Catopsbaatar

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Catopsbaatar
Temporal range:
Ma
↓
The most complete skeleton (specimen PM120/107) shown from above (left, A) and below (right, B) as preserved, with diagrams showing individual bones
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: †Multituberculata
Family: †Djadochtatheriidae
Genus: †Catopsbaatar
Kielan-Jaworowska
, 1994
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

epoch, 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 last is the genus' most complete specimen. Catopsbaatar 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 these animals' heads may have been similar to those of rodents. The skull was heavy-set and wide, with 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

omnivorous; Catopsbaatar had powerful jaw muscles, and its incisors were well adapted for gnawing hard seeds, using a backwards chewing stroke. Multituberculates are thought to have had a sprawling posture, and Catopsbaatar may have been able to jump. Catopsbaatar is known from the Barun Goyot Formation
, which is thought to be about 72 million years old.

Taxonomy

Preserved skeleton and diagram of bones
Cretaceous-aged fossil localities of Mongolia; Catopsbaatar was collected in area A (Khermeen Tsav I, II, and Khulsan at the left)

In 1970 and 1971, the Polish-Mongolian Palaeontological Expeditions collected mammalian fossils from the

tubercles) on their molars. In 1974, Polish palaeontologist Zofia Kielan-Jaworowska named a new species of the Mongolian multituberculate genus Djadochtatherium as D. catopsaloides, with specimen ZPAL MgM−I/78 from the Polish collection as the holotype. The specific name refers to the animal's similarity to the North American species Catopsalis joyneri, which Kielan-Jaworowska thought was a possible descendant. The specimen, collected at the Hermiin Tsav I locality, is an almost-complete skull of a juvenile with portions of the cranium damaged. Kielan-Jaworowska also assigned other specimens to the species: 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 with a fragment of jaw (ZPAL MgM−I/159 from Khulsan, the only specimen not from the Hermiin Tsav I and II localities).[1][2][3][4]

Collection of Catopsbaatar skull bones
Three skulls with jaws (A and B were collected in 1975, C in 1999), showing differences in size related to their individual ages.

Kielan-Jaworowska and American palaeontologist Robert E. Sloan considered the genus Djadochtatherium a

monotypic genus in 1994, Catopsbaatar. The word catops is derived from the Greek 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).[7] The name Catopsalis itself consists of the Greek words for "visible" and "cutting shears" (psalis).[8][7] The word baatar is used as a suffix in the names of many multituberculate genera, and alludes to the Mongolian capital Ulaanbaatar, which itself means "red hero".[9]

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

Natural History Museum of Oslo, where it was being 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.[11][13][3] Catopsalis joyneri, the basis of the name C. catopsaloides, was moved to the new genus Valenopsalis in 2015.[8][1]

Evolution

Catopsbaatar belonged to the order Multituberculata, a group within

Postcranial bones (the rest of the skeleton, other than the skull) remain rare.[13]

Kielan-Jaworowska originally classified Catopsbaatar as a member of the multituberculate family

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.[14] 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).[15]

djadochtatheriids Kryptobaatar (A), Djadochtatherium
(B), and Catopsbaatar (C), with 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:[14]

Ptilodus

Description

Skull

Drawing of Catopsbaatar, resembling a squirrel
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 (specimen PM 120/107) is 63 mm (2.5 in) long and 55 mm (2.2 in) wide, with a 41 mm-long (1.6 in) lower jaw. By comparison, the skull of the juvenile holotype (ZPAL MgM−I/78) is about 53 mm (2.1 inches) long and 56 mm (2.2 inches) wide, with a 35 mm-long (1.4 in) lower jaw. The largest adult skull (ZPAL MgM−I/79) is 70 mm (2.8 in) long but, since 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 those of rodents.[12]

The skull of Catopsbaatar was heavy-set, with a wide margin across the front. It was shorter along the midline than at the sides, because the

frontal bones was less pointed forwards in the middle than were those of Catopsbaatar's relatives.[3]

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

The

major palatine foramina had shallow grooves extending forward. The postpalatine torus (a bony protrusion on the palate) of Catopsbaatar was less prominent than was 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 in the middle which pointed backwards—similar to the condition in 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]

premolars
are included, although 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. 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 only marked by a depression. The

vascular system inside the skull of Catopsbaatar did not differ much from those of related genera.[16]

Catopsbaatar's

mandibular condyle (which articulated with the skull) was slightly above the level of the molars. The front part of the masseteric crest was very prominent, forming a bulge known as a masseteric protuberance. The robustness of this crest and the presence of the protuberance varies among related genera. The masseteric fovea (pit) in front of the masseteric fossa was probably more distinct than in other djadochtatherioids. Each half of the mandibular symphysis (where the two halves of the mandible connect) was shaped like an upside-down teardrop. 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, known as the pterygoideus shelf.[3]

Dentition

molars of the holotype
, including an isolated molar (B)

The

dental formula (the number of teeth of each type in the tooth row of a mammal) of Catopsbaatar was 2.0.3.21.0.2.2 (two incisors, no canines, three premolars and two molars in half of the upper tooth row, and one incisor, no canines, two premolars and two molars in half of 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 number by 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 of a tooth, from the outer to the inner side; each row is separated by a colon.[3][4][11]

