Erlikosaurus (meaning "Erlik's lizard") is a genus of therizinosaurid that lived in Asia during the Late Cretaceous period. The fossils, a skull and some post-cranial fragments, were found in the Bayan Shireh Formation of Mongolia in 1972, dating to around 96 and 89 million years ago. These remains were later described by Altangerel Perle and Rinchen Barsbold in 1980, naming the new genus and species Erlikosaurus andrewsi. It represents the second therizinosaur taxon from this formation (after Segnosaurus), with the most complete skull among members of this peculiar family of dinosaurs.
In contrast to most therizinosaurids, Erlikosaurus was a small member reaching nearly in length and in mass. It had a well-developed beak at the snout tip and toothed jaws that were used for its herbivorous diet. The feet ended in four toes with the first one articulated to the ankle—in contrast to the vestigial first toe of most theropods. Like other therizinosaurids, Erlikosaurus had a large gut for food processing, strong arms ending in elongated claws, and a backwards directed pelvis.
Erlikosaurus is classified as a therizinosaur within the Therizinosauridae. Therizinosaurs were long-enigmatic dinosaurs with unclear relationships during the early years of research. Subsequent studies proved their true nature as theropod dinosaurs and systematic position among maniraptorans. The beak and jaws of Erlikosaurus indicate a leaf-stripping feeding method characterized by the active use of the beak aided by the neck. Several differences with the sympatric Segnosaurus show that these related genera were niche partitioned.
Discovery and naming
The holotype specimen, MPC-D 100/111, was found in layers from the Bayshin Tsav locality on the Bayan Shireh Formation, consisting of an exceptionally well preserved skull, a virtually complete right pes only lacking the proximal end of metatarsals II, III and IV, and an almost complete left humerus. Other remains include some fragmentary cervical vertebrae, however, the count is not specified and they were not illustrated. These findings were made during a Soviet-Mongolian expedition in the Ömnögovi Province in 1972. Translated paper Eight years later, the genus and type species, Erlikosaurus andrewsi, was named and described (although very briefly) by paleontologists Rinchen Barsbold and Altangerel Perle in 1980, however, Barsbold was not indicated as the name-giver of this particular species. The generic name, Erlikosaurus, was taken from that of the demon king Erlik, from Turko-Mongolian mythology and the Greek (, meaning lizard). The specific name, andrewsi, is in honour to the American paleontologist Roy Chapman Andrews, who was the leader of the American Asiatic Expeditions from 1922 to 1930. Apparently, in the original description a left pes was claimed to be part of the holotype, however, this statement has not been mentioned again.
Confusingly, in 1981 Perle again named and described the species as if it were new, but this time in more detail and spelling the generic name as a Latinised "Erlicosaurus". It is today widely accepted by most authors that the original name, Erlikosaurus, is valid. At the time of its discovery it was the only known therizinosaur (then called segnosaurs Translated paper ) for which a complete skull had been discovered, and this helped shed light on a puzzling and poorly known group of dinosaurs. It still represents the most completely known therizinosaurian skull.
In 2010, Gregory S. Paul challenged the validity of this taxon, arguing that Erlikosaurus may be synonymous with Enigmosaurus (named in 1983 Translated paper ), since the remains of the latter were found in the same geologic formation, and only known from pelvic remains, whereas the pelvis in Erlikosaurus is unknown; this would make Enigmosaurus a junior synonym of Erlikosaurus. However, since the holotype hip of Enigmosaurus did not closely resemble that of the specimen in Segnosaurus as would be expected for the Segnosaurus-like remains of Erlikosaurus, and there is a considerable size difference, paleontologist Rinchen Barsbold disputed the alleged synonymy. Additional to this, the remains of Erlikosaurus and Enigmosaurus are known from upper and lower boundary, respectively. Consequently, Enigmosaurus and Erlikosaurus are generally considered separated genera. Additionally, some maniraptoran specimens from the Dinosaur Park Formation of Canada have been referred to as cf. E. andrewsi in the past by Philip J. Currie, but these specimens are most likely indeterminate troodontids.
Description
As the genus is only known from very fragmentary material, it has been problematic to determine the size of Erlikosaurus, especially as most of the vertebral column of the holotype is missing. The skull of the holotype specimen length is approximately long, indicating a very small individual. Overall, Erlikosaurus was a small-sized therizinosaurid, estimated to have reach about with a more lightly built than the ponderous Segnosaurus. Genus List for Holtz 2012 Weight Information In 2012 Stephan Lautenschlager and colleagues used theropod-specific equations to estimate the body mass of Erlikosaurus and other therizinosaurs. However, since the femur is unknown, they used bivariate regression analyses on log-transformed data for Erlikosaurus. The results ended up on a femoral length of and a weight of . Given the uncertainties of these estimates, they established an overall mass range between . Alternative estimations have suggested a maximum length of long, and a more conservative length of 4.5 metres and a weight of . Though Erlikosaurus largely lacks body remains, as a therizinosaurid it would have had a strong arm build with large claws, a broad and bulky torso, and an opisthopubic (directed backwards) pelvis. It is known that therizinosaurs were feathered animals based on the preserved feather impressions in specimens of Beipiaosaurus and Jianchangosaurus, so it is likely that Erlikosaurus was feathered as well.
