Microscopic imprints on the juvenile shells of Palaeozoic linguliform brachiopods

The juvenile shell of living discinid brachiopods is composed of valvular mosaics of rhombic, micrometric–sized siliceous tablets. The tablets are shed in adult growth stages leaving shallow imprints on the primary layer of the organophosphatic mature shell, which occur on an upper Silurian discinoid. Imprints also indent the first–formed shells of over 100 lingulate genera, including all acrotretides and Paterula . No micrometric bodies that could have made these imprints are known, but their structure and composition can be inferred from imprint morphology. Diagnostic features of imprints as casts include constancy of shape and size, and rigidity of the indenting bodies relative to the rheology of the polymerizing primary layer of chitin, glycosaminoglycans (GAGs) and apatite, in which they were embedded. Three kinds of discrete structures are distinguishable. Nanometric–sized, basinal pits that may be compound or deformed, are assumed to be casts of mucinous vesicles. Larger, flat–based and hemispherical imprints were almost certainly made by biomineralized discoids and spheroids. Additional evidence that discoids and spheroids were more soluble than apatite suggests that they were calcitic. Mineralized mosaics could have protected pelagic juveniles from solar radiation in the early Palaeozoic when the ozone layer was more rarified than today.