Yarasuchus
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''Yarasuchus'' (meaning "red crocodile") is an extinct
genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...
of
avemetatarsalia Avemetatarsalia (meaning "bird metatarsals") is a clade of diapsid reptiles containing all archosaurs more closely related to birds than to crocodilians. The two most successful groups of avemetatarsalians were the dinosaurs and pterosaurs. ...
n archosaur that lived during the
Anisian In the geologic timescale, the Anisian is the lower stage or earliest age of the Middle Triassic series or epoch and lasted from million years ago until million years ago. The Anisian Age succeeds the Olenekian Age (part of the Lower Triassic ...
stage of the
Middle Triassic In the geologic timescale, the Middle Triassic is the second of three epochs of the Triassic period or the middle of three series in which the Triassic system is divided in chronostratigraphy. The Middle Triassic spans the time between Ma and ...
of
India India, officially the Republic of India (Hindi: ), is a country in South Asia. It is the seventh-largest country by area, the second-most populous country, and the most populous democracy in the world. Bounded by the Indian Ocean on the so ...
.Bandyopadhyay, S. and Sengupta, D. P. (1999). Middle Triassic vertebrate faunas from India. ''
Journal of African Earth Sciences The ''Journal of African Earth Sciences'' is a peer-reviewed scientific journal published by Elsevier. It covers the earth sciences, primarily on issues that are relevant to Africa and the Middle East. The journal was established in 1983 and the ed ...
'' 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 In the geologic timescale, the Middle Triassic is the second of three epochs of the Triassic period or the middle of three series in which the Triassic system is divided in chronostratigraphy. The Middle Triassic spans the time between Ma and ...
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 In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...
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
ornithodira Avemetatarsalia (meaning "bird metatarsals") is a clade of diapsid reptiles containing all archosaurs more closely related to birds than to crocodilians. The two most successful groups of avemetatarsalians were the dinosaurs and pterosaurs. Dinos ...
ns. ''Yarasuchus'' has had a complicated taxonomic history, after originally being described as a "
prestosuchid Prestosuchidae (in its widest usage) is a polyphyletic grouping of carnivorous archosaurs that lived during the Triassic. They were large active terrestrial apex predators, ranging from around in length. They succeeded the Erythrosuchidae as th ...
rauisuchia "Rauisuchia" is a paraphyletic group of mostly large and carnivorous Triassic archosaurs. Rauisuchians are a category of archosaurs within a larger group called Pseudosuchia, which encompasses all archosaurs more closely related to crocodilians ...
n", it was later variously recovered as a
poposauroid Poposauroidea is a clade of advanced pseudosuchians (archosaurs closer to crocodilians than to dinosaurs). It includes poposaurids, shuvosaurids, ctenosauriscids, and other unusual pseudosuchians such as ''Qianosuchus'' and ''Lotosaurus''. How ...
pseudosuchia Pseudosuchia is one of two major divisions of Archosauria, including living crocodilians and all archosaurs more closely related to crocodilians than to birds. Pseudosuchians are also informally known as "crocodilian-line archosaurs". Prior to ...
n 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 ''Spondylosoma'' (meaning "vertebra body") is a genus of avemetatarsalian archosaur belonging to the clade Aphanosauria from the late Ladinian-age Middle Triassic Lower Santa Maria Formation in Paleorrota Geopark, Brazil. History Friedrich v ...
.'' Together, they belong to a group called
Aphanosauria Aphanosauria ("hidden lizards") is an extinct group of reptiles distantly related to dinosaurs (including birds). They are at the base of a group known as Avemetatarsalia, one of two main branches of archosaurs. The other main branch, Pseudosuc ...
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
dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...
s and
pterosaur Pterosaurs (; from Greek ''pteron'' and ''sauros'', meaning "wing lizard") is an extinct clade of flying reptiles in the order, Pterosauria. They existed during most of the Mesozoic: from the Late Triassic to the end of the Cretaceous (228 ...
s.


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 glenoid fossa of the scapula or the glenoid cavity is a bone part of the shoulder. The word ''glenoid'' is pronounced or (both are common) and is from el, gléne, "socket", reflecting the shoulder joint's ball-and-socket form. It is a sha ...
