Article: Growth analysis of Silurian orthoconic nautiloids
Evidence from orthocone septal strength implies approximate depth limits of 200 m for the near-shore dwelling actinocerids, 500 m for large orthocerids, and no more than 1500 m for the small pelagic orthocerids found in 'graptolitic shales'. These estimates refer to the initial depositional depth of fully septate conchs; not the habitat depth of pelagic species, nor the occurrence of fragmented and reworked specimens. The further interpretation of the autecology of the large orthocerids must be based either on the distantly related, coiled genus Nautilus, or their large coleoid descendants. Studies of the growth rate of large orthocerids test their ecological similarity to large predatory coleoids. Cycles of lirae spacing in Geisonocerina, annulation amplitude in Dawsonoceras, and septal spacing in a variety of Silurian orthocones, show annual periodicities implying growth rates of about 100 mm per year. Large conchs with a length of 1 -0 to 1 -5 m had a protracted growth phase for over fifteen years, followed by at least one year as a slowly growing mature stage. The adolescent increase in body weight and mantle cavity volume is even less than that of Nautilus, suggesting little ecological similarity to coleoids.