Trimerus (Ramiotis) iani, Sandford, 2005

Sandford, Andrew C., 2005, Homalonotid trilobites from the Silurian and Lower Devonian of south-eastern Australia and New Zealand (Arthropoda: Trilobita: Homalonotidae), Memoirs of Museum Victoria 62 (1), pp. 1-66 : 47-49

Trimerus (Ramiotis) iani sp. nov.

Trimerus sp. — Holloway and Sandford, 1993: 93, figs 4C–D, 4F–G,

4I–J, 4N–P, non fig. 4L.

Type material. Holotype NMV P137221 (cephalon lacking rostral plate, figured Holloway and Sandford, 1993: 4 F–G, 4J, Fig. 19.1 herein) from PL359 , Tiger Range, northwest of Maydena , Tasmania . Paratypes NMV P137222 (cranidium, figured Holloway and Sandford: 4I) , NMV P137227 (pygidium, figured Holloway and Sandford: 4N–P) , NMV P137224 (cephalon, figured Holloway and Sandford: 4C) , NMV P137223 (cephalon and two displaced thoracic segments) from PL359 . For locality see Holloway and Sandford: fig. 1

Previously figured material. NMV P137225 (librigena, figured Holloway and Sandford, 1993: 4 D) from PL 359.

Other material. NMV P137226 (cephalon) from PL 359.

Stratigraphic distribution. As for Brongniartella sp.

Derivation of name. For my brother Ian.

Diagnosis. Cephalon rounded triangular, length ~0.7 times width, sides (opposite midlength) converging at ~60˚. Glabella trapezoid in outline, length 1.2 times width, sides very weakly concave and tapering at 20˚ anteriorly, anterior glabellar margin transverse, without distinct medial indentation. S1-S3 and sagittal glabellar ridge very weakly defined. Preglabellar field of moderate length (0.2 times cranidial length), rostral suture transverse, dorsal section of rostral plate short, 0.05 times cephalic length. Eye placed opposite ~0.5 cranidial length (~0.6 glabellar length). Anterior branches of facial sutures converging at 60˚, curving abruptly adjacent to connective sutures. Ventral surface of rostral plate elongate pentangular in outline, with connective sutures converging posteriorly at 45˚. Pygidium short. Axis 0.45 times pygidial width, raised posteriorly, 10 rings, ring furrows moderately impressed. 6 ribs, pleural furrows shallow. Axial furrows moderately impressed.

Discussion. Trimerus (Ramiotis) iani is abundant at the type locality. Holloway and Sandford (1993) were uncertain whether the two dissimilar homalonotid pygidia from PL 359 represented different species, or whether the differences were attributable to size differences or deformation. In reviewing this conclusion, I cannot attribute the contrast between the narrow, concave-sided and strongly raised axis of the smaller specimen (see Holloway and Sandford, 1993: fig. 4L) and the wide, convex-sided and low axis of the larger specimen entirely to deformation (see Fig. 19.6). The more elongate (length=width) and parabolic outline and the indication of a distinct border suggest that the smaller pygidium represents a separate species, referable to Brongniartella (see above).

The diagnosis includes a number of features not listed in the brief description of the species by Holloway and Sandford (1993). Of the two cephala not figured by Holloway and Sandford, one specimen shows the anterior margin of the cephalon, with the impression of the rostral suture evident on the surface of the doublure (Fig. 19.4). The other specimen shows the posterior portion of the ventral surface of the rostral plate. The pygidial pleural furrows are shallow, although the greater depth of the first furrow appears to be partly exaggerated by longitudinal shearing.

Holloway and Sandford (1993) distinguished Trimerus (Ramiotis) iani from previously documented homalonotids from the Ludlow-Pragian strata of south-eastern Australia. Of the species discussed by Holloway and Sandford, only specimens described by Talent (1964) as “ Dipleura sp. ” can be referred to T. (Ramiotis).

In cephalic features Trimerus (Ramiotis) iani is similar to T. (R.) otisi from the Ludlow of Victoria. These species share a glabella with elongate proportions (L:W~1.2) and moderately tapering (~25˚) and weakly concave sides, a preglabellar field of similar length (0.2 cranidial length), similar cephalic proportions (L:W ~0.7), a transverse rostral suture, very short pygidial proportions and comparable pygidial segmentation. T. (R.) otisi differs in having well defined glabellar lobation and a distinctive tricuspid anterior cephalic margin. In cephalic features T. (R.) permutus from the Ludlow of Poland is also closely comparable, although it differs in having strongly expressed glabellar lobation. In pygidial features iani demonstrates its closer affinities to other Lower Silurian T. (Ramiotis), particularly in the weak expression of the pleural furrows, the deeper ring furrows, and the rounded tip. In pygidial morphology iani is closest to T. (R.) dyaulax , that differs in having weaker segmentation. T. (R.) iani is closest in age to the type species T. (R.) rickardsi , but differs in having a shorter preglabellar field, weaker glabellar lobation, a transverse rostral suture, and a rounded pygidial tip.