Acernaspis, , Ramskold and Werdelin, 1991

Sandford, Andrew C. & Holloway, David J., 2006, Early Silurian phacopide trilobites from central Victoria, Australia, Memoirs of Museum Victoria 63 (2), pp. 215-255 : 222-223

Acernaspis ? sp.

Figures 5A–D

Material. NMV P139804 (incomplete cephalon) from PL385, Costerfield. Wapentake Formation. For locality see fig. 9.

Description. Anterior and anterolateral cephalic margins moderately arched upwards medially in anterior view. Glabella weakly convex (sag., tr.), low anteriorly, sides of composite lobe diverging at about 55° between level of S2 and outer end of preglabellar furrow. S1 curving forward adaxially, connected medially by wide (sag.), shallow depression. S2 moderately impressed, convex forwards, subparallel to posterior branch of S3, its inner end connected to S1 by shallow exsagittal furrow. L2 about 75% length (exsag.) of L3 adaxially. S3 distinctly shallower than S2; posterior branch of S3 not extending as far adaxially and abaxially as S2; anterior branch not quite meeting posterior branch, oriented at about 60° to sagittal line and very weakly curved, meeting axial furrow anteriorly. Preglabellar furrow moderately impressed, continuous medially. Anterior cephalic border of almost uniform length (sag., exsag.) except abaxially. Preserved portion of eye (more than anterior half) with 12 files of lenses, formula (from anterior) 4 5 6 7 6 7 6 6 6 6 7 6 …, sclera depressed. Palpebral lobe raised high above palpebral area, with shallow but distinct rim furrow; anterior part of palpebral furrow deep and narrow. Librigenal field concave below eye and merging with lateral border furrow. Anterior branch of facial suture cutting across anterolateral corner of glabella (fig. 5B–C). Vincular furrow moderately impressed medially, deep laterally with strong notching. Medial part of cephalic doublure not raised as high above vincular furrow anteriorly as posteriorly, giving doublure a step-like profile or ʻbevelledʼ morphology. Composite glabellar lobe with sculpture of low, perforate tubercles of small to moderate size.

Remarks. The species is represented by a single, incomplete cephalon lacking the posterior portion more or less behind a transverse line through the medial part of S1. The specimen differs from Ananaspis typhlagogus from the same formation in that the glabella is much more weakly convex, not as strongly curved in anterior outline and does not overhang the anterior border; the anterior border is wider (sag., exsag.); S2 is deeper, morestronglycurved(nottransverseadaxially),runssubparallel to the posterior branch of S3, and is connected to S1 adaxially by a shallow exsagittal furrow; the posterior branch of S3 is narrower (tr.) and the anterior branch longer; the preglabellar furrow and preserved portion of the palpebral furrow are deeper; and the visual surface does not overhang the lateral border as strongly in dorsal view and is higher, with more lenses per file (up to seven instead of five as in typhlagogus ). Although it could be suggested that the low glabellar profile of the specimen is due to tectonic flattening, we consider that this is unlikely because of the other differences from typhlagogus listed, especially those in the eyes.

Acernaspis ? sp. differs from most species of Acernaspis except A.? rubicundula Ramsköld, 1985 , from the lower Wenlock of Sweden, in having tuberculate sculpture on the composite glabellar lobe (see discussion above under genus). A.? rubicundula differs from A.? sp. in that S2 is not as curved, does not extend as close to the sagittal line and is not connected to S1 adaxially by a weak longitudinal furrow; L2 is much shorter (about half length of L3 adaxially); S3 is deeper, with a shorter anterior branch not extending as close to the posterior branch; and S2 and S3 have weakly raised rims. These differences suggest that the Victorian species may not be most closely related to rubicundula , despite the similarity in glabellar sculpture.

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.