A CASE STUDY ON BIOEROSION IN FOSSIL CEPHALOPODS FROM THE UPPER CARBONIFEROUS BUCKHORN ASPHALT QUARRY LAGERSTÄTTE, OKLAHOMA, USA

Cephalopod remains from the Desmoinesian (Pennsylvanian, Upper Carboniferous) Buckhorn Asphalt Quarry (Oklahoma, USA) were studied intensively over the past decades. However, this is the first study that focusses exclusively on bioerosion affecting their shells. Research on bioerosion in these fossils was possible due to soaking of sediments and shells with hydrocarbons, during or shortly after time of deposition, that sealed open space and prevented strong diagenesis. The shells derive from the ‘cephalopod coquina’ that represents the deepest-water deposit within the single cycle of trans- and regression in the outcrop. The cephalopod remains comprise fragments of both, orthoconic and coiled nautiloid specimens as well as those from ammonoids.

The shells were cast and investigated with a scanning electron microscope. The study revealed a diverse association of ichnotaxa and ichnoforms (twenty-two in total, eight of them new – among them six ichnoforms could derive from foraminiferans - and twelve with their oldest record in the Carboniferous) comprising chlorophyte, cyanobacterial, and rhodophyte traces. Fungal and spongial traces as well as those of so far unknown origin are also recorded. Most common are Ichnoreticulina elegans (a chlorophyte trace), a new morphotype of Scolecia isp. (probably produced by cyanobacteria) as well as the fungal (Schizochytrium sp.) trace Flagrichnus profundus, the ‘extremely thin form’ (likely the Conchocelis-stage of Porphyra sp. or Bangia sp.), and the ‘superthin form’ that is produced by some unknown organotroph.

Straight nautiloids are intensively bored by I. elegans and Scolecia n. isp. while in the shells of coiled cephalopods F. profundus, the ‘extremely thin form’, and the ‘superthin form’ are most abundant. The ichnotaxon associations in the shells indicate different modes of deposition: Shells of coiled specimens seem to have drifted for a while and were therefore more prone to bioerosion by autotrophs until they sank to the seafloor, while those of the orthocones sank more rapidly with deposition in the deep euphotic to dysphotic zone of the ichnobathymetrical scheme. A comparison with the studies on extant Nautilus supports the idea that ichnotaxa present in Carboniferous cephalopods have gone lost with the extinction of these.