Mongolian Dinosaurs

R Barsbold. Encyclopedia of Dinosaurs. Editor: Philip J Currie & Kevin Padian. Amsterdam: Academic Press, 1997.

The discovery of dinosaurs in the Gobi Desert of Mongolia was one of the sensations of the natural sciences in the 1920s. Mongolian dinosaurs have been made famous thanks to the efforts of the Central Asiatic Expeditions (American Museum of Natural History, 1922, 1924/1925), the mongolian paleontological expeditions (Academy of Sciences of the USSR, 1946, 1948/1949), the polish-mongolian paleontological expeditions (Academy of Science, Mongolia and the Polish Academy of Science, 1963- 1965, 1967-1971), and the Mongolian-Russian Paleontological Expeditions (Academy of Science, Mongolia, and the Russian Academy of Science, since 1969). Since the first discovery, Mongolia has contributed more than 50 genera to the world’s dinosaur treasury, making up almost 20% of all valid genera of dinosaurs currently known (Dodson and Dawson, 1991). Most of these taxa are based on relatively complete specimens, and some are represented by hundreds of individuals. Mongolian dinosaurs are known from a comparatively narrow time interval, mainly from the Late Cretaceous, although some have been found in the Early Cretaceous too. Recently, the possibilities for Late Jurassic taxa have been opened in western Mongolia by the discovery of dinosaur bones.

The nonmarine Cretaceous, which produces most dinosaurs of Mongolia, is divided into stages with successive dinosaur communities (Jerzykiewicz and Russell, 1991). These are as follows: Tsagan-Tsav (Berriasian-Valanginean; dinosaurs include an indefinite psittacosaurid), Shine-Khuduk (Hauterivian-Barremian; Harpymimus, psittacosaurs, and“Iguanodon”), Barun-Bayan (Albian-Cenomanian; nests of large round eggs), Bayan-Shire (Cenomanian-Turonian; therizinosaurs, ornithomimids, and indefinite sauropods), Djadokhta (lower Senonian; dromaeosaurids, Saurornithoides, oviraptorids, Pinacosaurus, hadrosaur babies, and protoceratopsids), Barun-Goyot (upper Senonian; eggs, oviraptorids, ankylosaurs, pachycephalosaurs, and Bagaceratops), and Nemegt (upper Senonian; oviraptorids, Therizinosaurus, Deinocheirus, Mononykus, tyrannosaurids, hadrosaurs, Saichania and other ankylosaurs, pachycephalosaurs).

Late Cretaceous theropod dinosaurs are better known from Mongolia than anywhere else. Perhaps the most famous of these is Velociraptor mongoliensis, which was 1.5 m long. It had a low skull, a tail stiffened by rod-like extentions of the prezygapophyses and hemal arches, a semilunate (pulley-like) carpal, an opisthopubic pelvis, and a functionally didactylous foot in which the second pedal digit was raised above the ground to support a strongly enlarged ungual phalanx. It was one of the Mongolian dinosaurs discovered in 1923 and was one of the first to show evidence of the connections between dinosaur faunas of central Asia and North America. One specimen is well known as part of the “Fighting Dinosaurs” discovery, in which the predator’s sharp claws were found thrust into the tissues of the head and abdomen of a Protoceratops.

Other Mongolian dromaeosaurids include Adasaurus mongoliensis, which had a deep skull and a reduced pedal ungual on the second digit, and Hulsanpes perlei, which had a long and slender metatarsus.

Gallimimus bullatus was on average 3.5 m long, although one young specimen is 1.2 m long and the type specimen was almost 8 m. This was the first ornithomimid to show a parasphenoid capsule, a synapomorphy shared with troodontids and other ornithomimids. Other Mongolian ornithomimosaurs include Anserimimus planinychus, a 3.5-m-long form with flattened manual unguals; Harpymimus okladnikovi, a similarly sized species with reduced teeth in the front of the mandible and a short metacarpal I; and Garudimimus brevipes, which retained its first pedal digit. Mongolian ornithomimids are more diverse than their North American relatives.

Troodontids as a whole had the largest relative brain sizes of all known dinosaurs. The metatarsus was elongated, although most of the weight was carried by metatarsal IV. Like dromaeosaurids, the second pedal digit was specialized to carry the claw off the ground, although the ungual was not as well developed. Saurornithoides mongoliensis from the Djadokhta beds was approximately 2 m long, whereas the younger S. junior was larger and had more teeth. Two other troodontids, Borogovia gracilicrus and Tochisaurus nemegtensis, are only known from partial skeletons.

Oviraptorids are diverse, and include oviraptor, conchoraptor, ingenia, and a genus that will be described shortly. Nests and embryos are known for these animals, which appear to have brooded their eggs like birds.

Elmisaurus rarus was a small, gracile animal with the third metatarsal pinched proximally between metatarsals II and IV. Related forms have been found in North America.

