AMERICANt MUSEUM Novitates PUBLISHED BY THE AMERICAN MUSEUM OF NATURAL HISTORY CENTRAL PARK WEST AT 79TH STREET, NEW YORK, N.Y. 10024 Number 2870, pp. 1-8, figs. 1-3 April 6, 1987 Selected Features of the Desmostylian Skeleton and Their Phylogenetic Implications MICHAEL J. NOVACEK1 AND ANDRE R. WYSS2 ABSTRACT According to several standard descriptions, des- pertaining to these traits are either in error, or have mostylians lack certain specializations shared by alternative phylogenetic implications. Hence, proboscideans, sirenians, and hyracoids. These comparisons of these conditions do not exclude specializations are amastoidy and the serial ar- desmostylians from the superordinal group Teth- rangement ofthe carpals with the concomitant loss ytheria (proboscideans and sirenians) or the more of contact between the lunar and unciform. We inclusive Paenungulata (tethytheres and hyra- argue that original descriptions of desmostylians coids). INTRODUCTION The Desmostylia are an extinct order of be more conservative than sirenians (and, by mammals with specializations ofthe skeleton implication, proboscideans) with respect to suitable for an amphibious mode of life. Al- the reduction ofmastoid exposure (Hay, 1915; though this group was formerly associated Abel, 1922; VanderHoof, 1937) and the se- with the aquatic Sirenia (Simpson, 1945), and rial arrangement ofthe carpal elements (Shi- is now placed within a superordinal category kama, 1966). These differences have influ- Tethytheria, which also includes the Probos- enced more recent students of the problem cidea and Sirenia (McKenna, 1975), these (e.g., Tassy, 1981), who have opted for a close avowed relationships present some prob- association ofProboscidea (including the late lems. According to standard descriptions, Eocene-early Oligocene genus Moeritheri- desmostylians lack certain skeletal special- um) and Sirenia to the exclusion of Desmo- izations shared by Proboscidea and Sirenia. stylia. Specifically, desmostylians were claimed to Of significance is the fact that the above ' Chairman and Associate Curator, Department ofVertebrate Paleontology, American Museum ofNatural History. 2Graduate Student, Department of Vertebrate Paleontology, American Museum of Natural History; Graduate Student, Department of Geological Sciences, Columbia University. Copyright ©) American Museum of Natural History 1987 ISSN 0003-0082 / Price $ 1.00 2 AMERICAN MUSEUM NOVITATES NO. 2870 cited specializations, though supposedly ofthe ambiguities discussed below. The char- lacking in desmostylians, are found in hy- acters below are of note. racoids as well as proboscideans and sire- nians. Such features have been used as evi- dence for the monophyly ofthe Paenungulata, MASTOID ExPosuRE a superordinal group comprising tethytheres Typically (and primitively) within mam- and hyracoids (Gregory, 1910; Shoshani et mals the mastoid portion of the petrosal is al., 1978; Novacek, 1982, 1986). Hence, the well exposed on the exterior surface of the lack ofcertain apomorphies in desmostylians skull, as it forms a broad flange in the occiput not only presents a difficulty for recognition lateral to the exoccipital and medial and pos- of the Tethytheria but also for the more in- terior to the squamosal (Novacek and Wyss, clusive Paenungulata, if Desmostylia are to 1986). However, the mastoid is not exposed be retained within these groups. in this fashion in the occiput ofcertain mam- Here we offer arguments why published de- mals, including pholidotans, cetaceans, der- scriptions of desmostylians with respect to mopterans, advanced artiodactyls, probos- the characters in question are either incorrect, cideans, sirenians, and hyracoids. This, the or are subject to alternative phylogenetic in- "amastoid condition" is clearly derived for terpretations. Our emendations allow assign- mammals (Novacek and Wyss, 1986). ment of the Desmostylia to the Tethytheria. Sirenians show exposure ofthe mastoid via They are also consistent with a paenungulate a large fenestra in the dorsal occiput, but this grouping for tethytheres and hyracoids. seems a uniquely specialized condition. Al- though the mastoid is inflated and essentially ACKNOWLEDGMENTS fills this large fenestra, it does not extend around the base of the cranium to form a We thank Malcolm C. McKenna, John flange on the ventral occiput. Thus, sirenians Flynn, Daryl Domning, and Clayton Ray for do not show the continuous mastoid expo- discussion. Daryl Domning provided a par- sure between the horizontal basicranium and ticularly useful review ofthe manuscript. We ventral (vertical) occiput that is characteristic also thank Clayton Ray for access to the im- of most mammals. The opening of the oc- portant collection of desmostylian material cipital fenestra