN
HIPPOS AND WHALES
Author(s): JONATHAN H. GEISLER and MARK D. UHEN
Source: Journal of Vertebrate Paleontology, 23(4):991-996. 2003.
Published By: The Society of Vertebrate Paleontology
DOI: 10.1671/32
URL: http://www.bioone.org/doi/full/10.1671/32
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Journal of Vertebrate Paleontology 23(4):991–996, December 2003
� 2003 by the Society of Vertebrate Paleontology
991
RAPID COMMUNICATION
MORPHOLOGICAL SUPPORT FOR A CLOSE RELATIONSHIP BETWEEN
HIPPOS AND WHALES
JONATHAN H. GEISLER 1 and MARK D. UHEN 2
1 Department Geology/Geography and Georgia Southern Museum, Georgia Southern University, Statesboro, Georgia
30460-8149, geislerj@georgiasouthern.edu;
2 Cranbrook Institute of Science, 39221 Woodward Avenue, P.O. Box 801, Bloomfield Hills, Michigan 48303-0801,
uhen@umich.edu
Recent discoveries of the ankles of fossil whales, reported
by Gingerich et al. (2001) and Thewissen et al. (2001b), corroborated
the molecular hypothesis that Cetacea (whales, dolphins,
and porpoises) are closely related to artiodactyls (evenhoofed
mammals including hippopotami, pigs, deer, and camels);
however, major points of disagreement remain. A morphology-based
study incorporating some of these new data
(Thewissen et al., 2001b) supported the exclusion of Cetacea
from the clade of living artiodactyls. In contrast, a vast amount
of molecular data support placement of Cetacea within Artiodactyla,
as close relatives to Hippopotamidae (Gatesy et al.,
1996, 1999; Montgelard et al., 1997; Shimamura et al., 1997,
1999; Nikaido et al., 2001). Here we report that morphological
data from extinct and extant taxa support placement of Cetacea
within Artiodactyla as the closest relatives of Hippopotamidae
(Fig. 1B) and indicate that molecular and morphological evidence
for the phylogeny of these taxa are now much more congruent
than previously thought.
MATERIALS AND METHODS
Our result is based on a cladistic analysis of a modified version
of the character/taxon matrix of Geisler (2001a). The present
matrix incorporates new information on the early cetaceans
Artiocetus, Rodhocetus (Gingerich et al., 2001), and Pakicetus
(Thewissen et al., 2001b); includes some changes in the scoring
of Basilosaurus; adds the artiodactyls Amphirhagatherium weigelti
(Heller, 1934; Erfurt, 2000; Hooker and Thomas, 2001)
and Raoellidae (Kumar and Sahni, 1985; Thewissen et al.,
2001b); includes the mesonychid Ankalagon (AMNH-VP 776,
777, 2454; O’Leary et al., 2000); adds seven characters from
Thewissen et al. (2001b), and consists of 195 characters scored
for 69 mammalian taxa. The matrix and character list are available
on the internet at http://www.vertpaleo.org/jvp/JVPcontents.html.
Of the 195 characters, 121 are binary, 14 are unordered
multistate characters, and 60 are ordered multistate characters.
Although we do not claim to have included every published
character relevant to cetartiodactyl phylogeny, we note
that this matrix has substantially more taxa and more morphological
characters than previous phylogenetic analyses (e.g.,
Geisler and Luo, 1998; O’Leary, 1998, 2001; Luo and Gingerich,
1999; O’Leary and Geisler, 1999; O’Leary and Uhen,
1999; Thewissen et al., 2001b).
The computer program NONA 1.9 (Goloboff, 1994) was
used to find most parsimonious trees. An initial search was
conducted using two commands: mult*100; hold/10, which invoke
100 iterations of TBR (tree bisection and reconnection)
branch swapping and save only 10 trees per iteration. The most
parsimonious tree from the initial search was used as a starting
tree for 1,000 iterations of the parsimony ratchet (Nixon, 1999),
which was implemented with the command nix*1000. Bremer
support values were calculated using the programs PAUP 3.1.1
(Swofford, 1993) and TreeRot (Sorenson, 1996), with modifications
to the TreeRot commands file as described in Geisler
(2001a). Lists of unequivocal synapomorphies for each node
were compiled using the apo/ command in NONA 1.9. Where
we describe synapomorphies supported by our study, we cite
previous studies that have reached the same conclusion.
