Skull structure in Catopsbaatar and the zygomatic ridges in multituberculate mammals ZOFIA KIELAN−JAWOROWSKA, JØRN H. HURUM, and ALEXEY V. LOPATIN Kielan−Jaworowska, Z., Hurum, J.H., and Lopatin, A.V. 2005. Skull structure in Catopsbaatar and the zygomatic ridges in multituberculate mammals. Acta Palaeontologica Polonica 50 (3): 487–512. The late Campanian djadochtatherioid multituberculate Catopsbaatar catopsaloides was originally known from three skulls from Hermiin Tsav in the Gobi Desert (Mongolia). Three more skulls from Hermiin Tsav are now available, associ− ated with parts of the previously unknown postcranial skeleton, which will be described separately. We describe herein the skull and dentition of C. catopsaloides, based on all available material, housed in PIN, PM, and ZPAL collections. The genera Catopsbaatar, Djadochtatherium, and Kryptobaatar share several characters, unknown in Tombaatar, such as very long postorbital processes directed postero−laterally and downwards, parietal ridges extending from the posterior margins of the postorbital processes postero−medially, and nuchal crests with prominent lateral wings, incurved anteri− orly in the middle, so that the skull in dorsal view is shorter in the middle than laterally. Catopsbaatar shares with Djadochtatherium a very high and prominent anterior zygomatic ridge, and presence of the masseteric protuberance, but differs from it and from other djadochtatherioid genera in having the orbit situated more posteriorly, the intermediate zygomatic ridge adhering to the anterior ridge, and a smaller trapezoidal (rather than crescent−shaped) p4 without ridges; it differs from Kryptobaatar and Djadochtatherium in having three upper premolars (P2 being lost) and shares this last character with Tombaatar. Catopsbaatar is known not only from Hermiin Tsav, but also from Baruungoyot Formation of Khulsan, represented there by a single m2. We demonstrate that the separation of the masseter superficialis into two parts, the origins of which leave scars on the lateral wall of the zygomatic arch surrounded by zygomatic ridges, occurs in all the multituberculates (beginning with Paulchoffatiidae), and is regarded as a multituberculate autapomorphy. Key words: Mammalia, Multituberculata, Djadochtatheriidae, Catopsbaatar, Djadochtatherium, Kryptobaatar, zygo− matic ridges, Cretaceous, Gobi Desert. Zofia Kielan−Jaworowska [[email protected]], Instytut Paleobiologii, Polska Akademia Nauk, ul. Twarda 51/55, PL−00−818 Warszawa, Poland; Jørn H. Hurum [[email protected]], Naturhistorisk Museum, Boks 1172 Blindern, N−0318 Oslo, Norway; Alexey V. Lopatin [[email protected]], Paleontological Institute, Russian Academy of Sciences, Profsojuznaya 123, 117997 Moscow, Russia. Introduction situated some 50 km south−westward from Naran Bulak in the Gobi Desert (Barsbold et al. 1971; Gradziński and Jerzy− In 1997 Kielan−Jaworowska and Hurum erected the multi− kiewicz 1972; Kielan−Jaworowska and Barsbold 1972; Kie− tuberculate family Djadochtatheriidae to include four Late lan−Jaworowska et al. 2000). The only exception is the single Cretaceous Gobi Desert genera: Djadochtatherium Simpson, m2 from Khulsan (ZPAL MgM−I/159, see description of the 1925, Kryptobaatar Kielan−Jaworowska, 1970, Catopsbaatar dentition). Kielan−Jaworowska, 1994, and Tombaatar Rougier, Nova− The Polish−Mongolian Expedition of 1971, when collect− cek, and Dashzeveg, 1997. The family is well−defined and dif− ing mammals at Hermiin Tsav, recognized within the Upper fers not only from all other multituberculates, but also from all Cretaceous outcrops there two localities, designated Hermiin other mammals in having an unusual shape of the snout. While Tsav I and Hermiin Tsav II (Gradziński and Jerzykiewicz in mammals in general, the lateral margins of the snout are 1972: 27, figs. 6, 7; Kielan−Jaworowska and Barsbold 1972: incurved medially in front of the zygomatic arches, in Dja− 11). Of three C. catopsaloides skulls found by members dochtatheriidae the anterior margins of the snout are confluent of the Polish−Mongolian Expeditions in 1971, two (ZPAL with the zygomatic arches, which results in a subtrapezoid MgM−I/78 and /79) were found at Hermiin Tsav I and one shape of the snout in dorsal view. (ZPAL MgM−I/80) at Hermiin Tsav II. Specimen PM 120/107 The material of Catopsbaatar, which is a monotypic derives from Hermiin Tsav I. The exact location of the two genus represented by Catopsbaatar catopsaloides (Kielan− skulls (PIN 4537/4 and 4537/5) found by members of the Jaworowska, 1974), housed in three museums (ZPAL, PIN, South Gobi Group of the Soviet−Mongolian Expedition in and PM, in Warsaw, Moscow and Ulaanbaatar respectively), 1975 is not known, and we refer to them as Hermiin Tsav, derives almost exclusively