Pachycormiformes

Pachycormiformes is an extinct order of ray-finned fish that lived from the Jurassic to the Cretaceous periods. These fish were characterized by their deep bodies and large fins, and some species reached considerable sizes. The group is known from fossils found in marine sediments worldwide.

The anatomy of pachycormiforms shows a mixture of primitive and derived traits. They possessed ganoid scales, which are thick and rhomboidal, a feature common in many early bony fishes. However, their fin structures, particularly the pectoral and pelvic fins, were more advanced, suggesting adaptations for powerful swimming. Their skulls were robust, and many species had large mouths, indicating a predatory lifestyle.

Several notable genera are known within Pachycormiformes. Leedsichthys, from the Jurassic of Europe, is one of the largest fish ever discovered, with estimated lengths of up to 16 meters. This massive filter feeder likely consumed plankton. Other genera, such as Pachycormus and Pseudobates, were smaller and possessed teeth, suggesting they preyed on smaller fish and invertebrates.

The extinction of Pachycormiformes occurred at the end of the Cretaceous period, likely due to the same mass extinction event that wiped out the non-avian dinosaurs. Their unique adaptations and significant diversity made them an important component of Mesozoic marine ecosystems. The study of their fossils provides valuable insights into the evolution of bony fishes.