Orbaspina, Valentine & Brock, 2003

Valentine, James L. & Brock, Glenn A., 2003, A New Siphonotretid Brachiopod from the Silurian of Central-Western New South Wales, Australia, Records of the Australian Museum 55 (2), pp. 231-244 : 237

Orbaspina n.gen.

Type species. Orbaspina gelasinus n.gen. and n.sp., Llandovery to Wenlock, central-western New South Wales, Australia .

Diagnosis. A siphonotretid with large, keyhole-shaped pedicle foramen extending forward through resorption to form an elongate, broadly triangular, pedicle track; pedicle track covered posteriorly by a concave plate and anteriorly by short “listrum-like” plate; tubular hollow spines of uniform size arranged in concentric rows close to valve margin; post larval shell bearing numerous subcircular dimples loosely arranged in concentric rows.

Etymology. Orba L., orphan, refers to the occurrence of this taxon after the supposed demise of the siphonotretids during the end-Ordovician extinction event; spina L., thorny, in reference to the post-larval shell ornament of tubular hollow spines.

Remarks. Features considered to be of generic significance among the siphonotretids include the nature of the pedicle foramen and tube, the pseudointerarea of both valves and the presence of tubular hollow spines (Holmer & Popov, 2000). Based on these features, Orbaspina n.gen. appears closest morphologically to those genera grouped together by Havlíěek (1982) and Popov & Holmer (1994) in the subfamily Schizamboninae . The most characteristic feature of these siphonotretids is the large pedicle foramen that extends forward through resorption, forming an elongate triangular pedicle track that may be covered posteriorly by a plate. However, the dimpled post-larval shell of Orbaspina n.gen. is unknown in any previously described Cambrian or Ordovician siphonotretid. This feature is present on a number of post-Ordovician siphonotretids including,

Siphonotreta anglica Morris (1849) , and Siphonotretine sp. and Schizambonine sp. A Mergl (2001a,b). The short “listrum-like” plate anteriorly covering the pedicle track of O. gelasinus n.gen. and n.sp. ( Fig. 6g ) has not been previously documented among the siphonotretids.

Figures

Fig. 6. Orbaspina gelasinus n.gen. and n.sp.: a–d, paratype AMF120612, ventral valve from sample BM 14.30, external view (a), detail of spines on anterior slope (b), detail of larval shell (c), internal view (d); e, f, paratypeAMF120613, ventral valve from sample BM 14.30, internal view (e), detail of pseudointerarea (f); g, paratype AMF122213, ventral valve from sample BM 13.80, internal view, note the “listrum-like” plate covering the anterior portion of the pedicle track as indicated by arrow; h, paratypeAMF122214, ventral valve from sample BM 9.90, external view showing common state of presentation of recovered ventral valves; i, paratype AMF122215, fragment from sample BM 9.30, external view. Scale bars = 500 µm (g); 100 µm (a, c–f,h, i); 10 µm (b).