''Cricodon'' is an

extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

of trirachodontid

cynodont The cynodonts () (clade Cynodontia) are a clade of eutheriodont therapsids that first appeared in the Late Permian (approximately 260 mya), and extensively diversified after the Permian–Triassic extinction event. Cynodonts had a wide variety ...

s that lived during the

Early Triassic The Early Triassic is the first of three epochs of the Triassic Period of the geologic timescale. It spans the time between Ma and Ma (million years ago). Rocks from this epoch are collectively known as the Lower Triassic Series, which is a un ...

and

Middle Triassic In the geologic timescale, the Middle Triassic is the second of three epochs of the Triassic period or the middle of three series in which the Triassic system is divided in chronostratigraphy. The Middle Triassic spans the time between Ma and ...

periods of Africa.Abdala, F., J. Neveling, and J. Welman. 2006. A new trirachodontid cynodont from the lower levels of the Burgersdorp Formation (Lower Triassic) of the Beaufort Group, South Africa and the cladistic relationships of Gondwanan gomphodonts. ''Zoological Journal of the Linnean Society'' 147:383–413. A. W. Crompton named ''Cricodon'' based on the ring-like arrangement of the cuspules on the crown of a typical postcanine tooth.Crompton, A. W. 1955. On some Triassic cynodonts from Tanganyika. ''Proceedings of the Zoological Society of London'' 125(3–4):617–669. The epithet of the type species, ''C. metabolus'', indicates the change in structure of certain postcanines resulting from replacement.

Discovery

''Cricodon'' was first discovered in the

Tanzanian Tanzania (; ), officially the United Republic of Tanzania ( sw, Jamhuri ya Muungano wa Tanzania), is a country in East Africa within the African Great Lakes region. It borders Uganda to the north; Kenya to the northeast; Comoro Islands and ...

Manda Beds The Manda Formation (also known as the Manda Beds) is a Middle Triassic (Anisian?) or possibly Late Triassic (Carnian?) geologic formation in Tanzania. It preserves fossils of many terrestrial vertebrates from the Triassic, including some of the e ...

South Africa South Africa, officially the Republic of South Africa (RSA), is the southernmost country in Africa. It is bounded to the south by of coastline that stretch along the South Atlantic and Indian Oceans; to the north by the neighbouring countri ...

. Broili & Schröder (1936) were the first to describe ''Cricodon'', yet were not able to provide a name for the

taxon In biology, a taxon (back-formation from ''taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular nam ...

, which at the time was only known from 5 teeth. Extensive and in-depth descriptions of

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

s from the

were provided by A. W. Crompton (1955). Crompton provided the name ''Cricodon'' as more

discoveries were found and a more complete view of the

skeleton A skeleton is the structural frame that supports the body of an animal. There are several types of skeletons, including the exoskeleton, which is the stable outer shell of an organism, the endoskeleton, which forms the support structure inside ...

could be created.

Fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

evidence of ''Cricodon'' has also been discovered in the

Karoo The Karoo ( ; from the Afrikaans borrowing of the South Khoekhoe !Orakobab or Khoemana word ''ǃ’Aukarob'' "Hardveld") is a semi-desert natural region of South Africa. No exact definition of what constitutes the Karoo is available, so its ext ...

Beds, specifically in the

Cynognathus Assemblage Zone The ''Cynognathus'' Assemblage Zone is a tetrapod biozone utilized in the Karoo Basin of South Africa. It is equivalent to the Burgersdorp Formation, the youngest lithostratigraphic formation in the Beaufort Group, which is part of the fossilife ...

.Abdala, F., P. J. Hancox, and J. Neveling. 2005. Cynodonts from the uppermost Burgersdorp Formation, South Africa, and their bearing on the biostratigraphy and correlation of the Triassic ''Cynognathus'' Assemblage Zone. ''Journal of Vertebrate Paleontology'' 25:192–199. The ''Cynognathus'' Assemblage Zone encompasses the boundary between the late Early and early Middle

Triassic period The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest period ...

, and has been subdivided into three distinct subzones (Subzone A, Subzone B, and Subzone C)Hancox, P., Shishkin, M., Rubidge, B., & Kitching, J. (1995). A threefold subdivision of the ''Cynognathus'' assemblage zone (Beaufort Group, South Africa) and its palaeogeographical implications. South African Journal of Science,91(3), 143-144. based primarily on the spatial and temporal ranges of key

temnospondyl Temnospondyli (from Greek language, Greek τέμνειν, ''temnein'' 'to cut' and σπόνδυλος, ''spondylos'' 'vertebra') is a diverse order (biology), order of small to giant tetrapods—often considered Labyrinthodontia, primitive amphi ...

index

taxa In biology, a taxon (back-formation from ''taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular nam ...

