Rapid Communication Morphological Support For

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Journal of Vertebrate Paleontology 23(4):991±996, December 2003 ᭧ 2003 by the Society of Vertebrate Paleontology

RAPID COMMUNICATION

MORPHOLOGICAL SUPPORT FOR A CLOSE RELATIONSHIP BETWEEN HIPPOS AND WHALES

JONATHAN H. GEISLER1 and MARK D. UHEN2 1Department Geology/Geography and Georgia Southern Museum, Georgia Southern University, Statesboro, Georgia 30460-8149, [email protected]; 2Cranbrook Institute of Science, 39221 Woodward Avenue, P.O. Box 801, Bloom®eld Hills, Michigan 48303-0801, [email protected]

Recent discoveries of the ankles of fossil whales, reported tree for 1,000 iterations of the parsimony ratchet (Nixon, 1999), by Gingerich et al. (2001) and Thewissen et al. (2001b), cor- which was implemented with the command nix*1000. Bremer roborated the molecular hypothesis that Cetacea (whales, dol- support values were calculated using the programs PAUP 3.1.1 phins, and porpoises) are closely related to artiodactyls (even- (Swofford, 1993) and TreeRot (Sorenson, 1996), with modi®- hoofed mammals including hippopotami, pigs, deer, and cam- cations to the TreeRot commands ®le as described in Geisler els); however, major points of disagreement remain. A mor- (2001a). Lists of unequivocal synapomorphies for each node phology-based study incorporating some of these new data were compiled using the apo/ command in NONA 1.9. Where (Thewissen et al., 2001b) supported the exclusion of Cetacea we describe synapomorphies supported by our study, we cite from the clade of living artiodactyls. In contrast, a vast amount previous studies that have reached the same conclusion. of molecular data support placement of Cetacea within Artio- Institutional Abbreviations AMNH, American Museum dactyla, as close relatives to Hippopotamidae (Gatesy et al., of Natural History, Departments of Mammalogy and Vertebrate 1996, 1999; Montgelard et al., 1997; Shimamura et al., 1997, Paleontology (New York); GSM, Georgia Southern Museum, 1999; Nikaido et al., 2001). Here we report that morphological Vertebrate Collection (Statesboro, Georgia); IVPP, Institute of data from extinct and extant taxa support placement of Cetacea Vertebrate Paleontology and Paleoanthropology, Chinese Acad- within Artiodactyla as the closest relatives of Hippopotamidae emy of Sciences (Beijing, China). (Fig. 1B) and indicate that molecular and morphological evidence for the phylogeny of these taxa are now much more con- RESULTS gruent than previously thought. A total of 45 most parsimonious trees of 1,513 steps in length were found (within-taxon polymorphism was not counted as MATERIALS AND METHODS extra steps). All most parsimonious trees have Cetacea deeply Our result is based on a cladistic analysis of a modi®ed ver- nested within Artiodactyla as the sister-group to Hippopotami- sion of the character/taxon matrix of Geisler (2001a). The pre- dae (Fig. 1B), like molecular studies and unlike the most recent sent matrix incorporates new information on the early cetaceans morphological-based analysis (Thewissen et al., 2001b) (Fig. Artiocetus, Rodhocetus (Gingerich et al., 2001), and Pakicetus 1A). The novel morphological result reported here is primarily (Thewissen et al., 2001b); includes some changes in the scoring attributed to recently described cetacean fossils (Gingerich et of Basilosaurus; adds the artiodactyls Amphirhagatherium wei- al., 2001; Thewissen et al., 2001b), because a study with similar gelti (Heller, 1934; Erfurt, 2000; Hooker and Thomas, 2001) characters and taxa, but without the new ankle data, supported and Raoellidae (Kumar and Sahni, 1985; Thewissen et al., the exclusion of Cetacea from Artiodactyla and a close rela- 2001b); includes the mesonychid Ankalagon (AMNH-VP 776, tionship between Cetacea and Mesonychidae (Geisler, 2001a), 777, 2454; O'Leary et al., 2000); adds seven characters from an extinct group of hoofed mammals. We concur with recent Thewissen et al. (2001b), and