麻豆淫院

Researchers report three distinct tanystropheid taxa from the upper Blue Mesa Member of the Chinle Formation in the Petrified Forest National Park in Arizona, including a new genus and species, Akidostropheus oligos, with a distinctive back spike emanating from the spine.

Archosauromorph reptiles originated in the Permian and radiated extensively in the Triassic following the end-Permian extinction event, to become a dominant faunal group that includes clades such as crocodilians, nonavian dinosaurs, birds, and potentially turtles.

Non-archosauriform archosauromorphs hold key information about the anatomical origins of archosaurs and ecological dynamics during the Triassic Period.

Tanystropheids were highly diverse in body plans and ecologies, with taxa from marine-influenced and freshwater fluvial and lacustrine deposits, and with a small but expanding North American record spanning much of the Triassic. Cervical vertebra anatomy carries diagnostic value across archosauromorph groups.

In the study, "A diverse assemblage of tanystropheid archosauromorphs from the continental interior of Late Triassic Pangea includes a new taxon (Akidostropheus oligos gen. et sp. nov.)," in Palaeodiversity, researchers used cervical vertebrae morphotypes as a metric for diversity to recognize three distinct taxa at the locality, including aff. Tanystropheus and a novel taxon named Akidostropheus oligos, "recognizable by its autapomorphic spike emanating dorsally from the neural spine that may be a defensive structure."

Thunderstorm Ridge within Blue Mesa Member includes hundreds of tanystropheid bones that are disarticulated, dissociated, and lack statistical directionality. The age is bracketed between 223 and 218 million years ago, and the occurrence of A. oligos is also reported at other sites near St. Johns, Arizona.

Fossils were collected by painstakingly slow paleontological procedures, including hand quarrying and screenwashing, microscope-assisted picking, preparation using consolidants, adhesives, pneumatic air scribe, and pin vice.

Due to disarticulation, no elements were confidently associated and comparisons focused on cervical vertebrae described as "very small," measuring 6.7 mm tall and 5.5 mm long.

One (or possibly two) unidentified taxa held two morphotypes. Type 1 showed moderately elongated neck bones with a shallow ridge underneath and small pits near the spine. Morphotype 2 had very elongated neck bones with a long, low top ridge, faint side ridges, and a keel mainly near the back end.

The aff. Tanystropheus held ultra-elongated neck bones (ends flat to slightly cupped on both sides) with greatly reduced top ridges and a deep groove running along the underside. Dating places the specimen ~4 to 19 million years more recent than the previously described Tanystropheus.

Akidostropheus oligos presented tiny vertebrae, a distinctive back spike with fine striations and similar spikes present on trunk and tail vertebrae. Longitudinal spike striations are consistent with a keratinous sheath and the authors note it may be a neural-spine outgrowth or a co-ossified osteoderm.

The unexpected diversity of tanystropheid archosauromorphs indicates that semi-tropical nonmarine ecosystems may have provided opportunities for speciation and niche partitioning in Triassic vertebrates and provides valuable three-dimensional anatomical data for a group often characterized by two-dimensional preservation.

A probable hypothesis for the function of the dorsal spikes of A. oligos was that of defense against predation.