Acernaspis georgei, Sandford & Holloway, 2006

Sandford, Andrew C. & Holloway, David J., 2006, Early Silurian phacopide trilobites from central Victoria, Australia, Memoirs of Museum Victoria 63 (2), pp. 215-255 : 220-222

Acernaspis georgei sp. nov.

Figures 2 , 3A–B, D–E, H–I

Acernaspis .— Rickards and Sandford, 1998: 750.

Type material. Holotype NMV P515 (cephalon) from PL1964 , Geological Survey locality B25, Springfield . Paratypes NMV P138259 (cephalon) , NMV P138262 (pygidium) , NMV P138278 (pygidium) , NMV P147765 (cephalon) , NMV P147766 (cephalon) , NMV P147796 (pygidium) , NMV P312816 (cephalon) from PL256 , Wallan . Paratype NMV P138271 (cephalon) from PL 598, type section of the Chintin Formation, Springfield.

Other material. NMV P138260–P138261, NMV P147764, NMV P147767–P147768, NMV P147788–P147795, NMV P147797–P147806 from PL 256, Wallan. NMV P139447–P139453 from ʻLancefieldʼ (unknown locality in the Parish of Goldie). For localities see Thomas (1960), VandenBerg (1991), Sandford and Rickards (1999) (fig. 1), Sandford (2005) (fig. 11) and (fig. 4).

Derivation of name. After ACSʼs late father.

Diagnosis. Anterior cephalic border comprising 2.5% sagittal cephalic length in dorsal view. Lateral glabellar furrows moderately impressed, S2 extending close to axial furrow, posterior branch of S3 weakly convex forward, anterior branch placed posteriorly, with midlength opposite anterior margin of palpebral lobe. Palpebral furrow moderately to deeply impressed, distinct palpebral rim furrow, eye length (exsag.) 45% sagittal cephalic length, postocular area with length (exsag.) 5% sagittal cephalic length, visual surface with 15 files of lenses with up to 6 (mostly 5) lenses per file. Genal angle obtusely angular, lacking prominent point or genal spine. Second and third pygidial pleural furrows moderately impressed.

Description. Cephalon semicircular, in frontal view with anterior margin weakly arched. Glabella weakly convex (sag., tr.), not reaching anterior cephalic margin in dorsal view, maximum width across frontal lobe approximately equal to cephalic length, 50% maximum cephalic width and 160% occipital width (tr.). Axial furrow wide and very deep, weakly converging between posterior margin and L1, diverging at 60° between L1 and a point opposite midlength of palpebral lobe, thereafter diverging forward at 20°. Anterior margin gently rounded in front of glabella, forming arc centred on posterior margin. Occipital ring with length (sag.) 15% cephalic length and width (tr.) 33% cephalic width, with exsagittal furrow impressed in anterior margin defining obliquely directed lateral lobes. Occipital furrow deeply impressed.L1 high, lateral node isolated by deep exsagittal furrow. S1 deep laterally, weak across median part of glabella. S2 and S3 moderately impressed on external surface. S2 directed at about 10° to transverse, straight, its inner end situated more or less opposite 40% glabellar length from posterior. Posterior branch of S3 with anteriormost point more or less opposite 55% sagittal glabellar length from posterior. Anterior branch of S3 oriented at 40° to exsagittal line, weakly sigmoidal. L2 and L3 equal in length (exsag.). Anterior border furrow moderately impressed. Anterior border wide. Eye placed with midlength opposite 38% sagittal cephalic length from posterior. Palpebral area high, convex. Palpebral lobe steeply inclined, flat (tr.), raised above palpebral area. Visual surface large, with up to 71 lenses, formula ( NMV P138259, fig. 2D, H) 5 5 6 5 5 5 5 5 5 5 4 5 5 4 3. Posterior border furrow deep adaxially, terminating abruptly abaxially at a point in line (exsag.) with distal margin of eye. Posterior border narrow adaxially, widening slightly abaxially. Librigenal field concave below eye, not distinctly differentiated from lateral border furrow. Posterior branch of facial suture skirting posterior margin of eye abaxially, crossing genal area directed anterolaterally at about 25° to transverse, reaching a point opposite 33% sagittal cephalic length, deflected backwards at same angle abaxially. Cephalic doublure with vincular furrow deep throughout and strongly notched laterally. Cephalic sculpture finely granulate.

Pygidium lenticular in dorsal outline. Axis comprising 85% sagittal pygidial length and 30% maximum pygidial width anteriorly, with 6 rings that are well defined in anterior part of axis and poorly defined in posterior part, terminal piece merging with postaxial region. Pleurae with 5 pleural furrows, anterior one narrow (exsag.), deep, reaching 75% distance to margin, remaining pleural furrows successively shallower, posteriormost one very weak. Interpleural furrows very shallow. Border and furrow not defined.

