An Intersection in Time and Space: Significance of Modern Invertebrate Borings in Upper Cretaceous Echinoids

Abstract

Reworked fossils may contribute unique data to ichnology, stratigraphy, and paleoecology. Reworked Upper Cretaceous echinoids are found on Ocean Isle Beach, North Carolina, United States. Tests of Hardouinia sp. cf. H. mortonis were intensely bored by clionaid sponges (Entobia) and bivalves (Gastrochaenolites), with rare polychaete annelids (Caulostrepsis). Evidence for a high-energy environment in the Late Cretaceous is provided by the sandstone infill and a geopetal infill. Hardouinia were bored on all surfaces, largely lack surface detail and have been corraded, exposing the internal structure of some borings, indicating that these specimens were tumbled in a modern, high energy marine environment. Borers showed substrate preferences: Entobia is largely limited to the test, but do penetrate the sandstone infill in some specimens; Caulostrepsis mainly infested the crystalline calcite of the echinoid test; and Gastrochaenolites penetrated both tests and lithified sandstone infill. This Entobia-Gastrochaenolites assemblage in these echinoids differs from the Entobia-Caulostrepsis assemblage recognized from the approximately coeval, but spatially distant, tests of Upper Cretaceous Echinocorys ex gr. scutata found reworked on the North Sea coast of Norfolk, England. Unlike the Entobia-Gastrochaenolites association of North Carolina, the boring association in Norfolk is between Entobia and Caulostrepsis, which are found in both tests and chalk infill. Thus, the lithology of the infill is an important factor in determining these differences.