Romeriida is a clade

Clade

A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

 of reptiles that consists of diapsid

Diapsid

Diapsids are a group of reptiles that developed two holes in each side of their skulls, about 300 million years ago during the late Carboniferous period. Living diapsids are extremely diverse, and include all crocodiles, lizards, snakes, and tuatara...

s and the extinct protorothyridid

Protorothyrididae

Protorothyrididae is a family of small, lizard-like reptiles. Their skulls did not have fenestrae, as is also true of modern turtles and tortoises. Protorothyridids lived from the Late Carboniferous to Early Permian periods, in what is now North America. Many genera of primitive reptiles were...

 genus

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...

 Paleothyris

Paleothyris

Paleothyris was a small, agile, anapsid reptile which lived in the Middle Pennsylvanian epoch in Nova Scotia . Paleothyris had sharp teeth and large eyes, meaning that it was a nocturnal hunter. It was about a foot long. It probably fed on insects and other smaller animals found on the floor of its...

, if not the entire 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...

 Protorothyrididae. It is phylogenetically defined by Laurin & Reisz (1995) as the last common ancestor of Paleothyris and diapsids, and all its descendants. It is named after Alfred Romer

Alfred Romer

Alfred Sherwood Romer was an American paleontologist and comparative anatomist and a specialist in vertebrate evolution.-Biography:...

, a prominent vertebrate paleontologist of the twentieth century.

Protorothyridids were once placed in the family Romeriidae along with the captorhinid

Captorhinidae

Captorhinidae is one of the earliest and most basal reptile families.-Description:...

 Romeria

Romeria (genus)

Romeria is an extinct genus of Early Permian captorinid known from Texas of the United States. It was first named by Llewellyn Ivor Price in 1937 and the type species is Romeria texana. R. texana is known from the holotype MCZ 1480, a three-dimensionally preserved skull...

. Because Romeria is now considered to be a captorhinid, and Captorhinidae is placed outside of Romeriida, the genus is excluded from the clade. Protorothyridids were once the collective term for several romeriid genera of uncertain classification. However, more recent studies have proposed that Protorothyrididae is a paraphyletic taxon. Therefore, it is possible that many protorothyridids do not lay within the clade Romeriida.

Several synapomorphies characterize the romeriids. These include the separation of the tabular bone from the opisthotic bone, ventrally keeled anterior pleurocentra, long and slender carpi

Carpus

In tetrapods, the carpus is the sole cluster of bones in the wrist between the radius and ulna and the metacarpus. The bones of the carpus do not belong to individual fingers , whereas those of the metacarpus do. The corresponding part of the foot is the tarsus...

 and tarsi

Tarsus (skeleton)

In tetrapods, the tarsus is a cluster of articulating bones in each foot situated between the lower end of tibia and fibula of the lower leg and the metatarsus. In the foot the tarsus articulates with the bones of the metatarsus, which in turn articulate with the bones of the individual toes...

, and overlapping metapodial

Metapodial

Metapodials are long bones of the hand and feet which connect the digits to the centers. In humans, five are present in each hand and foot....

s.

Below is a cladogram

Cladogram

A cladogram is a diagram used in cladistics which shows ancestral relations between organisms, to represent the evolutionary tree of life. Although traditionally such cladograms were generated largely on the basis of morphological characters, DNA and RNA sequencing data and computational...

 showing the placement of Romeriida within Amniota, modified from Hill, 2005:
Cladogram after Müller & Reisz, 2006:
    Polyphyletic Protorothyrididae

External links

• Romeriida in the Paleobiology Database
  Paleobiology 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...