| Anomocephalus | |
|---|---|
 | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Synapsida |
| Clade: | Therapsida |
| Clade: | †Anomodontia |
| Genus: | †Anomocephalus Modestoet al.,1999 |
| Species: | †A. africanus |
| Binomial name | |
| †Anomocephalus africanus Modestoet al., 1999 | |
Anomocephalus is anextinctgenus of primitiveanomodonts and belongs to the cladeAnomocephaloidea. The name is said to be derived from the Greek wordanomos meaning lawless andcephalos meaning head.[1] The proper word for head in Greek is however κεφαλή (kephalē).[2] It is primitive in that it retains a complete set ofteeth in both jaws, in contrast to its descendants, thedicynodonts, whose dentition is reduced to only a single pair oftusks (and in many cases no teeth at all), with their jaws covered by a horny beak similar to that of a moderntortoise. However, they are in no way closely related.
Its discovery in 1999 from the earliest terrestrial rocks ofGondwana (fromWilliston in theKaroo of theNorthern Cape Province ofSouth Africa) has shown that this group ofherbivores originated inGondwana; notLaurasia, as had previously been supposed. It lived 260 million years ago during thePermian Period, inarid areas withrivers andlakes—almost like parts of modern-dayNamibia orBotswana.[1] It is most closely related toTiarajudens fromBrazil.[3]
Anomocephalus was discovered at the base of theBeaufort Group, which is a geographical stratum that consists of mostly sandstone and shales that have been deposited in theKaroo Basin.[1][4] The Beaufort Group dominated most of the basin with fluvial sedimentation, which is carried by streams and rivers that were most likely formed by ice masses such as glaciers.[5] The climate at this time during the Mid toLate Permian became warm and semi-arid with seasonal rainfall.[5] The central region of the basin is thought to have been drained by semi-permanent lakes and fine-grained meander belts.[5]
Anomocephalus was collected from a locality near Williston at the base of the Beaufort Group in the Karoo Basin, which is located within the Northern Cape Province of South Africa.[1] It was discovered during the continued program of B. Rubidge to determine the lateral extent of theEodicynodon (an extinct dicynodont therapsid) Assemblage Zone.[1] It was first described by Modesto in 1999 and is known only by a partial skull with distinctive dentition and was preserved in hard mud rock.[1]
The discovery ofAnomocephalus and itsphylogenetic position provided compelling evidence thatanomodonts initially diversified in Gondwana.[1] This conflicts with previous suggestions that anomodonts were freely dispersing between the northern and southern regions of the Late PermianPangea or thattherapsids first evolved in Euramerica and then moved toGondwana when climate became favorable.[1][6][7] Additionally, the basal phylogenetic position ofAnomocephalus suggests that herbivory was acquired initially by the anomodonts of Gondwana.[1]
Thepremaxilla contains a deep alveolar portion with room for two teeth and themaxilla is slightly elongated in comparison to other anomodonts.[8][1] On the posterior portion of the maxilla, the characteristic anomodont curvature is seen in thezygomatic arch.[8][1] The nasal, prefrontal, andlacrimal resemble in both form and position those of other more basal anomodonts.[1] Additionally, thejugal has a greater marginal exposure than other anomodonts and it tapers posteriorly.[1] The dorsal lamella of the quadratojugal more closely resembles dicynodonts than basal anomodonts.[1] The postorbital bone tapers ventrally and is visibly flat and curved.[1] Like in other anomodonts, thedentary is dorsoventrally deep, and the squamosal is triradiate as suggested by the ventral and anterior processes.[8][1]
Anomocephalus possess five upperincisors that have an ovoid-shapedcrown when observed from the occlusal view.[9] The dentition of the maxilla begins as tiny peg-like elements that become buccolingually wide and mesiodistally short.[9] Six teeth are located on the pterygoid/ epipterygoid with four additional empty/damaged alveoli which suggests that there were at least ten teeth that made up the rightpalatal dentition.[9] These palatal teeth have long, curved roots and the crowns are rectangular with an occlusal basin.[9] There are twoin situ lower incisiforms that are followed by two displaced lower teeth, the second of these teeth is transversally expanded and shows a saddle-like crown just like the palatal teeth.[1][9] Additionally, there are three posterior lower teeth on the dentary with an unerupted, replacement tooth evident below the last lower tooth, which is evidence of at least a second wave of tooth replacement.[9]
Although a post-cranial skeleton was not found with the partial skull ofAnomocephalus, its sister taxaTiarajudens eccentricus was discovered in 2011 with a partial left pectoral girdle and its left limb, an isolated lefttibia with the pes, and foot elements.[9][3] Of the axial elements, only two fragmentary ribs of parallel margin were found with no clear curvature and the most complete fragment was 8 mm wide and 86 mm long.[9] Thehumerus that was found withT. eccentricus is approximately 177 mm in length and displayed well-expanded proximal and distal portions.[9] The radius is 128 mm in length with expanded, flat proximal and distal surfaces, and the ulna is more robust than the radius and slightly longer at 137 mm.[9] The pes showed five partial digits and they were all robust witharthrodial joints between the distalmetatarsals and proximal phalanges as well as between the phalanges.[9] Additionally, 15 left and three right gastralia were preserved as long, thin, and delicate bones.[9]
Anomocephalus exhibits palatal teeth and the morphology of the teeth is consistent with a high-fiber herbivorous diet.[9] Cisneros and colleagues suggested thatAnomocephalus had an incipient propaliny during theocclusions due to the longitudinal dimensions of each facet of the quadrate being twice as large as the transversal dimension.[9][1] They suggest that this would allow for forward and backward movement of the lower jaw during chewing.[9] Propaliny is also suggested to be linked to improved capability for processing plant material.[10][11][12]
