Catopsbaatar

Catopsbaatar
Temporal range: Late Cretaceous
Catopsbaatar catopsaloides.jpg
The most completely preserved skeleton (specimen PM120/107) shown from above as preserved (left), with diagram showing individual bones
Scientific classification
Kingdom:
Phylum:
Class:
Order:
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Genus:
Catopsbaatar

Species:
C. catopsaloides
Binomial name
Catopsbaatar catopsaloides
(Kielan-Jaworowska, 1974)
Synonyms
  • Djadochtatherium catopsaloides Kielan-Jaworowska, 1974
  • Catopsalis catopsaloides Kielan-Jaworowska & Sloan, 1979

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 D. catopsaloides as a new species of the genus Djadochtatherium in 1974. The specific name refers to its similarity to the genus Catopsalis. The species was moved to the genus Catopsalis in 1979, and received its own genus (Catopsbaatar, "visible hero") in 1994. Five skulls, one molar, and one skeleton with a skull are known; the latter is the genus' most complete specimen. It was a member of the family Djadochtatheriidae.

The skull of Catopsbaatar was up to 70 mm (2.8 in) long and, as in other multituberculates, proportionally large. The external appearance of their heads may have been similar to that of rodents. The skull was heavy-set and wide, with the zygomatic arches strongly expanded to the sides. The eye sockets were smaller and placed further back than in its relatives, and the snout was more elongated. It had two semicircular ridges on the side of the skull, to which the jaw muscles were attached. The mandible was strong and very elongated. It had very robust incisors, and cheek teeth with multiple cusps (for which multituberculates are named). The pelvic bones differed from those of other multituberculates in that they were not fused to each other. Catopsbaatar had spurs on its ankles, like those of the male platypus and echidna, without evidence of a venom canal (present in the former).

The spurs of Catopsbaatar and other Mesozoic mammals may have been used for protection against dinosaurs and other predators. Multituberculates are thought to have given live birth, and the fact that they had hair indicates that they were homeothermic (warmblooded). Multituberculates would have been omnivorous, with a backwards chewing stroke; Catopsbaatar had powerful jaw muscles, and its incisors were well adapted for gnawing hard seeds. Multituberculates are thought to have had a sprawling posture, and Catopsbaatar may have been able to jump. Catopsbaatar is known from the Red Beds of Hermiin Tsav and the Barun Goyot Formation, which are thought to be the same geologic age.

Taxonomy

Three skulls with jaws (A and B were collected in 1975, C in 1999), showing differences in size related to the age of each individual

In 1970 and 1971, the Polish-Mongolian Palaeontological Expeditions collected mammalian fossils from the Red Beds of Hermiin Tsav formation in the Gobi desert of Mongolia. About 100 specimens were recovered from four localities, and are housed at the Polish Academy of Sciences in Warsaw. Two thirds of the collected specimens were multituberculates, an extinct order of mammals with rodent-like dentition, named for the multiple cusps (or tubercles) on their molar teeth. In 1974, the Polish palaeontologist Zofia Kielan-Jaworowska named a new species of the Mongolian multituberculate genus Djadochtatherium, D. catopsaloides, with specimen ZPAL MgM−I/78 from the Polish collection as the holotype. The specific name refers to its similarity to the North American species Catopsalis joyneri, which Kielan-Jaworowska thought to be a possible descendant. The specimen was collected at the Hermiin Tsav I locality, and is an almost complete skull of a juvenile individual, with some parts of the cranium damaged. In addition, Kielan-Jaworowska assigned a damaged skull missing lower jaws (ZPAL MgM−I/79, an adult), a skull with partial lower jaws (ZPAL MgM−I/80), and a molar tooth with a fragment of jaw (ZPAL MgM−I/159 from the Barun Goyot Formation of Khulsan, the only specimen not from Hermiin Tsav), to the species.[1][2][3][4]