(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 The jugal is a skull bone found in most reptiles, amphibians and birds. In mammals, the jugal is often called the malar or zygomatic. It is connected to the quadratojugal and maxilla, as well as other bones, which may vary by species. Anatomy ...
, quadrate and part of the
quadratojugal The quadratojugal is a skull bone present in many vertebrates, including some living reptiles and amphibians. Anatomy and function In animals with a quadratojugal bone, it is typically found connected to the jugal (cheek) bone from the front and ...
,
squamosal The squamosal is a skull bone found in most reptiles, amphibians, and birds. In fishes, it is also called the pterotic bone. In most tetrapods, the squamosal and quadratojugal bones form the cheek series of the skull. The bone forms an ancestral co ...
, both pterygoids and two
maxilla The maxilla (plural: ''maxillae'' ) in vertebrates is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. T ...
that included a portion of
premaxilla The premaxilla (or praemaxilla) is one of a pair of small cranial bones at the very tip of the upper jaw of many animals, usually, but not always, bearing teeth. In humans, they are fused with the maxilla. The "premaxilla" of therian mammal has ...
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 The ''postorbital'' is one of the bones in vertebrate skulls which forms a portion of the dermal skull roof and, sometimes, a ring about the orbit. Generally, it is located behind the postfrontal and posteriorly to the orbital fenestra. In some ...
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 The vertebral column, also known as the backbone or spine, is part of the axial skeleton. The vertebral column is the defining characteristic of a vertebrate in which the notochord (a flexible rod of uniform composition) found in all chordate ...
is represented in ''Yarasuchus'', including at least 8 cervicals (including the
atlas An atlas is a collection of maps; it is typically a bundle of maps of Earth or of a region of Earth. Atlases have traditionally been bound into book form, but today many atlases are in multimedia formats. In addition to presenting geograp ...
and
axis An axis (plural ''axes'') is an imaginary line around which an object rotates or is symmetrical. Axis may also refer to: Mathematics * Axis of rotation: see rotation around a fixed axis * Axis (mathematics), a designator for a Cartesian-coordinat ...
), 17 dorsals, 2
sacral Sacral may refer to: *Sacred Sacred describes something that is dedicated or set apart for the service or worship of a deity; is considered worthy of spiritual respect or devotion; or inspires awe or reverence among believers. The property ...
, 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 Centra is a convenience shop chain that operates throughout Ireland. The chain operates as a symbol group owned by Musgrave Group, the food wholesaler, meaning the stores are all owned by individual franchisees. The chain has three different ...
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 rib A cervical rib in humans is an extra rib which arises from the seventh cervical vertebra. Their presence is a congenital abnormality located above the normal first rib. A cervical rib is estimated to occur in 0.2% to 0.5% (1 in 200 to 500) of th ...
s 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
osteoderm Osteoderms are bony deposits forming scales, plates, or other structures based in the dermis. Osteoderms are found in many groups of extant and extinct reptiles and amphibians, including lizards, crocodilians, frogs, temnospondyls (extinc ...
s (although see below). The pectoral (shoulder) girdle is delicately built, with a tall and slender
scapula The scapula (plural scapulae or scapulas), also known as the shoulder blade, is the bone that connects the humerus (upper arm bone) with the clavicle (collar bone). Like their connected bones, the scapulae are paired, with each scapula on eith ...
that is expanded distally and constricted near the glenoid. The scapula has a continuous
acromion process In human anatomy, the acromion (from Greek: ''akros'', "highest", ''ōmos'', "shoulder", plural: acromia) is a bony process on the scapula (shoulder blade). Together with the coracoid process it extends laterally over the shoulder joint. The acr ...
, 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 A coracoid (from Greek κόραξ, ''koraks'', raven) is a paired bone which is part of the shoulder assembly in all vertebrates except therian mammals (marsupials and placentals). In therian mammals (including humans), a coracoid process is prese ...
