Typothorax

{{Short description|Extinct genus of reptiles}} {{Automatic taxobox | fossil_range = [[Late Triassic]], {{fossilrange|216|203}} | image = Typothorax coccinarum skeleton.jpg | image_upright = | image_caption = ''T. coccinarum'' skeleton | taxon = Typothorax | authority = [[Edward Drinker Cope|Cope]], 1875 | subdivision_ranks = Species | subdivision = * {{extinct}}''T. coccinarum'' Cope, 1875 ([[Type species|type]]) * {{extinct}}''T. antiquum'' Lucas ''et al.'', 2002 | synonyms = * ''Episcoposaurus'' Cope, 1887 * ''Episcoposaurus horridus'' Cope, 1887 }}

'''''Typothorax''''' is an [[extinct]] [[genus]] of [[typothoracine]] [[aetosaur]] that lived in the [[Late Triassic]]. Its remains have been found in [[North America]]. Two species are known: ''T. coccinarum'', the [[type species]], and ''T. antiquum''.

==Description== [[Image:Typothorax coccinarum scale.svg|thumb|left|Size of ''Typothorax coccinarum'' relative to a human.]] ''Typothorax'' was an [[aetosaur]], a [[pseudosuchian]] distantly related to modern crocodilians. Unlike modern crocodilians, aetosaurs were herbivorous. ''Typothorax'' and other aetosaurs possess small, leaf-shaped teeth that were unsuited for a diet consisting of meat.<ref name="typomorph">Martz, J.W. 2002. The morphology and ontogeny of Typothorax coccinarum (Archosauria, Stagonolepididae) from the Upper Triassic of the American southwest. M.S. thesis, Geosciences, Texas Tech University, Lubbock, 279 pp.</ref><ref name="heckertandlucas">{{cite journal | last1 = Heckert | first1 = A. B. | last2 = Lucas | first2 = S. G. | year = 1999 | title = A new aetosaur (Reptilia: Archosauria) from the Upper Triassic of Texas and the phylogeny of aetosaurs | journal = Journal of Vertebrate Paleontology | volume = 19 | issue = 1| pages = 50–68 | doi=10.1080/02724634.1999.10011122| citeseerx = 10.1.1.563.9516 }}</ref> Unlike some aetosaurs such as ''[[Desmatosuchus]]'', ''Typothorax'' does not have large shoulder spikes.<ref name="typomorph" /><ref name="heckertandlucas" /> It does, however, have a pair of enlarged spikes on the neck projecting from the third row of scutes. It has lateral scutes that bear horns that are posteriorly hooked along its back, while its sides and underbelly are covered with ornamented scutes. Although [[fossil]]s of aetosaurs are not as common as other Triassic [[archosaurs]], with their armor plates being the most common, ''Typothorax'' has been represented by fewer skeletal elements than other aetosaurs. The largest known specimen of ''T. coccinarum'', described in 2023, would have been similar in size to ''Desmatosuchus'', measuring over {{convert|5|m|ft}} long, though it has been identified as a skeletally immature individual (16 to 19 years old) based on the open neurocentral suture and the lines of arrested growth. However, the authors warn that, since smaller individuals have co-ossified neurocentral sutures, they may not be a reliable factor in determining maturity in aetosaurs. So this individual still may be an adult.<ref>{{cite journal |last1=Parker |first1=W. G. |last2=Reyes |first2=W. A. |last3=Marsh |first3=A. D. |year=2023 |title=Incongruent ontogenetic maturity indicators in a Late Triassic archosaur (Aetosauria: ''Typothorax coccinarum'') |journal=The Anatomical Record |doi=10.1002/ar.25343 |pmid=37937738 }}</ref>

