Exoskeletal structure in the Ordovician trilobiteFlexicalymene

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Abstract
Scanning Electron Microscope study of the exoskeletal ultrastructure of secondarily phosphatized material of Flexicalymene sp. from the Upper Ordovician Maquoketa Shale, Iowa, USA, shows that the exocuticle, comprising 20% of the total exoskeletal thickness, is composed of horizontal laminar units between 0.2 and 1 .mu.m thick. These units consist of primary mineralized organic fibers which form horizontal laminae interconnected by inter-laminae. The endocuticle is considerably more mineralized than the exocuticle, its original organic structure cannot be observed in untreated preparations. Etching with chromium sulfate reveals: horizontal organic laminar units, 0.2-2 .mu.m thick, and pore canals with non-twisted walls about 0.3 .mu.m in diameter. Exuvia cannot be distinguished from the exoskeletons of dead animals. The exoskeletal ultrastructure in trilobites agrees essentially with that in crustaceans.

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