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 The ulna (''pl''. ulnae or ulnas) is a long bone found in the forearm that stretches from the elbow to the smallest finger, and when in anatomical position, is found on the medial side of the forearm. That is, the ulna is on the same side of t ...
is arched somewhat, and is roughly equal in length to the humerus. The
femur The femur (; ), or thigh bone, is the proximal bone of the hindlimb in tetrapod vertebrates. The head of the femur articulates with the acetabulum in the pelvic bone forming the hip joint, while the distal part of the femur articulates wit ...
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 Adilabad district is a district located in the northern region of Telangana, India. It is known as the gateway district to South and Central India. The town of Adilabad is its headquarters. The district shares boundaries with Asifabad, Nirma ...
of
India India, officially the Republic of India (Hindi: ), is a country in South Asia. It is the seventh-largest country by area, the second-most populous country, and the most populous democracy in the world. Bounded by the Indian Ocean on the so ...
, 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 In classical geometry, a radius ( : radii) of a circle or sphere is any of the line segments from its center to its perimeter, and in more modern usage, it is also their length. The name comes from the latin ''radius'', meaning ray but also the ...
,
fibula The fibula or calf bone is a leg bone on the lateral side of the tibia, to which it is connected above and below. It is the smaller of the two bones and, in proportion to its length, the most slender of all the long bones. Its upper extremity i ...
, manus, and most of the pes and skull. The name is derived from 'Yara', meaning red in the local dialect, and the Greek ''suchos'' ("crocodile"), 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 Indian Statistical Institute (ISI) is a higher education and research institute which is recognized as an Institute of National Importance by the 1959 act of the Indian parliament. It grew out of the Statistical Laboratory set up by Prasanta ...
in
Kolkata Kolkata (, or , ; also known as Calcutta , the official name until 2001) is the capital of the Indian state of West Bengal, on the eastern bank of the Hooghly River west of the border with Bangladesh. It is the primary business, comme ...
, 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 Prestosuchidae (in its widest usage) is a polyphyletic grouping of carnivorous archosaurs that lived during the Triassic. They were large active terrestrial apex predators, ranging from around in length. They succeeded the Erythrosuchidae as th ...
", similar to ''
Prestosuchus ''Prestosuchus'' (meaning "Prestes crocodile") is an extinct genus of pseudosuchian in the group Loricata, which also includes '' Saurosuchus'' and '' Postosuchus''. It has historically been referred to as a " rauisuchian", and was the defining ...
'', ''
Ticinosuchus ''Ticinosuchus'' is an extinct genus of suchian archosaur from the Middle Triassic (Anisian - Ladinian) of Switzerland and Italy. Description One of only a handful of fossil reptiles that have been found in Switzerland, ''Ticinosuchus'' ( ...
,'' and ''
Mandasuchus ''Mandasuchus'' is an extinct genus of loricatan pseudosuchian from the Manda Formation of Tanzania, which dates back to the Anisian stage of the Middle Triassic. Although this genus was first mentioned by Alan Charig in 1956,Charig, A. J. (1956 ...
'' by Sen, however no
phylogenetic analysis In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...
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 Grade most commonly refers to: * Grade (education), a measurement of a student's performance * Grade, the number of the year a student has reached in a given educational stage * Grade (slope), the steepness of a slope Grade or grading may also ref ...
of
paracrocodylomorphs Paracrocodylomorpha is a clade of pseudosuchian archosaurs. The clade includes the diverse and unusual group Poposauroidea as well as the generally carnivorous and quadrupedal members of Loricata, including modern crocodylians. Paracrocodylomorp ...
, and that the supposed shared characteristics of the group were in fact found throughout
Pseudosuchia Pseudosuchia is one of two major divisions of Archosauria, including living crocodilians and all archosaurs more closely related to crocodilians than to birds. Pseudosuchians are also informally known as "crocodilian-line archosaurs". Prior to ...
