Trimerus (Ramiotis), 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 : 43-44

Trimerus (Ramiotis) subgen. nov.

Type species. Trimerus (Ramiotis) rickardsi sp. nov., from the upper Llandovery (crenulata Biozone) Chintin Formation, central Victoria, Australia .

Other species included. Platycoryphe dyaulax Thomas, 1977 , Trimerus (Ramiotis) iani sp. nov., T. permutus Tomczykowa, 1978 (nom. nov. for T. lobatus Tomczykowa, 1975 non Prouty, 1923), T. (R.) otisi sp. nov., T. (R.) tomczykowae sp. nov., T. (R.) thomasi sp. nov., T. sp. (in Edgecombe and Fortey, 2000), T. sp. (in Curtis and Lane, 1998), T. sp. A (in Wolfart, 1961),

Other species tentatively included. Dipleura salteri Morris, 1988 (nom. nov. for Homalonotus (Koenigia) ludensis Salter (1865) non H. ludensis Murchison, 1839 ).

Range. Silurian.

Derivation of name. For my son Otis Rami.

Diagnosis. Glabella trapezoid or weakly expanded across L1- L1 (tr.), low, with sides more or less straight and weakly to moderately convergent. S1-S3 weakly impressed. Preglabellar field of short to moderate length (~0.15–0.25 times cephalic length). Hypostomal middle furrow shallow. Pygidium short, length 0.5–1.0 times width. Postaxial ridge raised. Pygidium with moderately acute to obtusely convergent sides (>70˚), tip without process or spine. Pygidial border furrow and ridge-like border present in late species.

Discussion. Thomas (1977) noted that the rarity of Ashgill and Llandovery homalonotids emphasised the morphological gap between Ordovician and post-Ordovician genera. Reed (1918) suggested that links between Wenlock Trimerus and Ordovician homalonotid groups might be found in Llandovery strata. In this context Thomas interpreted his new Saudi Arabian Llandovery species Platycoryphe dyaulax as showing features of both Platycoryphe / Brongniartella and Trimerus . Thomas suggested that post-Ordovician homalonotines were derived from Platycoryphe / Brongniartella stock, rather than from Eohomalonotus as suggested by Sdzuy (1957). In assigning dyaulax to Platycoryphe, Thomas emphasised its wider and rounded pygidial outline, contrasting with the longer and posteriorly pointed outlines of ‘typical’ Trimerus , i.e. T. ( Trimerus ). Pygidia of dyaulax also exhibit shallow ring furrows and very shallow to effaced axial and pleural furrows, whereas pygidia of T. ( Trimerus ) exhibit pleural and ring furrows of moderate depth, and of equal depth.

New species from the Llandovery and lower Wenlock of south-eastern Australia show strong resemblances to Platycoryphe dyaulax and define a tight species group. Similarities in cephalic morphology between dyaulax and Trimerus noted by Thomas are emphasised by the new Llandovery-Wenlock taxa, and all share weakly tapering, straight-sided, trapezoid glabellar outlines. The new south-eastern Australia taxa exhibit pygidial proportions and segmentation intermediate between dyaulax and T. ( Trimerus ) and support Thomas’s suggestion of close affinities between dyaulax and Trimerus . These similarities, and the weak definition of thoracic trilobation, the width of the thoracic axis and the lower degree pygidial segmentation are emphasised here to justify assignment of dyaulax to Trimerus rather than to Platycoryphe or Brongniartella . T. (Ramiotis) is erected to embrace this group of species, which includes late Llandovery T. (R.) rickardsi from Victoria and T. (R.) iani from Tasmania, and the early Wenlock T. (R.) tomczykowae from Victoria.

In pygidial outline and segmentation there is a morphological gradient through time from Trimerus (Ramiotis) dyaulax (mid Llandovery) to T. (R.) rickardsi and T. (R.) iani (late Llandovery) with moderately impressed ring and axial furrows and shallow pleural furrows, to T. (R.) tomczykowae (early Wenlock) with pygidial furrows of subequal and moderate depth, and to T. ( Trimerus ) (Wenlock-early Ludlow). Although T. ( Trimerus ) appears to be derived from this group, there are a number of new or poorly known Ludlow-Pr˘ídolí Trimerus that share the distinctive cranidial morphology of the Llandovery-Wenlock T. (Ramiotis). In exhibiting elongate glabellar proportions, weakly tapered, straight-sided trapezoid glabellar outlines, low glabellar profiles and preglabellar fields of moderate length, these species are compatible with assignment to T. (Ramiotis) rather than to T. ( Trimerus ). These species include T. (R.) otisi and T. (R.) thomasi from Victoria, T. (R.) permutus from Poland, and T. ( R.) sp. (in Wolfart, 1961) from Bolivia. Compared with the Llandovery-Wenlock T. (Ramiotis), these Ludlow-Pr˘ídolí species encompass a wider range of pygidial morphologies, particularly in length:width proportions and depth of pygidial furrows.

Trimerus (Ramiotis) is easily distinguished from T. ( Trimerus ) in that it lacks most of the characteristic cephalic and pygidial features of the latter. Of the suite of features listed as diagnostic for T. ( Trimerus ), only T. (R.) permutus exhibits strongly expressed glabellar lobation. Several species exhibit sagittal glabellar ridges (albeit weak), several species exhibit distinct medial indentations of the anterior glabellar margin, and some specimens of T. (R.) otisi show a tendency towards outlines expanded across L1-L1 (tr.) and strongly tapered glabellar outlines. However, there are no species of T. (Ramiotis) that exhibit a more than a few of the characters of T. ( Trimerus ), justifying the grouping of the latter. However, with the exception of the deep pygidial border furrow and raised border, the characters distinguishing T. (Ramiotis) from T. ( Trimerus ) can be considered as primitive, and hence only subgeneric status is justifiable.

Dipleura salteri Morris, 1988 from the upper Ludlow (Ludfordian) of England is a poorly known species represented by a single cranidium (see Salter, 1865: p. 121, pl. 12 fig. 1). Following Salter’s (1865) suggestion, Tomczykowa (1975) and Morris (1988) assigned the species to Dipleura , but the forward eye position, the long preglabellar field and the quadrate course of the preocular facial sutures preclude assignment to that genus and are in accord with assignment to Trimerus . Subgeneric assignment of salteri is difficult as the species exhibits a very strongly tapered glabellar outline comparable to that of T. ( Trimerus ), but lacks the other diagnostic glabellar features of the subgenus such as the outline strongly expanded across L1-L1 (tr.) and the distinct lobation. However, the strong tapering of the glabella may be partly attributable to sagittally oriented tectonic compression evident in the apparent concavity (sag.) of the preglabellar field. In all other respects the specimen is compatible with assignment to T. (Ramiotis), to which it is tentatively assigned.

Trimerus (Ramiotis) dyaulax is from the Aeronian (mid- Llandovery) convolutus Biozone. A possibly earlier origin for Trimerus is suggested by a poorly known species from the lower Llandovery of Paraguay. Wolfart (1961) assigned the several cranidia representing this species to Trimerus . In having elongate glabellar proportions, weakly tapered, straight-sided trapezoid glabellar outlines, weak glabellar lobation and a preglabellar field of moderate length, these can be assigned to T. (Ramiotis). The only other Llandovery T. (Ramiotis) is represented by fragmentary pygidia from the Aeronian of Wales, documented as T. sp. by Curtis and Lane (1998). Although closer in age to dyaulax , the Welsh species differs markedly from it in having moderately impressed pleural, ring and axial furrows, in this respect closely resembling T. (R.) rickardsi .