EOPHILOBRYOIDELLA SINOANISICA NEW GENUS AND SPECIES, AN EARLY PHILOBRYID BIVALVE FROM THE UPPER ANISIAN (MIDDLE TRIASSIC) OF QINGYAN, SOUTHWESTERN CHINA

Journal of Paleontology, Mar 2004 by Stiller, Frank, Jinhua, Chen

With Eophilobryoidella sinoanisica n. gen. and sp., the fossil record of the Philobryidae is extended from the Eocene (Keen, 1969; Tevesz, 1977) or the Kimmeridgian (Upper jurassic) (Skelton and Benton, 1993, yet undescribed taxon) down to the lower upper Anisian of the Middle Triassic.

It has been proposed that the Philobryidae may have evolved from the Limopsidae (e.g., Tevecz, 1977; Carter, 1990). This was consistent with the fossil record known at that time [Philobryidae: Eocene to Recent (Keen, 1969; Tevecz, 1977; Carter, 1990); Limopsidae: Upper Triassic (Cox et al., 1969-1971) or Middle jurassic (Tevesz, 1977) or? jurassic, Albian (Lower Cretaceous) (Carter, 1990) to Recent]. However, Skelton and Benton (1993) had already mentioned an undescribed philobryid taxon from the Kimmeridgian and gave for the Limopsidae a fossil record since the Albian. This questions the formerly proposed origin of the Philobryidae. With the new philobryid taxon from the Anisian described herein, the Philobryidae appear to predate the Limopsidae.

PALEOECOLOGY

Modern philobryids are epibenthonic, suspensivorous, viviparous bivalves. By means of a short, thin byssus consisting of comparatively few threads and protruding between the anterior valve margins, they are attached to algae, bryozoans, sponges, gorgonaceans, and other stable, living or dead substrates (Cotton, 1961; Nicol, 1966; Tevesz, 1977; Kilburn and Rippey, 1982). Despite being epibyssate, juveniles as well as adults also are efficient epifaunal crawlers on hard substrates and stable soft substrates; using their extended foot, they are able to crawl very actively, relatively rapidly, and for extended periods of time (Tevesz, 1977; Kilburn and Rippey, 1982). They are able to break their byssal attachment and to rebuild it later. Recent philobryids occur from the intertidal zone down to depths of more than 1000 m, mostly however in the subtidal and shelf region. Most common and widely distributed in the southern part of the southern hemisphere, representatives of the Philobryidae, however, also live in other regions such as the Carribean and Mediterranean Seas.

Evidence suggests that Eophilobryoidella sinoanisica n. gen. and sp., like the Recent philobryids, lived epibenthonically and was probably byssate (although there is no distinct small byssal gape between the anterior valve margins), attaching to stable substrates such as macroalgae. Whether it was hemisessile, with periods of byssal attachment being interrupted by free crawling on the seafloor in search of places offering more suitable living conditions, is unknown. This epibenthonic mode of life in shallow marine habitats is suggested, among other indications, by an articulated, double-valved specimen with an epizoic calcareous red algae (Fig. 3.2-3.5); the calcareous red algae grew on the valve during the bivalve's lifetime.

Such interpretation fits well the paleosynecological analyses of the fossil assemblages of Leidapo/Bangtoupo and the reconstructions of the paleobiocoenoses (Stiller, 1997, 2001). At Leidapo/ Bangtoupo, various stable soft-bottom fossil assemblages of shallow-marine facies are preserved. These mainly epibenthonic assemblages are very rich in individuals and some show a high species diversity; brachiopods, bivalves, and gastropods dominate. These assemblages represent the remains of organisms that lived in a protected shallow-marine habitat situated at the transition from an extensive, more or less restricted carbonate platform to an open basin with mainly siliciclastic sedimentation. The organisms lived in an environment of normal salinity and generally low energy, with gently moving shallow water below the fair weather wave base and above the storm wave base. Only sporadic, shortlived storm events of higher turbulence interrupted these conditions. The clean and clear water was rich in oxygen and nutrients. The fine-grained sea bottom was soft but stable. Stiller (1997, 2001) suggested the presence of macroalgal vegetation on the seafloor. The macroalgae provided favorable living conditions for the multitude of small invertebrates. In the course of time, the density of the algal vegetation varied, sometimes forming algal meadows.