| Kwanasaurus | |
|---|---|
 | |
| Skeletal and life restoration | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria (?) |
| Clade: | †Ornithischia (?) |
| Family: | †Silesauridae |
| Clade: | †Sulcimentisauria |
| Genus: | †Kwanasaurus Martz & Small,2019 |
| Type species | |
| †Kwanasaurus williamparkeri Martz & Small, 2019 | |
Kwanasaurus is an extinctgenus ofsilesauriddinosauromorph reptiles from theLate Triassic ofColorado. 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 thefemur. It was described along with new specimens ofDromomeron from the Eagle Basin, the northernmost extent of theChinle Formation.[1][2]
Kwanasaurus hails from Triassic deposits in the Eagle Basin surrounding the town ofEagle, Colorado. This area contains the most northern exposures of theChinle Formation, which is famous for its Late Triassic fossils ofdinosaurs and otherreptiles. Tentative terrestrial reptilebiostratigraphy estimates that the Eagle Basin fossils, which were preserved in redsiltstone, belong to theRevueltian biozone of the mid to lateNorian stage of the Triassic, 215-207 million years ago. Theholotype ofKwanasaurus is a partialsilesaurid maxilla,DMNH EPV.65879. All other silesaurid maxillae recovered from the area seem to represent the same taxon, indicating thatKwanasaurus was likely the only silesaurid from the Eagle Basin. With this in mind, all other Eagle Basin fossils resembling those of silesaurids have been referred to the taxon. These include multipledentaries, teeth,ilia,femora, and ahumerus.Dinosauromorph-like tibia and scapulae from the area may also belong toKwanasaurus, though they have not been referred to the genus due to lacking any clear silesaurid features.Kwanasaurus was named in a 2019 paper byJeffrey W. Martz andBryan J. Small, along with the description of newDromomeron material. The genus name incorporateskwana, theUte name for eagle. The specific name commemorates paleontologistBill Parker.[1]
Themaxilla is much deeper and more robust inKwanasaurus than in any othersilesaurid. There are replacement pits on the inner edge of the tooth row similar to those ofthyreophorans, and smaller and more numerous pits on the outer surface of the maxilla. Five of the replacement pits at the midlength of the bone are set in a groove, a trait also present inSilesaurus and silesaurid skull material from theNtawere Formation. The front of the maxilla is similar to that ofLewisuchus andSilesaurus, with a triangularpremaxillary facet and thick, sharpvomerine flange. The ascending process of the maxilla is a thin, anteroposteriorly wide, and steeply-rising prong, and theantorbital fossa has a concave lower edge, both likeSilesaurus. The inner surface of the maxilla has a thick medial flange, which droops down to the tooth row as a smooth triangular blade. This medial flange is unique toKwanasaurus among silesaurids (and Triassicdinosauromorphs in general), and likely extended the maxilla's connection with thepalate behind the vomer. The rear portion of theholotype maxilla is characteristically complex and similar to that described forPlateosaurus. These complex traits include a posterolateral flange which likely shielded part of thejugal, a pair of deep dorsomedial grooves (likely articulating with thelacrimal and jugal), and a broad groove behind the medial flange which likely articulated with thepalatine.[1]
Lower jaw bones referred toKwanasaurus include DMNH EPV.63136, one of the most completedentaries found for any silesaurid. As in other sulcimentisaurians, themeckelian 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 ofSilesaurus andSacisaurus, along with several medial grooves. Further back, the dentary is relatively deep and develops a lateral ridge similar to one reported forDiodorus andEucoelophysis. 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 inSilesaurus,Eucoelophysis, andTechnosaurus.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 ofKwanasaurus 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 possiblyTechnosaurus are the only other silesaurids known to possess similar teeth, although leaf-shaped teeth are also common in various other herbivorousarchosaurs. 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 inLewisuchus, 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 withSilesaurus andTechnosaurus (which sometimes have several adjacent teeth replaced at once), but resembles the condition in some specimens ofSacisaurus,Diodorus, andAsilisaurus.[1]
