Exaeretodon

{{Automatic taxobox

| fossil_range = Ladinian-Norian
~{{Fossil range|235|205.6}}

| image = Cynodont.jpg

| image_caption = Exhibit at Mori Tower

| taxon = Exaeretodon

| authority = Cabrera, 1943

| subdivision_ranks = Species

| subdivision = * {{extinct}}E. argentinus {{small|Cabrera, 1943 (type) [originally Belesodon argentinus]}}

{{extinct}}?E. major {{small|Huene, 1936 [originally Traversodon]}}
{{extinct}}E. riograndensis {{small|Abdala et al., 2002}}
{{extinct}}?E. statisticae {{small|Chatterjee, 1982}}

| synonyms = * Theropsis {{small|Cabrera, 1943}}

Ischignathus {{small|Bonaparte, 1963}}

}}

Exaeretodon is an extinct genus of fairly large, low-slung traversodontid cynodonts from the southern parts of Pangea. Four species are known, hailing from various formations. E. argentinus is from the Carnian-age (Late Triassic) Cancha de Bochas Member of the Ischigualasto Formation in the Ischigualasto-Villa Unión Basin in northwestern Argentina. E. major and E. riograndensis are from the Carnian-age portion of the Santa Maria Formation of the Paraná Basin in southeastern Brazil.{{Cite journal|last1=Langer|first1=Max C.|last2=Ramezani|first2=Jahandar|last3=Da Rosa|first3=Átila A.S.|date=May 2018|title=U-Pb age constraints on dinosaur rise from south Brazil|journal=Gondwana Research|volume=57|pages=133–140|doi=10.1016/j.gr.2018.01.005|bibcode=2018GondR..57..133L }} E. statisticae is from the Carnian-age Lower Maleri Formation of India.

Description

File:Exaeretodon argentinus.png

File:August 1, 2012 - Skull of Exaeretodon on Display at the Royal Ontario Museum (MC2 4483).jpg]]

This genus was an herbivore up to {{convert|1.8|m|ft|abbr=off|sp=us}} long, 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.{{Citation needed|date=June 2011}}

Taxonomy

Exaeretodon is a gomphodontosuchine cynodont. 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 Ischignathus 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 bones.{{cite journal |last=Liu |first=J. |year=2007 |title=The taxonomy of the traversodontid cynodonts Exaeretodon and Ischignathus |url=http://www.sbpbrasil.org/revista/edicoes/10_2/liu.pdf |journal=Revista Brasileira de Paleontologia |volume=10 |issue=2 |pages=133–136 |doi=10.4072/rbp.2007.2.07|doi-access=free |bibcode=2007RvBrP..10..133L }}

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.

Proexaeretodon vince was considered a synonym of Exaeretodon argentinus but was accepted as a valid taxon in 2023.{{Cite journal |last1=Schmitt |first1=Maurício Rodrigo |last2=Martinelli |first2=Agustín Guillermo |last3=Kaiuca |first3=João Felipe Leal |last4=Schultz |first4=Cesar Leandro |last5=Soares |first5=Marina Bento |date=2023 |title=Old fossil findings in the Upper Triassic rocks of southern Brazil improve diversity of traversodontid cynodonts (Therapsida, Cynodontia) |journal=The Anatomical Record |volume=307 |issue=4 |pages=1474–1514 |doi=10.1002/ar.25244 |pmid=37246488 |s2cid=258960737 |name-list-style=amp }}

Phylogeny

Exaeretodon in a cladogram after Kammerer et al. (2012):{{Cite journal | last1 = Kammerer | first1 = C. F. | last2 = Flynn | first2 = J. J. | last3 = Ranivoharimanana | first3 = L. | last4 = Wyss | first4 = A. R. | title = Ontogeny in the Malagasy Traversodontid Dadadon isaloi and a Reconsideration of its Phylogenetic Relationships | doi = 10.3158/2158-5520-5.1.112 | journal = Fieldiana Life and Earth Sciences | volume = 5 | pages = 112–125 | year = 2012 | s2cid = 84651546 }}

{{clade| style=font-size:90%;line-height:95%

|label1=Cynognathia

|1={{clade

|1=Cynognathus

|label2=Gomphodontia

|2={{clade

|1=Diademodon

|2={{clade

|label1=Trirachodontidae

|1={{clade

|1=Trirachodon

|2={{clade

|1=Cricodon

|2=Langbergia}} }}

|label2=Traversodontidae

|2={{clade

|1={{clade

|1=Andescynodon

|2=Pascualgnathus}}

|2={{clade

|1={{clade

|1=Arctotraversodon

|2={{clade

|1=Boreogomphodon

|2=Nanogomphodon}} }}

|2={{clade

|label1=Massetognathinae

|1={{clade

|1=Massetognathus

|2={{clade

|1=Dadadon

|2=Santacruzodon}} }}

|label2=Gomphodontosuchinae

|2={{clade

|1=Gomphodontosuchus

|2={{clade

|1=Menadon

|2={{clade

|1=Protuberum

|2={{clade

|1=Exaeretodon

|2=Scalenodontoides

}} }} }} }} }} }} }} }} }} }} }}

Palaeoecology

Dental microwear evidence suggests that E. argentinus transitioned from utilising primarily crushing-based feeding in its early stages of development to primarily chewing-based feeding upon reaching adulthood. This suggests that juveniles of this species were to some degree faunivorous.{{Cite journal |last1=Wynd |first1=Brenen |last2=Abdala |first2=Fernando |last3=Nesbitt |first3=Sterling J. |date=21 October 2022 |title=Ontogenetic growth in the crania of Exaeretodon argentinus (Synapsida: Cynodontia) captures a dietary shift |journal=PeerJ |language=en |volume=10 |pages=e14196 |doi=10.7717/peerj.14196 |doi-access=free |issn=2167-8359 |pmc=9590418 |pmid=36299507 }}

Palaeopathology

A pathological specimen of E. riograndensis has been found containing callouses in the ribs that correspond to a healed transverse fracture that likely occurred after a fall or an episode of intraspecific or interspecific combat. These injuries would have significantly impacted the individual's mobility such that it would have been unable to survive alone, suggesting that the gregariousness of the species helped this individual survive such a debilitating injury.{{Cite journal |last=Doneda |first=Ana Laura |last2=Roese–Miron |first2=Lívia |last3=Kerber |first3=Leonardo |date=6 May 2025 |title=Bony injuries in a Late Triassic forerunner of mammals from Brazil |url=https://link.springer.com/10.1007/s00114-025-01984-2 |journal=The Science of Nature |language=en |volume=112 |issue=3 |doi=10.1007/s00114-025-01984-2 |issn=0028-1042 |access-date=28 May 2025 |via=Springer Nature Link}}