LISA SCHNETZ *1, RICHARD J. BUTLER 1, MICHAEL I. COATES 2& IVAN J. SANSOM 1
1 University of Birmingham; lxs821@student.bham.ac.uk
2 University of Chicago
There are many groups of fish that are solely represented by fossils, with no modern-day representatives. Among these, the acanthodians or ‘spiny sharks’that lived from 444 to 259 million years ago,appear to have an important role in understanding the early evolutionary history of modern-day sharks. However,interpretations of acanthodiansare often controversial and in a state of flux due to the limitations of their fossil record which is partially a consequence of their skeleton being made from cartilage rather than bone. We aimed to explore these problems by using scoring methods that establish how much of the fish is preserved in a fossil in order to quantify the quality of the acanthodian fossil record.Information from museum visits and literature were compiled into a dataset of more than 1600 occurrences comprising 300 species. We found that acanthodian completeness fluctuates through time but is lowest in the early Silurian (444-439 Ma) and highest in the early Permian (299-294 Ma). Fossils of acanthodians are less complete than other vertebrate groups but similarly complete to bats, while those that are preserved in fresh water environments are significantly more complete than marine environments.Completeness does not differ considerably between hemispheres but species numbers do show an uneven distribution. Our results reveal that the acanthodian fossil record is seemingly highly affected by environmental and sampling biases. This comprises the first study to deal with the completeness of fish fossils and identify some of the factors impacting on the quality of their fossil record.