Outdated reconstruction of a quadrupedal and prosauropod-like Erlikosaurus
Erlikosaurus is a genus of herbivorous theropod dinosaur from the late Cretaceous Period, belonging to the Therizinosauridae. Its fossils, a skull and some post-cranial fragments, were found in the Bayan Shireh Formation of Mongolia, dating to around 90 million years ago.
Discovery and naming[]
The remains of Erlikosaurus were discovered in 1972 at Bayshin Tsav, during a Soviet-Mongolian expedition in Ömnögovi Province. The type species, Erlikosaurus andrewsi, was named and described by Altangerel Perle in 1980, in an article co-authored with Rinchen Barsbold, who however, is not indicated as the name-giver of this particular species. Its generic name was taken from that of the demon king Erlik from Turko-Mongolian mythology and the specific name of the American paleontologist Roy Chapman Andrews.[2] Confusingly, in 1981 Perle again named the species as if it were new, this time spelling the generic name as a Latinised "Erlicosaurus".[3] It is today generally considered that the original name, Erlikosaurus, is valid.
Erlikosaurus
The holotype, IGM 100/111, was found in layers dating from the Cenomanian-Santonian. It consists of a complete skull with lower jaws, some cervical vertebrae fragments, the left humerus and the right foot. At the time it was the only known therizinosaur (then called segnosaurs) for which a skull had been discovered.[1] This helped shed light on a puzzling and poorly known group of dinosaurs. It still represents the most completely known therizinosaurian skull.[4]
Some scientists have speculated that Erlikosaurus may be the same animal as Enigmosaurus mongoliensis named in 1983,[5] since the latter was found in the same geologic formation, and was only known from part of a hip, whereas the pelvis of Erlikosaurus is unknown. This would make Enigmosaurus a junior synonym of Erlikosaurus.[1] However, since the Enigmosaurus hip did not resemble that of Segnosaurus as closely as would be expected for the Segnosaurus-like Erlikosaurus remains and there is a considerable size difference, paleontologist Rinchen Barsbold disputed the alleged synonymy.[1] Consequently, Enigmosaurus and Erlikosaurus are generally still considered separate genera.
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 25 cm (250 mm) long, indicating a very small individual. Overall, Erlikosaurus was a small-sized therizinosaurid, estimated to have reach about 3.4 m (11 ft) with a more lightly built than the ponderous Segnosaurus. 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 44.33 cm (443.3 mm) and a weight of 173.7 kg (383 lb). Given the uncertainties of these estimates, they established an overall mass range between 150 to 250 kg (330 to 550 lb). Alternative estimations have suggested a maximum length of 6 m (20 ft) long, and a more conservative length of 4.5 metres and a weight of 500 kg (1,100 lb). 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 2 to 5 mm (0.20 to 0.50 cm) in diameter. Isolated foramina are connected internally by a complex neurovascular canal. When restored, the skull measures 26 cm (260 mm) long and the mandible is about 24 cm (240 mm).
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 stock.
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 7 cm (70 mm) 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 11 cm (110 mm) 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 deltoideal process. It is robust with an estimated length of 30 cm (300 mm). 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[]
Erlikosaurus was by Perle assigned to the Segnosauridae, a group today known as the Therizinosauridae, confirmed by later cladistic analyses. Therizinosaurs were a strange group of theropods that ate plants instead of meat, and had a backward-facing pubis, like ornithischians. Also like ornithischians, their jaws were tipped by a broad rounded bony beak useful for cropping off plants.
The relationships of therizinosaurs were quite complicated when the first members were discovered. As an example, the first known therizinosaur taxon, Therizinosaurus, was interpreted to represent turtle-like animals that used the elongated claws to feed on seaweed. However, in 1970, Rozhdestvensky proposed the idea that therizinosaurs (then known as segnosaurs) instead of being non-dinosaur creatures, they were in fact, theropods. Later, in 1980, segnosaurs were thought to be slow, semiaquatic animals, with this, Gregory S. Paul claimed that these controversial animals had no theropod characteristics and they were prosauropods with ornithischian adaptations, also, they shared evolutionary relationships. However, with the description of more genera such as Alxasaurus, Nanshiungosaurus, and the redescription of the skull of Erlikosaurus, more theropod evidence began to be supported. With the discovery and description of the feathered Beipiaosaurus, therizinosaurs were utterly recognized as theropods, and started to be reconstructed in an accurate, bipedal posture.
Consequently, therizinosaurs are now classified as theropods, within the Coelurosauria. Lindsay Zanno was one of the first authors to examine in detail the relationships and affinites of therizinosaurs. Her work has been useful in many phylogenetic analyses. The cladogram below is the result of the phylogenetic analysis performed by Hartman et al. 2019 using the data provided by Zanno in 2010. Erlikosaurus occupied a very derived position in a clade with the two Nothronychus species:
| Therizinosauridae |
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