GROWTH OF PENTREMITES AND IMPLICATIONS FOR BLASTOID ONTOGENY

Ontogeny (growth of an individual) and phylogeny have been widely discussed over the past century in understanding the evolution of fossil forms. Evolutionary changes in ontogeny, such as the growth rate of skeletal elements, changing in onset time of morphologic change and the time of the onset of maturity defined the forms of heterochrony. Consequently, studying the ontogeny of species provides an opportunity to understand changes in morphology through changes in growth patterns. Heterochrony has been well documented in echinoderms, including blastoids, which are widely used as model organisms on which to assess morphological changes. They have a set number of plates that can be easily identified on each specimen facilitating the measurement and examination of minute changes in overall morphology. Herein we examine one of the most successful and abundant Mississippian blastoids, Pentremites. A common barrier to such a study is the low abundance of target specimens. However, the Upper Mississippian (Chesterian) Glen Dean Formation in Hopkinsville, Kentucky offers ample 1-5 mm high specimens and upwards of 35,000 specimens < 1mm high per kilogram of washed sediment. Ontogenetic studies often exclude specimens less than 5 mm in height, but here we include a full range of smaller specimens to provide a more complete ontogenetic sequence. Shale was washed, sieved, and picked for blastoids that were sorted by size and species for morphological analysis. Standard morphological measurements were made, such as length of ambulacra, vault to pelvis ratio, and thecal height and width) of all Pentremites specimens to fully capture ontogenetic changes. The data were examined first in bivariate plots to assess the variation of growth between species with the two major morphotypes plotting on distinct ontogenetic trajectories. These data show that the two common species were morphologically distinct early in ontogeny.