Skull
The snout is moderately elongated, with a premaxilla featuring elongated nasal processes. A fine, vertical lamina of bone is connected rostrally to the medial margin of the premaxilla, indicating that when the animal was alive, a cartilaginous internasal septum was present. Additional to this, the premaxilla features lateral and medial foramina that are connected by a complex system of vascular canals, which pervades the structure of the premaxilla and is probably associated with the sensory branches of the neurovasculature and ophthalmic nerve supporting the rhamphotheca (beak). The maxilla is triangular in shape and preserves 24 alveoli, the teeth are homodont with coarse serrations. The dentary is wedge-shaped elongated and preserves 31 alveoli. In a dorsal view, it is U-shaped and flattened at the back with an expansion lying across. The lateral and ventral surfaces in the symphyseal region bears a series of foramina that measure in diameter. Isolated foramina are connected internally by a complex neurovascular canal. When restored, the skull measures long and the mandible is about .
The well preserved braincase is very much complete, only missing the sphenethmoid-mesethmoid complex, whereas the laterosphenoids and orbitosphenoids are incompletely preserved in medial view. The bones around the braincase are strongly coossified, but the sutures between individual elements are not visible superficially, except for a few areas. However, these internal sutures can be traced in CT scans and therefore, braincase elements could be differentiated one from other. The restored brain of the specimen is somewhat elongated. The olfactory apparatus and the cerebral hemispheres are very notorious, with the olfactory tract being far larger than the actual brain. The cerebral hemispheres are large and broad. On the cerebral surface complex vascular grooves can be found, which are typically found in birds and mammals, as well as other dinosaurs. Lastly, the cerebellum is not very notorious as previous elements, it is elongated and stocky.
Keratinous beaks, or rhamphothecae, are well documented among diverse groups within the Dinosauria. Ornithomimosaurs have solid evidence for it. However, this is not an indicative to suggest the lack of this anatomical feature in other groups. Several characteristics are indicative of a rhamphothecae, such as an edentulous premaxilla with a thin, tapering lower edge, the successive loss of maxillary and dentary teeth, a mandibular concavity in the lower side, the displacement of the lower surface in the dentary, and a rostral projection of the mandibular symphysis.
In Erlikosaurus, the presence of a keratinous beak on the maxilla and premaxilla can be inferred by the presence of numerous neurovascular foramina on the rostral and lateral surfaces in the skull, furthermore, it bears all the mentioned features above, however, it is unclear the extension of the beak. The preserved rhamphotheca in specimens of Gallimimus and Ornithomimus evidences that the keratin sheath covered the premaxilla and overlapped it on the lower side by a few millimeters. In some extant birds, the rhamphotheca is typically restricted to the premaxilla and maxilla, although in some cases it partially covers the nasal process in some birds. Apparently, in Erlikosaurus the rhamphotheca covered the nasal process of the premaxilla.
Postcranial skeleton
Body remains of Erlikosaurus are very sparse compared to the cranial elements, consisting of a humerus, a right foot and some cervical vertebrae. The particular cervicals were not figured and counted but briefly described. The cervicals are platycoelus (slightly concave at both ends) with low neural arches. Being relatively robust, they have thick prezygapophyses and large parapophyses. Additional, the cervicals show some resemblance to those of Segnosaurus, however, being much smaller.
The preserved right pes is virtually complete, only missing the proximal end of the metatarsals II, III and IV. It is shortened in length, with robust metatarsals that bear widened articular extremities, and form a non-compact metatarsus. The metatarsal I is the shortest in comparison, it measures long and expands the laterally extended proximal articular surface of the metatarsus. All of the remaining metatarsals, are somewhat equal in size, metatarsal II covers in length. The pedal digits are very peculiar in structure; the first digit is reduced in length, with all the remaining digits being nearly equal in length, however the fourth digit is very thin compared to the others. The phalanges of the three first digits are shortened, robust with comparable structure. The second and third phalanx of fourth digit are discoidal and stocky. Lastly, the unguals are recurved, exceptionally large, and strongly flattened laterally. Gregory S. Paul surmised that the long, slender claws of the feet were used for self-defence mechanism.
The left humerus is the only preserved remain from the pectoral region. The humerus shows an elongated epiphyses and a relatively large deltoidal process. It is robust with an estimated length of . It has a reduced shaft. The proximal end of the humeurs is greatly broad. The humeral head features an articular surface that is convex and broad, in the middle it is reduced toward the margins. A prominent deltopectoral crest is present with the top located 1/3 at the length of the humerus from the proximal end. The articulation condyles for the radius and ulna are differentiated and divided by a shortened, furrow-like fossa and overall, they are very reduced in size. The fossa for the ulnar process is moderately deep and wide. The internal roughness of the head is prominent, as in the unrelated Dromaeosauridae.
Classification