Trimerus (Edgillia) jelli, 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 : 41-43

Trimerus (Edgillia) jelli sp. nov.

Homalonotidae gen. et sp. indet. 3— Holloway and Neil, 1982: 146, fig. 4M–R.

Type material. Holotype NMV P82873 (pygidium) from PL2288 , Thomas locality F25, Parish of Redcastle, Heathcote , Victoria (Fig. 17.9) . Paratypes NMV P78300 (ex GSV48930 , figured Holloway and Neil, 1982: fig. 4 R) , NMV P304610 (cranidia) , NMV P304612 (thoracic segment) , NMV P78298 (pygidium, ex GSV48942 , figured Holloway and Neil, 1982: figs 4M, 4N, 4Q) from PL2319 , Thomas locality F55, Parish of Redcastle , Heathcote . Paratypes NMV P304638 (cranidium) , NMV P304633–4 (librigenae) , NMV P304635 (hypostome) , NMV P304636 , P304637 (thoracic segments) , NMV P304639 (pygidium) from PL6652 , Heathcote . Paratype NMV P78299 (librigena, figured Holloway and Neil, 1982: figs 4O, 4P) from PL 2288. For localities see Fig. 8 .

Registered material. 45 specimens: 6 cranidia, 3 librigenae, 1 hypostome, 25 thoracic segments, 8 pygidia. NMV P82925 (ex GSV48928 ), NMV P304610 – P304625 , NMV P304626 (ex GSV48933 ), GSV48944 , GSV48938 from PL2319 . NMV P82873 , NMV P304628 – P304632 from PL2288 . NMV P304633 – P304647 from PL6652 . Unregistered specimen from PL2328 , Heathcote .

Stratigraphic distribution. Stoddart Member, Mt Ida Formation, 200 m to 500 m above the base of the unit, Boucotia australis Assemblage Zone , mid-late Lochkovian.

Derivation of name. For Peter Jell (Queensland Museum), for his contribution to Victorian palaeontology.

Diagnosis. Glabella with very weakly defined lobation, sides straight and converging at about 15˚, anterior margin well defined, transverse, with broad medial depression. Axial furrows poorly defined posteriorly. Preglabellar field long, length 0.45 times anterior width of glabella (estimated 0.25 times cranidial length). Anterior branches of facial suture converging at 40˚. Anterior margin of cranidium (including rostral suture) transverse. Palpebral lobe placed with midline opposite (estimated) 0.35 cranidial length. Hypostome with large, broadbased rounded lobes on the posterior margin. Thoracic pleurae with bilobed tips. Pygidium triangular, length equalling width, sides straight, converging at 60˚. Axis with 10 rings, width 0.46 times pygidial width, length 0.8 (estimated) times pygidial length. Axial furrows very shallow. 6 pleural ribs. Pleural offset at third rib. Ring furrows moderately impressed, pleural furrows very shallow to indistinct, increasingly so posteriorly.

Discussion. Although most specimens assigned to this species are fragmentary, the morphology is distinctive enough to warrant description. The weakly convergent glabellar sides, the transverse and well-defined anterior margin and the simple glabellar morphology support assignment to Edgillia.

In cephalic features Trimerus (Edgillia) jelli differs from T. (E.) kinglakensis and T. (E.) vanuxemi in the transverse course of the rostral suture. Pygidial axial, ring and pleural furrows of vanuxemi are moderate to shallow in depth. In kinglakensis , the axial furrows and ring furrows are deep, with pleural furrows shallower, and markedly shallowing distally. The pygidium of jelli also exhibits pleural furrows shallower than the ring furrows, but is distinctive in that the pleural furrows are very shallow. The weakly defined segmentation is comparable to that of the Llandovery-lower Wenlock species of T. (Ramiotis), although jelli differs from these in having a more elongate pygidial outline and in lacking the raised postaxial ridge typical of T. (Ramiotis). In these respects the pygidia of jelli more closely resemble pygidia of species assigned in this work to Burmeisteria , in particular B. clarkei and B. linares . However, the strong segmentation shared by clarkei and linares suggest that similarities in the pygidial outline and depth of pygidial axial ring and pleural furrows are not of high taxonomic significance. The distinctive (straight-sided, weakly furrowed) pygidial morphology shared by these three taxa and others such as as Dipleura garratti suggests the morphology is homologous.

Hypostomes are known for few species of Trimerus . The hypostome of T. (Edgillia) jelli (Fig. 17.12) closely resembles that of T. (E.) kinglakensis ( Fig. 16.2). These both differ from the hypostome of T. (T.) delphinocephalus ( Whittington, 1993: fig. 1B) and T. (Ramiotis) rickardsi ( Fig. 18.6) in having larger posterior border lobes and deeper middle furrows.

Environmental notes. Trimerus (Edgillia) jelli is considered to have inhabited high-energy, very shallow 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).