Hippuritida

Hippuritida is an extinct order of bivalve molluscs that lived during the Cretaceous period. These unusual bivalves, often referred to as rudists, are characterized by their irregular and often conical shells. Unlike most bivalves, rudists were cemented to the substrate by their left valve, which was typically much larger and more robust than the right valve. The right valve was often smaller, operculum-like, and sometimes reduced to a lid.

Rudists were reef-builders, forming extensive and diverse biogenic structures in shallow marine environments, particularly in the Tethyan region. Their skeletal material contributed significantly to carbonate deposition during the Cretaceous. The diversity of rudist forms is remarkable, with some species exhibiting simple conical or cylindrical shells, while others developed complex, elaborate structures. Their internal morphology also varied, with different patterns of internal septations and pallial lines.

The fossil record of Hippuritida is abundant in Cretaceous marine sedimentary rocks. Their rapid diversification and widespread distribution make them important index fossils for dating and correlating these rock layers. Rudists became particularly dominant in the late Cretaceous, but their ecological success was abruptly terminated by the Cretaceous-Paleogene extinction event, which wiped out the vast majority of marine life. Their extinction marked the end of an era of bivalve-dominated reef building.