Institutional Abbreviations AMNH, American Museum
of Natural History, Departments of Mammalogy and Vertebrate
Paleontology (New York); GSM, Georgia Southern Museum,
Vertebrate Collection (Statesboro, Georgia); IVPP, Institute of
Vertebrate Paleontology and Paleoanthropology, Chinese Academy
of Sciences (Beijing, China).
RESULTS
A total of 45 most parsimonious trees of 1,513 steps in length
were found (within-taxon polymorphism was not counted as
extra steps). All most parsimonious trees have Cetacea deeply
nested within Artiodactyla as the sister-group to Hippopotamidae
(Fig. 1B), like molecular studies and unlike the most recent
morphological-based analysis (Thewissen et al., 2001b) (Fig.
1A). The novel morphological result reported here is primarily
attributed to recently described cetacean fossils (Gingerich et
al., 2001; Thewissen et al., 2001b), because a study with similar
characters and taxa, but without the new ankle data, supported
the exclusion of Cetacea from Artiodactyla and a close relationship
between Cetacea and Mesonychidae (Geisler, 2001a),
an extinct group of hoofed mammals. We concur with recent
authors (Gingerich et al., 2001; Thewissen et al., 2001b) that a
suite of characters in the ankles of early whales supports a clade
comprised of Cetacea and Artiodactyla but not mesonychids.
The sister-group to the hippo/whale clade varies among the
most parsimonious trees; in 36 trees the sister-group is a clade
of suiform artiodactyls including Suina (pigs and peccaries),
Entelodontidae, and Anthracotheriidae (represented by Elomeryx);
and in the remaining nine trees it is Raoellidae. The latter
trees are interesting because like cetaceans, raoellids possess a
P4 paracone that is much higher than those of the succeeding
molars. Like pakicetids (Thewissen and Hussain, 1998) and the
protocetid Artiocetus (Gingerich et al., 2001), raoellids such as
Kunmunella kalakotensis (Kumar and Shani, 1985) also have a
single-rooted P1. Intriguingly, raoellid fossils are abundant in
the same Asian sites that produce pakicetids (Thewissen et al.,
2001a). Unfortunately, the non-dental anatomy of raoellids is
undescribed. Discoveries of new raoellid fossils would not only
test the hypothesis that they are closely related to hippos and

992 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 23, NO. 4, 2003
FIGURE 1. A comparison of the strict consensus tree of Thewissen et al. (2001b) (A) and the strict consensus tree for the present study (B).
Note that in the consensus of Thewissen et al. (2001b) the Cetacea are not inside the clade of extant artiodactyls, and the hippopotamus is not
closely related to whales. In contrast, the morphological data analyzed in this study support the inclusion of Cetacea within Artiodactyla as the
sister-group to Hippopotamidae, as suggested by numerous molecular studies (e.g., Gatesy et al., 1999). To facilitate comparison and to highlight
key features, some taxa have been collapsed into higher-level groups. In cases where a higher-level taxon includes three or more taxa, parentheses
are used to describe the phylogeny in the strict consensus of the present study: (all taxa with ‘‘*’’ are in the current study only): Cameloidea �
(Eotylops* (Poebrotherium (Llama*, Camelus*))); Entelodontidae (Archaeotherium); Hapalodectes � (H. hetangensis*, H. leptognathus*); Mesonychidae
� (Dissacus praenuntius*, Dissacus navajovius*, Mongolian Dissacus* (Ankalagon (Sinonyx (Pachyaena gigantea*, Pachyaena
ossifraga*, (Mesonyx, Harpagolestes, Synoplotherium))))); Mysticeti � (Balaenoptera); Odontoceti � (Physeter (Tursiops, Delphinapterus); Oreodontoidea
� (Agriochoerus, Merycoidodon*); Perissodactyla � ((Equus*, Mesohippus*) (Heptodon*, Hyracotherium*)); Phenacodontidae �
(Meniscotherium, Phenacodus); Protoceratidae � (Leptoreodon* (Heteromeryx*, Protoceras*)); Ruminantia � (Hypertragulus* (Leptomeryx*
(Tragulus (Bos* (Odocoileus*, Ovis*))))); Suina � (Perchoerus* (Sus, Tayassu*)); and Xiphodontoidea � (Xiphodon*, Amphimeryx*). In the

GEISLER AND UHEN—WHALES AND HIPPOS CLOSELY RELATED
FIGURE 2. Posterior view of the left squamosal, petrosal, and tympanic
bulla of a juvenile Hippopotamus amphibius (AMNH 130247).
