UNUSUAL PRESERVATION OF SILURIAN HOLOCYSTITIDS (DIPLOPORITA: BLASTOZOA): IMPLICATIONS FOR UNDERSTANDING EVOLUTIONARY TRENDS

The dynamics of Silurian echinoderms, especially non-crinoid echinoderms, are difficult to understand because of a significant lack of well-preserved fossil material. The diploporitan Holocystites Fauna is an exception to this; there is an unusually large collection of these fossils from the midcontinent of the United States. Hundreds of body fossils of these diploporitans have been collected since their discovery over a century ago; however, predictable taphonomic patterns have resulted in the disarticulation of morphologically important features, such as the anal and oral cover plates and the brachioles. This lack of data makes it difficult to understand how holocystitids relate evolutionarily to other diploporitans and to quantify morphological trends of diploporitans through time.

New holocystitid fossils from the middle Silurian Massie Formation of southern Indiana have been found with unusually well preserved morphological features. In particular, multiple specimens of Holocystites have been found preserving oral and anal plates that have previously never been found preserved on a holocystitid and are rarely preserved on other blastozoan echinoderm fossils. These specimens show that the mouth is covered by relatively few plates but otherwise has a configuration consistent with that of other Paleozoic stemmed echinoderms. Furthermore, a specimen of Triamara was discovered preserving a significant portion of a brachiole attached to the brachiole facet. This specimen shows that holocystitid brachioles are extremely large, as compared with other known blastozoan brachioles. The brachioles of Triamara are also uniserial, which is in line with other known blastozoan brachioles. This find is significant because very little information exists not just concerning diploporitan brachioles, but also for all blastozoan brachioles. Understanding these morphologies is critical for inferring the relationships of diploporitans to other blastozoan groups and to crinozoan echinoderms.