In 1979, Kielan-Jaworowska and the American palaeontologist Robert E. Sloan considered the genus Djadochtatherium a junior synonym of Catopsalis itself, thereby creating the new combination C. catopsaloides.[5] In 1986, the American palaeontologists Nancy B. Simmons and Miao Desui conducted a cladistic analysis which showed that Catopsalis was a paraphyletic taxon, an unnatural grouping of species, and that C. catopsaloides was therefore in need of its own generic name.[6] In 1994, Kielan-Jaworowska followed the suggestion by Simmons & Miao and moved C. catopsaloides to its own, monotypic genus; Catopsbaatar. Catops is derived from the Greek word katoptos, which means "visible" or "evident", while baatar is Mongolian for "hero", and the name is a reference to its similarity to the genus Catopsalis.[7]

PM120/107 shown from below

Later in 1994, Kielan-Jaworowska and the Polish palaeontologist Petr P. Gambaryan mentioned some caudal (tail) vertebrae which may have belonged to Catopsbaatar, but this attribution is uncertain, since they may also belong to the related Tombaatar, which was named in 1997.[8][9] A fourth skull (PIN 4537/4, a juvenile), discovered during the Soviet−Mongolian Expedition in 1975, was mentioned by Gambaryan and Kielan-Jaworowska in 1995.[10] In 1999, the Canadian palaontologist Phillip J. Currie found a new Catopsbaatar specimen, during the "Dinosaurs of the Gobi" expedition, organised by the American Nomadic Expeditions Company. Housed at the Mongolian Academy of Sciences in Ulanbaatar as PM120/107, it is the most completely preserved specimen yet known, and consists of a complete skull (which may be slightly flattened) and partial skeleton of an adult individual. The specimen preserves rather complete fore and hindlimbs, which were not known for the genus until that point, and which are generally rarely preserved in multituberculates. The ilia from its pelvis were stolen and destroyed by a schoolboy on tour at the Natural History Museum of Oslo when the specimen was prepared there in 2000. The specimen was reported in 2002 by Kielan-Jaworowska, the Norwegian palaeontologist Jørn Hurum, Currie, and the Mongolian palaeontologist Rinchen Barsbold, who also mentioned another skull (PIN 4537/5, a juvenile) found during the 1975 expedition.[9][11][3] In 2015, Catopsalis joyneri, which was the basis of the name C. catopsaloides, was moved to the new genus Valenopsalis instead.[12][1]

Evolution

Catopsbaatar belonged in the order Multituberculata, a group within Allotheria, an infraclass of mammals outside the group that contains placentals and marsupials, Metatheria. Multituberculata is characterized by their premolars and molars with multiple low cusps arranged in longitudal rows. Multituberculates are the best known group of mammals from the Mesozoic era (living alongside the dinosaurs); the earliest remains of multituberculates are from the Jurassic period, but the group is known from remains as recent as the Eocene epoch. The group may have become extinct due to competition with eutherians mammals, such as rodents. Multituberculates were mainly known from teeth and jaws until the 1920s, when more complete specimens were discovered in Asia, and later the rest of the world.[4] Postcranial bones (of the rest of the skeleton other than the skull) remain rare, however.[11]

Kielan-Jaworowska originally classfied Catopsbaatar as a member of the multituberculate family Taeniolabididae in 1974.[1] In 1994, she suggested that Djadochtatherium was close to the ancestry of Catopsbaatar.[7] In 1997, Kielan-Jaworowska and Hurum named a new family of multituberculates, Djadochtatheriidae, which they placed in the new suborder Djadochtatheria. The family included the genera Djadochtatherium, Catopsbaatar, Kryptobaatar, and Tombaatar, all from the Gobi desert. The family differs from all other multituberculates, as well as other mammals, in that the front margins of the snout were confluent with the zygomatic arches (or cheek bones), resulting in the snout being trapezoid in shape when seen from above. In general, other mammals have snouts where the margins at the sides of the snout are curved inward in front of the zygomatic arches.[13] In 2001, Kielan-Jaworowska and Hurum revised the higher ranks within Multituberculata and replaced the suborder Djadochtatheria with the superfamily Djadochtatherioidea, placed in the suborder Cimolodonta.[14]

