Aleodon

''Aleodon'' is an extinct genus of cynodonts that lived from the Middle to the Late Triassic. Relatively few analyses have been conducted to identify the phylogenetic placement of ''Aleodon'', however those that have place ''Aleodon'' as a sister taxon to ''Chiniquodon''. Two species of ''Aleodon'' are recognized: ''A. brachyramphus'' which was discovered in Tanzania, and ''A. cromptoni'' which was discovered most recently in Brazil.

The name for the genus ''Aleodon'' was created when Alfred W. "Fuzz" Crompton initially discovered the type species, ''Aleodon brachyramphus''. The genus name, "Aleodon" referred to the grinding nature of the postcanine teeth, while "brachyramphus" referred to the relatively short snout of the specimen. The most recently discovered species, ''A. cromptoni'' was named after Crompton.

Discovery and Classification

In 1955, ''Aleodon'' was initially classified as a gomphodont cynodont based on the partial skull and lower jaw fossils found in 1933 in Tanzania. The classification was based on the presence of three types of post-canines that were identified in the fossil that were similar to another well-known gomphodont, ''Diademodon tetragonus''.

It was later argued in 2001 by Hopson and Kitching that ''Aleodon'' be classified under Chiniquodontidae based on less-worn dentition of unpublished specimens. This classification was accepted by many sources. However, a study done the following year by Abdala and Gianni found, based on fossils found in Namibia, that ''Aleodon'' (and '' Cromptodon'') both had well developed lingual cingular platforms in their post-canines, a characteristic that Chiniquodontidae did not possess.

While not many analysis have been done to clarify the phylogenetic relationship of ''Aleodon'' thus far, those that have been done describe ''Aleodon'' as the sister taxon of ''Chiniquodon''.

Description

Dentition

The dentition of ''Aleodon'' is the most significant morphological feature to distinguish it against other genera. Crompton pointed out three distinct regions of the postcanine row: anterior circular, transversely expanded ovate, and sectorial posterior teeth (described as “shearing” by Crompton). These three regions of post canines were very similar to a well-known gomphodont, ''Diademodon tetragonus'' (which led Crompton to believe that ''Aleodon'' was part of Gomphodonta). Crompton also noted that ''Aleodon'' uniquely possessed two incisors on the lower jaw which differs from most cynodonts which primitively had four.
Based on more fossils, ''Aleodon'' was also found to have a long secondary palate, a characteristic that was shared with members of the monophyletic group Chiniquodontidae and resulted in Aleodon's reidentification to Probainognathia from Gomphodonta. Shortly after, specimens of ''Aleodon'' showed the possession of well-developed lingual cingular platforms on the post canines. This finding resulted in some researchers removing ''Aleodon'' from Chiniquodontidae due to the group not characterized in possessing well-developed lingual cingular platforms. The lingual cingular platform that is expanded in both labiolingual and mesiodistal dimensions relative to the labial portion of the crown is also the most developed of all Triassic Probainognathians. These observations led some researchers to believe that ''Aleodon'' (along with ''Candelariodon'') may be specimens that exhibit the gradual development of the cingular platform.

Skull

Based on specimens found in Brazil, ''Aleodon'' was found to share the same general cranial morphology of Chiniquodontidae. Diagnostic features of Chiniquodontidae are zygomatic arches flaring laterally, angulation between the ventral edge of the maxillary zygomatic process and the anteroventral margin of the jugal, elongated pterygoid flanges that end in a thin projection, and a long secondary palate. When ''Aleodon'' was first characterized by Crompton, he noted the presence of a pineal foramen, however later specimens showed no evidence of a pineal foramen on the skull of ''Aleodon''. ''Aleodon'' is also characterized by its relatively short snout.

Geological/Paleoenvironmental Information

Manda Formation (Tanzania)

Specimens of ''Aleodon'' were first recovered from the Triassic Manda Beds of Tanzania by Parrington in the 1930s. The Manda Beds were first surveyed by Gordon M. Stockley from June to October 1930. There was further collection by Parrington and Nowack in the 1930s in which ''Aleodon brachyramphus'' was later described by Crompton. The collections showed that the Manda Beds contained fauna thecodontians, cynodonts, and dicynodonts, as well as a large amphibian and a rhynchosaur. The formation was described as being 2000 meters thick with purple to brown mudstones with grey sandstones in between. From 60 to 150 meters below the top of the formation reptile bones were found while most (65%) of specimens were found 1180 meters and 1310 meters below the surface.

Omingonde Formation (Namibia)

Fossil vertebrates in the Omingonde Formation of Namibia were first found and published by Keyser in 1973. Among the fossils he described amphibians, cynodonts and dicynodonts. The cynodonts were initially represented by Cynognathus and herbivorous gomphodonts. After the discovery of Aleodon among other cynodont specimens such as ''Chiniquodon'', ''Luangwa'', and an unidentified traversodontid in 2009, the Omingonde Formation of Namibia is considered to possess the most diverse fauna of Middle Triassic cynodonts in the world. The discovery also provided researchers a definite link between two faunas in South America and East Africa. From this, the Omingonde formation was also considered the key in “reconstructing the biogeography of Southern Gondwana during the Middle Triassic”.

Rio Grande do Sul (Brazil)

Most recently a new species of ''Aleodon'' (''A. cromptoni'') was found in the Rio Grande do Sul in southern Brazil. The specimens were first collected in 1988 by Daniel and Abraão Cargnin.

References

{{Taxonbar, from1=Q2081682, from2=Q20716967

Prehistoric probainognathians
Prehistoric cynodont genera
Middle Triassic synapsids of Africa
Middle Triassic synapsids of South America
Late Triassic synapsids of South America
Fossils of Brazil
Triassic Tanzania
Fossils of Tanzania
Triassic Namibia
Fossils of Namibia
Omingonde Formation
Rio Grande do Sul
Fossil taxa described in 1955