''Baldwinonus'' is an extinct

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...

of basal

synapsid Synapsids + (, 'arch') > () "having a fused arch"; synonymous with ''theropsids'' (Greek, "beast-face") are one of the two major groups of animals that evolved from basal amniotes, the other being the sauropsids, the group that includes reptil ...

s from the Early Permian. The

type species In zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the species that contains the biological type specime ...

is ''Baldwinonus trux'', named in 1940 from the

Cutler Formation The Cutler Formation or Cutler Group is a rock unit that is exposed across the U.S. states of Arizona, northwest New Mexico, southeast Utah and southwest Colorado. It was laid down in the Early Permian during the Wolfcampian epoch. Descri ...

New Mexico ) , population_demonym = New Mexican ( es, Neomexicano, Neomejicano, Nuevo Mexicano) , seat = Santa Fe , LargestCity = Albuquerque , LargestMetro = Tiguex , OfficialLang = None , Languages = English, Spanish ( New Mexican), Navajo, Ke ...

. A second species, ''Baldwinonus dunkardensis'', was named in 1952 from

Ohio Ohio () is a state in the Midwestern region of the United States. Of the fifty U.S. states, it is the 34th-largest by area, and with a population of nearly 11.8 million, is the seventh-most populous and tenth-most densely populated. The sta ...

. ''Baldwinonus'' was first classified in the family Eothyrididae, but the group has since been recognized as a

wastebasket taxon Wastebasket taxon (also called a wastebin taxon, dustbin taxon or catch-all taxon) is a term used by some taxonomists to refer to a taxon that has the sole purpose of classifying organisms that do not fit anywhere else. They are typically defined ...

for many early synapsids.R. R. Reisz. 1986. Pelycosauria. Handbuch der Paläoherpetologie / Encyclopedia of Paleoherpetology 17A:1-102 More recently, ''Baldwinonus'' has been placed in the family

Ophiacodontidae Ophiacodontidae is an extinct family of early eupelycosaurs from the Carboniferous and Permian. '' Archaeothyris'', and ''Clepsydrops'' were among the earliest ophiacodontids, appearing in the Late Carboniferous. Ophiacodontids are among the mos ...

. Its

phylogenetic In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...

relationship to other early synapsids remains poorly understood because it is only known from a few fragments of bone.

Description

''Baldwinonus trux'' is known from a fragment of a right

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 ...

or upper jaw bone, part of a

quadrate bone The quadrate bone is a skull bone in most tetrapods, including amphibians, sauropsids (reptiles, birds), and early synapsids. In most tetrapods, the quadrate bone connects to the quadratojugal and squamosal bones in the skull, and forms upper pa ...

, and several

vertebra The spinal column, a defining synapomorphy shared by nearly all vertebrates, Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristi ...

e. The maxilla contains 25 tooth sockets, some with teeth. There are sockets for five precaniniform teeth at the front of the jaw, two caniniform teeth behind them, and eight postcaniniform teeth at the back of the jaw. The margin of the jaw is straight for most of its length but curves upward toward the front tip. The large caniniform teeth in ''B. trux'' were initially taken as evidence for its placement within Eothyrididae, since large teeth are one of the most distinctive features of the eothyridid '' Eothyris''. However, ''Eothyris'' only has one precanine tooth on each side of the jaw, and is probably not closely related to ''Baldwinonus trux''. A buttress of bone runs along the inside of the jaw above the sockets of the caniniform teeth, strengthening the jaw. This buttress extends upward like a flap to form a sharp ridge, a feature that ''Baldwinonus'' shares with a group of early synapsids called Ophiacodontidae. A groove of bone that presumably made space for a blood vessel runs horizontally along the inner surface of the jaw but turns downward near the buttress. In other groups of early synapsids such as Sphenacodontinae the buttress does not form a flap but grades smoothly with the surface of the jaw above it, and the groove for the blood vessel continues running horizontally onto the surface of the buttress. ''Baldwinonus trux'' is very similar in appearance to another poorly known synapsid called '' Stereophallodon''. Both have nearly identical vertebrae and maxillary bones. The only difference between the two is that the edge of the maxilla that contacts the palatine bone ends behind the caniniform tooth buttress in ''Baldwinonus'' and ends on the buttress in ''Stereophallodon''. In their description of the two synapsids, Brinkman and Eberth (1986) considered ''Baldwinonus'' and ''Stereophallodon'' to be closely related members of the family Ophiacodontidae. Some characteristics of the

centrum (Latin for ''center'') may refer to: Places In Greenland * Nuuk Centrum, a district of Nuuk, Greenland * Centrum Lake, Greenland In the Netherlands * Amsterdam-Centrum, the inner-most borough of Amsterdam, Netherlands * Rotterdam Centrum, a borou ...

or central part of the vertebra were interpreted as primitive characteristics of synapsids in general, and large caniniform teeth were taken as evidence that the two taxa were primitive among ophiacodontids as well. ''Baldwinonus dunkardensis'' is also known from a maxilla. Like ''B. trux'', it has two large canines with a buttress running above them. Unlike ''B. trux'', ''B. dunkardensis'' has only one precanine tooth and its maxilla is about half the size. In this respect it is more similar to eothyridids like ''Eothyris'' and '' Oedaleops'' than it is to ''B. trux'' or ophiacodontids.

References

{{Taxonbar, from=Q3279573 Ophiacodontids Prehistoric synapsid genera Cisuralian synapsids of North America Taxa named by Alfred Romer Fossil taxa described in 1940 Cisuralian genus first appearances Cisuralian genus extinctions