First report on burrowing traces in internal moulds of shells from the Middle Jurassic of western Argentina

DAMBORENEA, S.E.; MANCEÑIDO, M.O.

Santa Rosa, La Pampa, Argentina

Simposio; Primera Reunión Argentina de Icnología; 1993

Universidad Nacional de La Pampa

Among trace-fossil makers of the marine environment, a group of specialized, small-sized, deposit-feeding burrowers which probe at the interface between a shell and its internal mould, is increasingly attracting the attention of palaeoichnologists. New information is added on the basis of own observations carried out on well preserved steinkerns of brachiopods and molluscs from Middle Jurassic deposits of Neuquen Province (central western Argentina) which bear branched, slender, burrow systems. These are interpreted as forming part of a pre-lithification suite developed in  a remarkable microhabitat, namely, the particulate but firm substrate enclosed within dead shells. These are analyzed and compared to (a) the recently erected ichnogenus Arachnostega, thus far reported from Late Jurassic deposits of northern Germany in relatively restricted enclaves within a shallow-water reefal environment; and (b) extant analogues which are known from mud-filled valves of semi-infaunal endobyssate mussels in USA Atlantic shores, as well as from mud-filled valves of infaunal burrowing clams in the North Sea coasts. In both modern examples the environment corresponds to muddy intertidal flats and infaunal errant polychaetes have been identified as the living producers. They apparently display an opportunistic behaviour and generate a conspicuous burrow-pattern that may be closely matched by the fossil counterparts. Furthermore, the depositional setting independently inferred for the Argentinian occurrences reveals a wider bathymetric scope. The studied materials thus document a considerable extension in the stratigraphical and geographical range of such specialized burrowers and also add significantly to the knowledge of the facial and taphonomical conditions involved.