Propelleridae

Propelleridae is a family of extinct marine bivalve mollusks that lived during the Triassic and Jurassic periods. Their fossils are found in various marine sedimentary rocks from these geological eras.

The most distinctive feature of Propelleridae is the presence of a prominent, often spiral-shaped, external calcification on the shell, which gave the family its name. This structure, sometimes referred to as a "propeller," varied in size and shape among different species within the family. The function of this appendage is not fully understood, but it is hypothesized to have played a role in anchoring the organism to the substrate, perhaps in soft sediments, or it may have served a sensory or defense purpose.

These bivalves possessed equivalve shells, meaning both valves were of similar size and shape. The internal shell morphology would have been typical of bivalves, with muscle scars indicating attachment points for the adductor muscles used to close the shell. The precise ecological niche and diet of Propelleridae are inferred from their fossil context, suggesting they were likely infaunal or epifaunal organisms living in marine environments. Their extinction is part of the broader patterns of marine life changes at the end of the Jurassic period.