pachycormiforms

Pachycormiforms were an extinct order of ray-finned fish that lived from the Devonian to the Cretaceous periods. They are characterized by their thick, bony scales, which gave them their name, meaning "thick-bodied form." Many pachycormiforms were large, robust fish, with some species reaching lengths of several meters. Their bodies were typically deep and laterally compressed. Fossil evidence suggests a varied diet, with some species likely being predatory while others may have been filter feeders.

Key anatomical features of pachycormiforms include a homocercal tail fin, indicating they were active swimmers. Their pectoral and pelvic fins were often large and paddle-like, which may have aided in maneuvering. The skull structure varied, but generally showed a reduction in the number of bones compared to earlier fish. The scales were a defining characteristic, being large, rhomboidal, and interlocking, offering significant protection.

Pachycormiforms inhabited a range of marine environments, from shallow coastal waters to the open ocean. Their distribution was global, with fossils found on multiple continents. The group's longevity, spanning over 200 million years, suggests a successful evolutionary lineage. However, like many other fish groups, they eventually went extinct, likely due to a combination of environmental changes and competition from other evolving fish groups. Notable genera include Pachycormus and Leedsichthys, the latter being one of the largest fish ever to have lived.