Kwanasaurus
Kwanasaurus Temporal range: Late Triassic,
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Skeletal and life restoration | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Chordata |
Clade: | Dinosauriformes
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Clade: | Dracohors
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Family: | †Silesauridae |
Clade: | †Sulcimentisauria |
Genus: | †Kwanasaurus Martz & Small, 2019 |
Type species | |
†Kwanasaurus williamparkeri Martz & Small, 2019
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Kwanasaurus is an extinct genus of silesaurid dinosauromorph reptiles from the Late Triassic of Colorado. It is known from a single species, Kwanasaurus williamparkeri. Kwanasaurus had a deeper, stronger skull and greater specialization for herbivory compared to other silesaurids. It also possessed many unique characteristics of the snout, ilium, and lower part of the femur. It was described along with new specimens of Dromomeron from the Eagle Basin, the northernmost extent of the Chinle Formation.[1][2]
Discovery
Kwanasaurus hails from Triassic deposits in the Eagle Basin surrounding the town of
Description
Skull
The
Lower jaw bones referred to Kwanasaurus include DMNH EPV.63136, one of the most complete dentaries found for any silesaurid. As in other sulcimentisaurians, the meckelian groove is positioned close to the lower edge of the jaw and the teeth are constricted at the root. The front tip of the dentary is pointed, toothless, and has a lateral groove akin to that of Silesaurus and Sacisaurus, along with several medial grooves. Further back, the dentary is relatively deep and develops a lateral ridge similar to one reported for Diodorus and Eucoelophysis. The pattern of pitting and holes on the outer surface of the dentary also resembles those taxa. The tooth row is edged by a medial groove connecting a series of replacement pits; above the groove the bone is inset similar to the case in Silesaurus, Eucoelophysis, and Technosaurus. Kwanasaurus is the only silesaurid to preserve data on the mandibular fenestra. This hole in the jaw was triangular, edged from below by a posteroventral process of the dentary which also overlapped a partial angular. The dentary's posterodorsal process is sharp along its upper edge and notched along its lower edge.[1]
Teeth
Teeth of Kwanasaurus have been found both as isolated material and within maxillae and dentaries. Isolated teeth are leaf-shaped, with coarse denticles, slightly flattened sides, and crown tips more than halfway towards the rear of the tooth. The lingual (tongue) side of the tooth has a thick vertical ridge covered in striations. Sacisaurus, Eucoelophysis, and possibly Technosaurus are the only other silesaurids known to possess similar teeth, although leaf-shaped teeth are also common in various other herbivorous archosaurs. In some of the maxilla, the teeth are short and swollen (almost round in cross section) and become smaller towards the rear of the bone. The dentary teeth are similar but more asymmetrical. The middle of the dentary has the largest and most denticulate teeth in the jaw. There are 12 maxillary teeth and 14 dentary teeth. These teeth extend further back in the skull than most silesaurids, as is the case in Lewisuchus, but they are not as numerous as those of that taxon.[1]
Like other silesaurids, Kwanasaurus has ankylothecodont tooth implantation, meaning the teeth are set in sockets but also fused to the surrounding bone. At least in the maxilla, Kwanasaurus has a complex pattern of tooth replacement. This pattern involves replacement teeth being formed along the lingual edge of the tooth row, shifting outwards (at which point the original tooth's attachment dissolves and the tooth detaches), fusing to the leftover socket and leaving behind a replacement pit. The tooth row has alternating empty and full sockets, indicating that adjacent teeth were never replaced at the same time. This contrasts with Silesaurus and Technosaurus (which sometimes have several adjacent teeth replaced at once), but resembles the condition in some specimens of Sacisaurus, Diodorus, and Asilisaurus.[1]
Forelimbs
A long and slender
Hip and hindlimbs
