Phacopidae, Hawle and Corda, 1847

Sandford, Andrew C. & Holloway, David J., 2006, Early Silurian phacopide trilobites from central Victoria, Australia, Memoirs of Museum Victoria 63 (2), pp. 215-255 : 237

Phacopidae gen. indet. 3

Figures 5E–G , 10K

Material. NMV P139353 (partially enrolled exoskeleton), NMV P312077 (partly disarticulated thoracopygon) from PL1369, Springfield. Springfield Formation.

Remarks. These two small specimens are considered to belong to the same species because of similarities in the pygidia, which have six axial rings, two pseudo-articulating half rings (although the third ring is damaged in the thoracopygon), four distinct pleural furrows, and well-defined interpleural furrows extending almost to the margin. The partially enrolled exoskeleton has a cephalon with a moderately convex (sag.) glabella that does not overhang the preglabellar furrow and anterior border in dorsal view, very weak S2 and S3, a shallow fixigenal border furrow, an eye of moderate size situated well in front of the posterior border furrow, a weakly curved palpebral lobe, and a finely granulose sculpture. Compared to the cephalon described above as Phacopidae gen. indet. 2, which is from the same locality, the present specimen represents a larger individual (cephalic length 7.5 mm as opposed to 3 mm) and has a more weakly convex glabella and much smaller eye. The difference in the eyes cannot be attributed to ontogenetic changes as eye size increases during ontogeny rather than decreases, as documented by Ramsköld (1988) in Acernaspis . We therefore conclude that Phacopidae gen. indet. 2 and Phacopidae gen. indet. 3 belong to different taxa.

Figures

Figure 5. A–D, Acernaspis? sp., NMV P139804, cephalon, × 4, from PL385, Costerfield. E–G, Phacopidae gen.indet.3, NMV P139353, enrolled exoskeleton from PL1369, Springfield; E, × 4.0; F, × 4.5.

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).