Yarasuchus (meaning "red crocodile") is an extinct genus of avemetatarsalian archosaur that lived during the Anisian stage of the Middle Triassic of India.Bandyopadhyay, S. and Sengupta, D. P. (1999). Middle Triassic vertebrate faunas from India. Journal of African Earth Sciences 29: 233–241. 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, comprising at least eight cervicals (including the atlas and axis), 17 dorsals, two 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.
Unlike the cervicals, the dorsal (back) vertebra are short and compact, almost only half the length of the cervicals, as are the two sacral (hip) vertebra. The front-most two or three dorsal vertebra possess similar keels to the cervicals, unlike the condition in Teleocrater which has no keeled dorsals. The preserved proximal caudal (tail) vertebra are consistently elongate, however the distal caudals are missing and so the end of the tail is unknown. The vertebrae all have tall neural spines that run down the back, the tallest of which are over the hip where the spines are two times taller than the corresponding centra. The neck has similarly tall neural spines, however they are longer than tall and inclined forward so as to overhang the vertebra before it. The cervical neural spines are also unusually thickened and roughly textured at the top. The caudal neural spines are also tall and narrow, unlike the elongated centra, and are inclined backwards, decreasing in height further down the tail. The tall, broad neural spines were initially believed to be associated with rows of paramedian osteoderms (although see below).
The pectoral (shoulder) girdle is delicately built, with a tall and slender scapula that is expanded distally and constricted near the glenoid. The scapula has a continuous acromion process, as well as an unusual sharp, thin ridge of bone running down its posterior margin, a feature only found in aphanosaurs and silesaurids. The coracoid is small and rounded, and forms a down and rearward facing glenoid fossa with the scapula. The ilium of the pelvic (hip) girdle has a prominent posterior process and supraacetabular crest—a ridge of bone over the acetabulum. The acetabulum itself is relatively large and almost completely closed, unlike the fully open acetabulum of dinosaurs. The pubis is short and points downwards, with a thickened, flattened end. The ischium is very similar to that of Teleocrater, and is directed down and back from the hips, with a tapered shaft and slightly expanded distal end. The pelvic girdle articulates with the spine via the sacral ribs, of which the first is larger and more robust. The nature of the articulation between the ilium and the sacral ribs suggests the hip was held sub-horizontally and faced ventrolaterally, causing the legs to be positioned down and outwards from the body.
The limb bones of Yarasuchus are characteristically slender. The humerus is long and cylindrical, with a moderately developed elongated deltopectoral crest that occupies roughly 30% of the length of the bone, similar to the condition in dinosaurs. The ulna is arched somewhat, and is roughly equal in length to the humerus. The femur is similarly gracile, and has a sigmoidal (s-shaped) curve along its length. The head of the femur is not turned in. A moderately developed fourth trochanter is placed proximally on the femur, unlike the well developed fourth trochanters of dinosaurs. The manus and pes are almost entirely missing, except for the calcaneus of the ankle joint. The calcaneus indicates that Yarasuchus had a "crocodile-normal" ankle, which allowed for more rotation of the foot than the derived hinge-like avemetatarsalian ankle. The forelimb to hindlimb length ratio is roughly 3:5, with much longer back legs than the front. Although the manus is missing, the overall similarity of Teleocrater suggests it would have had a relatively small hand.
The osteoderms attributed to Yarasuchus are greatly sculptured with a ridge running anteroposteriorly along each one. This is unusual in comparison to Teleocrater, which has been interpreted as lacking any form of similar dermal armour. However, these osteoderms are larger in proportion to the presacral vertebra than would be expected and bear a close similarity to osteoderms of an erythrosuchid also found in the same formation. Nesbitt and colleagues considered it probable that the osteoderms do not belong Yarasuchus and so it may have also lacked osteoderms like other aphanosaurs and avemetatarsalians.
Discovery and naming Yarasuchus is known from at least two individuals collected from a single assemblage in the Yerrapalli Formation, located near the Bhimaram village in the Adilabad district of India, in a layer of fine red mudstone. The material was found disarticulated, however it represents the majority of the skeleton, missing only the distal caudal vertebrae, radius, fibula, manus, and most of the pes and skull. The name is derived from 'Yara', meaning red in the local dialect, referring to the red mudstones the fossils were discovered in. The specific name refers to the Deccan region of India, where the Yerrapalli Formation and surrounding Pranhita-Godavari Basin are situated. The fossils were also found in association with two specimens of the allokotosaur Pamelaria. It was initially reported on in a 1993 thesis by Dasgupta, who considered it an indeterminate rauisuchid, before being officially described and named as a new taxon by Sen in 2005. All the material is held at the Geology Museum of the Indian Statistical Institute in Kolkata, India.
In 2016, the material was re-examined by Nesbitt and colleagues in their description of Teleocrater, which revealed a number of previously unrecognised anatomical characteristics. This re-evaluation prompted the inclusion of Yarasuchus within the newly recognised avemetatarsalian clade Aphanosauria. In their examination, Nesbitt and colleagues were able to refer a number of previously undescribed calcanea collected at the site to the hypodigm of Yarasuchus based on their similarity to Teleocrater, and identified ischia that were originally reported as missing. They also removed a number of erroneously referred skull bones and osteoderms from the hypodigm. The relatively complete skeleton of Yarasuchus allowed Nesbitt and colleagues to confidently refer a number of isolated bones as all belonging to specimens of Teleocrater because of their close similarity to those of Yarasuchus, which provided a key reference point for understanding the anatomy of the previously enigmatic taxon.
Classification Yarasuchus was originally described as a "prestosuchid", similar to Prestosuchus, Ticinosuchus, and Mandasuchus by Sen, however no phylogenetic analysis was performed and the referral was made based on general morphological similarities, despite the already debated validity of "Prestosuchidae". In 2010, Brusatte and colleagues conclusively demonstrated that "Prestosuchidae" was a paraphyletic grade of paracrocodylomorphs, and that the supposed shared characteristics of the group were in fact found throughout Pseudosuchia. They also performed a detailed cladistic analysis of fossil Triassic archosaurs, which instead found Yarasuchus to be a basal member of Poposauroidea, although support for this position was weak.
The position of Yarasuchus in subsequent studies has been similarly unstable, though it has nonetheless typically been regarded as a pseudosuchian. A study by Ezcurra on archosauromorph phylogeny in 2016 found Yarasuchus in a position outside of Archosauria, clading together with Dongusuchus in a polytomy with Euparkeria and a clade made up of Proterochampsia and archosaurs. The left cladogram depicts the results of Brusatte and colleagues in 2010, while the right cladogram depicts that of Ezcurra in 2016: