Homalonotus talenti, 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 : 16-19

Homalonotus talenti sp. nov.

P304917–P304951, NMV P304959–305026 from PL6650. NMV P304952–P304957, NMV P305027–P305029 from PL6651. NMV P304958, NMV P305041 from PL2265, Thomas locality F46, Parish of Heathcote, Heathcote. NMV P78296, P78297 from PL2239. NMV P305030–P305040 from PL2207, Thomas locality F7, Parish of Dargile, Heathcote. For localities see Fig. 8 .

Stratigraphic distribution. Upper beds of the McIvor Sandstone and lowermost beds of the Cornella Member of the Mt Ida Formation, Notoparmella plentiensis Assemblage Zone , late Ludlow. The age of the McIvor Sandstone is constrained by early Ludlow (upper nilssoni Biozone) graptolite assemblages from the underlying Melbourne Formation ( Rickards and Sandford, 1998). In Europe, Homalonotus occurs in abundance in strata of Ludfordian age, occurring rarely in the Gorstian ( Thomas et al., 1989) and possibly ranging into the Pr˘ídolí. The first appearance of H. talenti about 380 m above the base of the McIvor Sandstone (at PL2265) is considered here to approximate the Gorstian-Ludfordian boundary. The Ludlow-Pr˘ídolí boundary can be placed within the interval between the last appearance of talenti (at PL2239) and the first appearance of the post-Ludlow brachiopod Cyrtina at a slightly higher horizon, 50–100 m above the base of Cornella Member.

Derivation of name. For John A. Talent (Macquarie University), for his contribution to Victorian palaeontology.

Diagnosis. Glabella trapezoid, sides straight and converging at 20º, anterior margin broadly rounded. Glabellar length about 1.1 times preoccipital glabellar width and 0.94 times cranidial length. Preglabellar furrow very deeply impressed. Eye placed with midline of palpebral lobe opposite 0.56 glabellar length/0.52 cranidial length. Dorsal surface of rostral plate 0.14 times cephalic length. Hypostome with length 0.85 width, lobes on the posterior border with length 0.17 times hypostomal length. Pygidium with length about 0.9 times width, postaxial ridge projecting posteriorly, tip rounded. Pygidial axis with width 0.5 times pygidial width, 11 axial rings. Axial furrows straight and tapering at about 30º. 7 pleural ribs, rib-ring medially offset at fourth rib. Ring, pleural and axial furrows shallow to moderately impressed.

Course of rostral suture broadly rounded, concentric to anterior margin of glabella. Dorsal surface of rostral plate rounded triangular, length (sag.) 0.4 times width, strongly concave (tr. sect.). On cephalic doublure connective sutures straight and weakly converging posteriorly. Librigenal doublure without distinct vincular furrow, strongly convex (exsag. sect.) adjacent to connective suture (i.e. accommodating preglabellar furrow). Hypostomal suture very broadly rounded.

Hypostome with middle furrow shallow to moderately impressed, anterior wing process small (width 0.1 times hypostomal width), lobes on posterior border parabolic in outline, deep medial notch with sides converging at about 75º.

Thorax with axial furrows poorly defined. Pleural furrows narrow (exsag.) and deep, pleural tips rounded.

Pygidium with triangular outline, sides weakly convex and converging at about 80º. In lateral view pygidium high, height equal to length. Pygidial axis reaching to about 0.8 times pygidial length, continuous posteriorly with wide postaxial ridge. Postaxial ridge raised, parallel-sided, wide, width (tr.) 0.45 times axial width, posterior margin with parabolic outline. Axial furrows tapering, curving to the exsagittal posteriorly, shallow anteriorly, moderately impressed posteriorly. Pleural furrows shallowing markedly adjacent to border furrow. Border furrow moderately impressed opposite pleural field, shallow opposite postaxial ridge. In dorsal view border narrow (tr.) but protuberant from margin of pleural field. In lateral view border very wide anteriorly, merging with wide articulating facet, narrowing markedly posteriorly. Lateral border continuous with narrow pygidial doublure. In posterior view anterior margin of pygidium strongly convex, posterior margin with distinct medial and lateral arches. In ventral view, inner margin of doublure parabolic in outline with deep medial indentation.

Dorsal exoskeleton finely granulose. Pygidial border with fine ridging. Discussion. Holloway and Neil (1982) suggested the affinities of an incomplete cephalon examined by them were with Homalonotus . Their suggestion is confirmed with the additional specimens listed. They noted similarities between the associated pygidium and that of the South American Lower Devonian species H. clarkei , but these similarities are superficial. Following Cooper (1982), and as discussed below, clarkei is assigned to Burmeisteria .

Hypostomes are also known for Homalonotus knightii and H. rhinotropis ( Salter, 1865: pl. 12 fig. 10; Angelin, 1878: pl. 20 fig. 1c). The hypostome of H. talenti most closely resembles that of the type species. The hypostome of rhinotropis is relatively elongate (length times 1.1 width) compared to that of knightii (length 0.81 times width) and talenti (length 0.85 times width). The projections of the posterior margin are broadly based and long in talenti and knightii , but short in rhinotropis .

Although the ventral surface of the rostral plate of Homalonotus talenti is not known, the course of the connective sutures (indicated by the shape of the librigenae) is more or less straight and slightly convergent posteriorly, indicating the rostral plate to be approximately pentagonal, with subequal sides. Apparent curvature of the connective sutures on Fig. 9.14 is due to the oblique orientation of the specimen.

The broad, rounded medial indentation of the inner margin of the pygidial doublure of Homalonotus talenti (Fig. 10.7e) differs from that of H. knightii , in which the margin is more or less straight laterally, and defines an acute angle medially (see Salter, 1865: pl. 12 fig. 9 a, Tomczykowa, 1975: pl. 1 fig. 5c).

Environmental notes. Throughout its range Homalonotus talenti occurs in thick-bedded fine- to medium-grained sandstones, in low diversity or monospecific assemblages. At the type locality where the species occurs in greatest abundance the proportion of articulated specimens is low (6%) and the proportion of broken specimens low (3%).The fauna can be assigned to taphofacies TII and indicates shallow environments around normal wave base.

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).