Alocorthis psygmatelos, Paterson & Brock, 2003

Paterson, John R. & Brock, Glenn A., 2003, Early Ordovician Orthide Brachiopods from Mount Arrowsmith, Northwestern New South Wales, Australia, Records of the Australian Museum 55 (2), pp. 221-230 : 227-229

Alocorthis psygmatelos n.gen. and n.sp.

Fig. 4A–J , Table 2

Type material. HOLOTYPE: AM F120719 , dorsal valve ( Fig. 4A,B ) . PARATYPES: AM F120720 , dorsal valve ( Fig. 4C,D ) ; AM F120721 , dorsal valve ( Fig. 4E ) ; AM F120722 , ventral valve ( Fig. 4F,G ) ; AM F120723 , ventral valve ( Fig. 4H,I ) ; AM F120724 , ventral valve ( Fig. 4J ) .

Type locality. Tabita Formation (spot locality MTA /IV/4, northern section of the syncline at 30°06'36"S 141°35'05"E) .

Etymology. Greek psygma, fan, and telos, end; referring to the fan-like anterior extremities of the brachiophores.

Diagnosis. As for genus by monotypy.

Description. Exterior: poorly preserved in available material, but apparently weakly ventribiconvex, transverse, subquadrate in outline. Ventral valve interarea low and weakly apsacline; delthyrium open; maximum length of ventral valve approximately 70% maximum width. Dorsal valve interarea anacline; notothyrium open; maximum length of dorsal valve approximately 55% maximum width. Fine ramicostellate ornament, with ribs arising by bifurcation.

Ventral valve interior: muscle field subtriangular with curved anterior margin, restricted to delthyrial cavity, bounded laterally by short receding dental plates; muscle field slightly raised above valve floor, about one-fifth valve length in larger specimens. Teeth relatively large, simple, deltidiodont. Strap-like vascula media weakly impressed, proximal portions well separated and divergent ( Fig. 4I ). Ribs impressed on periphery of valve.

Dorsal valve interior: notothyrial platform short and very wide, anteriorly elevated above valve floor, supported by short, wide, very low median ridge. Cardinal process absent. Brachiophores short, robust, with narrow furrows (0.25 mm in width) situated along dorsal surfaces and fan-like anterior extremities, widely divergent anterolaterally at about 100–110°. Dental sockets simple, subcircular and deep. Fulcral plates absent. Musculature poorly defined; posterior pair large and subcircular, situated on either side of a relatively wide, low median ridge, below the notothyrial platform. Internal ribbing as in ventral valve.

Discussion. The relatively small ( Table 2) subquadrate, weakly ventribiconvex, transverse shell, subtriangular ventral valve muscle scar ( Figs. 4F–J ), presence of a wide notothyrial platform, short, robust, widely divergent brachiophores, and absence of a cardinal process in the dorsal valve ( Figs. 4A–E ) indicate that Alocorthis psygmatelos could be placed in either the Nanorthidae (Superfamily Orthoidea ) or the Eoorthidae (Superfamily Plectorthoidea ). As noted by Williams & Harper (2000, p. 766) the eoorthids differ significantly from other typical plectorthoids in lacking brachiophore supporting structures and fulcral plates. Williams & Harper (2000) indicate that the eoorthids should be viewed as the link group between the plectorthoids and the orthoids. On balance, Alocorthis probably has more features in common with the Nanorthidae than with the Eoorthidae and we refer this genus, with some hesitation, to the Nanorthidae Havlíěek.

Of the genera lacking a cardinal process that are currently assigned to the Nanorthidae (see Williams & Harper, 2000, p. 742–745), Alocorthis is similar to both Nanorthis Ulrich & Cooper and Archaeorthis Schuchert & Cooper. Nanorthis has a similar ornament, shell shape and size range to Alocorthis , but Nanorthis can be discriminated from Alocorthis by its short, blade-like brachiophores and rudimentary notothyrial platform ( Laurie, 1980; Williams & Harper, 2000). The species N. brachymyaria recently described from the Late Tremadoc of Argentina (Benedetto & Carrasco, 2002, fig. 4) is very similar to Alocorthis in size and external ornament. However, N. brachymyaria can be distinguished from A. psygmatelos by its more acute brachiophore angle, lack of a raised muscle field in the ventral valve and more arched vascula media. It is interesting to note that Benedetto & Carrasco (2002, fig. 4.11) figure one specimen of N. brachymyaria that appears to show brachiophores with narrow furrows along the dorsal surfaces and fan-like terminations which indicate this species may be referable to Alocorthis .

[ Fig. 3 , caption continued] scale bar = 1.5 mm; (J) ventral valve internal mould, TAB1 / 25 m, AM F120716 , scale bar = 2.5 mm; (K) latex of ventral valve exterior, MTA /0.2, AM F120717 , scale bar = 2.5 mm; (L–N) Holotype, latex of ventral valve exterior (scale bar = 2.5 mm), lateral view (scale bar = 1.5 mm) and interarea (scale bar = 2 mm), TAB1 / 30 m, AM F120718 .

