| Erlikosaurus | |
|---|---|
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| Skeletal diagram of the holotype | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Superfamily: | †Therizinosauroidea |
| Family: | †Therizinosauridae |
| Genus: | †Erlikosaurus Barsbold &Perle,1980 |
| Type species | |
| †Erlikosaurus andrewsi Barsbold & Perle, 1980 | |
| Synonyms | |
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Erlikosaurus (meaning "Erlik's lizard") is agenus oftherizinosaurid that lived inAsia during theLate Cretaceous period. The fossils, a skull and some post-cranial fragments, were found in theBayan Shireh Formation ofMongolia 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 speciesErlikosaurus andrewsi. It represents the second therizinosaur taxon from this formation (afterSegnosaurus), 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 itsherbivorous diet. The feet ended in fourtoes with the first one articulated to theankle—in contrast to thevestigial first toe of most theropods. Like other therizinosaurids,Erlikosaurus had a large gut for food processing, strong arms ending in elongatedclaws, and a backwards directedpelvis.
Erlikosaurus is classified as a therizinosaur within theTherizinosauridae. 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 amongmaniraptorans. The beak and jaws ofErlikosaurus indicate aleaf-stripping feeding method characterized by the active use of the beak aided by the neck. Several differences with thesympatricSegnosaurus show that these related genera wereniche partitioned.
Theholotype specimen,MPC-D 100/111, was found in layers from the Bayshin Tsav locality on theBayan Shireh Formation, consisting of an exceptionally well preservedskull, a virtually complete rightpes only lacking the proximal end ofmetatarsals II, III and IV, and an almost complete lefthumerus. Other remains include some fragmentarycervical vertebrae, however, the count is not specified and they were not illustrated. These findings were made during a Soviet-Mongolian expedition in theÖmnögovi Province in 1972.[1][2] Eight years later, thegenus andtype species,Erlikosaurus andrewsi, was named and described (although very briefly) by paleontologistsRinchen Barsbold andAltangerel Perle in1980, however, Barsbold was not indicated as the name-giver of this particular species. Thegeneric name,Erlikosaurus, was taken from that of the demon kingErlik, from Turko-Mongolianmythology and theGreekσαῦρος (sauros, meaning lizard). Thespecific name,andrewsi, is in honour to the American paleontologistRoy 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]
Confusingly, in1981 Perle again named and described the species as if it were new, but this time in more detail and spelling the generic name as aLatinised "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, and this helped shed light on a puzzling and poorly known group of dinosaurs. It still represents the most completely known therizinosaurian skull.[4][5]
In2010,Gregory S. Paul challenged the validity of this taxon, arguing thatErlikosaurus may be synonymous withEnigmosaurus (named in1983[7]), since the remains of the latter were found in the samegeologic formation, and only known from pelvic remains, whereas thepelvis inErlikosaurus is unknown; this would makeEnigmosaurus ajunior synonym ofErlikosaurus.[8] However, since the holotype hip ofEnigmosaurus did not closely resemble that of the specimen inSegnosaurus as would be expected for theSegnosaurus-like remains ofErlikosaurus, and there is a considerable size difference, paleontologist Rinchen Barsbold disputed the alleged synonymy.[9] Additional to this, the remains ofErlikosaurus andEnigmosaurus are known from upper and lower boundary, respectively.[3][7][10] Consequently,Enigmosaurus andErlikosaurus are generally considered separated genera.[11] Additionally, some maniraptoran specimens from theDinosaur Park Formation ofCanada have been referred to ascf.E. andrewsi in the past byPhilip J. Currie, but these specimens are most likely indeterminatetroodontids.[12]
As the genus is only known from very fragmentary material, it has been problematic to determine the size ofErlikosaurus, especially as most of thevertebral 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 ponderousSegnosaurus.[13] In2012 Stephan Lautenschlager and colleagues used theropod-specific equations to estimate the body mass ofErlikosaurus and other therizinosaurs. However, since the femur is unknown, they used bivariate regression analyses on log-transformed data forErlikosaurus. 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).[14] ThoughErlikosaurus 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.[15] It is known that therizinosaurs werefeathered animals based on the preservedfeather impressions in specimens ofBeipiaosaurus andJianchangosaurus, so it islikely thatErlikosaurus was feathered as well.[16][17]
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The snout is moderately elongated, with apremaxilla featuring elongated nasalprocesses. A fine, vertical lamina ofbone is connected rostrally to the medial margin of the premaxilla, indicating that when the animal was alive, acartilaginousinternasal septum was present. Additional to this, the premaxilla features lateral and medialforamina that are connected by a complex system ofvascular canals, which pervades the structure of the premaxilla and is probably associated with the sensory branches of the neurovasculature andophthalmic nerve supporting therhamphotheca (beak). Themaxilla is triangular in shape and preserves 24alveoli, theteeth arehomodont with coarseserrations. Thedentary 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).[18]
The well preservedbraincase 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 stronglycoossified, but thesutures between individual elements are not visible superficially, except for a few areas.[3] However, these internal sutures can be traced inCT scans and therefore, braincase elements could be differentiated one from other.[18] The restoredbrain of the specimen is somewhat elongated. The olfactory apparatus and thecerebral hemispheres are very notorious, with theolfactory 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 inbirds andmammals, as well as other dinosaurs. Lastly, thecerebellum is not very notorious as previous elements, it is elongated and stocky.[5]
