Trimerus (Ramiotis) rickardsi, 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 : 44-47

Trimerus (Ramiotis) rickardsi sp. nov.

Figure 18

Type material. Holotype NMV P305556 (cephalon) from PL6361 , Springfield , Victoria ( Fig. 18.1) . Paratypes NMV P305553 , P305554 , NMV P305557 , NMV P305563 (cephala) , NMV P305559 – P305562 , NMV P305564 (cranidia) , NMV P305582 (thoracic segment) , NMV P305572 – P305574 (pygidia) from PL6361 . Paratype NMV P138204 (pygidium) from PL 667, Springfield. Paratype NMV P138207 (pygidium) from PL 599, Springfield. Paratypes NMV P147779 (hypostome) , NMV P147772 (pygidium) from PL256 , Wallan, Victoria. For localities see Fig. 11 .

Registered material. 66 specimens. 5 cephala, 15 cranidia, 6 librigenae, 1 rostral plate, 3 hypostomes, 12 thoracic segments, 24 pygidia. NMV P305553 – P305593 from PL6361 . NMV P305626 – P305628 from PL6390 , Springfield. NMV P138213 , NMV P147771 , P147772 , NMV P147774 – P147777 , NMV P147779 – P147780 , NMV P1477787 from PL256 . NMV P139442 – P139446 , NMV P305595 from “Lancefield”, Victoria. NMV P138205 – P138207 , NMV P138270 from PL599 . NMV P138204 , NMV P305594 from PL667 . NMV P138267 , NMV P147778 from PL1964 , GSV locality B25, Springfield. Lithological similarities suggests that the specimens labelled as coming from “Lancefield” almost certainly come from the Parish of Goldie , and possibly in the vicinity of allotments B19 and B23 (see Thomas, 1960: 1:31, 680 Lancefield Geological Sheet) .

Stratigraphic horizon. Chintin Formation, upper Llandovery. Rickards and Sandford (1998) suggested equivalence to the upper crenulata Biozone, based on the presence of Acernaspis sp. within the unit, and on the occurrence of crenulata Biozone graptolites in the uppermost beds of the underlying Springfield Sandstone.

Derivation of name. For R. B. Rickards (University of Cambridge), for his contribution to Victorian palaeontology.

Diagnosis. Cephalon rounded pentagonal, width about 1.6 times length, sides converging at 60˚ posteriorly, at 125˚ anteriorly. Glabella trapezoid, length 1.2 times width, sides subparallel opposite paraglabellar areas, converging anteriorly at about 22˚. S1-S3 moderately impressed adjacent to the axial furrow, shallow adaxially. Preglabellar field long, 0.23 times cephalic length. Palpebral lobes placed opposite 0.42 cranidial length/0.52 glabellar length. Anterior branches of facial suture straight, converging at about 45˚, anterior-most section curving gently to the diagonal, continuous in curvature with broadly rounded rostral suture. Dorsal section of rostral plate crescentic, length 0.1 times cephalic length. Ventral section of rostral plate with length 0.92 times width, connective sutures converging posteriorly at 50˚. Posterior border of hypostome with short lobes of length 0.1 times hypostomal length. Pygidium wide, length 0.78 times width, with sides weakly convex, converging posteriorly at about 100˚. Pygidial axis 0.42 times pygidial width, with 10 axial rings. Axial furrows straight, tapering at about 30˚, shallow anteriorly, moderately impressed posteriorly. Ring furrows deep to moderately impressed, pleural furrows shallow. 7–8 ribs, rib-ring medially offset at fourth rib.

Description. Exoskeleton small, maximum length 12 cm (estimated from NMV P138204), occipital and pygidial convexity (tr.) moderate. Dorsal exoskeleton and doublure finely granulose.

Cranidium with length (sag.) 0.9 times cephalic length. Glabella trapezoid, length about 0.77 times cranidial length, width about 0.43 times cranidial width, anterior margin moderately well defined, broadly rounded, medial indentation variably expressed (indistinct to strong, e.g. NMV P305559). Glabellar lobation well defined. Occipital ring 0.09 times glabellar length, slightly wider medially. Occipital furrow deeply impressed with broad forward flexure medially. L1 ~0.3 times glabellar length, L2 and L3 ~0.1 times glabellar length and frontal lobe ~0.25 times glabellar length. S1 weakly convex forwards and directed posteromedially at about 25˚ from the transverse. S2 transverse. S3 directed antero-medially at about 10˚ from the transverse. Axial furrows moderately impressed opposite genae, shallow and directed diagonally opposite occipital ring. Paraglabellar area very weakly defined. Length (exsag.) of posterior border equal to occipital length adaxially, lengthening (exsag.) markedly abaxially to about twice occipital length. Posterior border furrow transverse, very wide, moderately impressed, terminating distally. Postocular fixigenal area with length (exsag.) 0.22 times cranidial length. Palpebral lobe short, 0.1 times cranidial length, placed with b- b about 1.77 times preoccipital glabellar width/0.74 times cephalic width. Palpebral furrow wide and shallow. Preocular fixigenal area narrow (tr.), 0.17 times b- b, uniform in width anteriorly. Preglabellar field flat (tr. sect.). Anterior branches of facial suture straight from eye to a point opposite midlength (sag.) of preglabellar field, anterior section curved. Librigenal border furrow wide and shallow opposite genal field, not defined opposite preglabellar field, border furrow indistinct on fixigenae. Lateral border narrow and convex, narrowing posteriorly, indistinct opposite preglabellar field and on fixigenae. Dorsal surface of rostral plate with length 0.13 times width, anterior margin rounded. Dorsal section of connective sutures diverging forwards at about 30˚. Ventral surface of rostral plate flat (tr, sag. sect.), kite-shaped, sides very weakly sinusoidal. Librigenal doublure without distinct vincular furrow. Hypostomal suture transverse.

