Orbaspina gelasinus, Valentine, James L. & Brock, Glenn A., 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 : 237-242

Orbaspina gelasinus n.sp.

Figs. 5–7

“New genus A” Valentine, Brock & Molloy (2003):pl. 3, figs.18–29.

Type material. HOLOTYPE: AMF120610 ( Fig. 5a–g ): dorsal valve from sample BM 14.85, ranuliformis Zone, Boree Creek Formation ( Fig. 3 ). Figured PARATYPES: AMF122212 ( Fig. 5h–k ): dorsal valve from sample BM 14.85, ranuliformis Zone, Boree Creek Formation ( Fig. 3 ); AMF120612 ( Fig. 6a–d ): ventral valve from sample BM 14.30, ranuliformis Zone, Boree Creek Formation ( Fig. 3 ); AMF120613 ( Fig. 6e, f ): ventral valve from sample BM 14.30, ranuliformis Zone ( Fig. 3 ); AMF122213 ( Fig. 6g ): ventral valve from sample BM 13.80, ranuliformis Zone ( Fig. 3 ); AMF122214 ( Fig. 6h ): ventral valve from sample BM 9.90, amorphognathoides Zone ( Fig. 3 ); AMF122215 ( Fig. 6i ): fragment from sample BM 9.30, amorphognathoides Zone ( Fig. 3 ); AMF122216 ( Fig. 7a–d ): fragment from sample BM 13.80, ranuliformis Zone ( Fig. 3 ); AMF122217 ( Fig. 7e, f ): fragment from sample BM 14.30, ranuliformis Zone ( Fig. 3 ); AMF122218 ( Fig. 7g ): dorsal valve fragment from sample BM 11.20, ranuliformis Zone ( Fig. 3 ); AMF122220 ( Fig. 7i,j ): dorsal valve fragment from sample BM 14.40, ranuliformis Zone ( Fig. 3 ). Unfigured PARATYPES: 34 ventral valves, 101 dorsal valves and 140 fragments ( Fig. 3 ).

Type locality and horizon. Massive red and grey lensoidal limestones at “Kalinga”, along the BM section (samples BM 5.50 to BM 15.60 ) ( Fig. 3 ) through the Boree Creek Formation , central-western New South Wales, Australia ( Fig. 1 ) .

Age. Early Silurian: late Llandovery ( amorphognathoides Zone ) to early Wenlock (ranuliformis Zone) ( Bischoff, 1986; Cockle, 1999; Molloy, unpub. data).

Etymology. Gelasinus L., dimpled, in reference to the dimpled post-larval shell ornament.

Diagnosis. As for genus by monotypy.

Description. Ventral valve incompletely known, planar to weakly convex,?subcircular in outline. Lateral margins evenly curved; posterior margin weakly angular. Maximum width at, or slightly anterior of, valve midlength. In lateral profile, valve highest around midlength. Larval shell subrectangular, smooth, averaging 65 µm in width and 30 µm in length, separated from post-larval shell by distinct change in elevation. Foramen keyhole-shaped,?centrally located at anterior end of elongate triangular to subtriangular pedicle track. Pedicle track covered posteriorly by concave plate bearing numerous, closely spaced growth lamellae and anteriorly by very short, flat “listrum-like” plate. Post-larval shell ornament with low rounded concentric ridges, becoming stronger towards margins, spaced at intervals averaging 25 µm. Tubular hollow spines sparsely developed along tops of concentric ridges, projecting at low angle from valve surface, becoming more numerous towards valve margins.

Ventral valve interior with long, posteriorly convex, ridge-like pseudointerarea following line of posterior margin. Pseudointerarea extending up to 88% valve width. Propareas long, narrowly triangular, separated from central portion of pseudointerarea by long, anteriorly divergent, uniformly narrow grooves that “articulate” with raised portion of propareas on dorsal valve pseudointerarea. Central portion of pseudointerarea strongly arcuate, lacking median plate. Transmedian and umbonal muscle scars subcircular, posterocentrally located. Other muscle scars not observed. Inner surface of plate covering pedicle track bearing shallow median groove that terminates at posterior margin of foramen around valve midlength. Foramen bounded anteriorly and laterally by low ridge. No pedicle tube observed. Vascular system not observed.

