Comparative osteohistology of hyperelongate neural spines in the Edaphosauridae (Amniota: Synapsida)

Abstract

Abstract: The Permo‐Carboniferous Edaphosauridae is an extinct family of omnivorous and herbivorous basal synapsids in which a dorsal sail, comprised of a series of hyperelongate vertebral spinous processes (‘neural spines’), evolved convergently to their sphenacodontid contemporaries. Descriptive and quantitative microstructural analysis of these hypertrophied neural spines allows inferences of sail growth, allometry and structural properties and permits systematic comparisons with other groups (i.e. sphenacodontids). In Edaphosaurus and Ianthasaurus, neural spine microstructure is characterized by a lamellar‐zonal primary bone complex within the cortex, distal cross‐sectional bone density ranging from approximately 0.50 to 0.75, low cortical porosity ranging from 1.0 to 5.0 per cent, relative bone wall thickness ranging from 10 to 25 per cent and presence of a central cavity, fully developed with smooth endosteal margins in both genera. Tubercle growth, however, appears to have been slower and incremental in the specimen of Ianthasaurus examined here. Microstructural similarities further support the position of Lupeosaurus within Edaphosauridae as hypothesized by earlier authors, though most parsimoniously in a basal position (as a central cavity is only incipiently developed and tubercles are lacking). Histological properties of hyperelongate neural spines in Lupeosaurus and other edaphosaurids do not meet previously established requirements of the thermoregulatory hypothesis, either calling into question the sail’s thermoregulatory function in this group or indicating a reconsideration of those putative thermoregulatory adaptations and the purported soft‐tissue correlates of the sail. Conversely, microstructural patterns exhibit variation that is consistent with an intra‐ or interspecific display function. Given the diverse microstructural properties at the family and genus‐level, histological study of hyperelongate neural spines may also aid in recognizing taxonomic compositions of Permo‐Carboniferous microvertebrate assemblages in which only fragmentary materials are known.