Allometric variation in the genus Steinmanella (Trigoniodia, Bivalvia) fron the Lower Cretaceous of the Neuquén basin (West-Central Argentina)

MILLA CARMONA, PABLO; SOTO IM; LAZO, DARÍO

Riverside

Congreso; 11th North American Paleontological Convention; 2019

University of California

Because of the outstanding diversity and disparity they reached during the Mesozoic, the paleobiology of trigoniid bivalves has attracted considerable interest. In this work, we assessed the patterns of allometric variation within the genus SteinmanellaCrickmay (Myophorellinae, Trigonioida) as it occurs in the lower Valanginian ? upper Hauterivian (Lower Cretaceous) of the Neuquén Basin (west-central Argentina). The shells of 236 specimens belonging to 7 species ofSteinmanella (namely, S. quintucoensis, S. subquadrata,S. curacoensis, S. caicayensis, S. pehuenmapuensis, S.transitoria and S. vacaensis) were digitized in three dimensions, and variation in two prominent external morphological characters, general valve geometry and sculpture, was subsequently analyzed. Shell surface shape and size were measured by means of geometric morphometrics, whereas sculpture was quantified using counts of ribs and nodes. The trajectories of the studied species through different size categories(intended to represent meaningful ontogenetic stages) were compared using phenotypic trajectory analysis.Our results show that early and late growth changes differ in nature between species. The former seems to be far more plastic, being characterized by changes in the direction and magnitude of the allometric trajectory through the shell surface and sculpture morphospaces respectively. On the other hand, late growth seems to be more conserved and channeled, showing more infrequent changes which mainly involve themagnitude of the trajectory across the shell surface morphospace. Therefore, the distinctive features of each species would had been acquired early in life,with later changes involving a general trend towards shell elongation, thus challenging the view that early development is more conserved in evolution. These findings can have important implications for the evolution of Steinmanella, as heterochronic processes acting upon ontogenetic variation is thought to be a major driver of bivalve evolution.