Typothorax Coccinarum
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''Typothorax'' is an
extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...
genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...
of typothoracine
aetosaur Aetosaurs () are heavily armored reptiles belonging to the extinct order (biology), order Aetosauria (; from Ancient Greek, Greek, (aetos, "eagle") and (, "lizard")). They were medium- to large-sized Omnivore, omnivorous or Herbivore, herbivor ...
that lived in the
Late Triassic The Late Triassic is the third and final epoch (geology), epoch of the Triassic geologic time scale, Period in the geologic time scale, spanning the time between annum, Ma and Ma (million years ago). It is preceded by the Middle Triassic Epoch a ...
. Its remains have been found in
North America North America is a continent in the Northern Hemisphere, Northern and Western Hemisphere, Western hemispheres. North America is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South Ameri ...
. Two species are known: ''T. coccinarum'', the
type species In International_Code_of_Zoological_Nomenclature, zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the spe ...
, and ''T. antiquum''.


Description

''Typothorax'' was an
aetosaur Aetosaurs () are heavily armored reptiles belonging to the extinct order (biology), order Aetosauria (; from Ancient Greek, Greek, (aetos, "eagle") and (, "lizard")). They were medium- to large-sized Omnivore, omnivorous or Herbivore, herbivor ...
, a
pseudosuchian Pseudosuchia, from Ancient Greek ψεύδος (''pseúdos)'', meaning "false", and σούχος (''soúkhos''), meaning "crocodile" is one of two major divisions of Archosauria, including living crocodilians and all archosaurs more closely relat ...
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.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. Unlike some aetosaurs such as ''
Desmatosuchus ''Desmatosuchus'' (, from Greek language, Greek δεσμός ''desmos'' 'link' + σοῦχος ''soûkhos'' 'crocodile') is an extinct genus of archosaur belonging to the Order Aetosauria. It lived during the Late Triassic. Description ''Desma ...
'', ''Typothorax'' does not have large shoulder spikes. 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 A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserve ...
s of aetosaurs are not as common as other Triassic
archosaurs Archosauria () or archosaurs () is a clade of diapsid sauropsid tetrapods, with birds and crocodilians being the only extant taxon, extant representatives. Although broadly classified as reptiles, which traditionally exclude birds, the cladistics ...
, 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 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. The vertebral column of ''Typothorax'' is shortened, with individual vertebrae being reduced in length. However, the
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 (extinct amph ...
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 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. In ''T. coccinarum'', there are 10
thoracic The thorax (: thoraces or thoraxes) or chest is a part of the anatomy of mammals and other tetrapod animals located between the neck and the abdomen. In insects, crustaceans, and the extinct trilobites, the thorax is one of the three main ...
columns and four
caudal Caudal may refer to: Anatomy * Caudal (anatomical term) (from Latin ''cauda''; tail), used to describe how close something is to the trailing end of an organism * Caudal artery, the portion of the dorsal aorta of a vertebrate that passes into th ...
columns of ventral osteoderms on the underside. Unlike all other aetosaurs, ''Typothorax'' possesses spiked osteoderms on the underside of the tail, near the
cloaca A cloaca ( ), : cloacae ( or ), or vent, is the rear orifice that serves as the only opening for the digestive (rectum), reproductive, and urinary tracts (if present) of many vertebrate animals. All amphibians, reptiles, birds, cartilagin ...
. This region is not covered by osteoderms to the extent that it is in other aetosaurs such as ''
Aetosaurus ''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 a ...
'' and ''
Coahomasuchus ''Coahomasuchus'' is an extinct genus of aetosaurine aetosaur. Remains of the genus have been found from deposits in Texas and North Carolina that date to the Otischalkian faunachron (lower late Carnian) of the Late Triassic. It was small for a ...
''.


