Evolutionary ecology of nine sympatric species of the pelecypod Limopsis in Cretaceous chalk

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Abstract
The ecology and functional morphology of 9 spp. of Limopsis [L. maggae, L. helenae, L. augustae, L. ravni, L. nanae, L. alvildae, L. misjae, L. amandae and L. magdae] that lived together in muddy bryozoan bioherms in the Cretaceous of Denmark are discussed. Two independent lines of limopsids entered the white-chalk environment: a non-byssate line represented by 1 flat circular burrowing species and a byssate line which gave rise to the formation of 8 spp. Seven of these, ranging from a large globose infaunal species to a small mytiliform epibyssate monomyarian, constitute a morphocline. The morphocline reflects adaptations along an environmental gradient, possibly related to substrate size and water flow. The morphocline contains 2 dominant species. One of these is epifaunal and the other infaunal, which demonstrates a trophic-group distribution sensu Turpaeva, reducing competition. A 3rd, infaunal, species, not a member of the cline, reached a level of dominance which underlines the evolutionary distance between this species and those of the morphocline. The sympatrical formation of the species appears to have taken place through specific habitat selection, resulting in non-competitive units. The morphological modifications resulted in a series of species ranging from near isomyarian forms over increasingly heteromyarian species to a monomyarian species, a unique case within the arcids.