Comparison between the skulls of the djadochtatheriids Kryptobaatar (A), Djadochtatherium (B), and Catopsbaatar (C), with the muscle scars surrounded by the zygomatic ridges shaded

The following cladogram shows the placement of Catopsbaatar among other multituberculates according to Kielan-Jaworowska and Hurum, 1997:[13]

Ptilodus

Buginbaatar

Lambdopsalis

Taeniolabis

Stygimys

Eucosmodon

Nemegtbaatar

Bulganbaatar

Kamptobaatar

Sloanbaatar

Nessovbaatar

Chulsanbaatar

Kryptobaatar

Djadochtatherium

Catopsbaatar

Tombaatar

Description

Skull

Restoration of an individual in aggressive posture, with sprawling limbs, and spurs on the ankles similar to those of the male platypus

The most complete adult Catopsbaatar skull (of specimen PM 120/107) is 63 mm (2.5 in) long, 55 mm (2.2 in) wide, with a 41 mm (1.6 in) long lower jaw. By comparison, the skull of the juvenile holotype (ZPAL MgM−I/78) is about 53 mm (2.1 inches) long, 56 mm (2.2 inches) wide, with a 35 mm (1.4 in) long lower jaw. The largest adult skull (ZPAL MgM−I/79) is 70 mm (2.8 in) long, but as it is incomplete, its other measurements are unknown. Catopsbaatar was larger than its relatives Kryptobaatar and Djadochtatherium.[3] Multituberculates had relatively large skulls and short necks; their skulls were proportionally longer and wider than those of similarly sized rodents and marsupials.[4] The external appearance of their heads may have been similar to that of rodents.[10]

Overall, the skull of Catopsbaatar was heavy-set, with a wide margin across the front. The skull was shorter along the midline than at the sides, because the nuchal crest at the back of the head curved inwards at the middle. The zygomatic arches were strongly expanded to the sides, with the skull-width across the arches being about 85% of the skull-length. Seen from above, the front part of the orbits (eye sockets) were placed further back than in other djadochtatheriids, resulting in a more elongated snout (65% of the skull-length) and small orbits. The premaxilla (the front bone of the upper jaw) extended less than two thirds of the part of the snout in front of the eyes, shorter than that of Kryptobaatar. The premaxilla consisted of parts of the face and the palate; djadochtatheriids had a ridge on the boundary between the two called the premaxillary ridge. The palatal side of the ridge on each premaxilla was narrow, flat, and covered in small foramina (openings). The palatal part of the premaxilla was concave and had many randomly arranged nutrient foramina. The nasal bone (which formed the upper part of the snout) was relatively wide, becoming wider towards the back, and its front was covered with irregularly spaced vascular foramina. The suture between its nasal and frontal bones was less pointed forwards in the middle than those of its relatives.[3]

Stereo photographs showing the most complete adult skull PM 120/107 from above, the side, and below

The maxilla (the main part of the upper jaw) was extensive, and formed most of the side of the snout. It bore all the upper teeth, except the incisors. The infraorbital foramen (an opening at the lower front of the maxilla) was slit-like in some specimens, but rounded in others, and varied in numbers from one to three. One of the most characteristic features on the face of Catopsbaatar was the very large anterior zygomatic ridge on the sides of the upper jaw (a site for jaw muscle attachment). It was much higher than in other djadochtatheriids, except for Djadochtatherium, from which it differed in that the ridge was semicircular rather than roughly trapezoidal (other genera have elliptical ridges). The front margin of the anterior zygomatic ridge was thickened, which produced a bulge on the side margin of the snout when viewed from the side and above. The lower part of the suture between the maxilla and the squamosal bone extended along the hind border of the anterior zygomatic ridge. The palatal processes of the maxilla formed most of the palate. The major palatine foramina had shallow grooves extending forwards from them. The postpalatine torus (a bony protusion on the palate) of Catopsbaatar was less prominent than that of Tombaatar. The frontal bone was large and formed most of the cranial-roof. The suture between the frontal and parietal bones formed a U-shaped part at the middle that pointed backwards, similar to Kryptobaatar but less deep, with smaller U-shaped structures at the sides. This differed from Djadochtatherium, which had a narrower, V-shaped suture between the frontal and parietal bones.[3]

