Dipleura garratti, 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 : 30-32

Dipleura garratti sp. nov.

Figure 14 Homalonotus sp. — Chapman, 1908: 220.

Type material. Holotype NMV P308644 (pygidium) from PL6615 , Eden Park , Victoria ( Fig. 14.13) . Paratypes NMV P308638 , NMV P308641 (cranidia) , NMV P308639 , NMV P308642 , NMV P308645 (cephala) , NMV P308643 (librigena) , NMV P308637 , NMV P308640 , NMV P308646 (pygidia) from PL6615 . Paratypes NMV P308635-6 (cephalothoraxes) , NMV P308649 , NMV P308651 , NMV P308663 , (cephala) , NMV P308657 (cranidium) from PL6614 , Eden Park. NMV P308648 from “ Whittlesea ”, Victoria. For localities see Fig. 11 .

Registered material. 47 specimens: 4 cephalothoraxes, 8 cephala, 17 cranidia, 2 librigenae, 2 thoracic segments, 14 pygidia. NMV P304513 – P304515 , NMV P304572 , P304573 , NMV P308633 – P308636 , NMV P308649 – P308657 , NMV P308673 from PL6614 . NMV P308637 – P308647 , NMV P308659 – P308672 from PL6615 . NMV P308658 from PL6625 , Wandong, Victoria. NMV P308648 from “Whittlesea”. Unregistered specimens from PL1793 , Clonbinane and Kenley locality 14c, Upper Plenty. For localities see Fig. 11 .

Stratigraphic distribution. As for Homalonotus williamsi .

Derivation of name. For Michael J. Garratt, for his contribution to Victorian palaeontology and stratigraphy.

Diagnosis. Glabella trapezoid, length 1.05 times preoccipital glabellar width, sides straight and converging at about 25˚, anterior margin well defined, very broadly rounded to transverse. Length (sag.) of preglabellar field 0.15–0.18 times cranidial length. Palpebral lobe placed with midline opposite 0.47 times glabellar length/0.38 times cranidial length. Anterior branches of facial suture angular, subparallel to axial furrows to a point opposite 0.75 glabellar length, anteriorly converging at about 80˚. Rostral suture broadly curved. Ventral surface of rostral plate with length about equal to width, flat, connective sutures angular, anterior section converging at 40 degrees, posterior section at 80˚. Pygidium triangular, length equal to width, sides straight and converging at 65˚, tip acutely angular. Pygidial axis with width 0.5 times pygidial width, 12 axial rings, ring furrows moderately impressed, axis moderately swollen posteriorly, continuous with wide but poorly defined postaxial ridge. Axial furrows straight and tapering at about 33˚, weakly impressed to indistinct. Pleural furrows weakly impressed to indistinct. 6 pleural ribs, rib-ring medially offset at second rib.

Description. Exoskeleton small (maximum length estimated 8 cm from NMV P308644), occipital and pygidial convexity (tr.) moderate. Dorsal exoskeleton finely granulose.

Cephalon with width about 1.6 times length, with semielliptic outline, sides moderately convex. Cranidial width about 1.64 times length. Glabellar length about 0.8 times cranidial length. Occipital ring 0.12 times glabellar length, slightly wider medially. Occipital furrow deeply impressed, with weak forward flexure medially. Glabellar lobation extremely weak to indistinct, best seen on NMV P308638 as very shallow depression at adaxial end of S1 placed opposite 0.3 times glabellar length. Paraglabellar area very weakly defined. Length (exsag.) of posterior border equal to occipital length adaxially, lengthening slightly abaxially. Posterior border furrow transverse adaxially, curving gently forwards abaxially, very wide, moderately impressed, terminating distally. Postocular fixigenal area very short, length (exsag.) 0.15 times cranidial length. Palpebral lobes placed remotely (b- b 1.65 times preoccipital glabellar width). Palpebral lobe length (exsag.) 0.15 times cranidial length, palpebral furrow indistinct. Preocular fixigenal area of moderate width, 0.18 b- b, narrowing slightly anteriorly, eye ridges very weakly defined. Librigena without distinct border furrow or lateral border. Dorsal surface of rostral plate very short (sag.), crescentic in outline. Ventral surface of rostral plate kite-shaped, posterior width 0.2 times maximum width. Hypostomal suture extremely weakly curved.

Thorax with thirteen segments. Axial furrows extremely shallow, expressed as diagonally directed furrows on each segment, each meeting posterior margin at axial articulating process on internal mould. Pleural furrows wide and deep across axis, narrower (exsag.) and deeper across pleural field.

Pygidial border furrow and border poorly defined. In posterior view posterior margin of pygidium horizontal. In lateral view dorsal profile evenly inclined, interrupted by swelling of terminal piece.

Discussion. The assignment of this species to Dipleura is indicated by the course of the facial sutures (anterior branches converging at 90˚), the posterior position of the eye and the almost effaced pygidial axial furrows and pleural furrows. The short preglabellar field and indistinct glabellar lobation are in accord with this assignment. D. garratti is the earliest representative of the genus. A number of differences with later Dipleura can be interpreted as reflecting the closer affinities of garratti with Trimerus , particularly its triangular pygidial outline and its posteriorly raised axis that is continuous with a wide postaxial ridge ( Fig. 14.12).

In many features Dipleura garratti is most closely comparable to the German upper Pragian-lower Emsian D. laevicauda , sharing a moderately tapered glabellar outline, more elongate pygidial proportions and a relatively wide pygidial axis. The course of the cephalic sutures is most like that of the Bolivian Givetian D. boliviensis . Originally described as a subspecies of D. dekayi , the relatively longer cephalic proportions, more tapering glabella, more anteriorly and less remotely placed eyes are significant in regarding boliviensis as an independent species.

Environmental notes. Dipleura garratti occurs in a trilobite fauna dominated by proetids and dalmanitids, with Homalonotus williamsi , aulacopleurids, odontopleurids and phacopids in very low abundance. Although known from various localities between Clonbinane and Eden Park, the fauna is well represented at two richly fossiliferous localities in the Macropleura band at Eden Park, a horizon characterised by an abundance of the large, thick-shelled brachiopod M. densilineata . At PL6615, dalmanitids (relative abundance 50%) are more abundant than proetids (relative abundance 21%), whereas at PL6614 the proetids are dominant (relative abundance 55%) over dalmanitids (relative abundance 26%). The relative abundances of garratti and williamsi are similar at both localities (17-19% and 2% respectively).

The taphonomy and facies of Dipleura garratti differs markedly between PL6614 and PL6615. At PL6615, 87% are isolated tergites and the remainder are cephala. With the exception of the cephalon of Homalonotus williamsi all other trilobites associated with garratti are represented by isolated tergites, with the proportion of broken specimens 16%. The lithology is best described as a bioclastic coquina in a siltstone matrix. Assignment to shallow taphofacies TII is in accord with Garratt’s (1983) assignment of the brachiopod fauna to the shallow-water Notoconchidium Community (BA2). A moderate energy environment at depths around normal wave base is indicated. Slightly deeper conditions are indicated by the taphonomy and lithofacies at PL6614, where 21% of specimens of garratti are cephala and 21% are cephalothoraxes. The proportion of broken specimens (9%) is lower than at PL6615. The lithology at PL6614 is a poorly bedded micaceous siltstone, and although bioclasts are extremely abundant they have not been winnowed to form a coquina. The preservation suggests an environment at depths below normal wave base.

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).