Typothorax
Encyclopedia
Typothorax is an extinct genus
of 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.
, a crurotarsan
related to modern crocodile. Unlike modern crocodiles, however, aetosaurs were herbivorous. They possessed small, leaf-shaped teeth which were unsuited for a diet consisting of meat.
Unlike some aetosaurs like Desmatosuchus
, 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
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. Typothorax was about 2.5 metres (8.2 ft) long and weighed 100 kilogram.
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 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 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
.
of western North America but has been poorly understood since that naming. Paleontologists have found the characteristic armor plates for decades, but only recently has a comprehensive study of Typothorax appeared. 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
.
, an anteriorly directed pes (foot), and the projection of the lateral surface of the illium 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, 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
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 a 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. The upturned snout of Typothorax and other aetosaurs suggests that they may have searched for food by rooting around in soil.
of aetosaurs called the Typothoracisinae
, 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. 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.
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
of aetosaur
Aetosaur
Aetosaurs are an extinct order of heavily armoured, medium- to large-sized Late Triassic herbivorous archosaurs. They have small heads, upturned snouts, erect limbs, and a body covered by plate-like scutes. All aetosaurs belong to the family Stagonolepididae...
that lived in the Late Triassic
Late Triassic
The Late Triassic is in the geologic timescale the third and final of three epochs of the Triassic period. The corresponding series is known as the Upper Triassic. In the past it was sometimes called the Keuper, after a German lithostratigraphic group that has a roughly corresponding age...
. Its remains have been found in North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
. Two species are known: T. coccinarum, the type species
Type species
In biological nomenclature, a type species is both a concept and a practical system which is used in the classification and nomenclature of animals and plants. The value of a "type species" lies in the fact that it makes clear what is meant by a particular genus name. A type species is the species...
, and T. antiquum.
Description
Typothorax was a classic aetosaurAetosaur
Aetosaurs are an extinct order of heavily armoured, medium- to large-sized Late Triassic herbivorous archosaurs. They have small heads, upturned snouts, erect limbs, and a body covered by plate-like scutes. All aetosaurs belong to the family Stagonolepididae...
, a crurotarsan
Crurotarsi
The Crurotarsi are a group of archosauriformes, represented today by the crocodiles,...
related to modern crocodile. Unlike modern crocodiles, however, aetosaurs were herbivorous. They possessed small, leaf-shaped teeth which were unsuited for a diet consisting of meat.
Unlike some aetosaurs like Desmatosuchus
Desmatosuchus
Desmatosuchus is an extinct genus of archosaur belonging to the Order Aetosauria. It was one of the largest aetosaurs, being long and about 1.50 m high. It lived during the Late Triassic in Texas....
, 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
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
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. Typothorax was about 2.5 metres (8.2 ft) long and weighed 100 kilogram.
The vertebral column of Typothorax is shortened, with individual vertebrae being reduced in length. However, the osteoderm
Osteoderm
Osteoderms are bony deposits forming scales, plates or other structures in the dermal layers of the skin. Osteoderms are found in many groups of extant and extinct reptiles, including lizards, various groups of dinosaurs , crocodilians, phytosaurs, aetosaurs, placodonts, and hupehsuchians...
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 columns and four caudal
Tail
The tail is the section at the rear end of an animal's body; in general, the term refers to a distinct, flexible appendage to the torso. It is the part of the body that corresponds roughly to the sacrum and coccyx in mammals, reptiles, and birds...
columns of ventral osteoderms on the underside. Unlike all other aetosaurs, Typothorax possesses spiked osteoderms on the underside of the tail, near the cloaca
Cloaca
In zoological anatomy, a cloaca is the posterior opening that serves as the only such opening for the intestinal, reproductive, and urinary tracts of certain animal species...
. This region is not covered by osteoderms to the extent that it is in other aetosaurs such as Aetosaurus
Aetosaurus
Aetosaurus is an extinct genus of archosaur 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...
and Coahomasuchus
Coahomasuchus
Coahomasuchus is an extinct genus of aetosaurine stagonolepidid. Fossils have been found from the Dockum Group in western Texas and date back to the Otischalkian faunachron of the Late Triassic. It was small for an aetosaur, being less than 1 meter long. The dorsal plates are distinctively flat...
.
History
Typothorax was one of the earliest vertebrates named from the TriassicTriassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...
of western North America but has been poorly understood since that naming. Paleontologists have found the characteristic armor plates for decades, but only recently has a comprehensive study of Typothorax appeared. It has been found in Arizona
Arizona
Arizona ; is a state located in the southwestern region of the United States. It is also part of the western United States and the mountain west. The capital and largest city is Phoenix...
, in the Chinle Formation
Chinle Formation
The Chinle is a geologic formation that is spread across the U.S. states of northern Arizona, Nevada, Utah, western New Mexico, and western Colorado. The Chinle is controversially considered to be synonymous to the Dockum Group in eastern Colorado, eastern New Mexico, southwestern Kansas, the...
, as well as in New Mexico
New Mexico
New Mexico is a state located in the southwest and western regions of the United States. New Mexico is also usually considered one of the Mountain States. With a population density of 16 per square mile, New Mexico is the sixth-most sparsely inhabited U.S...
and Texas
Texas
Texas is the second largest U.S. state by both area and population, and the largest state by area in the contiguous United States.The name, based on the Caddo word "Tejas" meaning "friends" or "allies", was applied by the Spanish to the Caddo themselves and to the region of their settlement in...
in the Bull Canyon Formation of the Dockum Group
Dockum Group
The Dockum is a Late Triassic geologic group found primarily on the Llano Estacado of western Texas and eastern New Mexico with minor exposures in southwestern Kansas, eastern Colorado, and Oklahoma panhandle. The Dockum reaches a maximum thickness of slightly over 650 m but is usually much...
