When was evidence of the Catopsbaatar first found?
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Catopsbaatar is a genus of multituberculate, an extinct order of rodent-like mammals. It lived in what is now Mongolia during the late Campanian age of the Late Cretaceous Period, about 72 million years ago. The first fossils were collected in the early 1970s, and the animal was named as a new species of the genus "Djadochtatherium " in 1974, "D. catopsaloides". The specific name refers to the animal's similarity to the genus "Catopsalis". The species was moved to the genus "Catopsalis" in 1979, and received its own genus ("Catopsbaatar", Greek and Mongolian for "visible hero") in 1994. Five skulls, one molar, and one skeleton with a skull are known; the latter is the genus' most complete specimen. "Catopsbaatar" was a member of the family Djadochtatheriidae.
Catopsbaatar
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Kielan-Jaworowska originally classfied "Catopsbaatar" as a member of the multituberculate family Taeniolabididae in 1974. In 1994, she suggested that "Djadochtatherium" was close to "Catopsbaatar" ancestry. She and Hurum named a new family of multituberculates, Djadochtatheriidae (which they placed in the new suborder Djadochtatheria), in 1997. The family included the genera "Djadochtatherium", "Catopsbaatar", "Kryptobaatar", and "Tombaatar", all from the Gobi Desert. The family differs from other multituberculates (and other mammals) in that the front margins of its snout were confluent with the zygomatic arches (cheekbones), giving the snout a trapezoid shape when seen from above. In general, other mammals have snouts where the side margins are curved inward in front of the zygomatic arches. Kielan-Jaworowska and Hurum revised the higher ranks within Multituberculata in 2001, replacing the suborder Djadochtatheria with the superfamily Djadochtatherioidea (placed in the suborder Cimolodonta).
The following cladogram shows the placement of "Catopsbaatar" among other multituberculates according to Kielan-Jaworowska and Hurum, 1997:
The most complete adult "Catopsbaatar" skull (specimen PM 120/107) is long and wide, with a lower jaw. By comparison, the skull of the juvenile holotype (ZPAL MgM−I/78) is about long and wide, with a lower jaw. The largest adult skull (ZPAL MgM−I/79) is long but, since it is incomplete, its other measurements are unknown. "Catopsbaatar" was larger than its relatives, "Kryptobaatar" and "Djadochtatherium". Multituberculates had relatively large skulls and short necks; their skulls were proportionally longer and wider than those of similarly sized rodents and marsupials. The external appearance of their heads may have been similar to those of rodents.
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Kielan-Jaworowska and American palaeontologist Robert E. Sloan considered the genus "Djadochtatherium" a junior synonym of "Catopsalis", and created the new combination "C. catopsaloides" in 1979. American palaeontologists Nancy B. Simmons and Miao Desui conducted a 1986 cladistic analysis which indicated that "Catopsalis" was a paraphyletic taxon (an unnatural grouping of species), and "C. catopsaloides" required its own generic name. Kielan-Jaworowska followed Simmons and Miao's suggestion, moving "C. catopsaloides" to its own monotypic genus ("Catopsbaatar") in 1994. "Catops" is derived from the Greek word "katoptos" ("visible" or "evident"); "baatar" is Mongolian for "hero", and the name refers to "Catopsbaatar"'s similarity to the genus "Catopsalis" (as is the case for the specific name). The name "Catopsalis" itself consists of the Greek words for "visible" and "cutting shears" ("psalis").
Later in 1994, Kielan-Jaworowska and the Russian palaeontologist Petr P. Gambaryan mentioned caudal (tail) vertebrae which may have belonged to "Catopsbaatar"; this attribution is uncertain, since they may instead belong to the related "Tombaatar" (named in 1997). A fourth skull (PIN 4537/4, a juvenile), discovered during the 1975 Soviet−Mongolian Expedition, was mentioned by Gambaryan and Kielan-Jaworowska in 1995. Canadian palaontologist Phillip J. Currie found a new "Catopsbaatar" specimen during the 1999 Dinosaurs of the Gobi expedition, organised by the American Nomadic Expeditions Company. Housed at the Mongolian Academy of Sciences in Ulaanbaatar as PM120/107, this most completely preserved known specimen consists of the complete skull (which may be slightly flattened) and partial skeleton of an adult individual. The specimen has rather complete fore- and hind limbs, which were unknown for the genus until then and which are generally rarely preserved in multituberculates. Its pelvic ilia were stolen and destroyed by a schoolboy on tour at the Natural History Museum of Oslo, when it was prepared in 2000. The specimen was reported in 2002 by Kielan-Jaworowska, Norwegian palaeontologist Jørn Hurum, Currie and Mongolian palaeontologist Rinchen Barsbold, who also mentioned another skull (PIN 4537/5, a juvenile) found during the 1975 expedition. "Catopsalis joyneri", the basis of the name "C. catopsaloides", was moved to the new genus "Valenopsalis" in 2015.
Catopsbaatar
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"Catopsbaatar" belonged to the order Multituberculata, a group within Allotheria (an infraclass of mammals outside Theria, the group that contains modern placentals and marsupials). Multituberculates are characterized by having premolars and molars with multiple low cusps, arranged in longitudal rows. Multituberculates are the best-known group of mammals from the Mesozoic Era, when the dinosaurs dominated; although the earliest multituberculate remains are from the Jurassic Period, the group is known as recently as the Eocene Epoch (thereby surviving the Cretaceous–Palaeogene extinction event). The group may have become extinct due to competition with eutherian mammals, such as rodents. Multituberculates were mainly known from teeth and jaws until the 1920s, when more complete specimens were discovered—first in Asia, and then elsewhere. Postcranial bones (the rest of the skeleton, other than the skull) remain rare.
