ONTOGENETIC AND SYSTEMATIC STUDY OF EUCYSTIS (DIPLOPORITA: ECHINODERMATA)

Our understanding of growth patterns, paleoecology, and systematics of the diploporitan echinoderm Eucystis, is poorly understood, leaving a number of questions concerning their evolutionary history. Eucystis has the longest temporal range (Ordovician-Devonian) among diploporitan echinoderms and one of the widest geographical ranges, with fossil collections from Laurentia, Gondwana, and Baltica. Eucystis is identifiable by comparatively long, multibrancing ambulacra that end in a varying number of brachiole facets, ovate to spherical theca, pentagonal thecal plates, and a holdfast attachment structure of varying size and shape.

Juvenile specimens are rare and often are absent from research collections; further, the preservation of most diploporitans is poor, leaving many morphologically important features taphonomically weathered or disarticulated. However, Eucystis angelini, collected from the Upper Ordovician of Sweden, is preserved in uncharacteristically high numbers and preserve a wide range of ontogenetic stages.

A morphometric study of E. angelini was performed on a large collection of specimens. Measurements of the thecal body, peristome (mouth), periproct (anus), feeding ambulacra, plates, respiratory structures, and holdfast were taken and analyzed to quantify this species’ growth patterns. The study suggests that the ambulacra grew at a faster rate than the theca. A second study used the previously collected data to determine which variables are driving the overall changes in size and shape from juvenile to adult form, and which variables are driving the separation between proposed species of Eucystis. Certain variables cannot be used to determine growth patterns and delineating species; the morphology of the holdfasts are likely controlled by external environmental factors and are not related to the growth of the organism.

The results from this study will aid in better understanding diploporitan growth patterns, which can be compared with other early Paleozoic echinoderm ontogenies, and will also allow for better understanding of diploporitan paleoecology. This study highlights the importance of finding juvenile fossils, and how critical they are in assessing ontogenetic growth of echinoderms.