Comparative anatomy and osteohistology of hyperelongate neural spines in the sphenacodontids Sphenacodon and Dimetrodon (Amniota: Synapsida)

Abstract

Osteohistological investigations of hyperelongate vertebral spinous processes (neural spines) are presented to elucidate previously unknown aspects of dorsal sail form and function in two, closely related genera of "sail-backed" synapsids: Sphenacodon and Dimetrodon. Although recent and classic surveys of bone histology in extinct vertebrates have sampled the genus Dimetrodon, new sectioning of Sphenacodon material allows a comparative analysis of these structures among Sphenacodontidae for the first time. Variability within the histological organization of the neural spine is assessed by examining multiple regions along its length, and implications for soft tissue correlates, growth and mechanics are considered here. Both genera exhibit extensive parallel-fibered and fibrolamellar bone, in addition to lamellar bone. Several features vary along the length of the spine in each species. Muscle scars and extensive Sharpey's fibers are present at the base of the spine; no scars and fewer fibers are manifested ∼55-60 mm above the zygapophyses in mature individuals. The distal cortex of the spine does not exhibit greater vascularity than the proximal region in either genus. However, both genera manifest distinct vascular grooves of variable size along the distal periosteal surface, some of which become incorporated into the distal cortex. The observed histovariability appears to record the transition from the proximal (epaxial muscle embedded) to the distally protruding portion of the spine. These observations and independent pathological evidence support the existence of a short dorsal crest in Sphenacodon and possibly other basal sphenacodontids. Although the thermoregulatory capacity of such a crest remains uncertain, developmental and mechanical features are readily interpretable and are discussed with respect to the origins and early evolution of the dorsal sail in sphenacodontid synapsids.

Microstructural features of the femur in early ophiacodontids: A reappraisal of ancestral habitat use and lifestyle of amniotes

“…(Huttenlocker and Rega, 2012) and Dimetrodon natalis (Shelton et al, 2013; see also Huttenlocker et al, 2010).…”

Selection towards larger body size in both herbivorous and carnivorous synapsids during the Carboniferous

“…Sphenacomorpha is the only group of medium-to-large bodied non-mammalian synapsids to have developed elongate neural spines forming a dorsal sail, the function of which has been subject to intense scrutiny (Romer 1927;Rodbard 1949;Pivorunas 1970;Bramwell and Felqett 1973;Haack 1986;Tracy et al 1986;Bennett 1996;Florides et al 2001;Huttenlocker et al 2010Huttenlocker et al , 2011Tomkins et al 2010;Rega et al 2012). It has been suggested that sphenacomorphs were the largest terrestrial vertebrates of the Permo-Carboniferous because of their dorsal sails, as the sail would act to strengthen the vertebral column at large body masses and increase the efficiency of thermoregulation (Romer 1927;Pivorunas 1970;Florides et al 2001).…”

“…It has been suggested that sphenacomorphs were the largest terrestrial vertebrates of the Permo-Carboniferous because of their dorsal sails, as the sail would act to strengthen the vertebral column at large body masses and increase the efficiency of thermoregulation (Romer 1927;Pivorunas 1970;Florides et al 2001). Recent studies have questioned the use of the dorsal sail as a thermoregulatory organ based on histological analyses of the neural spines (Huttenlocker et al 2010(Huttenlocker et al , 2011, the suggestion that Dimetrodon was nocturnal (Angielczyk and Schmitz 2014), and the positive allometry of neural spine height through evolutionary time as evidence that the sail was used for sexual selection (Bakker 1986;Tomkins et al 2010). However, the idea of whether or not dorsal sails are related to body size in sphenacomorphs has never been quantitatively tested.…”

Pathological Features in Upper Permian and Middle Triassic Dicynodonts (Synapsida, Therapsida)

“…These include the possible occurrence of cysts in Brazilian rhynchosaurs (Schultz 1999), avascular necrosis in mosasaurs (Rothschild and Martin 1987), osteoarthritis in plesiosaurs (Wells 1964), osteomyelitis in crocodilians (Ferigolo 1993a), bone infections in aetosaurs (Lucas 2000), fractures and infections in dinosaurs (Rothschild 1988;Hanna 2002;Peterson et al 2009), and luxations, exostoses and arthrosis in mammals (Moodie 1930;Choquette et al 1975;Ferigolo 1985Ferigolo , 1993bLucas and Schoch 1987;Wang and Rothschild 1992;Gillette and Madsen 1993;Henriques et al 1998;Scott and Rooney 2001). Recently, there has been increasing interest in pathological features of basal synapsids (Huttenlocker et al 2010;Huttenlocker and Rega 2012) and other Paleozoic tetrapods (Reisz et al 2011). Among dicynodonts, pathologies have been reported on several occasions.…”