Eupelycosauria is a large

clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

of animals characterized by the unique shape of their

skull The skull, or cranium, is typically a bony enclosure around the brain of a vertebrate. In some fish, and amphibians, the skull is of cartilage. The skull is at the head end of the vertebrate. In the human, the skull comprises two prominent ...

, encompassing all

mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...

s and their closest extinct relatives. They first appeared 308million years ago during the Early Pennsylvanian epoch, with the fossils of '' Echinerpeton'' and perhaps an even earlier genus, '' Protoclepsydrops'', representing just one of the many stages in the evolution of mammals,Kemp. T.S., 1982, ''Mammal-like Reptiles and the Origin of Mammals''.

Academic Press Academic Press (AP) is an academic book publisher founded in 1941. It launched a British division in the 1950s. Academic Press was acquired by Harcourt, Brace & World in 1969. Reed Elsevier said in 2000 it would buy Harcourt, a deal complete ...

, New York in contrast to their earlier

amniote Amniotes are tetrapod vertebrate animals belonging to the clade Amniota, a large group that comprises the vast majority of living terrestrial animal, terrestrial and semiaquatic vertebrates. Amniotes evolution, evolved from amphibious Stem tet ...

ancestors.

Taxonomy

Eupelycosaurs are

synapsid Synapsida is a diverse group of tetrapod vertebrates that includes all mammals and their extinct relatives. It is one of the two major clades of the group Amniota, the other being the more diverse group Sauropsida (which includes all extant rept ...

s, animals whose skull has a single opening behind the eye. They are distinguished from the

Caseasauria Caseasauria is one of the two main clades of early synapsids, the other being the Eupelycosauria. Caseasaurs are currently known only from the Late Carboniferous and the Permian, and include two superficially different families, the small insect ...

n synapsids by having a long, narrow supratemporal bone (instead of one that is as wide as it is long) and a

frontal bone In the human skull, the frontal bone or sincipital bone is an unpaired bone which consists of two portions.'' Gray's Anatomy'' (1918) These are the vertically oriented squamous part, and the horizontally oriented orbital part, making up the bo ...

with a wider connection to the upper margin of the

orbit In celestial mechanics, an orbit (also known as orbital revolution) is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an ...

. Laurin, M. and Reisz, R. R., 1997
Autapomorphies of the main clades of synapsids
-

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The only living descendants of basal eupelycosaurs are the

s. The group was originally considered a suborder of pelycosaurs or "mammal like reptiles",Reisz, R. R., 1986, ''Handbuch der Paläoherpetologie – Encyclopedia of Paleoherpetology, Part 17A Pelycosauria'' Verlag Dr. Friedrich Pfeil, but it was redefined in 1997, and the term pelycosaur itself has fallen into disfavor. We now know that the eupelycosaurs were not in fact reptiles nor of reptile lineage - the modern term stem mammal is used instead. Some recent studies suggested that one of its subgroups,

Varanopidae Varanopidae is an extinct family (biology), family of amniotes known from the Late Carboniferous to Middle Permian that resembled monitor lizards (with the name of the group deriving from the monitor lizard genus ''Varanus'') and may have filled ...

, are really nested within sauropsids, leaving the other defined subgroup of it, Metopophora, as its synonym.

Evolution

Many non-therapsid eupelycosaurs were the dominant land animals from the latest

Carboniferous The Carboniferous ( ) is a Geologic time scale, geologic period and System (stratigraphy), system of the Paleozoic era (geology), era that spans 60 million years, from the end of the Devonian Period Ma (million years ago) to the beginning of the ...

to the end of the

Early Permian 01 or 01 may refer to: * The year 2001, or any year ending with 01 * The month of January * 1 (number) Music * '01 (Richard Müller album), ''01'' (Richard Müller album), 2001 * 01 (Urban Zakapa album), ''01'' (Urban Zakapa album), 2011 * ''01011 ...

epoch. Ophiacodontids were common from their appearance in the late

( Pennsylvanian) to the early Permian, but they became progressively smaller as the early Permian progressed. The edaphosaurids, along with the caseids, were the dominant

herbivore A herbivore is an animal anatomically and physiologically evolved to feed on plants, especially upon vascular tissues such as foliage, fruits or seeds, as the main component of its diet. These more broadly also encompass animals that eat ...

s in the early part of the Permian. The most renowned edaphosaurid is '' Edaphosaurus'', a large [] herbivore which had a sail on its back, probably used for thermoregulation and mating. Sphenacodontidae, Sphenacodontids, a family of carnivorous eupelycosaurs, included the famous ''Dimetrodon'', which is sometimes mistaken for a

dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic Geological period, period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the #Evolutio ...

, and was the largest predator of the period. Like ''Edaphosaurus'', ''Dimetrodon'' also had a distinctive sail on its back, and it probably served the same purpose - regulating heat. The varanopid family passingly resembled today's monitor lizards and may have had the same lifestyle.

Therapsids Therapsida is a clade comprising a major group of eupelycosaurian synapsids that includes mammals and their ancestors and close relatives. Many of the traits today seen as unique to mammals had their origin within early therapsids, including li ...

descended from a clade closely related to the sphenacodontids. They became the succeeding dominant land animals for the rest of the Permian, and in the latter part of the

Triassic The Triassic ( ; sometimes symbolized 🝈) is a geologic period and system which spans 50.5 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.4 Mya. The Triassic is t ...

, descendants of the

cynodont Cynodontia () is a clade of eutheriodont therapsids that first appeared in the Late Permian (approximately 260 Megaannum, mya), and extensively diversified after the Permian–Triassic extinction event. Mammals are cynodonts, as are their extin ...

s, an advanced group of therapsids, gave rise to the first true

s. All non-therapsid synapsids, including all basal eupelycosaurs, as well as many other life forms, became extinct at the end of Permian period.

Classification

The following

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

is modified from Huttenlocker ''et al''. (2021):

References

External links

* * {{Taxonbar, from=Q1349838 Synapsida Tetrapod unranked clades Extant Pennsylvanian first appearances