Erlikosaurus

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Erlikosaurus
Temporal range: Late Cretaceous, 97–83 Ma
Erlikosaurus skull and foot.jpg
Skull and right pes of the holotype specimen MPC-D 100/111
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Clade: Dinosauria
Clade: Saurischia
Clade: Theropoda
Superfamily: Therizinosauroidea
Family: Therizinosauridae
Genus: Erlikosaurus
Barsbold & Perle, 1980
Type species
Erlikosaurus andrewsi
Barsbold & Perle, 1980

Erlikosaurus (meaning "Erlik's lizard") is a genus of therizinosaur theropod dinosaur that lived in Asia during the Late Cretaceous period. Its fossils, a skull and some post-cranial fragments, were found in the Bayan Shireh Formation of Mongolia, dating to around 97 million and 83 million years ago. Erlikosaurus represents the second therizinosaurian taxa from this Mongolian formation (alongside Enigmosaurus and Segnosaurus) with the most complete skull among other relatives of this peculiar family of dinosaurs.

Discovery and naming

Fossil localities in Mongolia. Locality of Erlikosaurus in Bayshin Tsav, at Area C
Known elements from holotype MPC-D 100/111

The holotype, MPC-D 100/111, was found in layers from the Bayshin Tsav locality on the Bayan Shireh Formation, consisting of a well preserved skull with lower jaws, some cervical vertebrae fragments and remains of the forelimb and hindlimb; these findings were made during a Soviet-Mongolian expedition in the Ömnögovi Province in 1972.[1][2] 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 σαῦρος (sauros, 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.[1]

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".[2] 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[3]) for which a complete skull had been discovered, this helped shed light on a puzzling and poorly known group of dinosaurs. It still represents the most completely known therizinosaurian skull.[4][5]

In 2010, Gregory S. Paul challenged the validity of this taxon, arguing that Erlikosaurus may be the same animal as Enigmosaurus mongoliensis (named in 1983[6]), since the latter was found in the same geologic formation, and was only known from pelvic remains, whereas the pelvis of Erlikosaurus is unknown; this would make Enigmosaurus a junior synonym of Erlikosaurus.[7] 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.[8] Additional to this, Erlikosaurus and Enigmosaurus are known from upper and lower beds, respectively.[9][6][10] Consequently, Enigmosaurus and Erlikosaurus are generally still considered separate genera.[11]

Description

Partial forelimb of the basal therizinosaur Beipiaosaurus with impressions of feather-like structures, Paleozoological Museum of China
Life restoration of Erlikosaurus

The holotype is fragmentary, 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.[1][2] Apparently, in the original description a left pes was claimed to be part of the holotype[1], however, this statement has not been mentioned again.[2][9][4][5] It is currently now know that some therizinosaurs were feathered based on the preserved skin impressions of Beipiaosaurus, so it is likely that Erlikosaurus was feathered as well.[12] 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 holotype skull length is 25 cm (250 mm) long; the humerus is approximately 30 cm (300 mm) long, this indicates a very small individual. Erlikosaurus may have been more lightly built than close relative Segnosaurus with an estimated adult size of 6 m (20 ft) long.[8] Other estimates are lower: in 2010 Gregory S. Paul gave a length of 4.5 metres and a weight of 500 kg (1,100 lb).[7] Nevertheless, Thomas Holtz in 2012 estimated its length at 3.4 m (11 ft) long with a weight between 227 to 454 kg (500 to 1,001 lb).[13] Stephan et al. 2012 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; 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).[5]

Skull

The most studied element from Erlikosaurus is its articulated, complete skull. The snout is moderately elongated[1], 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 dorsal view, it is U-shaped and dorsoventrally flattened with a transverse expansion. 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.[14]

Reconstructed brain of Erlikosaurus

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 few exceptions.[9] However, these internal sutures can be traced in CT scans and therefore, braincase elements could be differentiated one from other.[14] The restored brain of Erlikosaurus 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. Interestingly, 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.[5]

Keratinous beaks, or rhamphothecae, are well documented among diverse groups within the Dinosauria.[14] Ornithomimosaurs have solid evidence for it[15][16], however, this is not an evidence to suggest the lack of this anatomical feature in other groups. Several characteristics are indicative of a rhamphothecae:[14]

