A GROWTH STUDY OF ORDOVICIAN SPHAERONITES (DIPLOPORITA: ECHINODERMATA)

Ontogenetic patterns among Paleozoic blastozoan echinoderms are poorly understood. Growth studies involving these extinct groups are hampered by poor preservation and a lack of available specimens, especially juveniles. This issue is highlighted in the diploporitan blastozoans, a group diagnosed by having double pore (diplopore) respiratory structures that pierce the wall of the theca. Often, diploporitan taxa are represented by few poorly preserved specimens, commonly with morphologically distinctive areas (e.g., oral area) taphonomically weathered.

This study examines the ontogeny of Sphaeronites globulus rossicum from specimens collected near the Mishina Gora impact crater south of St. Petersburg, Russia. This Ordovician age diploporitan is characterized by a noticeable reduction in size of the oral area, with extremely short, branched ambulacra that terminate in numerous brachiole facets. This species is further characterized by a combined hydropore and gonopore located left of the anal pyramid and a spherical theca. Measurements of the ambulacral lengths, number and size of brachioles, average size of diplopores, dimensions of the peristome (mouth), and dimensions of the plates were taken to quantify ontogenetic changes in the species. Volumetric changes in the theca were measured utilizing 3D laser scanner technology. Qualitative and quantitative observations of the holdfast indicate that the morphology, as has been recently noted in other blastozoan groups, is largely driven by ecophenotypic plasticity. Specimens encompass a range of ontogenetic stages, although the earliest stages of juvenile growth are not represented, as is the case with all diploporitans. Ambulacral length and mouth size do not grow proportionally to the size of the theca and grow with negative allometry.

This study, one of the few focused on the growth of diploporitan echinoderms, sheds light on the ontogenetic changes in this species. These results can be used to better quantify diploporitan growth as a whole and can be compared with closely related blastozoan taxa to understand major evolutionary changes in growth through the Ordovician. The variation in holdfast dimensions likely results from ecophenotypic plasticity and should not be used as a diagnostic for systematics, or as a measurement in ontogenetic studies.