Struveria, Rickards, 1965

Sandford, Andrew C. & Holloway, David J., 2006, Early Silurian phacopide trilobites from central Victoria, Australia, Memoirs of Museum Victoria 63 (2), pp. 215-255 : 246-249

Struveria ? plinthourgos sp. nov.

Figure 18

Eudolatites sp.— Rickards and Sandford, 1998: 751.

Type material. Holotype NMV P79125 (internal mould of dorsal exoskeleton) from PL1374 , old Camberwell brick pit, Rose Street, Camberwell. Anderson Creek Formation.

Derivation of name. Greek, ʻbrickmakerʼ, in reference to the type locality.

Diagnosis. Glabella subpentagonal in outline, expanding strongly and rather uniformly forward, width across frontal lobe about twice width across occipital ring and approximately equal to sagittal length.Eye short (exsag.), with posterior edge opposite S3. Pygidium with strongly convex (tr.) axis of 10 or 11 rings, 9 pleural furrows, and relatively broad, concave border on which pleural and interpleural furrows are very weakly expressed.

Description. Cephalon with length (sag.) about 60% maximum width. Glabella comprising a little less than 30% cephalic width posteriorly. Occipital furrow shallowing and deflected forward medially. S1 and S2 transverse, meeting axial furrow distinctly, adaxial portions with slit-like apodemal pits situated in line exsagittally, S2 comprising 33% glabellar width at this level and with apodemal pit occupying half width (tr.) of furrow. L1 approximately 80% length (exsag.) of L2 laterally. S3 diverging forward at about 125°, shallow adaxially and deepening slightly abaxially but not expanding appreciably. L3 a little more than twice as long (exsag.) abaxially as adaxially. Frontal lobe comprising approximately half sagittal length of glabella, rhombic in outline, apparently with short (exsag.) longitudinal depression at about midlength. Eyes and palpebral lobes not preserved, palpebral furrow apparently shallow, weakly curved and oriented slightly oblique to exsagittal line. Posterior branch of facial following gently sigmoidal course, directed slightly forward across most of genal field and deflected backward laterally. Posterior border furrow deeply impressed, apparently deflected forward slightly abaxially and dying out distally before reaching lateral border furrow. Backward deflection of posterior cephalic margin distally indicates that genal spine was present (fig. 18D) but its length is unknown. Fixigenal field densely pitted.

Thorax of 11 segments. Axis strongly convex (tr.), increasing slightly in width from 1st to 4th or 5th segment and thereafter narrowing at similar rate, width on 1st segment equal to that on 8th segment and almost 120% that on last segment. Axial rings gently convex (sag., exsag.), without lateral lobes. Axial furrow deeply impressed. Pleural lobe slightly wider (tr.) than axis in anterior half of thorax and becoming increasingly wider towards back. Pleurae strongly downturned beyond fulcrum, with large articulating facets and pointed tips becoming backwardly deflected towards back of thorax. Pleural furrows very deep, extending weakly onto articulating facets distally where they curve slightly forward.

Posterior termination of pygidium unknown; if it is rounded rather than mucronate or angular, pygidium is about 150% as wide as long (sag.) with parabolic posterior outline; anterolateral corner well-rounded. Axis strongly convex (tr.), width 25% maximum pygidial width anteriorly, possibly not tapering uniformly backward but a little more strongly across first 3 segments than next 2 or 3, thereafter at similar rate as initially. 1st ring with broad, shallow median excavation in posterior margin for pseudo-articulating half ring on 2nd segment, much weaker excavations present on 2nd and 3rd rings. Inter-ring furrows 1–6 with apodemal pits laterally, remaining inter-ring furrows very weak. Pleural field rather strongly convex (tr.), 1st segment with wide articulating facet extending 66% width of pleura and with deep pleural furrow not extending onto facet distally, subsequent segments with successively shallower pleural furrows more closely approaching interpleural furrows in depth and length (exsag.). Concave border comprising almost 33% pleural width anteriorly, not bounded adaxially by distinct border furrow, outer margin not rolled in transverse section.

Remarks. The only known specimen is an internal mould of a very large and substantially complete dorsal exoskeleton almost 130 mm long (assuming the pygidium lacks a mucro). The cephalon is crushed, especially the frontal lobe of the glabella which bears radiating fractures, so that its flattened profile is not indicative of the original convexity. The anterior outline of the glabella in dorsal view is formed by an arcuate fracture, below which the cephalon slopes steeply downward to a lower margin that is a smoothly curved line (figs 18C–D); it is not clear whether this line is the facial suture or the hypostomal suture, but the former is more likely. The lateral part of the right cheek has been pushed downward and inward along a posterolaterally-directed fracture that has destroyed the palpebral lobe, and of the right librigena only the border and possibly a small portion of the adjacent field are preserved. On the left cheek the palpebral lobe has been broken off along the palpebral furrow, the flattened appearance of the genal spine base may not be indicative of its original shape in cross-section, and all that remains of the librigena is the lateral outline (fig. 18D). The distal parts of the last few thoracic segments are missing on the left side, together with most of the left pleural lobe and posterior termination of the pygidium.

Generic assignment of the species is problematic, partly because certain important characters cannot be determined, including the form of the cephalic borders and genal spine, and the nature of the pygidial termination. The specimen was assigned to Eudolatites by Rickards and Sandford (1998), although that genus is not known to range above the Ordovician, but features such as the transverse orientation of S1 and the presence of an apodemal pit in S2 are incompatible with such an assignment. On the basis of these characters and the form of the pygidial pleural and interpleural furrows we tentatively assign plinthourgos to Struveria . The species differs from other members of the genus in that the glabella is subpentagonal in outline rather than club-shaped (i.e. it is not concave in lateral outline behind a transverse line through the inner end of S3), the eye is shorter (exsag.) and does not extend as far backward, the palpebral furrow is more weakly curved, the genal spine appears to be smaller at its base, and the pygidium has a concave border; however,we can not be certain that the last feature is not caused by compression of the dorsal exoskeleton onto the doublure.

Figures

Figure 18. Struveria? plinthourgos sp. nov., holotype NMV P79125, internal mould of dorsal exoskeleton, from PL1374, Camberwell; A, × 1; B–E, × 1.25.