Trimerus (Edgillia) kinglakensis ( Gill, 1949 )

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 : 39-41

Trimerus (Edgillia) kinglakensis ( Gill, 1949)

Homalonotus sp. — Gill, 1947: 13. Trimerus kinglakensis Gill, 1949: 67 , pl. 8 figs 1–3, pl. 9 figs 3, 5–6, text-fig. 1E.— Williams, 1964: table 1.— Wenndorf, 1990: 16.— Jell, 1992: 11, text-fig.1B.— Schraut, 2000: 382.

Type material. Holotype NMV P14580 (cephalon, figured Gill, 1949: pl. 8 figs 1, 2, pl. 9 fig. 3, text-fig. 1E, Fig. 16.12 herein, counterpart previously registered NMV P14581 ) from PL252 , Williams locality W3, Middendorps Quarry, Kinglake West , Victoria . Paratype NMV P14582 (thoracopygon, figured Gill, 1949: pl. 8 fig. 3, counterpart previously registered NMV P14583 ) from PL252 . For locality see Fig. 11 .

Previously figured material. NMV P14584 (thoracopygon, figured Gill, 1949: pl. 9 fig. 5), NMV P14585 (pygidium, figured Gill, 1949: pl. 9 fig.6, counterpart previously registered NMV P14586) from PL 252.

Registered material. 307 specimens: 2 dorsal exoskeletons, 22 articulated thorax-pygidia with displaced cephala, 29 articulated thoraxes with displaced cephala, 12 articulated thorax-pygidia with inverted cephala, 3 articulated thoraxes with inverted cephala, 3 articulated thoraxes with displaced cephala and pygidia, 110 articulated or partly disarticulated thoraxes with attached pygidia, 10 articulated or partly disarticulated thoraxes with displaced pygidia, 32 cephala, 4 cranidia, 1 librigena, 30 articulated or partly disarticulated thoraxes, 50 pygidia. NMV P149634 , P149635 , NMV P305248 – P305545 from PL252 . NMV P305546 from “ Kinglake ”, Victoria . NMV P305547 from “King Parrot Creek”, Kinglake West . NMV P304849 from PL6640 , Garratt locality M8, Heathcote Junction . NMV P304850 from PL6641 , Williams locality G96, Clonbinane . NMV P304853 , P304854 ? from PL6645 , Williams locality W47, Humevale . NMV P304852 ? from PL6648 , Humevale . NMV P304857 ? from PL6659 , Williams locality W8, Kinglake West . NMV P304856 from PL6660 , Humevale. Unregistered specimen from PL229 , Williams locality W7, Collins Quarry, Kinglake West. For localities see Fig. 11 . NMV P305548 – P305550 from “Christmas Hills”, Victoria (probably in the vicinity of PL261 ) . For locality see Jell and Holloway, 1983: fig. 1. NMV P305551 , P305552 from old Scoresby Brick Pit, Wantirna South , Victoria .

Stratigraphic distribution. Humevale Siltstone (from basal horizons at the old Scoresby Brick Pit to the Collins Quarry Sandstone Member at the top of the unit), upper Notoparmella plentiensis-Boucotia janae- Boucotia australis assemblage zones, Pr˘ídolí -Lochkovian.

Revised diagnosis. Cranidial width 1.5 times length. Glabella trapezoid, length more or less equal to width, sides straight, converging at about 15˚, anterior margin well defined. Proximal section of S1 moderately impressed, directed diagonally, glabellar lobation otherwise poorly defined. Preglabellar field 0.2 times cranidial length. Palpebral lobes placed opposite glabellar midlength/0.4 cranidial length and remote (b- b 1.55 times preoccipital glabellar width). Anterior branches of facial suture angular, parallel to axial furrows for a short distance posteriorly, converging at about 80˚ anteriorly. Librigenal border furrow weakly defined. Course of rostral suture angular, defining an obtuse angle of about 140˚, dorsal section of rostral plate boomerang-shaped. Hypostome with middle furrow very deep, posterior border with wide, short triangular lobes (length 0.1 times hypostomal length) and deep, angular medial notch. Pygidium triangular, length 0.95 times width, with long pointed tip, sides straight, converging at about 75˚. Axial furrows moderately impressed opposite first ring, deep posteriorly. Axial width 0.45 times pygidial width. 13–14 axial rings. 7 pleural ribs, rib-ring medially offset at third rib, pleural furrows markedly shallowing abaxially. Dorsal exoskeleton very strongly pitted.

Description. Exoskeleton large, maximum size estimated 25 cm (from NMV P305258), occipital/pygidial convexity (tr.) moderate. Pygidial and librigenal doublure and hypostome finely ornamented with short ridges.

