Trimerus (Edgillia), 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 : 38-39

Trimerus (Edgillia) subgen. nov.

Type species. Trimerus kinglakensis Gill, 1949 from the Lochkovian Humevale Siltstone , central Victoria .

Other species included. Trimerus (Trimerus) grandis Benedetto and Martel (in Baldis et al., 1976), T. (Edgillia) jelli sp. nov., Homalonotus (Trimerus) mongolicus Tchernycheva, 1937 , H. vanuxemi Hall, 1859 .

Other species tentatively included. Homalonotus major Whitfield, 1881 .

Range. Upper Silurian-Lower Devonian.

Derivation of name. For the late E. D. Gill, for his contribution to Victorian palaeontology.

Diagnosis. Glabella subquadrate, length about 1.0 times width, sides straight and weakly tapered (converging at around 15–20˚), anterior margin transverse, well defined. S2 and S3 indistinct, S1 indistinct or weakly defined, not extending to axial furrows. Eye placed more or less opposite glabellar midlength. Hypostomal middle furrow with deep impressions abaxially. 10–14 pygidial axial rings. Pygidial postaxial ridge poorly defined. Ring furrows deep, pleural furrows shallow. Pygidial length 0.85–1.0 times width.

Discussion. Represented by a population sample of over 300 partly articulated exoskeletons, Trimerus (Edgillia) kinglakensis is undoubtedly the best represented species of Trimerus from the Pr˘ídolí-Lower Devonian interval. T. (E.) kinglakensis differs markedly from many earlier representatives of Trimerus in its glabellar morphology, principally in having a subquadrate outline and a raised but evenly-vaulted surface. Species assigned here to T. (Ramiotis) differs in having a more strongly tapered and generally more elongate glabellar outline, with many exhibiting weak glabellar lobation and rounded anterior glabellar margins and pygidia with raised postaxial ridges. The glabellar outline strongly expanded across L1-L1 (tr.), distinct glabellar lobation, paraglabellar areas, and the presence of a glabellar longitudinal ridge and glabellar antero-median indentation in T. ( Trimerus ) distinguishes kinglakensis from that group. The morphology exhibited by kinglakensis is shared with several poorly known Upper Silurian-Lower Devonian taxa and defines a species group for which T. (Edgillia) is established.

Homalonotus vanuxemi from North America is roughly contemporary with Trimerus (Edgillia) kinglakensis , occurring in Lower Devonian strata of New York State. The species was originally described from its thorax and pygidium ( Hall, 1859). The later documentation of the cranidium ( Hall and Clarke, 1888: p. 11, plate 5b, Williams and Breger, 1916: plate 22, figs 12, 21, Shimer and Shrock, 1944: plate 272, fig. 32) demonstrates a cephalic morphology closely comparable with kinglakensis . In particular, the straight sides, transverse anterior margin and weakly tapered outline and proportions of the glabella of vanuxemi are almost identical to the Victorian species. With respect to the morphology of the preglabellar field, the illustrations of the cranidium by Hall and Clarke and Shimer and Shrock are not in close agreement. Hall and Clarke noted an exceptionally long preglabellar field and a transverse rostral suture, although their observations were based on a single fragment. Shimer and Shrock illustrated a complete cranidium with a somewhat shorter preglabellar field, and although the rostral suture is not clearly depicted it appears forwardly convex in outline. In these respects the cephalon illustrated by Shimer and Shrock is very close to kinglakensis and supports assignment of vanuxemi to T. (Edgillia). The pygidia of these two species also share a relatively high degree of segmentation and both lack a raised postaxial ridge. The relative depth of the pygidial pleural furrows of vanuxemi is difficult to ascertain from the illustrations available. However, Hall and Clarke’s illustration of a large specimen in lateral view (Hall and Clarke: pl. 5b fig. 2) suggests shallower pleural furrows than the ring furrows, as in kinglakensis .

Trimerus (Trimerus) grandis from the Upper Silurian-Lower Devonian of Argentina was described by Benedetto and Martel in Baldis et al. (1976) from a single specimen. Assignment of this species to T. (Edgillia) is supported by the raised, evenly vaulted glabellar profile, the subquadrate glabellar outline, the poorly defined glabellar lobation, and the relative shallowness of the pygidial pleural furrows relative to the ring furrows. The higher degree of pygidial segmentation (11-12 axial rings) is in accord with assignment to T. (Edgillia).

A new species represented almost entirely by fragmented tergites from the Lochkovian of Victoria can also be assigned to Trimerus (Edgillia) . The material available is closely comparable to T. (E.) kinglakensis and T. (E.) vanuxemi and is described in this work as T. (E.) jelli . There are few other homalonotids recorded from the Lower Devonian that can be confidently assigned to T. (Edgillia). Homalonotus major was described from a single, incomplete but very large thoracopygon. Whitfield (1881) estimated the specimen to represent an individual 15.5 inches (31 cm) and in its very large size the species is comparable to kinglakensis , vanuxemi and T. (E.) grandis . The depth of the axial ring and pleural furrows of major and kinglakensis are comparable, although those of major do not appear to shallow abaxially. The specimen has 10 axial rings, although the posterior part of the pygidium is broken off and whether there were further rings or a postaxial ridge cannot be determined. On these few features the species is only doubtfully assigned to Edgillia.

Gill (1949) considered Trimerus (Edgillia) kinglakensis to be most closely related to, and directly descended from, the earliest Ludlow T. (T.) vomer . Gill constructed his phylogeny erroneously considering these species to be much closer in age than they actually are. Redescription of both vomer and kinglakensis shows that these species are not closely related. T. (T.) vomer is a highly derived species and kinglakensis exhibits none of its characteristic features. With respect to the presumably ancestral T. (Ramiotis), morphoclines of increasing glabellar complexity defining T. ( Trimerus ) are contrary to morphoclines of decreasing glabellar complexity characterising T. (Edgillia) and suggest independent derivation of these subgenera.