Palaeoecology and palaeogeographic relations of the Silurian phragmoceratids (Nautiloidea, Cephalopoda) of the Prague Basin (Bohemia)

Abstract

Phragmoceras and Tubiferoceras are discosorid nautiloid genera with endogastric cyrtoconic to orthoconic breviconic shells possessing a contracted aperture. Because of the constricted aperture phragmoceratids have usually been considered as microphages. The constriction of the aperture appears during the late ephebic stage. Preceding ontogenetic stages possessed a brevicone shell with an open aperture, usually considered to be indicative of nectobenthic predatory lifestyle. The apertural constriction probably improved hydrodynamic control and served as protection for the soft body. Attachment sites for distinct retractor muscles suggest evidence of potentially fast movement of the head-arm complex out of the aperture with consequent capture of larger prey. Phragmoceratids are a characteristic component of Silurian nautiloid faunas, which inhabited the mainly tropical carbonate platforms of Baltica and Laurentia. Phragmoceratids closely related to those of the Baltic and Avalonia occasionally appeared in the Prague Basin, which was located in the temperate zone at the northern margin of peri-Gondwana. The distribution pattern of phragmoceratids suggests that immigration into the Prague Basin occurred in three stages: (1) first appeared stray immigrants (or occasional visitors) from warmer seas, this stage of immigration reflecting activation of sea currents after the early Silurian widespread anoxia in peri-Gondwana (latest Llandovery-early Wenlock); (2) appearance of small endemic palaeo-populations of migrants (with evidence of local hatching of phragmocerids) that persisted only briefly during the early Homerian and early Gorstian low stands; (3) stable palaeo-populations appeared in the Early Ludfordian, with occurrence of endemic taxa related to the forms known from Baltica-Avalonia, suggesting stable conditions for nautiloid evolution. The Middle Ludfordian Kozlowskii Event caused the extinction of the last phragmocerid taxa in the Prague Basin as elsewhere. Nautiloid immigration to the Prague Basin (Perunica microplate) indicates that Perunica, in contrast to peri-Gondwanan areas, was within reach of the South Tropical Current since the latest Llandovery. Many other nautiloid families show a similar migration pattern although timing of the immigrations do not necessarily correlate. Thus the nautiloids provide useful data for palaeobiogeographic and climate reconstruction, such analysis can be made precise by comparison of nautiloid clades.