Convergent discoidal sand dollars from isolated regions: A geometric morphometric analyses of Dendraster and Arachnoides

  • Analysis suggests ontogenetic controls for the acquisition of discoidal morphology
  • The signal of ontogenetic variation in petaloids and posteriors is limited
  • Analysis demonstrates the use of geometric morphometrics for fossil clypeasteroids

Sand dollars (Clypeasteroids) have developed a unique discoidal morphology along the perimeter curvature or ambitus. The distinctive round and flattened morphology appears adaptive to their shallow water habitat and appears in multiple species throughout the group. This analysis applies geometric morphometric analysis to fossil clypeasteroid specimens from the genus Dendraster from the western United States coastline, California, and examines the following: 1) Quantifying morphological, ontogenetic, and developmental variation between the clypeasteroid genera Dendraster and Arachnoides; 2) Quantifying discoidal morphology, petaloid structures, and posterior morphological variation within and between the examined genera; 3) Assessing how regional endemicity may effect variation between and within the assessed Clypeasteroids. This study begins to quantify how this distinctive morphology developed in fossil and extant clypeasteroids and how morphological variation during ontogeny compares between the two clypeasteroid genera. Results demonstrate strongest ontogenetic trends occur in the aboral/oral profiles for the landmark and semilandmark analyses of ambitus morphology and curvature shape. Other morphologic traits including the petaloid structures and posterior profile do not readily demonstrate ontogenetic trends. Interspecific variation or potentially morphological variation due to environmental controls appear to play a larger role in observed morphological variation for these traits. This analysis also demonstrates the utility of geometric morphometric analysis for clypeasteroid genera; building a foundation for broader analysis of discoidal morphologies.

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