Diagrams showing a reconstructed adult skull from above, below, and the side. Note that the upper P1 and P3 premolars are included, though these disappeared as an individual aged

The intermediate zygomatic ridge on the squamosal bone (also for jaw muscle attachment) was much smaller and lower than the anterior zygomatic ridge in front of it, and Catopsbaatar differed from other djadochtatheriids in that the intermediate ridge contacted the anterior ridge with its front edge. The posterior zygomatic ridge at the lower back of the squamosal bone was the weakest of the three ridges, and was only marked by a depression. The postorbital processes behind the orbit on the parietal bone were very long, and the parietal ridges extended from the hind margin of the postorbital processes towards each other, but without reaching. The nuchal crest was very prominent, and extended to the sides to form "wings". The occipital plate was almost vertical, yet slightly concave, and was obscured by the nuchal crest when the skull was viewed from above. Though incompletely preserved, it is assumed the occipital plate sloped back and upwards from the occipital condyles, as in some relatives.[3] The orbito-temporal vascular system inside the skull of Catopsbaatar did not differ much from those of related genera.[15]

The mandible of Catopsbaatar was robust and very elongated. The diastema (gap between the front and cheek teeth) was concave, and extended for 20% of the dentary bone (the main bone of the lower jaw). When seen from above, the diastema formed a wide shelf, which sloped downwards on he inner side of the jaw. The small mental foramen was close to the upper middle margin of the diastema. The coronoid process of the mandible appears to have been relatively longer and narrower than in other djadochtatherioideans. It was separated from the alveolar process (where the teeth are contained) by a wide groove. The mandibular condyle (which articulated with the skull) was placed slightly above the level of the molars. The front part of the masseretic crest was very prominent, and formed a bulge called the masseretic protuberance. The robustness of this crest and the presence of the protuberance varies between related genera. The masseteric fovea (or pit) in front of the masseteric fossa was probably more distinct than in other dochtatherioids. Each half of the mandibular symphysis (where the two halves of the mandible connect) was shaped like an upside down tear-drop. The pterygoid fossa on the inner side of the mandible was very large, and occupied most of the hind part of the dentary. The lower part of this fossa had a boundary called the pterygoideus shelf.[3]

Dentition

Stereo micrographs showing premolars and molars of the holotype, with an assigned molar inserted (B)

The dental formula (which shows the number of teeth of each type in the tooth-row of a mammal) of Catopsbaatar was 2.0.3.21.0.2.2 (2 incisors, 0 canines, 3 premolars, and 2 molars in half the upper tooth-row, and 1 incisor, 0 canines, 2 premolars and 2 molars in half the lower). By comparison, the dental formula of humans is 2.1.2.2-32.1.2.2-3. Each tooth in a mammal is designated with a letter and a number for its position (I for incisor, C for canine, P for premolar, M for molar); the letters are capitalised for the teeth of the upper jaw, but not for those in the lower jaw. The cusp formula shows the arrangement and number of cusps in consecutive rows on a tooth, from the outer to the inner side, each row separated by a colon.[3][4][9]

