''Eodicynodon'' (''eo-,'' early or primitive, dicynodont) is an extinct

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...

dicynodont Dicynodontia is an extinct clade of anomodonts, an extinct type of non-mammalian therapsid. Dicynodonts were herbivorous animals with a pair of tusks, hence their name, which means 'two dog tooth'. Members of the group possessed a horny, typic ...

therapsid Therapsida is a major group of eupelycosaurian synapsids that includes mammals, their ancestors and relatives. Many of the traits today seen as unique to mammals had their origin within early therapsids, including limbs that were oriented more ...

s, a highly diverse group of herbivorous synapsids that were widespread during the middle-late

Permian The Permian ( ) is a geologic period and System (stratigraphy), stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last ...

and early

Triassic The Triassic ( ) is a geologic period and system (stratigraphy), system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago (Year#Abbreviations yr and ya, Mya), to the beginning of the Jurassic Period 201.36 ...

. As its name suggests, ''Eodicynodon'' is the oldest and most primitive dicynodont yet identified, ranging from the middle to late Permian and possessing a mix of ancestral Anomodont/therapsid features and derived dicynodont synapomorphies. First described by paleontologist T. H. Barry in 1974, its only associated species, ''E. oosthuizeni'', is named after Roy Oosthuizeni, the South African farmer who discovered the type specimen (a partial skull without the mandible) on his

Cape Province The Province of the Cape of Good Hope ( af, Provinsie Kaap die Goeie Hoop), commonly referred to as the Cape Province ( af, Kaapprovinsie) and colloquially as The Cape ( af, Die Kaap), was a province in the Union of South Africa and subsequen ...

farm between 1964 and 1970.

Description

''Eodicynodon'' was a medium-sized dicynodont, at about 450 mm long and 150 mm high. While it had many features common to all dicynodonts, such as canine tusks and jaw structures related to the "cheek pivot system" of mastication, it also displayed a number of ancestral features more similar to some of its primitive therapsid relatives, which are listed below.

Cranial

Skull roof

Palate/snout

While the premaxillary bones are fused in more derived dicynodonts, a thin suture extending dorsally up from the palatal facet reveals that they are paired in ''Eodicynodon,'' an ancestral feature they share with their primitive relatives

Venyukovia ''Venyukovia'' (named after its discoverer, Pavel N. Venyukov) is an extinct genus of venyukovioid therapsid, a basal anomodont from the Middle Permian of Russia. The type and sole species, ''V. prima'', is known only by a partial lower jaw with ...

Otsheria ''Otsheria'' is an extinct genus of anomodont from the Permian of Russia Russia (, , ), or the Russian Federation, is a transcontinental country spanning Eastern Europe and Northern Asia. It is the largest country in the world, with ...

, and

Pelycosaur Pelycosaur ( ) is an older term for basal or primitive Late Paleozoic synapsids, excluding the therapsids and their descendants. Previously, the term ''mammal-like reptile'' had been used, and pelycosaur was considered an order, but this is ...

ia. Similarly, while the vomers of later dicynodonts are fused, they are paired in ''Eodicynodon'' and together with the pterygoid border the intervomero-pterygoidal vacuity; in more derived dicynodonts, this vacuity is more posteriorly located and exclusively bordered by the pterygoid. These features are also present in more primitive relatives of dicynodonts, including Pristerodon, sphenacodont pelycosaurs,

cotylosaur Captorhinidae (also known as cotylosaurs) is an extinct family of tetrapods, traditionally considered primitive reptiles, known from the late Carboniferous to the Late Permian. They had a cosmopolitan distribution across Pangea. Description C ...

s, and Venyukovia.

Mastication

Dicynodonts were specialized herbivores that employed a unique “cheek pivot system” of mastication that created powerful shearing action upon closure of the jaw and subsequently ground mouth contents through a system of interlocking ridges and grooves formed from the palate and dentary. Two morphological features, present already in ''Eodicynodon'', made this motion possible. The first was a double convex jaw joint, wherein both the quadrate and articular formed convex condyles. As the jaw closed, the articular condyle of the lower jaw slid anterio-dorsally along the quadrate condyle, resulting in closure of the mouth from back to front as the posterior end of the mandible was elevated dorsally relative to the anterior end. Forward slide of the lower jaw was limited by the second morphological feature unique to dicynodonts, a pivot point created between the dentary groove and palatal notch upon closure of the jaw. The lower jaw would then move so that the articular condyle slid anterio-ventrally along the quadrate condyle, which would cause the mandible to pivot in such a way that the front of the mouth closed and the back opened.

Postcranial

Limb morphology

Discovery and geology

The South African

Karoo Supergroup The Karoo Supergroup is the most widespread stratigraphic unit in Africa south of the Kalahari Desert. The supergroup consists of a sequence of units, mostly of nonmarine origin, deposited between the Late Carboniferous and Early Jurassic, a peri ...

is a fossil-rich series of bedded shales that was continuously deposited beginning in the late Carboniferous through the early Jurassic. Though a diverse assemblage of dicynodonts appears early on in the Beaufort Group, the immediately preceding Ecca Group, Ecca was long understood to be barren of fossils, despite a lack of geological evidence for a change in paleoenvironment from Ecca to Beaufort. From 1964 to 1970, the farmer Roy Oosthuizen, whose land was located in an area firmly established as Upper Ecca (Middle Permian), collected a number of nodules containing the remains of several therapsids, including several small dicynodonts and the partial skull that is the type specimen of ''Eodicynodon''.

Classification

'''E.' oelofseni - discussion of species validity''

Phylogeny

Synapsida

Dicynodontia

Below is a cladogram modified from Angielczyk and Rubidge (2010) showing the phylogenetic relationships of Dicynodontia:

References

Sources

* ''The Origin and Evolution of Mammals'' (Oxford Biology) by T. S. Kemp {{Taxonbar, from=Q2326463 Dicynodonts Guadalupian synapsids of Africa Fossils of South Africa Fossil taxa described in 1974 Anomodont genera