Origin, evolution and classification of the new superorder Nepiomorphia (Mollusca, Bivalvia, Lower Palaeozoic)

Abstract

A number of bivalve taxa defined in the past as 'Cryptodonten' by Neumayr, 1884 were grouped together in the high-level taxon Palaeoconchae Neumayr, 1884. Cox (1969) noted that Palaeoconchae and Cryptodonta were synonymous and Newell (1969) used Cryptodonta as a subclass of bivalves. However, for the past 120 years, the Cryptodonta has been poorly conceptualized and the name was used for poorly understood genera or those lacking dentition. As used by Newell, Cryptodonta included taxa now placed in the subclasses Protobranchia Pelseneer and Autolamellibranchiata Grobben, and to the class Rostroconchia Pojeta, Runnegar, Morris and Newell. The bulk of Newell's use of Cryptodonta was made up of Silurian and Devonian taxa first described by Barrande (1881) from Bohemia; Newell placed these in the order Praecardioida Newell. In effect, Cryptodonta became a 'wastebasket' grouping for what, at the time, were poorly understood taxa. Many of the formerly poorly understood praecardioids are now better known and are herein placed in the new superorder Nepiomorphia. The Nepiomorphia contains two orders: (1) order Praecardioida that includes the families Slavidae Kříž, Cardiolidae Hoernes, Praecardiidae Hoernes and Buchiolidae Grimm; and (2) new order Antipleuroida that includes the families Stolidotidae fam. nov., Spanilidae fam. nov. and Antipleuridae Neumayr. The Nepiomorphia originated probably in the early Silurian as result of r-selection progenesis. When the marine current system became re-established after the late Ordovician glaciation and in the early Silurian, an at least temporary ventilation of the shallow waters by surface currents was renewed in higher latitudes of peri-Gondwana and Siberia, producing acceptable sea-bottom environments. Larvae were distributed by surface currents from the warm tropical regions of Laurentia and Baltica and were among the first benthic organisms to colonize the new environments. Temporary ventilation created frequent density-independent catastrophic mortalities of early ontogenetic stages, with no competitors and with super-abundant resources. During the Silurian and early Devonian, the Nepiomorphia underwent several diversifications in the recurring cephalopod limestone biofacies characteristic of peri-Gondwana, and evolved infaunal, semi-infaunal and epifaunal modes of life in several lineages. The Nepiomorphia most probably became extinct during the early Carboniferous and had no role in the future evolution of the Bivalvia. © 2007 The Palaeontological Association.