First records of sublethal and lethal injuries in Late Jurassic (Tithonian) ammonoids from Argentina

VENNARI, V.

Heidelberg

Simposio; 22º Colloquium on Latin American Earth Sciences; 2011

Late Jurassic-Early Cretaceous ammonite shells are the most common invertebrate remainsin the marine sedimentites of the Vaca Muerta Formation, Neuquén Basin, west-centralArgentina. Despite of their high abundance and well preservation, sublethal and lethal injuriesin Argentinian ammonoids have been overlooked by previous workers, hence this is the first approach to their study. From a total of 2,000 Tithonian ammonites recovered from different localities along the basin only 1.2% of them have evidences of shell regeneration and only one specimen shows a ventral bite mark on its living chamber comparable to the cases studied by Klompmaker et al. (2009) and Andrew et al. (2010), and consequently interpreted as a successful predatory attack. 86% of the damaged specimens exhibit lateral, latero-ventral and ventral injuries which are correlated in most cases with sculpture disruptions in the form of backwards rib bending (Rippenscheitelung) with or without a median furrow associated (Rippenknickung). This morphology -forma verticata- Hölder (1956) is the result of a punctual injury on the mantle edge, the one in charge of sculptured shell secretion. Shell breakage areas on other specimens are inferred from abrupt direction changes in the ornamentation elements or little shell portions unornamented or with a lowest relief sculpture -forma substructa of Hölder (1973)-. When only the internal mould is preserved, these kind of injuries are preceded by a slight depression as a consequence of extra shell material accumulation to heal the traumatism. Regenerated shell areas in ammonites have been frequently associated with frustrated predation attempts, although injuries around the peristome during mating or feeding on prey, have been also considered. Many potential predators are found on the same stratigraphic levels than the injured ammonoids. These include marine reptiles (crocodiles, sea turtles, ichthyosaurs and pliosaurs), fishes (Pachycormids, Ichthyodectiforms and Caturidae-like) and other groups of cephalopods (nautilids and belemnites). Taking into consideration the feeding habits of those taxa and the small to medium size of the injured ammonites shells (diameter < 90 mm), only the smaller durophagous active predators, or their hatchlings, with strong beaks or jaws may have been the ones who attacked but not succeed in killing the ammonites.