macrosemiids

Macrosemiids are an extinct family of ray-finned fish that lived during the Jurassic and Cretaceous periods. They are known from fossils found in marine sedimentary rocks across Europe and North Africa. These fish are characterized by their deep, laterally compressed bodies and a single, long dorsal fin that extends along much of their back. Their scales were typically ganoid, meaning they were thick and rhomboidal, giving the fish a heavily armored appearance.

The mouth of macrosemiids was generally small and positioned ventrally, suggesting they fed on small invertebrates or algae from the seabed. Their pectoral fins were often broad and fan-shaped, and their pelvic fins were located relatively far back on the body. The caudal fin was usually forked. Fossil specimens sometimes show evidence of gastroliths, or stomach stones, which may have aided in digestion.

While their exact ecological role is debated, macrosemiids were likely demersal or benthopelagic, living near the seafloor. They are considered to be a basal group within the teleostean fishes, meaning they represent an early lineage in the evolution of modern bony fishes. Their relatively primitive features, such as ganoid scales and a less specialized fin structure, place them in a significant position for understanding fish evolution during the Mesozoic Era.