Abstract

The Cretaceous/Tertiary boundary event exerted a deep influence on the life of the benthic invertebrates in the Danish Basin. The density and taxonomic diversity of most groups fell abruptly and the lowermost Danian strata are almost barren of invertebrates. The echinoderms, and among them in particular crinoids of the family Bourgueticrinidae, are the only group that deviates form this general pattern and show an increase in the strata immediately above the boundary. This exceptional development is accompanied by a heterochronic change in which the largest element of the bourgueticrinid theca, the proximale, is lost by paedomorphosis (neoteny). In the Late Cretaceous bourgueticrinids the proximale is formed during early ontogeny by the incorporation of a columnal element into the theca, and the addition of new columnals stops once the proximale is formed. The paedomorphic change and the elimination of the proximale allow, in principle, columnals to be added throughout life. This new development is probably the result of a selection for longer stems. It is successful from the very beginning and Democrinus maximus, the earliest bourgueticrinid species in the Danish Basin without a proximale, makes up about 90% of the crinoid fauna in the lowermost Danian deposits.

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