The I2 upper front incisors of Catopsbaatar were very robust, and had a sharply limited band of enamel. These two incisors converged slightly towards the middle and touched each other. The smaller I3 incisor behind was cone shaped. The alveolus (tooth socket) of its I3 incisor was formed by the premaxilla rather than both the premaxilla and maxilla, unlike in Tombaatar. The front upper premolars P1 and P3 were only present in juveniles (deciduous), and disappeared in older individuals along with their alveoli. The P1 appears to have had two cusps, was single-rooted, and had a cone-like, blunt crown. The P3 was single-rooted and smaller than the P1. The cusp formula of the P4 premolar was 5−4:1, with the central cusp being the largest. The P4 of Catopsbaatar was almost trapezoidal in shape, unlike in Djadochtatherium and Kryptobaatar where it is crescent-shaped, and it was also smaller and lacked ridges. Catopsbaatar also differed in only having three upper premolars, due to lacking the P2, a feature shared with Tombaatar. Other mammals usually evolve the loss of teeth at the beginning or end of a tooth row, not in the middle, as in multituberculates. The cusp formula of the M1 molar was 5−6:5−6:4, with the inner ridge extending about 75% of the tooth's length. The cusps of the M1 were sharp and unworn in juveniles, but worn and concave in older animals. The cusp formula of the M2 molar was 2:2−3:2−3.[3][4]

As was characteristic for multituberculates, the lower pair of incisors of Catopsbaatar was very strong and compressed sideways. It had a sharply limited band of enamel, and grew continually. The lower p3 premolar was very small and adhered entirely to the lower diastema under the larger p4. The blade-like p4 was roughly trapezoidal in side view, and had three cusps along the horizontal upper margin, and one cusp on outer back side. The p4 did not have the ridges on the outer and inner side as are present in other multituberculates. The m1 molar was almost symmetrical, and its cusp formula was 4:4, with the size of the cusps decreasing towards the back. The m2 had a cusp formula of 2−3:2, with most specimens having 2:2. The cusps on the inner side were wider than those on the outer side, the inner row of cusps was shorter than the outer one, and the hind margin of the tooth was arranged obliquely.[3]

Postcranial skeleton

Lumbar vertebra of PM120/107 in multiple views

The only preserved postcranial skeleton of Catopsbaatar is that of specimen PM120/107, which is fragmentary. It includes elements that are unknown or incompletely preserved in other multituberculates. One lumbar vertebra (the fifth or sixth, from between the ribcage and the pelvis) had a spinuous process which was stout in side view, and long when seen from above. The clavicle was slightly less curved than that of Kryptobaatar, resembled a bent rod which was widened in each end, and measured about 24.8 mm (0.98 in). The upper part of the scapulocoracoid was relatively wide in side view, and the lower part was very narrow. It was probably about 60 mm (2.4 in) long when complete. The preserved part of the humerus (upper arm bone) was about 37.5 mm (1.48 in) long. The shaft of the humerus was triangular in cross-section, relatively narrow when seen from above, and most of its width was occupied by the intertubercular groove. This groove was delimited on the side by the crest of the greater tubercle, whose middle part formed the deltopectoral crest. The ulnar condyle (where the ulna bone of the lower arm articulated with the humerus) was more prominent than the radial condyle (where the radius articulated), oval, and delimited from the radial condyle by a groove. The radius was about 26 mm (1.0 in) long, with a prominent head. The shaft of the radius was smooth, compressed from top to bottom, and oval in cross-section. The ulna was compressed sideways, flatter than the radius, and was about 40 mm (1.6 in) long as preserved.[11]

Stereo photos and diagrams showing the femora of PM120/107

The contact from the ischium to the ilium and pubis of the pelvis was not fused, and the front end of the ischium was a rugose suture. The pubis was roughly triangular and had a rough suture for the ilium above, and a deep groove for the ischium at the lower front. The pelvic bones of Catopsbaatar specimen PM120/107 differed from those of other multituberculates in that they were not fused to each other. The presence of sutures in the pelvis of PM120/107 indicates that it was a juvenile, though the skull appears to be adult, and the meaning of this discrepancy is unknown. The femur (thigh bone) was proportionally similar to that of Eucosmodon and Nemegtbaatar, being smaller than the former but larger than the latter. The femur was stout in proportion to its length, and it may have been about 56 mm (2.2 in) long. The tibia of the lower leg was about 35.8 mm (1.41 in) long. Seen from behind the upper side, the tibia had a deep excavation, which may be characteristic of all multituberculates.[11]