Edentulous premaxilla with a thin, tapering ventral margin, successive loss of maxillary and dentary teeth, a mandibular ventral concavity, ventral displacement of the dentary, and a rostral projection of the mandibular symphysis.[14]

In Erlikosaurus, the presence of a keratinous beak on the maxilla and premaxilla can be inferred by the presence of neurovascular foramina on the rostral and lateral surfaces. The skull of Erlikosaurus bears all the mentioned features indicative of a rhamphotheca, however it is unclear the extension of the beak.[14] The preserved rhamphotheca in Ornithomimus[15] evidences that the keratin sheath covered the premaxilla, and ventrally overlapped it 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.[17] Apparently, in Erlikosaurus the rhamphotheca covered the nasal process of the premaxilla. To deal with this enigmatic topic, they reconstructed a conservative and extensive rhamphotheca.[14]

Postcrania

Body remains of Erlikosaurus are very sparse compared to the cranial elements, consisting of a humerus, a foot and some cervical vertebrae. The particular cervicals were not figured and counted, but briefly described. The cervicals are platycoelous with low neural arches.[1] Being relatively robust, they have thick prezygapophyses and large parapophyses. Additional, the cervicals show some resemblance to those of Segnosaurus, however, being much smaller.[2]

Hindlimb

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, expanding the laterally extended proximal articular surface of the metatarsus. All of the remaining metatarsals, are somewhat equal in size. 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 compressed laterally.[1] Gregory S. Paul surmised that the long, slender claws of the feet were used for self-defence mechanism.[7]

Forelimb

The left humerus is the only preserved remain from the pectoral region. The humerus shows an elongated epiphyses and a relatively large deltoideal process.[1] As pointed by Perle 1981, it is robust with an estimated maximal 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[2], as in the unrelated Dromaeosauridae.[18]

Classification

Outdated quadrupedal, prosauropod-like restoration of Erlikosaurus

Erlikosaurus was by Perle assigned to the Segnosauridae[1], a group today known as the Therizinosauridae, confirmed by later cladistic analyses.[4] The 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.[8]

The relationships of therizinosaurs were quite complicated when members were first discovered. As an example, the first known therizinosaur, Therizinosaurus, was interpreted to be as a turtle-like animal that used the elongated claws to feed on seaweed.[19] However, in 1970, Rozhdestvensky proposed the idea that therizinosaurs (then known as segnosaurs) instead of being non-dinosaur creatures, they were in fact, theropods.[20] 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.[21] However, with the description of genera Alxasaurus[22], Nanshiungosaurus[23], and the redescription of the skull of Erlikosaurus[9], 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.[12]

Skull of Erlikosaurus (below) and endocranial elements of other therizinosaurs

Consequently, therizinosaurs are now classified as theropods, within the Coelurosauria. The cladogram below is the result of the recently performed phylogenetic analysis of the Therizinosauria by Hartman et al. 2019.[24]

Therizinosauria

Falcarius

Jianchangosaurus

Beipiaosaurus

"Chilantaisaurus" zheziangensis

Enigmosaurus

Alxasaurus

Therizinosauridae

Suzhousaurus

Neimongosaurus

Erliansaurus

Therizinosaurus

Nanchao taxon

Nanshiungosaurus

AMNH 6368

Segnosaurus

Erlikosaurus

Nothronychus graffami

Nothronychus mckinleyi

Paleobiology

Dentition of the lower jaw in Erlikosaurus
Dentition of the lower jaw in Segnosaurus

Erlikosaurus is poorly known from postcranial material, but its skull became the focus of study in Computed Tomography (CT) scans that were published back in 2012 by Stephen Lautenschlager and Dr Emily Rayfield of Bristol University School of Earth Sciences, Professor Lindsay Zanno of the North Carolina Museum of Natural History and North Carolina State University, and Lawrence Witmer, Chang Professor of Paleontology at the Ohio University Heritage College of Osteopathic Medicine. Analysis of the brain cavity revealed that Erlikosaurus, and quite likely most other therizinosaurids, had well developed senses of smell, hearing, and balance, traits better associated with carnivorous theropods. The enlarged forebrain of Erlikosaurus may also have been useful in complex social behavior and predator evasion.[5]