.
Paleobiology
Like all aetosaurs and many other early crurotarsans, Typothorax had erect hindlimbs held beneath the body. This is evident by a straight femurFemur
The femur , or thigh bone, is the most proximal bone of the leg in tetrapod vertebrates capable of walking or jumping, such as most land mammals, birds, many reptiles such as lizards, and amphibians such as frogs. In vertebrates with four legs such as dogs and horses, the femur is found only in...
, an anteriorly directed pes (foot), and the projection of the lateral surface of the illium over the femur. Because the length of the femur is almost equal to that of the tibia and fibula (lower leg) and astragalus
Astragalus
Astragalus is a large genus of about 3,000 species of herbs and small shrubs, belonging to the legume family Fabaceae, subfamily Faboideae. The genus is native to temperate regions of the Northern Hemisphere...
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, ceratopsia
Ceratopsia
Ceratopsia or Ceratopia is a group of herbivorous, beaked dinosaurs which thrived in what are now North America, Europe, and Asia, during the Cretaceous Period, although ancestral forms lived earlier, in the Jurassic. The earliest known ceratopsian, Yinlong downsi, lived between 161.2 and 155.7...
ns, and the early cynodont
Cynodont
Cynodontia or cynodonts are a taxon of therapsids which first appeared in the Late Permian and were eventually distributed throughout all seven continents by the Early Triassic . This clade includes modern mammals and their extinct close relatives. They were one of the most diverse groups of...
Procynosuchus
Procynosuchus
Procynosuchus was a cynodont from the Late Permian. It is considered to be one of the earliest and most basal cynodonts. Remains of Procynosuchus have been found in Germany, Zambia and South Africa...
.
Several aspects of the forelimbs have been interpreted as adaptations to digging. Like many digging tetrapod
Tetrapod
Tetrapods are vertebrate animals having four limbs. Amphibians, reptiles, birds and mammals are all tetrapods; even snakes and other limbless reptiles and amphibians are tetrapods by descent. The earliest tetrapods evolved from the lobe-finned fishes in the Devonian...
s, the radius
Radius
In classical geometry, a radius of a circle or sphere is any line segment from its center to its perimeter. By extension, the radius of a circle or sphere is the length of any such segment, which is half the diameter. If the object does not have an obvious center, the term may refer to its...
is significantly shorter than the humerus
Humerus
The humerus is a long bone in the arm or forelimb that runs from the shoulder to the elbow....
. Like other aetosaurs, there is a prominent deltopectoral crest on the humerus. The manus
Manus (zoology)
The manus is the zoological term for the distal portion of the fore limb of an animal. In tetrapods, it is the part of the pentadactyl limb that includes the metacarpals and digits . During evolution, it has taken many forms and served a variety of functions...
is short and wide, a characteristic of digging animals. There is also a entepicondyle on the humerus, which is the origin of forearm pronator
Pronation
In anatomy, pronation is a rotational movement of the forearm at the radioulnar joint, or of the foot at the subtalar and talocalcaneonavicular joints. For the forearm, when standing in the anatomical position, pronation will move the palm of the hand from an anterior-facing position to a...
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
Fossorial
A fossorial organism is one that is adapted to digging and life underground such as the badger, the naked mole rat, and the mole salamanders Ambystomatidae...
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 cladeClade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
of aetosaurs called the Typothoracisinae
Typothoracisinae
Typothoracisinae is a clade of aetosaurs within the subfamily Aetosaurinae. It is a stem-based taxon defined as all aetosaurs closer to Typothorax than to Stagonolepis or Desmatosuchus. As with many aetosaur taxa, most of the synapomorphies that diagnose the clade are found in the osteoderms...
, within the subfamily Aetosaurinae
Aetosaurinae
Aetosaurinae is one of the two subfamilies of aetosaurs, the other being Desmatosuchinae. It is a stem-based taxon defined as all aetosaurs more closely related to Aetosaurus than to the last common ancestor of Aetosaurus and Desmatosuchus. The only synapomorphy that diagnoses the clade...
. It is closely related to Redondasuchus
Redondasuchus
Redondasuchus is an extinct genus of aetosaur. It may 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...
. Redondasuchus reseri was first named in 1991, but was later synonymized
Synonym (taxonomy)
In scientific nomenclature, a synonym is a scientific name that is or was used for a taxon of organisms that also goes by a different scientific name. For example, Linnaeus was the first to give a scientific name to the Norway spruce, which he called Pinus abies...
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.
External links
- Typothorax in the Paleobiology DatabasePaleobiology Database' is an online resource for information on the distribution and classification of fossil animals, plants, and microorganisms.-History:The Paleobiology Database was founded in 2000. It has been funded by the National Science Foundation and the Australian Research Council...
- Jeff Martz's Master's Thesis from Texas Tech UniversityTexas Tech UniversityTexas Tech University, often referred to as Texas Tech or TTU, is a public research university in Lubbock, Texas, United States. Established on February 10, 1923, and originally known as Texas Technological College, it is the leading institution of the Texas Tech University System and has the...
.