. ''Cricodon'' fossils have been found in the youngest of the three subzones, Subzone C. However, upper postcanines resembling those of ''Cricodon'' are also known from deposits corresponding to Subzone B. The trirachodontid ''"Trirachodon" kannemeyeri'' Seeley, 1895 is now referred to ''Cricodon'' as ''C. kannemeyeri''.

Description

Skull

Cranial bone The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, the ...

s recovered from the

consist of a badly crushed orbito-ethmoidal region, a practically complete right

mandible In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower tooth, teeth in place. The mandible sits beneath the maxilla. It is the only movabl ...

, two fragments of the left mandible, several loose teeth, a portion of the

occiput The occipital bone () is a cranial dermal bone and the main bone of the occiput (back and lower part of the skull). It is trapezoidal in shape and curved on itself like a shallow dish. The occipital bone overlies the occipital lobes of the cereb ...

, and several unidentified fragments. In the

upper jaw The maxilla (plural: ''maxillae'' ) in vertebrates is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. The t ...

, only the posterior ends of the

maxillae The maxilla (plural: ''maxillae'' ) in vertebrates is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. The t ...

, a portion of the

palate The palate () is the roof of the mouth in humans and other mammals. It separates the oral cavity from the nasal cavity. A similar structure is found in crocodilians, but in most other tetrapods, the oral and nasal cavities are not truly sepa ...

, and the floor of the orbits are well preserved. The maxillary postcanines are transversely ovate and have three main

cusp A cusp is the most pointed end of a curve. It often refers to cusp (anatomy), a pointed structure on a tooth. Cusp or CUSP may also refer to: Mathematics * Cusp (singularity), a singular point of a curve * Cusp catastrophe, a branch of bifurca ...

s arranged upon the same transverse plane. The three main cusps are composed of the lingual, central, and labial cusp. There are also small peripheral cuspules present on the

anterior Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...

and posterior borders of the crown. Roots of the upper postcanines are long, transversely flattened near the crown and taper away to a short point distally. The maximum known transverse diameter of the maxillary postcanines is 13mm. A characteristic feature of the

is the sharp angle formed by the junction of the lower margin of the

dentary In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower tooth, teeth in place. The mandible sits beneath the maxilla. It is the only movabl ...

and the anterior surface of the dental

symphysis A symphysis (, pl. symphyses) is a fibrocartilaginous fusion between two bones. It is a type of cartilaginous joint, specifically a secondary cartilaginous joint. # A symphysis is an amphiarthrosis, a slightly movable joint. # A growing together ...

. A diagnostic difference between the

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. A

mammalian Mammals () are a group of vertebrate animals constituting the class (biology), class Mammalia (), characterized by the presence of mammary glands which in Female#Mammalian female, females produce milk for feeding (nursing) their young, a ...

feature that can be seen in gomphodont

cynodonts The cynodonts () (clade Cynodontia) are a clade of eutheriodont therapsids that first appeared in the Late Permian (approximately 260 mya), and extensively diversified after the Permian–Triassic extinction event. Cynodonts had a wide variety ...

is that each root is surrounded by a pocket of

cancellous bone A bone is a rigid organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red and white blood cells, store minerals, provide structure and support for the body, and ...

. The maximum transverse diameter of the mandibular postcanines is 9mm. The main cusps of the mandibular and maxillary postcanines form a transverse ridge across the center of the crown. The transverse ridge of a tooth would fit into the depression formed between the transverse ridges of two adjoining teeth. Recent research conducted by Hendrickx, Abdala, and Choiniere (2016) has revealed new information in regard to the distribution of enamel

microstructure Microstructure is the very small scale structure of a material, defined as the structure of a prepared surface of material as revealed by an optical microscope above 25× magnification. The microstructure of a material (such as metals, polymers ...

in non-mammaliform cynodonts, specifically in ''Cricodon 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 in

cynognathia Cynognathia ("dog jaw") is one of two major clades of cynodonts, the other being Probainognathia. Cynognathians included the large carnivorous genus '' Cynognathus'' and the herbivorous traversodontids. Cynognathians can be identified by several ...

ns.Hendrickx, C., Abdala, F., & Choiniere, J. (2016). Postcanine microstructure in ''Cricodon metabolus'', a Middle Triassic gomphodont cynodont from south-eastern Africa. ''Palaeontology'', 59(6), 851-861. The newfound discovery in regards to the thickened enamel has many ecological implications. In ''Cricodon metabolus'', the enamel layer of the gomphodont tooth is around 11.5 times thicker than the sectorial tooth. The postcanine gomphodont teeth (labiolingually expanded teeth with large occlusal surfaces) were used for chewing, crushing, and grinding fibrous plant material, meaning that they were under higher loads and apically oriented stresses. Sectorial teeth on the other hand were used to shear plant material and were not subjected to the same types of occlusal stresses, therefore the enamel thickness was not maintained. Another reason proposed by Hendrickx, Abdala, and Choiniere (2016) to explain the thin enamel in sectorial teeth is due to replacement timing and patterns, as they will be sequentially replaced by gomphodont teeth. Another characteristic that was observed in the gomphodont teeth were enamel and

dentine Dentin () (American English) or dentine ( or ) (British English) ( la, substantia eburnea) is a calcified tissue of the body and, along with enamel, cementum, and pulp, is one of the four major components of teeth. It is usually covered by ena ...