from the locality of Hermiin Tsav without further qualification (see also Kurochkin and Bars− Acta Palaeontol. Pol. 50 (3): 487–512, 2005 http://app.pan.pl/acta50/app50−487.pdf 488 ACTA PALAEONTOLOGICA POLONICA 50 (3), 2005 bold 2000). For lists of mammals found at the two localities superfamily Djadochtatherioidea within Cimolodonta. We see Kielan−Jaworowska et al. (2003: table 2). follow this division in the present paper. In the present paper we re−describe the skull of Catops− As most of the djadochtatherioidean genera are mono− baatar catopsaloides, mostly on the basis of three new, fairly typic (Kryptobaatar being an exception), in the descriptions complete specimens. The masticatory musculature of the that follow, for the sake of brevity we often use only generic specimen PIN 4537/5 has been reconstructed by Gambaryan names. and Kielan−Jaworowska (1995). Specimen PM 120/107, Institutional abbreviations.—HMNS, Hayashibara Museum found during the 1999 Nomadic Expedition at the locality of of Natural Sciences, Okayama, Japan; IMM, Inner Mongo− Hermiin Tsav I, was briefly reported by Kielan−Jaworowska lian Museum, Hohhot, China; PIN, Paleontological Institute, et al. (2002). The red beds of Hermiin Tsav yielding these Russian Academy of Sciences, Moscow, Russia; PM, Pale− skulls are regarded as a stratigraphic equivalent of the ontological Center of the Mongolian Academy of Sciences, Baruungoyot Formation, which is apparently of early or late Ulaanbaatar, Mongolia; PSS−MAE, Mongolian−American Campanian age (Gradziński and Jerzykiewicz 1972; Gra− Museum Expeditions (the collections are for the time being dziński et al. 1977; Averianov 1997; Kielan−Jaworowska et housed at the American Museum of Natural History in New al. 2003). In addition to the three new skulls mentioned York, USA); PU, Princeton University, Princeton, New Jer− above, we figure in this paper the two specimens from the sey, USA; ZPAL, Institute of Paleobiology, Polish Academy 1971 Polish−Mongolian Expedition, mentioned by Kielan− of Sciences, Warsaw, Poland. Jaworowska (1974: 41), but not figured previously. These Other abbreviations are the skull ZPAL MgM−I/80 from Hermiin Tsav and the .—We use abbreviations I, P, M, and i, p, m, for upper and lower incisors, premolars and molars single m2 from Khulsan (ZPAL MgM−I/159). respectively; m, muscle. The new skulls described herein are more complete than those previously known and allow us to recognize some structures not known previously in Catopsbaatar. Nonethe− less the state of preservation of all the skulls of Catopsbaatar Terminology is not exquisite, and often we base our interpretations on a comparison with better preserved material of Kryptobaatar Wherever possible we use the terminology of Nomina Anato− mica Veterinaria (Schaller 1992) and Evans (1995). For recently described in meticulous detail by Wible and Rougier osteological characters, which do not occur in extant mam− (2000). Comparisons with Kryptobaatar and Djadochtathe− mals we follow Kielan−Jaworowska et al. (1986) and Wible rium (the latter based on the holotype AMNH 20440; photo− and Rougier (2000), with the exception of usage of “minor graphs of a specimen from Tögrög, kindly sent by Dr. Mahito palatine foramen” by the latter. With respect to “lateral Watabe to ZK−J in 1996; and the photograph of a skull, flange” we use this term differently than Wible and Hopson collected by Mongolian−American Museum Expeditions at (1995). Ukhaa Tolgod, figured by Webster 1996) show that the three The term lateral flange was introduced by Crompton genera in question are closer to one another than previously (1958) for Diarthrognathus, and then adopted by Kermack thought, which allows us to enlarge the diagnosis of the fam− (1963) for Mesozoic mammals (“triconodonts”). Of the ily Djadochtatheriidae. The relevant details of skull structure specimens described by Kermack (1963) it is best seen in are, however, not known in Tombaatar, which is less Trioracodon ferox extending parallel to the lateral boundary complete than other djadochtatheriid taxa. of the lateral trough, which adjoins the promontorium (pl. 2: The genus Catopsbaatar has a long history. Originally 3 of Kermack’s paper). Kielan−Jaworowska (1971: fig. 12) Catopsbaatar catopsaloides was referred to Djadochtathe− compared the ventral view of the petrosals in two multi− rium (Kielan−Jaworowska 1974) and then to Catopsalis (Kie− tuberculate taxa, Kamptobaatar and Sloanbaatar, with that lan−Jaworowska and Sloan 1979; Kielan−Jaworowska et al. of Trioracodon and identified the multituberculate lateral 1986). Cladistic analysis of Catopsalis and related forms al− flange placed exactly as in Trioracodon. Wible and Hopson lowed Simmons and Miao (1986: 87) to demonstrate the (1993: fig.