''Etjoia'' is an

extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

traversodontid Traversodontidae is an extinct family of herbivorous cynodonts. Traversodonts were primarily Gondwanan, with many species known from Africa and South America. Recently, traversodonts have also been found from Europe and North America. Traversodon ...

cynodont Cynodontia () is a clade of eutheriodont therapsids that first appeared in the Late Permian (approximately 260 Megaannum, mya), and extensively diversified after the Permian–Triassic extinction event. Mammals are cynodonts, as are their extin ...

s that lived during the

Middle Triassic In the geologic timescale, the Middle Triassic is the second of three epoch (geology), epochs of the Triassic period (geology), period or the middle of three series (stratigraphy), series in which the Triassic system (stratigraphy), system is di ...

Late Triassic The Late Triassic is the third and final epoch (geology), epoch of the Triassic geologic time scale, Period in the geologic time scale, spanning the time between annum, Ma and Ma (million years ago). It is preceded by the Middle Triassic Epoch a ...

period in southern Africa. This medium-sized omnivorous cynognathian provides important information on the dental evolution of early diverging

gomphodont Gomphodontia is a clade of cynognathian cynodonts that includes the families Diademodontidae, Trirachodontidae, and Traversodontidae. Gomphodonts are distinguished by wide and closely spaced molar-like postcanine teeth, which are convergent wi ...

s and traversodontids.

Discovery and etymology

''Etjoia'' was discovered in the upper beds of the

Omingonde Formation The Omingonde Formation is an Early Triassic, Early to Middle Triassic (Anisian to Ladinian) geologic Formation (geology), formation, part of the Karoo Supergroup, in the western Otjozondjupa Region and northeastern Erongo Region of north-central N ...

Namibia Namibia, officially the Republic of Namibia, is a country on the west coast of Southern Africa. Its borders include the Atlantic Ocean to the west, Angola and Zambia to the north, Botswana to the east and South Africa to the south; in the no ...

(dated to the

Ladinian The Ladinian is a stage and age in the Middle Triassic series or epoch. It spans the time between Ma and ~237 Ma (million years ago). The Ladinian was preceded by the Anisian and succeeded by the Carnian (part of the Upper or Late Triassic ...

Carnian The Carnian (less commonly, Karnian) is the lowermost stage (stratigraphy), stage of the Upper Triassic series (stratigraphy), Series (or earliest age (geology), age of the Late Triassic Epoch (reference date), Epoch). It lasted from 237 to 227.3 ...

) by American

paleontologist Paleontology, also spelled as palaeontology or palæontology, is the scientific study of the life of the past, mainly but not exclusively through the study of fossils. Paleontologists use fossils as a means to classify organisms, measure geolo ...

Charles Schaff in 1996. The specimen was unearthed on the south-eastern side of the Etjo Mountain, in the Waterberg Basin of the Otjozondjupa region, in central north-west

. The holotype is deposited in the Geological Survey of Namibia of

Windhoek Windhoek (; ; ) is the capital and largest city of Namibia. It is located in central Namibia in the Khomas Highland plateau area, at around above sea level, almost exactly at the country's geographical centre. The population of Windhoek, which ...

with the specimen number GSN F1591. The generic name refers to Mount Etjo from which the new genus was unearthed. The type species, ''E. dentitransitus'', refers to the possession of a transitional dentition with sub-circular upper postcanines and a large number of sectorial teeth representing the diademodontid/trirachodontid postcanine pattern, and with sub-rectangular lower postcanines with a mesially positioned transverse crest, which is the classical morphology of lower traversodontid postcanines.

Description

''Etjoia'' is a medium-sized non-strictly herbivorous gomphodont represented by an almost complete skull and a few cervical vertebrae. It was estimated to have a body length ranging from 40 to 55 cm and a body mass of 3.2 kg. This cynodont mainly differs from other

s by the morphology of its gomphodont postcanines and the high number of its sectorial teeth.

Skull

The cranial material of ''Etjoia'' consists of an almost complete skull only missing the right zygoma. The cranium, which has a basal skull length of 88.5 mm, is characterized by the absence of a labial shelf in the maxilla.

Dentition

The dentition comprises four upper and three lower

incisor Incisors (from Latin ''incidere'', "to cut") are the front teeth present in most mammals. They are located in the premaxilla above and on the mandible below. Humans have a total of eight (two on each side, top and bottom). Opossums have 18, wher ...

s, one upper and one lower

canine Canine may refer to: Zoology and anatomy * Animals of the family Canidae, more specifically the subfamily Caninae, which includes dogs, wolves, foxes, jackals and coyotes ** ''Canis'', a genus that includes dogs, wolves, coyotes, and jackals ** Do ...

, and nine upper and eight to nine lower postcanines. The lower incisors bear large apically inclined denticles whereas the upper incisors only have minute denticles. Longitudinal ridges are present in the upper canines. Both upper and lower postcanine teeth include one conical, four gomphodont, and four sectorial postcanines, which is the highest number of sectorial teeth among all

s. Unlike other gomphodonts, the upper gomphodont postcanines are subcircular to elliptical whereas the lower gomphodont teeth are subrectangular and mesiodistally (anteroposteriorly) elongated. The third lower gomphodont postcanine has the particularity of having a low ridge (known as a mesial cingulum) anterior to the prominent transverse crest. The last upper sectorial postcanines are also strongly offset lingually (medially).

Postcranial skeleton

The postcranial skeleton is only represented by the first five cervical vertebrae.

Environment

The environment in which ''Etjoia'' was living in was characterized by meandering streams on a semi-arid loessic floodplain with shallow saline lakes. The floodplain was subject to short, wet winters with heavy thunderstorms followed by long, warm, and dry summers during which dust storms occurred frequently. ''Etjoia'' was coexisting with small herbivorous kannemeyeriid

dicynodont Dicynodontia is an extinct clade of anomodonts, an extinct type of non-mammalian therapsid. Dicynodonts were herbivores that typically bore a pair of tusks, hence their name, which means 'two dog tooth'. Members of the group possessed a horny, t ...

s and the

apex predator An apex predator, also known as a top predator or superpredator, is a predator at the top of a food chain, without natural predators of its own. Apex predators are usually defined in terms of trophic dynamics, meaning that they occupy the hig ...

Cynognathus ''Cynognathus'' is an extinct genus of large-bodied cynodontian therapsids that lived in the Middle Triassic. It is known from a single species, ''Cynognathus crateronotus''. ''Cynognathus'' was a predator closely related to mammals and had a s ...

'', a large carnivorous cynognathian.

Classification

Below is a

cladogram A cladogram (from Greek language, Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an Phylogenetic tree, evolutionary tree because it does not s ...

from Hendrickx ''et al.'' (2020) showing the

phylogenetic In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical dat ...

relationships of Diademodontidae, Trirachodontidae (here recovered as paraphyletic), and early diverging

Traversodontidae Traversodontidae is an extinct family of herbivorous cynodonts. Traversodonts were primarily Gondwanan, with many species known from Africa and South America. Recently, traversodonts have also been found from Europe and North America. Traversodon ...

References

External links

Etjoia dentitransitus
3d model of the holotype on the site Morphomuseum. {{Taxonbar, from=Q99551255 Traversodontidae Middle Triassic synapsids of Africa Late Triassic synapsids of Africa Triassic Namibia Fossils of Namibia Omingonde Formation Fossil taxa described in 2020