Typothorax

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Typothorax
Temporal range: Late Triassic, 216–203  Ma
Typothorax coccinarum skeleton.jpg
T. coccinarum skeleton
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Clade: Archosauria
Clade: Pseudosuchia
Order:† Aetosauria
Family:† Stagonolepididae
Subfamily:† Typothoracinae
Genus:† Typothorax
Cope, 1875
Species
  • †T. coccinarumCope, 1875 (type)
  • †T. antiquumLucas et al., 2002
Synonyms
  • EpiscoposaurusCope, 1887
  • Episcoposaurus horridusCope, 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.

Contents

Description

Size of Typothorax coccinarum relative to a human. Typothorax coccinarum scale.svg
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. [1] [2] Unlike some aetosaurs such as Desmatosuchus , Typothorax does not have large shoulder spikes. [1] [2] 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 fossils 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 5 metres (16 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. [3]

The vertebral column of Typothorax is shortened, with individual vertebrae being reduced in length. However, the osteoderms 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 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. [4]

In T. coccinarum, there are 10 thoracic columns and four 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 [5] but has been poorly understood since that naming. Paleontologists have found the characteristic armor plates for decades, [6] [7] but only recently has a comprehensive study of Typothorax appeared. [1] [2] 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

Life restoration of two T. coccinarum Typothorax coccinarum.jpg
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 (foot), and the projection of the lateral surface of the ilium over the femur. Because the length of the femur is almost equal to that of the tibia and fibula (lower leg) and 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 ankylosaurs, ceratopsians, and the early cynodont Procynosuchus .

Several aspects of the forelimbs have been interpreted as adaptations to digging. Like many digging tetrapods, the radius is significantly shorter than the humerus. Like other aetosaurs, there is a prominent deltopectoral crest on the humerus. The manus is short and wide, a characteristic of digging animals. There is also an entepicondyle on the humerus, which is the origin of forearm pronator and manual flexor muscles. Typothorax also possesses a relatively long olecranon process for the insertion of the 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. [4] 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 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. [7] [8] Martz (2002) suggested that R. reseri belongs to Typothorax, but is still its own species, called T. reseri. [1] More recently, the distinctiveness of Redondasuchus from Typothorax has been supported with the description of a new species, R. rineharti, in 2006. [9] 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. [4]

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. [10]

Related Research Articles

<span class="mw-page-title-main">Aetosaur</span> Extinct order of heavily armoured reptiles

Aetosaurs are heavily armored reptiles belonging to the extinct order Aetosauria. They were medium- to large-sized omnivorous or herbivorous pseudosuchians, part of the branch of archosaurs more closely related to crocodilians than to birds and other dinosaurs. All known aetosaurs are restricted to the Late Triassic, and in some strata from this time they are among the most abundant fossil vertebrates. They have small heads, upturned snouts, erect limbs, and a body ornamented with four rows of plate-like osteoderms. Aetosaur fossil remains are known from Europe, North and South America, parts of Africa, and India. Since their armoured plates are often preserved and are abundant in certain localities, aetosaurs serve as important Late Triassic tetrapod index fossils. Many aetosaurs had wide geographic ranges, but their stratigraphic ranges were relatively short. Therefore, the presence of particular aetosaurs can accurately date a site in which they are found.

<i>Aetosaurus</i> Extinct genus of reptiles

Aetosaurus is an extinct genus of pseudosuchian reptile belonging to the order Aetosauria. It is generally considered to be the most primitive aetosaur. Three species are currently recognized: A. ferratus, the type species from Germany and Italy; A. crassicauda from Germany; and A. arcuatus from eastern North America. Additional specimens referred to Aetosaurus have been found in the Chinle Group of the southwestern United States, and the Fleming Fjord Formation of Greenland. Specimens of Aetosaurus occur in Norian-age strata.

<i>Desmatosuchus</i> Extinct genus of reptiles

Desmatosuchus is an extinct genus of archosaur belonging to the Order Aetosauria. It lived during the Late Triassic.

Calyptosuchus is an extinct genus of aetosaur from the Late Triassic of North America. Like other aetosaurs, it was heavily armored and had a pig-like snout used to uproot plants.

<i>Acaenasuchus</i> Genus of reptiles

Acaenasuchus is an extinct genus of pseudosuchian, endemic to what would be presently be known as Arizona during the Late Triassic, specifically during the Carnian and Norian stages of the Triassic. Acaenasuchus had a stratigraphic range of approximately 11.5 million years. Acaenasuchus is further categorized as one of the type fauna that belong to the Adamanian LVF, based on the fauna of the Blue Mesa Member of the Chinle Petrified Forest Formation of Arizona, where Acaenasuchus was initially discovered.