Lateral is to the left, dorsal is to the top, and the occipital bones have
been removed. Note the elongate mastoid process of the petrosal, a trait
also seen in cetaceans. Abbreviations: VII, stylomastoid foramen for
the facial nerve (cranial nerve VII); mp, mastoid process of the petrosal;
pet, petrosal; sq, squamosal; sqs, sutural surface on the squamosal for
the exoccipital; tyb, ectotympanic bulla.
whales but would also help resolve the ambiguity in the optimization
of several characters across higher-level cetartiodactyl
clades.
Only two of the characters that support a sister-group relationship
between Cetacea and Hippopotamidae are unequivocal
synapomorphies: absence of paraconules on upper molars and
absence of crest between the hypoconid and entoconid (i.e.,
hypolophid) on lower molars. Several other characters are
equivocal synapomorphies of the hippo/whale clade because
they cannot be scored in raoellids, and thus could be a synapomorphy
of Raoellidae � Cetacea � Hippopotamidae in nine
of the most parsimonious trees. These characters include salient
features such as the near absence of hair and the loss of sebaceous
glands (Gatesy et al., 1996), as well as previously unrecognized
features such as a low dentary condyle, wide fifth
metatarsal, nasals that terminate between the orbits, and elongate
mastoid process of the petrosal. In adults of Hippopotamus
amphibious the mastoid process appears to be absent; however,
it has only fused to the squamosal. An elongate, anteroposteriorly-compressed
mastoid process is visible in skulls of juvenile
specimens (Fig. 2). Interestingly, mesonychids also have
an elongate mastoid process and a posterior position of the nasals.
Although these similarities are most-parsimoniously interpreted
as convergent on the shortest trees for this study, it is
clear that morphological data do not universally support a Hippopotamidae
and Cetacea clade. Further evidence for disagreement
between some morphological characters is the low Bremer
support for clades that render Artiodactyla paraphyletic: Cetar-
←
present study, coding for Cebochoeridae is based on Cebochoerus only, while Thewissen et al. (2001b) include data from Gervachoerus. Similarly,
Thewissen et al. (2001b) created a composite of Amphirhagatherium (Anthracobunodon) and Haplobunodon, while we used Amphirhagatherium
weigelti only. For specifics of the phylogenies within Mesonychidae in strict consensus A, readers are referred to Thewissen et al. (2001b). The
simplified consensus of the current study (B) includes clades common to all 45 most-parsimonious trees, each 1,513 steps in length.
993
tiodactyla has a Bremer support of 3; the Cetacea and Hippopotamidae
clade has a Bremer support of 2.
In all most-parsimonious trees, many ‘‘suiform’’ artiodactyls
(e.g., anthracotheres, entelodontids, suids, tayassuids) are closely
related to the Hippopotamidae and Cetacea clade. Even
though the exact phylogenetic arrangement of the ‘‘suiform’’
artiodactyls varies among our shortest trees, their proximity to
cetaceans and hippopotamids is supported by an enlarged facial
portion of the lacrimal. In Hippopotamus and early whales such
as Georgiacetus (Hulbert et al., 1998) and Remingtonocetus
(Kumar and Sahni, 1986:fig. 4) the distance between the anteriormost
point of the lacrimal and the anterior edge of the orbit
is greater than the anteroposterior diameter of the orbit (Fig.
