PEDOMORPHOSIS OF AMMONOIDS AS A RESULT OF SEALEVEL FLUCTUATIONS IN THE LATE DEVONIAN WOCKLUMERIA STUFE

A remarkable diversification of several independent ammonoid lineages with high evolutionary rates occurred in the Late Devonian Wocklumeria Stufe. Many speciation events led to paedomorphic ammonoids that disp lay a striking range of conch shapes, sculpture, and ornamentation. In the goniatite family Prionoceratidae, the transition from normal Mimi mitoceras species to paedomorphic Balvia species provides an example o f rapid size decrease combined with an early character developmental o ffset arising from progenesis. Adults of early Balvia species largely have the preadult ancestral morphology of Mimimitoceras, but later evo lving species acquire distinct conch and ornamentation types. Progenet ic ammonoid species also appeared within the clymeniid family Kosmocly meniidae and probably in the Glatzielliidae. In the clymeniid family P arawock-lumeriidae, evolution is characterized by the extension of tri -segmented and triangularly coiled whorls found only in juveniles of e arlier species, to the adults of later species. This is interpreted as resulting from neoteny. The distribution of paedomorphic ammonoids in the Late Devonian Wocklumeria Stufe is closely correlated to relative sealevel changes The regressive trend in the lower two-thirds of the Wocklumeria Stufe is interpreted as the cause of a diversification of the pelagic habitat during unstable conditions, and as an extrinsic fa ctor inducing heterochronic change. Some ammonoids reacted by rapid ma turation and faster reproductive rates, giving the opportunity to expl oit a wider range of niches. The apparent consequence was the formatio n of several allopatric species.