Trimerus (Ramiotis) tomczykowae, 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 : 54

Trimerus (Ramiotis) tomczykowae sp. nov.

Type material. Holotype NMV P138203 (pygidium) from the “ Illaenus band”, Costerfield , Victoria (Fig. 19.8) . Paratype NMV P305191 (cranidium) from PL 2263, Thomas locality F44, Parish of Dargile , Costerfield . Paratypes NMV P138292 , P138293 (rostral plates) from PL 1460, Thomas locality F43A, Parish of Dargile , Costerfield. For localities see Fig. 10 .

Registered material. 8 specimens: 1 cephalon, 3 cranidia, 2 rostral plates, 2 pygidia. NMV P138291 – P138294 , NMV P138296 , NMV P139358 from PL1460 . NMV P305191 from PL2263 . NMV P138203 from the “ Illaenus band” .

Stratigraphic distribution. Illaenus band, Wapentake Formation. Rickards and Sandford (1998) interpreted Ananaspis typhlagogus to indicate a Wenlock age for these beds.

Derivation of name. For Ewa Tomczykowa (Instytut

Geologiczny, Poland) for her contribution to the study of homalonotids.

Diagnosis. Glabella trapezoid, elongate, length ~1.1 times width, sides parallel opposite paraglabellar area, straight and converging at about 20˚ anteriorly, with extremely weak lobation, anterior margin strongly indented. Paraglabellar area distinct. Palpebral lobe placed at about 0.55 glabellar length. Rostral suture more or less transverse. Dorsal surface of rostral plate long, one third length of ventral surface, anterior margin of rostral plate with sides converging at 115˚, rounded medially. Connective sutures straight, diverging forwards at 55˚. Ventral surface of rostral plate flat, length equalling width. Pygidial axial furrows moderately impressed. Axis about 0.4 times pygidial width, sides straight and tapering at 25˚, posterior third of axis moderately convex and raised, terminal piece continuous with and not distinguishable from wide, raised, subparallel-sided postaxial ridge. 10 axial rings, ring furrows moderately impressed, 7 pleural ribs, pleural furrows moderately impressed and equal in depth to ring furrows, shallowing distally. Pleural offset at fourth rib.

Discussion. Trimerus (Ramiotis) tomczykowae is a very rare element of the Illaenus band fauna. Despite the very few and fragmentary specimens available, a fairly complete reconstruction of the cephalon can be made, and together with the single, well-preserved pygidium clearly represents a new species of homalonotid and a full description of the species is possible with the material at hand. Significant cephalic characters include the elongate, trapezoid, straight-sided, moderately tapering glabella, the weakly expressed glabellar lobation, the moderate length of the preglabellar field (0.25 cranidial length) and the short pygidial proportions (L/W~0.9) indicate assignment to T. (Ramiotis). In cephalic features tomczykowae differs from the type species T. (R.) rickardsi in having a transverse rostral suture and strong medial indentation of the anterior glabellar margin. In these respects tomczykowae is closest to T. (R.) dyaulax .

The equal depth of pleural and ring furrows attained by Trimerus (Ramiotis) tomczykowae is considered here to represent a significant stage of homalonotine development. This feature distinguishes Wenlock-Ludlow T. (Ramiotis) from Llandovery members of the group. Although in its short pygidial proportions and rounded outline the affinities of tomczykowae to Llandovery T. (Ramiotis) are clear, it represents a transitional morphology between these and Upper Silurian species of T. (Ramiotis) and T. ( Trimerus ).

Indeterminate homalonotids from the Wenlock of central Victoria include a partly disarticulated exoskeleton from the old Tooronga Brick Pit, high in the Anderson Creek Formation, only briefly examined by the author. The abundant and diverse trilobite fauna occurring in the Bylands Siltstone at PL 206, Wallan has yielded only an single homalonotid thoracic segment.

Environmental notes. Trimerus (Ramiotis) tomczykowae is a very rare element of a moderately diverse fauna occurring in the nodular mudstones of the Illaenus band. The fauna is dominated by the blind illaenid Thomastus thomastus Öpik, 1953 . Together with Ananaspis typhlagogus the composition of the fauna closely resembles a contemporary fauna from Argentina described by Waisfeld and Sanchez (1993). The taphonomy of the Illaenus band is characterised by an abundance of complete exoskeletons of Thomastus in the ‘bumastoid stance’, presumed to be individuals preserved in their infaunal life position ( Sandford and Holloway, 1998). The taphonomy and the autecology of Thomastus in the mudstones suggest a deeper water environment for tomczykowae . Several specimens of tomczykowae occur in a richly fossiliferous sandstone presumably interbedded within the mudstone, although the bed is not known in outcrop.