ECHINOID PRESERVATION IN A MIDDLE JURASSIC SHALLOW MARINE EQUATORIAL ENVIRONMENT: TESTING MODELS OF TAPHONOMY AND ENCRUSTATION

Abundant echinoids in the Matmor Formation (Callovian) of southern Israel provide the first opportunity to test models of taphonomy and encrustation of echinoids in a shallow marine assemblage that lived in Jurassic equatorial waters. The Matmor Formation, mostly marls and patch reefs of sponges and corals, was deposited in a lagoonal setting on the western coast of the Neotethys in the Ethiopian Faunal Province. The benthic fauna was diverse and numerous, and cidaroid echinoids (especially Rhabdocidaris) were significant components. Most are preserved as disarticulated spines and coronal fragments, but there are a few intact coronas. Of the 438 echinoid specimens studied, 10.1% are encrusted (9.4% of spines and 22.7% of coronal fragments), 4.3% show unhealed fish bite marks, and a single spine has barnacle borings (Rogerella). The encrusters include calcisponges, foraminiferans (Bullopora), serpulids (Serpula), sabellids (Glomerula gordialis), cyclostome bryozoans (Stomatopora), thecideide brachiopods (Moorellina negevensis), plicatulid bivalves (Plicatula), and oysters (Nanogyra nana). The Matmor echinoids are consistent with taphonomic models for Jurassic cidaroids where there is a dominance of disarticulated spines and a significant number of denuded but intact coronas. The taphonomy of the Matmor echinoids suggests that the tests spent more than the threshold of decay on the seafloor (estimated by experiment to be about seven days) before final burial. This conclusion is supported by the numbers and diversity of encrusters on the echinoids. Since cidaroid spines lack an epithelium during life, which may be demonstrated by encrustation in fossils, it is impossible to tell if most encrusters on them were pre- or postmortem. Some encrusters, though, extended across the spine base, showing that they arrived after disarticulation. The tests had an epithelium producing biocides, preventing encrustation, so encrusters on them are post-mortem. Echinoid encrustation is known since the Carboniferous. There is thus a consistent substrate with which we can analyze marine encrustation patterns for the past 325 million years, testing models of productivity, selectivity and diversification in a novel way. This data from the Matmor Formation begins to fill a significant gap in our information.