Klug, C., Kröger, B., Kiessling, W., Mullins, G.L., Servais, T., Frýda, J., Korn, D. & Turner, S. 2009: The Devonian nekton revolution. Lethaia, 10.1111/j.1502-3931.2009.00206.x Traditional analyses of Early Phanerozoic marine diversity at the genus level show an explosive radiation of marine life until the Late Ordovician, followed by a phase of erratic decline continuing until the end of the Palaeozoic, whereas a more recent analysis extends the duration of this early radiation into the Devonian. This catch-all approach hides an evolutionary and ecological key event long after the Ordovician radiation: the rapid occupation of the free water column by animals during the Devonian. Here, we explore the timing of the occupation of the water column in the Palaeozoic and test the hypothesis that ecological escalation led to fundamental evolutionary changes in the mid-Palaeozoic marine water column. According to our analyses, demersal and nektonic modes of life were probably initially driven by competition in the diversity-saturated benthic habitats together with the availability of abundant planktonic food. Escalatory feedback then promoted the rapid rise of nekton in the Devonian as suggested by the sequence and tempo of water-column occupation. □Devonian, diversity, ecology, food webs, nekton, plankton, radiation.

During the Devonian Nekton Revolution, ammonoids show a progressive coiling of their shell just like many other pelagic mollusk groups. These now extinct, externally shelled cephalopods derived from bactritoid cephalopods with a straight shell in the Early Devonian. During the Devonian, evolutionary trends toward tighter coiling and a size reduction occurred in ammonoid embryonic shells. In at least three lineages, descendants with a closed umbilicus evolved convergently from forms with an opening in the first whorl (umbilical window). Other lineages having representatives with open umbilici became extinct around important Devonian events whereas only those with more tightly coiled embryonic shells survived. This change was accompanied by an evolutionary trend in shape of the initial chamber, but no clear trend in its size. The fact that several ammonoid lineages independently reduced and closed the umbilical window more or less synchronously indicates that common driving factors were involved. A trend in size decrease of the embryos as well as the concurrent increase in adult size in some lineages likely reflects a fundamental change in reproductive strategies toward a higher fecundity early in the evolutionary history of ammonoids. This might have played an important role in their subsequent success as well as in their demise.

Mollusks in general and ammonoids in particular are known to display a sometimes profound morphological intraspecific variability of their shell. Although this phenomenon is of greatest importance, it has rarely been investigated and quantified. It is especially crucial for taxonomy and incidentally for biodiversity analyses to account for it, because otherwise, the number of described species might exceed that of actual species within any group. Early ammonoids (Early Devonian, Paleozoic) typically suffer from this bias. For instance, most specimens from the same layer and the same region (e.g., the Erbenoceras beds of the Moroccan eastern Anti-Atlas studied here) differ morphologically from each other. Depending on the importance given to certain morphological characters, therefore, one could create a new species for almost every specimen. In this study, we measured nearly 100 such specimens from a restricted stratigraphic interval and quantified their intraspecific variability. There is a variable but strong overlap of the quantified shell characters at most ontogenetic stages, and only two species are here separated rather than the four previously recognized in Morocco. When ontogenetic trajectories of the Moroccan specimens are compared with coeval faunas from other regions (assigned to other species), a strong overlap between the morphospaces occupied by these taxa becomes apparent. The justification of some of these latter species is thus questionable even if their mean values in some conch parameters differ considerably from the mean values of the Moroccan species. Hence, the number of currently valid species of these loosely coiled early ammonoids is probably much too high. Extreme caution must therefore be taken when examining the diversity of groups in which the intraspecific variability is poorly known.