consists of 195 characters scored authors (Gingerich et al., 2001; Thewissen et al., 2001b) that a for 69 mammalian taxa. The matrix and character list are avail- suite of characters in the ankles of early whales supports a clade able on the internet at http://www.vertpaleo.org/jvp/JVPcon- comprised of Cetacea and Artiodactyla but not mesonychids. tents.html. Of the 195 characters, 121 are binary, 14 are unor- The sister-group to the hippo/whale clade varies among the dered multistate characters, and 60 are ordered multistate char- most parsimonious trees; in 36 trees the sister-group is a clade acters. Although we do not claim to have included every pub- of suiform artiodactyls including Suina (pigs and peccaries), lished character relevant to cetartiodactyl phylogeny, we note Entelodontidae, and Anthracotheriidae (represented by Elome- that this matrix has substantially more taxa and more morpho- ryx); and in the remaining nine trees it is Raoellidae. The latter logical characters than previous phylogenetic analyses (e.g., trees are interesting because like cetaceans, raoellids possess a Geisler and Luo, 1998; O'Leary, 1998, 2001; Luo and Ginger- P4 paracone that is much higher than those of the succeeding ich, 1999; O'Leary and Geisler, 1999; O'Leary and Uhen, molars. Like pakicetids (Thewissen and Hussain, 1998) and the 1999; Thewissen et al., 2001b). protocetid Artiocetus (Gingerich et al., 2001), raoellids such as The computer program NONA 1.9 (Goloboff, 1994) was Kunmunella kalakotensis (Kumar and Shani, 1985) also have a used to ®nd most parsimonious trees. An initial search was single-rooted P1. Intriguingly, raoellid fossils are abundant in conducted using two commands: mult*100; hold/10, which in- the same Asian sites that produce pakicetids (Thewissen et al., voke 100 iterations of TBR (tree bisection and reconnection) 2001a). Unfortunately, the non-dental anatomy of raoellids is branch swapping and save only 10 trees per iteration. The most undescribed. Discoveries of new raoellid fossils would not only parsimonious tree from the initial search was used as a starting test the hypothesis that they are closely related to hippos and

991 992 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 ϭ (Eotylops*(Poebrotherium (Llama*, Camelus*))); Entelodontidae (Archaeotherium); Hapalodectes ϭ (H. hetangensis*, H. leptognathus*); Me- sonychidae ϭ (Dissacus praenuntius*, Dissacus navajovius*, Mongolian Dissacus*(Ankalagon (Sinonyx (Pachyaena gigantea*, Pachyaena ossifraga*, (Mesonyx, Harpagolestes, Synoplotherium))))); Mysticeti ϭ (Balaenoptera); Odontoceti ϭ (Physeter (Tursiops, Delphinapterus); Or- eodontoidea ϭ (Agriochoerus, Merycoidodon*); Perissodactyla ϭ ((Equus*, Mesohippus*) (Heptodon*, Hyracotherium*)); Phenacodontidae ϭ (Meniscotherium, Phenacodus); Protoceratidae ϭ (Leptoreodon*(Heteromeryx*, Protoceras*)); Ruminantia ϭ (Hypertragulus*(Leptomeryx* (Tragulus (Bos*(Odocoileus*, Ovis*))))); Suina ϭ (Perchoerus*(Sus, Tayassu*)); and Xiphodontoidea ϭ (Xiphodon*, Amphimeryx*). In the GEISLER AND UHENÐWHALES AND HIPPOS CLOSELY RELATED 993

tiodactyla has a Bremer support of 3; the Cetacea and Hippo- potamidae 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:®g. 4) the distance between the anter- iormost 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 sim- ilarity 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, FIGURE 2. Posterior view of the left squamosal, petrosal, and tym- molecular data do not support a close relationship between the panic bulla of a juvenile Hippopotamus amphibius (AMNH 130247). cetacean/hippopotamid clade and ``suiform'' artiodactyls. In- Lateral is to the left, dorsal is to the top, and the occipital bones have stead, vastly different molecular characters including nucleotide been removed. Note the elongate mastoid process of the petrosal, a trait sequences (Gatesy et al., 1996, 1999; Montgelard et al., 1997), also seen in cetaceans. Abbreviations: VII, stylomastoid foramen for SINEs (short interspersed nuclear elements) (Shimamura et al., the facial nerve (cranial nerve VII); mp, mastoid process of the petrosal; 1997, 1999; Nikaido et al., 1999), and multiple base-pair de- pet, petrosal; sq, squamosal; sqs, sutural surface on the squamosal for letions (Gatesy et al., 1996; Geisler, 2001b) support a clade the exoccipital; tyb, ectotympanic bulla. 