Yarasuchus: Difference between revisions

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'''''Yarasuchus''''' (meaning "red crocodile") is an [[extinct]] [[genus]] of [[avemetatarsalia]]n [[archosaur]] that lived during the [[Anisian]] stage of the [[Middle Triassic]] of [[India]].<ref name=BS99>Bandyopadhyay, S. and Sengupta, D. P. (1999). Middle Triassic vertebrate faunas from India. ''[[Journal of African Earth Sciences]]'' '''29''': 233–241.</ref> 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.<ref name="Sen, 2005">{{cite journal |last1=Sen |first1=Kasturi |title=A new rauisuchian archosaur from the Middle Triassic of India. |journal=[[Palaeontology (journal)|Palaeontology]] |volume=48 |issue=1 |date=2005 |doi=10.1111/j.1475-4983.2004.00438.x |pages=185–196 |bibcode=2005Palgy..48..185S |doi-access=free }}</ref> The [[type species|type]] and only [[species]] is '''''Y. deccanensis'''''. ''Yarasuchus'' was a quadruped roughly {{Convert|2–2.5|m|feet}} 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]]ns.

''Yarasuchus'' has had a complicated taxonomic history, after originally being described as a "[[Prestosuchidae|prestosuchid]] [[rauisuchia]]n", it was later variously recovered as a [[Poposauroidea|poposauroid]] [[pseudosuchia]]n and a non-archosaurian [[Archosauriformes|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 [[dinosaur]]s and [[pterosaur]]s.<ref name="Teleo">{{cite journal |last1=Nesbitt |first1=S.J. |last2=Butler |first2=R.J. |last3=Ezcurra |first3=M.D. |last4=Barrett |first4=P.M. |last5=Stocker |first5=M.R. |last6=Angielczyk |first6=K.D. |last7=Smith |first7=R.M H. |last8=Sidor |first8=C.A. |last9=Niedźwiedzki |first9=G. |last10=Sennikov |first10=A.G. |last11=Charig |first11=A.J. |title=The earliest bird-line archosaurs and the assembly of the dinosaur body plan |journal=Nature |volume=544 |issue=7651 |pages=484–487 |date=2017 |doi=10.1038/nature22037 |pmid=28405026 |bibcode=2017Natur.544..484N |s2cid=9095072 |url=http://pure-oai.bham.ac.uk/ws/files/39495711/Nesbitt_et_al._in_press.pdf }}</ref>

Almost the whole [[vertebral column]] is represented in ''Yarasuchus'', including at least 8 [[Cervical vertebrae|cervicals]] (including the [[Atlas (anatomy)|atlas]] and [[Axis (anatomy)|axis]]), 17 [[Thoracic vertebrae|dorsals]], 2 [[Sacral vertebra|sacral]], and at least 11 proximal [[Caudal vertebra|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 [[Vertebral centrum|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.<ref name=NJKP07>{{cite journal |last1=Jalil |first1=Nour-Eddine |last2=Peyer |first2=Karin |year=2007 |title=A new rauisuchian (Archosauria, Suchia) from the Upper Triassic of the Argana Basin, Morocco |journal=[[Palaeontology (journal)|Palaeontology]] |volume=50 |issue=2 |pages=417–430 |doi=10.1111/j.1475-4983.2007.00640.x|bibcode=2007Palgy..50..417J |doi-access=free }}</ref> The posterior [[cervical rib]]s have three heads, an unusual condition in archosaurs, and the corresponding cervicals possess an accessory articular facet to accommodate this.<ref name="Sen, 2005"/><ref name=Teleo/><ref name=":0"/><ref name=Teleo2/>

The [[Pectoral girdle|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 [[Silesauridae|silesaurids]].<ref name="Teleo" /> The [[coracoid]] is small and rounded, and forms a down and rearward facing glenoid fossa with the scapula. The [[Ilium (bone)|ilium]] of the [[Pelvic girdle|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 (bone)|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.<ref name="Teleo2" /> 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.<ref name="Sen, 2005" /><ref name=":0" />

