Hippuritoida

Hippuritoida is an extinct suborder of bivalve mollusks that lived during the Cretaceous period. These organisms are notable for their unusual, often conical or irregularly shaped shells, which differed significantly from the typical flattened shape of modern bivalves. They are commonly referred to as rudist bivalves. The shell of a hippuritoid typically consists of two valves, with the larger, lower valve being attached to the substrate and the smaller, upper valve acting as a lid.

These bivalves were reef-builders, meaning they formed large fossil accumulations that contributed significantly to the marine ecosystems of the Cretaceous. Their shells were often thick and robust, and their growth patterns allowed them to form complex, three-dimensional structures. Many hippuritoid species developed specialized morphologies, with some appearing horn-like, others resembling stacks of coins, and some forming large, irregular masses.

Hippuritoids were particularly abundant in the tropical and subtropical seas of the Tethys Ocean during the Late Cretaceous. They formed extensive reefs in shallow marine environments, providing habitats for other marine life. Their ecological role was similar to that of modern corals in building reef structures. The suborder became extinct at the end of the Cretaceous period, along with many other marine organisms, during the Cretaceous-Paleogene extinction event. Their fossilized shells are common in Cretaceous sedimentary rocks and are important index fossils for biostratigraphy.