Mark J. Macdougall

The oldest parareptile and the early diversification of reptiles

Amniotes, tetrapods that evolved the cleidoic egg and thus independence from aquatic larval stages, appeared ca 314 Ma during the Coal Age. The rapid diversification of amniotes and other tetrapods over the course of the Late Carboniferous period was recently attributed to the fragmentation of coal-swamp rainforests ca 307 Ma. However, the amniote fossil record during the Carboniferous is relatively sparse, with ca 33% of the diversity represented by single specimens for each species. We describe here a new species of reptilian amniote that was collected from uppermost Carboniferous rocks of Prince Edward Island, Canada. Erpetonyx arsenaultorum gen. et sp. nov. is a new parareptile distinguished by 29 presacral vertebrae and autapomorphies of the carpus. Phylogenetic analyses of parareptiles reveal E. arsenaultorum as the closest relative of bolosaurids. Stratigraphic calibration of our results indicates that parareptiles began their evolutionary radiation before the close of the Carboniferous Period, and that the diversity of end-Carboniferous reptiles is 80% greater than suggested by previous work. Latest Carboniferous reptiles were still half as diverse as synapsid amniotes, a disparity that may be attributable to preservational biases, to collecting biases, to the origin of herbivory in tetrapods or any combination of these factors.

A New Species of the Parareptile Genus Delorhynchus, Based on Articulated Skeletal Remains from Richards Spur, Lower Permian of Oklahoma

ABSTRACT Description of a new species of the parareptile genus Delorhynchus is based on a well-preserved partial subadult skeleton, an isolated adult skull, and disarticulated elements recently collected from the Lower Permian Richards Spur locality of Oklahoma, U.S.A. Delorhynchus cifellii, sp. nov., is distinguished from Delorhynchus priscus by the lack of an accessory articulating anterodorsal flange of the maxilla. The hypodigm of Delorhynchus cifellii reveals that Delorhynchus is distinguished from other parareptiles by cranial dermal sculpture consisting of a system of low, smooth tuberosities and a pattern of diffuse shallow, circular dimples. In a phylogenetic analysis of parareptiles, Delorhynchus cifellii is positioned as the sister species of Lanthanosuchoidea. Recognition of Delorhynchus cifellii. sp. nov., and its phylogenetic position among parareptiles highlights the significance of the Richards Spur locality in our understanding of the early evolutionary history of reptiles.

New Information on the Skull of the Early Triassic Parareptile Sauropareion anoplus, with a Discussion of Tooth Attachment and Replacement in Procolophonids

ABSTRACT A partial reptile skull collected from the Lower Triassic (Induan) Barendskraal locality of South Africa is referable to the procolophonid parareptile Sauropareion anoplus. The specimen comprises the left side of the skull roof, palate, and mandible, and preserves areas not preserved or accessible in the skulls of previously published specimens, including fragmentary remains of the premaxilla, the anterior end of the maxilla, and the anterior end and lingual aspect of the mandible. The maxilla exhibits a maxillary depression that is similar in both shape and relative size to that seen in Procolophon trigoniceps, an anterolateral maxillary foramen that is indistinguishable from those seen in other procolophonoids, and 11 conical teeth. The suborbital foramen is formed ventrally by the palatine and the ectopterygoid. The dentary and the coronoid are fused together indistinguishably in lateral aspect. The marginal teeth appear to be anchored to the jaws by bone of attachment. The dentary teeth are exposed in lingual aspect and appear to be ankylosed to the summit of the bone, suggesting acrodont tooth implantation. Tooth replacement is seen in these teeth, where the lingual surface of a single tooth crown base in the middle of the dentary series is excavated for the apex of a replacement tooth crown, but the excavation does not extend ventrally onto the lingual surface of the bone. Considering that no extant reptiles with acrodont tooth implantation exhibit tooth replacement or bone of attachment, we tentatively regard protothecodonty to be present in S. anoplus.

A new parareptile (Parareptilia, Lanthanosuchoidea) from the Early Permian of Oklahoma

ABSTRACT An exquisitely preserved skull of Feeserpeton oklahomensis, gen. et sp. nov., collected from the Lower Permian fissure-fill deposits near Richards Spur, Oklahoma, is the newest parareptile from the locality. The specimen comprises an almost complete skull, lacking most of the premaxillae, parts of the jugals, and parts of the braincase. The maxillae and dentaries possess caniniform regions, including large caniniform teeth that are at least twice as large as any other tooth in the jaws. A phylogenetic analysis of Parareptilia that includes the new taxon indicates that the Early Permian F. oklahomensis is a member of the poorly known Lanthanosuchoidea, positioning it as the sister taxon of a clade consisting of Acleistorhinus pteroticus and Lanthanosuchus watsoni. This new taxon adds significantly to our limited knowledge of the early stages of parareptile evolution, as well as the large terrestrial tetrapod diversity found at Richards Spur.