3A, B). By contrast, mesonychids such as Sinonyx (Zhou et al.,
1995) have a much smaller exposure of the lacrimal on the face
(Fig. 1C). A large lacrimal also occurs on the face of extant
ruminants (e.g., Odocoileus, Ovis, Tragulus); however, this similarity
is interpreted as convergent because the basal ruminants
Hypertragulus (e.g., AMNH 53802, 1341) and Leptomeryx
(e.g., AMNH 11870) have a small facial portion of the lacrimal.
Although our morphological data and molecules agree on a
sister-group relationship between Hippopotamidae and Cetacea,
molecular data do not support a close relationship between the
cetacean/hippopotamid clade and ‘‘suiform’’ artiodactyls. Instead,
vastly different molecular characters including nucleotide
sequences (Gatesy et al., 1996, 1999; Montgelard et al., 1997),
SINEs (short interspersed nuclear elements) (Shimamura et al.,
1997, 1999; Nikaido et al., 1999), and multiple base-pair deletions
(Gatesy et al., 1996; Geisler, 2001b) support a clade
including Ruminantia (deer, cows, antelope), Hippopotamidae,
and Cetacea, but excluding Suina. As in previous studies, the
morphological data analyzed here continue to support monophyly
of Neoselenodontia, which includes the sister-groups
Ruminantia and Cameloidea. Neoselenodontia is supported by
several characters, including small supraspinatus fossa of the
scapula, tibia and fibula fused at proximal ends, and middle
portions of 2nd and 5th metatarsals absent (Geisler, 2001a).
Even so, the degree to which the morphological data of the
present study contradict Ruminantia � Hippopotamidae � Cetacea
is much less than previously published matrices. According
to the matrix of Geisler (2001a), this clade had a Bremer
support of �23, while in the present study its Bremer support
has increased by 9 steps to �14.
Unlike Geisler (2001a) but like many previous studies (e.g.,
Geisler and Luo, 1998; O’Leary, 1998; O’Leary and Geisler,
1999), the present study supports monophyly of Mesonychia
(sensu O’Leary, 1998) and Mesonychidae. Mesonychia has a
Bremer support of one and is diagnosed by seven unequivocal
synapomorphies: well developed parastyle on M1 (O’Leary and
Geisler, 1999), M2 metacone half the size of paracone, m3 hypoconulid
absent, molar paraconules absent, paraconid directly
anterior to protoconid (O’Leary, 1998), protoconid nearly twice
the height of talonid, and narrow talonid basin. Mesonychidae
has a Bremer support of four and is supported by three unequivocal
synapomorphies: postglenoid foramen small or absent
(Luo and Gingerich, 1999; O’Leary and Geisler, 1999),
foramen ovale anterior to glenoid fossa (O’Leary and Geisler,
1999), and hypocone and metaconule absent on upper molars.
Considering the paraphyly of Mesonychidae in the trees of
Geisler (2001a), it appears that the monophyly of Mesonychi-

994 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 23, NO. 4, 2003
FIGURE 3. Comparison of the lacrimals in a hippopotamus, a mesonychid, and a primitive cetacean. A, Hippopotamus amphibious (AMNH
15898), right side of skull in anterolateral, and slightly dorsal, view; B, the Eocene cetacean Georgiacetus vogtlensis (GSM 350), skull in right
lateral view; C, the mesonychid Sinonyx jiashanensis (IVPP V10760), skull in right lateral view, but image reversed to facilitate comparison.
Note the enlarged facial portion of the lacrimal in Hippopotamus and Georgiacetus, but not Sinonyx. Scale bar in each equals 5 cm. See Materials
and Methods for institutional abbreviations.
dae is contingent upon placing Cetacea within the clade of extant
artiodactyls.