Cephalon with rounded triangular outline with weakly convex sides, converging forwards at about 70˚ opposite genal field and at 115˚ opposite rostral plate, length 0.66 times width. Glabellar length about equal to width (although difficult to quantify due to the poor definition of the axial furrows posteriorly), width about 0.5 times cranidial width, sides straight to weakly convex, length 0.8 times cranidial length, anterior margin transverse, medial indentation moderately impressed to absent. Glabellar lobation very weakly defined, generally only the proximal part of S1 distinct, S2 distinct only on smallest specimens (e.g. NMV P305272, see Fig. 16.10). S3 indistinct. Occipital ring 0.1 times cranidial length, slightly wider medially. Occipital furrow deep with weak forward flexure medially. L1 0.35 times glabellar length. S1 strongly convex, weakly impressed to indistinct adjacent to axial furrow, moderately impressed and directed diagonally (posteromedially) abaxially. S2 a very weak depression adjacent to axial furrow or indistinct, rarely extending adaxially, transverse. Axial furrows shallow to indistinct and directed obliquely opposite paraglabellar area and across occipital furrow, deep anteriorly. Paraglabellar area weakly defined. Preglabellar field flat (tr. sect.). Length (exsag.) of posterior border equal to occipital length adaxially, lengthening to 1.5 times occipital length abaxially. Posterior border furrow transverse, very wide, moderately impressed, terminating distally. Postocular fixigenal area long, length (exsag.) 0.21 times cranidial length. Palpebral lobe 0.14 times cranidial length, b- b 1.64 times glabellar width, palpebral furrow indistinct. Preocular fixigenal area of moderate width, 0.17 times b- b, narrowing markedly anteriorly, eye ridges not distinct. Anterior branches of facial suture meeting connective sutures opposite 0.9 cephalic length. Librigena with wide, moderately impressed border furrow anterior of eye, not defined opposite preglabellar field, librigenal field weakly convex, steeply inclined. Dorsal section of connective sutures diverging at about 90˚. Dorsal surface of rostral plate length (sag.) 0.5 times cranidial length, very weakly concave (tr. sect.). Rostral suture moderately convex. Ventral surface of rostral plate kite-shaped, with length 0.9 times width, posterior width 0.1 maximum times width. Connective sutures angular, anterior section converging posteriorly at about 30˚, posterior section converging posteriorly at about 70˚. Librigenal doublure with wide, shallow vincular furrow, indistinct anteriorly. Hypostomal suture broadly concave.

Hypostome with length 0.75 times width, anterior wings large, width 0.35 times hypostomal width.

Thorax with thirteen segments. Axial furrows defined by shallow to moderately impressed, diagonally directed furrows on each segment, each meeting posterior margin at indentation that correspond to deep pits (axial articulating process) on internal mould. Pleural furrows wide and deep, pleural tips rounded.

Pygidium triangular, with sides weakly concave. Pygidial axis with sides straight and tapering at about 30˚. Axis reaching 0.85 times pygidial length. Terminal piece parabolic in outline, length 0.08 times axial length, posterior margin moderately to weakly defined. Postaxial ridge not defined. Ring furrows short (sag.) and deep. Pleural furrows wide and deep adjacent to axial furrows, shallowing markedly abaxially, reaching wide and very shallow border furrow. Interpleural furrows very weakly defined. Border weakly defined. Pygidial doublure flat and weakly inclined posteriorly, laterally with angular profile (tr. section), distal surface horizontal, proximal surface increasingly inclined anteriorly. In posterior view anterior margin of pygidium moderately convex, posterior margin without distinct medial arch.

Remarks. Gill’s (1949) original description is not detailed but documents most of the characters listed in the revised diagnosis. Gill described the glabellar furrows as absent, but the proximal portion of S1 is manifest as a short, moderately impressed diagonal furrow.

Trimerus (Edgillia) kinglakensis can be closely compared to the poorly known T. (E.) vanuxemi from the Helderbergian of North America. In addition to the shared cephalic and pygidial features listed above as defining T. (Edgillia), the species share a forwardly convex rostral suture, a similar pygidial rib-ring offset (third–fourth? rib). The species differ in that vanuxemi exhibits longer glabellar proportions, shorter cephalic and pygidial proportions, a greater number of pleural ribs, a wider pygidial axis, and lacks a produced pygidial tip. In the depth of the pygidial axial furrows and ring and pleural furrows, kinglakensis shows closer resemblance to T. (E?) major , also from the Helderbergian of North America. However, major differs from kinglakensis in that the pleural furrows do not shallow abaxially. Pygidial morphology of the Late Silurian T. (E.) mongolicus is closely comparable to that of kinglakensis . The Mongolian species exhibits similar proportions, 13 pygidial axial rings, deep ring furrows and shallow pygidial pleural furrows and an acuminate tip. T. (E.) kinglakensis differs in having deeper pygidial axial furrows, a shorter preglabellar field, and in the strong convexity of the rostral suture. Although the course of the rostral suture has been emphasised in the diagnoses of higher taxa (e.g. medial flexure in Burmeisteria , concavity in Digonus : see discussion above), variability in the forward curvature of the rostral suture amongst Trimerus is considered only of specific significance. Apart from kinglakensis and vanuxemi , few species of Trimerus exhibit strongly forwardly convex rostral sutures, the most notable exceptions being the type species T. (T.) delphinocephalus and T. (T.) flexuosus . Other features of these latter species are considered characteristic of T. ( Trimerus ) and T. (Ramiotis) respectively, and indicate that they are not closely related to kinglakensis .

Environmental notes. See discussion on taphonomy and biofacies. Trimerus (Edgillia) kinglakensis is considered to have inhabited mid- to outer-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).