for dorsal exposure of the in- at the United States National Museum flated mastoid is therefore likely a secondary (USNM), Smithsonian Institution. Figures feature, and one that could well derive from were prepared by Lisa Lomauro (figs. 1B, 3) the amastoid condition exemplified by hy- and Chester Tarka (fig. 2). This work was racoids and proboscideans. supported by the Frick Laboratory Endow- The broad occipital exposure of the des- ment Fund in Vertebrate Paleontology. mostylian mastoid was explicitly described by Hay (1915, p. 387), figured but not de- scribed by Abel (1922, fig. 3), and figured CHARACTER ANALYSIS and discussed by VanderHoof(1937, figs. 1, Desmostylians include the nominal genera 12). These descriptions are based on a skull Desmostylus, Paleoparadoxia, Cornwallius, ofDesmostylus hesperus (USNM 8191). Van- VanderhooJius, Kronokotherium, and Be- derHoof's (1937) figure 12 of this specimen hemotops (Hay, 1915; VanderHoof, 1937; shows a mastoid element only on the left side Pronina, 1957; Reinhart, 1959; Ijiri and Ka- ofthe skull. Although he did not label sutures, mei, 1961; Shikama, 1966; Domning et al., it appears from his figure 12 that the mastoid 1986; Kronokotherium and Vanderhoofius on the left side of the skull is isolated on its may be junior synonyms of Desmostylus, D. lateral border by a short, curved suture and Domning, personal commun.). Ofthese, only on its medial border by a long, vertically ori- Desmostylus and Paleoparadoxia are well ented suture that runs to the lambdoidal rim represented by nearly complete skulls and (fig. 1A herein). However, the "lateral su- skeletons, and even in such cases damage ture" is more likely a contact between the during preservation has contributed to some occipital exposure of the squamosal and the 1987 NOVACEK AND WYSS: DESMOSTYLIAN SKELETON 3 exoccipital (fig. IB). Contrary to Vander- Hoof's figure, this suture runs all the way to A the occipital foramen. The suture is also pre- served on the right side of the skull where it appears to extend beyond the occipital fo- ramen and join the oblique supraoccipital- exoccipital suture either at the lambdoidal crest or at a somewhat more medial junction (fig. 1B). The "medial suture," which VanderHoof (1937) and others seem to have taken as the mastoid-exoccipital suture, is an artifact, a crack in the bone. It is clearly not present on the right side of USNM 8191. It crosses, rather thanjoins, the "lateral suture" (cf. fig. 1A, 1B). It trends lateral to the oc- cipital foramen; this foramen typically lies within the mastoid or exoccipital or in a su- B ture separating these elements from the su- Focc praoccipital. The "medial suture" also marks the lateral edge ofa damaged area where sur- face bone has been removed. We conclude, then, that earlier descrip- tions of this important specimen are in error and there is clearly no exposure of the mas- toid on the occiput in Desmostylus hesperus. Moreover, published figures and descriptions of Desmostylus hesperus japonicus (a senior synonym of Desmostylus mirabilis Nagao, Ijiri and Kamei, see Shikama, 1966) and Pa- leoparadoxia tabatai (Ijiri and Kamei, 1961, pl. 1, fig. 4 and pl. 3, fig. 5) do not indicate a mastoid in the occiput. Instead, these fig- Exocc Cocc ures correspond closely with the geometry of elements reconstructed in figure 1 B. Finally, immature specimens (USNM 181744) of 4 cm Desmostylus hesperus (fig. 2) and Cornwallius Fig. 1. Posterior views of occipital region in sp. (USNM 181788) show neither mastoid USNM 8191, Desmostylus hesperus. (A) Recon- exposure nor VanderHoof's (1937) "medial struction from VanderHoof(I 937, fig. 12). (B) Re- suture" in the occiput. construction favored in this paper. Symbols are The broad invasion of the squamosal on Exocc, exoccipital; Focc, occipital foramen; Ma, the occipital surface in Desmostylus (figs. 1 B, mastoid process (of the petromastoid); Cocc, oc- 2) is a curious feature. This arrangement, cipital condyle; Socc, supraoccipital; Sq, squa- however, bears striking resemblance to that mosal. Dashed line indicates crack in the bone in Sirenia where the occipital exposure ofthe surface. Hatching indicates damaged area. squamosal is even more marked. It is clear, then, that the ventral mastoid is concealed in these taxa by the expansive paroccipital ly derived feature. It probably represents the apophysis formed by broad contact between marked expansion of the occipital foramen, the squamosal and exoccipital. Only in si- which in Desmostylus is also atypically large renians does a large fenestra expose, more (figs. 1A, 1B, 2). The amastoid condition not- dorsally, the inflated mastoid process. As ed for tethytheres and hyracoids is thus at- noted above, we regard this fenestra as a high- tained in