Dorsal valve convex, subcircular in outline with evenly rounded lateral margins and gently curved anterior and posterior margins. Maximum width at, or slightly anterior of, valve midlength. In lateral profile, valve highest at midlength, flattening anteriorly. Sulcate in anterior view, particularly in larger specimens. Larval shell smooth, subcircular to subrectangular, averaging 140 µm in length and 200 µm in width. Larval shell separated from postlarval shell by change in elevation and onset of post-larval shell ornament. Post-larval shell ornament of irregular concentric growth lamellae becoming better developed, occasionally frill-like, towards margins. Lamellae spaced at intervals of 50 to 250 µm, averaging 115 µm. Post-larval shell bearing numerous subcircular dimples loosely arranged in concentric rows, ranging from 20 to 40 µm, averaging 30 µm, in diameter. Short tubular hollow spines of uniform size, averaging 175 µm in length, projecting at low angle from valve surface. Spines developed on lamellae, sparsely in older portions of valve, becoming more numerous and arranged in rows toward margins, including posterior margin. Spines bearing occasional, faintly developed, transverse grooves.

Dorsal valve interior with large triangular, orthocline to anacline, shelf-like pseudointerarea, extending up to 85% valve width. Median plate triangular, concave, flatbottomed, bearing numerous, closely spaced growth lamellae. Propareas narrowly subtriangular, raised above pseudointerarea with rounded crests, also bearing numerous growth lamellae. Some specimens with two hollow spines projecting from underneath pseudointerarea into body cavity. Outside lateral and transmedian muscle scars located immediately anterior of pseudointerarea, weakly impressed, elongately suboval, extending to approximately 25% valve length. Muscle scars separated by very low, broadly rounded, indistinct median ridge, widening anteriorly, reflecting external sulcus. Median ridge occasionally forming a very short, but distinct ridge at posterior end of muscle field. Central muscle scars indistinct, located around valve midlength. Remaining muscle scars not observed. Vascular system not observed. Numerous holes, occasionally with a very low, narrow ridge around them, connect spines with valve interior, particularly around margins.

Measurements. Following the morphological measurements for organophosphatic brachiopods used by Popov & Holmer (1994: 35), the average dimensions and ratios of Orbaspina gelasinus n.gen. and n.sp. are given in Tables 1 and 2. All dimensions given are in millimetres. Abbreviations used are as follows: FP, point of origin of pedicle foramen from posterior margin; L, length; LI, maximum length of pseudointerarea; ML, median length of pseudointerarea; N, number of specimens measured; SD, standard deviation; W, width; WI, maximum width of pseudointerarea; WP, maximum width of pedicle track.

Discussion. Tubular hollow spines are the one diagnostic feature of the Siphonotretida uniting them as a monophyletic clade (Holmer & Popov, 2000). Although not numerous, particularly on the ventral valve, tubular hollow spines are clearly discernible on both valves of O. gelasinus n.gen. and n.sp. ( Figs. 5a,b,h,i , 6a,b,i , 7a–f ). The hollow internal spines projecting from underneath the pseudointerarea of the dorsal valve in some specimens ( Fig. 7i,j ) has not previously been observed in other siphonotretids. As they would have presumably been surrounded by soft tissue throughout life, their function remains unknown. Although not unique to the siphonotretids, all siphonotretids possesses a smooth, unpitted larval shell (Holmer & Popov, 2000), a feature also present in O. gelasinus n.gen. and n.sp. ( Figs. 5a,j , 6c ). The shell structure of O. gelasinus n.gen. and n.sp. is consistent with that described by Biernat & Williams (1970) and Ushatinskaya et al. (1988) for other siphonotretids ( Fig. 5g ).

Recently, Mergl (2001a,b) documented four indeterminate post-Ordovician siphonotretids from central Bohemia, two of which were described as possessing a pitted post larval shell. Siphonotretine sp., represented by a single fragment from the Ludlow Kopanina Formation of Řeporyje, has deep, regular, closely spaced pits, somewhat similar to the pitting found on acrotretid larval shells. The spines of this species are also more numerous and erect than in O. gelasinus n.gen. and n.sp. The second taxon, Schizambonine sp. A, is represented by a dorsal valve fragment recovered from the Pragian Dvorce- Prokop Limestone of Klukovice. It possesses minute, scattered spines, somewhat similar to those observed on the pedicle valve of O. gelasinus n.gen. and n.sp. The post-larval shell pitting, however, is finer and more regular than the dimpling observed in O. gelasinus n.gen. and n.sp. Little is known about the internal features of these two species.

