Exaeretodon
Encyclopedia
Exaeretodon is a genus
of traversodontid cynodont
; several species are known, from various formations
. E. argentinus, E. frenguelli, and E. vincei are from the Carnian
-age (Upper Triassic) Ischigualasto Formation
of Argentina
. E. major and E. riograndensis are from the Ladinian
-age (Middle Triassic
) portion of the Santa Maria Formation
of Brazil
. E. statisticae is from the Carnian-age Lower Maleri Formation
of India
. This genus was an herbivore
up to 1.8 meters long (5.9 ft), with a specialized grinding action when feeding.
An analysis of the size of the bones of calves collected in Paleorrota
concluded that the mother Exaeretodon had one or two calves, for one pregnancy
. Another point of interest is that these cynodonts had deciduous teeth
, which is a characteristic of mammals and means that babies could not chew, and required specialized parental care. Only older juveniles had permanent teeth.
Exaeretodon is one of the largest traversodontids, but the traversodontid species Ischignathus sudamericanus was reported in 1967 to be even larger. I. sudamericanus was based on a single skull from the Ischigualasto Formation in the same deposit as Exaeretodon argentinus.
When he first named the species, Argentine paleontologist José Bonaparte
mentioned several features that distinguish it from all other traversodontids. The tooth rows of the upper jaw are more parallel to each other in I. sudamericanus than they are in Exaeretodon, and they are also inset closer to the inside of the mouth. There are also more postcanine teeth oriented toward middle of the subtemporal fenestrae (two holes in the bottom of the skull) and ascending rama of the dentary (projections of the lower jaw that extend up to the skull). The ascending rama are also wider and taller in the I. sudamericanus specimen. The orbit
or eye socket is longer than that of Exaeretodon, as are the palatine bone
s.
Despite the differences, a 2007 study concluded that the I. sudamericanus skull represents the same species as E. argentinus. Using allometry, paleontologist Jun Liu found I. sudamericanus to be the largest known example of a growth series in E. argentinus. Thus, Ischignathus is now regarded as a junior synonym of Exaeretodon. As the animal grew, the proportions of bones changed. These differing proportions were initially seen as species-distinguishing characters, but are now regarded as natural ontogenic variation.
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 traversodontid 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...
; several species are known, from various formations
Geologic formation
A formation or geological formation is the fundamental unit of lithostratigraphy. A formation consists of a certain number of rock strata that have a comparable lithology, facies or other similar properties...
. E. argentinus, E. frenguelli, and E. vincei are from the Carnian
Carnian
The Carnian is the lowermost stage of the Upper Triassic series . It lasted from about 228.7 till 216.5 million years ago . The Carnian is preceded by the Ladinian and is followed by the Norian...
-age (Upper Triassic) Ischigualasto Formation
Ischigualasto
Ischigualasto is a geological formation and a natural park associated with it in the province of San Juan, north-western Argentina, near the border with Chile...
of Argentina
Argentina
Argentina , officially the Argentine Republic , is the second largest country in South America by land area, after Brazil. It is constituted as a federation of 23 provinces and an autonomous city, Buenos Aires...
. E. major and E. riograndensis are from the Ladinian
Ladinian
The Ladinian is a stage and age in the Middle Triassic series or epoch. It spans the time between 237 ± 2 Ma and 228 ± 2 Ma...
-age (Middle Triassic
Middle Triassic
In the geologic timescale, the Middle Triassic is the second of three epochs of the Triassic period or the middle of three series in which the Triassic system is divided. It spans the time between 245 ± 1.5 Ma and 228 ± 2 Ma...
) portion of the Santa Maria Formation
Santa Maria Formation
The Santa Maria Formation is a sedimentary rock formation found in Rio Grande do Sul, Brazil. It has a late Ladinian – early Carnian age , and is notable for its fossils of early dinosaurs, including the herrerasaur Staurikosaurus, the basal saurischian Teyuwasu, and the basal sauropodomorph...
of Brazil
Brazil
Brazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...
. E. statisticae is from the Carnian-age Lower Maleri Formation
Lower Maleri Formation
The Lower Maleri Formation is a sedimentary rock formation found in Andhra Pradesh, India. It is the lowermost member of the Pranhita–Godavari Basin. It is of late Carnian to early Norian age , and is notable for its fossils of early dinosaurs, including the basal saurischian ...
of India
India
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...
. This genus was an herbivore
Herbivore
Herbivores are organisms that are anatomically and physiologically adapted to eat plant-based foods. Herbivory is a form of consumption in which an organism principally eats autotrophs such as plants, algae and photosynthesizing bacteria. More generally, organisms that feed on autotrophs in...
up to 1.8 meters long (5.9 ft), with a specialized grinding action when feeding.
An analysis of the size of the bones of calves collected in Paleorrota
Paleorrota
Paleorrota , is a geopark located in the center of the Brazilian state of Rio Grande do Sul. The rocks and fossils found along the route date back to the times when there was only one supercontinent Pangaea....
concluded that the mother Exaeretodon had one or two calves, for one pregnancy
Pregnancy
Pregnancy refers to the fertilization and development of one or more offspring, known as a fetus or embryo, in a woman's uterus. In a pregnancy, there can be multiple gestations, as in the case of twins or triplets...
. Another point of interest is that these cynodonts had deciduous teeth
Deciduous teeth
Deciduous teeth, otherwise known as reborner teeth, baby teeth, temporary teeth and primary teeth, are the first set of teeth in the growth development of humans and many other mammals. In some Asian countries they are referred to as fall teeth as they will eventually fall out, while in almost all...
, which is a characteristic of mammals and means that babies could not chew, and required specialized parental care. Only older juveniles had permanent teeth.
Taxonomy
When he first named the species, Argentine paleontologist José Bonaparte
José Bonaparte
José Fernando Bonaparte, Ph.D. , is an Argentine paleontologist who discovered a plethora of South American dinosaurs and mentored a new generation of Argentine paleontologists like Rodolfo Coria...
mentioned several features that distinguish it from all other traversodontids. The tooth rows of the upper jaw are more parallel to each other in I. sudamericanus than they are in Exaeretodon, and they are also inset closer to the inside of the mouth. There are also more postcanine teeth oriented toward middle of the subtemporal fenestrae (two holes in the bottom of the skull) and ascending rama of the dentary (projections of the lower jaw that extend up to the skull). The ascending rama are also wider and taller in the I. sudamericanus specimen. The orbit
Orbit (anatomy)
In anatomy, the orbit is the cavity or socket of the skull in which the eye and its appendages are situated. "Orbit" can refer to the bony socket, or it can also be used to imply the contents...
or eye socket is longer than that of Exaeretodon, as are the palatine bone
Palatine bone
The palatine bone is a bone in many species of the animal kingdom, commonly termed the palatum .-Human anatomy:...
s.
Despite the differences, a 2007 study concluded that the I. sudamericanus skull represents the same species as E. argentinus. Using allometry, paleontologist Jun Liu found I. sudamericanus to be the largest known example of a growth series in E. argentinus. Thus, Ischignathus is now regarded as a junior synonym of Exaeretodon. As the animal grew, the proportions of bones changed. These differing proportions were initially seen as species-distinguishing characters, but are now regarded as natural ontogenic variation.
External links
- Exaeretodon at Palaeos.com.
- Dinosaurs of Rio grande do Sul.