Phacopidella, Reed, 1905

Sandford, Andrew C. & Holloway, David J., 2006, Early Silurian phacopide trilobites from central Victoria, Australia, Memoirs of Museum Victoria 63 (2), pp. 215-255 : 235

Phacopidella ? sp.

Figure 12A

Asaphus .— Selwyn, 1863: map note.

Phacops crosslei .— Chapman, 1913: 210, 229.— Chapman, 1915: 169.

Phacopidella ?.— Rickards and Sandford, 1998: 749.

Material. NMV P1218 (dorsal exoskeleton with displaced cephalon), probably from PL 1393, Geological Survey locality Ba5, Keilor. Springfield Formation, in siltstone underlying the Lintons Creek Conglomerate Member; late Llandovery, Telychian ( Spirograptus turriculatus – Monograptus crispus biozones). For locality see Selwyn (1863), Rickards and Sandford (1998) (fig. 5).

Description. Exoskeleton, of estimated original length about 35 mm, has been flattened tectonically and slightly sheared. The preserved portion of the cephalon consists of the glabella behind about the middle (sag.) of the frontal lobe together with the posteromedian part of cheeks. The glabella is slightly narrower across L1 than across the occipital ring and expands gently forwards in front of L1, the axial furrow here diverging at about 40°. The occipital ring is almost twice as long sagittally as exsagittally, with lateral lobes defined by very short exsagittal incisions in the anterior edge. The median portion of the occipital furrow is arched forward. L1 has large, quadrate nodes laterally and is slightly shorter (sag., exsag.) than the occipital ring medially. S1 is deep laterally, is deflected forwards adaxial to the short exsagittal furrow defining the lateral node of L1 and shallowing rapidly, becoming indistinct medially. S2 is short (exsag.), very faint and weakly convex forward, directed slightly obliquely backward laterally. L2 is about as long (exsag.) as L1. The posterior branch of S3 is indistinctly preserved on the left side of the glabella, running subparallel to S2. Axial furrow deep. Left palpebral lobe possibly partly preserved, seemingly relatively long (exsag.) and obliquely oriented. Thorax with 11 segments. Axial rings with short, deep exsagittal incisions in anterior margin defining large, quadrate lateral lobes. Pleurae with deep pleural furrow situated at middle (exsag.) of segment at fulcrum; distal parts of pleurae not preserved but overall width of pleural lobe appears not to exceed width of axis. Pygidium large, approximately 160% as wide as long (sag.). Axis about 30% maximum pygidial width anteriorly and about 66% sagittal pygidial length, with 5 well defined rings and weak 6th ring. Pleurae with 5 deep pleural furrows and 2 weak ones posteriorly; interpleural furrows moderately impressed. Dorsal surface of pleurae not intact abaxially on internal mould, so appearance of wide, smooth border is largely an artefact.

Remarks. The lithology of this specimen is identical to that of a specimen of Hadromeros from Keilor, one of two trilobites collected by C.D. Aplin in the 1850s that were noted on the geological quarter sheet by Selwyn (1863) as Cheirurus and Asaphus . NMV P1218 is apparently the second trilobite collected by Aplin, and that recorded by Chapman (1913) as Phacops crossleii Etheridge and Mitchell, 1896 from Keilor. However, the museum label with the specimen records the locality as ʻSaltwater River, one mile west of Gisborneʼ, as published by Chapman (1915). Chapman (1915) noted the lithology of the trilobite specimen as ʻbearing a strong resemblance to the Keilor...mudstonesʼ. Chapman, the museum palaeontologist at the time, appears to have confused Gisborne with Keilor. Only Early to Middle Ordovician graptolitic beds outcrop in the Gisborne area, with a lithology quite unlike that of the trilobite specimen.

The low pygidial convexity (largely a result of tectonic flattening) and well defined pygidial segmentation explain Selwynʼs (1863) preliminary assignment of the specimen to Asaphus . The glabellar segmentation, especially S1 that is continuous though very weak medially and the shallowness and orientation of S2 and the posterior branch of S3, as well as the overall form of the pygidium, indicate that the specimen belongs to the Phacopidae . The gentle forward expansion of the glabella, the relatively long (sag., exsag.) L1 with large lateral nodes, the well-defined nodes on the occipital ring and thoracic axial rings, and the relatively large and well-segmented pygidium are all consistent with assignment to Phacopidella . A characteristic feature of Phacopidella (and of Eophacops ) is the abrupt shallowing and adaxial flexure of the axial furrow at the anterolateral extremity of L3. This part of the glabella is not preserved in the specimen; however, so that the presence of this feature and assignment to Phacopidella cannot be confirmed. This uncertainty aside, the specimen differs from the type species of Phacopidella , P. glockeri , in having a wider axis on the thorax and pygidium, the pygidial axis extends closer to the posterior margin, and the pygidial pleural and interpleural furrows are deeper.

Figures

Figure 12. A, Phacopidella? sp., NMV P1218, dorsal exoskeleton, × 2.7, from PL1393, Keilor. B–C, Struveria sp. 1, NMV P139357, partly disarticulated exoskeleton with hypostome and cephalic doublure exposed, from PL1386, Broadmeadows; B, × 3.4; C, × 4. D–F, Phacopidae gen. indet. 2, NMV P312076, cephalon, × 8, from PL1369, Springfield. (A, E–F are internal moulds).