Plicidentine in the Early Permian Parareptile Colobomycter pholeter, and Its Phylogenetic and Functional Significance among Coeval Members of the Clade

Once thought to be an exclusively anamniote characteristic, plicidentine, a pattern of infolding of dentine, is now known to be found in various amniote clades, including Parareptilia. In the absence of detailed analyses of parareptilian dentition, most parareptiles were assumed to lack plicidentine due to the absence of external indicators, such as plications on the tooth base. The clear presence of this dentinal feature in the largest premaxillary and maxillary teeth of Colobomycter pholeter, led us to the present detailed study within the dentition of this unusual parareptile, and those of coeval members of this clade. Our study reveals that there is large variability in the degree of dentine infolding within C. pholeter dentition, as well as within those of closely related parareptiles. This variability ranges from a lack of plications, to very complex anamniote-like plicidentine. Utilizing computed tomography scans in conjunction with histological sections we also demonstrate the utility of computed tomography scans in conducting non-destructive sampling in the identification of plicidentine. Given the variability of plicidentine in this sample of parareptiles, we hypothesize that one function of parareptilian plicidentine is to increase the surface area for attachment tissues, and we suggest that the use of plicidentine as a character in phylogenetic analyses of parareptiles may be misleading.

A new reptile from the Richards Spur locality, Oklahoma, U.S.A., and patterns of Early Permian parareptile diversification

ABSTRACT The Lower Permian Richards Spur locality is the most speciose Paleozoic continental vertebrate assemblage currently known, and a significant proportion of the tetrapod diversity found at the locality is made up of parareptiles. The first Richards Spur parareptile to be described was Colobomycter pholeter. It has been characterized by its enlarged premaxillary tooth and paired enlarged maxillary teeth, unique dentition that grants it an appearance quite distinct from other parareptiles at Richards Spur. Here we describe new cranial material from Richards Spur that is referable to Colobomycter. This new material differs from that of C. pholeter in that it possesses at least three more teeth on its maxilla, the enlarged premaxillary and maxillary teeth are more gracile than those in C. pholeter, and the lacrimal is restricted externally to the orbital margin and does not exhibit an extra lateral exposure. We infer that these differences merit specific distinction and assign the new fossil to Colobomycter vaughni, sp. nov. The discovery of C. vaughni at Richards Spur is important, because it reveals the presence of another member of the clade Lanthanosuchoidea in Oklahoma, making it the sixth to be found in the state. The large number of taxa from this clade found in Oklahoma suggests that during the Early Permian, this area of western Laurasia was the center of a radiation of small, predatory lanthanosuchoids.

Ontogenetic Change in the Temporal Region of the Early Permian Parareptile Delorhynchus cifellii and the Implications for Closure of the Temporal Fenestra in Amniotes

A juvenile specimen of Delorhynchus cifellii, collected from the Early Permian fissure-fill deposits of Richards Spur, Oklahoma, permits the first detailed study of cranial ontogeny in this parareptile. The specimen, consisting of a partially articulated skull and mandible, exhibits several features that identify it as juvenile. The dermal tuberosities that ornament the dorsal side and lateral edges of the largest skull of D. cifellii specimens, are less prominent in the intermediate sized holotype, and are absent in the new specimen. This indicates that the new specimen represents an earlier ontogenetic stage than all previously described members of this species. In addition, the incomplete interdigitation of the sutures, most notably along the fronto-nasal contact, plus the proportionally larger sizes of the orbit and temporal fenestrae further support an early ontogenetic stage for this specimen. Comparisons between this juvenile and previously described specimens reveal that the size and shape of the temporal fenestra in Delorhynchus appear to vary through ontogeny, due to changes in the shape and size of the bordering cranial elements. The jugal of the juvenile specimen is tri-radiate and similar in outline with those found in other amniotes with temporal fenestrae. The available growth series of D. cifellii shows that the jugal gradually becomes a more robust, tetra-radiate element, as the proportionate size of the temporal fenestra is reduced. Ontogenetic changes of other elements that form the border of the fenestra also contribute to its reduction. This growth series provides valuable new information regarding the ontogenetic trajectory of the temporal fenestra in a Palaeozoic reptile, which may be applicable to the evolutionary event of loss of temporal fenestration in other amniotes.