Unlike most other multituberculates and other mammals, the calcaneus bone at the back of the foot had a short tuber calcanei (like some tree kangaroos), with an expanded, anvil-shaped proximal process, which was strongly bent downwards and to the side. Catopsbaatar had an os calcaris bone on the inner side of its ankle, a feature also seen in males of the modern monotremes, the platypus and the echidna, as well as in other Mesozoic mammals. The os calcaris bone was plate-like and rectangular in outline, and as in monotremes, it supported the cornu calcaris, together forming an extratarsal spur. Unlike other Mesosoic mammals, these two elements were not fused together in multituberculates. The cornu calcaris was triangular, with a concavity in the middle, and was 13 mm (0.51 in) long. The spur was flattened, and was thicker at its conjunction with the os calcaris, where they connected via several ridges. As the spur may have been moved from its original position, it is unknown whether it faced inwards, as in the platypus. Unlike the platypus, there was no impression of a canal for venom. The cornu calcaris of Catopsbaatar was ossified (turned to bone) and would have been covered in keratin (the horny covering which nails are also made of). The cornu calcaris of the platypus is not ossified, consisting only of keratin, and is hollow.[11][16]

Palaeobiology

Stereo photos and diagram showing the left foot of PM120/107; the os calcaris bone, which formed the base of the spur, is at the upper left

In 2006, Hurum, Zhe-Xi Luo, and Kielan-Jaworowska suggested that the spurs found in Mesozoic mammals, such as those of Catopsbaatar, were homologous with those of monotremes, and that this feature was a basal (or "primitive") feature later lost in therian mammals. The male platypus uses the spur to deliver venom from a gland, but it is unknown if the extinct groups would have been venomous as well. Mesozoic mammals were mostly small (with exceptions such as Repenomamus being the size of a fox, or the marten-sized deltatheroidans that were contemporary to Catopsbaatar[17]), and though they were too small to be prey for large theropod dinosaurs, but smaller therpods, large lizards, crocodiles, and birds could have fed on them. Mammal jaws have been found inside the abdomen of a specimen of the small theropod Sinosauropteryx, for example, belonging to Zhangheotherium (which had spurs) and the multituberculate Sinobaatar. Since dinosaurs dominated the world during the Mesozoic, this period has been called the "dark ages" of mammal history. The spur was probably used as a defensive weapons in small, early mammals, which would have been even more effective if venomous. The spur could also have been used during intraspecific competition or predation.[16]

Stereo photos showing pelvic bones of PM120/107

The lack of fusion of the pelvic bones in Catopsbaatar specimen PM120/107 may either be because this occurred late in development, because it was a sexually dimorphic feature only occurring in males (unfused pelvic bones might enable expansion of the birth canal in females), or a taxonomic difference between Catopsbaatar and other multituberculates.[11] Unlike in other mammals, the pelvis of multituberculates was very narrow, and 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 this keel indicates that the pelvis could not have spread during birth. Because there would be little space for the passage of an egg (the 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.[4]

Hair associated with bones of the Mongolian multituberculate Lambdopsalis (structurally similar to hair of modern mammals) 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 all fossil mammals too), a feature probably related to homeothermy (or "warmbloodedness").[4][18]

Feeding and diet

Reconstructed jaw musculature; B1 shows superficial layers, B2 shows second layers

Multituberculates are historically thought to have been either carnivores or herbivores, but since the 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).[4] Uniquely among mammals, multituberculates employed a backward chewing stroke, which resulted in the masticatory muscles (the muscles that move the mandible) being inserted more to the front than in other groups, including in rodents. In 1995, Gambaryan and Kielan-Jaworowska reconstructed the masticatory musculature of various multituberculates, 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. Due to being relatively larger than various other multituberculates, Catopsbaatar would only have to open its mouth 25° to be sufficient to crush hard seeds that were 12–14 mm (0.47–0.55 in) in diameter (a gape of 40° would have caused dislocation). After the incisors were finished cutting, the premolars and molars began to grind with a "power stroke".[10]

