Improving elasmobranch baselines in the California Channel Islands through ZooMS and stable isotope analyses

Sharks are among the world's most endangered vertebrates, having declined by over 70% in abundance over the past 50 years—a collapse driven primarily by modern fishing. However, the extent of this impact is difficult to assess without improved records of past species distributions. Zooarchaeological records offer valuable baselines for shark diversity, distribution, and exploitation and provide essential historical context for assessing contemporary declines and vulnerability to extinction. Yet these records lag behind those for ray-finned fishes and marine mammals, largely because shark remains are difficult to identify to species level in the archaeological record. Here, we explore the use of Zooarchaeology by Mass Spectrometry (ZooMS) for species identification in this under-studied group, using an archaeological case study from the marine environments of Santa Rosa Island, California, USA. ZooMS analyses were paired with stable isotope analysis (SIA) for a subset of specimens, allowing us to assess both taxonomic resolution and past trophic ecology and habitat use of regional elasmobranchs. Of the 27 samples analysed that could not be identified beyond family level by morphological analysis, mostly as Triakidae (houndsharks) or Rajidae (skates), ZooMS grouped them into three distinct taxa that included the school shark ( Galeorhinus galeus ), leopard shark ( Triakis semifasciata ), and Pacific spiny dog shark ( Squalus suckleyi ). Among three additional samples morphologically identified as the Pacific angel shark ( Squatina californica ), two were confirmed but another distinct angel shark with a collagen fingerprint more closely resembling but distinct from the Chilean angel shark ( Squatina armata ) was also recovered—the latter possibly from a previously unknown species. Our findings demonstrate that ZooMS can distinguish between smoothhounds, leopard sharks, and other morphologically similar species and, when paired with stable isotope analysis, can reconstruct both the taxonomic composition and ecological structure of past shark communities. Furthermore, by demonstrating that ZooMS can detect species previously unknown to the region, and potentially new to science, these results hold promise for reconstructing the historical composition of nearshore marine ecosystems and establishing appropriate historical baselines for the future management of at-risk species.