A NEW EARLY JURASSIC RHYNCHONELLID BRACHIOPOD FROM THE WESTERN TETHYS AND IMPLICATIONS FOR SYSTEMATICS OF RHYNCHONELLIDS FROM THE TRIASSIC–JURASSIC BOUNDARY

Abstract

A new multicostate rhynchonellid brachiopod Jakubirhynchia occurs in the Hettangian (Early Jurassic) carbonate deposits of the West Carpathians. A comparison with externally similar multicostate rhynchonellids from the Hettangian-Sinemurian carbonate deposits of the Alps shows their congeneric and conspecific status, indicating relatively wide regional distribution in the Early Jurassic of the western Tethys. Diagnostic characters include conjunct deltidial plates, divided, vertically inclined inner hinge plates forming sessile septalium, and subfalciform crura with relatively complex, serrate terminations and broad outline in lateral view. Although the shape of hinge plates and crura may vary according to shell convexity and shell orientation during serial sectioning, two-dimensional restorations of cardinalia and crura show their distinctive morphology. It is important to note that some of the cardinalia details and crural shape can be detected only with very short between-section distances. Based on external characters, Jakubirhynchia is a homeomorph to another multicostate rhynchonellid, Calcirhynchia, typical of the Hettangian and Early Sinemurian of the northwestern Europe province and reported also from the Tethyan province. This suggests that original distribution of Calcirhynchia might not be so wide and postextinction multicostate rhynchonellids were taxonomically more diverse as was previously supposed. Jakubirhynchiaconsistently occupies offshore, low-energy, micritic-rich habitats below storm wave base. It is the first reported Hettangian member of the family Basiliolidae and may bridge the gap between the Carnian Veghirhynchia and Early Jurassic basiliolids. Preextinction, Norian and Rhaetian brachiopods that are close morphologic relatives of Jakubirhynchia are known from outside the western Tethyan area only.