WATERWAYS FORMATION: THE INFLUENCE OF PREDATION ON TURNOVER IN DEVONIAN CONCAVO-CONVEX BRACHIOPODS

Strophodontoidea and Productidina are two morphologically similar clades of strophomenate (~ concavo-convex) brachiopods; the primary distinction between the two higher taxa is the presence of spines on the productidines. During the Middle to Late Devonian, global diversity of the two groups manifested a classic double-wedge pattern, in which the richness of productidines increased as strophodontoid richness declined, culminating in the extinction of the latter taxon at the Frasnian-Famennian boundary. The similar morphology, and the double-wedge pattern, suggest the possibility that the two groups may have competed directly for resources, or indirectly through predation vulnerability. Previous case-studies have demonstrated that productidine spines could inhibit both drilling and crushing predation in the Devonian, and that productidines were consequently less vulnerable to predation than their strophodontoid cousins, thus potentially giving productidines an evolutionary advantage. However, no previous research has investigated predation intensity through time in conjunction with changes in the relative abundance of the two groups at the community scale.

The Waterways Formation (Givetian-Frasnian, Devonian, Western Canada Sedimentary Basin) of Alberta captures the critical interval in time during which productidines begin to replace strophodontoids, and both groups are present within Waterways communities. Within the Waterways, strophodontoids, common in the Calumet Member, are replaced through time by productidines, which are more common in the Moberly Member. The most abundant productidine is Praewaagenoconcha n. sp., which was one of the oldest productidine species in North America to have spines on both valves. Praewaagenoconcha spines are arranged in dense concentric rows and the spines of Waterways specimens are also frequently organized on strong rugae. Both repair frequency (a proxy for crushing predation) and drilling frequency on strophomenates were significantly greater in the Moberly than in the Calumet, suggesting that predation may have been an additional factor facilitating faunal turnover during the Waterways, and supporting the hypothesis that the replacement of strophodontoids by productidines through time may have been influenced by predation.