Biomechanics of <i>Machaeracanthus</i> pectoral fin spines provide evidence for distinctive spine function and lifestyle among early chondrichthyans

Acanthodians are a major group of Paleaozoic jawed vertebrates that constitute a paraphyletic assemblage of stem-chondrichthyans (Brazeau and Friedman, 2015). Representatives of this group are characterized, among other traits, by the presence of bony spines in front of all paired and median fins except the caudal (Denison, 1979), which has given rise to their colloquial name of 'spiny sharks'. The occurrence of pectoral fin spines is recognized as a potential gnathostome synapomorphy (Miller et al., 2003) or symplesiomorphy (Coates, 2003), being also present in other major groups of Paleaozoic jawed vertebrates, including placoderms (Young, 2010), 'non-acanthodian' chondrichthyans (Miller et al., 2003), and osteichthyans (Zhu et al., 1999). However, this trait was independently lost in the later evolutionary history of these lineages and is absent in most living representatives (Coates, 2003; Miller et al., 2003), with the exception of catfishes (Siluriformes), that acquired pectoral fin spines as an evolutionary reversion (Price et al., 2015). As a consequence, the paucity of living analogsue precludes deriving functional interpretations of those structures and the role that they fulfilled in life remains unclear, despite this having the potential to enrich our understanding on the ecologies and lifestyles of groups of early jawed vertebrates. Machaeracanthus constitutes a genus of acanthodians that ranged from the Late Silurian to the Middle Devonian, which is known from fin spines, scales, and a few endoskeletal remains (Burrow et al., 2010; Botella et al., 2012). The spines of this genus differ from the fin spines of all other acanthodians and sharks in presenting a marked cross-sectional asymmetry and a totally enclosed central canal, which is usually open along the proximal end of the trailing edge in other taxa (Burrow et al., 2010). The description of wear patterns at the tips of pectoral fin spines of Machaeracanthus and their peculiar arrangement in pairs has led some authors to propose that these elements could have been used as 'snow-shoes' to lay on and prevent sinking into the substrate below or even to propel itself along the bottom (Südkamp and Burrow, 2007). Here, we test this hypothesis through beam theory analyses and provide evidence that the biomechanical properties of Machaeracanthus pectoral fin spines are compatible with this interpretation, thus shedding light on the diversity of the functions of these intriguing anatomical structures and the lifestyles of some of the earliest jawed vertebrates

Divergent strategies in cranial biomechanics and feeding ecology of the ankylosaurian dinosaurs

Abstract Ankylosaurs were important megaherbivores of Jurassic and Cretaceous ecosystems. Their distinctive craniodental anatomy and mechanics differentiated them from coexisting hadrosaurs and ceratopsians, and morphological evidence suggests dietary niche partitioning between sympatric ankylosaurids and nodosaurids. Here, we investigate the skull biomechanics of ankylosaurs relative to feeding function. First, we compare feeding functional performance between nodosaurids and ankylosaurids applying finite element analysis and lever mechanics to the skulls of Panoplosaurus mirus (Nodosauridae) and Euoplocephalus tutus (Ankylosauridae). We also compare jaw performance across a wider sample of ankylosaurs through lever mechanics and phylogenetic comparative methods. Mandibular stress levels are higher in Euoplocephalus, supporting the view that Panoplosaurus consumed tougher foodstuffs. Bite force and mechanical advantage (MA) estimates indicate that Panoplosaurus had a relatively more forceful and efficient bite than Euoplocephalus. There is little support for a role of the secondary palate in resisting feeding loads in the two ankylosaur clades. Several ankylosaurs converged on similar jaw mechanics, while some nodosaurids specialised towards high MA and some ankylosaurids evolved low MA jaws. Our study supports the hypothesis that ankylosaurs partitioned dietary niches in Late Cretaceous ecosystems and reveals that the two main ankylosaur clades evolved divergent evolutionary pathways in skull biomechanics and feeding habits

Ballell et al Dental form and function in the early feeding diversification of dinosaurs Sci Adv

Supplementary Data from Ballell, A., Benton, M. J. and Rayfield, E. J. Dental form and function in the early feeding diversification of dinosaurs. Science Advances. Computed Tomography dataset of the Thecodontosaurus antiquus specimen BRSUG 28221, containing a maxillary tooth. Three-dimensional surface models of the early dinosaur teeth included in the biomechanical and morphometric analyses

Ballell et al Thecodontosaurus braincase and endocast

Computed Tomography dataset of the Thecodontosaurus braincase specimen YPM 2192. Three-dimensional surface models of the braincase and endocast of the Thecodontosaurus specimen YPM 2192. Supporting Information from: A. Ballell, J. L. King, J. M. Neenan, E. J. Rayfield and M. J. Benton. The braincase, brain and palaeobiology of the basal sauropodomorph dinosaur Thecodontosaurus antiquus. Zoological Journal of the Linnean Society. DOI: 10.1093/zoolinnean/zlaa157

Ballell et al Thecodontosaurus braincase and endocast

Computed Tomography dataset of the Thecodontosaurus braincase specimen YPM 2192. Three-dimensional surface models of the braincase and endocast of the Thecodontosaurus specimen YPM 2192. Supporting Information from: A. Ballell, J. L. King, J. M. Neenan, E. J. Rayfield and M. J. Benton. The braincase, brain and palaeobiology of the basal sauropodomorph dinosaur Thecodontosaurus antiquus. Zoological Journal of the Linnean Society. DOI: 10.1093/zoolinnean/zlaa157