Peltura cf. transiens ( Brögger, 1882 )

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

Peltura cf. transiens ( Brögger, 1882)

Fig. 3D–F .

Material.—Three pygidia, 0.4, 0.6, and 0.8 mm long (the latter estimated).

Description.—The pygidia are subsemicircular in outline and more than twice as wide as they are long. The axis is wider than the pleural fields, slightly tapering backwards, and broadly rounded posteriorly, not reaching the posterior margin. It consists of an articulating half−ring, two axial rings, and a terminal piece. The pleural field narrows backwards and has two shallow furrows extending almost to the postero−lateral margins. A very narrow, curving articulating facet is present on the antero−lateral margin. Three pairs of marginal spines, directed backwards and inwards, are present lateral to the posterior margin behind the axis.

Remarks.—In the smallest pygidium ( Fig. 3F ) the marginal spines are relatively longer than in the larger pygidia. The development of the surface sculpture can be traced from the smallest to the largest specimen. In the smallest specimen the ornament consists of granules, slightly elongated and, on the pleural fields, subparallel with the postero−lateral margin. On the axial rings these granules have coalesced into incipient transverse terrace lines. Less developed terrace lines are present at the posterior margin. In the next larger specimen ( Fig. 3D ) the terrace lines are much more distinctly developed on the pleural fields and behind the axis. In the largest specimen ( Fig. 3E ) the terrace lines are very well developed as raised ridges forming short, linked sections, but continuous behind the axis. On the terminal piece of the axis they are again continuous and subconcentric with the axial furrow surrounding the posterior end of the axis.

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.