Acrobrochus marotiriensis, Bitner, 2007

Bitner, Maria Aleksandra, 2007, Recent brachiopods from the Austral Islands, French Polynesia, South-Central Pacific, Zoosystema 29 (3), pp. 491-502 : 492-495

Acrobrochus marotiriensis n. sp. ( Figs 1 A-H; 2A, B; Table 1)

HOLOTYPE. — BENTHAUS, stn DW 1886, specimen on Figure 1 C-E ( MNHN BRA-3043 ).

PARATYPES. — BENTHAUS, stns DW 1884, DW 1886 ( MNHN BRA- 3044-3046).

TYPE LOCALITY. — Austral Islands, Marotiri, BENTHAUS, stn DW 1886, 27°51.27’S, 143°32.39’W, 620-1000 m.

ETYMOLOGY. — Geographic name, from the Marotiri Island where the holotype was collected.

MATERIAL EXAMINED. — Austral Islands. Marotiri, BENTHAUS, stn DW 1884, 8 complete specimens, 41 ventral valves, 32 dorsal valves. — Stn DW 1885, 5 complete specimens, 1 ventral valve, 1 dorsal valve. — Stn DW 1886, 1 complete specimen, 2 ventral valves, 2 dorsal valves. — Stn DW 1887, 1 dorsal valve. — Neilson Reef, stn DW 1923, 4 ventral valves, 3 dorsal valves. All complete specimens are empty shells.

DEPTH RANGE. — 360-1000 m.

MEASUREMENTS. — See Table 1.

DIAGNOSIS. — Acrobrochus of medium size, strongly biconvex, loop with subparallel descending branches and very broad, gently folded transverse band.

DESCRIPTION

The shell is of medium size (max. length 20.7 mm), strongly biconvex, smooth with numerous weakly defined growth lines. The outline is variable, from elongate oval to subpentagonal, dorsal valve often subcircular. The shell is relatively thick with a greatly thickened muscle region. The beak is erect to slightly curved, no beak ridges are observed. The pedicle opening is of medium size, permesothyrid, labiate. The symphytium is small, partly visible to nearly concealed. The lateral commissures are straight to slightly ventrally curved, while the anterior commissure is rectimarginate to slightly uniplicate in adult.

The ventral valve interior has small, short teeth not supported by dental plates. The pedicle collar is short, excavated, forming a tube. The dorsal valve interior has short inner socket ridges. The cardinal process is prominent, semicircular. The outer hinge plates are triangular, relatively narrow, slightly concave and margined by a small elevation of the crural bases. The crural processes are blunt and short. The loop is short, about onethird of valve length, and narrow. The descending branches are wide, subparallel; the transverse band is very broad and gently folded. The muscle scars are deeply impressed.

Ultrastructural analysis was carried out on the transverse section made perpendicular to the plane of symmetry in the centro-anterior part of the pedicle valve. The shell is composed of three layers ( Fig. 2A, B ). The primary layer is 16-38 µm thick and built of microgranular calcitic crystallites. The secondary layer is thicker than the primary one (28- 55 µm) and made up of sheaves of fibres which are anvil-like in transverse section. The tertiary layer, much thicker than the two previous (727-790 µm), is composed of large calcitic prisms perpendicular to the shell surface. The total thickness of the shell is 817-890 µm at the observed section.

REMARKS

The loop character and cardinalia of the investigated specimens are consistent with those of the genus Acrobrochus ( Cooper 1973, 1983). Acrobrochus differs from Liothyrella Thomson, 1916 in loop characters; Liothyrella has a widely triangular loop with relatively narrow transverse band ( Cooper 1983). Gryphus Megerle von Mühlfeldt, 1811 lacks elevated crural bases, characterized for Acrobrochus . The crural bases in Gryphus flush with the inner margin of the outer hinge plates giving the appearance of a wide, flat plate. Also its loop is wider with slightly diverging descending branches ( Cooper 1983). Acrobrochus differs also from two other short-looped genera, Dolichozygus Cooper, 1983 and Dysedrosia Cooper, 1983 , in having shorter crural processes without sharp points ( Cooper 1983).

Three species of Acrobrochus have been hitherto described from Recent seas ( Cooper 1973, 1982, 1983; Foster 1974, 1989). All of them occur in the bathyal zone at high latitudes in the South Atlantic and Antarctica. The newly described species differs from the type species, A. vema in being much smaller, more convex and in having a smaller foramen and straight lateral commissure ( Cooper 1973, 1982, 1983). Acrobrochus blochmanni (Jackson, 1912) from off Antarctica differs from the studied material in being larger, more elongate and less convex ( Foster 1974, 1989; Cooper 1983). Acrobrochus hendleri ( Cooper, 1982) from the southern Atlantic is nearly twice as large as A. marotiriensis n. sp., more elongate in outline and less convex.

The shell of terebratulide brachiopods is usually built of primary and secondary layers but some of them possess also a tertiary layer which modifies from the secondary layer. The presence of the tertiary layer in Acrobrochus is not surprising as this layer is found in closely related genera such as Liothyrella , Gryphus and Dallithyris Muir-Wood, 1959 ( MacKinnon & Williams 1974; present paper). MacKinnon & Williams (1974) examined 33 fossil and two living genera of short-looped brachiopods. The tertiary layer was identified in the majority of the examined genera, only six of them lack this layer.

With its smooth shell and short loop Acrobrochus marotiriensis n. sp. somewhat resembles Dallithyris pacifica , a species also present in the collection. It can be distinguished from the latter species by its smaller size, greater convexity and thicker shell. Also the outer hinge plates in D. pacifica are wider, and the transverse band of the loop is angularly arched. Acrobrochus marotiriensis n. sp. is the most common species in the studied collection.

The genus Acrobrochus is also reported from the Tertiary of Australia and New Zealand ( Cooper 1983; MacKinnon et al. 1993).

Figures

FIG. 1. — A-H, Acrobrochus marotiriensis n. sp., Marotiri Island; A, B, dorsal and lateral views of complete specimen,paratype (MNHN BRA-3044), stn DW 1884, 570-620 m; C-E, dorsal, lateral and anterior views of complete specimen, holotype (MNHN BRA-3043), stn DW 1886, 620-1000 m; F, G, paratype (MNHN BRA-3045), stn DW 1884, 570-620 m, SEM enlargement of the loop of brachial skeleton of the dorsal valve (F) and enlargement of brachial skeleton tilted (G) to show gently folded transverse band; H, inner view of ventral valve,paratype (MNHN BRA-3046),stn DW 1886, 620-1000 m; I-K, Dallithyris pacifica Bitner, 2006, dorsal,lateral and anterior views of complete specimen (MNHN BRA-3048), Marotiri Island, stn DW 1885, 700-800 m, note gastropod boring on I. Scale bars: A-E, H, 0.5 cm; F, G, 2 mm; I-K, 1 cm.

FIG. 2. — A, B, Acrobrochus marotiriensis n. sp. (MNHN BRA-3047), SEM micrographs, Marotiri Island, stn DW 1884, 570-620 m, transverse sections of the entire shell showing thin primary layer (top),secondary layer built of anvil-shaped fibres and prismatic tertiary layer;C, D, Dallithyris pacifica Bitner,2006 (MNHN BRA-3049),SEM micrographs,Marotiri Island,stn DW 1885,700-800 m; C, transverse section of the entire shell showing the acicular primary layer (top) underlain by the fibrous secondary layer, passing into the prismatic tertiary layer; D, section of the shell showing anvil-shaped fibres of the secondary layer. Scale bars: A, B, 50 µm; C, D, 20 µm.