Functional Morphology of the Feeding Apparatus of the Snaggletooth Shark, Hemipristis elongata (Carcharhiniformes: Hemigaleidae)

Evidence for high-performance suction feeding in the Pennsylvanian stem-group holocephalan Iniopera

The Carboniferous (358.9 to 298.9 Ma) saw the emergence of marine ecosystems dominated by modern vertebrate groups, including abundant stem-group holocephalans (chimaeras and relatives). Compared with the handful of anatomically conservative holocephalan genera alive today-demersal durophages all-these animals were astonishingly morphologically diverse, and bizarre anatomies in groups such as iniopterygians hint at specialized ecological roles foreshadowing those of the later, suction-feeding neopterygians. However, flattened fossils usually obscure these animals' functional morphologies and how they fitted into these important early ecosystems. Here, we use three-dimensional (3D) methods to show that the musculoskeletal anatomy of the uniquely 3D-preserved iniopterygian Iniopera can be best interpreted as being similar to that of living holocephalans rather than elasmobranchs but that it was mechanically unsuited to durophagy. Rather, Iniopera had a small, anteriorly oriented mouth aperture, expandable pharynx, and strong muscular links among the pectoral girdle, neurocranium, and ventral pharynx consistent with high-performance suction feeding, something exhibited by no living holocephalan and never clearly characterized in any of the extinct members of the holocephalan stem-group. Remarkably, in adapting a distinctly holocephalan anatomy to suction feeding, Iniopera is more comparable to modern tetrapod suction feeders than to the more closely related high-performance suction-feeding elasmobranchs. This raises questions about the assumed role of durophagy in the evolution of holocephalans' distinctive anatomy and offers a rare glimpse into the breadth of ecological niches filled by holocephalans in a pre-neopterygian world.

Long-Axis Rotation of Jaws of Bamboo Sharks (Chiloscyllium plagiosum) During Suction Feeding

Long-axis rotation (LAR) of the jaws may be an important component of vertebrate feeding mechanisms, as it has been hypothesized to occur during prey capture or food processing across diverse vertebrate groups including mammals, ray-finned fishes, and sharks and rays. LAR can affect tooth orientation as well as muscle fiber direction and therefore muscle power during feeding. However, to date only a handful of studies have demonstrated this LAR in vivo. Here, we use XROMM to document LAR of the upper and lower jaws in white-spotted bamboo sharks, Chiloscyllium plagiosum, during suction feeding. As the lower jaw begins to depress for suction expansion, both the upper jaw (palatoquadrate) and lower jaw (Meckel's cartilage) evert, such that their toothed surfaces move laterally, and then they invert with jaw closing. Eversion has been shown to tense the dental ligament and erect the teeth in some sharks, but it is not clear how this tooth erection would contribute to suction feeding in bamboo sharks. Two recent XROMM studies have shown LAR of the lower jaws during mastication in mammals and stingrays and our study extends LAR to suction feeding and confirms its presence in shark species. Examples of LAR of the jaws are becoming increasingly widespread across vertebrates with unfused mandibular symphyses. Unfused lower jaws are the plesiomorphic condition for most vertebrate lineages and therefore LAR may be a common component of jaw mechanics unless the mandibular symphysis is fused.