Wenndorfia lilydalensis ( 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 : 60-62

Wenndorfia lilydalensis ( Gill, 1949)

Figures 5C , 24

Homalonotus harrisoni .— Cresswell, 1894: 156.

Homalonotus sp. — Chapman, 1907: 239.— Gill, 1938: 170.

Trimerus lilydalensis Gill, 1949: 69 , pl. 8 figs 4–5, pl. 9 fig. 7, text–fig.1F— Holloway and Neil, 1982: 145.

Type material. Holotype NMV P14587 (cephalon, figured Gill, 1949: pl. 8 figs 4–5, text–fig. 1F, Fig. 24.1 herein, counterpart previously registered NMV P14588 ) from PL1801 , Gill locality 1, Mooroolbark , Victoria . Paratype NMV P14589 (pygidium, figured Gill, 1949: pl. 9 fig. 7) from PL 1801. For locality see Fig. 23 .

Registered material. 57 specimens: 1 articulated dorsal exoskeleton, 1 disarticulated dorsal exoskeleton, 1 enrolled cephalothorax, 9 cephala, 7 cranidia, 3 librigenae, 12 thoracic segments, 23 pygidia. NMV P304527 from PL1805 , Gill locality 5, Coldstream, Victoria. NMV P304531 , P304532 from PL1990 , Coldstream. NMV P304528 – P304530 from PL1813 , Gill locality 13, Mooroolbark. NMV P304524 – P304526 from PL1887 , Gill locality 87, Mooroolbark. NMV P3204533 , NMV P304570 from PL1869 , Gill locality 69, Mooroolbark. NMV P304549 – P304558 from PL1801 . NMV P304567 , P304569 from PL6661 , Lilydale , Victoria. NMV P457 , NMV P304559 – P304562 from PL1802 , Gill locality 2, Lilydale. NMV P304516 – P304518 from PL1810 , Gill locality 10, Lilydale. NMV P304522 from PL1858 , Gill locality 58, Lilydale. NMV P304534 – P304548 , NMV P304566 from PL1859 , Gill locality 59, Lilydale. NMV P304523 from PL1862 , Gill locality 62, Lilydale. NMV P304520 , P304521 from PL1829 , Gill locality 29, Kilsyth , Victoria. NMV P304563 – P304565 from “Lilydale”. NMV P304519 from “within 1 km of Mooroolbark Railway Station”. NMV P304571 probably from the Lilydale area . For localities see Fig. 23 .

Stratigraphic distribution. Humevale Siltstone, from 1750 m above the base of the unit up to a horizon 3700 m above the base of the unit, upper Boucotia australis Assemblage Zone to lower Boucotia loy-olensis Assemblage Zone , late Lochkovian–earliest Pragian.

Diagnosis. Cephalon weakly pentangular in outline, with length 0.56 times width. Glabellar length equalling width, sides straight, weakly tapering (15˚), strongly rounded anteriorly. Glabellar lobation weakly defined. Palpebral lobe placed with midline opposite 0.5 cranidial length. Preglabellar field (excluding rostral plate) 0.15 times cranidial length. Anterior cranidial margin evenly rounded, concentric with glabellar anterior margin. Dorsal surface of rostral plate crescentic in shape, length (sag.) 0.05 times cephalic length, anterior margin obtusely angular (130˚). Ventral surface of rostral plate wide, kite-shaped, width 1.15 times length. Pygidium triangular, length 0.8 times width, tip obtusely pointed (~100˚). Axis very wide anteriorly, width 0.66 times pygidial width, length 0.92 times pygidial length, sides straight. 9-11 axial rings, 8 pleural ribs, ring and pleural furrows deep. Surface of cephalon with fine tubercles, more distinct and densely distributed adjacent to the cephalic margin.

Description. Exoskeleton with maximum size moderate (estimated 20 cm from NMV P304562).

