Meeting Abstract
The subclass Holocephali (chimaeras) is comprised of deep-water cartilaginous fishes and is the sister group to Elasmobranchii (sharks, skates, rays). The jaws and tooth plates of chimaeras diverged from elasmobranchs in shape and structure. Chimaeras have a holostylic jaw suspension (upper jaw fused to the cranium) and 3 pairs of tooth plates. In contrast, elasmobranchs have a hyostylic jaw suspension (mobile upper jaw) and individual or tightly interconnected teeth. There are 3 extant families of holocephalans, and of the extinct holocephalans, the fossils contain only tooth plates and spines. As a result, holocephalans are primarily classified on tooth plate characteristics. In Hydrolagus colliei, the tooth plates are thin and vertically aligned with the anterior plates forming a beak, and the diet contains a range of hard and soft prey. How H. colliei are able to feed on such a diversity of prey and how their distinct tooth plates function during feeding is perplexing. The tooth plates of H. colliei were analyzed for shape variation using 2D and 3D geometric morphometrics. Also, the second moment of area (I) was calculated for the lower jaw using CT scans. This revealed that tooth plate shape varies greatly within species and even among individuals of similar lengths. The lower jaw has a higher I in the posterior half compared to the anterior half. The values for I in the posterior half imply that this is where H. colliei cracks or crushes prey and are comparable to durophagous elasmobranchs. Examining the tooth plates of other holocephalans may reveal that species share a general tooth plate shape, but the variation appears in the details: edge outline, height, tritors, grooves. This suggests that while shape and structure of the feeding apparatus can be specialized in chondrichthyes, the function can be generalized.
