Family NITOSAURIDAE
Delorhynchus priscus new genus and new species
(delos, Gr., evident; rhynchos, Gr., neuter, nostril; priscus, L., ancient. Delorhynchus is masculine because of the ending that it acquires when transliterated into Latin.)
Type specimen.—Fragmentary left maxilla, bearing four teeth, KU 11117.
Referred specimens.—Fragmentary right maxilla having four teeth, KU 11118; fragmentary left maxilla having four teeth, the most posterior of which has been broken, KU 11119.
Horizon and locality.—A fissure deposit in the Arbuckle limestone at the Dolese Brothers Limestone Quarry, approximately six miles north of Fort Sill, in sec. 31, T. 4 N, R. 11 W, Comanche County, Oklahoma. These sediments are of early Permian age, possibly equivalent to the Arroyo formation, Lower Clear Fork Group of Texas (Vaughn, 1958: 981).
Diagnosis.—Small; marginal teeth conical, slender and recurved at tips; marginal tooth-row without caniniform enlargement; narial opening enlarged and bordered dorsally, posteriorly and ventrally by maxilla; maxilla with foramen opening laterally at posteroventral corner of naris.
Description (based on 3 maxillary fragments, see Table 1).—Each of the maxillary fragments bears four thecodont teeth. These are conical, slender and sharply pointed; in their distal third they are slightly recurved, laterally compressed, and have anterior and posterior non-serrated cutting edges. In medial aspect at their bases, the teeth are longitudinally striated. The bases of the teeth are circular in cross-section and are slightly bulbous. There is no caniniform enlargement of any of the teeth, the longest tooth of each fragment being differently placed in the series of teeth and little longer than the others. There is no swelling on either the internal or external surfaces of the maxillae. The teeth are in a continuous series; no diastema or maxillary step is evident.
The fragments have been broken along similar lines of fracture, and each is approximately rhomboidal in shape. The maxilla encircles the posterior border of the naris and extends dorsally above the naris to an extent sufficient to indicate the probable exclusion of the lacrimal bone from the narial border. At the posteroventral corner of the naris a foramen opens onto the lateral surface of the maxilla. The opening is the entrance to a canal that runs posteriorly above the tooth-row throughout the length of each specimen. Beneath the naris the maxilla extends as a broad tapering shelf, the ventral surface of which articulates with the premaxilla. The narial rim is wide, but wider ventrally than dorsally. The plane of the narial rim is oblique to the lateral surface of the maxilla. The external surface of each fragment is grooved and pitted. The ossification of each fragment appears to have been complete.
Table 1.—Dimensions, in Millimeters, of Three Maxillary Fragments of Delorhynchus priscus
Key:
A. Anterior height of fragment
B. Posterior height of fragment
C. Length of fragment at tooth-row
D. Dorsal length of fragment
E. Mean length of teeth
F. Anterior width of naris
| Catalogue Number and Mean | A. | B. | C. | D. | E. | F. |
| KU 11117 | 6.0 | 8.0 | 6.0 | 8.0 | 3.0 | 3.0 |
| KU 11118 | 6.0 | 6.0 | 9.0 | 8.0 | 2.0 | 3.0 |
| KU 11119 | 6.6 | 8.0 | 10.0 | 11.0 | ? | 4.6 |
| Mean | 6.2 | 7.3 | 8.3 | 9.0 | 2.5 | 4.5 |
Discussion.—The Nitosauridae are small primitive edaphosaurs with a moderately elongate face, sharp subisodont teeth, little development of canines and few specializations. The jaw is of a primitive type and articulates on a level with the tooth-row. The palatal dentition is primitive (Romer, 1956:280). The nitosaurids are thought to be related to the later Caseidae, and the most obvious structural similarities are found in the postcranial skeleton (Vaughn, 1958:989). Cranial resemblances between the families are fewer, but nevertheless indicate that a nitosaurid-caseid relationship exists.
Vaughn (1958) described a small pelycosaur, Colobomycter pholeter (Eothyrididae, Ophiacodontia) that structurally resembles the Caseidae. This individual also was obtained from the Fort Sill locality. In Vaughn's opinion the features of Colobomycter indicate a close relationship between eothyridids and caseids and the possibility that the caseids may well have been of eothyridid rather than nitosaurid derivation.
In view of this historical uncertainty of the relationships between the Nitosauridae, the Eothyrididae and the Caseidae, it is well to consider how the maxillary fragments described above differ from and resemble representatives of each of these three families as reported in the literature.
Delorhynchus resembles Colobomycter in size. The mean extra-maxillary length of the undamaged teeth of the three fragments is 2.5 mm., equal to that reported by Vaughn (1958:985) for teeth about midway in the postcanine series of Colobomycter. None of the teeth of Delorhynchus extends beyond the maxillary rim as far as does the canine of Colobomycter (3.5 mm.).
The teeth in both genera are conical and sharply pointed. The naris in each is enlarged, and the lacrimal is excluded from the narial margin in each (by inference in Delorhynchus.)
The differences between the maxillae of Colobomycter and Delorhynchus are most striking in the lack of canines in the latter and the correlated absence of modifications of the maxillary for support of canines. Additionally, Delorhynchus bears an infraorbital canal in contrast to the groove in similar position in Colobomycter. The recurvature of the four teeth present in the fragments of Delorhynchus differs from that in the teeth of Colobomycter in which only the canine and precanine are recurved. Vaughn implies that anterior and posterior cutting edges extend the length of the teeth in Colobomycter; these are restricted to the distal third of the teeth in Delorhynchus. The external surfaces of the maxillae of Delorhynchus are pitted and ridged; Vaughn was unable to discern sculpturing of the corresponding surfaces in Colobomycter.
Delorhynchus resembles the nitosaurids in size, the shape and sharpness of the teeth, their recurvature and the slight enlargement of their bases, the exclusion of the lacrimal bone from the narial margin (in Mycterosaurus) and the apparent lack of a special canine pair of teeth. Resemblances to the caseids are to be noted in the enlargement of the naris (4.5 mm. in height as opposed to 1.7 mm. in Colobomycter), lack of development of canines, presence of an infraorbital canal (in Cotylorhynchus) and absence of many replacement gaps in the marginal row of teeth.
The absence of caniniform enlargement and the extension of the maxilla dorsad of the naris exclude Delorhynchus from the Eothyrididae (Ophiacodontia) but are no bar to its inclusion in the Nitosauridae (Edaphosauria). The marginal teeth of Delorhynchus are simple and primitive, being much like those of the nitosaurids that are described in the literature.
The large narial opening and its posterior, dorsal and ventral enclosure by the maxilla, the infraorbital canal, and the sculptured external surfaces of the maxillary fragments indicate that Delorhynchus, in these features at least, is close to achieving the caseid grade.