The vertebral column of ''Typothorax'' is shortened, with individual vertebrae being reduced in length. However, the [[osteoderm]]s that overly the vertebrae are not shortened. Instead, they are reduced in number so that each dorsal paramedian osteoderm (osteoderm that covers the back) overlies several dorsal vertebrae. In nearly all other crurotarsans, there is one row of osteoderms per vertebra. ''T. coccinarum'' has around 20 rows of presacral osteoderms and about 26 presacral vertebrae. If the cervical spikes of ''Typothorax'' are [[Homology (biology)|homologous]] to those of ''Desmatosuchus'', it is likely that rows of osteoderms were removed from the front. This is because in ''Desmatosuchus'' the spikes are present in the fifth row, while in ''Typothorax'' they are present in the third.<ref name=Hetal10>{{cite journal |last=Heckert |first=A.B. |author2=Lucas, S.G. |author3=Rinehart, L.F. |author4=Celesky, M.D. |author5=Spielmann, J.A. |author6= Hunt, A.P. |year=2010 |title=Articulated skeletons of the aetosaur ''Typothorax coccinarum'' Cope (Archosauria: Stagonolepididae) from the Upper Triassic Bull Canyon Formation (Revueltian: early-mid Norian), eastern New Mexico, USA |journal=Journal of Vertebrate Paleontology |volume=30 |issue=3 |pages=619–642 |doi=10.1080/02724631003763524 |s2cid=140536594 |url=http://pdfserve.informaworld.com/852191__922419027.pdf}}</ref>

In ''T. coccinarum'', there are 10 [[thoracic]] columns and four [[tail|caudal]] columns of ventral osteoderms on the underside. Unlike all other aetosaurs, ''Typothorax'' possesses spiked osteoderms on the underside of the tail, near the [[cloaca]]. This region is not covered by osteoderms to the extent that it is in other aetosaurs such as ''[[Aetosaurus]]'' and ''[[Coahomasuchus]]''.

==History== ''Typothorax'' was one of the earliest vertebrates named from the [[Triassic]] of western North America<ref name="geologynm">Cope, E.D. 1875. The geology of New Mexico. Philadelphia Academy of Natural Sciences Proceedings, pp. 263-267.</ref> but has been poorly understood since that naming. [[Paleontologist]]s have found the characteristic armor plates for decades,<ref name="case22">Case, E.C. 1922. New reptiles and stegocephalians from the Upper Triassic of western Texas. Carnegie Institution of Washington Publication, no. 321 (October, 1922). Carnegie Institution of Washington, Washington D.C., 84 pp.</ref><ref name="longandmurray">Long, R.A., and Murry P.A. 1995. Late Triassic (Carnian and Norian) tetrapods from the southwestern United States. New Mexico Museum of Natural History and Science Bulletin 4, 254 pp.</ref> but only recently has a comprehensive study of ''Typothorax'' appeared.<ref name="typomorph" /><ref name="heckertandlucas" /> It has been found in [[Arizona]], in the [[Chinle Formation]], as well as in [[New Mexico]] and [[Texas]] in the [[Bull Canyon Formation]] of the [[Dockum Group]].

==Paleobiology== [[File:Typothorax coccinarum.jpg|thumb|Life restoration of two ''T. coccinarum'']] Like all aetosaurs and many other early crurotarsans, ''Typothorax'' had erect hindlimbs held beneath the body. This is evident by a straight [[femur]], an anteriorly directed [[Pes (anatomy)|pes]] (foot), and the projection of the lateral surface of the [[ilium (bone)|ilium]] over the femur. Because the length of the femur is almost equal to that of the tibia and fibula (lower leg) and [[Talus bone|astragalus]] and [[calcaneum]] (ankle), ''Typothorax'' was probably slow-moving. The forelimbs are reduced in size and were directed outward in a sprawling position. This posture is also seen in [[ankylosaur]]s, [[ceratopsia]]ns, and the early [[cynodont]] ''[[Procynosuchus]]''.

Several aspects of the forelimbs have been interpreted as adaptations to digging. Like many digging [[tetrapod]]s, the [[Radius (bone)|radius]] is significantly shorter than the [[humerus]]. Like other aetosaurs, there is a prominent [[deltopectoral crest]] on the humerus. The [[manus (zoology)|manus]] is short and wide, a characteristic of digging animals. There is also an entepicondyle on the humerus, which is the origin of forearm [[pronation|pronator]] and manual flexor muscles. ''Typothorax'' also possesses a relatively long [[olecranon process]] for the insertion of the ''[[Triceps|M. triceps]]'' muscle, but not as long as those of digging animals. Heckert ''et al.'' (2010) concluded that ''Typothorax'' was not specifically adapted for a [[fossorial]] lifestyle but had an ability to dig that other aetosaurs didn't have.<ref name=Hetal10/> The upturned snout of ''Typothorax'' and other aetosaurs suggests that they may have searched for food by rooting around in soil.

