Delorhynchus

Delorhynchus is an Early Permian parareptile from the Richards Spur fissure fills of Oklahoma. Its best-known species, D. cifellii, is represented by a skull associated with vertebrae, ribs, shoulder bones and a humerus. Neutron tomography revealed a braincase and small elements hidden inside the rock without physically removing them. The specimen is still incomplete, but it provides an unusually detailed view of an acleistorhinid beyond the head. The profile is part of the other fossil reptile catalogue.

Quick facts

From isolated jaws to a partial skeleton

Everett C. Fox established Delorhynchus in 1962 for fragmentary jaw material called D. priscus. The diagnostic value of those old pieces is difficult to assess. In 2014, a much more informative specimen from Richards Spur was named D. cifellii. Its holotype, OMNH 73515, preserves a skull together with parts of the axial skeleton and shoulder region, giving researchers the first detailed postcranial anatomy for an acleistorhinid parareptile.

Later work named D. multidentatus from another fragmentary skull. The genus therefore has a complex record: the species differ in completeness, and the status of the original D. priscus material is not as straightforward as the newer, better-described specimens. A genus name may encompass fossils of very different evidential quality.

Neutron images inside the rock

Parts of OMNH 73515 remained embedded in dense matrix or overlapped other bones. Neutron computed tomography was used to distinguish internal structures in three dimensions. The 2023 redescription documented a sphenethmoid, paired epipterygoids, a more complete neurocranium and a stapes visible in three dimensions. The same analysis described vertebrae, ribs, shoulder girdle and humerus.

Neutrons interact differently with some materials than X-rays do, allowing boundaries that are hard to resolve in one imaging method to become clearer in another. The digital result depends on segmentation: specialists trace the boundary between fossil and matrix, and tightly joined or damaged areas can remain uncertain. The method does not cut new bones into existence; it helps read structures already preserved inside the specimen.

Age-related change in the skull

Several Delorhynchus skulls from Richards Spur differ in the proportions and closure of the temporal region behind the eye. A 2016 study interpreted the pattern across seven specimens as ontogenetic change: as individuals matured, surrounding bones expanded and reduced the size of the opening. This cautions against naming every differently shaped skull as a separate species without considering growth.

The sample is valuable but not a complete life history. Fossil size alone does not provide an exact age, and the sequence of specimens cannot identify how many years each growth stage lasted. Differences may also reflect preservation and individual variation. The strongest conclusion is that skull form changed within the sampled material, so age must be considered in classification.

What the fossil says about relationships

The cranial and postcranial data revised the placement of Delorhynchus among early parareptiles. The 2023 analysis recovered it as the closest sampled relative of Colobomycter and broadened the anatomical picture of acleistorhinids. Phylogenetic trees summarise shared character patterns; they are not direct records of a population splitting in the past.

The fossils support a small-bodied, land-dwelling reptile with a broad skull and short marginal teeth. A precise body length, complete gait and social habits are not preserved. As with Colobomycter, a skull can reveal a great deal about teeth and classification while leaving most of the body unknown. The fissure deposits at Richards Spur preserve a rich Permian community, but co-occurrence in a deposit does not prove direct ecological interaction.