Sphenacodontia is a stem-based

clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English ter ...

of derived synapsids. It was defined by Amson and Laurin (2011) as "the largest clade that includes '' Haptodus baylei'', '' Haptodus garnettensis'' and '' Sphenacodon ferox'', but not '' Edaphosaurus pogonias''". They first appear during the Late Pennsylvanian ( Upper Carboniferous) epoch. From the end of the

Carboniferous The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carboniferou ...

to the end of the

Permian The Permian ( ) is a geologic period and stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last period of the Paleo ...

, most of them remained large, with only some secondarily becoming small in size. Basal Sphenacodontia constitute a transitional

evolution Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...

ary series from early pelycosaurs to ancestral

therapsid Therapsida is a major group of eupelycosaurian synapsids that includes mammals, their ancestors and relatives. Many of the traits today seen as unique to mammals had their origin within early therapsids, including limbs that were oriented more ...

s (which in turn were the ancestors of more advanced forms and finally the

mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur ...

s). One might say that the sphenacodontians are proto-therapsids (even though there is almost a 30-million-years gap between the separation of the ancestors of therapsids from other sphenacodontians and the first appearance of therapsids in the fossil record).

Characteristics

The defining characteristics include a thickening of the

maxilla 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. T ...

visible on its internal surface, above the large front (

caniniform In mammalian oral anatomy, the canine teeth, also called cuspids, dog teeth, or (in the context of the upper jaw) fangs, eye teeth, vampire teeth, or vampire fangs, are the relatively long, pointed teeth. They can appear more flattened however ...

) teeth; and the

premaxilla The premaxilla (or praemaxilla) is one of a pair of small cranial bones at the very tip of the upper jaw of many animals, usually, but not always, bearing teeth. In humans, they are fused with the maxilla. The "premaxilla" of therian mammal has ...

ry teeth being set in deep sockets. All other (sister group and more primitive) synapsid clades have teeth that are set in shallow sockets.

Classification

The following taxonomy follows Fröbisch ''et al.'' (2011), Benson (2012) and Spinder (2016) unless otherwise noted. Class Synapsida *

Eupelycosauria Eupelycosauria is a large clade of animals characterized by the unique shape of their skull, encompassing all mammals and their closest extinct relatives. They first appeared 308million years ago during the Early Pennsylvanian epoch, with the fos ...

** Sphenacodontia *** †'' Haptodus'' *** †''

Hypselohaptodus ''Hypselohaptodus'' is a genus of sphenacodont synapsid from the Cisuralian of England. It contains a single species, ''Hypselohaptodus grandis'', and is known only from a single specimen, a partial left maxilla, which is hosted at the Warwick ...

'' *** Pantherapsida **** †'' Kenomagnathus'' **** †'' Tetraceratops'' **** † Palaeohatteriidae ****

Sphenacodontoidea Sphenacodontoidea is a node-based clade that is defined to include the most recent common ancestor of Sphenacodontidae and Therapsida and its descendants (including mammals). Sphenacodontoids are characterised by a number of synapomorphies conc ...

***** † Sphenacodontidae ***** Therapsida

References

* Laurin, M. and Reisz, R. R., 1997
Autapomorphies of the main clades of synapsids
- Tree of Life Web Project

External links

Synapsida: Sphenacodontia
{{Taxonbar, from=Q930410 Extant Pennsylvanian first appearances Taxa named by Llewellyn Ivor Price Taxa named by Alfred Romer

Characteristics

Classification

See also

References

External links