incremental lines. The enamel and

incremental lines,

odontoblast In vertebrates, an odontoblast is a cell of neural crest origin that is part of the outer surface of the dental pulp, and whose biological function is dentinogenesis, which is the formation of dentin, the substance beneath the tooth enamel on the ...

tubules In biology, a tubule is a general term referring to small tube or similar type of structure. Specifically, tubule can refer to: * a small tube or fistular structure * a minute tube lined with glandular epithelium * any hollow cylindrical body stru ...

in dentine, and discontinuous columnar divergence units in enamel support the consistent presence of synapsid columnar enamel in Cynognathia.

Postcranial skeleton

Twenty-five

vertebrae The spinal column, a defining synapomorphy shared by nearly all vertebrates,Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristic i ...

were discovered which belong to the

dorsal Dorsal (from Latin ''dorsum'' ‘back’) may refer to: * Dorsal (anatomy), an anatomical term of location referring to the back or upper side of an organism or parts of an organism * Dorsal, positioned on top of an aircraft's fuselage * Dorsal co ...

and

sacral region The sacrum (plural: ''sacra'' or ''sacrums''), in human anatomy, is a large, triangular bone at the base of the spine that forms by the fusing of the sacral vertebrae (S1S5) between ages 18 and 30. The sacrum situates at the upper, back part ...

s with only one vertebrae having a well preserved

neural arch The spinal column, a defining synapomorphy shared by nearly all vertebrates,Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristic i ...

from the sacral region. There are additional apophyses below the posterior

zygapophysis The articular processes or zygapophyses (Greek ζυγον = "yoke" (because it links two vertebrae) + απο = "away" + φυσις = "process") of a vertebra are projections of the vertebra that serve the purpose of fitting with an adjacent vertebr ...

which articulate with concavities on the lateral surface of the neural arch, posterior and inferior to the

zygapophysis. ''Cricodon metabolus'' has the typical cynodont expanded

ribs The rib cage, as an enclosure that comprises the ribs, vertebral column and sternum in the thorax of most vertebrates, protects vital organs such as the heart, lungs and great vessels. The sternum, together known as the thoracic cage, is a semi- ...

of which thirteen dorsal ribs were discovered. The entire

humerus The humerus (; ) is a long bone in the arm that runs from the shoulder to the elbow. It connects the scapula and the two bones of the lower arm, the radius and ulna, and consists of three sections. The humeral upper extremity consists of a roun ...

was 12.5 cm long and had typical cynodont characterizations such as the twisted bone and the plane of the

distal Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position pro ...

end forming an angle of 40 degrees with that of the

proximal Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position pro ...

. The discovered

femur The femur (; ), or thigh bone, is the proximal bone of the hindlimb in tetrapod vertebrates. The head of the femur articulates with the acetabulum in the pelvic bone forming the hip joint, while the distal part of the femur articulates with ...

was 12.5 cm long with the

capitulum capitulum (plural capitula) may refer to: *the Latin word for chapter ** an index or list of chapters at the head of a gospel manuscript ** a short reading in the Liturgy of the Hours *** derived from which, it is the Latin for the assembly known ...

directed at an angle of 45 to 50 degrees to the main axis of the slender shaft. Another characteristic of the femur noted is that it projects well forward from the main body of the shaft. Although little is known of the hand of cynodonts, it is believed that ''Cricodon metabolus'' has a phalangeal formula of 23443, with the second phalange reduced in digits three and four.

Classification

Below is a

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

from Gao ''et al.'' (2010)Gao, K., Fox, R., Zhou, C., & Li, D. (2010). A New Nonmammalian Eucynodont (Synapsida: Therapsida) from the Triassic of Northern Gansu Province, China, and its Biostratigraphic and Biogeographic Implications. ''American Museum Novitates'', 1-25. showing the

phylogenetic In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...

relationships of one part of the Cynodontia relative to ''Cricodon'':

References

External links

Tritylodontoidea Cope, 1884
{{Taxonbar, from=Q30593303 Cynognathians Prehistoric cynodont genera Early Triassic synapsids of Africa Middle Triassic synapsids of Africa Early Triassic genus first appearances Middle Triassic genus extinctions Triassic Tanzania Fossils of Tanzania Fossil taxa described in 1955