| Alxasaurus | |
|---|---|
 | |
| Skeletal mount at theFukui Prefectural Dinosaur Museum | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Clade: | †Therizinosauria |
| Superfamily: | †Therizinosauroidea |
| Genus: | †Alxasaurus Russell &Dong,1993 |
| Type species | |
| †Alxasaurus elesitaiensis Russell & Dong, 1993 | |
Alxasaurus (/ˌɑːlʃəˈsɔːrəs/; meaning "Alxa lizard") is agenus oftherizinosauroidtheropoddinosaurs from theEarly Cretaceous (Albian age)Bayin-Gobi Formation ofInner Mongolia. It is known from five specimens, recovered from the Bayin-Gobi in 1988, as part of theChina-Canada Dinosaur Project. During their preparation, palaeontologistsDong Zhiming andDale Russell noted strong similarities toSegnosaurus. In 1993, they describedAlxasaurus and named itstype species,A. elesitaiensis. While therizinosaurs had previously been tentatively seen as late-survivingbasalsauropodomorphs, the description ofAlxasaurus lent credence to the idea that they were instead highly derivedcoelurosaurs.
While outside ofTherizinosauridae itself,Alxasaurus had many of the traits characteristic of derived therizinosaurs, such as an abbreviated tail, shortenedmetatarsals and broad feetconvergent with those of basal sauropodomorphs, and fairly long, recurved hand claws. Like related genera, it likely would have had a beak and a large gut, both adaptations for aherbivorous diet.
Between 21 August and 2 September 1988, an expedition to theBayin-Gobi Formation, inInner Mongolia, was carried out by as part of theChina-Canada Dinosaur Project. The expedition uncovered five theropod specimens:IVPP 88402, actually two specimens (one larger and one smaller);IVP 88301, consisting of scattered axial elements;IVPP 88510, consisting of limb elements; andIVP 88501. The specimens were transported to theInstitute of Vertebrate Paleontology and Paleoanthropology in Beijing. While the specimens were under preparation,Dong Zhiming noted similarities toSegnosaurus.[1] This conclusion was supported by his Canadian colleagueDale Russell, who notified George Olshevsky that the specimen provided "conclusive evidence" that segnosaurs were derived theropods.[2] This contradicted the contemporary view that segnosaurs were late-surviving basal sauropodomorphs.[2][3] In a paper released in the last issue of the 1993 volume of theCanadian Journal of Earth Sciences,[1] Russell and Dong described the Alxa specimens, assigning them thebinomial nameAlxasaurus elesitaiensis. The genus is named after theAlxa Desert of Inner Mongolia, also known as the "Alashan"desert, and the name also includes theGreek wordsauros ("lizard").Alxa (or Alashan) is also the name of theleague, oradministrative division, of the Inner Mongolia (Nei Mongol Zizhiqu) region of thePeople's Republic of China. The single knownspecies,elesitaiensis, is named after Elesitai, a village found in this region, near which thefossil remains ofAlxasaurus were located. The larger IVPP 88402 specimen was designated as theholotype, while the others were designated asparatypes. Together, they represent most of the specieshypodigm, aside from the skull.[1]
Alxasaurus is among the earliest known members of the Therizinosauroidea, but it already possessed the body shape — including the long neck, short tail, and relatively large claws — of later therizinosauroids. Like other members of this group, it was abipedalherbivore with a large gut to process plant material.[1] The fiveAlxasaurus specimens all vary in size. In 1993, Russell and Dong estimated a body length of 3.8 m (12 ft), a hip height of 1.5 m (4.9 ft), and a body mass of 380 kg (840 lb).[1] In 2010,Gregory S. Paul estimated its length to be 4 m (13 ft), and its body mass to be 400 kg (880 lb).[4]
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The skull ofAlxasaurus is represented solely by a moderately well-preserved rightdentary, belonging to the holotype. Thealveolar margin, the part of the dentary that bore teeth, measured 18.5 cm (7.3 in) in length. Several teeth, all partly erupted, are preserved, though there was room for around forty teeth (more than the thirty-one ofErlikosaurus and the twenty-four ofSegnosaurus). The teeth in theanterior (front) half of the alveolar margin bore between eight and tendenticles on the anteriorcarinae; thoseposterior to (rearward of) those teeth had similar denticle shapes and sizes, though their exact number is unclear. Though poorly preserved,interdental plates appear to have been present. While not preserved inAlxasaurus, its presence in other therizinosaurs suggests that a beak likely would have been present.[1]