The relationships of some Mongolian carnivorous dinosaurs are difficult to determine. These include Deinocheirus mirificus, which has 2.4-m-long forelimbs. The metacarpals and fingers are subequal in length and have massive, curved ungual phalanges. It is generally assigned to the Ornithomimosauria because of resemblance of manual and forelimb structure, but there are some significant differences in the outlines of the forelimb bones, and unfortunately nothing else is known of this dinosaur. The functional significance of the gigantic forelimbs is also unknown. Rozhdestvensky (1970) supposed that the “terrible hand” dinosaur fed on termites and used its massive claws to break open termite nests.

Therizinosaurus cheloniformis had 2-m-long front limbs with a three-fingered hand. There is a pulley- like semilunate carpal in the wrist, and the first metacarpal is short. The laterally compressed unguals are gigantic, and the one on the first finger can be 0.7 m long. The claws on the second and third digits were shorter. The first remains of Therizinosauruswere thought to belong to a “turtle-like lizard” (Maleev, 1954) and were later assigned to chelonians. Recently, however, it has been included with Segnosaurus, Erlikosaurus, and others within the Therizinosauroidea (Russell and Dong, 1993).

Erlikosaurus andrewsi is a medium-sized form with a well-preserved skull. The jaws are toothless at the front, the external nares are greatly elongated, a secondary palate is well developed, the basicranium and ear region is enlarged and pneumatized, the pelvis is opisthopubic, the ilium is broad and short with a flaring anterior flange, and four toes of each foot contacted the ground. Segnosaurus galbinensis and Enigmosaurus mongoliensis are two other therizinosaurs. Collectively these are one of the most unusual types of dinosaurs discovered within the past two decades. At first they were conditionally considered as aberrant theropods but were subsequently designated as Late Cretaceous relicts of the “prosauropod-ornithischian transition” (Paul, 1984) or as the sister group of sauropodomorphs (Barsbold and Maryanska, 1990). Recently, therizinosaurs have been unambiguously shown to be theropods (Russell and Dong, 1993; Clark et al., 1994). Clearly, the morphology of group is very peculiar.

Avimimus portentosus is a small (less than 1 m) animal with some bird-like characters, including a proximally fused carpometacarpus and ilia that are inclined medially. The ulna is interpreted by Kurzanov (1981, 1982) as showing evidence of feather attachments.

Mononykus olecranus is a similar sized animal that has biconvex posterior dorsal vertebrae and keeled posterior synsacral vertebrae. The forelimb is very short and robust, with a prominent deltopectoral crest, a large and massive olecranon, a flat carpometacarpus, and a functionally monodactylous manus with a robust ungual phalanx. The pelvis is opisthopubic, and the third metatarsal is proximally reduced. The first fragmentary remains of Mononykus were found by the Central Asiatic Expeditions but remained unrecognized in storage until more complete material had been found from Bugin-Tsav. One of the most interesting finds in recent times, Mononykus has been classified as a basal bird (Perle et al., 1994). Although it displays many avian features, it has also retained many theropod characters and is one of the best examples showing the complexity of the theropod-bird transition. The forelimb specializations are comparable to those of some fossorial tetrapods, but the shortness of the front limb argues against this.

Tyrannosaurids are characteristic of Late Cretaceous faunas of the Northern Hemisphere. Closely related toTyrannosaurus, Tarbosaurus bataar is the 12- to 13-m-long Mongolian representative of this group (Maleev, 1974). It is very common at some sites in the Nemegt beds. Alectrosaurus olseni and Alioramus remotus are the other two tyrannosaurids known from Mongolia.

Several sauropods have been described from the Upper Cretaceous strata of Mongolia. Nemegtosaurus mongoliensis has a lightly built but elongate skull with slender, peg-like teeth at the front of the jaws. The nasal does not contact the maxilla, but the squamosal reaches the quadratojugal. Quaesitosaurus orientalis has a rather broad snout, and the short squamosal does not contact the quadratojugal. The third genus, Opisthocoelicaudia skarzynskii, was 12 or 13 m long. The dorsal vertebrae are strongly opisthocoelous, bearing prominent ball-and-socket articulations. The tail is very short for a sauropod and has only approximately 35 vertebrae. Of the three sauropod species, the first two are represented by incomplete skulls of diplodocid character, whereas Opisthocoelicaudia lacks the skull and neck but has a camarasaurid skeleton.

Psittacosaurus mongoliensis was approximately 1.5 m long. The snout is short and tapers anteriorly into a tall, narrow, parrot-like beak. The external naris is small and is positioned high on the snout, whereas the antorbital fossa and fenestra are both absent. The lightly built postcranial skeleton has a manual fourth digit with only small terminal phalanx. Other species have been described from successive Lower Cretaceous horizons.

Protoceratopsian dinosaurs are the most frequently recovered Mongolian dinosaurs. Protoceratops andrewsi is 2 m long. Its skull tapers anteriorly into a narrow edentulous beak, although the premaxilla retains two teeth. There is an antorbital fossa. The parietosquamosal frill extends behind the skull, exceeding about half of its total length in mature animals. The most common and well-known Mongolian dinosaur, more than a hundred specimens of P. andrewsi have been found since its first discovery in 1922. Babies were recognized first for this dinosaur, and recently at Tugrigin-Shire a flock of 15 hatchlings was recovered. All of the tiny skeletons lay closely packed with their bodies bent but similarily oriented. No doubt this regularity in disposition of the babies will provide information about the behavior and paleoecology of P. andrewsi. The abundance of specimens has allowed several researchers to attempt to analyze ontogenetic growth and sexual dimorphism. Protoceratops is one of the “fighting dinosaurs” and, even though it was the victim, it holds in its mouth the right arm of the predator and was undoubtedly the cause of its death.