Previously described dental/masticatory characters that support
a close relationship between mesonychids and cetaceans
(Thewissen, 1994; Geisler and Luo, 1998; O’Leary, 1998;
O’Leary and Geisler, 1999) are optimized as convergent on the
most parsimonious trees of the present study. The claim that
dental characters are convergent is not unprecedented. Three of
the characters that support a mesonychid and cetacean clade
(i.e., reduction of lower molar metaconids and talonid basins;
embrasure pits) also occur in several other distantly related
groups of carnivorous mammals (Muizon and Badre, 1997;
Uhen, 1996). We suspect that the convergence in dental characters
between cetaceans and mesonychians is due to a similarity
in function and that several dental characters may not be
independent. For example, the embrasure pits on the maxillae
in early cetaceans, many mesonychians, and some carnivorans
accommodate the protoconids of the lower teeth when the
mouth is closed. In living carnivorans and basilosaurid cetaceans,
the embrasure pits form in direct response to the approximation
of the teeth and the bones of the palate and jaw
during chewing (Uhen, 1996). Therefore, embrasure pits and
the height of the protoconid of the lower molars may not be
evolving independently. The hypothesis that other dental character
states (e.g., absence of molar metaconids, absence of paraconules
and metaconules) are interdependent is supported by
statistical studies that show them to be correlated among artiodactyls,
cetaceans, and mesonychians (Naylor and Adams,
2001; Geisler, 2001b). Even so, the correlation is not perfect,
indicating that a limited degree of independence remains and
that dental characters should remain in phylogenetic analyses
as separate characters.
The topology common to all our most parsimonious trees is
relatively robust to the exclusion of some anatomical characters.
Like O’Leary and Geisler (1999) and Geisler (2001a), but unlike
Thewissen et al. (2001b), we included soft-tissue morphological
characters (e.g., absence of hair, absence of sebaceous
glands). Although Thewissen et al. (2001b) did not explain why
they chose to exclude these features, their reason may be the
widely held view that the loss of hair and sebaceous glands is
prone to convergence and thus unreliable (see Luckett and
Hong, 1998, and references therein). Although we think that
homoplasy is best viewed as an a posteriori interpretation and
not an a priori assumption, we experimented with our matrix
by removing all six soft tissue characters (i.e., characters 181–
186) and re-running the phylogenetic analysis. Using the smaller
matrix, we found 297 most-parsimonious trees of 1,505 steps
each. As in our analyses with all characters included, some
most parsimonious trees supported by the osteology-only version
of our matrix had Cetacea within the clade of extant artiodactyls
as the sister-group to Hippopotamidae; however, the remaining
shortest trees had a monophyletic Cetartiodactyla with
Cetacea as the sister-group to a monophyletic Artiodactyla, a
result similar to that found by Thewissen et al. (2001b).
DISCUSSION
It is reasonable to assume that in the cetartiodactyl transition
from a terrestrial to an aquatic habitat, a semi-aquatic existence
was a necessary intermediate step. Hypotheses on how this
transition occurred and which taxa were semi-aquatic have relied
on the perceived sister group to Cetacea (Gatesy and
O’Leary, 2001; O’Leary, 2001). Based on the most-parsimonious
trees for our morphological data set, we infer that all

GEISLER AND UHEN—WHALES AND HIPPOS CLOSELY RELATED
cetaceans and the common ancestor of hippopotamids and cetaceans
were at least semi-aquatic. In support of this view, hippopotamids
are semi-aquatic, and they share with cetaceans
some aquatic adaptations (e.g., loss of sebaceous glands and
loss of hair). Our conclusions are contrary to those of Thewissen
et al. (2001b:278) who state that pakicetids were ‘‘no
more amphibious than a tapir.’’ Unlike a recent reconstruction
of Rodhocetus (Gingerich et al., 2001, cover illustration), character
optimizations on our hypotheses of relationships indicate
that the earliest whales had little to no hair.
