Pachygenelus
Encyclopedia
Pachygenelus is an extinct genus
of tritheledontid cynodont. Fossil
s have been found from the Karoo basin
in South Africa
and date back to the Early Jurassic
.
Pachygenelus had both an articular
-quadrate
and dentary-squamosal
jaw joint characteristic of ictidosaurs. Only mammal
s possess the dentary-squamosal articulation, while all other tetrapods possess the typical arcticular-quadrate articulation. Thus the jaw of Pachygenelus can be seen as transitional between non-mammalian synapsids and true mammals. Another feature of Pachygenelus that is shared with mammals is plesiomorphic prismatic enamel, or enamel
arranged into strengthened prisms. The upper and lower tooth rows occluded
with one another, although not as close as what is seen in true mammals. Wear facets are present on the lingual sides of the upper and external faces of the lower postcanines, and are seen as evidence for the occlusion. Despite all of the derived adaptation seen in its teeth, the dentition of Pachygenelus is believed to have been the least specialized of all tritheledontids.
The genus was named in 1913 on the basis of a partial lower jaw found from South Africa, with the type species
being named P. monus. A new species, P. milleri, was named in 1983 and distinguished from the type species in possessing an accessory posterior cusp on the lower postcanines.
The use of the family
Tritheledontidae to include Pachygenelus and several other chiniquodontoids
has been questioned, and it has been suggested that only Trithelodon
meets the qualifications required for a true tritheledontid. The authors of the paper that first proposed the idea suggested that all other tritheledontids be reassigned to a new family called Pachygenelidae, named after Pachygenelus.
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
of tritheledontid cynodont. Fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
s have been found from the Karoo basin
Karoo
The Karoo is a semi-desert region of South Africa. It has two main sub-regions - the Great Karoo in the north and the Little Karoo in the south. The 'High' Karoo is one of the distinct physiographic provinces of the larger South African Platform division.-Great Karoo:The Great Karoo has an area of...
in South Africa
South Africa
The Republic of South Africa is a country in southern Africa. Located at the southern tip of Africa, it is divided into nine provinces, with of coastline on the Atlantic and Indian oceans...
and date back to the Early Jurassic
Early Jurassic
The Early Jurassic epoch is the earliest of three epochs of the Jurassic period...
.
Pachygenelus had both an articular
Articular
The articular bone is part of the lower jaw of most tetrapods, including amphibians, sauropsids and early synapsids. In these animals it is connected to two other lower jaw bones, the suprangular and the angular...
-quadrate
Quadrate bone
The quadrate bone is part of a skull in most tetrapods, including amphibians, sauropsids , and early synapsids. In these animals it connects to the quadratojugal and squamosal in the skull, and forms part of the jaw joint .- Evolutionary variation :In snakes, the quadrate bone has become elongated...
and dentary-squamosal
Squamosal
The squamosal is a bone of the head of higher vertebrates. It is the principal component of the cheek region in the skull, lying below the temporal series and otic notch and bounded anteriorly by postorbital. Posteriorly, the squamosal articulates with the posterior elements of the palatal complex,...
jaw joint characteristic of ictidosaurs. Only mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
s possess the dentary-squamosal articulation, while all other tetrapods possess the typical arcticular-quadrate articulation. Thus the jaw of Pachygenelus can be seen as transitional between non-mammalian synapsids and true mammals. Another feature of Pachygenelus that is shared with mammals is plesiomorphic prismatic enamel, or enamel
Tooth enamel
Tooth enamel, along with dentin, cementum, and dental pulp is one of the four major tissues that make up the tooth in vertebrates. It is the hardest and most highly mineralized substance in the human body. Tooth enamel is also found in the dermal denticles of sharks...
arranged into strengthened prisms. The upper and lower tooth rows occluded
Occlusion (dentistry)
Occlusion, in a dental context, means simply the contact between teeth. More technically, it is the relationship between the maxillary and mandibular teeth when they approach each other, as occurs during chewing or at rest....
with one another, although not as close as what is seen in true mammals. Wear facets are present on the lingual sides of the upper and external faces of the lower postcanines, and are seen as evidence for the occlusion. Despite all of the derived adaptation seen in its teeth, the dentition of Pachygenelus is believed to have been the least specialized of all tritheledontids.
The genus was named in 1913 on the basis of a partial lower jaw found from South Africa, with the type species
Type species
In biological nomenclature, a type species is both a concept and a practical system which is used in the classification and nomenclature of animals and plants. The value of a "type species" lies in the fact that it makes clear what is meant by a particular genus name. A type species is the species...
being named P. monus. A new species, P. milleri, was named in 1983 and distinguished from the type species in possessing an accessory posterior cusp on the lower postcanines.
The use of the family
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
Tritheledontidae to include Pachygenelus and several other chiniquodontoids
Chiniquodontoidea
Chiniquodontoidea is a clade of cynodonts that is defined as including all probainognathians closer to mammals than to Probainognathus which is the more primitive form in Probainognathia....
has been questioned, and it has been suggested that only Trithelodon
Trithelodon
Trithelodon was an extinct genus of a group of mammal-like reptiles called cynodonts. Like cynodonts, Trithelodon had many traits shared by mammals. Trithelodon was probably an insectivore, and nocturnal....
meets the qualifications required for a true tritheledontid. The authors of the paper that first proposed the idea suggested that all other tritheledontids be reassigned to a new family called Pachygenelidae, named after Pachygenelus.
External links
- Pachygenelus in the Paleobiology DatabasePaleobiology Database' is an online resource for information on the distribution and classification of fossil animals, plants, and microorganisms.-History:The Paleobiology Database was founded in 2000. It has been funded by the National Science Foundation and the Australian Research Council...