History

''Typothorax'' was one of the earliest vertebrates named from the
Triassic The Triassic ( ; sometimes symbolized 🝈) is a geologic period and system which spans 50.5 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.4 Mya. The Triassic is t ...
of western North AmericaCope, E.D. 1875. The geology of New Mexico. Philadelphia Academy of Natural Sciences Proceedings, pp. 263-267. but has been poorly understood since that naming.
Paleontologist Paleontology, also spelled as palaeontology or palæontology, is the scientific study of the life of the past, mainly but not exclusively through the study of fossils. Paleontologists use fossils as a means to classify organisms, measure geolo ...
s have found the characteristic armor plates for decades,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.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. but only recently has a comprehensive study of ''Typothorax'' appeared. It has been found in
Arizona Arizona is a U.S. state, state in the Southwestern United States, Southwestern region of the United States, sharing the Four Corners region of the western United States with Colorado, New Mexico, and Utah. It also borders Nevada to the nort ...
, in the
Chinle Formation The Chinle Formation is an Upper Triassic continental geological formation of fluvial, lacustrine, and palustrine to eolian deposits spread across the U.S. states of Nevada, Utah, northern Arizona, western New Mexico, and western Colorado. In ...
, as well as in
New Mexico New Mexico is a state in the Southwestern United States, Southwestern region of the United States. It is one of the Mountain States of the southern Rocky Mountains, sharing the Four Corners region with Utah, Colorado, and Arizona. It also ...
and
Texas Texas ( , ; or ) is the most populous U.S. state, state in the South Central United States, South Central region of the United States. It borders Louisiana to the east, Arkansas to the northeast, Oklahoma to the north, New Mexico to the we ...
in the
Bull Canyon Formation The Bull Canyon Formation is a geological Formation (geology), formation of Late Triassic (Norian) age in eastern New Mexico and the Texas Panhandle. It is one of several formations encompassed by the Dockum Group. The Bull Canyon Formation prese ...
of the
Dockum Group The Dockum is a Late Triassic (approximately late Carnian through Rhaetian, or 223–200 Ma) geologic group found primarily on the Llano Estacado of western Texas and eastern New Mexico with minor exposures in southwestern Kansas, eastern Color ...
.


