Lukeneder A 2007 Cephalopod evolution A new perspective Implications from two E Cretac ammonoid suborders英文电子书.pdf

© Biologiezentrum Linz, download unter www.biologiezentrum.at Cephalopod evolution: A new perspective – Implications from two Early Cretaceous ammonoid suborders (Northern Calcareous Alps, U er Austria) A . L U K E N E D E R Abstract: The status of two Early Cretaceous ammonoid groups from Upper Austria (Northern Calcareous Alps) is examined with respect to the evolution of their shape as well as to their morphology and environmental preference. The Valanginian Olcoste- phanus guebhardi (Verrucosum Zone, 137 mya) shows the evolutionary separation of sex in two different environments and has adapted its shape to the somewhat different environmental conditions. The heteromorph Barremian ammonoid Karsteniceras tern- bergense (Coronites Zone, 124 mya) is shown to have evolved during times with intermittent oxygen-depleted conditions associa- ted with stable, salinity-stratified water masses. Based on lithological and geochemical analysis combined with investigations of trace fossils, microfossils and macrofossils, an invasion of an opportunistic (r-strategist) Karsteniceras biocoenosis during unfavou- rable conditions is assumed. Both examples are chosen to demonstrate evolutionary trends in the Early Cretaceous which can be observed in the cephalopod group as a whole. Key words: Evolution, ammonoids, palaeobiology, Northern Calcareous Alps, Early Cretaceous. Introduction phological change and therefore appears as a new evo- lutionary trend. Radial or linear evolutions are the ABEL (1916) was the first to show a strong corres-