. They also performed a detailed
cladistic Cladistics (; ) is an approach to biological classification in which organisms are categorized in groups ("clades") based on hypotheses of most recent common ancestry. The evidence for hypothesized relationships is typically shared derived char ...
analysis of fossil Triassic archosaurs, which instead found ''Yarasuchus'' to be a basal member of
Poposauroidea Poposauroidea is a clade of advanced pseudosuchians (archosaurs closer to crocodilians than to dinosaurs). It includes poposaurids, shuvosaurids, ctenosauriscids, and other unusual pseudosuchians such as ''Qianosuchus'' and ''Lotosaurus''. How ...
, 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 An internal node of a phylogenetic tree is described as a polytomy or multifurcation if (i) it is in a rooted tree and is linked to three or more child subtrees or (ii) it is in an unrooted tree and is attached to four or more branches. A tr ...
with ''
Euparkeria ''Euparkeria'' (; meaning "Parker's good animal", named in honor of W.K. Parker) is an extinct genus of archosauriform from the Middle Triassic of South Africa. It was a small reptile that lived between 245-230 million years ago, and was close to ...
'' and a clade made up of
Proterochampsia Proterochampsia is a clade of early archosauriform reptiles from the Triassic period. It includes the Proterochampsidae (e.g. ''Proterochampsa'', ''Chanaresuchus'' and ''Tropidosuchus'') and probably also the Doswelliidae. Nesbitt (2011) define ...
and archosaurs. The left cladogram depicts the results of Brusatte and colleagues in 2010, while the right cladogram depicts that of Ezcurra in 2016: Further complicating the issue were suggestions that the hypodigm of ''Yarasuchus'' was a chimeric assemblage of material from both a "rauisuchian" archosaur and a prolacertiform archosauromorph, if not including material from yet other archosauromorphs. However, later examination of the material by Ezcurra found that none of it could be assigned to either a "rauisuchian" or prolacertiform, and that they all likely pertained to a single non- suchian archosauriform taxon. In 2017, ''Yarasuchus'' was included in an updated analysis of Triassic archosauromorphs by Nesbitt and colleagues in their official description of ''Teleocrater'', utilising two modified datasets, those of Nesbitt (2011) and Ezcurra (2016). Both analyses recovered ''Yarasuchus'' in a newly recognised clade they named Aphanosauria, which included ''Yarasuchus'' in a polytomy with ''Teleocrater'' and ''Dongusuchus'', as well as ''Spondylosoma''. Indeed, the clade Aphanosauria was cladistically defined as "the most inclusive clade containing ''Teleocrater rhadinus'' and ''Yarasuchus deccanensis'' but not ''
Passer domesticus The house sparrow (''Passer domesticus'') is a bird of the sparrow family Passeridae, found in most parts of the world. It is a small bird that has a typical length of and a mass of . Females and young birds are coloured pale brown and grey, an ...
'' or ''
Crocodylus niloticus ''Crocodylus'' is a genus of true crocodiles in the family Crocodylidae. Taxonomy The generic name, ''Crocodylus'', was proposed by Josephus Nicolaus Laurenti in 1768. ''Crocodylus'' contains 13–14 extant (living) species and 5 extinct spe ...
"''. Aphanosaurs were found to be the earliest diverging clade of
Avemetatarsalia Avemetatarsalia (meaning "bird metatarsals") is a clade of diapsid reptiles containing all archosaurs more closely related to birds than to crocodilians. The two most successful groups of avemetatarsalians were the dinosaurs and pterosaurs. ...
, sister taxon to the clade
Ornithodira Avemetatarsalia (meaning "bird metatarsals") is a clade of diapsid reptiles containing all archosaurs more closely related to birds than to crocodilians. The two most successful groups of avemetatarsalians were the dinosaurs and pterosaurs. Dinos ...
that includes the Pterosauria and Dinosauromorpha. The results of their analyses are reproduced and simplified below, combining the general topology of the Ezcurra dataset with the Nesbitt dataset results for Avemetatarsalia.


Evolutionary significance

The recognition of a close relationship between ''Yarasuchus'', ''Teleocrater'' and other aphanosaurs and their relation to other avemetatarsalians settled a number of the unusual anatomical features of ''Yarasuchus''. Many of the previously unique features of ''Yarasuchus'' unite it with other aphanosaurs, including the elongated neck, high neural spines, three-headed cervical ribs and slender
appendicular skeleton The appendicular skeleton is the portion of the skeleton of vertebrates consisting of the bones that support the appendages. There are 126 bones. The appendicular skeleton includes the skeletal elements within the limbs, as well as supporting shou ...