Paleobiology

Like all aetosaurs and many other early crurotarsans, ''Typothorax'' had erect hindlimbs held beneath the body. This is evident by a straight
femur The femur (; : femurs or femora ), or thigh bone is the only long bone, bone in the thigh — the region of the lower limb between the hip and the knee. In many quadrupeds, four-legged animals the femur is the upper bone of the hindleg. The Femo ...
, 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 Astragalus may refer to: * ''Astragalus'' (plant), a large genus of herbs and small shrubs *Astragalus (bone) The talus (; Latin for ankle or ankle bone; : tali), talus bone, astragalus (), or ankle bone is one of the group of foot bones known ...
and
calcaneum In humans and many other primates, the calcaneus (; from the Latin ''calcaneus'' or ''calcaneum'', meaning heel; : calcanei or calcanea) or heel bone is a bone of the tarsus of the foot which constitutes the heel. In some other animals, it is t ...
(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 Ankylosauria is a group of herbivorous dinosaurs of the clade Ornithischia. It includes the great majority of dinosaurs with armor in the form of bony osteoderms, similar to turtles. Ankylosaurs were bulky quadrupeds, with short, powerful l ...
s,
ceratopsia Ceratopsia or Ceratopia ( or ; Ancient Greek, Greek: "horned faces") is a group of herbivore, herbivorous, beaked dinosaurs that thrived in what are now North America, Asia and Europe, during the Cretaceous Period (geology), Period, although ance ...
ns, and the early
cynodont Cynodontia () is a clade of eutheriodont therapsids that first appeared in the Late Permian (approximately 260 Megaannum, mya), and extensively diversified after the Permian–Triassic extinction event. Mammals are cynodonts, as are their extin ...
''
Procynosuchus ''Procynosuchus'' (Greek: "Before dog crocodile") is an extinct genus of cynodonts from the Late Permian. It is considered to be one of the earliest and most basal (phylogenetics), basal cynodonts. It was 60 cm (2 ft) long. Remains of ...
''. Several aspects of the forelimbs have been interpreted as adaptations to digging. Like many digging
tetrapod A tetrapod (; from Ancient Greek :wiktionary:τετρα-#Ancient Greek, τετρα- ''(tetra-)'' 'four' and :wiktionary:πούς#Ancient Greek, πούς ''(poús)'' 'foot') is any four-Limb (anatomy), limbed vertebrate animal of the clade Tetr ...
s, the
radius In classical geometry, a radius (: radii or radiuses) of a circle or sphere is any of the line segments from its Centre (geometry), center to its perimeter, and in more modern usage, it is also their length. The radius of a regular polygon is th ...
is significantly shorter than the
humerus The humerus (; : humeri) is a long bone in the arm that runs from the shoulder to the elbow. It connects the scapula and the two bones of the lower arm, the radius (bone), radius and ulna, and consists of three sections. The humeral upper extrem ...
. Like other aetosaurs, there is a prominent deltopectoral crest on the humerus. The
manus Manus may refer to: Relating to locations around New Guinea *Manus Island, a Papua New Guinean island in the Admiralty Archipelago ** Manus languages, languages spoken on Manus and islands close by ** Manus Regional Processing Centre, an offshore ...
is short and wide, a characteristic of digging animals. There is also an entepicondyle on the humerus, which is the origin of forearm
pronator Motion, the process of movement, is described using specific anatomical terms. Motion includes movement of organs, joints, limbs, and specific sections of the body. The terminology used describes this motion according to its direction relative ...
and manual flexor muscles. ''Typothorax'' also possesses a relatively long
olecranon process The olecranon (, ), is a large, thick, curved bony process on the proximal, posterior end of the ulna. It forms the protruding part of the elbow and is opposite to the cubital fossa or elbow pit (trochlear notch). The olecranon serves as a lever ...
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 A fossorial animal () is one that is adapted to digging and which lives primarily (but not solely) underground. Examples of fossorial vertebrates are Mole (animal), moles, badgers, naked mole-rats, meerkats, armadillos, wombats, and mole salamand ...
lifestyle but had an ability to dig that other aetosaurs didn't have. 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 In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...
of aetosaurs called the
Typothoracinae 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 ...
, within the
subfamily In biological classification, a subfamily (Latin: ', plural ') is an auxiliary (intermediate) taxonomic rank, next below family but more inclusive than genus. Standard nomenclature rules end botanical subfamily names with "-oideae", and zo ...
Aetosaurinae 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 ''Aetosaur ...
. It is closely related to ''
Redondasuchus ''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 mi ...
''. ''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''. Martz (2002) suggested that ''R. reseri'' belongs to ''Typothorax'', but is still its own species, called ''T. reseri''. More recently, the distinctiveness of ''Redondasuchus'' from ''Typothorax'' has been supported with the description of a new species, ''R. rineharti'', in 2006. 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. Parker (2013) redescribed the
holotype A holotype (Latin: ''holotypus'') is a single physical example (or illustration) of an organism used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of s ...
of the type species of ''Typothorax'', ''T. coccinarum''. It was found to be too fragmentary to be diagnosable and therefore represents a ''
nomen dubium In binomial nomenclature, a ''nomen dubium'' (Latin for "doubtful name", plural ''nomina dubia'') is a scientific name that is of unknown or doubtful application. Zoology In case of a ''nomen dubium,'' it may be impossible to determine whether a ...
''. '' Episcoposaurus horridus'', the type species of its genus usually considered to be a
junior synonym In taxonomy, the scientific classification of living organisms, a synonym is an alternative scientific name for the accepted scientific name of a taxon. The botanical and zoological codes of nomenclature treat the concept of synonymy differently. ...
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 In biology, a type is a particular specimen (or in some cases a group of specimens) of an organism to which the scientific name of that organism is formally associated. In other words, a type is an example that serves to anchor or centralizes ...
for ''T. coccinarum'' from well preserved specimens which have been previously referred to it.


References


External links


''Typothorax''
in the
Paleobiology Database The Paleobiology Database (PBDB) is an online resource for information on the distribution and classification of fossil animals, plants, and microorganisms. History The Paleobiology Database originated in the NCEAS-funded Phanerozoic Marine Pale ...
*Jeff Martz'
Master's Thesis
from
Texas Tech University Texas Tech University (Texas Tech, Tech, or TTU) is a public university, public research university in Lubbock, Texas, United States. Established on February 10, 1923, and called Texas Technological College until 1969, it is the flagship instit ...
. {{Taxonbar, from=Q1012908 Aetosaurs of North America Chinle fauna Prehistoric pseudosuchian genera Taxa named by Edward Drinker Cope