Histological evidence for dermal-endochondral co-ossification of the dorsal blades in the late Paleozoic amphibian Platyhystrix rugosus (Temnospondyli: Dissorophidae)

ABSTRACT

We provide a detailed histological report of the dorsal blades of the Permo-Carboniferous dissorophid Platyhystrix rugosus. The blades, originally interpreted as hyperelongated neural spines, show compelling histological evidence of a dermal contribution that includes mineralized fibers and plywood-like tissue remarkably similar to extant osteoderm-bearing ceratophryid frogs and trionychid turtles. Their fusion to the vertebrae suggests homology with the osteoderm series of related Permian dissorophids. Although the mode of ossification is difficult to ascertain, both dermal ossification and metaplasia of the surrounding fibrous connective tissue (such as ligaments interconnecting the blades) likely combined to produce the fused blades observed in the closely allied Carboniferous platyhystricine Astreptorhachis. These represent the oldest examples of the phenomenon of dermal-endochondral co-ossification of osteoderms and vertebrae in the postcranial skeleton of a Paleozoic tetrapod. Our findings also highlight the adoption of different developmental mechanisms to produce convergent ecomorphology in sail-backed Paleozoic dissorophid amphibians (dermal blades derived from osteoderms) versus contemporary synapsids (hyperelongated neural spines). Future histological studies should sample additional dissorophid taxa to map the development of osteoderms within a broader phylogenetic context and to better understand their functional diversity.

ACKNOWLEDGMENTS

We thank the Bureau of Land Management for access to field sites in San Juan County, Utah (BLM permit UT17-001S) and D. S. Berman and A. Henrici of the Carnegie Museum for curating these (CM 96544, 96545) and for providing access to additional specimens for thin-sectioning (CM 96546). A.K.H. thanks E. Rega of Western University, Pomona, for access to thin-sectioning equipment for CM 96546 early on in the project, S. Quinzio for Ceratophrys histology images used in Fig. 4A, and E. Patellos for comments on an early draft. Funding for this project was provided in part by NSF-EAR-2219947 to A.K.H. and by the University of Southern California’s Undergraduate Research Associates Program. Finally, we thank the handling editor, referee T. Scheyer and one anonymous referee for their constructive reviews.