Cephalon with pentangular outline, sides converging forwards at 70-75˚ opposite glabella and at about 135˚ opposite preglabellar field, length about 0.57 times width. Glabella with length 0.75 times cephalic length, width 0.45 times cephalic width, sides converging at 13-17˚, length ranging between 1.06-0.91 times width. Occipital ring length 0.1 times cranidial length, slightly wider medially. Occipital furrow deep on internal moulds, shallow on external moulds, weak forward flexure medially. Glabellar anterior margin well defined, with arc of curvature centred at 0.55 times glabellar length. Glabella weakly convex (tr., sag.). Glabellar lobation very weakly defined (best seen on NMV P304562 and NMV P304516, Figs 24.2, 24.4), L1 0.22 times glabellar length, L2 and L3 0.11 times glabellar length. S3 transverse, S2 directed inwards-backwards at about 7˚ from the transverse and S1 at 15˚ from the transverse. Axial furrows wide, shallow posteriorly, moderately impressed anteriorly, paraglabellar areas poorly defined. Preglabellar field of uniform width, flat. Length (exsag.) of posterior border equal to occipital length abaxially, lengthening adaxially to 2.0 times occipital length. Posterior border furrow transverse, shallow and very wide, meeting lateral border furrow distally. Postocular fixigenal area short, length (exsag.) 0.14 times cranidial length. Palpebral lobe 0.13 times cranidial 1.75 times preoccipital glabellar width, very shallow b- b length, palpebral furrow. Preocular fixigena area wide, 0.22 narrowing forwards anteriorly. Anterior branches of facial suture broadly curved, meeting connective sutures almost at margin opposite 0.9 cephalic length. Librigena with wide, moderately impressed border furrow, librigenal area weakly convex, steeply inclined, lateral border wide and convex, shallow, wide subocular furrow. Dorsal surface of rostral plate slightly inclined (in lateral view, see Figs 24.2c, 24.3c), length (exsag.) 0.18 times width (tr.). Ventral surface of rostral plate kite-shaped, with slightly medial flexure (tr.) giving anterior margin of cephalon a very weak M-shaped outline, flat posteriorly. Connective sutures weakly sinusoidal, diverging at 90˚ posteriorly and at 50˚ anteriorly. Posteriorly, connective sutures are either confluent immediately anterior to the meeting the hypostomal suture ( NMV P304536, Fig. 24.3a), or meet the hypostomal suture separately ( NMV P304531, Fig. 24.5).

Thorax with 13 segments. On external moulds axial furrows and the pleural flexure are not defined, although the axis is marked on internal moulds by exsagittal line of deep depressions (axial articulating processes) on the posterior margin of each segment. Pleural furrows wide and deep, pleural tips rounded.

Pygidium with sides weakly and uniformly convex. Axial furrows converging at about 45˚, indistinct opposite first and second rings, shallow posteriorly. Axis poorly defined posteriorly without postaxial ridge. Pleural furrows not reaching margin, border poorly defined and lacking independent convexity from pleural field, without border furrow. In posterior view anterior margin strongly convex, posterior margin weakly arched to accommodate rostral flexure. In lateral view dorsal margin steep (~45˚) and gently curved, ventral margin more or less horizontal.

Discussion. Trimerus lilydalensis exhibits several features indicative of assignment to Wenndorfia , including the strongly rounded anterior glabellar margin and the concentric anterior cranidial margin, the antero-medial rostral node, the weak pygidial trilobation, and the termination of the pygidial axis at about 0.9 pygidial length. The species was originally described from a limited number of poorly preserved specimens, but is redescribed from a much larger population.

Gill’s (1949) description of the species includes several of the diagnostic characters listed above, although the revised diagnosis differs in a number of points. Gill described the cephalon as being markedly inflated (it is only moderately convex, see Fig. 24.2c), and the exoskeleton as being smooth (it bears fine tubercules and pits, see Fig. 24.2a).

Holloway and Neil (1982) considered specimens of Digonus wenndorfi from the Heathcote area to be possibly conspecific with Wenndorfia lilydalensis . W. lilydalensis can be easily distinguished from wenndorfi in having pygidia lacking a raised postaxial ridge, and cranidia with a U-shaped rather than quadrate anterior margin, a more weakly tapered glabella with a more rounded anterior margin, and a forwardly convex rather than transverse rostral suture.