<i>Aetosauroides</i> Extinct genus of reptiles

Aetosauroides is an extinct genus of aetosaur from the Late Triassic of South America. It is one of four aetosaurs known from South America, the others being Neoaetosauroides, Chilenosuchus and Aetobarbakinoides. Three species have been named: the type species A. scagliai, A. subsulcatus and A. inhamandensis. Fossils have been found in the Cancha de Bochas Member of the Ischigualasto Formation in the Ischigualasto-Villa Unión Basin in northwestern Argentina and the Santa Maria Formation in the Paraná Basin in southeastern Brazil. The strata date to the late Carnian and early Norian stages, making Aetosauroides one of the oldest aetosaurs.

Lucasuchus is an extinct genus of aetosaur. Fossils have been found from the Bull Canyon Formation of the Dockum Group outcropping in the Revuelto Creek locality in Quay County, New Mexico. All specimens date back to the Norian stage of the Late Triassic. The genus was named in 1995 after the American paleontologist Spencer G. Lucas.

<i>Paratypothorax</i> Genus of reptiles

Paratypothorax is an extinct genus of aetosaur, known from a single species, Paratypothorax andressorum. It was a broadly distributed member of the group found in Germany, North America, and possibly parts of Gondwana. The best specimens come from Germany, though for more than a century they were mistakenly considered phytosaur armor. Paratypothorax was a large and wide-bodied typothoracine aetosaur, as well as the namesake of the tribe Paratypothoracisini.

Tecovasuchus is an extinct genus of aetosaur. It is known primarily from osteoderms found from the Tecovas Formation in Texas, which is Late Triassic in age, dating back to the lower Norian. Material is also known from several other localities of the Chinle Group in New Mexico and Arizona, such as older Carnian outcrops and younger Rhaetian outcrops. Specimens of Tecovasuchus have been collected from the Tecovas Formation, the Bluewater Creek Formation, and the Los Esteros Member of the Santa Rosa Formation.

<span class="mw-page-title-main">Typothoracinae</span> Extinct clade of reptiles

Typothoracinae is a clade of aetosaurs within the subfamily Aetosaurinae. It was originally defined as a stem-based taxon including all aetosaurs closer to Typothorax than to Stagonolepis or Desmatosuchus. This definition was later expanded to specifically exclude Aetosaurus; as of 2016, Typothoracinae is defined as the least inclusive clade containing Typothorax and Paratypothorax, but not Aetosaurus,Stagonolepis, or Desmatosuchus. The clade was first named in 2007 under the spelling Typothoracisinae, after its namesake Typothorax. However, this spelling was based on incorrect taxonomic nomenclature, and the clade's name was corrected to Typothoracinae in 2016.

<span class="mw-page-title-main">Aetosaurinae</span> Extinct subfamily of reptiles

Aetosaurinae is one of the two main clades of aetosaurs, the other being Desmatosuchia. It is a stem-based taxon defined as all aetosaurs more closely related to Aetosaurus than Desmatosuchus. Aetosaurinae currently comprises Aetosaurus, similar forms such as Coahomasuchus and Stenomyti, and the widespread and successful aetosaur clade Typothoracinae.

<span class="mw-page-title-main">Paratypothoracini</span> Extinct clade of reptiles

Paratypothoracini is a clade of aetosaurs within the group Typothoracinae. It is a node-based taxon that includes Rioarribasuchus (=Heliocanthus), Paratypothorax, Tecovasuchus, and all descendants of their most recent common ancestor. The clade was first named in 2007 under the spelling Paraypothoracisini, after its namesake Paratypothorax. However, this spelling was based on incorrect taxonomic nomenclature, and the clade's name was corrected to Paratypothoracinae in 2016.

<span class="mw-page-title-main">Desmatosuchinae</span> Subfamily of reptiles

Desmatosuchinae is a major subfamily of aetosaurs within the clade Desmatosuchia. It is a stem-based taxon defined as all aetosaurs more closely related to Desmatosuchus than to Stagonolepis,Aetosaurus, or Paratypothorax.

Redondasuchus is an extinct genus of aetosaur. It may be a junior synonym of Typothorax coccinarum, another aetosaur. Redondasuchus is a member of the clade Typothoracisinae within the subfamily Aetosaurinae, and lived during the middle Norian stage of the Late Triassic. Material belonging to the genus has been found from the Redonda Formation in east-central New Mexico. The type species, R. reseri, was named in 1991 after having been referred to as a species of Typothorax since 1985. A second species, R. rineharti, was described in 2006.

Aetobarbakinoides is an extinct genus of stagonolepidid aetosaur known from the Late Triassic of Rio Grande do Sul state, southern Brazil. Fossils have been found from the Santa Maria Supersequence of the late Carnian and early Norian stages, making Aetobarbakinoides one of the oldest aetosaurs. The type species, A. brasiliensis, was named in 2012, and is notable for being described primarily by features of the vertebrae; most other aetosaurs are diagnosed by features in bony plates called osteoderms, which are by far the most common material. Although placed in a basal phylogenetic position among aetosaurs, Aetobarbakinoides is closely related to both desmatosuchines and typothoracisines, two derived clades of aetosaurs.

