Eureptilia

Captorhinus is the type genus of the family Captorhinidae. This genus comprises the single-tooth-rowed species C. laticeps and C. magnus as well as the multiple-tooth-rowed C. aguti. All members of the genus are exclusively known from Lower Permian terrestrial deposits of Texas and Oklahoma.

Scientific History: Captorhinus is the longest-known genus of the family and its scientific record dates back to the 1880s. In 1882 E. D. Cope described Ectocynodon aguti and later assigned it to his genus Pariotichus leading to the new combination P. aguti. In 1909 S. W. Williston described the species P. laticeps.

A first revision of these "cotylosaurs" by E. C. Case in 1911 revealed that most of the representatives of Pariotichus belong to another genus erected by Cope in 1895: Captorhinus. Case identified four species of Captorhinus: C. aguti, C. angusticeps, C. isolomus, and C. aduncus. In that course P. laticeps was synonymized with C. isolomus. Moreover Case de facto declared Ectocynodon a nomen dubium due to the bad state of preservation of its type material. Strictly speaking the state of preservation of the holotype of Captorhinus is not much better, however, Case deemed it diagnostic at that time. Seen that Ectocynodon is thus the senior synonym proper it provides a good example for a nomen oblitum, because it has never been used in captorhinid taxonomy since 1911.

In 1959 a second revision of the captorhinid fossils by R. J. Seltin lead to the recognition of only one species of Captorhinus: C. aguti. Oddly Seltin (1959) erected the new species Labidosaurus oklahomensis on the basis of a small single-tooth-rowed skull with Captorhinus-like appearance. Later doubts were casted on the assignment of the material formerly referred to P. laticeps to C. aguti because of the presence of a single tooth row, unlike to all other remains of C. aguti which show multiple tooth rows in at least one branch of the lower or upper jaw. In 1979 M. J. Heaton thus erected the genus Eocaptorhinus for these exclusively single-tooth-rowed remains leading to the new combination E. laticeps. In addition Heaton assigned Seltin's L. oklahomensis to this species. Finally E. laticeps was reassigned to Captorhinus by several authors because of its general similarity to C. aguti (see below), but this time retained its status as distinct species.

In 2001 the scientifically youngest species of Captorhinus was reported by Kissel et al. and subsequently, in 2002, was given the name C. magnus.

Characteristics of the genus: Captorhinus is characterized by a series of cranial traits that occur in other basal captorhinids as well, e.g. relatively large eye openings, shagreen dentiton on palatal bones, a distinctive sculptural pattern on the bones of the dermal skull roof and on the bones of the external surface of the mandible, and a down-curved, hook-like premaxilla. However within the genus one species existst that has multiple tooth rows, a feature not known from other basal captorhinids, such as Romeria or Protocaptorhinus. In addition the posterior teeth of the maxillary are not simple cones as in more basal captorhinids but are either chisel-shaped or ogival. Also the posterior part of the skull is clearly more expanded laterally, being reminiscent rather to the moradisaurines than to the basal members of the family. A unique feature of Captorhinus may be the retroarticular process of the lower jaw being conspiciously longer than broad, since this is as yet not seen in any other captorhinid taxon.

Type species: C. aguti is the type species of the genus. It is without doubt the by far best known captorhinid species. Hundreds of well preserved, articulated skulls and skull fragments are collected at the famous Dolese Brothers quarry near Fort Sill in Oklahoma.

C. aguti is distinguished primarily from C. laticeps and C. magnus in having multiple rows of teeth on the maxillary and dentary bones. These rows are, in contrast to the tooth rows seen in moradisaurines, not oriented parallel to the lateral margins of the jaws but are oriented obliquely to that margins, in a way that each row is followed posterolaterally by the next one. Thus, up to eight rows are observed in C aguti without the dentigerous bones forming the strongly medially expanded tooth plates which characterize the jaws of the moradisaurines. Nevertheless the toothed areas of maxillary and dentary are broader mediolaterally than in the single-tooth-rowed captorhinids.

Second species: C. laticeps is distinguished form C. aguti in lacking multiple tooth rows in all four branches of the jaws. In almost all other respects it is very similar to C. aguti. C. laticeps differs from C. magnus in being considerably smaller and in having chisel-shaped instead of ogival teeth on the posterior portion of maxillary and dentary.

Synonymy of Eocaptorhinus: Eocaptorhinus is considered a synonym of Captorhinus. The genus was erected by Malcolm J. Heaton in 1979 including fossils hitherto referred to Pariotichus laticeps Williston leading to the new combination Eocaptorhinus laticeps. Heaton chose this name meaning "early Captorhinus because the remains of this animal are striking similar to Captorhinus aguti, with the exception of lacking the characteristic multiple tooth rows, but come from an geologically older horizon. Disregarding the principles of cladistics Heaton made an stratophenetic approach in elucidating the evolution of captorhinids, postulating an anagenetic Romeria -> Protocaptorhinus -> Eocaptorhinus -> Captorhinus model.

More recent examinations of Eocaptorhinus and Captorhinus aguti revealed that they share such a great number of non-dentitional traits that distinction on a generic level became obsolete. Moreover in one locality (Ft. Sill, Oklahoma) captorhinid remains with both, single tooth rows and multiple tooth rows are present without any possibility of stratigraphic differentiation of the fossils found at that time. In addition it has been shown that in C. aguti some specimen have a single tooth row on one and a double tooth row on the other branch of the jaw. Thus, despite minor differences in postcranial body structure, it has been ambiguous wether C. laticeps and C. aguti are truly distinct species and if not in which way both forms are related: as sympatric subspecies, as males and females, or afterall as distinct ontogenetic stages of one single species.

Then a significant difference in the shape of the teeth of both forms was recognized: C. aguti was discovered to have teeth that resemble rather a pointed arch (also called ogive) whereas C. laticeps has teeth that are rather chisel-shaped. However this seemed not sufficient to legitimate the "two-species-concept".

The discovery of a new species, Captorhinus magnus, throwed light on this obscure relationship in a cladistical way. C. magnus is approximately as twice as large as C. aguti and its jaws are equipped with only one single row of, however ogival, teeth. Most important is the fact that C. magnus is unequivocally distinct from C. aguti in having an concave articulation surface at the distal end of the femur whereas in C. aguti this articulation surface is convex, a feature hardly relatable to ontogeny. Sexual dimorphism could also be excluded because further collections at the Ft. Sill site obtained that specimen of C. magnus are more abundant in the lower parts of the section whereas C. aguti is more abundant in the higher part. In case of sexual dimorphism a by far more regular distribution would be expected. Finally, inclusion of the new taxon in the phylogenetic analyses yielded that C. laticeps and C. aguti are no longer sister clades but C. aguti is sister to C. magnus. Thus, any conspecific or closer relationship of C. laticeps and C. aguti can be ruled out due to the close relationship of C. aguti to another, species of Captorhinus that is clearly not conspecific with C. aguti.

Third species: C. magnus differs from C. aguti in having only one single row of teeth on dentary and maxillary. From C. laticeps it is distinct in being twice as large and in having ogival teeth in the posterior portion of maxillary and dentary. Moreover the distal articulation surface of the femur is concave whereas in C. aguti and C. laticeps it is convex.

Phylogeny: Captorhinus comprises rather derived representatives of basal captorhinids. The dentition, accompanied with the laterally widened posterior portion of the skull, shows a certain degree of specialization compared to the more basal forms.

Below follow succinct accounts on the recognized species of Captorhinus