Archaeorthis is also similar to Alocorthis in basic dimensions, but most species of this genus have the ventral muscle scar impressed on a well-developed callosity which extends forward as a wide median ridge (Ulrich & Cooper, 1938, p. 92). Alocorthis psygmatelos does not possess this feature ( Figs. 4F–J ). Laurie (1987) described the species Archaeorthis waratahensis from the Early Ordovician Digger Island Formation in south Gippsland, Victoria, that is characterized by the presence of a weakly developed callosity in the ventral valve (at least in early growth stages). This species can be discriminated from Alocorthis psygmatelos by its fascicostellate ornament, deep notothyrial cavity and well-developed dorsal sulcus in the dorsal valve. The brachiophores are also much simpler than those in Alocorthis psygmatelos ( Figs. 4A–E ).

Other nanorthid genera such as Cyrtonotella Schuchert & Cooper , Diplonorthis Mitchell , Nicoloidea Zeng , Nothorthis Ulrich & Cooper , Pleurorthis Cooper , Riograndella Kobayashi , Shoshonorthis Jaanusson & Bassett and Xinanorthis Xu, Rong & Liu can be distinguished from Alocorthis by a combination of features such as planoconvex to concavoconvex shape, presence of a welldeveloped cardinal process, weakly developed notothyrial platform or bilobed muscle field in the ventral valve (see Williams & Harper, 2000, pp. 742–745).

Of the genera within the Eoorthidae that lack a cardinal process, Alocorthis is most similar to the type species of Robertorthis , R. holoubkovensis Havlíěek (1977 , pl. V, figs. 8–10, 17), from the Early Ordovician (Tremadoc) of Bohemia. The internal morphology of the dorsal valve is almost identical in both species. The major differences are that the notothyrial platform of R. holoubkovensis is not supported by a median ridge, and the anterior extremities of the brachiophores in R. holoubkovensis bear fine ridges running parallel with the hinge line to define slit-like dental sockets. The ventral valve interiors of both taxa are also very similar, in that they possess a subtriangular muscle field which is raised above the valve floor and restricted to the delthyrial cavity.

Brahimorthis Havlíěek from the Middle Cambrian of north Africa and Europe is also somewhat similar to Alocorthis in that it has ramicostellate ornament and lacks a cardinal process (see Havlíěek, 1977, pl. II, figs. 15–19).

Brahimorthis can be distinguished from Alocorthis by its lack of dental plates, and the presence of a raised transverse ridge in the delthyrial cavity (Williams & Harper, 2000).

ACKNOWLEDGMENTS. The authors thank Ms Annie O’Connor for access to “Mount Arrowsmith” Station and for providing accommodation during fieldwork in February 2001. Dr Yong-Yi Zhen kindly identified the conodonts collected from the TAB 1 section and, in conjunction with Dr Ian Percival and Prof Barry Webby, provided access to an in press manuscript on the conodonts from Mount Arrowsmith. James Valentine kindly read an earlier draft of this manuscript. Drs Ian Percival and Des Strusz provided helpful reviews of the manuscript. Partial funding for this work was provided by a Betty Mayne Grant from the Linnean Society of New South Wales to JRP, and a Macquarie University Research Grant to GAB. Dean Oliver drafted Figs . 1–2 with characteristic accuracy and efficiency.

Figures

Fig. 4. Alocorthis psygmatelos n.gen. and n.sp. Scale bars = 2 mm, unless otherwise stated; (A,B) holotype, latex of dorsal valve interior, and detail of cardinalia (B, scale bar = 1 mm); furrows extending along dorsal surface of brachiophores are arrowed in B; Note wide notothyrial platform lacking cardinal process, MTA/IV/4,AM F120719; (C,D) latex of dorsal valve interior, and detail of cardinalia (D, scale bar = 1.5 mm); fan-like termination of left brachiophore is arrowed in D, TAB1/55 m, AM F120720; (E) latex of dorsal valve interior, TAB1/45 m, AM F120721; (F,G) latex cast and original mould of ventral valve interior, MTA/0.2,AM F120722; (H,I) latex cast and original mould of ventral valve interior, MTA/0.2,AM F120723; (J) latex cast of ventral valve interior, TAB1/45 m,AM F120724.

Fig. 3. Celsiorthis bulancis n.gen. and n.sp. (A,B) dorsal valve interior and anterior, locality T3, AM F120710, scale bar = 1.5 mm; (C) latex of dorsal valve interior, PING1, AM F120711, scale bar = 1 mm; (D–F) lateral, dorsal and posterior views of complete abraded shell, TAB1/20 m, AM F120712, scale bar = 3 mm; (G) latex of ventral valve interior, PING1, AM F120713, scale bar = 2.5 mm; (H) latex of ventral valve interior, PING1, AM F120714, scale bar = 2 mm; (I) latex of ventral valve interior, PING1, AM F120715,

Fig. 1. Geological map of the southwest portion of the MountArrowsmith Inlier,northwestern New South Wales.All measured stratigraphic sections and spot localities are shown.

Fig. 2. Stratigraphic ranges of Celsiorthis bulancis n.gen. and n.sp. and Alocorthis psygmatelos n.gen. and n.sp. in the Tabita Formation (TAB 1 section) and Pingbilly Formation (PING section). First outcrop of the PING section is located 2.6 km NNW from the top of the TAB 1 section (see Fig. 1). The base and top of the Pingbilly Formation are undefined due to poor outcrop caused by alluvial cover.