Oligokyphus, a Cynodont
Oligokyphus is an extinct genus of advanced herbivorous cynodonts of the late Triassic to early Jurassic periods. Originally considered to be an early mammal, it is now classified as a Mammaliamorph (nearly a mammal) because Oligokyphus does not have the mammalian jaw attachments and it retains a vestigial joint between the quadrate bone and the squamosal bone in the skull.
Description[]
Oligokyphus (meaning "small curved animal"), was a small animal, around 50 centimetres (20 in) in length, belonging to the herbivorous Tritylodontidae family. It resembled a weasel in appearance, with a long and slim body. Oligokyphus was found widely across North America, Europe, and China.
Skull and jaw[]
The teeth of the upper and lower jaw contain bump rows that fit together perfectly in order to maintain an accurate bite. Oligokyphus had a face similar to that of modern mammals, although there were differences in the cheekbones and eye sockets. It had a bony secondary palate and double-rooted cheek teeth. Unlike mammals, the teeth of Oligokyphus did not occlude. The jaw was double jointed, and the neck was flexible, with an atlas and axis and a double occipital condyle.
The teeth were different from those of related cynodonts; there were no canine teeth, and unusually large, rodent-like incisors. There is a large gap, or diastema, separating the cheek teeth from the incisors. The lower jaw of these animals moved back and forth when the mouth was shut so that the food could be chopped up. Oligokyphus had no premaxilla, but did have a lateral extension of the maxilla.
While the postcanines in non-mammalians, such as Oligokyphus, are difficult to differentiate from canines, the lower postcanines of Oligokyphus (also considered to be pre-molars) are defining from other Tritylodonts. On lower postcanine teeth of Trityldonts, two cusps can be found per row; however, Oligokyphus have two rows with three cusps in each row. These cusps, specific to Oligokyphus Tritylodonts, allowed for a well-fitting bite that was particularly good at shredding plant material dense in fiber. The foremost incisors are similar to those of today's rodents, extremely intensified and enlarged. The typical location of canine teeth is left empty with Oligokyphus. Instead, a gap is inserted in this area of the jaw as Oligokyphus lack the teeth commonly known as canines.
Cladistics/Phylogeny[]
Oligokyphus is in the family Tritylodontidae. The family is named after the shape of their teeth. Tritylodontidae means "three knob teeth". The members of this family were all small to medium-sized advanced synapsids with combined specialized structures for herbivorous eating. The first Tritylodont was found in South Africa in upper Jurassic rocks. It was first thought to be one of the earliest mammals. This classification has since been adjusted. These non-mammals became progressively more mammal-like. They are now classified as the closest relatives to the mammals and this is supported by their high, flat, crested jaw, large zygomatic arches, well developed secondary palate, and specialized dentition.
There have also been comparisons between the cranial nerves of Tritylodonts and mammals. The shoulder girdle and forelimb structures were suggestive of digging animals. These animals were extremely active and burrowed in leaf litter and dirt, which suggests characteristics of rodents and rabbits. They naturally had a metabolism that was partially or completely endothermic. They were thought to be driven out by relatives such as mammals competing for the same ecological niches.[citation needed] Another reason that this animal could have gone extinct was due to new plant development. Some flowering plants, or angiosperms, could have been detrimental to these animals since they may not have been used to eating new plants.[citation needed]
Oligokyphus is placed into the subgroup Probainognathia. This forms a monophyletic group with the tritheledontid Pachygenelus.
Fossil finds[]
Though Oligokyphus is very widespread, it was not until 1953 that representatives of this group were found. Information was first collected from the Kayenta Formation on Comb Ridge in northeastern Arizona. Numerous specimens of Oligokyphus were obtained by Harvard University and the Museum of Northern Arizona in the "Silty Facies". Many fossils have also been found throughout the UK, Germany and China. Some very small fragment remains have also been found in Antarctica. By these fossil records, one can see that Oligokyphus have a vertical humerus and a minor trochanter. This broad distribution indicates that there were no barriers to separate this terrestrial vertebrate.
Paleoecology[]
Habitat[]
Oligokyphus were small tetrapod, terrestrial animals. They have long been considered as mammaliomorphs, a link between earlier synapsids and modern mammals.