Data from: Trends in the sand: directional evolution in the shell shape of recessing scallops (Bivalvia: Pectinidae)

Directional evolution is one of the most compelling evolutionary patterns
observed in macroevolution. Yet, despite its importance, detecting such
trends in multivariate data remains a challenge. In this study, we
evaluate multivariate evolution of shell shape in 93 bivalved scallop
species, combining geometric morphometrics and phylogenetic comparative
methods. Phylomorphospace visualization described the history of
morphological diversification in the group; revealing that taxa with a
recessing life habit were the most distinctive in shell shape, and
appeared to display a directional trend. To evaluate this hypothesis
empirically, we extended existing methods by characterizing the mean
directional evolution in phylomorphospace for recessing scallops. We then
compared this pattern to what was expected under several alternative
evolutionary scenarios using phylogenetic simulations. The observed
pattern did not fall within the distribution obtained under multivariate
Brownian motion, enabling us to reject this evolutionary scenario. By
contrast, the observed pattern was more similar to, and fell within, the
distribution obtained from simulations using Brownian motion combined with
a directional trend. Thus, the observed data are consistent with a pattern
of directional evolution for this lineage of recessing scallops. We
discuss this putative directional evolutionary trend in terms of its
potential adaptive role in exploiting novel habitats.