Ananaspis typhlagogus ( Öpik, 1953 )

Sandford, Andrew C. & Holloway, David J., 2006, Early Silurian phacopide trilobites from central Victoria, Australia, Memoirs of Museum Victoria 63 (2), pp. 215-255 : 227-230

Ananaspis typhlagogus ( Öpik, 1953)

Figures 3C, F–G, J–K , 7–8 ,?10E–G,?10J

Phacops typhlagogus .— Öpik, 1953: 26, pl. 10, figs 81–84.—

Talent, 1964: 50.— Sherwin, 1971: 94. Ananaspis typhlagogus .— Campbell, 1967: 32.— Ramsköld, 1988:

312, 313.— Rickards and Sandford, 1998: 751.— Sandford and Holloway,

1998: 915. ʻ Phacops ʼ typhlagonus [sic].— Chlupáč, 1977: 77. ʻ Ananaspis ʼ typhlagoga [sic].— Ramsköld and Werdelin, 1991:

72, 74.

Type material. Holotype CPC 686 (incomplete and partially enrolled exoskeleton), figured Öpik (1953: pl. 10, figs 81–84), figs 7C–E, from the ʻ Illaenus bandʼ (exact locality unknown), Costerfield. Wapentake Formation.

Other material. NMV P138228, NMV P138234–P138238, GSV36558 (missing), GSV46658 (missing), unregistered Geoscience Australia collection specimen from the ʻ Illaenus bandʼ (exact locality unknown), Costerfield. NMV P138229 from PL385, Costerfield. NMV P138279–P138289, NMV P140039, NMV P147835–P147837 from PL1460 (Thomas locality F43A), Costerfield. NMV P140152 from PL390, Costerfield. NMV P147049 from PL2263, Costerfield. Wapentake Formation. NMV P312813–P312815 from PL6361, Springfield. Chintin Formation. A single crushed cephalon NMV P147055 (figs. 10E–G, J) from PL206, Wallan, in the Bylands Siltstone, is tentatively assigned to the species.

Diagnosis. Glabella comprising a little more than 33% maximum cephalic width posteriorly and about 58% across frontal lobe. L2 not markedly shorter (exsag.) than L3 adaxially, S2 transverse adaxially and curving backwards abaxially. Composite lobe with densely and evenly distributed tubercles, larger ones of moderate size. Eye large, posterior edge very close to posterior border furrow, lower edge separated from lateral border furrow anteriorly by narrow band of librigenal field, visual surface with 16–18 rows of up to 5 lenses each. Fixigenal lateral border furrow shallow and wide. Pygidium with 6 axial rings and 4–5 pleural furrows.

Description. Exoskeleton of estimated maximum length 40 mm. Cephalon semi-elliptical in outline, length (sag.) approximately 58% maximum width, in frontal view anterior margin broadly arched upward adaxially with very weak downward sag medially. Glabella of moderate to strong convexity (sag., tr.), slightly overhanging anterior cephalic border in dorsal view, anterior margin forming arc centred approximately level with inner ends of S2. Width of glabella across frontal lobe almost twice width across L1 and approximately equal to sagittal cephalic length. Axial furrow shallow opposite occipital ring, very deep in front of occipital furrow and diverging forward at 55°–70° in front of S1. Occipital ring of uniform length (sag. exsag.) except distally where it is contracted, deflected slightly obliquely forward and bears weak lateral lobes. Occipital furrow very deep behind lateral node on L1, transverse and moderately impressed medially. L1 about 60% as long as occipital ring medially, with small, subquadrate, depressed lateral nodes. S1 deflected slightly forward adaxial to lateral node on L1, expanding and shallowing medially but continuous across glabella. L2 with maximum length (exsag.) 70%–80% maximum length of L3. S2 placed opposite midlength of eye, not reaching axial furrow. Posterior branch of S3 oriented transversely but gently convex forwards, inner end level with glabellar midlength (sag.). Anterior branch of S3 oriented diagonally, weakly convex forward. Preglabellar furrow moderately impressed. In anterior view anterior border narrow medially, wider laterally. Eye 45% sagittal cephalic length, distance of posterior edge from posterior border furrow less than or equal to length (exsag.) of posterior border directly behind. Visual surface on NMV P138236 (figs 7F–H, K) with lens formula 3? 5? 5 4? 5 4? 4 4? 4 5 4 4 4 4 3? 4 3 2; NMV P140152 (fig. 8J) with formula 2? 3? 4? 4 5 4 5 5? 5? 5? 4 5 5 5 4 3? Palpebral area weakly convex (tr., exsag.), palpebral furrow moderately impressed. Palpebral lobe crescentic in outline, very weakly convex (tr.), slightly elevated above palpebral area, with distinct rim furrow. Posterior border furrow deep, posterior border expanding (exsag.) backward adaxially so that genal angle is produced posteriorly. Librigenal field weakly concave below eye, steeply inclined, indistinctly separated from very weakly convex lateral border. Posterior branch of facial suture directed slightly obliquely forward across genal field and deflected backward across border to meet cephalic margin approximately level with median part of occipital furrow. Medial part of cephalic doublure steeply inclined backwards, weakly concave (sag.) in posterior part and weakly convex anteriorly. Hypostomal suture transverse medially. Vincular furrow moderately impressed, separated from anterior margin by narrow band, lateral notching moderately expressed.

Hypostome about 150% as wide anteriorly as long (sag.), parabolic in outline behind large, equilaterally-triangular anterior wings, with weakly defined shoulder opposite midlength (sag.). Middle body ovate in outline, moderately and evenly convex (sag., tr.), middle furrow and maculae indistinct. Lateral border narrow, in lateral view sloping gently dorsally from anterior wing to shoulder where it is gently deflected ventrally. Posterior border expanding slightly medially, posterior border furrow shallower than lateral border furrow.

