| Ahshislepelta | |
|---|---|
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| Skeletal diagram | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | †Ornithischia |
| Clade: | †Thyreophora |
| Clade: | †Ankylosauria |
| Genus: | †Ahshislepelta Burns & Sullivan,2011 |
| Species: | †A. minor |
| Binomial name | |
| †Ahshislepelta minor Burns & Sullivan, 2011 | |
Ahshislepelta (meaning "Ah-shi-sle-pah Wash shield") is amonospecificgenus ofankylosaurdinosaur fromNew Mexico that lived during theLate Cretaceous (lateCampanian stage, 74.5 Ma) in what is now the Hunter Wash Member of theKirtland Formation.[1] The type and only species,Ahshislepelta minor, is known only from an incompletepostcranial skeleton of a small subadult or adult individual.[1] It was named in2011 by Michael Burns andRobert M. Sullivan.[1] Based on the size of the humerus,Ahshislepelta is larger thanPinacosaurus mephistocephalus but smaller thanTalarurus andPinacosaurus grangeri.[1]
In 2005, a partial postcranialskeleton of anankylosaur was discovered from the Hunter Wash Member in the lower part of the Kirtland Formation, New Mexico at the Ah-shi-sle-pah Wilderness Study Area. The specimen was later collected over consecutive field seasons from 2005 to 2009 and was subsequently described in 2011 by Michael Burns andRobert M. Sullivan. Theholotype specimen,SMP VP-1930, consists of a partialgirdle, partialscapulocoracoid, a proximal portion of theradius, numerouscervical and/ordorsal vertebrae fragments, complete and fragmentary thoracicosteoderms, and other unidentifiable postcranial fragments. The holotype specimen is currently housed at theState Museum of Pennsylvania,Harrisburg.[1]
The generic name,Ahshislepelta, is derived fromAh-shi-sle-pah Wash, the locality from which the holotype came, and theAncient Greek word "peltè" (shield). The specific name, minor, is in reference to its small adult size relative to otherNorth American ankylosaurids.[1]
The holotype specimen represents asubadult or adult individual based on the complete fusion of the scapulocoracoids, centra andneural arches of the vertebrae, and the re-modelled osteoderm cores. The holotype falls within the same size range as adult specimens of otherankylosaurid taxa outside of North America and represents an individual that was larger than juvenile specimens ofPinacosaurus grangeri andPinacosaurus mephistocephalus but smaller than Talarurus and adult specimens ofP. grangeri based on humeral size; however, the scapulocoracoid is larger than that ofTalarurus.[1]
Burns & Sullivan (2011) diagnosedAhshislepelta based on the dorsolateral overhang of the scapular acromion process to 25% of the dorsoventral width of thescapula.Ahshislepelta differs from other ankylosaurids, with the exception ofEuoplocephalus, by the superficial osteodermal surface texture, characterized by uniformly distributed pitted rugosity, and sparse distribution of reticular neurovascular grooves with neurovascular foramina extending perpendicularly to obliquely into the bone.[1]
The right scapula preserves osteoderms on the lateral side at the most distal portion and is associated with fragments ofribs that are mostly visible on the medial side. The scapula is fully fused to thecoracoid and has a blunt, rugose process along the anterodorsal margin for the attachment ofmuscles. The coracoid is roughly 16 cm long and the coracoid foramen is anterodorsal to theglenoid cavity. The glenoid cavity forms an arc that projects the scapular margin posteroventrally at an 80° angle relative to the coracoid margin. As innodosaurids, the scapula does not narrow towards the upper surface and sides relative to the coracoid and does not exhibit a scapular neck. A rugose, ridge-like structure that is identified as theacromion is present along the dorsal margin of the scapula opposite the glenoid cavity. The acromion attains its front most extension towards the back and sides to the most anterior extent of the glenoid cavity. Due totaphonomic shearing, the prominent overhang of the acromion on the left scapula is absent, although this feature is not a taphonomic artefact on the right scapulocoracoid. However, the right scapulocoracoid is cracked, its junction with the scapular blade is visible, well-preserved, and verifies its natural projection towards the lower edge and sides. Towards the sides of the acromion, the borders of theinfraspinous fossa are prominent and creates an area or the attachment of muscles. The left forelimb is more than 50% complete. As in ankylosaurids, the lefthumerus is massive and has a length of 31 cm. The left humerus preserves a deltopectoral crest that is well-developed and measures 15 cm across the widest portion of the humerus. The lateral margin of the deltopectoral crest forms a 23° angle in posterior view with the long axis of the humerus. Bounded by the deltopectoral crest and humeral head is a broad, bicipital fossa that is present on the anterior face of the humerus. On the anterior face of the humerus is a hemispherical radial condyle. The proximal articular surface of the leftradius is oval in proximal view and is concave. The articular surface is 8 cm long, proportionally twice the diameter of thediaphysis.[1]
Numerousvertebrae fragments that probably represent parts of the cervicals and/or dorsals were recovered and the position of the vertebrae are based on their association with elements of the pectoral girdles and forelimb. A few vertebrae were dorsoventrally tall dorsal vertebrae, based on a few of the more complete vertebrae appearing to have laterally compressed neural canals. All other vertebrae lack open sutural facets for the neural arches.[1]
Numerous osteoderms and associated osteoderm fragments were associated with the holotype specimen. Most osteoderms pertain to the thoracic region while others might pertain to the pelvic region andforelimbs. All osteoderms are either keeled or circular with an off-center apex. The distal portion of the right scapula preserves anin situ osteoderm and numerous ossicles. The osteoderm and ossicles of the right scapula and another series of osteoderms preserved in situ display a rosette arrangement of small ossicles surrounding the osteoderms. The largest osteoderm pertains to the thoracic region behind the cervical region based on the similar morphology to the median thoracic osteoderms of the holotype specimen of Scolosaurus. The osteoderm has a length of 15 cm, a width of 12 cm and a height of 10 cm. The smallest ossicle has a diameter of 1 cm. The surface texture of the osteoderms are smooth to uniformly pitted with a sparse patterns of grooves as inEuoplocephalus. One osteoderm has a histological condition typical of that of ankylosaurids.[1]
Burns & Sullivan (2011) originally placedAhshislepelta as an ankylosaurid within the cladeAnkylosaurinae, although this was not based on the result of aphylogenetic analysis.[1] Likewise, Arbour & Currie (2015) placedAhshislepelta as sister taxon toGastonia at the base of Ankylosauridae, and Wiersma and Irmis (2018) placed it in a polytomy consisting ofEuoplocephalus,Oohkotokia,Scolosaurus,Ziapelta,Anodontosaurus,Ankylosaurus andDyoplosaurus.[2]
On the other hand, several other studies have favored a nodosaurid position. Arbouret al. (2016) recovered it in apolytomy withNiobrarasaurus and a juvenile nodosaurid from thePaw Paw Formation,[3][4] while Zhenget al. (2018) positionedAhshislepelta in a polytomy containingTatankacephalus,Silvisaurus,Niobrarasurus,Nodosaurus and more deeply nested taxa. Rivera-Sylvaet al. (2018) placed it as sister taxon toNiobrarasurus.[5][6]
A phylogenetic analysis conducted by Rivera-Sylvaet al. (2018) is reproduced below, favoring a nodosaurid position. Clade names have been added following Madziaet al. (2021).[6][7]
| Nodosauridae |
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In contrast, an earlier analysis by Arbour & Currie (2015) favors an ankylosaurid position. Their results are reproduced below.[3]
| Ankylosauridae |
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Ahshislepelta is known from the Hunter Wash Member of the Kirtland Formation which has been dated to the upperCampanian stage, 74.5 Ma.[1][8] The Kirtland Formation consists of interbeddedsandstone,siltstone,mudstone,coal andshale.[9] The Kirtland Formation lies on the western margin of theWestern Interior Seaway and represents afloodplain that was abundant withferns,conifers andflowering plants.[10] Based on the abundance of angiosperms with leaves that have entire or nearly entire margins and drip points, the Kirtland Formation may have had a warm temperate to subtropical climate.[10] The Kirtland Formation was better drained than the underlyingFruitland Formation due to the lack of coal swamps.[10]
Ahshislepelta coexisted with thepachycephalosauridStegoceras,[11] thechasmosaurineceratopsidsNavajoceratops andTerminocavus,[12] theankylosaurine ankylosauridZiapelta,[8] theeutyrannosaurtyrannosauroidBistahieversor,[13] and theazhdarchidpterosaurNavajodactylus.[14]
