Despite the paleontological relevance and paleobiological interest of trigoniid bivalves, our knowledge of their ontogeny –an aspect of crucial evolutionary importance– remains limited. Here, we assess the intra- and interspecific ontogenetic variations exhibited by the genus Steinmanella Crickmay (Myophorellidae: Steinmanellinae) during the early Valanginian – late Hauterivian of Argentina, and explore some of their implications. The (ontogenetic) allometric trajectories of seven species recognized for this interval were estimated from longitudinal data using 3D geometric morphometrics, segmented regressions and model selection tools, and then compared using trajectory analysis and allometric spaces. Our results show that within-species shell shape variation describes biphasic ontogenetic trajectories, decoupled from ontogenetic changes shown by sculpture, and with a gradual decay in magnitude as ontogeny progresses. The mode of change characterizing each phase (crescentic growth and anteroposterior elongation, respectively) is conserved across species, thus representing a feature of Steinmanella ontogeny; its evolutionary origin is inferred to be a consequence of the rate modification and allometric repatterning of the ancestral ontogeny. Among species, trajectories are more variable during early ontogenetic stages, becoming increasingly conservative at later stages. Trajectories’ general orientation allows recognition of two stratigraphically-consecutive groups of species, hinting at a potentially higher genus-level diversity in the studied interval. In terms of functional morphology, juveniles had a morphology more suited for active burrowing than adults, whose features are associated with a sedentary lifestyle. The characteristic disparity of trigoniids could be related to the existence of an ontogenetic period of greater shell malleability betrayed by the presence of crescentic shape change.

Shell morphology was digitized in three dimensions with a HandyScan3D laser scanner and digitally processed in MeshLab to remove irregularities from the surface. Longitudinal variation in shell shape and size, as well as sculpture shape, were quantified using 3D geometric morphometrics (curve and surface semilandmarks). Ontogenetic (allometric) trajectories were reconstructed for each species using segmented regressions, with the number of segments (i.e., number of ontogenetic phases) estimated using model selection methods. Finally, allometric spaces were constructed by applying ordination methods (PCA) on regression's coefficients.

The data uploaded here include:

1) MillaCarmonaetal_FigureS1_1col.pdf: a supplementary figure showing results of a post hoc analysis on shape variance.

2.1) Supplementary materials 1_meshes.zip: a zip file containing the 278 meshes used as raw material for our analyses.

2.2) Supplementary materials 1_R files and data.zip: a zip file containing 2.2.1) a csv file containing data of specimens, 2.2.2-2.2.7) five scripts with the code necessary to run all analyses, and 2.2.8-2.2.9) two R objects (to be imported directly into the R session) containing the morphometric data necessary to run all analyses.

3) Supplementary materials 2_animated trajectories.zip: a folder containing several animations intending to aid interpretation of ontogenetic trajectores.

4) Supplementary materials 3_extended description of changes.doc: an exhaustive description of our species-level findings.