Being a cimolodont, Catopsbaatar did not have I1 incisors.[14] The I2 upper-front incisors of Catopsbaatar were very robust and had a sharply limited band of enamel. The two incisors converged slightly towards the middle, touching each other. The smaller I3 incisor behind was cone-shaped. The alveolus (tooth socket) of Catopsbaatar's I3 incisor was formed by the premaxilla, rather than the premaxilla and maxilla (unlike in Tombaatar). The front upper premolars P1 and P3 were only present in juveniles (deciduous), disappearing (with their alveoli) in older individuals. P1 appears to have had two cusps, was single-rooted, and had a cone-like, blunt crown. P3 was single-rooted and smaller than P1. The cusp formula of the P4 premolar was 5−4:1, 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), smaller, and lacking ridges. Catopsbaatar also differed by having only three upper premolars, 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 percent of the tooth's length. The cusps of 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]

Catopsbaatar had a single lower pair of incisors, characteristic of multituberculates, which was very strong and compressed sideways. It had a sharply limited band of enamel, and grew continually. The 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 the 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, the size of the cusps decreasing towards the back. The m2 had a cusp formula of 2−3:2, most specimens being 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 Catopsbaatar specimen which preserves the postcranial skeleton is PM120/107, which is fragmentary. It includes elements that are unknown, or incompletely preserved, in other multituberculates. One

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 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. Its shaft was smooth, compressed from top to bottom, and oval in cross-section. The ulna was compressed sideways, flatter than the radius, and about 40 mm (1.6 in) long as preserved.[13]

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 formed a rugose suture. The pubis was roughly triangular, with a rough suture for the ilium above and a deep groove for the ischium at the lower front. Specimen PM120/107's pelvic bones differed from those of other multituberculates in not being fused together. The presence of sutures in the pelvis of PM120/107 indicates that it was a juvenile, although the skull appears adult; the meaning of this discrepancy is unknown. The femur (thigh bone) was proportionally similar to that of Eucosmodon and Nemegtbaatar—smaller than the former, but larger than the latter. The femur was stout relative 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 (cavity) which may be characteristic of multituberculates.[13]

Unlike most other multituberculates and other mammals, the

ossified (turned into bone) and would have been covered in keratin (the horny covering seen in nails and hoofs). The cornu calcaris of the platypus consists only of keratin, and is hollow.[13][17]

Palaeobiology

See caption
Stereo photos and diagram of PM120/107's left foot; the os calcaris bone, which formed the base of the spur, is at the upper left.

Hurum, Zhe-Xi Luo, and Kielan-Jaworowska suggested in 2006 that the spurs on the ankles of Mesozoic mammals (such as Catopsbaatar) were

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.[17]

Six photos of three small pelvic bones
Stereo photos of PM120/107's ischia and pubis

The pelvic bones of Catopsbaatar specimen PM120/107 may not have been fused because fusion occurred late in development, because it was a

viviparous (gave live birth) and that the newborns were extremely small—similar to those of marsupials.[4]

Hair, structurally similar to that of modern mammals and associated with bones of the Mongolian multituberculate

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).[4][18]

Feeding and diet

Two diagrams of a jaw and its muscles
Reconstructed jaw musculature; B1 shows superficial layers, and B2 shows second layers.

Although multituberculates were thought to have been carnivores or herbivores, since American palaeontologist

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 have to open its mouth only 25 degrees to crush hard seeds 12–14 mm (0.47–0.55 in) 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".[12]

dentaries
of two specimens

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

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).[3][12]

Posture and locomotion

See caption
Stereo photos and diagrams of PM120/107's arm bones

The limb posture of multituberculates has been debated. According to some researchers, they employed a parasagittal stance with the erect limbs under the body; others consider a sprawling stance more likely. Kielan-Jaworowska and Hurum supported the latter theory in 2006 based on the presence of hind-leg spurs, a feature they considered present only in sprawling mammals. They pointed out that all early mammals preserved in

lacustrine (lake) deposits were compressed from top to bottom, suggesting a sprawling stance, whereas later mammals were preserved on their flanks. Earlier arguments for a sprawling stance include deep pelvises and features of the legs. They also suggested that the feet of multituberculates would have been plantigrade (the sole touching the ground) at rest, but digitigrade (the sole not touching the ground) when jumping and running quickly; they dismissed the idea that the forelimbs of multituberculates and other early mammals were more parasagittal than their hindlimbs. Kielan-Jaworowska and Hurum depicted Catopsbaatar with plantigrade, sprawling legs, with mobile spurs which pointed inward when preparing for attack.[19][4]

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.

arboreal (lived in trees), most Asian taxa were probably terrestrial; some others were fossorial, digging and living underground.[4]

Palaeoenvironment

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

All specimens of Catopsbaatar are known from the Barun Goyot Formation of the Gobi Desert, which probably dates to the late

Ostracods include Limnocythere, Cypridea, and Eucypris.[26]

References

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External links

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