The anatomy of the bifurcated neural spine and its occurence within Tetrapoda

Abstract

Vertebral neural spine bifurcation has been historically treated as largely restrictive to sauropodomorph dinosaurs; wherein it is inferred to be an adaptation in response to the increasing weight from the horizontally extended cervical column. Because no extant terrestrial vertebrates have massive, horizontally extended necks, extant forms with large cranial masses were examined for the presence of neural spine bifurcation. Here, I report for the first time on the soft tissue surrounding neural spine bifurcation in a terrestrial quadruped through the dissection of three Ankole-Watusi cattle. With horns weighing up to a combined 90 kg, the Ankole-Watusi is unlike any other breed of cattle in terms of cranial weight and presence of neural spine bifurcation. Using the Ankole-Watusi as a model, it appears that neural spine bifurcation plays a critical role in supporting a large mobile weight adjacent to the girdles. In addition to neural spine bifurcation being recognized within nonavian dinosaurs, this vertebral feature is also documented within many members of temnospondyls, captorhinids, seymouriamorphs, diadectomorphs, Aves, marsupials, artiodactyls, perissodactyls, and Primates, amongst others. This phylogenetic distribution indicates that spine bifurcation is more common than previously thought, and that this vertebral adaptation has contributed throughout the evolutionary history of tetrapods. Neural spine bifurcation should now be recognized as an anatomical component adapted by some vertebrates to deal with massive, horizontal, mobile weights adjacent the girdles.

Vertebral Adaptations to Large Body Size in Theropod Dinosaurs

“…This irregular zone in both species is indicative of the metaplastic transitionary tissue known as the enthesis, which is the hypermineralized calcified fibrocartilage from connective tissue (ligament or tendon) to the spine, confirming the identity of the spinal projections as metaplastic [ 4 , 32 , 33 ]. Similar vertebral entheses have been described from neosauropod neural spines [ 1 , 4 , 32 , 34 ]. In the Tyrannosaurus sample, this “veneer”-like enthesis pervades into the highly pneumatized neural spine, composed of highly fibrous secondary reconstructions, which agrees with the observation of Horner et al 2015 [ 4 ] showing that the bulk of the neural spine is composed of metaplastic tissue.…”

“…Because these structures do serve to decrease interspinal space and maximize bony support along the spinal column, it is possible that they represent an additional, independent evolution of a mechanism for spinal rigidity. Sauropods also exhibit neural spine modifications, such as bifurcating neural spines, which have been hypothesized to be an adaptation for increased vertebral column mobility [ 1 ]. Some sauropod specimens (most notably Diplodocus longus USNM 10865) possess spinal projections in the caudal series that are identical to those observed in large-bodied theropods [ 42 ].…”

“…Many large-bodied amniotes, from bovids to crocodilians, exhibit rugose, spur-like projections on the neural spines within the dorsal and caudal regions of the spinal column [ 1 ]. These projections are thought to arise from metaplasia of the supraspinous and interspinous ligaments [ 1 – 4 ] caused by tensile loading in the apical portion of the neural spines, as would be predicted from ventrally directed bending stresses of the vertebral column [ 5 ]. Metaplasia, in the paleontological and evolutionary biological sense, is the permanent transformation of tissue types, and cannot be described as “ossification” of the ligaments, as metaplasia is not the process of bone formation [ 4 ].…”

Evolution and function of anterior cervical vertebral fusion in tetrapods

“…The head of the pacarana is relatively large (White & Alberico, 1992), and a head-support hypothesis has been applied to ceratopsians and hornbills. Indeed, vertebrae may fuse in response to stress from a heavy head in some extant bovids (Woodruff, 2014). In bovids and other mammals and reptiles a dorsal ligament supports their large skulls (e.g.…”

“…In bovids and other mammals and reptiles a dorsal ligament supports their large skulls (e.g. Nishi, 1916;Tsuihiji, 2004;Woodruff, 2014), yet they do not show signs of a syncervical or a similar structure and fusion does not appear to be reactive, rather than adaptive, in other taxa with large skulls. Alternatively, it is possible that syncervicals evolved in response to different lifestyles present in the ancestors of these clades, such as fossoriality, or may have arisen by chance.…”