Thecervical (neck)vertebrae ofAlxasaurus' holotype are badly crushed, though as far as can be gathered, theircentra wereamphiplatyan, meaning that their surfaces were relatively flat at both ends. Theirneural spines were small and narrow. Thezygapophyses, the articular processes of the vertebrae, were broad, and curved anteroventrally (forward and downward). From either side of theanterior (front) portion of thevertebral centra,alar processes extended, bearing capitular facets. The smaller IVPP 88402 specimen preserves three articulated vertebrae, two from the base of the neck, and one being the firstdorsal (back) vertebra. The latter had a more robust neural spine than any of the preceding vertebrae, intermediate morphologically between the cervical and dorsal vertebrae. The zygapophyses of the dorsal vertebrae had planes of articulation that sloped medially, and theprezygapophyses were buttressed by the roof of theneural canal. Thesacrum is very crushed. The sacral vertebrae have centra that are relatively flatventrally (at the bottom), diminishing in size the more posterior they were. As inSegnosaurus, the sacral spines did not extend above the highest point of theilium.Alxasaurus'caudal (tail) series is fairly well-preserved, though some of the distal (far) vertebrae are absent. From what can be gathered, the vertebral count was decreased. Like other therizinosaurs, the tail was short, relative to that of other theropods.[1]
Thescapula ofAlxasaurus was long and slender, and its blade was only slightly extended distally. The anteroproximal (front, near the body axis) margin is not preserved. The posterior part of thecoronoid was subcircular, tapering to a blunt point. Theacrocoracoid process was situated close to the ventral margin of the glenoid articulation. The arm bones were crushed, and so details of thehumerus,radius andulna are difficult to discern. The manus (hand), on the other hand, is preserved in detail, though the manualphalanges (digit bones; in this case, finger bones) are disarticulated and have incomplete shafts. The distalcarpals, the bones of the wrist closest to the hand itself, were similar to those ofTherizinosaurus, though the second distal carpal had an additional articular lobe on its posterior surface. Themetacarpals, the bones of the manus itself, were also very similar to those ofTherizinosaurus, though less powerfully constructed. The phalanges bore ligament pits which extended to the medial surfaces, unlike in that genus. Overall, the phalanges were very similar to those ofSegnosaurus. Theunguals (claws) were badly crushed, partly because they were hollow proximally. They were similar in shape to those ofDeinocheirus, but were narrower.[1] In comparison to taxa likeTherizinosaurus, they were small, and likely served a more generalised function.[5]
The anterior portion of theilium is unpreserved, as is thepubis. Dorsal to the posterodistal end of the ilium was a large crest, probablyhomologous to a rugose area found in the same position inSegnosaurus. Two flattened, strap-like elements are preserved, tentatively interpreted asischia. The shaft ofAlxasaurus' femur was slightly bowed laterally, unlike inSegnosaurus, where it was straight. Thefemoral head is inclined dorsomedially. The bones of thetarsus (ankle) are unknown. Many pedal (foot) elements are known, though identifications are largely dubious. The proximal end of the thirdmetatarsal was probably broad, as inSegnosaurus. The metatarsus overall was abbreviated, converging on basal sauropodomorphs. The foot overall was likely quite broad.[1]
While exhibiting many typical therizinosaur features in overall body shape and in the teeth, the skeleton ofAlxasaurus also shows several features present in more typical theropods, and the discovery of this animal provided significant evidence that therizinosaurs were aberrant theropods. Specifically, the semilunatecarpal bone of the wrist is found only inmaniraptoran theropods, which also includeoviraptorosaurs,dromaeosaurs,troodontids, andbirds.[1] Even morebasal therizinosaurs such as thefeatheredBeipiaosaurus and primitiveFalcarius have since been discovered with more theropod features and have helped to solidify this arrangement.[6][7]Alxasaurus is now thought to occupy a position between the earlyBeipiaosaurus and later therizinosaurids such asErlikosaurus,Segnosaurus, orTherizinosaurus.[8][9]
Although Rusell and Dong coined the Alxasauridae to containAlxasaurus,[1] the family has not been widely corroborated in most analyses. In 2010,Lindsay E. Zanno noted that, while technically still valid, the group currently consists of only a single species and is thus of dubious utility.[8]
The followingcladogram is based on the phylogenetic analysis conducted by Hartman et al., 2019:[9]
| Therizinosauria |
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