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 calcanuescalcaneus 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.<ref name="Sen, 2005" /><ref name="Teleo" /><ref name=":0" />

''Yarasuchus'' is known from at least two individuals collected from a single {{convert|1|m2|sqft}} 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 (bone)|radius]], [[fibula]], 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 (disambiguation)|Deccan region]] of India, where the Yerrapalli Formation and surrounding [[Pranhita-Godavari Basin]] are situated.<ref name="Sen, 2005"/> The fossils were also found in association with two specimens of the allokotosaur ''[[Pamelaria]].''<ref name="Sen, 2005" /><ref name=":0" /><ref name="SK03" /> It was initially reported on in a 1993 thesis by Dasgupta, who considered it an indeterminate [[Rauisuchidae|rauisuchid]],<ref name=":0">{{Cite journal|last=Kasturi|first=Dasgupta|date=1993|title=New reptiles diapsida archosauromorpha from the triassic yerrapalli formation Deccan India their importance in geology and palaeontology|journal=University|hdl=10603/159402?mode=simple|url=http://shodhganga.inflibnet.ac.in/handle/10603/159402?mode=simple}}</ref> before being officially described and named as a new taxon by Sen in 2005.<ref name="Sen, 2005" /> All the material is held at the Geology Museum of the [[Indian Statistical Institute]] in [[Kolkata]], India.<ref name="Sen, 2005"/><ref name=Teleo/><ref name=":0"/>

''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".<ref name="Sen, 2005"/> In 2010, [[Stephen L. Brusatte|Brusatte]] and colleagues conclusively demonstrated that "Prestosuchidae" was a [[paraphyletic]] [[evolutionary grade|grade]] of [[paracrocodylomorpha|paracrocodylomorphs]], and that the supposed shared characteristics of the group were in fact found throughout [[Pseudosuchia]]. They also performed a detailed [[Cladistics|cladistic]] analysis of fossil Triassic archosaurs, which instead found ''Yarasuchus'' to be a basal member of [[Poposauroidea]], although support for this position was weak.<ref name=Betal10>{{cite journal |last=Brusatte |first=S.L. |author2=Benton, M.J. |author3=Desojo, J.B. |author4= Langer, M.C. |year=2010 |title=The higher-level phylogeny of Archosauria (Tetrapoda: Diapsida) |journal=Journal of Systematic Palaeontology |volume=8 |issue=1 |pages=3–47 |doi=10.1080/14772010903537732 |bibcode=2010JSPal...8....3B |hdl=11336/69099 |s2cid=59148006 |url=https://www.pure.ed.ac.uk/ws/files/8232155/PDF_Brusatteetal2010ArchosaurPhylogeny.pdf }}</ref>

