Encrusting patterns in Cretaceous mollusks: comparisons between benthic and nektobenthic hosts.

LUCI, LETICIA

Heidelberg

Congreso; 22nd International Coloquium on Latin American Earth Sciences; 2011

The encrusting fauna of nektobenthic nautilids (genus Cymatoceras Hyatt), semiinfaunal trigonioids (genus Steinmanella Crickmay) and cementing epifaunal pectinids (genus Prohinnites Gillet) were surveyed, and compared to each other. The hosts have been collected from Hauterivian beds of the Agrio Formation in the Neuquén Basin, Argentina. The unit is composed of grey shales, calcareous shales, limestones, and coquinas (Leanza et al., 2006), and has been interpreted as shoreface and offshore shelf deposits (Lazo et al., 2005).Differences in composition of the encrusting fauna and relative abundance of encrusters where found between hosts. The encrusting fauna is dominated by oysters (53% to 86%), followed by uncoiled serpulids (18% to 13%). Minor components of the encrusting fauna vary, but are represented by bryozoans (most diverse and abundant in pectinids), coiled serpulids (absent in trigonioids), foraminifers (found only in trigonioids, and possibly nautilids) and other serpulids (with one or three keels, found only in pectinids).It is significant that the two dominant encrusting groups, oysters and uncoiled serpulids, are commonly clustered together or overgrown. Oysters rarely exceed 3cm in length, while serpulid size varies. Both groups frequently present a gregarious behavior; the attachment of pioneer individuals triggered the attachment of other specimens and this is a factor to be considered when evaluating their great abundance as encrusters. Differences in the abundance and presence of the other groups probably respond to the interplay of the requirements of each kind of encruster, and the environmental conditions provided by each host. Taphonomy also plays an important role, since the original patterns of encrustation are obscured by taphonomic modification, which follows different pathways in each host; for example, cementing pectinids may lose the upper valves, but preserve the cemented ones, which may provide shelter for attaching organisms; nautilids, on the other hand, may go through a nekroplanctonic phase, undergoing extensive modification before reaching the sea bottom, where further modification takes place. Unveiling these pathways is essential to attempt an adequate paleobiological interpretation; and thus, this is the current line of investigation.