HOMEOMORPHY IN AN EARLY JURASSIC RHYNCHONELLIDE-DOMINATED BRACHIOPOD COQUINA FROM SMOKOVAC, MONTENEGRO: AN INCERTAE SEDIS FINDS A HOME IN THE SUBORDER TEREBRATELLIDINA

Brachiopod coquinas from the Early Jurassic (?Hettangian–Sinemurian) of Smokovac, Montenegro contain rhynchonellide brachiopods. Liassic brachiopods from Smokovac were first described in the nineteenth century (e.g., Hauer 1868, Eichenbaum 1883, Bittner 1895), and later (Mihajlović, 1955). Most species recorded from Smokovac have been referred to the rhynchonellide genus Rhynchonellina. Current taxonomic practice would likely place the costate species in Sulcirostra and the smooth-shelled forms in Rhynchonellina.

Our study of a recently discovered talus block shows this fauna also includes an externally homeomorphic terebratellidine. We recovered Rhynchonellina bilobata Gemmellaro and a terebratellidine, Arzonellina stachei (Bittner), the latter a new combination, previously referred to Rhynchonellina.

This study initiated because: 1) new material became available and; 2) it contained brachiopods considered to be Rhynchonellina, which has been speculated to be associated with cold, hydrocarbon seeps. To date no samples with Rhynchonellina had been investigated to ascertain this claim. However, carbon stable isotope data of carbonate cements from this coquina do not display signatures that indicate precipitation in the vicinity of hydrocarbon seeps.

Arzonellina exotica, the type species, considered of Sinemurian age from Arzo, Southern Alps, Switzerland, is probably contemporaneous with the Smokovac material (Sulser 2004). Sulser was not able to place it in an order (incertae sedis). This was due to the unusual internal characters that Arzonellina exhibited. New information on internal structures include: 1) the presence of a hood in the posterior of the dorsal valve; 2) developing descending branches of the brachidium; 3) a septal pillar that anteriorly develops into a median septum that bifurcates anteriorly (two parallel, vertically-oriented septa result); 4) ascending branches that develop anteriorly.

Primarily on the basis of new data Arzonellina should be referred to the order Terebratulida, suborder Terebratellidina, superfamily Kingenoidea. The new material from Smokovac is assigned to Arzonellina stachei (Bittner). The genus is presently known from the western end of Tethys. This study further illustrates the intricacies of homoeomorphy between brachiopod orders.