The position of ''Yarasuchus'' in subsequent studies has been similarly unstable,<ref name=Butleretal11>{{Cite journal|last1=Butler |first1=R.J. |last2=Brusatte |first2=S.L. |last3=Reich |first3=M. |last4=Nesbitt |first4=S.J. |last5=Schoch |first5=R.R. |last6=Hornung |first6=J.J. |year=2011 |title=The sail-backed reptile ''Ctenosauriscus'' from the latest Early Triassic of Germany and the timing and biogeography of the early archosaur radiation |journal=PLOS ONE |volume=6 |issue=10 |pages=e25693 |doi=10.1371/journal.pone.0025693 |pmid=22022431 |pmc=3194824|bibcode=2011PLoSO...625693B |doi-access=free }}</ref><ref name=Decuriasuchus>{{Cite journal|author1=Marco Aurélio G. França |author2=Jorge Ferigolo |author3=Max C. Langer |year=2011 |title=Associated skeletons of a new middle Triassic "Rauisuchia" from Brazil |journal=Naturwissenschaften |volume=98 |issue=5 |pages=389–395 |doi=10.1007/s00114-011-0782-3 |pmid=21445632|bibcode=2011NW.....98..389F |s2cid=4811617 }}</ref><ref name=Nundasuchus>{{cite journal |last1=Nesbitt |first1=Sterling J.|last2=Sidor |first2=Christian A. |last3=Angielczyk |first3=Kenneth D. |last4=Smith |first4=Roger M. H. |last5=Tsuji |first5=Linda A. |date=November 2014 |title=A new archosaur from the Manda beds (Anisian, Middle Triassic) of southern Tanzania and its implications for character state optimizations at Archosauria and Pseudosuchia |journal=Journal of Vertebrate Paleontology |volume=34 |issue=6 |pages=1357–1382 |doi=10.1080/02724634.2014.859622 |bibcode=2014JVPal..34.1357N |s2cid=129558756}}</ref> though it has nonetheless typically been regarded as a pseudosuchian.<ref name=Desojo09>{{cite journal |last1=Desojo |first1=J.B. |last2=Arcucci |first2=A.B. |year=2009 |title=New material of ''Luperosuchus fractus'' (Archosauria: Crurotarsi) from the Middle Triassic of Argentina: the earliest known South American "rauisuchian" |journal=Journal of Vertebrate Paleontology |volume=29 |issue=4 |pages=1311–1315 |doi=10.1671/039.029.0422|bibcode=2009JVPal..29.1311D |s2cid=132648252 }}</ref><ref name=Rauisuchus>{{cite journal |last1=Lautenschlager |first1=S. |last2=Rauhut |first2=O.W.M. |title=Osteology of Rauisuchus tiradentes from the Late Triassic (Carnian) Santa Maria Formation of Brazil, and its implications for rauisuchid anatomy and phylogeny |year=2015 |journal=[[Zoological Journal of the Linnean Society]] |volume=173 |issue=1 |pages=55–91 |doi=10.1111/zoj.12196 |url=https://academic.oup.com/zoolinnean/article/173/1/55/2453081|doi-access=free }}</ref><ref name="Chatterjee2017">{{Cite book|url=https://books.google.com/books?id=0DI5DwAAQBAJ&lpg=PP1&pg=PA28|title=Indian Plate and Its Epic Voyage from Gondwana to Asia: Its Tectonic, Paleoclimatic, and Paleobiogeographic Evolution|last1=Chatterjee|first1=Sankar|last2=Scotese|first2=Christopher R.|last3=Bajpai|first3=Sunil|publisher=Geological Society of America|year=2017|isbn=978-0813725291|edition=Illustrated|volume=529|pages=28–29}}</ref> 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.<ref name=Ezcurra2016>{{Cite journal|last=Ezcurra |first=M.D. |date=2016 |title=The phylogenetic relationships of basal archosauromorphs, with an emphasis on the systematics of proterosuchian archosauriforms |journal=[[PeerJ]] |language=en |volume=4 |pages=e1778 |doi=10.7717/peerj.1778 |pmid=27162705 |pmc=4860341 |issn=2167-8359 |doi-access=free }}</ref> The left cladogram depicts the results of Brusatte and colleagues in 2010,<ref name=Betal10/> while the right cladogram depicts that of Ezcurra in 2016:<ref name=Ezcurra2016/>

