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The Endocranial Cast and Encephalization Quotient of Ptilodus (Mul Tituberculata, Mammalia)

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The Endocranial Cast and Encephalization Quotient of Ptilodus (Mul Tituberculata, Mammalia) THE ENDOCRANIAL CAST AND ENCEPHALIZATION QUOTIENT OF PTILODUS (MUL TITUBERCULATA, MAMMALIA) by David W. KRAUSE' & Zofia KIELAN·JAWOROWSKA'* CONTENTS Page Abstract, Resume . .. 100 Introduction ..................................................................... 100 Structure of the endocranial cast '" . .. .. .. .. 101 EncephaIization quotient .. .. 104 Revised estimate of brain mass. .. .. .. 105 Revised estimate of body mass. .. 106 Revised encephalization quotient . .. 110 Acknowledgements ............................................................... 110 References cited. .. 110 Plate * Deparunent of Anatomical Sciences, State University of New York, Stony Brook, New York 11794- 8081, U.S.A. ** Paleontologisk Museum, Universitctet i Oslo, Sars gate I, N-0562 Oslo 5, Nonvay. Key-words: Multituberculata, PtilOdllS, endocranial cast, olfactory bulbs, encephaIization quotient, Paleocene, Montana. Mots-cles: Muititubercules, Ptilodus, moulage endocranien, bulbes olfactifs, quotient d'encephalisation, Paleoc~ne, Montana. Palaeovertebrata. Montpellier, 22 (2-3): 99-112. 1 fig., 1 pI. (Re~ le 29 Novembre 1991, accepte le 28 Janvier 1992, public le 11 Fevrier 1993) ABSTRACT Simpson (l937a) reconstructed the brain of the Paleocene multituberculate Ptilodus as having tile olfactory bulbs widest anteriorly, an unusual condition among mammals. Slight preparation and reexamination of tlw endocast specimens of Ptilodus reveals that tile olfactory bulbs in fact taper anteriorly, as in the recently-described endocranial casts of three multituberculate species from the Late Cretaceous of Asia, and as in tllose of otller mammals. The brain of Ptilodlls was tllerefore essentially of the same general pattern as in other multituberculates. Pti/odus differs, however, in having tile vennis more deeply inserted between the cerebral hemispheres. Other details concerning the endocast morphology of Ptilodlls are clarified. TIle encephalization quotient (EQ) of Ptilodus is recalculated from revised estimates of both endocranial volume and body mass to fall somewhere within a broad range of values; from 0.41 to 0.62 if the olfactory bulbs are included and from 0.37 to 0.55 if they are not. In any case, the relative brain size of Ptifodus is roughly equivalent to tlrat of Chulsanbaatar, the only other multituberculate for which EQ has been calculated, but considerably smaller tlran tlrat of an average extant marrunal. RESUME Dans la reconstitution de I'encephale du multitubercule paleocene Ptilodus donnee par Simpson (1937a), les bulbes olfactifs sont elargis anterieurement, ce qui represente une condition inhabituelle chez les mammiteres. Une preparation minutieuse ainsi qu'un recxarnen du moulage endocranien de Ptilodlls revelent qu'en fait 1es bulbes olfactifs s'effilent anterieurement comme chez trois especes de multituberculCs recemment decrites du Cretace superieur d'Asie, et plus generalement comme chez les autres marruniteres. Si I'encephale de Ptilodlls avait done la meme structure generale que les autres multituberculCs, iI en diftere par un vennis plus profondement insere entre les deux hemispheres cerebrales. D'autres details concemant la morphologie du moulage endocranien ont ete precises. Le coefficient d'encephalisation (EQ) de Ptilodlls a ete recalcule it partir de nouvelles estimations du volume endocranien et du poids de I'animal; ce coefficent se situe dans un large intervalle de valeurs: de OAI it 0,62 si les bulbes olfactifs sont pris en compte, sinon, de 0,37 it 0,55. Dans les deux cas, la taille relative de I'encephale de Ptilodus est sensiblement celle de Chlllsanbaalar, le seul autre multitubercule pour lequel un coefficient d'encephalisation a ete calcule, mais elle est considerablement plus petite que celle d'un marnmitere actuel moycn. INTRODUCTION Simpson (1937a) described the endocranial cast and reconstructed the brain of the ptilodontoid multituberculate PtilOdllS montanus DOUGLASS on the basis of four fragmentary specimens from the middle Paleocene of the Crazy Mountains Basin, south-central Montana. PtilOdlls is the only NOlth American multituberculate genus and the only member of the suborder Ptilodontoidea for which endocranial casts have been described. Simpson (1937a: 752-753) noted that the olfactOlY bulbs are "enormous, relatively larger than in any other mammalian brain known to me" and are "peculiar in shape, being pyriform but with the greatest width anterior." Since Simpson's paper, endocranial casts have been described and reconstructed 100 for cynodonts (Hopson 1979; Quiroga 1979, 1980, 1984), Cretaceous eutherian mammals (Kielan-Jaworowska 1984, 1986; Kielan-Jaworowska & Trofimov 1986), and Jurassic and Cretaceous multituberculates. Hahn (1969) reconstructed the endocranial cast for the Late Jurassic paulchoffatoid Paulchoffatia delgadoi from Portugal. Kielan­ Jaworowska (1983, 1986) and Kielan-Jaworowska et al. (1986) described and reconstructed endocranial casts of two taeniolabidoid multituberculate species, Chulsanbaater vulgaris and Nemegtbaatar gobiellsis, from the Late Cretaceous of Mongolia. (Kielan-Jaworowska [1983] also mentioned a partial endocranial cast of Kamptobaatar kuczynskii and Kielan-Jaworowska & Dashzeveg [1978] illustrated but did not describe the endocranial cast of Tugrigbaatar saichanensis, both also taeniolabidoids from the Late Cretaceous of Mongolia.) These papers give much infOlmation that is new on the evolution of the brain at the reptilian-mammalian transition and in early mammals. In all of these multituberculates, as well as in previously described triconodonts (Simpson 1927, 1928; see also Hopson 1979 and Kielan-Jaworowska 1986), the olfactory bulbs are large and taper anteriorly; they are not abruptly U'uncated anteriorly as reconsU'ucted for Ptilodus by Simpson (1937a, fig. 9; see also fig. la of this paper). In an attempt to ascertain whether or not Pti/odus did indeed have olfactory bulbs differently shaped from those in cynodonts and all other known early mammals, we reexamined all of Simpson's specimens. Slight preparation and examination of one specimen (USNM 9710) demonstrated that the shape of the olfactory bulbs is not "peculiar" relative to those of either cynodonts or other mammals, or particularly, relative to those of Late Cretaceous Asian multituberculates. We also take this opportunity to make other comparisons of the endocranial cast of Ptilodus with those of more recently discovered multituberculate endocranial casts and to provide a recalculated estimate of the encephalization quotient (EQ) of Ptilodus lIlontanus. The latter is based on revised estimates of both brain size and body size for the species. We employ the following abbreviations: AMNH American Museum of Natural History, New York UA University of Alberta, Edmonton USNM United States National Museum, Washington, D.C. ZPAL Institute of Paleobiology, Polish Academy of Sciences (Instytut Paleobiologii), Warsaw. STRUCTURE OF THE ENDOCRANIAL CAST The specimens studied by Simpson (1937a) consisted of AMNH 35490 (crushed skull with only a part of the cerebral hemispheres exposed - illustrated in Hopson et al. 1989, fig. 1), AMNH 35491 (part of cerebral hemispheres and cerebellum), USNM 6076 (cerebrum and cerebellum - illustrated in Wall & Krause, 1992, fig. 2), and 101 USNM 9710 (olfactory bulbs and anteIior part of the cerebral hemispheres). We reexamined all of the specimens upon which Simpson based his reconstlUction of the brain; USNM 9710 and AMNH 35491, in particular, provide new information and are illustrated here for the first time (pI. l). The endocranial casts of the olfactory bulbs in USNM 9710 were somewhat obscured by overlying matrix, which was carefully removed to reveal the dorsal borders of the olfactory bulbs (pI. 1, fig. I). This procedure permits a revised reconstlUction of the olfactory bulbs (fig. 1b). The olfactolY bulbs in USNM 9710 taper anteriorly, with their greatest width being at about mid-length, not far anteIiorly as reconstructed by Simpson (l937a). They are separated from the cerebral hemispheres by a wide transverse fissure. The olfactory bulbs are of similar shape in other multituberculates (e.g., Nemegtbaatar gobiensis - see Kielan-Iaworowska et al. 1986, pI. 6, fig. 18B; and Chulsanbaatar vulgaris - see Kielan-Iaworowska 1983, pI. 1, figs. 1-4) and are not atypical among mammals in this regard. As noted and emphasized by Simpson (l937a), the olfactOlY bulbs of Ptilodus are indeed large but, relative to the cerebral hemispheres, not as large as originally reconstlUcted (fig. I). Large olfactory bulbs have been noted for other multituberculates as well (Kielan-Iaworowska 1983, 1986; Kielan-Jaworowska et al. 1986). The presence of large olfactOlY bulbs in the brain of PtilodllS is concordant with other evidence indicating that this genus and at least some other multituberculates were small-eyed, olfactory-dominated, noctumal forms (e.g., Krause 1986). AMNH 35491 contains the cast of the posteIior part of the cerebral hemispheres and the anteIior prut of the central lobe of the cerebellum (vermis) (pI. 1, figs. 2, 3). The cross-section of the left fossa subarcuata (housing the pru'aflocculus) and the section of the posterior semicircular canal is also preserved on the left side of the specimen (pI. 1, fig. 3). The vermis is strongly convex dorsally and more deeply inserted between the cerebral hemispheres than in other multituberculate endocranial casts. The cast of the transverse sinus is very distinct. In the shape of the vermis, Ptilodus resembles more
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