The hip is only represented by the ilium, which had a low, saddle-shaped upper blade like silesaurids such as Silesaurus, Eucoelophysis, and Ignotosaurus. It also resembles these taxa in its elongated and expanded preacetabular process. Kwanasaurus takes this trend further, lengthening the preacetabular process so that it projects further forwards than the pubic peduncle, an adaptation otherwise only seen in several lineages of dinosaurs among dinosauromorphs. The postacetabular process is large and possesses a well-developed brevis shelf and brevis fossa, traits standard for sulcimentisaurians. Like many basal dinosauromorphs, the brevis shelf merges with the edge of the acetabulum and the rear edge of the postacetabular process has a small pointed extension. The acetabulum is deep and has a very thin and concave lower edge. This contrasts with other silesaurids, which have a straight lower edge to the acetabulum, and instead may suggest a partially perforated acetabulum akin to that of dinosaurs. The inner surface of the ilium has several facets for the sacral ribs. The second facet is twice as long as the first and may have encompassed two sacral ribs, suggesting that Kwanasaurus possessed three sacral vertebrae.[1]
Multiple
Classification
Kwanasaurus was added into a phylogenetic analysis to test its relationship to other silesaurids. The codings for the taxon were based on both all the Eagle Basin silesaurid material as well as the dinosauromorph tibiae and scapulae which may additionally belong to it. The strict consensus tree (average result of all most parsimonious trees) was poorly resolved, with practically all silesaurids in a polytomy along with ornithischians and sauropodomorphs. The adams consensus tree (in which unstable taxa cluster at the base of the smallest group they are always within) has better resolution. In this tree, the highly unstable Ignotosaurus shifts to a polytomy with dinosaurs and other silesaurids, while Silesauridae starts with Lewisuchus at the base, followed by Soumyasaurus and Asilisaurus, and finally a clade containing all other silesaurids. This clade was named Sulcimentisauria by the authors of the paper. Removing Ignotosaurus, Soumyasaurus, and Technosaurus led to higher resolution within Silesauridae. Kwanasaurus was found to be the sister taxon to the other North American taxon, Eucoelophysis. Here is the consensus cladogram (specifically the portion focusing on dinosauromorphs) after the removal of the three unstable taxa:[1]
Dinosauromorpha |
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Paleoecology
Kwanasaurus was the northernmost and youngest silesaurid found in North America. Its short, leaf-shaped, and heavily denticulated teeth were adapted for eating plants, a trait shared by several other advanced silesaurids. This diet represents the culmination of a series of adaptations within Silesauridae, starting with carnivory in Lewisuchus (evidenced by recurved and finely serrated teeth), leading to omnivory and/or insectivory in Asilisaurus and Silesaurus (conical teeth with few serrations), and eventually the evolution of a specialized herbivorous diet in advanced sulcimentisaurians. Kwanasaurus in particular had robust skull bones ornamented with ridges, indicating that it likely fed on tougher plants than other herbivorous silesaurids. The timing of silesaurid dietary evolution mirrors the acquisition of herbivory in sauropodomorph dinosaurs, which diversified in southern and eastern portions of Pangea in the Norian stage. The absence of herbivorous dinosaurs in the Chinle Formation may indicate that they had not yet colonized the northwestern region of Pangea that would eventually become North America. This would leave herbivorous niches available for other amniotes, explaining the diversity of non-dinosaur herbivores in the Chinle Formation. These include allokotosaurs (Trilophosaurus), pseudosuchians (Revueltosaurus, aetosaurs, and shuvosaurids), and dicynodonts (Placerias). Kwanasaurus was one of the silesaurids that was a part of this Norian herbivore guild, along with its relative Eucoelophysis (which lived in further south in New Mexico).[1]
Gallery
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Additional maxillae
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Additional dentaries
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Teeth
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Additional ilia
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A complete femur
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Large proximal femora
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Small proximal femora
References
- ^ PMID 31534843.
- ^ "Meet Kwanasaurus, an early relative of dinosaurs - La Provincia". Spain's News. 2019-10-30. Retrieved 2019-12-09.