Bronzoria recta, Ishizaki & Shiino, 2024

Bronzoria recta View in CoL sp. nov.

Figs. 5–8 View Fig View Fig View Fig View Fig .

1973 Orbiculoidea sp. cf. O. sibirica Moisseiev ; Murata 1973: pl. 29: 16, 17.

2023 Orbiculoidea sp. cf. O. sibirica Moisseiev ; Ishizaki and Shiino 2023b: fig. 12G.

ZooBank LSID: urn:lsid:zoobank.org:act:73725EC9-9A3E-41DF-85B5-6D3A6948E166

Etymology: From Latin recta , straight; a narrow and straight pedicle track is the most notable characteristic of this species.

Type material: Holotype: dorsal and ventral valves, originally conjoined shell, IGPS 112854 ( Fig. 5A View Fig ) . Paratypes: four supposed conjoined shells, IGPS 92672 , 112855–112857 ( Figs. 5B, C View Fig , 6B, E View Fig ); three dorsal valves, IGPS 112858–112860 ( Fig. 6 View Fig ); two ventral valves, IGPS 112861–112862 ( Figs. 7 View Fig , 8 View Fig ); all from type locality and horizon .

Type horizon: Middle part of the Osawa Formation (unit-2: Spathian, Olenekian), Lower Triassic .

Type locality: Tatezaki area in Minamisanriku Town, Miyagi Prefecture, Japan ( Fig. 3 View Fig ) .

Material.—Eight articulated shells, 33 dorsal valves, 15 ventral valves and one unknown valve (dorsal or ventral) were collected (IGPS 112854–112910). We also used four specimens (IGPS 92672–92675) reported as Orbiculoidea sp. cf. O. sibirica by Murata (1973). Although the original height of the shell and the 3D morphology of the shell slope were not preserved due to deformation, the overall morphology was well preserved. The shells were thin and blackish in colour.

Diagnosis.—Convexoplane to convexoconcave shell with a low conical dorsal valve. Both dorsal and ventral valves have comparatively dense, regularly arranged fila on the outer surface. The dorsal valve has a convex anterior slope and a concave posterior slope. The dorsal apex is subposterior to the centre. The ventral apex is almost central, with a posterior slope curving dorsally. The pedicle track is narrow and straight, extending to the posterior margin along the midline of a V-shaped large depressed area.

Description.—The shell convexity is convexoplane–convexoconcave with a subcircular outline ( Figs. 5–8 View Fig View Fig View Fig View Fig ). The length and width of the shell are 7–13 mm and 6–14 mm, respectively. The outline of the smaller specimens tends to be a true circle, while that of the larger specimens changes to a weak trapezoid with a short, straight posterior margin ( Figs. 5 View Fig , 6 View Fig ).

The dorsal valve is low conical in shape with 1.08– 2.09 mm height, which is supposed to be lower than the original height. The apex is located posteriorly at one-third to one-fourth of its length from the posterior margin ( Figs. 5 View Fig , 6 View Fig ). The anterior slope is moderately convex ( Fig. 6A View Fig 2, C 2 View Fig ). The posterior slope is overall concave ( Fig. 6A–D View Fig ), though its convexity changes during growth. The slope just posterior to the apex is moderately convex, and subsequently bent posteriorly to become a plane to a gently concave slope ( Fig. 6A View Fig 2 View Fig , A 3 View Fig , C). The shell margin, including the posterior slope, is slightly bent ventrally ( Fig. 6A View Fig 2 View Fig , C). In this configuration, the position of the apex moves backwards, with the larval shell facing the posterior direction ( Figs. 5A View Fig , 6A, C View Fig ). The lateral slopes are gently convex and smoothly transition into the convex anterior slope and concave posterior slope. The external surface of the dorsal valve exhibits numerous, distinct, and slightly elevated concentric growth lines. These growth lines are somewhat discontinuous and are arranged at intervals of approximately 0.06–0.07 mm ( Fig. 6A View Fig 4 View Fig , D). Several growth lines are inserted into the anterior part. The growth lines periodically develop into higher-elevated concentric fila. The concentric fila interval is the broadest in the antero-median part, with 2–3 fila of 1 mm in length ( Fig. 6A View Fig 4 View Fig ). Both the growth lines and concentric fila are faint around the apex, while they are distinct near the shell margin. The concentric fila are more continuous than the growth lines. The internal surface of the dorsal valve is comparatively smooth with broad concentric undulations ( Fig. 6A View Fig 3 View Fig ). The undulations show low-height wavy outlines in cross sectional view, of which the width increases from 0.1 mm to 0.35 mm toward the shell margin ( Figs. 5 View Fig , 6 View Fig ). The larger specimen has a median septum just anterior to the apex ( Fig. 6E View Fig ). In the peripheral part of the internal surface, narrow and shallow furrows 0.02–0.04 mm in width extend radially ( Fig. 5A 5 View Fig ). From these furrows, network-like thinner furrows branch and extend radially, all of which are similar to mantle canals ( Mergl and Massa 2005; Williams et al. 1997).