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 whales but would also help resolve the ambiguity in the opti- Ruminantia and Cameloidea. Neoselenodontia is supported by mization of several characters across higher-level cetartiodactyl several characters, including small supraspinatus fossa of the clades. scapula, tibia and ®bula fused at proximal ends, and middle Only two of the characters that support a sister-group rela- portions of 2nd and 5th metatarsals absent (Geisler, 2001a). tionship between Cetacea and Hippopotamidae are unequivocal synapomorphies: absence of paraconules on upper molars and Even so, the degree to which the morphological data of the absence of crest between the hypoconid and entoconid (i.e., present study contradict Ruminantia ϩ Hippopotamidae ϩ Ce- hypolophid) on lower molars. Several other characters are tacea is much less than previously published matrices. Accord- equivocal synapomorphies of the hippo/whale clade because ing to the matrix of Geisler (2001a), this clade had a Bremer they cannot be scored in raoellids, and thus could be a syna- support of Ϫ23, while in the present study its Bremer support pomorphy of Raoellidae ϩ Cetacea ϩ Hippopotamidae in nine has increased by 9 steps to Ϫ14. of the most parsimonious trees. These characters include salient Unlike Geisler (2001a) but like many previous studies (e.g., features such as the near absence of hair and the loss of seba- Geisler and Luo, 1998; O'Leary, 1998; O'Leary and Geisler, ceous glands (Gatesy et al., 1996), as well as previously unrec- 1999), the present study supports monophyly of Mesonychia ognized features such as a low dentary condyle, wide ®fth (sensu O'Leary, 1998) and Mesonychidae. Mesonychia has a metatarsal, nasals that terminate between the orbits, and elon- Bremer support of one and is diagnosed by seven unequivocal gate mastoid process of the petrosal. In adults of Hippopotamus synapomorphies: well developed parastyle on M1 (O'Leary and amphibious the mastoid process appears to be absent; however, Geisler, 1999), M2 metacone half the size of paracone, m3 hy- it has only fused to the squamosal. An elongate, anteroposte- poconulid absent, molar paraconules absent, paraconid directly riorly-compressed mastoid process is visible in skulls of juve- anterior to protoconid (O'Leary, 1998), protoconid nearly twice nile specimens (Fig. 2). Interestingly, mesonychids also have the height of talonid, and narrow talonid basin. Mesonychidae an elongate mastoid process and a posterior position of the na- has a Bremer support of four and is supported by three un- sals. Although these similarities are most-parsimoniously inter- equivocal synapomorphies: postglenoid foramen small or ab- preted as convergent on the shortest trees for this study, it is sent (Luo and Gingerich, 1999; O'Leary and Geisler, 1999), clear that morphological data do not universally support a Hip- foramen ovale anterior to glenoid fossa (O'Leary and Geisler, popotamidae and Cetacea clade. Further evidence for disagree- 1999), and hypocone and metaconule absent on upper molars. ment between some morphological characters is the low Bremer Considering the paraphyly of Mesonychidae in the trees of support for clades that render Artiodactyla paraphyletic: Cetar- Geisler (2001a), it appears that the monophyly of Mesonychi-

