Feeding traces related to shells from the Prague Basin, Czech Republic (Tremadocian to early Darriwilian, Ordovician)

Abstract

Post-mortem feeding activities along or within the interiors of shallowly buried shells can be preserved by sediment infilling and be visible on internal moulds of shelly fossils. Almost no such activities were recorded in three lowermost Ordovician formations (Třenice, Mílina and Klabava; Tremadocian to Dapingian) of the Prague Basin in the Barrandian area (Czech Republic). In contrast, a distinct increase occurs in the Šárka Formation (early and middle Darriwilian). Šárka feeding traces are common in shells preserved within siliceous nodules. They are associated with almost all taxonomic groups composing a rich and diversified fossil assemblage within that formation; but there is no evidence of similar traces produced by feeding on soft-bodied fauna. Arachnostega, Pilichnus and Palaeophycus are the most abundant ichnogera observed. Their trace makers selectively oriented on decaying soft tissues. Palaeophycus producers apparently preferred an easily accessible and nourishing food that was easily consumed. These traces typically intersect a digestive tract, especially the gut. The Arachnostega and Pilichnus traces are oriented in a manner suggesting systematic feeding. These tracemakers spent more time in a carcass, reaching lower nutrient tissues, including various membranes, that degraded more slowly and were more difficult to access. The preferential distribution of feeding traces in each shell reflects the topology of the soft tissues serving as a food. All studied feeding traces were found in nodules, never in shales of the Šárka Formation. This reflects a taphonomical bias influencing occurrences of these traces. Therefore, a lack of the feeding traces in the units underlying the Šárka Formation could be the result of taphonomic differences related to depositional conditions and diagenetic processes. Nevertheless, the abundance of those traces in the Šárka Formation illustrates the prosperity of this feeding behavior during the Darriwilian. An increase in the abundance, diversity and complexity of communities that fed within the internal ecospace of shells while utilizing feeding strategies developed during previous periods is considered to be a primary effect related to increased ecosystem complexity during the Great Ordovician Biodiversification Event.