==Classification== ''Typothorax'' belongs to a [[clade]] of aetosaurs called the [[Typothoracinae]], within the [[subfamily]] [[Aetosaurinae]]. It is closely related to ''[[Redondasuchus]]''. ''Redondasuchus reseri'' was first named in 1991, but was later [[synonym (taxonomy)|synonymized]] with ''T. coccinarum'' in 1995 on the basis of the similarity between the osteoderms used to describe the species and the anterior osteoderms of ''T. coccinarum''.<ref name="longandmurray"/><ref name=HL91>{{cite journal |last=Hunt |first=A.P. |author2=Lucas, S.G. |year=1991 |title=A new aetosaur from the Upper Triassic of eastern New Mexico |journal= Neues Jahrbuch für Geologie und Paläontologie - Monatshefte |volume=1991 |pages=728–736|doi=10.1127/njgpm/1991/1992/728 }}</ref> Martz (2002) suggested that ''R. reseri'' belongs to ''Typothorax'', but is still its own species, called ''T. reseri''.<ref name="typomorph"/> More recently, the distinctiveness of ''Redondasuchus'' from ''Typothorax'' has been supported with the description of a new species, ''R. rineharti'', in 2006.<ref name=Setal06>{{cite journal |last=Spielmann |first=J.A. |author2=Hunt, A.P. |author3=Lucas, S.G. |author4= Heckert, A.B. |year=2006 |title=Revision of ''Redondasuchus'' (Archosauria: Aetosauria) from the Upper Triassic Redonda Formation, New Mexico, with description of a new species |journal=New Mexico Museum of Natural History and Science Bulletin |volume=37 |pages=583–587 |url=http://www.miketaylor.org.uk/dino/nm/SpielmannEtAl2006-revision-of-redondasuchus.pdf}}</ref> With new skeletal material from ''T. coccinarum'', Heckert ''et al.'' (2010) claim that ''Redondasuchus'' is distinct from ''Typothorax'' because it has strongly flexed paramedian osteoderms, while ''Typothorax'' has more gently arching paramedian osteoderms.<ref name=Hetal10/>

Parker (2013) redescribed the [[holotype]] of the type species of ''Typothorax'', ''T. coccinarum''. It was found to be too fragmentary to be diagnosable and therefore represents a ''[[nomen dubium]]''. ''[[Episcoposaurus]] horridus'', the type species of its genus usually considered to be a [[junior synonym]] of ''T. coccinarum'', is the next available name for material referred to ''T. coccinarum''. Therefore, other species of ''Typothorax'' as well as material referred to ''T. coccinarum'', can be reassigned to ''Episcoposaurus''. Nevertheless, to preserve taxonomic stability Parker (2013) suggested to attempt to petition for a [[neotype]] for ''T. coccinarum'' from well preserved specimens which have been previously referred to it.<ref name=TypothoraxRev>{{Cite journal | last1 = Parker | first1 = W. G. | title = Redescription and taxonomic status of specimens of ''Episcoposaurus'' and ''Typothorax'', the earliest known aetosaurs (Archosauria: Suchia) from the Upper Triassic of western North America, and the problem of proxy "holotypes" | doi = 10.1017/S1755691013000212 | journal = Earth and Environmental Science Transactions of the Royal Society of Edinburgh | volume = 103 | issue = 3–4 | pages = 313–338 | year = 2013 | s2cid = 130822568 }}</ref>

==References== {{Reflist}}

==External links== *[https://paleobiodb.org/classic/checkTaxonInfo?taxon_no=64792 ''Typothorax''] in the [[Paleobiology Database]] *Jeff Martz's [http://etd.lib.ttu.edu/theses/available/etd-07312008-31295017084384/unrestricted/31295017084384.pdf Master's Thesis] from [[Texas Tech University]].

[[Category:Aetosaurs of North America]] [[Category:Chinle fauna]] [[Category:Prehistoric pseudosuchian genera]] [[Category:Taxa named by Edward Drinker Cope]]