Body size of orthoconic cephalopods from the late Silurian and Devonian of the Anti-Atlas (Morocco)

Abstract

Fluctuations in body size of orthoconic cephalopods are reported from late Silurian to Late Devonian sediments at several locations in the Tafilalt and in the Dra-Valley (Anti-Atlas, Morocco). The combination of measurements of diameters and apical angles allows the reconstruction of their total conch size (length and volume), which revealed a strongly right-skewed size distribution with an average length of 278 mm, while the largest Devonian actinocerids exceeded 2 m. Within the examined groups (Actinocerida, Orthocerida, Pseudorthocerida), there is no uniform trend, but rather frequent fluctuations with maximum sizes in the late Lochkovian and early Emsian. Body size decreased in times of extinction events, while stable periods are mostly associated with a size increase. Additionally, conch size correlates well with gamma diversityand global d13C values. Furthermore, the apical angle and septal diameter of orthocones appear to correlate, but only when their mean values are compared across beds. Anti-Atlas, body size, Cephalopoda, Devonian, extinction events, Morocco, orthocones.