The Shell Microstructure of the Earliest Paterinids (Brachiopoda) and Its Postmortem Alteration
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Shell Microstructure of the Earliest Paterinids (Brachiopoda) and Its Postmortem Alteration G. T. Ushatinskaya* Borissiak Paleontological Institute, Russian Academy of Sciences, Moscow, 117647 Russia *e-mail: [email protected] Received December 3, 2019; revised December 12, 2019; accepted December 12, 2019
Abstract—The shell microstructure and its postmortem alteration were studied for two Early–Middle Cambrian paterinid families constituting the order Paterinida. The paterinid shells during life were organophosphate; most of the shell substance was organic. The organic matter after the death played an important role in the processes of postmortem alteration of the original microstructure. The conditions near the sea bottom also strongly influenced the type of preservation of paterinid shells. Keywords: oldest brachiopods, shell composition, microstructure, fossilization, burial conditions DOI: 10.1134/S0031030120050147
INTRODUCTION The class Paterinata (order Paterinida) consists of Early Paleozoic brachiopods, which had organophosphate shells during life. Like other organophosphateshelled brachiopods of the subphylum Linguliformea, the paterinids lacked shell articulation of mineral outgrowths at the posterior margin of the ventral valves and corresponding sockets in the dorsal valve. Unlike other linguliforms, they had a true interarea with delthyrium and notothyrium partly or completely closed by deltidium and chilidium respectively, and a straight or almost straight posterior margin that brings them together with the articulated brachiopods of the class Rhynchonelliformea (or Articulata). Besides, some of the oldest paterinids had a rounded pedicle opening near the ventral umbone, which was closed posteriorly by a concave plate, the colleplax. The colleplax is also known for the Early Cambrian brachiopods of the order Chileida from the class Rhynchonelliformea (Holmer et al., 2009). This combination of features has complicated the assignment of paterinids to a higher taxon. In “Osnovy paleontologii” (Gorjansky, 1960), paterinids were considered as the superfamily Paterinacea and assigned along with the superfamily Kutorginacea to the order Kutorginida. The brachiopods of the superfamily Kutorginacea had a calcareous shell; the whole order was assigned to the class Inarticulata. In the first edition of “Treatise…” (Rowell, 1965), the paterinids were distinguished as a separate order Paterinida Rowell, 1965 and also assigned to the class Inarticulata together with the orders of carbonate-shelled brachiopods Craniida and Obolellida.
Gorjansky and Popov (1985) established the class Lingulata Gorjansky et Popov, 1985 that comprised all brachiopods with organophosphate shells including the paterinids. Finally Williams et al. (1996) distinguished the paterinids as a separate class Paterinata based on the following characteristic features of the earliest paterinids: organophosphate shell, structure of the shell posterior margin, presence of interareas on both valves, patterns of muscle scars and mantle canals, and pres
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