MORPHOLOGICAL ANALYSIS OF HETEROCHRONY OF THE BLASTOID GENERA METABALASTUS AND TRICOELOCRINUS

Metablastus and Tricoelocrinus were uncommon blastoids that ranged throughout the North American Mississippian age formations, with the former found in older formations such as the Edwardsville Formation. These genera exhibit distinct variation in their developmental growth progressions, however, this variation appears to relate to overall size rather than taxonomic identity. This is particularly apparent between the taxa Metablastus wortheni and Tricoelocrinus woodmani. The objective of this study is to assess the hypothesis that T. woodmani exhibits shared ontogenic characteristics with M. wortheni resultant of heterochrony as an outcome of divergence from the Metablastus genus.

In order to establish the relationship between these species and disentangle the potential role that heterochrony has played in their taxonomic delineation a representative sample of 17 M. wortheni species and twenty seven T. woodmani species were analyzed using geometric morphometric techniques, including Procrustes technique with relative warp analysis. Longitudinal and summit orientations were used in accordance with a series of fixed and repeatable landmarks. Additionally, a series of eight linear measurements were taken in longitudinal profile consistent with previous morphological blastoid literature. Relative warp analyses of the longitudinal and summit profiles explained 80% and 60% of the variation, respectively, between individuals. Subsequent regression analysis indicated strong allometric relationships between shape and overall size (gauged by length of theca). Significant differences between species, however, did not become evident until theca sizes exceeded 20mm whereby T. woodmani consistently developed significant differences in morphology of longitudinal and summit profiles compared with M. wortheni and T. woodmani under 20mm in theca length. Our hypothesis is consistent with T. woodmani having evolved from M. wortheni as a result of heterochrony.