SOFT-TISSUE PRESERVATION IN SMALL SHELLY FAUNAS

Soft-tissue preservation in small shelly faunas (SSFs) appears contradictory because these faunas often have been considered exclusively as skeletal fauna. Taphonomy of SSFs indicates that phosphatization is mostly of early to late diagenetic origin. Cambrian SSFs embrace phosphatization in biomineralized skeletons, sklerotized organic cuticules, and a variety of volatile tissues. Phosphatization of skeletons and labile organic tissues principally followed similar preservational mechanisms but was selective according to available time and chemical gradients.

Majority of Cambrian SSFs were primarily biomineralized (mostly aragonitic, calcitic) or sklerotized. Due to preferred preservation of skeletons, the sequence of fossil appearance may be partially biased too. The skeletal record in the Ediacaran- Cambrian implicated a three-fold radiation of metazoans during the Cambrian bioradiation event. First Ediacaran skeletal fossils were Porifera and Cnidaria, while bilaterians radiated during Cambrian Series 1-2. Here first skeletalized faunas were siphogonuchitids, molluscs, stem-group brachiopods, all related to lophotrochozoans, as well as chaetognaths. According to previous studies the major increase in metazoan diversity occurred in Cambrian Stage 3 to 4. This is mostly due to rapid diversification in Archaeocyatha, ecdysozoan clades, such as athropods, priapulids, and deuterostoms, such as echinoderms.

Soft-tissued phosphatized remains now shed more light on the diversification sequence of metazoans than solely derived from skeletal fossil record. Phosphatized embryos (Pseudooides, Olivooides etc.) and putative larvae support a Fortunian appearance of ecdysozoans related to arthropods, lobopodians, and scalidophorans. These data corroborate previous reports of arthropod-type trace fossils prior to the appearance of skeletonized forms during Cambrian Stage 3. It can be concluded that majority of protostomian clades appeared nearly simultaneously during the Fortunian. The strong and rapid appearance of ecdysozoan clades in the fossil record during Cambrian Stage 3 may be due to the appearance of more heavily cuticularized and partially biomineralized skeletons and the previously recognized general switch from mostly aragonitic toward calcitic skeletal mineralogy.