← 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 speci®cs of the phylogenies within Mesonychidae in strict consensus A, readers are referred to Thewissen et al. (2001b). The simpli®ed consensus of the current study (B) includes clades common to all 45 most-parsimonious trees, each 1,513 steps in length. 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 ex- The topology common to all our most parsimonious trees is tant artiodactyls. relatively robust to the exclusion of some anatomical characters. Previously described dental/masticatory characters that sup- Like O'Leary and Geisler (1999) and Geisler (2001a), but un- port a close relationship between mesonychids and cetaceans like Thewissen et al. (2001b), we included soft-tissue morpho- (Thewissen, 1994; Geisler and Luo, 1998; O'Leary, 1998; logical characters (e.g., absence of hair, absence of sebaceous O'Leary and Geisler, 1999) are optimized as convergent on the glands). Although Thewissen et al. (2001b) did not explain why most parsimonious trees of the present study. The claim that they chose to exclude these features, their reason may be the dental characters are convergent is not unprecedented. Three of widely held view that the loss of hair and sebaceous glands is the characters that support a mesonychid and cetacean clade prone to convergence and thus unreliable (see Luckett and (i.e., reduction of lower molar metaconids and talonid basins; Hong, 1998, and references therein). Although we think that embrasure pits) also occur in several other distantly related homoplasy is best viewed as an a posteriori interpretation and groups of carnivorous mammals (Muizon and Badre, 1997; not an a priori assumption, we experimented with our matrix Uhen, 1996). We suspect that the convergence in dental char- by removing all six soft tissue characters (i.e., characters 181± acters between cetaceans and mesonychians is due to a simi- 186) and re-running the phylogenetic analysis. Using the small- larity in function and that several dental characters may not be er matrix, we found 297 most-parsimonious trees of 1,505 steps independent. For example, the embrasure pits on the maxillae each. As in our analyses with all characters included, some in early cetaceans, many mesonychians, and some carnivorans most parsimonious trees supported by the osteology-only ver- accommodate the protoconids of the lower teeth when the sion of our matrix had Cetacea within the clade of extant artio- mouth is closed. In living carnivorans and basilosaurid ceta- dactyls as the sister-group to Hippopotamidae; however, the re- ceans, the embrasure pits form in direct response to the ap- maining shortest trees had a monophyletic Cetartiodactyla with proximation of the teeth and the bones of the palate and jaw Cetacea as the sister-group to a monophyletic Artiodactyla, a during chewing (Uhen, 1996). Therefore, embrasure pits and result similar to that found by Thewissen et al. (2001b). the height of the protoconid of the lower molars may not be evolving independently. The hypothesis that other dental char- DISCUSSION acter states (e.g., absence of molar metaconids, absence of paraconules and metaconules) are interdependent is supported by It is reasonable to assume that in the cetartiodactyl transition statistical studies that show them to be correlated among artio- from a terrestrial to an aquatic habitat, a semi-aquatic existence dactyls, cetaceans, and mesonychians (Naylor and Adams, was a necessary intermediate step. Hypotheses on how this 2001; Geisler, 2001b). Even so, the correlation is not perfect, transition occurred and which taxa were semi-aquatic have re- indicating that a limited degree of independence remains and lied on the perceived sister group to Cetacea (Gatesy and that dental characters should remain in phylogenetic analyses O'Leary, 2001; O'Leary, 2001). Based on the most-parsimo- as separate characters. nious trees for our morphological data set, we infer that all GEISLER AND UHENÐWHALES AND HIPPOS CLOSELY RELATED 995 cetaceans and the common ancestor of hippopotamids and ce- 188 for several specimens in the American Museum of Natural taceans were at least semi-aquatic. In support of this view, hip- History. Michael Morlo helped with the translation of key pas- popotamids are semi-aquatic, and they share with cetaceans sages in