Acrosaccus squamosus ( BARRANDE , 1879)

Mergl, Michal, 2012, Lingulate And Craniate Brachiopods From The Top Of The Králův Dvůr Formation (Latest Katian) And Their Contribution To Palaeogeography, Acta Musei Nationalis Pragae Series B 68 (1 - 2), pp. 35-46 : 36-38

Acrosaccus squamosus ( BARRANDE, 1879)

Text-figs 2 A-L

1879 Discina squamosa BARR. ; BARRANDE, pl. 96, case 6. 1982 Orbiculoidea squamosa ( BARRANDE, 1879) ; Havlíček and Mergl, pl. 1, figs 2, 3.

M a t e r i a l: Fifteen specimens from the collection of V . Havlíček (dorsal valves 3RO 64968-64971, 3RO 64973, 3RO 64975, 3RO 64977, ventral valves 3RO 64964, 3RO 64967, 3RO 64972, 3RO 64974, 3RO 64976, 3RO 64978- 64980), one dorsal and one ventral valve ( MM 047 , MM

048), and three dorsal and three ventral valves from new material (PCZCU 1844 to 1849).

D e s c r i p t i o n: Shell is small, biconvex, thin-walled, 4.5 mm wide in the largest specimen, showing gentle deviation from the bilateral symmetry.

Dorsal valve is flat conical, subcircular, with the apex situated between 20 to 25% of the shell length. The maximum shell width is slightly posterior to midlength. Anterior margin and lateral margins are evenly rounded. The posterior margin is less curved. Anterior slope is straight with gently convex apical area. Lateral slopes are straight. Posterior slope is gently convex. Dorsal larval shell is circular, with less rounded posterior margin, about 500 µm long and 600 µm wide, slightly and evenly convex without distinct growth lines. Sides of the larval shell are distinguished by a weak halo and a periphery distinctly elevated above the postlarval shell surface.

Ventral valve is low conical, subcircular with the apex near the centre of the valve. Anterior slope is straight to gently concave. Lateral slopes are straight. Posterior slope is weakly to distinctly convex. Pedicle track is small, elongate oval, widest along its posterior third, occupying 15% of the length and c. 5% of the width of the shell. The listri-

PCZCU 1824. P–R – Philhedra ? sp., Králův Dvůr Formation, ‘Perník bed’, Jezerka locality. Bar = 1 mm. Dorsal valve internal mould, oblique view and external mould, PCZCU 1850. S–U – Acanthambonia ? sp., Králův Dvůr Formation, ‘Perník bed’, Jezerka locality. Bar = 0.5 mm. Dorsal valve internal mould, external mould and detail of internal surface, MM 027.

um is divided into a narrow inner listrial plate and broader weakly concave outer listrial plates. External pedicle opening is at the deepest posterior end of the pedicle track.

Interior of dorsal valve has deep apical pit from which a thin weak median septum extends up to one-third of the anterior slope. Muscle imprints and vascular system are not clearly impressed. Ventral valve interior shows distinct internal pedicle tube which has a constant width along its whole length. The tube is opened posteriorly by an unrestricted internal foramen just anterior to weak internal brim.

Ornamentation consists of fine regular concentric rugellae. These are weaker and less distinct on the dorsal valve than on the ventral valve. Size of rugellae mildly but regularly increase toward shell periphery, peripheral rugellae being about double-sized that of early concentric fila near the apex. Rugellae are uninterrupted, undivided and almost uniformly sized along their courses except for rugellae on the posterior slope of the dorsal valve. Some of these rugellae are taller and thicker but some of the rugellae distinct on shell flanks and anterior slope are weaker or disappearing. Interspaces having broadly U-shaped profile are narrower than bases of rugellae. There are about 40 rugellae on large shells. Several early concentric fila have a posterior course hidden below the larval shell. These early fila are substitut- ed by coarser and taller rugellae anteriorly. Microornamentation is unknown.

R e m a r k s: The species was erected by Barrande (1879) based on the ventral valve collected in Králův Dvůr (Koenigshof in original German spelling), but the species has never been formally described despite its illustration by subsequent authors ( Havlíček and Mergl 1982). The variation in shell shape is likely to be original, but variation of shell convexity maybe accounted for by post-mortal deformation of the somewhat flexible organophosphatic shell in the sediment.

There are numerous but generally poorly known Late Ordovician discinoids similar to Acrosaccus squamosus ( BARRANDE, 1879) . Dorsal valves of Acrosaccus shuleri WILLARD, 1928 illustrated by Cooper (1956; pl. 18, fig. B7) and re-illustrated by Holmer and Popov (2000; p. 87, figs 2c and 2g) have clearly coarser ornamentation and a much more posteriorly located dorsal apex compared with A. squamosus . The ventral valve of A. shuleri displays a much longer and acutely closed pedicle track. The figured type specimen of A. posteroconvexus ( COOPER, 1956; pl. 28B) has a taller ventral valve with a more concave anterior slope than A. squamosus and its pedicle track is narrower. Nikitin et al. (1996) described and illustrated A. aff. posteroconvexus ( COOPER, 1956) from the Late Ordovician of Central Kazakhstan. This small discinoid shows some resemblance by its fine ornamentation, but differs from A. squamosus by its taller ventral valve. Another species, Acrosaccus sp. , from the Late Ordovician of Kazakhstan (Popov 2000) differs by a narrower ventral pedicle track but the comparison is somewhat uncertain due to its fragmental preservation. Sutton et al. (2000) re-described and illustrated several species of Schizotreta KUTORGA, 1848 from the Middle Ordovician (Llanvirn to Upper Llandeilo of the British regional subdivision, in Sutton et al. 1999). The species S.? transversa WILLIAMS, 1974 redescribed by these authors has a similar shell morphology but displays a coarser ornamentation than A. squamosus . Among the discinoids described by Holmer (1989) from the Middle Ordovician of Sweden the species Orbiculoidea ? sp. resembles the Bohemian species in its finely rugellate ornamentation.

Orbiculoidea concentrica ( WAHLENBERG, 1818) described from the Dalmanitina Beds (Hirnantian) of Wästergötland, Sweden by Bergström (1968) differs from A. squamosus by a longer pedicle track, coarser rugellate ornamentation and a larger shell size. Orbiculoidea radiata TROEDSSON, 1918 described from the Hirnantian of Poland ( Temple 1965) differs by its almost flat ventral valve and longer pedicle track.

There are several late Ordovician species from the British Isles which are similar to A. squamosa . The species Orbiculoidea shallochensis REED, 1917 from the Ashgill Series, Pushgillian, Dicellograptus complanatus Zone of the Girvan District, Scotland ( Harper 1984), also reported by Wright (1963) from the Portrane Limestone of Ireland, differs from A. squamosus by a larger shell size and having a larger and longer lanceolate-shaped pedicle track. Orbiculoidea inopia HARPER, 1984 from the Dicellograptus complatus Zone of Girvan District resembles A. squamosa by its small shell size and ornamentation. Although the ornamentation of the Scottish species is similarly fine, the rugellae are more distantly spaced than in A. squamosus . The outline of O. inopia is apparently more regularly circular than of A. squamosus . Orbiculoidea armstrongi REED, 1917 from the Ashgill Series, Rawtheyan, Dicellograptus anceps Zone of Girvan District, Scotland ( Harper 1984) differs by a flatly conical pedicle valve, a more centrally situated dorsal apex and a more convexly conical dorsal valve.

O c c u r r e n c e: Except for the type specimen figured by Barrande (1879), which was sampled from the Králův Dvůr area (=Koeningshof), all other specimens were collected in Jezerka where the species belongs among the moderately common fossils .