Phymatoderma granulata from the Oligocene-Miocene of Tierra del Fuego: Morphology and ethology.

E. B. OLIVERO; J. J. PONCE; M. I. LÓPEZ CABRERA; D. R. MARTINIONI

Trelew

Congreso; First international Congress on Ichnology - Ichnia 2004; 2004

Museo Paleontológico Egidio Feruglio

ABSTRACT.   Phymatoderma is rare in the fossil record and it is mainly known from shelf and deep-sea deposits in Europe and Ecuador. Deep-sea Oligocene-Miocene deposits from Tierra del Fuego are characterized by few, but highly bioturbated horizons choked with large Phymatoderma granulata. Associated ichnogenera include Zoophycos; Chondrites; Tasselia; and a vertical spreite structure. The P. granulata burrow system consists mainly of subhorizontal, irregularly branched tunnels completely filled with ovoid fecal pellets. Small feather-like branches, also filled with pellets are occasionally present at the tube margins. In horizontal view, well-preserved specimens show that the longer axis of the pellets are arranged within the tube into sigmoidal and/or short convex bends, which most likely represent a retrusive spreite formed by the joined lateral and proximal migration of a J-shaped causative burrow. In vertical section the tube shows concave-upward laminae suggesting that the causative burrow also moved upwards. Branching frequently occurs toward the inner bend of the J-shaped spreite, implying that new branches were formed by a sudden, distal extension of the causative burrow. The study of thin sections shows that the finest grains -silt grains, foraminifera and radiolaria--, and a very fine carbonate micrite were selectively concentrated within the fecal pellets. On the contrary, the non-bioturbated sediment consists of abundant fine-medium sand grains, a coarser-grained carbonate micrite, and few microfossils. The evidence is coherent with the food-mining behavior proposed for Phymatoderma even though in our material the structure was built from deeper to shallower levels within the sediment.