EOCENE GYROLITHES-THALASSINOIDES ICHNOCOENOSES FROM ALABAMA AND PERU: IMPLICATIONS FOR MARINE VERTEBRATE TAPHONOMY

Ichnocoenoses dominated by Gyrolithes and Thalassinoides were observed at two middle Eocene (Bartonian) localities, one in the coastal plain of Alabama and the other in the Pisco Basin of Peru. These localities are significant because both preserve archaeocete cetaceans and other important marine vertebrates. Gyrolithes is a vertical, helically-coiled burrow found in marginal marine settings. This burrow is commonly attributed to crustaceans and often grades into other ichnotaxa linked to crustacean trace-makers, particularly Thalassinoides and Ophiomorpha. At both localities described in this study, Gyrolithes and Thalassinoides are the only trace fossils present in the units they are observed.

At a locality in the upper Lisbon Formation of Alabama, Gyrolithes and Thalassinoides exhibit bioglyphs parallel to the axis of the burrows, suggesting the trace-maker burrowed into a chalky firmground. This may be indicative of erosion of surface softground substrates during regression, exposing underlying firmground to bioturbation by Gyrolithes-producing organisms. A specimen of the protocetid cetacean Georgiacetus was collected from shoreface sands within a transgressive sequence at this locality. In the Paracas Formation of Peru, Thalassinoides are preserved as convex hyporelief on the base of a sandstone event bed overlying laminated siltstones and claystones. Gyrolithes descend from this network of Thalassinoides into the mudstone. The Peruvian traces likely represent “doomed pioneers” that at least temporarily colonized a muddy softground below wave base following a high-energy event. Similar horizons to the Gyrolithes-bearing mudstone within the Eocene of the Pisco Basin preserve protocetids, basilosaurids, and a giant basal penguin, the latter of which exhibits soft tissue preservation. The sedimentology and ichnology of these localities suggest preservation in a shelf setting frequently affected by high-energy events.

The ichnocoenosis of Gyrolithes and Thalassinoides, in concert with sedimentologic observations, provides insights into the taphonomy of these marine vertebrates. This repeated trace fossil association in different sedimentary sequences may imply common environmental conditions during the middle Eocene at these localities.