Further complicating the issue were suggestions that the hypodigm of ''Yarasuchus'' was a chimeric assemblage of material from both a "rauisuchian" archosaur and a [[Prolacertiformes|prolacertiform]] archosauromorph, if not including material from yet other archosauromorphs.<ref name=Chimera>{{cite journal |last1=Lautenschlage |first1=S. |last2=Desojo |first2=J.B. |title=Reassessment of the Middle Triassic rauisuchian archosaurs ''Ticinosuchus ferox'' and ''Stagonosuchus nyassicus'' |journal=[[Paläontologische Zeitschrift]] |year=2011 |volume=85 |issue=4 |pages=357–381 |doi=10.1007/s12542-011-0105-1 |s2cid=86671911 |issn=1867-6812|hdl=11336/68929 |hdl-access=free }}</ref><ref name=Rauisuchia>{{cite journal|last1=Nesbitt |first1=S.J. |last2=Brusatte |first2=S.L. |last3=Desojo |first3=J.B. |last4=Liparini |first4=A. |last5=De França |first5=M.A.G. |last6=Weinbaum |first6=J.C. |last7=Gower |first7=D.J. |title=Rauisuchia |journal=[[Geological Society, London, Special Publications]] |volume=379 |issue=1 |pages=241–274 |year=2013 |doi=10.1144/SP379.1 |bibcode=2013GSLSP.379..241N |s2cid=219193351 |url=http://sp.lyellcollection.org/content/379/1/241}}</ref> 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-[[suchia]]n archosauriform taxon.<ref name=Ezcurra2016/>

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)<ref name="NSJ11">{{cite journal |last=Nesbitt |first=S.J. |year=2011 |title=The early evolution of archosaurs: relationships and the origin of major clades |url=http://digitallibrary.amnh.org/dspace/bitstream/2246/6112/1/B352.pdf |journal=Bulletin of the American Museum of Natural History |volume=352 |pages=1–292 |doi=10.1206/352.1|hdl=2246/6112 |s2cid=83493714 |doi-access=free}}</ref> and Ezcurra (2016).<ref name="Ezcurra2016" /> 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 ''[[House sparrow|Passer domesticus]]'' or ''[[Nile crocodile|Crocodylus niloticus]]"''. Aphanosaurs were found to be the earliest diverging clade of [[Avemetatarsalia]], sister taxon to the clade [[Ornithodira]] that includes the [[Pterosauria]] and [[Dinosauromorpha]].<ref name="Teleo"/> The results of their analyses are reproduced and simplified below, combining the general topology of the Ezcurra dataset with the Nesbitt datsetdataset results for Avemetatarsalia.

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]]. ''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.<ref name="Teleo" /> 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.<ref>{{Cite journal|last=Cau|first=Andrea|date=2018|title=The assembly of the avian body plan: a 160-million-year long process|url=http://paleoitalia.org/media/u/archives/01_Cau_2018_BSPI_571.pdf|journal=Bollettino della Società Paleontologica Italiana|volume=57|issue=1|pages=1–25|doi=10.4435/BSPI.2018.01|via=}}</ref>

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.<ref name="Teleo2"/> The convergence between ''Yarasuchus'' and poposauroids could be attributed to the broader trend of poposauroids converging on [[coelurosaur]]ian [[theropods]], a derived clade of avemetatarsalians.<ref name="Nesbitt2007">{{Cite journal |last=Nesbitt |first=Sterling J. |date=2007 |title=The anatomy of ''Effigia okeeffeae'' (Archosauria, Suchia), theropod-like convergence, and the distribution of related taxa. |journal=[[Bulletin of the American Museum of Natural History]] |volume=302 |issue=302 |pages=1–84 |doi=10.1206/0003-0090(2007)302[1:TAOEOA]2.0.CO;2|hdl=2246/5840 |s2cid=55677195 }}</ref> 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]] ranges broadly overlap with each other.<ref name="Teleo2" />

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''.<ref name="SK03">Sen, K. (2003). ''Pamelaria dolichotrachela'', a new prolacertid reptile from the Middle Triassic of India. ''Journal of Asian Earth Sciences'' '''21''': 663–681.</ref> Other vertebrate remains include those of the [[lungfish]] ''[[Ceratodus (genus)|Ceratodus]]'', the [[Actinopterygii|actinopterygian]] fish ''[[Saurichthys]]'', the [[temnospondyl]] ''[[Parotosuchus]]'', the [[dicynodont]]s ''[[Rechnisaurus]]'' and ''[[Wadiasaurus]]'', the [[rhynchosaur]] ''[[Mesodapedon]]'', and a large undescribed erythrosuchid.<ref name="BS99"/><ref name=":0" /><ref name="Chatterjee2017"/>