Hypostome with moderately impressed middle furrow.

Thoracic segments with wide (sag.), deep articulating furrows, deep and narow (exsag.) pleural furrows. Axis very weakly defined by very shallow, wide, diagonally directed furrow. Pleural tips weakly bilobed with larger lobe opposite posterior band and smaller lobe opposite anterior band.

Pygidium triangular. Pygidial axis extending to about 0.9 pygidial length, with semicircular terminal piece, length 0.1 times axial length, continuous posteriorly with raised postaxial ridge. Ring furrows deep to moderately impressed, pleural furrows shallow. Furrows markedly shallower on external casts. Pleural furrows not reaching margin. Border furrow and border poorly defined. In posterior view posterior margin of pygidium horizontal. In lateral view dorsal profile of axis moderately convex, postaxial ridge steeply inclined.

Discussion. Trimerus (Ramiotis) rickardsi is the most completely known homalonotid from the Llandovery. The species is of primary significance in the understanding of relationships between poorly known Llandovery homalonotids and betterknown Wenlock and Ludlow taxa.

In pygidial morphology Trimerus (Ramiotis) rickardsi is typical of other Llandovery-Wenlock species, particularly in having short pygidial proportions and much shallower ring furrows relative to the pleural furrows. In the latter feature rickardsi most closely resembles the very poorly known Welsh T. (R.) sp. (see Curtis and Lane, 1998), but can be distinguished by its higher segmentation (7 rings, 6 ribs in T. (R.) sp., cf. 10 rings, 8 ribs in rickardsi ). T. (R.) dyaulax differs from rickardsi in having even shorter proportions, a rounded pygidial tip, a low postaxial ridge, even shallower axial, ring and pleural furrows and lower segmentation (6 rings, 5 ribs) than counted for rickardsi . Pygidial morphology of T. (R.) iani from the upper Llandovery of Tasmania is poorly known. The single pygidium known exhibits segmentation comparable (9 rings, 6 ribs) to that of rickardsi , but differs in having shorter proportions and a rounded tip. The relative depth of the ring furrows compared to the pleural furrows distinguishes rickardsi from the Wenlock T. (R.) tomczykowae from Victoria, and from other Ludlow species of T. (Ramiotis). T. (R.) tomczykowae further differs in having a more rounded tip. Of the Upper Silurian species, T. (R.) thomasi from Victoria differs markedly in having elongate proportions (L:W~1.0), whilst pygidia of T. (R.) otisi from Victoria differ markedly in the depth of the pygidial pleural and ring furrows. Despite the pygidial differences, in the more elongate glabellar outline (L/W~1.2) the longer preglabellar field (0.25 cranidial length), and the expression of glabellar lobation, rickardsi shows closest resemblance to otisi . T. (R.) rickardsi differs from most other T. (Ramiotis) in having a forwardly convex rostral suture, comparable to that of the Llandovery Bolivian T. (R.) sp.

In having a relatively longer preglabellar field, lateral glabellar furrows of moderate length, a weak sagittal glabellar ridge and, in some specimens, a distinct medial indentation of the anterior glabellar margin, and higher pygidial segmentation Trimerus (Ramiotis) rickardsi shows closer affinities to T. ( Trimerus ) than other Llandovery T. (Ramiotis). However, in the absence of so many of the other derived features distinguishing T. ( Trimerus ), it would be over stated to regard rickardsi as a transitional species between the subgenera.

Environmental notes. Articulated specimens are only known from the type locality, and represented only by cephala. The proportion of articulated and broken specimens is 13% respectively, indicative of a taphofacies TII. Further east at PL256, Wallan, breakage of T. (R.) rickardsi and other trilobites is extremely high and indicates significant reworking. Associated tempestite lithologies comprising thick coquinal bioclastic lags and abundant mud rip-up clasts indicate deposition between normal wave base and storm wave base. The trilobite fauna may have been transport, possibly in a tempestite-generated mass-flow, although the profound compositional differences between the Springfield, Wallan and Lancefield faunas do not support this and suggest distinct, depth-related assemblages. Trimerus (Ramiotis) rickardsi is considered to inhabit inner shelf environments.

Figures

Figure 11. Geological sketch map of the Springfield-Kinglake West area showing Llandovery-Lochkovian fossil localities yielding homalonotids. For other fossil localities see also Jutson (1908, pl. 3), Thomas (1960), Talent (1964, fig. 1), Williams (1964, fig. 2), Garratt (1972, 1977), Rickards and Sandford (1998, fig. 6), Sandford (2002, fig. 1C; 2005, figs 2–3).