Erlikosaurus

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Erlikosaurus
Temporal range:
Ma
Skeletal diagram of the holotype
Scientific classification Edit this classification
Domain: Eukaryota
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
Synonyms
  • Erlicosaurus andrewsi Perle, 1981 (
    lapsus calami
    )

Erlikosaurus (meaning "

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 million 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 (alongside Enigmosaurus and 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 3.4 m (11 ft) in length and 150 to 250 kg (330 to 550 lb) in mass. It had a well-developed beak at the snout tip and toothed jaws that were used for its

claws, and a backwards directed pelvis
.

Erlikosaurus is classified as a therizinosaur within the

niche partitioned
.

Discovery and naming

Fossil localities in Mongolia. Locality of Erlikosaurus in Bayshin Tsav, at Area C

The

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] 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][3][4][5]

Skull and right pes of the holotype

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[6]) 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

junior synonym of Erlikosaurus.[8] 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.[9] Additional to this, the remains of Erlikosaurus and Enigmosaurus are known from upper and lower boundary, respectively.[3][7][10] Consequently, Enigmosaurus and Erlikosaurus are generally considered separated genera.[11]

Description

Life restoration

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.[12] 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 and 250 kg (330 and 550 lb).[5] Alternative estimations have suggested a maximum length of 6 m (20 ft) long,[9] and a more conservative length of 4.5 metres and a weight of 500 kg (1,100 lb).[13] 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.[14] 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.[15][16]

Skull

Reconstructed skull of the holotype in lateral (A) and dorsal (C) views
Holotype right (A) and left (B-C) hemimandibles

The snout is moderately elongated, with a

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).[17]

The well preserved

mammals, as well as other dinosaurs. Lastly, the cerebellum is not very notorious as previous elements, it is elongated and stocky.[5]

Teeth from the dentary bone

Ornithomimosaurs have solid evidence for it.[18][19] 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.[17]

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.[17] 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.[18] 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.[20] Apparently, in Erlikosaurus the rhamphotheca covered the nasal process of the premaxilla.[17]

Postcranial skeleton

Holotype right foot of Erlikosaurus with unguals removed

Body remains of Erlikosaurus are very sparse compared to the cranial elements, consisting of a

prezygapophyses and large parapophyses. Additional, the cervicals show some resemblance to those of Segnosaurus, however, being much smaller.[2]

The preserved right

claws of the feet were used for self-defence mechanism.[13]

The left

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.[21]

Classification

Main article: Timeline of therizinosaur research
Outdated quadrupedal, prosauropod-like restoration of Erlikosaurus

Erlikosaurus was by Perle assigned to the

ornithischians. Also like ornithischians, their jaws were tipped by a broad rounded bony beak useful for cropping off plants.[9][4]

The relationships of therizinosaurs were quite complicated when the first members were discovered. As an example, the first known therizinosaur taxon,

prosauropods with ornithischian adaptations, also, they shared evolutionary relationships.[24] However, with the description of more genera such as Alxasaurus,[25] Nanshiungosaurus,[26] and the redescription of the skull of Erlikosaurus, more theropod evidence began to be supported.[3] 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.[15]

Consequently, therizinosaurs are now classified as theropods, within 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:[11]

Holotype skull of Erlikosaurus (below) and endocranial elements of other therizinosaurs
Therizinosauridae
unnamed
unnamed

Nanchao embryos

unnamed

Nanshiungosaurus

unnamed
unnamed

Segnosaurus

AMNH 6368

unnamed

Erlikosaurus

unnamed

Nothronychus graffami

Nothronychus mckinleyi

Paleobiology

Senses

Reconstructed brain of Erlikosaurus

Erlikosaurus is poorly known from postcranial material, but the holotype skull became the focus of study in

theropods. The enlarged forebrain of Erlikosaurus may also have been useful in complex social behavior and predator evasion. These senses were also well-developed in earlier coelurosaurs and other theropods, indicating that therizinosaurs may have inherited many of these traits from their carnivorous ancestors and used them for their different and specialized dietary purposes.[5]

In

olfaction as the main sensory modality in large-bodied non-avian dinosaurs.[27]

Feeding and bite force

Proposed feeding method by Lautenschlager: Erlikosaurus reaches vegetation (A) and then rips it off with its beak (B)

In

rhamphotheca (keratinous beak), finding that this structure in the jaws acted as a stress-mitigating structure. They concluded that keratinous beaks are beneficial to enhance the stability of the skull making it less susceptible to bending and/or deformation during feeding.[29]

Optimal and maximum jaw gapes of Allosaurus (A), Tyrannosaurus (B) and Erlikosaurus (C)

The well preserved jaws also allowed a study by the University of Bristol to determine how its feeding style and dietary preferences were linked to how wide they could open the mouth. In the study, performed by Lautenshlager and colleagues in 2015, 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.[30]

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 and 121 N, with a skull characterised by high susceptibility to stress and deformation that indicates a feeding behaviour specialized in the active use of the beak. The results further support that Erlikosaurus relied on postcranial musculature to compensate the low bite force and to relieve stresses on cranial structure.[31]

Paleoenvironment

Dinosauria of the Bayan Shireh Formation
(Erlikosaurus in red, fourth from right)

The holotype of Erlikosaurus was unearthed from the Bayshin Tsav locality at the upper boundary of the

U–Pb measurements estimate the age of the Bayan Shireh Formation from 95.9 ± 6.0 million to 89.6 ± 4.0 million years ago, Cenomanian through Santonian ages.[33][34]

nanhsiungchelyid turtles.[42][43]

Coexistence with Segnosaurus

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

Erlikosaurus lived alongside a larger species of therizinosaurid in the Bayan Shireh Formation,

niche differentiation in food acquisition, processing, or resources. This conclusion is strengthened by the large difference in estimated body masses, which is up to 500%.[44]

Life restoration of the larger and sympatric Segnosaurus

In a 2017 study of niche partitioning in therizinosaurs through digital simulations, Lautenschlager found the straighter and more elongated dentaries of primitive therizinosaurs had the highest magnitudes of stress and strain during extrinsic feeding scenarios. In contrast, Erlikosaurus and Segnosaurus were aided by the down-turned tip of the lower jaws and symphyseal (bone union) regions, and probably also by stress and strain-mitigating beaks. The results also showed a difference in bite forces between Segnosaurus and Erlikosaurus, indicating the former would have been able to feed on tougher vegetation, while the overall robustness of the latter suggests greater flexibility in its manner of feeding. Lautenschlager pointed out the two taxa were adapted to different modes food acquisition, and that the difference in size and heights between the two therizinosaurids further separated their niches. While Segnosaurus was adapted to use its specialized dentition to procure or process food, Erlikosaurus mostly relied on its beak and neck musculature for cropping while foraging.[45]

In addition to these cranial differences, in 2019 Button and Zanno note that herbivorous dinosaurs followed two main distinct modes of feeding. One of these was processing food in the gut which is characterized by gracile skulls and relatively low bite forces, and the second was oral food processing, characterized by features associated with extensive processing such as the lower jaws or dentition. Segnosaurus was found to be in the former mode, whereas Erlikosaurus was more likely to fall in the second group, further supporting that these two therizinosaurids were separated by a well-defined niche differentiation.[46]

See also

References

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  2. ^ a b c d e f Perle, A. (1981). "Novyy segnozavrid iz verkhnego mela Mongolii" [New Segnosauridae from the Upper Cretaceous of Mongolia]. Transactions of the Joint Soviet-Mongolian Paleontological Expedition (in Russian). 15: 50–59. Translated paper
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