. ''Yarasuchus'' and the other aphanosaurs play a significant role in our understanding of early avemetatarsalian evolution, exemplified in the relatively completely known anatomy of ''Yarasuchus''. The structure of the foot, particularly the bones of the ankle (such as the calcaneus), demonstrate that avemetatarsalians evolved from ancestors with 'crocodile-normal' ankles, unlike the simple hinge-like ankles characteristic of derived ornithodirans. The 'crocodile-normal' ankle was once thought to be unique to pseudosuchians, but its presence in aphanosaurs like ''Yarasuchus'' imply that the evolution of the avemetatarsalian ankle was a more complicated process than initially believed. The anatomy of ''Yarasuchus'' also demonstrates that other typical avemetatarsalian features, such as slender limb girdles, had evolved prior to the eponymous 'advanced mesotarsal' ankles. Unusually, ''Yarasuchus'' and other aphanosaurs share a number of features convergently evolved with poposauroids. Aphanosaurs and poposauroids share only one unique trait (the presence of an accessory articulation facet just above the parapophysis of the cervicals for the three-headed cervical rib), however they have also convergently acquired a similar set of traits that are found throughout archosaurs. The previous phylogenetic position of ''Yarasuchus'' as a poposauroid by Brusatte and colleagues was likely due to this convergence. The convergence between ''Yarasuchus'' and poposauroids could be attributed to the broader trend of poposauroids converging on
coelurosaur Coelurosauria (; from Greek, meaning "hollow tailed lizards") is the clade containing all theropod dinosaurs more closely related to birds than to carnosaurs. Coelurosauria is a subgroup of theropod dinosaurs that includes compsognathids, tyran ...
ian
theropods Theropoda (; ), whose members are known as theropods, is a dinosaur clade that is characterized by hollow bones and three toes and claws on each limb. Theropods are generally classed as a group of saurischian dinosaurs. They were ancestrally ca ...
, a derived clade of avemetatarsalians. The similarity between poposauroids and aphanosaurs like ''Yarasuchus'' means it is difficult to determine the identity of isolated archosaur material that has features present in both groups, particularly as aphanosaurs are the earliest diverging avemetatarsalians while poposauroids are the oldest known pseudosuchians and so their
stratigraphic Stratigraphy is a branch of geology concerned with the study of rock layers (strata) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks. Stratigraphy has three related subfields: lithostra ...
ranges broadly overlap with each other.


Palaeobiology

''Yarasuchus'' was proposed to be facultatively bipedal by Dasgupta in 1993 on the basis of its gracile body, slender shoulder girdle and proportionately short forelimbs, among other features, and this suggestion was repeated by Sen in its official description in 2005. The closely related ''Teleocrater'' has since been interpreted to have been a quadruped, and as ''Yarasuchus'' has similar limb proportions, it likely was as well. The lack of definitive jaw material leaves the diet of ''Yarasuchus'' ambiguous, however the teeth of ''Teleocrater'' imply aphanosaurs were carnivorous, as with other early avemetatarsalians. Furthermore,
histological Histology, also known as microscopic anatomy or microanatomy, is the branch of biology which studies the microscopic anatomy of biological tissues. Histology is the microscopic counterpart to gross anatomy, which looks at larger structures vis ...
samples from ''Teleocrater'' show that its growth rates were more similar to those of other avemetatarsalians compared to pseudosuchians and stem-archosaurs, and so ''Yarasuchus'' may have also had a similarly higher growth rate.


