Yarasuchus (meaning "red crocodile") is an extinct genus of avemetatarsalian archosaur that lived during the Anisian stage of the Middle Triassic of India. The genus was named and described in 2005 from a collection of disarticulated but fairly complete fossil material found from the Middle Triassic Yerrapalli Formation. The material is thought to be from two individuals, possibly three, with one being much more complete and articulated than the other. The type and only species is Y. deccanensis. Yarasuchus was a quadruped roughly long, with an elongated neck and tall spines on its vertebrae. Unlike other quadrupedal Triassic reptiles, the limbs and shoulders of Yarasuchus were slender, and more like those of ornithodirans.
Yarasuchus has had a complicated taxonomic history, after originally being described as a "prestosuchid rauisuchian", it was later variously recovered as a poposauroid pseudosuchian and a non-archosaurian archosauriform of unstable position. In 2017 it was determined to be related to the similarly enigmatic Triassic reptiles Teleocrater, Dongusuchus and Spondylosoma. Together, they belong to a group called Aphanosauria and are placed at the base of Avemetatarsalia, sister to Ornithodira, making Yarasuchus one of the earliest diverging bird-line archosaurs known. The relative completeness of Yarasuchus and its evolutionary position helps to shed light on the origins of later, well known bird-line archosaurs such as the dinosaurs and pterosaurs.
Description
Yarasuchus was a relatively small, gracile, low-slung quadruped with a long neck and a small head, estimated to be around long. Despite its appearance, Yarasuchus has a number of features in common with other avemetatarsalians, such as its elongated neck vertebrae. It has long, slender limb bones, unlike the more robust limbs of most pseudosuchians, but similar to other avemetatarsalians. Unlike later avemetatarsalians though, Yarasuchus has a "crocodile-normal" ankle configuration, more similar to pseudosuchians and some stem-archosaurs. Yarasuchus is very similar in appearance to its close relative Teleocrater, and differs in only a few minor anatomical features of the skeleton, such as having a less of a rear facing glenoid (the bony shoulder socket) than that of Teleocrater.
Skull
The skull of Yarasuchus is poorly represented, known from only a few isolated pieces. A number of bones were initially identified by Sen (2005), including a jugal, quadrate and part of the quadratojugal, squamosal, both pterygoids and two maxilla that included a portion of premaxilla attached to one of them, as well as an associated tooth. However both maxillae differ from the known maxilla in Teleocrater and instead more closely resemble those of an allokotosaur. Nesbitt and colleagues also regarded the jugal, quadrate and quadratojugal as indeterminate bones, and re-identified the squamosal as a postorbital belonging to a larger individual than the holotype specimen. As of 2017, the right postorbital and both pterygoids are the only positively identified cranial material belonging to Yarasuchus.
Skeleton
Almost the whole vertebral column is represented in Yarasuchus, including at least 8 cervicals (including the atlas and axis), 17 dorsals, 2 sacral, and at least 11 proximal caudal vertebrae. The long neck of Yarasuchus is made up of a series of characteristically elongated cervical (neck) vertebrae, in contrast to its proportionately small skull. The articulating surfaces of the zygapophyses that connect between each vertebra are inclined and the centra are strongly curved along the bottom margin with offset faces, indicating that the neck was held raised up from the body and arched along its length. The cervicals are uniquely characterised by a prominent midline keel that runs along the front half of the underside of each centrum, followed by two separate keels that continue to the rear edge. The posterior cervical ribs have three heads, an unusual condition in archosaurs, and the corresponding cervicals possess an accessory articular facet to accommodate this.