Lower jaw of Erlikosaurus (bottom) and Segnosaurus (top) compared

The well preserved jaws of Erlikosaurus also allowed a study by the University of Bristol to determine how its feeding style and dietary preferences were linked to how wide it could open its mouth. In the study, performed by Stephen Lautenshlager et al., It was revealed that Erlikosaurus could open its mouth to a 43 degree angle at maximum. Also included in the study for comparison were the carnivorous theropods Allosaurus and Tyrannosaurus. From the comparisons, it was indicated that carnivorous dinosaurs had wider jaw gapes than herbivores, much as modern carnivorous animals do today.[25]

More findings about its exceptional skull have been made. Based on comparisons between the highly specialized mandible of Segnosaurus and the simple one from Erlikosaurus, these taxa were separated by niche partitioning. For instance, the mandible of Segnosaurus features very complex specializations, such as a gracile, recurved dentary, and a thickened lateral shelf. The dentition bears a triple carinae on the lower teeth, lingually folded mesial carinae appressed against triangular denticulate facets on the distal carinae, and extracarinal accessory denticles. In contrast, the dentition of Erlikosaurus seems to be more "standar", with a more simplistic morphology compared to the latter and other relatives.[26] In 2016, using Finite Element Analysis (FEA) and a Multibody Dynamics Analysis (MDA), the bite forces of Erlikosaurus, Plateosaurus and Stegosaurus were tested in order to estimate dietary habits. The resulting bite force for Erlikosaurus was between 50–121 N, with a skull characterised by high susceptibility to stress and deformation. This indicates a feeding behaviour that was specialised in an active use of the beak.[27] Erlikosaurus may have relied on postcranial musculature to compensate the low bite force and to relieve stresses on cranial structure, this is consistent with a herbivorous diet as suggested by other authors and this study.[28][29][30][27]

Paleoecology

Erlikosaurus compared to the known Dinosauria of the Bayan Shireh Formation (Erlikosaurus in dark green, next to Segnosaurus)

Erlikosaurus was unearthed from the Bayshin Tsav locality[1], which represents part of the Upper Bayan Shireh.[9][10] The definitive age of the Bayan Shireh Formation has been difficult to establish, however, there are various competing hypotheses: based on comparisons with other formations, the Bayan Shireh fauna seems to correspond best with the Turonian through early Campanian stages of the Late Cretaceous, about 93 million to 80 million years ago.[31] Through the examination of the magnetostratigraphy of the formation conducted by Hicks et al. 1999, seems to confirm that the entire Bayan Shireh lies within the Cretaceous Long Normal, which lasted only until the end of the Santonian stage, giving a Cenomanian-Santonian age, or, 98.5 million and 83.5 million years ago.[32] Dr. Thomas R. Holtz, JR. has suggested that Erlikosaurus lived between 99.6-85.8 million years ago.[13] However, according to Averianov and Sues in 2012, based on biostratigraphic occurrences and previous datings, the Upper Bayan Shireh correlated the Iren Dabasu Formation, approximately between 97 million and 83 million years ago.[33]

Environment and paleoflora

Fluvial, lacustrine and caliche-based boundary indicates a semi-arid environment and climate, with the presence of large rivers and lakes.[34][35][33] As interpreded by Hicks et al. 1999, during the times of the Bayan Shireh Formation, large rivers drained the eastern part of the Gobi Desert.[32] The Bayan Shireh Formation is generally considered to be lacustrine[36], however, largescale cross-stratification in many of the sandstone layers at the Bayn Shireh and Burkhant localities seems to indicate a meandering fluvial system.[32] Many fossil fruits remains have been recovered from the Bor Guvé and Khara Khutul localities (Upper and Lower Bayan Shireh, respectively), indicating the lpresence of Angiosperm plants. The collected fruits were identified to have an Okra-like appareance.[37]

Contemporaneous paleofauna

Erlikosaurus co-existed with a vast diversity of animals in the upper part of the formation, compromising dinosaur and non-dinosaur genera. Fellow theropods include the medium-sized Achillobator[38], Alectrosaurus[39], Garudimimus[40] and Segnosaurus[1]. The ankylosaurs Talarurus[41] and Tsagantegia.[42] Small marginocephalians Amtocephale[43] and Graciliceratops.[44] The basal hadrosauroid Gobihadros[10], and the large sauropod Erketu[37].[31][45] Other fauna includes the turtle Lindholmemys[46], crocodylomorph Paralligator[47], unnamed azhdarchids[48] and the shark Hybodus.[33]

See also

References

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