TAPHONOMIC FEEDBACK AND FACILITATED SUCCESSION IN A MIDDLE JURASSIC SHALLOW MARINE CRINOID COMMUNITY (MATMOR FORMATION, SOUTHERN ISRAEL)

The Matmor Formation (Middle Jurassic, Callovian-Oxfordian) contains a remarkable crinoid-dominated fauna patchily preserved in a series of calcareous marls exposed in Hamakhtesh Hagadol, an erosional crater in southern Israel. One of the most common taxa is Apiocrinites negevensis, a large articulate crinoid with the ability to colonize soft sediments with a spreading root-like holdfast. Ossicles of this crinoid were a major source of hard substrate for the development of a diverse sclerobiont community. Encrusting organisms include thecideide brachiopods, cyclostome bryozoans, microconchids, oysters, plicatulids, serpulids, sabellids, scleractinian corals, calcisponges, and foraminiferans. Ichnofossils associated with the crinoid skeletons include the barnacle boring Rogerella, the bivalve boring Gastrochaenolites, the worm borings Talpina and Trypanites, and the echinoid grazing marks Gnathichnus. In this study we collected specimens from a single highly fossiliferous layer (known informally as Subunit 51) exposed throughout the makhtesh in the Quenstedtoceras (Lamberticeras) lamberti Zone (Callovian). Most of the encrustation and boring occurred after the death and disarticulation of the crinoids, as shown by sclerobionts on articulating and broken surfaces, but there was a significant amount of encrustation and boring on the living stems, which produced a variety of swellings and overgrowths. This crinoid debris accumulated on the soft marls to provide essential hard substrate for later organisms, demonstrating taphonomic feedback and a facilitated ecological succession. Among the most common encrusted objects are the thick calyces of A. negevensis. These rested upside-down on the seafloor so that the stem facets faced upwards, making a conical benthic island for sclerobionts. This is analogous to a Silurian community described by Liddell and Brett (1982) in which the articulated calyces of camerate crinoids served in a similar fashion. The Jurassic analogs described here are not as densely encrusted and have fewer bryozoans, but otherwise show significant ecological similarities. This Jurassic sclerobiont community is also important because it was very close to the paleoequator in the Ethiopian Province of the Tethyan Faunal Realm, and so many of the fossils represent new taxa.