Trimerus (Trimerus) Green, 1832

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 : 34

Trimerus (Trimerus) Green, 1832

Type species. Trimerus delphinocephalus Green, 1832 , from the Wenlock Rochester Shale , New York State, by monotypy .

Other species included. Homalonotus (Trimerus) cylindricus Salter, 1865 , T. (T.) flexuosus Benedetto and Martel (in Baldis et al., 1976), H. harrisoni McCoy, 1876 , H. (T.) johannis Salter, 1865 , H. vomer Chapman, 1912 , T. ( T.) sp. A in Alberti (1970), T. sp. in Owens (1994).

Range. Wenlock-Ludlow, Pr˘ídolí?

Diagnosis. Glabella moderately to strongly expanded across L1-L1 (tr.), strongly raised, markedly narrower (tr.) anteriorly than posteriorly (sides tapering at 33-55˚, width opposite S3 less than 0.65 times L1-L1). S1 moderately to strongly expressed, S2 and S3 distinct or fused with S1 to form a shallow exsagittal depression. L1 and L2 with low ovoid swellings. Sagittal ridge of glabella well defined. Preglabellar field long to very long (0.22–0.35 times cranidial length), distinctly concave (tr. sect.). Pygidial length equal to or greater than width. Pygidial ring furrows and pleural furrows of similar depth, 10–12 axial rings, 7–9 ribs. Pygidium with acutely produced tip or with short, spine-like process.

Discussion. The type species Trimerus delphinocephalus belongs to a distinct species group within Trimerus , restricted in distribution to the mid Wenlock-lower Ludlow interval. The group is easily recognised in having distinctive glabellar features, particularly the glabella with an outline markedly expanded across L1-L1 (tr.), raised profile, distinct lobation, swellings on L1 and L2, and the sagittal ridge. In comparison to other members of Trimerus , these species also have more elongate preglabellar fields, more elongate and more highly segmented pygidia, and bear a short spine or acute process on the pygidial terminus. The species assigned are easily distinguished from other Trimerus , in which the characters listed are only occasionally or weakly expressed.

The characteristic cephalic features of Trimerus (Trimerus) are expressed most strongly in T. (T.) johannis from the upper Wenlock of England and T. (T.) vomer from the basal Ludlow in Victoria. These species are interpreted as the most highly derived of those assigned. Populations of T. (T.) delphinocephalus show wide morphological variability and the cephalic features characteristic of the subgenus are variably expressed, although they are clearly present in specimens figured by Whittington (1993: figs 1, 2). Pygidia of delphinocephalus differ from those of johannis and vomer in having shallower ring furrows and pleural furrows, and a short spine-like process posteriorly. In outline, the depth of the pygidial furrowing and presence of a posterior spine a single pygidium documented from the Upper Silurian of Morocco ( Alberti, 1970, Schraut, 2000) closely resembles pygidia of delphinocephalus and is also assigned to the subgenus.

Trimerus (Trimerus) flexuosus from the Wenlock-Ludlow? of Argentina is poorly known but is clearly attributable to T. ( Trimerus ). Benedetto and Martel (in Baldis et al., 1976) describe the species as having a raised glabella with distinct S1-S3, subcircular-ovate swellings on L1, and strongly defined paraglabellar areas, features characteristic of the subgenus. The pygidium closely resembles that of T. (T.) sp. (in Alberti, 1970) from Morocco in outline, in the depth of the pygidial furrowing and in the presence of a posterior spine. The rostral suture is convex, as in the type species.

An incomplete cranidium figured as Trimerus sp. from the upper Wenlock Brinkmarsh Beds of England ( Owens, 1994: pl. 2 fig. J) can be assigned to T. ( Trimerus ). In outline and height the glabella is not unlike that of T. (T.) johannis from the nearby area, to which it may belong.

The anterior margin of the cephalon is variably expressed in Trimerus (Trimerus) . In T. (T.) vomer , the angle of convergence of the sides of the cephalon increases abruptly opposite the antero-lateral corners of the cranidium, giving the cephalon an irregular pentagonal outline. In T. (T.) johannis (and possibly T. (T.) flexuosus ) the anterior margin of the cephalon is tricuspate, with the sides of the cephalon invaginated opposite the connective suture. The weakly trilobate anterior margin of the cephalon of T. (T.) delphinocephalus is intermediate between these morphologies.