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In 2008, Kielan-Jaworowska and Hurum suggested that the long spinous process on a "Catopsbaatar" vertebra and the long transverse processes in "Nemegtbaatar" may indicate that some multituberculates were saltatorial (had the ability to jump). "Catopsbaatar" probably had strong muscles attaching to the tuber calcanei, which further supports the jumping hypothesis. Although it has been suggested that multituberculates were arboreal (lived in trees), most Asian taxa were probably terrestrial; some others were fossorial, digging and living underground.
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Although multituberculates were thought to have been carnivores or herbivores, since American palaeontologist William A. Clemens and Kielan-Jaworowska suggested modern rat kangaroos as analogues for the group in 1979 they have been considered omnivores (feeding on both plants and animals). Uniquely among mammals, multituberculates employed a backward chewing stroke which resulted in the masticatory muscles—the muscles which move the mandible—being inserted more to the front than in other groups (including rodents). Gambaryan and Kielan-Jaworowska reconstructed the masticatory musculature of various multituberculates in 1995, and found that "Catopsbaatar" and its relatives had very powerful masticatory musculature, due to their high zygomatic arches and large anterior and intermediate zygomatic ridges and coronoid processes. Their powerful incisors, with limited bands of enamel, would have been well adapted to gnawing and to cutting hard seeds (similar to rodents). Since it was larger than some other multituberculates, "Catopsbaatar" would only have to open its mouth 25 degrees to crush hard seeds in diameter; a 40-degree gape would have caused dislocation. After the incisors cut, the premolars and molars would begin to grind with a "power stroke".
According to Gambaryan and Kielan-Jaworowska, the adaptation for crushing hard seeds sometimes—as in "Catopsbaatar"—opposed the benefit of a low condylar process (which discourages mandibular dislocation). The anterior and intermediate zygomatic ridges of the skull were the origin of the superficial masseter muscle, which facilitates chewing. The separation of the origin of this muscle into two parts and the rounded muscle scars left by them are unique among mammals to multituberculates. The masticatory muscles of multituberculates independently evolved features shared with rodents and small herbivorous marsupials. As with rodents, multituberculates may have been capable of bilateral mastication—where both rows of teeth in the mandible perform the same function simultaneously—and unilateral mastication (where the rows on one side are used).
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All specimens of "Catopsbaatar" are known from the Red Beds of Hermiin Tsav formation, except for one molar from the Barun Goyot Formation. The formations are considered coeval (of the same geological age), contain many of the same animals, and probably date to the late Campanian age of the Late Cretaceous Period (about 72 million years ago). The rock facies of the Red Beds of Hermiin Tsav formation consist of orange-coloured, thick-bedded sandstone, with a thin interbedding of light-coloured silt stones and claystones. The rock facies of the Barun Goyot Formation are considered to be the result of an arid or semi-arid environment, with aeolian (deposited by wind) beds.
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Hurum, Zhe-Xi Luo, and Kielan-Jaworowska suggested in 2006 that the spurs on the ankles of Mesozoic mammals (such as "Catopsbaatar") were homologous with those of monotremes, and were a basal (or "primitive") feature lost by later therian mammals. The male platypus uses the spur to deliver venom from a gland, but it is unknown if the extinct groups were venomous as well. Mesozoic mammals were mostly small (with exceptions such as the fox-sized "Repenomamus") and, although they were too small to be prey for large theropod dinosaurs, smaller theropods, large lizards, crocodiles, and birds could have fed on them. For example, mammal jaws have been found in the abdomen of a specimen of the small theropod "Sinosauropteryx"; the jaws belonged to "Zhangheotherium", which also had spurs, and the multituberculate "Sinobaatar". Since dinosaurs dominated Earth during the Mesozoic, this period has been called the "dark ages" of mammalian history. The spur, which would have been more effective if venomous, was probably used as a defensive weapon by small, early mammals. It could also have been used during intraspecific competition or predation.
The pelvic bones of "Catopsbaatar" specimen PM120/107 may not have been fused because fusion occurred late in development, because it was a sexually dimorphic feature only occurring in males (unfused pelvic bones might have enabled expansion of the birth canal in females), or pelvic fusion may be a taxonomic difference between "Catopsbaatar" and other multituberculates. Unlike with other mammals, the pelvis of multituberculates was very narrow; in other genera where the pelvis is known, each half of the pubis and ischium were fused together, forming a keel. The length and rigidity of the keel indicate that the pelvis could not have spread during birth. Because there would be little space for the passage of an egg (egg-laying monotremes have wide ischial archs), Kielan-Jaworowska suggested in 1979 that multituberculates were viviparous (gave live birth) and that the newborns were extremely small—similar to those of marsupials.
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Hair, structurally similar to that of modern mammals and associated with bones of the Mongolian multituberculate "Lambdopsalis", has been identified in coprolites (fossilised feces) of carnivorous mammals from the Palaeocene Epoch. This indicates that multituberculates had hair for insulation, similar to modern mammals (and possibly fossil mammals), a feature probably related to homeothermy (warm-bloodedness).
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At one time, the genus was also formally reported from the upper Cretaceous of Mongolia. However, that material was subsequently referred to the genera of "Djadochtatherium" and "Catopsbaatar". "Catopsalis" is within the suborder of Cimolodonta and a member of the superfamily Taeniolabidoidea.
Catopsalis