oncoids

Oncoids are a type of biological fossil, specifically fossilized remnants of organisms that possessed a shell or exoskeleton. The term "oncoid" is derived from the Greek word "onkos," meaning mass or lump, and refers to the characteristic rounded or irregularly shaped morphology of these fossils. They are commonly found in marine sedimentary rocks and are typically composed of calcium carbonate or other mineralized tissues.

These fossils are distinct from true shells or skeletal elements as they often represent fragmented or aggregated pieces of multiple organisms, or in some cases, the internal molds of cavities left by dissolved shells. Their preservation can vary widely, from well-preserved internal and external casts to mere impressions. The study of oncoids provides valuable insights into the paleoenvironment and paleocommunity structure of ancient marine ecosystems.

Oncoids can be indicative of specific depositional settings and can help paleontologists reconstruct past environments. Their presence and abundance can suggest levels of biological activity, energy of the water currents, and the types of organisms that inhabited these ancient seas. While not a formal taxonomic classification, oncoids are a useful descriptive term for a common type of fossil assemblage encountered in paleontology. They are often found alongside other marine fossils such as brachiopods, mollusks, and corals.