Keratinousbeaks, or rhamphothecae, are well documented among diverse groups within theDinosauria.[18]Ornithomimosaurs have solid evidence for it.[19][20] 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.[18]
InErlikosaurus, 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.[18] The preserved rhamphotheca in specimens ofGallimimus andOrnithomimus evidences that the keratin sheath covered the premaxilla and overlapped it on the lower side by a few millimeters.[19] 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.[21] Apparently, inErlikosaurus the rhamphotheca covered the nasal process of the premaxilla.[18]
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Body remains ofErlikosaurus are very sparse compared to the cranial elements, consisting of ahumerus, a right foot and somecervical vertebrae. The particular cervicals were not figured and counted but briefly described. The cervicals are platycoelus (slightly concave at both ends) with lowneural arches.[1] Being relatively robust, they have thickprezygapophyses and large parapophyses. Additional, the cervicals show some resemblance to those ofSegnosaurus, however, being much smaller.[2]
The preserved rightpes is virtually complete, only missing the proximal end of themetatarsals 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 proximalarticular surface of the metatarsus. All of the remaining metatarsals, are somewhat equal in size, metatarsal II covers 11 cm (110 mm) in length. The pedaldigits 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. Thephalanges of the three first digits are shortened, robust with comparable structure. The second and third phalanx of fourth digit are discoidal and stocky. Lastly, theunguals are recurved, exceptionally large, and strongly flattened laterally.[1][2] Gregory S. Paul surmised that the long, slenderclaws of the feet were used for self-defence mechanism.[14]
The lefthumerus is the only preserved remain from the pectoral region. The humerus shows an elongatedepiphyses and a relatively largedeltoidal process.[1] 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. Thehumeral head features anarticular surface that is convex and broad, in the middle it is reduced toward the margins. A prominentdeltopectoral crest is present with the top located 1/3 at the length of the humerus from the proximal end. The articulationcondyles for theradius andulna are differentiated and divided by a shortened, furrow-likefossa 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 unrelatedDromaeosauridae.[22]
Erlikosaurus was by Perle assigned to theSegnosauridae,[1] a group today known as theTherizinosauridae, confirmed by latercladistic analyses.[4]Therizinosaurs were a strange group oftheropods that ate plants instead of meat, and had a backward-facingpubis, likeornithischians. Also like ornithischians, their jaws were tipped by a broad rounded bonybeak 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,Therizinosaurus, was interpreted to representturtle-like animals that used the elongated claws to feed on seaweed.[23] However, in 1970, Rozhdestvensky proposed the idea that therizinosaurs (then known as segnosaurs) instead of being non-dinosaur creatures, they were in fact,theropods.[24] 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 wereprosauropods with ornithischian adaptations, also, they shared evolutionary relationships.[25] However, with the description of more genera such asAlxasaurus,[26]Nanshiungosaurus,[27] and the redescription of the skull ofErlikosaurus, more theropod evidence began to be supported.[3] With the discovery and description of the featheredBeipiaosaurus, therizinosaurs were utterly recognized as theropods, and started to be reconstructed in an accurate, bipedal posture.[16]
Consequently, therizinosaurs are now classified as theropods, within theCoelurosauria. 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.[4] The cladogram below is the result of thephylogenetic analysis performed by Hartmanet al.2019 using the data provided by Zanno in 2010.Erlikosaurus occupied a very derived position in a clade with the twoNothronychus species:[11]
| Therizinosauridae |
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Erlikosaurus is poorly known from postcranial material, but the holotype skull became the focus of study inComputed Tomography (CT) scans that were published back in2012 by the paleontologistStephan Lautenschlager and Dr Emily Rayfield of Bristol University School of Earth Sciences, ProfessorLindsay Zanno of theNorth Carolina Museum of Natural History andNorth 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 thatErlikosaurus, and quite likely most other therizinosaurids, had well developed senses of smell, hearing, and balance, traits better associated withcarnivoroustheropods. The enlarged forebrain ofErlikosaurus may also have been useful in complexsocial behavior andpredator 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]
In2019, Graham M. Hughes and John A. Finarelli analyzed theolfactory bulb ratio in modern birds and preserved skulls of several extinct dinosaur species to predict how manygenes would have been involved in the olfactory strength of these extinct species. Their analysis found thatErlikosaurus had about 477 genes encoding its olfactory receptors and an olfactory bulb ratio of 40, indicating moderate senses of smell. The scores ofErlikosaurus were higher than mostdromaeosaurids despite the herbivorous life-style in this taxon, and may reflect a transition to complex sociality and/or reduced visual capacities. Hughes and Finarelli pointed out that as dinosaurian lineages became larger, the size of the olfactory bulb increased, which may suggestolfaction as the main sensory modality in large-bodied non-avian dinosaurs.[28]