several German papers, which allowed us to incorpo- some aquatic adaptations (e.g., loss of sebaceous glands and rate Amphirhagatherium into our study. Kyle Staulter assisted loss of hair). Our conclusions are contrary to those of Thew- with earlier versions of Figure 1. We also thank Philip Ginger- issen et al. (2001b:278) who state that pakicetids were ``no ich for access to the type specimen of Sinonyx jiashanensis, more amphibious than a tapir.'' Unlike a recent reconstruction Alfred Mead and William Wall for access to specimens in the of Rodhocetus (Gingerich et al., 2001, cover illustration), char- mammalogy and fossil vertebrate collections of Georgia Col- acter optimizations on our hypotheses of relationships indicate lege & State University, and David Bohaska and Robert Purdy that the earliest whales had little to no hair. for access to specimens at the United States National Museum. A close relationship between Hippopotamidae and Cetacea Glenn Feldake and Tony Barthel (Smithsonian National Zoo- also challenges hypotheses that give feeding behavior a central logical Park) provided vital information on the feet of extant role in the initial cetartiodactyl transition from land to water. species of hippos. In particular we thank Mr. Fledake for send- Previous authors have suggested that a change to a piscivorous ing photographs of both species. diet occurred prior to locomotor adaptations in Cetacea (Gaskin, 1985; O'Leary and Uhen, 1999), and that this dietary switch, LITERATURE CITED as indicated by a novel form of tooth wear, was ``. . . the im- petus for the transition from life on land to life in water for this Erfurt, J. 2000. Reconstruction of the skeleton and biology of Anthra- cobunodon weigelti (Artiodactyla, Mammalia) from the Eocene of clade'' (O'Leary and Uhen, 1999:546). In contrast, optimizing the Geiseltal (Germany). Hallesches Jahrbuch fuÈr Geowissenschaf- habitat and diet onto our trees indicates that the common an- ten, B 12:57±141. cestor of hippos and cetaceans was herbivorous and spent con- Gaskin, D. E. 1985. The Ecology of Whales and Dolphins. Heinemann, siderable time in the water. The carnivory and piscivory ob- Portsmouth, 459 pp. served in many extant cetaceans is then interpreted to have Gatesy, J., and M. A. O'Leary. 2001. Deciphering whale origins with evolved later. Even so, O'Leary and Uhen (1999) may still be molecules and fossils. Trends in Ecology and Evolution 16:562± correct in hypothesizing that changes in diet may have been a 570. distal cause of additional anatomical adaptations (e.g., loss of ÐÐÐ, C. Hayashi, A. Cronin, and P. Arctander. 1996. Evidence from ilia/sacrum contact) to an aquatic environment. milk casein genes that cetaceans are close relatives of hippopotamid artiodactyls. Molecular Biology and Evolution 13:954±963. Previously, morphologists studying cetartiodactyl phylogeny ÐÐÐ, M. Milinkovitch, V. Waddell, and M. Stanhope. 1999. Stability cited long-branch attraction, missing data in fossils, and char- of cladistic relationships between Cetacea and higher-level artio- acter polarity as potential problems when using molecular data dactyl taxa. Systematic Biology 48:6±20. to test phylogenetic hypotheses (e.g., Luckett and Hong, 1998; Geisler, J. H. 2001a. New morphological evidence for the phylogeny of O'Leary and Geisler, 1999). Recent paleontological discoveries Artiodactyla, Cetacea, and Mesonychidae. American Museum Nov- (Gingerich et al., 2001; Thewissen et al., 2001b) and the current itates 3344:1±53. phylogenetic analysis suggest instead that some morphological ÐÐÐ 2001b. Morphological and molecular evidence for the phylog- characters were misleading, and that anatomists should consider eny of Cetacea and Artiodactyla: explaining incongruence between molecular data to be a reliable source of phylogenetic infor- types of data. Ph.D. dissertation, Columbia University, New York, 475 pp. mation. While in the case of whale origins much of the con¯ict ÐÐÐ, and Z. Luo. 1998. Relationships of Cetacea to terrestrial un- between morphological and molecular data was resolved by the gulates and the evolution of cranial vasculature in Cete; pp. 163± discovery of new fossils that brought the morphological hy- 212 in J. G. M. Thewissen (ed.), The Emergence of Whales. Ple- pothesis more in line with the molecular hypothesis, this has num, New York. not always been the case. A recent controversy over the position Gingerich, P. D., M. u. Haq, I. S. Zalmout, I. H. Khan, and M. S. of the sperm whale (Physeter catodoni; Milinkovitch et al., Malakani. 