Gorgetosuchus is an extinct genus of aetosaur from the Late Triassic of the North Carolina, represented by the type species Gorgetosuchus pekinensis. It is mainly known from osteoderms, including the front half of an articulated carapace. Gorgotesuchus is typically considered a basal desmatosuchin, though alternative interpretations exist.

<i>Scutarx</i> Extinct genus of reptiles

Scutarx is an extinct genus of Aetosauriformes, most commonly regarded by its species name Scutarx deltatylus. Scutarx lived around 230 million years ago during the Carnian and Norian stage of the Late Triassic. Scutarx are “medium sized” paramedian osteoderms belonging to the clade Aetosauria, a heavily armored and more herbivorous cousin of crocodiles.

Apachesuchus is an extinct genus of aetosaur from the Late Triassic of New Mexico. It is only known from several paramedian osteoderms discovered in Quay County in eastern New Mexico. This area belongs to the late Norian-age Quay Member of the Redonda Formation. Unique among aetosaurs, its osteoderms are nearly completely smooth, without strong pits or grooves. The left dorsal paramedian has a relatively high width-to-length ration, suggesting that Apachesuchus is a wide-bodied aetosaur within the clade Typothoracinae.

Kryphioparma is an extinct genus of aetosaur from the Late Triassic Blue Mesa Member of the Chinle Formation, Arizona. It is the oldest known member of the subfamily Typothoracinae, and is currently only known from five isolated and incomplete dorsal osteoderms. Regardless, said osteoderms show a clear mix of features that do not match any other known aetosaur and were thus used as the basis for a new genus and species in 2023. The genus is monotypic, only including a single species, Kryphioparma caerula.

Garzapelta is an extinct genus of aetosaur from the Late Triassic Cooper Canyon Formation containing a single species, G. muelleri. Garzapelta is known primarily from an associated collection of osteoderms, although some other bones such as ribs are also known. The anatomy of Garzapelta's armour displays a mix of features otherwise seen in Rioarribasuchus chamaensis, a member of the Paratypothoracini, and taxa of the subfamily Desmatosuchinae. This mix of characters is so distinct that phylogenetic analysis yielded different results based on what parts of the osteoderms were used, suggesting that the current dataset does not account for convergent evolution in osteoderm anatomy. Reyes, Martz and Small suggest that Garzapelta was likely a paratypothoracin that simply evolved lateral osteoderms similar to those of desmatosuchins, reasoning that its armour does not articulate in the way seen in members of the latter group.

References

  1. 1 2 3 4 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.
  2. 1 2 3 Heckert, A. B.; Lucas, S. G. (1999). "A new aetosaur (Reptilia: Archosauria) from the Upper Triassic of Texas and the phylogeny of aetosaurs". Journal of Vertebrate Paleontology. 19 (1): 50–68. CiteSeerX   10.1.1.563.9516 . doi:10.1080/02724634.1999.10011122.
  3. ↑ Parker, W. G.; Reyes, W. A.; Marsh, A. D. (2023). "Incongruent ontogenetic maturity indicators in a Late Triassic archosaur (Aetosauria: Typothorax coccinarum)". The Anatomical Record. doi:10.1002/ar.25343. PMID   37937738.
  4. 1 2 3 Heckert, A.B.; Lucas, S.G.; Rinehart, L.F.; Celesky, M.D.; Spielmann, J.A.; Hunt, A.P. (2010). "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" (PDF). Journal of Vertebrate Paleontology. 30 (3): 619–642. doi:10.1080/02724631003763524. S2CID   140536594.
  5. ↑ Cope, E.D. 1875. The geology of New Mexico. Philadelphia Academy of Natural Sciences Proceedings, pp. 263-267.
  6. ↑ 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.
  7. 1 2 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.
  8. ↑ Hunt, A.P.; Lucas, S.G. (1991). "A new aetosaur from the Upper Triassic of eastern New Mexico". Neues Jahrbuch für Geologie und Paläontologie - Monatshefte. 1991: 728–736. doi:10.1127/njgpm/1991/1992/728.
  9. ↑ Spielmann, J.A.; Hunt, A.P.; Lucas, S.G.; Heckert, A.B. (2006). "Revision of Redondasuchus (Archosauria: Aetosauria) from the Upper Triassic Redonda Formation, New Mexico, with description of a new species" (PDF). New Mexico Museum of Natural History and Science Bulletin. 37: 583–587.
  10. ↑ Parker, W. G. (2013). "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"". Earth and Environmental Science Transactions of the Royal Society of Edinburgh. 103 (3–4): 313–338. doi:10.1017/S1755691013000212. S2CID   130822568.
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