| Cricodon | |
|---|---|
 | |
| The partial skeletal reconstruction ofCricodon metabolus with a tool as reference for size. | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Synapsida |
| Clade: | Therapsida |
| Clade: | Cynodontia |
| Clade: | †Neogomphodontia |
| Genus: | †Cricodon A. W. Crompton, 1955 |
| Type species | |
| †C. metabolus | |
Cricodon is anextinctgenus oftrirachodontidcynodonts that lived during theEarly Triassic andMiddle Triassic periods of Africa.[1][2] A. W. Crompton namedCricodon based on the ring-like arrangement of the cuspules on the crown of a typical postcanine tooth.[3] The epithet of the type species,C. metabolus, indicates the change in structure of certain postcanines resulting from replacement.[3]
Cricodon was first discovered in theTanzanianManda Beds ofSouth Africa.[3][1] Broili & Schröder (1936)[4] were the first to describeCricodon, yet were not able to provide a name for thetaxon, which at the time was only known from 5 teeth. Extensive and in-depth descriptions offossils from theManda Beds were provided by A. W. Crompton (1955).[3] Crompton provided the nameCricodon as morefossil discoveries were found and a more complete view of theskeleton could be created.[3]
Fossil evidence ofCricodon has also been discovered in theKaroo Beds, specifically in theCynognathus Assemblage Zone ofSouth Africa.[5] TheCynognathus Assemblage Zone encompasses the boundary between the late Early and early MiddleTriassic period, and has been subdivided into three distinct subzones (Subzone A, Subzone B, and Subzone C)[6] based primarily on the spatial and temporal ranges of keytemnospondyl indextaxa.[5]Cricodon fossils have been found in the youngest of the three subzones, Subzone C.[5] However, upper postcanines resembling those ofCricodon are also known from deposits corresponding to Subzone B.[5]
The trirachodontid"Trirachodon" kannemeyeri Seeley, 1895 is now referred toCricodon asC. kannemeyeri.[7]
Cranial bones recovered from theManda Beds consist of a badly crushed orbito-ethmoidal region, a practically complete rightmandible, two fragments of the left mandible, several loose teeth, a portion of theocciput, and several unidentified fragments.[3]
In theupper jaw, only theposterior ends of themaxillae, a portion of thepalate, and the floor of the orbits are well preserved.[3] The maxillary postcanines are transversely ovate and have three maincusps arranged upon the same transverse plane.[3] The three main cusps are composed of the lingual, central, and labial cusp.[3] There are also small peripheral cuspules present on theanterior and posterior borders of the crown.[3] Roots of the upper postcanines are long, transversely flattened near the crown and taper away to a short point distally.[3] The maximum known transverse diameter of the maxillary postcanines is 13mm.[3]
A characteristic feature of themandible is the sharp angle formed by the junction of the lower margin of thedentary and the anterior surface of the dentalsymphysis.[3] A diagnostic difference between theupper jaw postcanine teeth and mandibular postcanine teeth is that mandibular postcanine teeth tend to be squarer in horizontal sections in contrast to the transversely ovate maxillary postcanines.[3] Amammalian feature that can be seen ingomphodontcynodonts is that each root is surrounded by a pocket ofcancellous bone.[3] The maximum transverse diameter of the mandibular postcanines is 9mm.[3]
The main cusps of the mandibular and maxillary postcanines form a transverse ridge across the center of the crown.[3] The transverse ridge of a tooth would fit into the depression formed between the transverse ridges of two adjoining teeth.[3] Recent research conducted by Hendrickx, Abdala, and Choiniere (2016)[8] has revealed new information in regard to the distribution ofenamelmicrostructure in non-mammaliform cynodonts, specifically inCricodon metabolus. Their research uncovered the presence of columnar divergence units in both the sectorial and gomphodont teeth of a trirachodontid along with the consistent presence of synapsid columnar enamel incynognathians.[8]
The newfound discovery in regards to the thickened enamel has many ecological implications.[8] InCricodon metabolus, the enamel layer of the gomphodont tooth is around 11.5 times thicker than the sectorial tooth.[8] The postcanine gomphodont teeth (labiolingually expanded teeth with largeocclusal surfaces) were used for chewing, crushing, and grinding fibrous plant material, meaning that they were under higher loads and apically oriented stresses.[8] Sectorial teeth on the other hand were used to shear plant material[3] and were not subjected to the same types of occlusal stresses, therefore the enamel thickness was not maintained.[8] Another reason proposed by Hendrickx, Abdala, and Choiniere (2016)[8] to explain the thin enamel in sectorial teeth is due to replacement timing and patterns, as they will be sequentially replaced by gomphodont teeth.[3] Another characteristic that was observed in the gomphodont teeth were enamel anddentine incremental lines.[8] The enamel anddentine incremental lines,odontoblasttubules in dentine, and discontinuous columnar divergence units in enamel support the consistent presence of synapsid columnar enamel in Cynognathia.[8]
Twenty-fivevertebrae were discovered which belong to thedorsal andsacral regions with only one vertebra having a well preservedneural arch from the sacral region.[3] There are additionalapophyses below the posteriorzygapophysis which articulate with concavities on the lateral surface of the neural arch, posterior and inferior to theanterior zygapophysis.[3]Cricodon metabolus has the typical cynodont expandedribs of which thirteen dorsal ribs were discovered.[3]
The entirehumerus was 12.5 cm long and had typical cynodont characterizations such as the twisted bone and the plane of thedistal end forming an angle of 40 degrees with that of theproximal.[3] The discoveredfemur was 12.5 cm long with thecapitulum directed at an angle of 45 to 50 degrees to the main axis of the slender shaft.[3] Another characteristic of the femur noted is that it projects well forward from the main body of the shaft.[3] Although little is known of the hand of cynodonts, it is believed thatCricodon metabolus has a phalangeal formula of 23443, with the second phalange reduced in digits three and four.[3]
Below is acladogram from Gaoet al. (2010)[9] showing thephylogenetic relationships of one part of the Cynodontia relative toCricodon:
| Cynognathia |
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