The ventral valve is flattened overall, and its apex is situated at the centre of the ventral valve ( Figs. 5 View Fig , 7 View Fig ). The margin of the ventral valve is slightly geniculated toward the ventral direction ( Figs. 5A View Fig 1 View Fig , B, 7A View Fig 1 View Fig ). The anterior and lateral slopes are flattened, while the posterior slope is concave, with its concavity change during growth ( Fig. 7A, B View Fig ). Along the median line, the posterior slope just posterior to the larval shell is strongly depressed and flattened ( Figs. 7B View Fig , 8 View Fig ). In the posterior part, the posterior slope bends ventrally, forming a concave outline ( Figs. 7B View Fig , 8 View Fig ). The peripheral part of the posterior slope is flattened with a geniculated margin. In the horizontal view, the concave area of the posterior slope exhibits a symmetric, V-shaped depression known as the large depressed area ( Fig. 7A View Fig 3, B 3 View Fig ). The angle of the V shape is approximately 70°. In the anterior part of the large depressed area, the boundary with the lateral slope is geniculated, while in the posterior part is smooth ( Fig. 7A, B View Fig ). Similar to the dorsal valve, the external surface of the ventral valve exhibits numerous and distinct, slightly elevated concentric growth lines ( Fig. 7A, B View Fig ). These growth lines are irregular and arranged in approximately 0.02–0.04 mm intervals. The growth lines periodically develop into higher elevated and concentric fila, with intervals of 3–4 ridges of 1 mm in length ( Fig. 7B View Fig 4 View Fig ). The concentric growth lines and the fila of the ventral valve are elevated more vertically than those of the dorsal valve. Growth lines and fila are faint around the apex, while they become distinct near the shell margin.

The narrow, longitudinal pedicle track is positioned just posterior to the ventral apex, which is approximately 0.3 mm wide, extending to the posterior margin ( Figs. 5C View Fig , 7 View Fig , 8 View Fig ). On the external surface, the listrium is in the depressed area toward the midline of the posterior slope, although it is filled with muddy sediments ( Figs. 7C View Fig , 8A View Fig ). Thus, detailed information regarding the morphology and surface ornamentation of the listrium is unknown. The pedicle foramen is also invisible due to filled sediments. However, given that the pedicle track extends to the shell margin, the pedicle foramen is undoubtedly located at the posterior margin and opens outwards ( Figs. 7A View Fig 2 View Fig , B, 8 View Fig ). On the internal surface, the listrium has a shape different from that seen in the external view. An anterior part of the listrium is convex, corresponding to the depression observed on the external surface Fig. 8B View Fig : arrowhead near the apex). In the middle part of the listrium, around the most depressed part on the posterior slope, the shells beside the pedicle track are elevated to the ridges; therefore, the middle part of the listrium appears depressed ( Fig. 8B View Fig : arrowhead of the middle). In the posterior part, around the shell margin, the elevated ridges parallel to the listrium become weak and obscure ( Fig. 8B View Fig : arrowhead near the posterior margin).

The larval shells are well-preserved in the dorsal and ventral valves, both of which have unique ornamentation with distinct boundaries ( Fig. 5A View Fig ). In the dorsal valve, the larval shell has a subcircular outline and was approximately 600 µm in diameter ( Fig. 5A View Fig 2 View Fig , A 3 View Fig ). All slopes of the larval shell are convex with a smooth surface. In the ventral valve, the larval shell has a subcircular outline, approximately 600 µm in diameter ( Fig. 5A View Fig 6 View Fig ). In the best-preserved specimen, the outline of the larval shell is heart-shaped. Similar to the dorsal larval shell, the ventral larval shell has a smooth surface but shows a variety of convexities. The peripheral part of the anterior to postero-lateral slope is moderately concave. The postero-median part has a slit-like ridge 40 µm wide, extending from the centre to the posterior margin of the halo. The slit-like ridge changes wider and lower toward the posterior margin of the halo, connecting to the pedicle track. The central part of the larval shell is gently convex.

Remarks.— Bronzoria recta gen. et sp. nov. is similar to Bronzoria sibirica ( Moisseiev, 1947) , in having a subcircular outline approximately 10 mm in size with convexoplane convexity. In particular, the narrow, longitudinal pedicle track extending to the posterior margin with the V-shaped large depressed area in Bronzoria sibirica is extremely similar to that of the present species. Murata (1973) described this species as Orbiculoidea sp. cf. O. sibirica based on its size, flattened ventral valve, and narrow pedicle track. However, the position of the dorsal apex in Bronzoria recta gen. et sp. nov. is more anterior than that in Bronzoria sibirica , with its apex located posteriorly at one-fifth to one-ninth of its length Dagys 1965). Moreover, the dorsal valve of the present species has a posterior slope with a concave outline, which differs from the gently convex posterior slope of Bronzoria sibirica . The dorsal convexity around the apex can be strongly related to the muscular system and the volume of soft parts

( Masunaga and Shiino 2021); therefore, this is not Bronzoria sibirica but a previously undescribed species.

In the Early Triassic, some discinid brachiopods exhibited a ventral morphology similar to that of Bronzoria recta gen. et sp. nov. ( Kittl 1904; Wagner 1913; Dagys 1965). According to Radwański and Summesberger (2001), three discinid species are categorised as “slightly (?) older Triassic forms”: Bronzoria sibirica , Bronzoria bosniaca ( Kittl, 1904) and Bronzoria major ( Wagner, 1913) . Bronzoria bosniaca and Bronzoria major are characterised by a narrow, longitudinal pedicle track extending to the posterior margin with the V-shaped large depressed area, as observed in Bronzoria recta gen. et sp. nov. ( Kittl 1904; Wagner 1913). These two species are also similar to this species in their low conical, convexoplane shell and the position of the dorsal apex, but they differ in their larger shell size ( Bronzoria bosniaca : 30 mm in diameter, Bronzoria major : 43 mm in diameter) and wider intervals of the fila compared to the present species. A similar morphology of convexity has also been reported in the Late Triassic Bronzoria rhaetica ( Andreae, 1893) . However, the latter species differs from Bronzoria recta gen. et sp. nov. in its larger size (32 mm in diameter), a pedicle track widening toward the posterior margin, and a trapezoid outline with a long, straight posterior margin ( Andreae 1893).

Stratigraphic and geographic range.—Osawa Formation, Spathian (upper Olenekian, Lower Triassic); probably within the Southern Kitakami Terrane, Japan.