2001. Origin of whales from early artiodactyls: hands 1993) was resolved by the discovery of new gene sequences, and feet of Eocene Protocetidae from Pakistan. Science 293:2239± which brought molecular data in line with the morphological 2242. and paleontological data (Nikaido et al., 2001). Goloboff, P. 1994. NONA version 1.9. Computer program and docu- mentation. Available at ftp.unt.edu.ar/pub/parsimony. Based on studies that appear to show that morphological and Heller, F. 1934. Anthracobunodon weigelti n. gen. et n. sp., ein Artiod- molecular data are at times fallible, we think that morpholo- actyle aus dem MitteleozaÈn Geiseltales bei Halle a. S. PalaÈontolo- gists, paleontologists, and molecular biologists should collect gische Zeitschrift 16:247±263. additional data to determine whether or not ruminants are the Hooker, J. J., and K. M. Thomas. 2001. A new species of Amphirha- sister-group of the Cetacea and Hippopotamidae clade. Despite gatherium (Choeropotamidae, Artiodactyla, Mammalia) from the the enormous amount of data collected so far, much work re- Late Eocene Headon Hill Formation of southern England and phy- mains to be done. For example, although there are studies that logeny of endemic European `anthracotheroids'. Palaeontology 44: compare the teeth (O'Leary, 1998), basicranium (Luo and Gin- 827±853. gerich, 1999), and vascular foramina (Geisler and Luo, 1998) Hulbert, R. C., Jr., R. M. Petkewich, G. A. Bishop, D. Bukry, and D. P. Aleshire. 1998. A new middle Eocene protocetid whale (Mam- of cetaceans and mesonychians, similar studies that compare malia: Cetacea: Archaeoceti) and associated biota from Georgia. whales to hippopotamids and other artiodactyls are lacking. In Journal of Paleontology 72:907±927. turn, molecular biologists can test hypotheses of nucleotide ho- Kumar, K., and A. Sahni. 1985. Eocene mammals from the Upper Su- mology by sampling more taxa and by sequencing new genes. bathu Group, Kashmir Himalaya, India. Journal of Vertebrate Pa- Regardless of the data employed, we remain optimistic that new leontology 5:153±168. data will continue to bring morphological and molecular hy- ÐÐÐ, and A. Sahni. 1986. Remingtonocetus harudiensis, new com- potheses for the origin of whales into congruence, and out of bination, a middle Eocene archaeocete (Mammalia, Cetacea) from con¯ict. Western Kutch, India. Journal of Vertebrate Paleontology 6:326± 349. Luckett, W. P., and N. Hong. 1998. Phylogenetic relationships between ACKNOWLEDGMENTS the orders Artiodactyla and Cetacea: a combined assessment of morphological and molecular evidence. Journal of Mammalian We greatly appreciate the help of Mick Ellison and Bolort- Evolution 5:127±181. setseg Minjin in photographing the hippopotamus skull in Fig- Luo, Z., and P. D. Gingerich. 1999. Terrestrial Mesonychia to aquatic ure 3. Bolortsetseg Minjin also helped code characters 187 and Cetacea: transformation of the basicranium and evolution of hear- 996 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 23, NO. 4, 2003

