Periostracized

Periostracized is an adjective used in malacology and paleontology to describe shells or organisms that possess or display the periostracum, the outer organic layer of a mollusk shell. The periostracum is secreted by the mantle and forms the outermost part of the shell before, or in some cases alongside, the calcareous layers.

The periostracum is primarily composed of organic material, including conchiolin and other proteins, and it may contain pigments. Its thickness and persistence vary widely among species. In some mollusks the periostracum is thick, pigmented, and remains intact for life, contributing to coloration and texture. In many others it is thin and easily eroded, so the underlying calcareous surface is more visible.

Function and development: The periostracum serves as a protective organic shield during early shell formation and can influence surface texture and patterning. It can affect how the shell interacts with the environment, including resistance to dissolution and biofouling. The shell’s calcareous layers subsequently form or beneath the periostracum, and erosion or diagenesis may remove the organic layer in fossils.

Significance: In descriptions of living mollusks and in fossil specimens, the presence or absence of periostracum is noted to aid in identification and interpretation of preservation. The term periostracized helps distinguish shells that retain this outer organic covering from those that expose only the mineralized inner layers.