Palaeoecology

Many other vertebrate remains have been found from the Yerrapalli Formation alongside those of ''Yarasuchus'', and would have coexisted with it during the Middle Triassic. Remains of the allokotosaur ''Pamelaria'' in particular have been found in close proximity to those of ''Yarasuchus''.Sen, K. (2003). ''Pamelaria dolichotrachela'', a new prolacertid reptile from the Middle Triassic of India. ''Journal of Asian Earth Sciences'' 21: 663–681. Other vertebrate remains include those of the
lungfish Lungfish are freshwater vertebrates belonging to the order Dipnoi. Lungfish are best known for retaining ancestral characteristics within the Osteichthyes, including the ability to breathe air, and ancestral structures within Sarcopterygii, i ...
'' Ceratodus'', the actinopterygian fish '' Saurichthys'', the
temnospondyl Temnospondyli (from Greek τέμνειν, ''temnein'' 'to cut' and σπόνδυλος, ''spondylos'' 'vertebra') is a diverse order of small to giant tetrapods—often considered primitive amphibians—that flourished worldwide during the Carbo ...
'' Parotosuchus'', the
dicynodont Dicynodontia is an extinct clade of anomodonts, an extinct type of non-mammalian therapsid. Dicynodonts were herbivorous animals with a pair of tusks, hence their name, which means 'two dog tooth'. Members of the group possessed a horny, typic ...
s '' Rechnisaurus'' and '' Wadiasaurus'', the
rhynchosaur Rhynchosaurs are a group of extinct herbivorous Triassic archosauromorph reptiles, belonging to the order Rhynchosauria. Members of the group are distinguished by their triangular skulls and elongated, beak like premaxillary bones. Rhynchosaurs ...
'' Mesodapedon'', and a large undescribed erythrosuchid. The anatomical similarity between ''Yarasuchus'' and ''Teleocrater'' is mirrored by the similarities in fauna and environment shared between the Yerrapalli Formation and the
Manda Formation The Manda Formation (also known as the Manda Beds) is a Middle Triassic (Anisian?) or possibly Late Triassic (Carnian?) geologic formation in Tanzania. It preserves fossils of many terrestrial vertebrates from the Triassic, including some of the e ...
in
Tanzania Tanzania (; ), officially the United Republic of Tanzania ( sw, Jamhuri ya Muungano wa Tanzania), is a country in East Africa within the African Great Lakes region. It borders Uganda to the north; Kenya to the northeast; Comoro Islands ...
, and both are estimated to be Anisian in age. At the time, India was still a part of the supercontinent Gondwana in
Pangaea Pangaea or Pangea () was a supercontinent that existed during the late Paleozoic and early Mesozoic eras. It assembled from the earlier continental units of Gondwana, Euramerica and Siberia during the Carboniferous approximately 335 million y ...
, and was located directly adjacent to East Africa. This demonstrates that early avemetatarsalians like ''Yarasuchus'' were geographically widespread in the Middle Triassic, as with other archosauriforms, in contrast to previous suggestions that pseudosuchians were more diverse''.'' The close similarity between ''Yarasuchus'' and the Russian ''Dongusuchus'' further supports this, indicating biogeographical affinities between India and Russia despite their widely separated palaeolatitudes. The sediments of the Yerrapalli Formation are interpreted as fluvial deposits, indicative of a broad, interchannel floodplain environment with seasonal ephemeral stream channels. The climate is thought to have been hot and dry with seasonal rainfall. This is consistent with the preservation state of the fossils, as the remains of ''Yarasuchus'' were found dismembered and disarticulated, suggesting the material was left exposed at the surface for a period before being buried by suspended fluvial sediments. There are few plant fossils known from the Yerrapalli Formation, however this is not believed to be due to it being an arid environment, but rather due to the hot and dry conditions being unsuitable to the fossilisation of plant material.


References


External links


''Yarasuchus''
in the
Paleobiology Database The Paleobiology Database is an online resource for information on the distribution and classification of fossil animals, plants, and microorganisms. History The Paleobiology Database (PBDB) originated in the NCEAS-funded Phanerozoic Marine Pale ...
{{Taxonbar, from=Q28647753 Middle Triassic archosaurs Prehistoric reptile genera Anisian life Middle Triassic reptiles of Asia Triassic India Fossils of India Fossil taxa described in 2005 Prehistoric avemetatarsalians