Peltura sp. B

Ahlberg, Per, Szaniawski, Hubert, Clarkson, Euan N. K. & Bengtson, Stefan, 2005, Phosphatised olenid trilobites and associated fauna from the Upper Cambrian of Västergötland, Sweden, Acta Palaeontologica Polonica 50 (3), pp. 429-440 : 434-437

Peltura sp. B

Fig. 3J .

Material.—One transitory pygidium with a length of about 0.5 mm and an estimated width of 1.0 mm.

Remarks.—The pygidium is subtriangular in outline with three axial rings and a terminal piece. There are four pairs of stout marginal spines, the outermost directed backwards and the innermost directed backwards and slightly inwards. While this pygidium is noted here as a separate species, it may merely represent a different growth stage of Peltura sp. A .

Indeterminate olenid fragments

Figs. 3K–P , 4A–H .

Material.—Three broken occipital rings, two thoracic “axial rings”, one broken thoracic tergite, four thoracic pleurae, one hypostome, one partial librigena with the eye, and two detached visual surfaces.

Remarks.—None of these fragments can be referred with certainty to any genus. The occipital rings ( Fig. 3K–M ), of width 0.6, 0.9, and 1.0 mm respectively, all of which have an occipital spine, appear to belong to three different species. They differ in their surface sculpture. The occipital ring shown in Fig. 3L has broadly spaced nodules along the posterior margin, the one shown in Fig. 3M is granulose, whereas the occipital ring shown in Fig. 3K has dense, closely packed and prominent granules.

The axial rings ( Fig. 3N, O ) again appear to belong to two different species. The axial ring shown in Fig. 3O has a smooth, lensoidal articulating half−ring (0.7 mm wide), approximately the same length (sag.) as the main part of the axial ring, which has a straight posterior margin, a distinct medial node, and a granular sculpture. The axial ring in Fig. 3N is seen edge on and is strongly arched with a broad−based and stout axial spine.

The incomplete thoracic tergite ( Fig. 3P ; estimated width 1.3 mm) has a smooth external surface. The axial ring proper is slightly wider than the pleura and about the same length (sag.) as the articulating half−ring. It has a conspicuous medial node. The pleurae lack a distinct pleural furrow and the lateral margins are somewhat rounded anteriorly, posteriorly making an angle of about 80 ° with the posterior margin.

Three pleurae (1.2, 1.3, and 1.6 mm wide; Fig. 4A–C ) are all broken off at the axial furrow, but can be referred to the same species. They are 0.4 times as long (exsag.) as they are wide with nearly straight and transversely parallel margins. The curving antero−lateral margin terminates in a broad−based spine, directed posteriorly and slightly laterally. Articulating facets are absent. An oblique pleural furrow traverses the pleura almost as far as the base of the spine. The external surface is granulose, except antero−laterally. Three to five larger nodes, irregularly spaced, are present along a transverse line in front of the posterior margin. A thin ridge, subparallel with the pleural furrow, extends from the anterior margin about a third of the way from the axial furrow to the middle part of the antero−lateral margin. Terrace lines run along the outer edge of the pleura, particularly on the spine.

The fourth pleura ( Fig. 4D ; 0.6 mm long exsag.) consists of the lateral part only and is seen in ventral view. It appears to have been very long (exsag.) and extends laterally into a sharp, postero−laterally directed spine. The doublure is clearly visible as a subtriangular area joining with the spine. It has an almost straight inner edge in the exsagittal plane, sharply curved posteriorly into a narrow shelf along the posterior margin of the pleura. Anteriorly the inner edge of the doublure curves slightly inwards to meet the anterior margin of the pleura. Seven terrace lines are arranged subparallel with the inner edge of the doublure. The inner surface of the pleura is smooth. There seems to be a hollow space between the doublure and the dorsal exoskeleton.

The two detached visual surfaces ( Fig. 4E, F ) and the librigena with an intact eye ( Fig. 4G ) probably belong to the same species, maybe a species of Ctenopyge . These detached eyes (0.55 and 0.60 mm long) are nearly spherical with between a hundred and two hundred lenses. In one specimen there is clear lens packing discontinuity (running from northeast to southwest in Fig. 4E ). The librigena has a regularly curved outer margin with a raised lateral border, about the same width as the narrowest distance from the border furrow to the base of the eye. The posterior branch of the facial suture is short, strongly convex forwards, and cuts the lateral border at a sharp angle. That part of the anterior branch of the facial suture still preserved curves slightly forwards from the top of the eye. The palpebral suture is straight adaxially. The external surface is smooth.

A poorly preserved hypostome ( Fig. 4H ), 0.5 mm long, is frayed laterally and its original external contour cannot be distinguished. The anterior margin is slightly convex forwards. Only part of the posterior border is preserved, but it is evidently longer (sag.) than the anterior border. The ovoid and convex middle body is distinct.

Figures

Fig. 3. A. Sphaerophthalmus sp. A, cranidium, SMNH Ar59729 (2.21), sample 2. B. Sphaerophthalmus sp. B, right librigena with intact eye, SMNH Ar59730 (3.11), sample 2. C. Sphaerophthalmus sp. C, incomplete thoracic tergite, SMNH Ar59731 (2.19), sample 2. D–F. Peltura cf. transiens (Brögger, 1882). D. Pygidium, SMNH Ar59732 (1.3), sample 1. E. Pygidium, SMNH Ar59733 (2.7), sample 2. F. Small pygidium, SMNH Ar59734 (3.12), sample 2. G. Peltura cf. paradoxa (Moberg and Möller, 1898), pygidium, SMNH Ar59735 (1.7), sample 1. H, I. Peltura sp. A. H. Pygidium, SMNH Ar59736 (1.2), sample 1. I. Transitory pygidium, SMNH Ar59737 (1.21), sample 1. J. Peltura sp. B, transitory pygidium, SMNH Ar59738 (1.6), sample 1. K–P. Indeterminate olenid fragments. K–M. Occipital rings. K. SMNH Ar59739 (1.19), sample 1. L. SMNH Ar59740 (1.15), sample 1. M. SMNH Ar59741 (2.14), sample 2. N. Axial ring seen edge on, SMNH Ar59742 (1.18), sample 1. O. Axial ring, SMNH Ar59743 (2.22), sample 2. P. Incomplete thoracic tergite, SMNH Ar59744 (1.24), sample 1. Scale bars 0.1 mm.

Fig. 4. Indeterminate olenid fragments. A. Left pleura, SMNH Ar59745 (2.13), sample 2. B. Right pleura, SMNH Ar59746 (1.17), sample 1. C. Left pleura, SMNH Ar59747 (1.16), sample 1. D. Pleura in ventral view, SMNH Ar59748 (2.16), sample 2. E. Detached eye, SMNH Ar59749 (2.17), sample 2. F. Detached eye, SMNH Ar59750 (2.18), sample 2. G. Librigena with intact eye, SMNH Ar597451 (2.15), sample 2. H. Poorly preserved hypostome, SMNH Ar59752 (1.22), sample 1. Scale bars 0.1 mm.