Ananaspis sp. 1

Sandford, Andrew C. & Holloway, David J., 2006, Early Silurian phacopide trilobites from central Victoria, Australia, Memoirs of Museum Victoria 63 (2), pp. 215-255 : 230-232

Ananaspis sp. 1

Figures 10A, 10D, 10I

Material. NMV P139354 (partly disarticulated cephalothorax), NMV P139355 (cephalon), NMV P139356 (right cheek) from PL1338, Wallan. Bylands Siltstone. For locality see Sandford and Rickards (1999: fig. 1).

Description. Cephalon subsemicircular, length 50% width. Glabella strongly convex (tr.), with maximum width situated at about 66% sagittal glabellar length and equal to 56% maximum cephalic width, 110% sagittal cephalic length and 170% occipital width. Outline of front of glabella forming arc centred at 33% sagittal cephalic length from posterior. Axial furrow wide and very deep in front of occipital ring, diverging strongly at about 75° between S1 and anterior end of palpebral furrow, thereafter diverging forward at about 45° to widest part of frontal lobe. Occipital ring 30% maximum cephalic width, short, 10% glabellar length sagittally and slightly shorter laterally, lacking lateral lobes defined by notches in anterior margin. Occipital furrow deep, transverse. L1 as high as composite lobe medially, lateral node of moderate size and height, isolated by deep exsagittal furrow from preoccipital ring, width of glabella across L1 75% occipital width. S1 deep, directed anteromedially from inner end of lateral node on L1, at about 22° to transverse, connected medially by a shallower, wide (sag.), transverse furrow. L2 65% length (exsag.) of L3 and 11% sagittal cephalic length. S2 and S3 moderately impressed. S2 directed at 45° distally, curving abruptly to transverse direction proximally, anteriormost point opposite 40% sagittal glabellar length from posterior. Posterior branch of S3 weakly convex forward, oriented transversely opposite glabellar midlength (sag.). Anterior branch of S3 straight, not connected to posterior branch, placed anteriorly opposite anterior margin of eye, directed anterolaterally at 60° to exsagittal line. Frontal lobe high. Preglabellar furrow very shallow. Anterior border very short (sag.). Eye large, 50% sagittal cephalic length, placed with midlength of eye opposite 33% sagittal glabellar length, occupying entire length of genal field. Palpebral area low, weakly convex, palpebral furrow moderately impressed. Palpebral lobe steeply inclined, raised above palpebral area, with shallow rim furrow. Visual surface large, higher anteriorly than posteriorly, with more than 18 files of up to 7 lenses each, without raised sclera. Posterior border short (exsag.) adaxially, approximately 300% as long distally. Posterior border furrow deep and wide, continuous with deep and wide fixigenal lateral border furrow. Genal angle obtuse, with small point at angle. Lateral border moderately convex, wide (tr.) posteriorly, narrowing strongly anteriorly. Librigenal lateral border furrow moderately impressed. Posterior branch of facial suture skirting back of eye to lateral border furrow, strongly convex forward across lateral border, anteriormost point opposite 25% cephalic length. Cephalic doublure with extremely shallow vincular furrow anteriorly, very shallow and very weakly notched laterally. Cephalic tubercles variable in size from small to moderate, with superimposed granules, densely distributed on composite lobe of glabella, L1 and palpebral area, remainder of exoskeleton with granulose sculpture.

Remarks. Ananaspis sp. 1 is the only trilobite known from PL1338. Stratigraphically, the locality is about 50 m above PL206, which lies 750 m further to the south and yields a more diverse fauna.

Although the above description is based only on one complete cephalon and two fragments, the material is sufficient to distinguish the species from other Victorian Silurian phacopids. The species is most easily distinguished by its fine, dense cephalic tuberculation, by the large size of the eye which extends the entire length of the genal field, and by the very shallow vincular furrow medially. Assignment to Ananaspis is indicated by features including short L2 (relative to the length of L3), a narrow glabella, the variably sized glabellar tubercles, a deep lateral border furrow continuous with the posterior border furrow and the obtuse genal angle with a small point. The eye is larger than those of other Ananaspis , although eye length is a character that varies significantly between species here assigned to the genus (eye length up to about 47% sagittal cephalic length in A. amelangi , 45% in A. fecunda and A. crossleii , 40% in A. decora , 37% in A. calvescens , 30% in A. aspera ). Ananaspis sp. 1 most closely resembles amelangi , sharing large eye size, similar cephalic ornament and cephalic proportions. The species differ in that amelangi has a glabella that does not always extend to the anterior margin of the cephalon, the occipital ring has more strongly defined lateral lobes, the visual surface has only six lenses per file and 15–16 files, the preglabellar and vincular furrows are deeper and the vincular notching is deeper. Ananaspis sp. 1 differs from the type species in having finer cephalic tuberculation, a larger eye and fewer lens files in the visual surface (19–21 in fecunda ). In lens formula A. sp. 1 is most similar to A. guttulus (16 files of up to seven lenses per file) and decora (16–17 files of seven, rarely eight, lenses per file).

Figures

Figure 10. A, D, I, Ananaspis sp. 1 from PL1338, Wallan. A, D, NMV P139354, cranidium, × 6.5. I, NMV P139356 cephalon (fragment), enlargement of eye, × 9. B–C, Ananaspis sp. 2, NMV P136142, damaged cephalothorax, from PL1371, Coburg; B, × 1.4; C, × 1.2. E–G, J, Ananaspis typhlagogus? (Öpik, 1953), NMV P147055, cephalon (crushed) from PL206, Wallan; E–G, × 3; J, × 2.5. H, Ivops wallanensis gen. et sp. nov., paratype NMV P139326, thoracopygon, × 3, from PL206, Wallan. K, Phacopidae gen. indet. 3, NMV P312077, partly disarticulated thoracopygon, × 4.5, from PL1369, Springfield. (A, C, E–H, J–K are internal moulds).