THE SILURIAN NEKTON REVOLUTION: PALEOZOIC JAWLESS FISHES EXHIBITED MODERN FORM-DEPENDENT MODES OF HABITAT USE

The mid-Paleozoic is widely called the “Age of Fishes”, marked by increased vertebrate diversity in terms of species, major clades, abundance and especially ecomorphologies. These innovations have been primarily associated with Devonian jawed fishes (gnathostomes). In contrast, Silurian jawless fishes, including stem-gnathostomes, were considered restricted in their habitat use (e.g. nearshore marine) and lifemode (e.g. benthic). Yet, agnathans came in a variety of body plans, ranging from heavily armored to streamlined to angelfish-like, that suggest greater ecological range. In order to quantify the habitat distributions of these mid-Paleozoic fishes, we assembled benthic assemblage zone data for the fossil records of major agnathan lineages. We used Bayesian methods of discrete ancestral state reconstruction to determine dispersal abilities and base habitat preference through time. We found that Silurian jawless fishes occupied a range of zones from freshwater to reef to open ocean. As may be expected from modern jawed fishes, distinct jawless body forms were associated with different segments of the depth and salinity gradients. The shallow benthos was occupied by osteostracans and the earliest heterostracans. These exhibited the ancestral body plan, being robust, heavily armored, and dorsoventrally-flattened. The lagoon and forereef were occupied by deep-bodied, laterally-flattened, manuverable forms such as anaspids and cyathaspid heterostracans. Both these groups developed long overlapping plates, likely resulting from a trade-off between flexibility and control in a complex environment. Later reef fishes achieved the same through stiffening of fin rays, ribs and backbones. Finally, the back reef, shelf, and open ocean were occupied primarily by thelodonts and pteraspid heterostracans. Both these clades exhibited streamlined bodies, flexible tails and hydrofoils, all features of modern pelagic fishes. Our results show that much of modern aquatic vertebrate habitat space was occupied by the Silurian, primarily by the forms of jawless fishes convergent on their later jawed relatives.