Chondrichthyans (cartilaginous fishes, including sharks, rays, skates, and chimaeras) first appeared more than 450 million years ago in the Ordovician and diversified into many of the groups that still exist today. However, their biodiversity patterns across the rest of the Paleozoic (Silurian–Permian) are obscured by gaps in their fossil record, caused by several biases. For example, chondrichthyan skeletons are predominantly made of cartilage, which rarely fossilizes, therefore limiting the quality of their fossil record. In our study, we use a newly created dataset of chondrichthyan fossil occurrences and apply statistical methods that aim to estimate patterns of diversity from incomplete fossil samples. Through this approach, we found that chondrichthyan diversity was initially low in the Ordovician and Silurian, then increased substantially in the Early Devonian, about halfway through the Paleozoic. Diversity peaked in the middle Carboniferous before decreasing across the remainder of the Paleozoic. This peak in diversity is dominated by stem-holocephalan chondrichthyans (a major group of Paleozoic chondrichthyans). Conversely, acanthodian chondrichthyan (early shark-like fish) diversity is highest in the Early Devonian before declining rapidly by the end of the Devonian. This suggests that there were two radiations in chondrichthyan diversity during the Paleozoic: the first in the earliest Devonian, led by acanthodian chondrichthyans, and the second in the earliest Carboniferous, led by holocephalans. Early in the Paleozoic, chondrichthyans lived in shallower waters, but after the Devonian, they increasingly branched out into deeper waters. This transition coincides with the Hangenberg extinction event at the end of the Devonian, suggesting that the dispersal of chondrichthyans, specifically holocephalans, into deeper-water environments and expansion of their niches was a response to the impacts of the Hangenberg extinction event on other species in the oceans.

The Paleozoic represents a key time interval in the origins and early diversification of chondrichthyans (cartilaginous fishes), but their diversity and macroevolution are largely obscured by heterogenous spatial and temporal sampling. The predominantly cartilaginous skeletons of chondrichthyans pose an additional limitation on their preservation potential and hence on the quality of their fossil record. Here, we use a newly compiled genus-level dataset and the application of sampling standardization methods to analyze global total-chondrichthyan diversity dynamics through time from their first appearance in the Ordovician through to the end of the Permian. Subsampled estimates of chondrichthyan genus richness were initially low in the Ordovician and Silurian but increased substantially in the Early Devonian. Richness reached its maximum in the middle Carboniferous before dropping across the Carboniferous/ Permian boundary and gradually decreasing throughout the Permian. Sampling is higher in both the Devonian and Carboniferous compared with the Silurian and most of the Permian stages. Shark-like scales from the Ordovician are too limited to allow for some of the subsampling techniques. Our results detect two Paleozoic radiations in chondrichthyan diversity: the first in the earliest Devonian, led by acanthodians (stem-group chondrichthyans), which then decline rapidly by the Late Devonian, and the second in the earliest Carboniferous, led by holocephalans, which increase greatly in richness across the Devonian/ Carboniferous boundary. Dispersal of chondrichthyans, specifically holocephalans, into deeper-water environments may reflect a niche expansion following the faunal displacement in the aftermath of the Hangenberg extinction event at the end of the Devonian.