A long and slenderhumerus is the only forelimb bone safely assigned toKwanasaurus, based on its similarity to that ofSilesaurus andDiodorus. The proximal portion is slightly expanded, but thehumeral head is not as thick or straight as that of other silesaurids. Unlikedinosaurs (but in line with other silesaurids), the deltopectoral crest is small and extends less than a third down the length of the shaft. The distal portion of the humerus is simple and barely expanded, twisted relative to the proximal portion (similar toSilesaurus), but also with a groove running up its anterolateral surface (similar toDiodorus).[1]
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The hip is only represented by theilium, which had a low, saddle-shaped upper blade like silesaurids such asSilesaurus,Eucoelophysis, andIgnotosaurus. 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 theacetabulum 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 thesacral ribs. The second facet is twice as long as the first and may have encompassed two sacral ribs, suggesting thatKwanasaurus possessed three sacral vertebrae.[1]
Multiplefemora of various sizes and conditions have been referred toKwanasaurus. Thefemoral head was similar to that of other advanced silesaurids, according to several traits. These include a longitudinal groove on its upper surface, a straight (rather than rounded) articular facet on the medial surface, a distinct notch on its underside, and an overall triangular cross section due to the lack of a distinct posteromedial tuber. Directly below the head extends a ridge known as a dorsolateral trochanter. Adjacent to it is theanterior trochanter, a pronounced vertical crest characteristic ofdinosauriforms (and a fewDromomeron specimens). The crest was blade-like, triangular, and straight-edged, proportionally most similar toSilesaurus among dinosauriforms. Only one femur referred toKwanasaurus (DMNH EPV.125924) possessed additional muscle attachment structures such as a trochanteric shelf and a swelling that likely represented the iliotrochanteris caudalis attachment site.Asilisaurus andSilesaurus are the only other silesaurids with trochanteric shelves, and its development is sometimes considered to be related to skeletal maturity. However, the largestKwanasaurus femur (DMNH EPV.34579) lacks a trochanteric shelf, despite its presence in the smaller 125924 specimen. All specimens had a lowfourth trochanter, sometimes edged by an anterior depression as inSacisaurus andDiodorus. The distal portion of the femur possessed several unique features. Themedial condyle was a sharp flange, notably thinner than thelateral condyle and crista tibiofibularis. This is similar tolagerpetids but in contrast to the broader medial condyle of all other silesaurids.Kwanasaurus also possesses a characteristic depression on the distal surface of the femur, in front of the crista tibiofibularis. Nevertheless, the notably deep and extensive sulcus present between the medial and lateral condyles is in line with that of other silesaurids.[1]
Kwanasaurus was added into aphylogenetic 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 allmost parsimonious trees) was poorly resolved, with practically all silesaurids in apolytomy along withornithischians andsauropodomorphs. 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 unstableIgnotosaurus shifts to a polytomy with dinosaurs and other silesaurids, while Silesauridae starts withLewisuchus at the base, followed bySoumyasaurus andAsilisaurus, and finally a clade containing all other silesaurids. Thisclade was named Sulcimentisauria by the authors of the paper. RemovingIgnotosaurus,Soumyasaurus, andTechnosaurus led to higher resolution within Silesauridae.Kwanasaurus was found to be thesister 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]
Kwanasaurus was the northernmost and youngest silesaurid found inNorth 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 inLewisuchus (evidenced by recurved and finely serrated teeth), leading to omnivory and/or insectivory inAsilisaurus andSilesaurus (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 insauropodomorphdinosaurs, which diversified in southern and eastern portions ofPangea in theNorian 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 otheramniotes, explaining the diversity of non-dinosaur herbivores in the Chinle Formation. These includeallokotosaurs (Trilophosaurus),pseudosuchians (Revueltosaurus,aetosaurs, andshuvosaurids), anddicynodonts (Placerias).Kwanasaurus was one of the silesaurids that was a part of this Norian herbivore guild, along with its relativeEucoelophysis (which lived in further south inNew Mexico).[1]
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