Abstract
Neural spine hyper-elongation in tetrapods is a unique morphological adaptation that creates a dorsal sail. While this extreme morphology has appeared several times in the evolutionary history of tetrapods, it was first experimented with by the non-mammalian synapsid group known as ‘Pelycosaurs’, famously represented by the Permian apex predator Dimetrodon. Here we provide new fossil data tracing back the evolution of this morphological innovation back to the initial Carboniferous radiation of the synapsid clade. We describe a new hyper-elongated neural spine belonging to the earliest ophiacodontid synapsid Echinerpeton intermedium, from the Pennsylvanian-aged deposits of Florence, Nova Scotia. The new fossil unveils the rapid convergence and repeated evolution of the ‘dorsal sail’ morphology in Synapsida. The development of this trait may have given early synapsids a unique advantage in their early radiation over the other early amniotes, the reptiles.
Highlights
Vertebral neural spine hyper-elongation (NSH) is a rare phenomenon in tetrapod backbones that gives the appearance of a dorsally protruding “sail” or “fin” on the animal
Long vertebral neural spines are present in mammals such as the American Bison (Bison bison), these are often covered by a soft tissue combination of epaxial musculature and adipose tissue, which creates the appearance of a “hump-back” (Bailey, 1997)
While in living tetrapods NSH is limited to reptiles, examination of the fossil record of amniotes reveals that this morphology has appeared several times in the evolutionary history of tetrapods
Summary
Vertebral neural spine hyper-elongation (NSH) is a rare phenomenon in tetrapod backbones that gives the appearance of a dorsally protruding “sail” or “fin” on the animal These bony “sails” protrude from the bulk of the trunk, including muscle and adipose tissue, and may be covered by a thin epithelium and collagenous integumentary webbing. A few extant squamates have true “sail-backs,” including the agamid genus Hydrosaurus, the chameleonid genus Trioceros as well as the corytophanid genus Basiliscus and its species (Bramwell and Fellgett, 1973) These sails are variable in where they appear on the body and are covered by a thin integumentary webbing. NSH is observed in various iconic archosaur lineages, including both carnivorous and herbivorous dinosaurs such as Spinosaurus and Ouranosaurus, respectively (Hone and Holtz, 2017) It is present in other groups such as the poposaurid Arizonasaurus (Nesbitt, 2003). The function of NSH in fossil amniotes remains enigmatic, we are able to comment on current hypotheses, such as thermoregulatory function and intra-species recognition, based on the amniote dominated fossil fauna of Florence, Nova Scotia
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