Trimerus (Ramiotis) thomasi, 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 : 53-54

Trimerus (Ramiotis) thomasi sp. nov.

Figure 21

Trimerus ( Dipleura ?) sp..— Talent, 1964: 49 (pars), fig. 6 non pl. 26 figs 1–2 (= Digonus wenndorfi sp. nov.)

Type material. Holotype NMV P305067 (pygidium) from PL6653 , Heathcote , Victoria ( Fig. 21.15) . Paratypes NMV P305046 – P305047 (cephala) , NMV P305048 , NMV P305053 – P305056 , NMV P305058 , P305059 , (cranidia) , NMV P305072 – P305073 (librigena) , NMV P305061 , NMV P304063 , NMV P305068 – P305070 , (pygidia) from PL2264 , Thomas locality F45, Parish of Heathcote , Heathcote . Paratype NMV P63385 (cranidium) , NMV P305043 (pygidium) from PL2265 , Thomas locality F46, Parish of Heathcote, Heathcote. For localities see Fig. 8 .

Previously figured material. NMV P63385 (ex GSV 38132, cranidium, figured Talent, 1964: text–fig. 6) from PL 2265.

Registered material. 94 specimens. 3 cephala, 40 cranidia, 5 librigenae, 1 thoracic segment, 45 pygidia. NMV P305042 – P305045 , NMV P305127 – P305130 from PL2265 . NMV P305046 – P305053 , NMV P305055 – P305066 , NMV P305068 – P305123 , NMV P305126 from PL2264 . NMV P305124 – P305125 , NMV P305054 , NMV P305067 from PL6653 . NMV P305131 from PL6654 , Heathcote. NMV P305132 from PL6655 , Heathcote. NMV P305133 from PL6656 , Heathcote. Unregistered specimens from PL2234 , Thomas locality F21, Parish of Heathcote , Heathcote , PL6727 , Thomas locality F28, Parish of Redcastle, Heathcote. For localities see Fig. 8 .

Stratigraphic distribution. McIvor Formation, ~ 500 m above the base of the unit, lower Notoparmella plentiensis Assemblage Zone , mid Ludlow.

Derivation of name. For Alan T. Thomas (University of Birmingham), for his contribution to the study of homalonotids.

Diagnosis. Cephalon with length about 0.7 times width. Glabella with length equalling width, trapezoid in outline, sides more or less straight, tapering weakly forwards at about 25˚. Glabellar lobation distinct, S1-S3 very shallow, L1, L2, L3 and frontal lobe subequal in length (exsag.). Anterior margin of glabella well defined, transverse or with broad medial indentation. Preglabellar field long, 0.25 times cranidial length, slightly concave (tr. sect.). Palpebral lobe placed with midline opposite 0.45 cranidial length (0.6 glabellar length) and with b- b 1.6 times preoccipital glabellar width. Anterior margin of cranidium transverse to very broadly convex. Pygidium with length equal to width, sides converging at about 70˚, angular tip. Pygidial axis wide, 0.43 times pygidial width, continuous with wide, raised postaxial area. 9 distinct axial rings, 7 distinct pleural ribs, axial furrows shallow, pleural furrows moderately impressed, rib-ring offset at fifth-sixth rib.

Remarks. Although specimens are abundant, the quality of the material of Trimerus (Ramiotis) thomasi and its similarity to T. (R.) otisi does not warrant a full description. The trapezoid, straight-sided and moderately tapered glabellar outline indicate assignment to Trimerus (Ramiotis) . The moderate length of the preglabellar field and the weak glabellar lobation are in accord with this assignment. The cranidium figured by Talent (1964: fig. 6) as T. ( Dipleura ?) sp. is significantly larger than cranidia from the type locality of T. (R.) thomasi which lies about 500m south-east of PL 2265 and more or less on strike. The larger specimen differs conspicuously in that the anterior branches of the facial sutures are more strongly convergent (~70˚) and the rostral suture correspondingly shorter (tr.), the preglabellar field is slightly shorter (0.22 cranidial length), and the medial flexure of the occipital furrow is strongly expressed (see Fig. 21.12). Whether these differences are attributable to the size or to variability within the species cannot be resolved without further specimens of large individuals. Within the range of intraspecific variability, there are specimens of thomasi exhibiting very weak lobation, and short and more strongly tapered glabellar outlines (e.g. Figs 21.4, 21.5). These morphs are closely comparable to the contemporary T. (R.) salteri from England. Original illustrations of the holotype [ Salter, 1865: pl. 12 fig. 1, as Homalonotus (Koenegia) ludensis ] indicates that the English species differs from thomasi in having a wider (tr.) preocular fixigenal field and deeper axial furrows.

Trimerus (Ramiotis) permutus from the upper Ludlow of Poland is close in age to T. (R.) thomasi . In cephalic morphology permutus differs markedly from thomasi in exhibiting strong glabellar lobation, an elongate glabellar outline, more strongly convergent facial sutures, and a short preglabellar field. The species are more closely comparable on pygidial features. Although pygidia of permutus are poorly known, they share with thomasi the elongate pygidial proportions and moderately and equally impressed axial, pleural and ring furrows. The pygidia differ in that thomasi exhibits a markedly raised postaxial ridge, whilst that of permutus is low.

Trimerus (Ramiotis) otisi from southern central Victoria is another contemporary of T. (R.) thomasi . Despite their geographic and temporal proximity, otisi and thomasi can be easily distinguished. The pygidia of thomasi are much more elongate, have a wider axis, and exhibit shallower pleural and ring furrows and deeper axial furrows. The cranidia of thomasi generally exhibit a more straight-sided and shorter glabellar outline, more weakly defined glabellar lobation, and a longer preglabellar field. Despite these differences, otisi and thomasi are considered to be closely related. Together with the poorly known T. (R.) sp. A from Bolivia, the species share a distinct pygidial morphology including pleural furrows that continue to a deep border furrow, and a raised ridge-like border. This pygidial morphology distinguishes this species group from Llandovery-Wenlock T. (Ramiotis). In other respects T. (R.) sp. A is morphologically intermediate between otisi and thomasi , exhibiting longer pygidial proportions of the latter and deeper pygidial pleural and ring furrows of the former.

Of the Llandovery-Wenlock species of Trimerus (Ramiotis) , T. (R.) thomasi shows greatest resemblance to T. (R.) tomczykowae from the Wenlock of Victoria (described below), that also exhibits very weak lobation, a transverse rostral suture, shorter glabellar proportions and a long preglabellar field. The species also share moderate and subequal depth of the pygidial pleural and ring furrows, distinguishing them from other Llandovery-Wenlock T. (Ramiotis). T. (R.) tomcykowae differs in having a short, rounded pygidial outline, slightly longer glabellar proportion, and a deep medial indentation of the glabellar anterior margin.

Environmental notes. Trimerus (Ramiotis) thomasi is the dominant element of monospecific or low diversity assemblages. Three cephala from PL2264 are the only known articulated specimens, isolated tergites representing 97% of the entire population, typical of taphofacies TII. The species occurs in medium-grained sandstones, associated with a shelly fauna dominated by gastropods and brachiopods. T. (R.) thomasi is considered to have inhabited inner shelf environments.

Figures

Figure 8. Geological sketch map of the Heathcote area showing Wenlock-Lochkovian fossil localities yielding homalonotids. For other fossil localities see also Thomas (1940a, 1940b, 1941, 1956), Talent (1964, fig. 1), Sandford (2002, fig. 1A; 2005, fig. 4).