A close relationship between Hippopotamidae and Cetacea
also challenges hypotheses that give feeding behavior a central
role in the initial cetartiodactyl transition from land to water.
Previous authors have suggested that a change to a piscivorous
diet occurred prior to locomotor adaptations in Cetacea (Gaskin,
1985; O’Leary and Uhen, 1999), and that this dietary switch,
as indicated by a novel form of tooth wear, was ‘‘. . . the impetus
for the transition from life on land to life in water for this
clade’’ (O’Leary and Uhen, 1999:546). In contrast, optimizing
habitat and diet onto our trees indicates that the common ancestor
of hippos and cetaceans was herbivorous and spent considerable
time in the water. The carnivory and piscivory observed
in many extant cetaceans is then interpreted to have
evolved later. Even so, O’Leary and Uhen (1999) may still be
correct in hypothesizing that changes in diet may have been a
distal cause of additional anatomical adaptations (e.g., loss of
ilia/sacrum contact) to an aquatic environment.
Previously, morphologists studying cetartiodactyl phylogeny
cited long-branch attraction, missing data in fossils, and character
polarity as potential problems when using molecular data
to test phylogenetic hypotheses (e.g., Luckett and Hong, 1998;
O’Leary and Geisler, 1999). Recent paleontological discoveries
(Gingerich et al., 2001; Thewissen et al., 2001b) and the current
phylogenetic analysis suggest instead that some morphological
characters were misleading, and that anatomists should consider
molecular data to be a reliable source of phylogenetic information.
While in the case of whale origins much of the conflict
between morphological and molecular data was resolved by the
discovery of new fossils that brought the morphological hypothesis
more in line with the molecular hypothesis, this has
not always been the case. A recent controversy over the position
of the sperm whale (Physeter catodoni; Milinkovitch et al.,
1993) was resolved by the discovery of new gene sequences,
which brought molecular data in line with the morphological
and paleontological data (Nikaido et al., 2001).
Based on studies that appear to show that morphological and
molecular data are at times fallible, we think that morphologists,
paleontologists, and molecular biologists should collect
additional data to determine whether or not ruminants are the
sister-group of the Cetacea and Hippopotamidae clade. Despite
the enormous amount of data collected so far, much work remains
to be done. For example, although there are studies that
compare the teeth (O’Leary, 1998), basicranium (Luo and Gingerich,
1999), and vascular foramina (Geisler and Luo, 1998)
of cetaceans and mesonychians, similar studies that compare
whales to hippopotamids and other artiodactyls are lacking. In
turn, molecular biologists can test hypotheses of nucleotide homology
by sampling more taxa and by sequencing new genes.
Regardless of the data employed, we remain optimistic that new
data will continue to bring morphological and molecular hypotheses
for the origin of whales into congruence, and out of
conflict.
ACKNOWLEDGMENTS
We greatly appreciate the help of Mick Ellison and Bolortsetseg
Minjin in photographing the hippopotamus skull in Figure
3. Bolortsetseg Minjin also helped code characters 187 and
995
188 for several specimens in the American Museum of Natural
History. Michael Morlo helped with the translation of key passages
in several German papers, which allowed us to incorporate
Amphirhagatherium into our study. Kyle Staulter assisted
with earlier versions of Figure 1. We also thank Philip Gingerich
for access to the type specimen of Sinonyx jiashanensis,
Alfred Mead and William Wall for access to specimens in the
mammalogy and fossil vertebrate collections of Georgia College
& State University, and David Bohaska and Robert Purdy
for access to specimens at the United States National Museum.
Glenn Feldake and Tony Barthel (Smithsonian National Zoological
Park) provided vital information on the feet of extant
species of hippos. In particular we thank Mr. Fledake for sending
photographs of both species.
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APPENDIX
Supplemental data available from SVP website: http://www.vertpaleo.
org/jvp/JVPcontents.html.