Remarks. Acernaspis georgei occurs at several localities in the Chintin Formation, although its relative abundance, faunal associates, preservation and associated lithology vary markedly. In Slab Hut Creek at PL 256, Wallan, the species is common (relative abundance 44%) and together with Calymene (relative abundance 35%) dominates the fauna preserved almost entirely as fragmentary cephala and pygidia. The trilobites at PL 256 occur within bioclastic sandstone coquinas which contain abundant disarticulated crinoid elements, gastropods, bryozoans and clasts of siltstone incorporated into the coquina in the plastic state, presumably as rip-up clasts. The coquinas are bedded but form large irregular bodies within massive siltstones, with which they have sharp contacts. At ʻLancefieldʼ the species occurs (relative abundance 14%) in a fauna dominated by Bessazoon sp. and Calymene sp. , preserved as isolated and generally undamaged exoskeletal elements in medium-grained sandstone.

The species can be confidently assigned to Acernaspis . Significant characters in this assignment include the low glabellar profile, non-tuberculate cephalic ornament, long (exsag.) L2 (equal in length to L3), wide anterior border visible in dorsal view, poorly defined lateral cephalic border furrow, short postocular area, strong notching of the vincular furrow laterally and the weak expression of the pygidial interpleural furrows and the fourth and fifth pleural furrows.

A poorly known, unnamed species of Acernaspis was described by Holloway and Sandford (1993) from the late Llandovery Richea Siltstone of Tasmania. The present species differs from the Tasmanian one in having a longer postocular area, a deeper palpebral furrow and a longer anterior cephalic border. ʻ Phacops ʼ macdonaldi Fletcher, 1950 , from the upper Llandovery to lower Wenlock of the Orange district, New South Wales, was tentatively assigned to Acernaspis by Sherwin (1971), but is now assigned to Ananaspis along with its junior subjective synonym ʻ Acernaspis ?ʼ oblatus [sic] Sherwin (see discussion of Ananaspis typhlagogus ).

Acernaspis georgei is most similar to the type species A. orestes . The two species share a relatively long (sag.) anterior cephalic border, a long postocular area and a non-spinose genal angle. A. georgei has one fewer lens file in the eye but otherwise shares a similar lens formula to small-eyed morphs of orestes , which have 16 files of up to six (mostly five) lenses per file. The species differ most notably in the arrangement of the lateral glabellar furrows and in the deeper palpebral furrows of georgei .

Species less closely related to A. georgei and A. orestes include Männilʼs (1970) Estonian species A. semicircularis , A. estonica , A. sulcata , A. rectifrons and A. incerta , and the Scottish A. xynon Howells, 1982 , all of which are easily distinguished in having acutely angular genal angles or short genal spines. In the depth and lateral extension of S2 (to a point close to the axial furrow), A. georgei resembles A. besciensis LespéranceandLetendre, 1982 fromtheRhuddanian of Anticosti Island, Canada (= A. salmoensis Lespérance, 1988 fide Chatterton and Ludvigsen, 2004), the oldest known species of the genus. A. besciensis differs from A. georgei in many other features, notably in having fewer lenses per file but more files in the eye (up to four lenses in 16 files), a shorter (tr.) and more forwardly convex S3, a narrow preoccipital ring and a shallower preglabellar furrow.

Figures

Figure 2. Acernaspis georgei sp. nov. A–C, E–F, holotype NMV P515, cephalon (fragment), from PL1964, Springfield; A–C, × 5; E, × 6.5; F, × 6. D, H, paratype NMV P138259, cephalon, from PL256, Wallan; D, × 5; H, × 4. G, paratype NMV P138261, pygidium, × 8, from PL256, Wallan. I, paratype NMV P138271, cephalon (fragment), × 7, from PL598, Springfield. (C, D are internal moulds).

Figure 3. A–B, D–E, H–I, Acernaspis georgei sp. nov. A, paratype NMV P147766, incomplete cephalon, × 4, from PL256, Wallan. B, NMV P139448, damaged cephalon with cephalic doublure exposed, × 3.25, from ʻLancefieldʼ. D, I, paratype NMV P312816, incomplete cephalon from PL256; D, × 5; I, × 4.5. E, NMV P139451, pygidium, × 4, from ʻLancefieldʼ. H, paratype NMV P138262, pygidium, × 5, from PL256, Wallan. C, F–G, J–K, Ananaspis typhlagogus (Öpik, 1953) from PL6361, Springfield. C, G, J–K, NMV P312813, cephalon; C, J, K × 3.75, G × 4.75. F, NMV P312814, thorax, × 5. (A–B, H are internal moulds).