Wenndorfia lilydalensis is most closely comparable to the contemporary W. bostoviensis from the upper Lochkovian of Poland. The species share a similar trapezoid, moderately tapering, straight-sided glabellar outline and a very wide (tr.) palpebral area. Glabellar lobation is variably expressed in lilydalensis , and although Tomczykowa (1975) noted an absence of glabellar lobation for bostoviensis , shallow S1-S3 can be seen on specimens considered here to be conspecific with bostoviensis (see discussion above). More significantly, bostoviensis also exhibits a relatively underived pygidium, with a triangular pygidial outline with an acutely angled tip. Pygidial axial furrows are moderately impressed on bostoviensis , distinguishing it from lilydalensis and younger species of Wenndorfia , that share very shallow to effaced pygidial axial furrows. The only other Wenndorfia to exhibit a triangular pygidial outline is the lower Pragian Polish W. nova, with less elongate pygidial proportions and a very narrow pygidial axis. In cranidial morphology nova shows some resemblance to lilydalensis , although the former differs in having narrow palpebral areas.

Wenndorfia lilydalensis is confined to the Lilydale sequence. One specimen labelled as coming from the Seville area , collected by F. Chapman, was found to have a counterpart labelled as coming from the Lilydale area, collected by J. Jutson. The lithology of the specimen matches lithologies from the Lilydale sequence rather than those from Seville. Gill’s (1938) record of the species from PL1835 , Gill locality 35, Seville is also probably erroneous, as the lithology of the specimen is unlike that at PL1835 , but again closely matches lithologies of the Lilydale sequence .

Environmental notes. The faunal associations of Wenndorfia lilydalensis are complex. W. lilydalensis first appears as a rare element in the trilobite fauna (relative abundance 2%), but through its range steadily increases in relative abundance, reaching maximum relative abundance in the upper horizons of its range. In the lower part of its range lilydalensis frequently occurs in phacopid-dominated assemblages, but in the upper parts of its range lilydalensis is frequently part of a distinct acastid/homalonotid dominated association

With the exception of a complete exoskeleton from a bioclastic coquinal sandstone at PL1805 (taphofacies TIII), all specimens of Wenndorfia lilydalensis are found in a siltstone lithology preserved as isolated sclerites or partly disarticulated specimens including cephala, a cephalon with 7 thoracic segments, and a moult assemblage, being a cephalon lying obliquely over the thorax. A cranidium and nearby pygidium on the same bedding plane might also be interpreted as a moult assemblage (taphofacies TIV). An outer shelf setting is suggested as the preferred environment of W. lilydalensis , which ranges only as a rare faunal element to mid shelf settings and to even deeper settings where it occurs with a calymenid-phacopid association. The taphonomy of the latter, with a high degree of articulation and containing sparsely distributed moult assemblages and rare enrolled specimens represents a new taphofacies for central Victoria, designated taphofacies TV, equivalent to Speyer and Brett’s (1986) taphofacies 4B (see Fig. 6 ).

Figures

Figure 5 A, D. Moult assemblages of Trimerus (Edgillia) kinglakensis in the upper siltstones at PL252, Middendorps Quarry, Kinglake West, Humevale Siltstone. A, upper right, displaced and inverted cephalon lying underneath thoracopygon (only anterior margin visible); left, thoracopygon with displaced and rotated cephalon. D, thoracopygon with displaced and rotated cephalon. B–C. Complete dorsal exoskeletons in sandstone bioclastic coquinas. B, Trimerus (Trimerus) harrisoni from PL1820, Brunswick, Melbourne Formation. C, Wenndorfia lilydalensis from PL1805, Coldstream. Humevale Siltstone.

Figure 6. Facies and environmental distribution of Australian homalonotids.

Figure 23. Geological sketch map of the Lilydale area showing Lochkovian-basal Pragian fossil localities yielding homalonotids. For other fossil localities see also Gill (1940, fig.1, 1945, fig. 2), Moore (1965, fig.1), VandenBerg (1970), Garratt (1972), Wall et al. (1995, fig. 1), Sandford (2003, text-fig. 1A, 2004, fig. 1).