In2013, Lautenschlager performed digital reconstructions for the cranialmusculature inErlikosaurus and found a relatively weak bite force compared to other theropods. As a whole, the adductor musculature of the jaws—which primarily function to close the jaws—generates a total force of 374 and 570 N but only a small portion is actually used when biting because the bite force starts to decline as the more the distance of the bite point is to the jaw joint. Lautenschlager found the lowest force at the snout tip with 43–65 N, and the highest at the last maxillar tooth region, with 90–134 N. Factors like the presence of a large gut to process vegetation and the lack of damage patterns on the teeth suggest thatErlikosaurus used only the tip of the snout and thepremaxillary region to reach for soft foliage or fruits, and the lesser bite force forErlikosaurus better served inleaf-stripping and plant-cropping feeding mechanism, rather than activemastication. In this study, Lautenschlager also suggested thatErlikosaurus may have been able to process mainly thin branches and plant matter based onStegosaurus. Moreover, the comparably narrow width of the snout could indicate selective feeding in this therizinosaurid. Lastly, the branch‐stripping behaviour ofErlikosaurus may have been compensated by the postcranial musculature.[29] During the same year, Lautenschlager and team made digital models of the skull ofErlikosaurus to test the function therhamphotheca (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.[30]
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 in2015, it was revealed thatErlikosaurus could open its mouth to a 43 degree angle at maximum. Also included in the study for comparison were the carnivorous theropodsAllosaurus andTyrannosaurus. From the comparisons, it was indicated that carnivorous dinosaurs had wider jaw gapes than herbivores, much as modern carnivorous animals do today.[31]
In2016, usingFinite Element Analysis (FEA) and aMultibody Dynamics Analysis (MDA), the bite forces ofErlikosaurus,Plateosaurus andStegosaurus were tested in order to estimate dietary habits. The resulting bite force forErlikosaurus was between 50 and 121N, 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 thatErlikosaurus relied on postcranial musculature to compensate the low bite force and to relieve stresses on cranial structure.[32]
The holotype ofErlikosaurus was unearthed from the Bayshin Tsav locality at the upper boundary of theBayan Shireh Formation, in a quarry composed of graysands withconglomerates,gravels, and grayclaystones. Bayshi Tsav is thought to have been deposited bymeandering rivers.[33][3][34] The examination of themagnetostratigraphy of the formation seems to confirm that the entire Bayan Shireh lies within theCretaceous Long Normal, which lasted only until the end of theSantonian stage. Moreover,calciteU–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.[34][35]
Fluvial,lacustrine andcaliche-based sedimentation indicates a lessersemi-arid climate, with the presence of wet environments composed of largemeanders andlakes. Largescale cross-stratification in many of thesandstone layers at the Bayn Shireh and Burkhant localities seems to indicate largemeandering rivers, and these large water bodies may have drained the eastern part of theGobi Desert.[36][34] Numerousfossilizedfruits have been recovered from the Bor Guvé and Khara Khutul localities.[37] A vast diversity offauna is known in the formation, compromising dinosaur and non-dinosaur genera. Fellow theropods include the tyrannosauroidKhankhuuluu,[38] the large dromaeosauridAchillobator,[39] and thedeinocheiridGarudimimus.[40] Other herbivorous dinosaurs are represented by theankylosaursTalarurus andTsagantegia,[41] smallmarginocephaliansAmtocephale andGraciliceratops,[42][43] thehadrosauroidGobihadros,[10] and thesauropodErketu.[37] Other fauna includesemiaquatic reptiles likecrocodylomorphs andnanhsiungchelyid turtles.[44][45]
Erlikosaurus lived alongside a larger species of therizinosaurid in the Bayan Shireh Formation,Segnosaurus. In 2016, Zanno and colleagues re-examined the lower jaws anddentition ofSegnosaurus making direct comparisons with those ofErlikosaurus in the process. They identified rather complex features in thedentary teeth ofSegnosaurus, which are represented by the presence of numerouscarinae (cutting edges) and folded carinae with denticulated front edges, and the enlargement ofdenticles (serrations). These traits together create a roughened, shredding surface near the base of the tooth crowns that was unique toSegnosaurus and suggest it consumed unique food resources or used highly specialized feeding strategies, with the addition of a higher degree of oral food processing than thesympatric—related species that lived in the same area at the same time—Erlikosaurus. On the contrary, the latter has verysymmetrical teeth with moderate denticles. The respective indistinct and specialized dentition ofErlikosaurus andSegnosaurus indicates that these two therizinosaurids were separated byniche differentiation in food acquisition, processing, or resources. This conclusion is strengthened by the large difference in estimated body masses, which is up to 500%.[46]
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 andSegnosaurus were aided by the down-turned tip of the lower jaws andsymphyseal (bone union) regions, and probably also by stress and strain-mitigating beaks. The results also showed a difference in bite forces betweenSegnosaurus andErlikosaurus, 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 theirniches. WhileSegnosaurus was adapted to use its specialized dentition to procure or process food,Erlikosaurus mostly relied on its beak and neck musculature for cropping whileforaging.[47]
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, whereasErlikosaurus was more likely to fall in the second group, further supporting that these two therizinosaurids were separated by a well-defined niche differentiation.[48]
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