ing in whales. University of Michigan Papers on Paleontology 31: Ankalagon (Mammalia, Mesonychia) and evidence of sexual di- 1±98. morphism in mesonychians. Journal of Vertebrate Paleontology 20: Milinkovitch, M. C., G. Orti, and A. Meyer. 1993. Revised phylogeny 387±393. of whales suggested by mitochondrial ribosomal DNA sequences. Shimamura, M., H. Yasue, K. Ohshima, H. Abe, H. Kato, T. Kishiro, Nature 361:346±348. M. Goto, I. Munechika, N. Okada. 1997. Molecular evidence from Montgelard, C., F. M. Catze¯is, and E. Douzery. 1997. Phylogenetic retroposons that whales form a clade within even-toed ungulates. relationships of artiodactyls and cetaceans as deduced from the Nature 388:666±670. comparison of cytochrome b and 12S RNA mitochondrial sequenc- ÐÐÐ, H. Abe, M. Nikaido, K. Ohshima, N. Okada. 1999. Genealogy es. Molecular Biology and Evolution 14:550±559. of families of SINEs in cetaceans and artiodactyls: the presence of Muizon, C. de., and B. Lange-Badre. 1997. Carnivorous dental adap- a huge superfamily of tRNAGlu-derived families of SINEs. Molec- tations in tribosphenic mammals and phylogenetic reconstruction. ular Biology and Evolution 16:1046±1060. Lethaia 30:353±366. Sorenson, M. D. 1996. TreeRot. University of Michigan, Ann Arbor. Naylor, G. J. P., and D. C. Adams. 2001. Are the fossil data really at Available at http://people.bu.edu/msoren/TreeRot.html. odds with the molecular data? Morphological evidence for Cetar- Swofford, D. L. 1993. PAUP: phylogenetic analysis using parsimony tiodactyla phylogeny reexamined. Systematic Biology 50:7±16. (3.1.1). Privately distributed by Illinois Natural History Survey, Nikaido, M., F. Matsuno, H. Hamilton, R. L. Brownell, Jr., Y. Cao, W. Champaign, Illinois. Ding, Z. Zuoyan, A. M. Shedlock, R. E. Fordyce, M. Hasegawa, Thewissen, J. G. M. 1994. Phylogenetic aspects of cetacean origins: a and N. Okada. 2001. Retroposon analysis of major cetacean line- morphological perspective. Journal of Mammalian Evolution 2: ages: the monophyly of toothed whales and paraphyly of river dol- 157±183. phins. Proceedings of the National Academy of Sciences of the ÐÐÐ, and S. T. Hussain. 1998. Systematic review of Pakicetidae, United States of America 98:7384±7389. early and middle Eocene Cetacea (Mammalia) from Pakistan and ÐÐÐ, A. P. Rooney, and N. Okada. 1999. Phylogenetic relationships India. Bulletin of the Carnegie Museum of Natural History 34:220± among cetartiodactyls based on insertions of short and long inter- 238. spersed elements: hippopotamuses are the closest extant relatives ÐÐÐ, E. M. Williams, and S. T. Hussain. 2001a. Eocene mammal of whales. Proceedings of the National Academy of Sciences of faunas from northern Indo-Pakistan. Journal of Vertebrate Paleon- the United States of America 96:10261±10266. tology 21:347±366. Nixon, K. C. 1999. The parsimony ratchet, a new method for rapid ÐÐÐ, ÐÐÐ, L. J. Roe, and S. T. Hussain. 2001b. Skeletons of parsimony analysis. Cladistics 15:407±414. terrestrial cetaceans and the relationship of whales to artiodactyls. O'Leary, M. A. 1998. Phylogenetic and morphometric reassessment of Nature 413:277±281. the dental evidence for a mesonychian and cetacean clade; pp. 133± Uhen, M. D. 1996. Dorudon atrox (Mammalia, Cetacea): form, func- 161 in J. G. M. Thewissen (ed.), The Emergence of Whales. Ple- tion, and phylogenetic relationships of an archaeocete from the Late num, New York. Middle Eocene of Egypt. Ph.D. dissertation, University of Michi- ÐÐÐ 2001. The phylogenetic position of cetaceans: further combined gan, Ann Arbor, Michigan, 608 pp. data analyses, comparisons with the stratigraphic record and a dis- Zhou, X., R. Zhai, P. D. Gingerich, and L. Chen. 1995. Skull of a new cussion of character optimization. American Zoologist 41:487±506. mesonychid (Mammalia, Mesonychia) from the late Paleocene of China. Journal of Vertebrate Paleontology 15:387±400. ÐÐÐ, and J. H. Geisler. 1999. The position of Cetacea within Mam- malia: phylogenetic analysis of morphological data from extinct Received 21 February 2003; accepted 2 July 2003. and extant taxa. Systematic Biology 48:455±490. ÐÐÐ, and M. D. Uhen. 1999. The time of origin of whales and the role of behavioral changes in the terrestrial-aquatic transition. Pa- APPENDIX leobiology 25:534±556. Supplemental data available from SVP website: http://www.vertpaleo. ÐÐÐ, S. G., Lucas, and T. E. Williamson. 2000. A new specimen of org/jvp/JVPcontents.html.