OPHIACODON LONG BONE HISTOLOGY: SUPPORTING THE BRINKMAN HYPOTHESIS

Histological analysis is the best method of species determination based on ontogenetic stages by examining the histology of the diaphyseal mid-shaft of long bones, usually femora or humeri. This area is known to contain the best record of growth. In non-therapsid synapsids (“pelycosaurs”), some species have been identified solely on the basis of size regardless of the degree of ossification observed in the epiphysis. Brinkman attempted to overcome this problem by establishing size-independent morphological criteria on putative growth series of “pelycosaurs”, including Ophiacodon. Based on the degree of ossification of the epiphyses and the development of specific anatomical structures, he subdivided the putative growth series into five stages. Here we test Brinkman’s hypothesis of a growth series using histological indicators based on material which comes from the Rattlesnake Canyon locality (Nocona Formation, Lower Permian) in Archer County, Texas, USA.

The smallest humerus examined was determined by Brinkman to be a stage two, and its histology preserves the neonatal line in the deep cortex. This is an indicator of the time when the animal presumably hatched, and is the boundary between original embryonic bone and tissue deposited postnatally. The largest humerus (stage five) examined contains within the outer most cortex an external fundamental system (second occurrence in “pelycosaurs”) and a sharp decrease in vascularization. This unequivocally indicates that the animal had reached skeletal maturity.

We conclude that the histology of the Ophiacodon humeral growth series of Brinkman does support his morphological ontogenetic stages. The general histology of the humeri consist of a cortical bone matrix made up of parallel-fibered and woven bone and is highly vascularized by radial and longitudinal canals with varying degrees of lamellar bone infilling. The histology and vascularization of the cortex is very similar to that seen in Dimetrodon natalisand, surprisingly, basal sauropterygians. The tissue described here has been named incipient fibro-lamellar bone. It is the precursor to the true fibro-lamellar bone seen later in therapsids and mammals.