There are several other Mongolian protoceratopsians. Microceratops gobiensis was a small, lightly built, cursorial form, with a short but fenestrated frill. Bagaceratops rozhdestvensky has a short snout and frill, an additional antorbital fenestra, and a prominent nasal horn core. Breviceratops kozlowskii has a short and narrow snout, a large, deep antorbital fossa, and a short, flattened nasal horn. The largest protoceratopsian is Udanoceratops tschizhovi, with a rostral to quadrate length of 65-70 cm and a probable total length of more than 3 m. The rostral bone is tall and narrow, there is no premaxillary dentition, and the nasals do not form a horn core.

Several pachycephalosaurids have been described on the basis of Mongolian specimens. Homalocephale calathocercoswas 1.5 m long and had a flat, table-like, ornamented skull roof. The almost round supratemporal fenestrae are broadly separated. Goyocephale lattimorei was an animal of similar size, but the supratemporal fenestrae are longitudinally oval, and the bridge between them is narrow. The medial part of the skull roof is ornamented. Tylocephale gilmorei was also about the same size, but the skull has a strongly elevated dome that incorporates the postorbitals and supraorbitals. Because the parietosquamosal shelf is narrow, supratemporal fenestrae are probably absent. Prenocephale prenes was also approximately 1.5 m long, but the skull has a high dome that incorporates the prefrontals, supraorbitals, postorbitals, and parts of the squamosals. The parietosquamosal shelf is not developed, and the supratemporal fenestrae are closed. Pachycephalosaurs were perhaps a rare component of Mongolian dinosaur communities, and their fossils were identified later than those of most other Mongolian dinosaur taxa.

“Iguanodon” orientalis was an 8- to 10-m-long iguanodontid. The skull has an unusually large external naris, its margins formed mostly by the highly arched nasal. This animal has a heavily constructed postcranial skeleton with powerfully built limbs and an enlarged conical pollex on the hand. The generic assignment of this species is conditional pending a more complete study.

Only two hadrosaurs have been described so far. Saurolophus angustirostris was a huge animal, up to 14 m long. Like the North American species, the nasals extend posterodorsally above the orbit to form a solid crest, which is supported from behind by upraised frontal buttresses. This is one of the most common Mongolian dinosaurs from the latest Cretaceous beds, and there are many fine specimens that even include abundant skin impressions. Barsboldia sicinskii is a lambeosaurine with very long and club-shaped neural spines in the dorsal vertebrae.

Mongolia has one of the best fossil records for ankylosaurid ankylosaurs. Pinacosaurus grangeri was the first Mongolian species described on the basis of adequate material. Its skull is rather short and broad, with anteriorly positioned nares. Each of these is subdivided by a premaxillary septum that separates a dorsal foramen leading to the respiratory passage from a ventral one that is connected to a premaxillary sinus. The occipital condyle is oriented posteroventrally, and the postcranial skeleton is rather light with slender limb bones. Talarurus plicatospineus is approximately 5 or 6 m long and has an elongate but relatively small skull. The occipital condyle is oriented posteroventrally, the exoccipital is high and perpendicular to the skull roof, and the occiput has shifted slightly backward beyond skull roof. As in Pinacosaurus, the maxillary teeth have cingulum-like thickenings, cut externally by furrows. The postcranial skeleton is rather heavily built, and the limb bones are massive. Originally described as Talarurus disparoserratus, Tumanova (1987) has redescribed the type material as Maleevus disparoserratus. Saichania chulsanensis was approximately 7 m long. The skull is short and broad, and each terminally situated large nostril is subdivided by a septum into a large, oval foramen leading into the air passage and a more ventral second opening that connects with a premaxillary sinus. The occipital condyle is oriented ventrally, the exoccipital is low, and the postcranial skeleton is heavily built with rather massive limb bones. Tarchia gigantea is another large ankylosaurid, but it has a high, short exoccipital that is perpendicular to the skull roof. The occipital condyle is oriented posteroventrally, and the occiput has shifted slightly behind the skull roof.Amtosaurus magnus is a poorly known form based on an incomplete skull. Shamosaurus scutatus is a large ankylosaurid in which the skull roof is completely obscured by fusion of the osteoderms. The postorbital osteoderms are not horn-like, osteoderms do not close the quadrate cotylus, the anterior part of the snout is narrow, the occipital condyle is oriented ventrally, and the quadrate and paroccipital process are fused. Shamosaurus is generally considered to be the earliest and most primitive of the ankylosaurids.

This quick survey of Mongolian dinosaurs shows how rich the faunas were in central Asia during Cretaceous times. Field programs have been intensified in recent years, and new species of dinosaurs continue to be recovered. However, the recovery of data and specimens is also making great contributions to our understanding of the behavior, ecology, growth, and physiology of dinosaurs.