Siphonotreta anglica (Morris) , from the Wenlock Coalbrookdale Formation of Dudley, England, was described by Morris (1849: 321) as possessing spines which were “…regularly and transversely sulcated or centrated,

giving the spines a beaded or jointed appearance”; a feature also noted by Davidson (1866). Some spines of O. gelasinus

n.gen. and n.sp. bear the occasional, faintly developed,

transverse groove ( Fig. 7f ), whereas no trace of similar grooving can be observed on any of the spines in Mergl’s

(2001a,b) material. Siphonotreta anglica also has more numerous spines and a more regular post-larval shell pitting than O. gelasinus n.gen. and n.sp. A comparison of internal features is not possible as these have not been described in

Siphonotreta anglica .

ACKNOWLEDGMENTS. The authors gratefully acknowledge the enthusiastic assistance with fieldwork provided by Peter Molloy, David Mathieson, Carl Valentine, John Talent, Ruth Mawson and Graham Felton on various occasions. David Mathieson assisted with scanning electron microscopy and digital photography. Peter Molloy, Andrew Simpson and Peter Cockle provided many useful insights into Silurian conodonts. David Holloway, Museum Victoria, kindly loaned us samples of Chapman’s “siphonotretid” collection. Dean Oliver skillfully drafted the figures. This project would not have been possible without the kindness and generosity of Keith Goodridge, who allowed us access to his property, “Kalinga”. Ian Percival (Sydney) and Michal Mergl (Plzen) are thanked for their constructive reviews of this manuscript.

Figures

Fig. 5. Orbaspina gelasinus n.gen. and n.sp.: a–g, holotype AMF120610, dorsal valve from sample BM 14.85, external view (a), detail of spines along posterior margin (b), detail of larval shell (c), internal view (d), detail of pseudointerarea in plan view (e), and anterior view (f), detail of shell lamina along broken section of anterior margin (g); h–k paratype AMF122212, dorsal valve from sample BM 14.85: external view (h), detail of anterior margin showing frill-like nature of growth lamellae (i), deatil of larval shell (j), detail of post-larval shell dimpling (k). Scale bars = 1 mm (a, d, h); 100 µm (b, c, e, f, j, i); 10 µm (g, k).

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).

Fig. 7. Orbaspina gelasinus n.gen. and n.sp.; a–d, paratype AMF122216, fragment from sample BM 13.80, external view (a), detail of spines (b), anterior view (c), interior view with spine openings on internal surface indicated by arrows (d); e, f, paratype AMF122217, fragment from sample BM 14.30, external view (e), detail of spines with weak transverse grooves on spines indicated by arrows (f); g, paratypeAMF122218, dorsal valve from sample BM 15.40, external view; h, paratypeAMF122219, dorsal valve fragment from sample BM 11.20, external view showing well developed post-larval shell dimpling; i, j, paratype AMF122220, fragment of dorsal valve pseudointerarea from sample BM 14.40, with two spines projecting from underneath the pseudointerea as indicated by arrows, in plan (i) and anterior views (j). Scale bars = 1 mm (a, d, g, h); 100 µm (c, e, f, i, j); 10 µm (b).

Fig. 3. Stratigraphic column of the BM section showing lithology and all sampled horizons.Lithological details of massive red and grey lensoidal limestones and sampled horizons for the 5.87 metres of section around the Llandovery-Wenlock boundary, from sample BM 3.65 to BM 16.80, is enlarged in the middle. Key to lithology as for Fig. 2; blank areas indicate no exposure. Modified after Valentine et al. (2003). Distribution and abundance of Orbaspina gelasinus n.gen. and n.sp. recovered from each sampled horizon in the massive red and grey lensoidal limestones is shown to the right. Sample size in kilograms for each horizon is given in brackets after each sample number.

Fig. 1. Regional geological map of the Boree Creek area. Stratigraphical section BM is indicated. Location of Fig. 2 is indicated by boxed area. (From Valentine et al., 2003)