Thorax of 11 segments. Axis wider (tr.) than pleurae, strongly convex (tr.), rings with lateral lobes weakly defined by slight expansion, by shallow notch in anterior margin, and by slightly oblique orientation of articulating furrow. Pleurae steeply inclined beyond fulcrum, well rounded distally, with deep pleural furrow terminating distally at edge of articulating facet, approximately 25% distance from fulcrum to tip.

Pygidia all incomplete or poorly preserved, with large, strongly oblique articulating facet and broadly rounded posterior margin. Axis gently tapering backwards and decreasing in height, bluntly rounded posteriorly, 1st ring standing higher than remainder and with shallow medial embayment in posterior edge to accommodate pseudo-articulating half ring on 2nd segment, 2nd and 3rd inter-ring furrows slightly expanded sagittally but without pseudo-articulating half-rings, 6th and 7th inter-ring furrows very poorly defined close together. Axial furrow deeper alongside axis than behind it. Pleural furrows short (exsag.) and rather sharply incised, 1st interpleural furrow shallow but distinct, remaining interpleural furrows very weak. Border very poorly defined.

Remarks. Ananaspis typhlagogus is not very common in the Wapentake Formation. Öpik (1953) reported that he had numerous fragments of the species but that only the holotype was well enough preserved to be used for description. Specimens such as the holotype that are preserved in the siliceous nodules of the Wapentake Formation are undeformed and the exoskeleton may be partly preserved as a mineralised crust (see Sandford and Holloway, 1998: 921). In contrast, moulds from mudstones of the Wapentake are crushed and fractured (fig. 8J), and those from a sandstone clast are fragmentary (figs 8F, H–I).

Öpik (1953) placed typhlagogus with the Phacops orestes group (i.e. Acernaspis ) but noted that the latter species differs in ʻmany important pointsʼ including the lower glabellar profile and stronger vincular notching. Campbell (1967) assigned typhlagogus to Ananaspis on the basis of its glabellar tuberculation and the bevelled profile of the cephalic anterior margin and doublure.

Ananaspis typhlagogus is very closely related to A. macdonaldi Fletcher, 1950 [= Acernaspis ? oblatus [sic] Sherwin, 1971; see Holloway, 1980: 64; Ramsköld and Werdelin, 1991:73], from the upper Llandovery to lower Wenlock of central-western New South Wales, and to ʻ Ananaspis ʼ sp. of Waisfeld and Sánchez, 1993, from strata of presumed similar age in Argentina. Sherwin (1971) stated that typhlagogus has more definite tuberculation and a more strongly inflated glabella than macdonaldi . However, the significance of the latter difference is difficult to assess in view of the wide variation in glabellar profile in typhlagogus , and we cannot see any clear difference between the species in sculpture. Ramsköld and Werdelin (1991) regarded macdonaldi and typhlagogus as possible synonyms, but we consider that typhlagogus can be distinguished by a higher visual surface of the eye with a greater number of lenses (five in the longest file instead of four in macdonaldi ), a lower eye socle, a larger L1, and a more strongly curved posteromedial outline of the hypostome. ʻ Ananaspis ʼ sp. from Argentina, known only from an internal mould of a single cephalon ( Waisfeld and Sánchez, 1993, pl. 1, figs 7–10), is difficult to distinguish from typhlagogus but it has an eye with up to six lenses per file and a vincular furrow that appears to be slightly deeper medially and has a distinct flexure anterolaterally.

Also similar morphologically to A. typhlagogus is A. aspera ( Hawle and Corda, 1847) from the Ludlow of the Czech Republic (see Chlupáč, 1977, pl. 5, figs 9–25). The similarities include: relatively narrow (tr.) cheeks; L2 not markedly shorter (exsag.) than L3; adaxial half of S2 transverse; lower edge of eye situated above lateral border furrow anteriorly; visual surface very steeply inclined; and first order glabellar tubercles of moderate size. A. aspera differs from A. typhlagogus in that the cheeks are approximately as wide as the glabella posteriorly instead of slightly narrower than it; the eyes are much shorter (exsag.); the palpebral furrow is deeper; the lateral border furrow is deeper on the fixigena; and the pygidium has a greater number of more strongly defined segments and a narrower, subparallel-sided axis.

Figures

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).

Figure 7. Ananaspis typhlagogus (Öpik, 1953). A–B, J, NMV P138229, cephalon × 7, from PL385, Costerfield. C–E, holotype CPC 686, dorsal exoskeleton, views of cephalon, × 3, from the ʻIllaenus bandʼ, Costerfield.F–H, K, NMV P138236, cephalon, × 4, from the ʻIllaenus bandʼ, Costerfield. I (and fig. 10C), NMV P138237, dorsal exoskeleton, × 4.5, from the ʻIllaenus bandʼ, Costerfield. L–M, unregistered specimen in AGSO collection, cephalon, view of hypostome, × 2.5, from the ʻIllaenus bandʼ, Costerfield. (I, L–M are latex casts).

Figure 8. Ananaspis typhlagogus (Öpik, 1953). A–B, D–E, NMV P147049, partially enrolled exoskeleton from PL2263, Costerfield; A, × 2.8; B, × 3; D–E, × 3.5. C, NMV P138237, view of pygidium, × 9. F, NMV P138287, pygidium, × 4.3, from PL1460, Costerfield. G, NMV P138233, thorax, × 2.5, from the ʻIllaenus bandʼ, Costerfield. H–I, NMV P138285, pygidium, × 4.5, from PL1460, Costerfield. J, NMV P140152, cephalon, × 3.9, from PL390, Costerfield. (C, F, H–J are latex casts).