Stereo photos showing the dentaries of two specimens in multiple views

Gambaryan and Kielan-Jaworowska concluded that the adaptation for crushing hard seeds in some cases, such as Catopsbaatar, worked against the benefits of having the condylar process of the mandible in a low position (a feature which works against dislocation of the mandible). The anterior and intermediate zygomatic ridges of the skull served as the origin of the superficial masseter muscle, which facilitates chewing. The separation of the origin of this muscle in two parts and the rounded muscle scars left by them are unique to multituberculates among mammals. The masticatory muscles of multituberculates independently evolved various features with rodents and small herbivorous marsupials. Similar to rodents, multituberculates may have been capable of both bilateral mastication, where both rows of teeth in the mandible perform the same function simultaneously, and unilateral mastication, where only one the rows on one side is used.[3][10]

Posture and locomotion

Stereo photos and diagrams showing arm bones of PM120/107

There has been debate about the limb-posture in multituberculates. Some researchers have advocated for them having employed a parasagittal stance, wherein the limbs were kept erect under the body, while others have considered a sprawling stance more likely. In 2006, Kielan-Jaworowska and Hurum supported a sprawling stance, based on the presence of spurs on their hindlegs, a feature they considered to have only been present in sprawling mammals. They also pointed out that all early mammals that have been preserved in lacustrine (lake) deposits have been compressed from top to bottom which suggests a sprawling stance, whereas later mammals are preserved on their flanks. Earlier arguments for a sprawling stance include their deep pelves and features in the legs. They also suggested that the feet of multituberculates would have been plantigrade (where the sole touched the ground) in resting posture, but digitigrade (where the sole did not touch the ground) when jumping and running fast. They also dismissed the idea that forelimbs of multituberculates and other early mammals were more parasagittal than their hindlimbs. In their article, they depicted Catopsbaatar with plantigrade, sprawling legs, with mobile spurs that pointed inwards when ready for attack.[19][4]

Kielan-Jaworowska and Hurum suggested in 2008 that the long spinuous process seen on a Catopsbaatar vertebra and the long transverse processes seen in Nemegtbaatar may indicate that some multituberculates had the ability to jump (were saltorial). Catopsbaatar probably also had strong muscles attaching to the tuber calcanei, which further supports jumping abilities.[11][20] Though it has been suggested that multituberculates were arboreal (lived in trees), most Asian taxa were probably terrestrial, with some being fossorial (digging and living underground).[4]

Palaeoecology

Stereo photos of a clavicle and possible interclavicle of PM120/107

All specimens of Catopsbaatar are known from the Red Beds of Hermiin Tsav formation, except for the single molar from the Barun Goyot Formation. These two 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.[21] The rock facies of the Red Beds of Hermiin Tsav formation consist of orange-coloured, thick-bedded sandstones, with interbedding light-coloured thin-bedded silt stones and claystones.[22][3] The rock facies of the Barun Goyot Formation are thought to represent an arid or semi-arid environment, with aeolian (affected by wind) beds.[23][24]

Other mammals known from the Red Beds of Hermiin Tsav include the multituberculates Nemegtbaatar, Chulsanbaatar, and Nessovbaatar, and the therians Deltatheridium, Asioryctes, and Barunlestes.[25] Dinosaurs include Ajancingenia, Velociraptor, Saichania, Platyceratops, Gobiceratops, and some indeterminate theropods. Reptiles include the turtle Mongolemys, and the lizards Gobinatus, Tchingisaurus, Prodenteia, Gladidenagama, and Phrynosomimus, and an indeterminate crocodile. In addition, the frog Gobiates is known, as well as an indeterminate alexornithiform bird. Ostracods include Limnocythere, Cypridea, and Eucypris.[26]

References

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