Balnibarbi erugata Fortey, 1974

Hopkins, Melanie J., 2019, Phylogenetic analysis and revision of the trilobite subfamily Balnibarbiinae (Olenidae), American Museum Novitates 2019 (3928), pp. 1-1 : 1-

Balnibarbi erugata Fortey, 1974

Figure 2

TYPE SUBSPECIES: Balnibarbi erugata erugata Fortey, 1974 .

TYPE LOCALITY: Profilstranda, Olenidsletta Member, Valhallfonna Formation, Ny Friesland, Spitsbergen, Svalbard.

SUBSPECIES: Balnibarbi erugata erugata Fortey, 1974 , Balnibarbi erugata sombrero ( Fortey, 1974) .

EMENDED DIAGNOSIS: Balnibarbi species with long (sag.) anterior border rounded on midline. Anterior border with 25–30 pits; pits expressed only on internal mold. Glabella rounded anteriorly; preglabellar field broad. Palpebral lobes long, with anterior end forward of 2p and posterior end at occipital furrow. Moderately large pygidium with four axial rings.

DISCUSSION: In both subspecies, distinct pits are visible only on the internal mold of the anterior border furrow (fig. 2E, F). In this case (where the pit is not expressed on the dorsal surface), this trait may be better described as a series of “protuberances” on the ventral surface that form pits on the internal mold. These protuberances meet nodes on the doublure of the librigena beneath the anterior border of the cranidium (see Fortey, 1974: pl. 5, fig. 10). On other Balnibarbi species , the structures are visible on the anterior border (e.g., Balnibarbi pulvurea , see Fortey, 1974: pl. 1, fig. 4), and more in keeping with the idea of a “pit.”

The holotype of Balnibarbi erugata erugata is almost completely exfoliated (fig. 2A), but specimens that otherwise fit the diagnosis (including some listed by Fortey, 1974) show very fine granulation across the glabella (fig. 2D), in the posterior border furrow (fig. 2C), and more rarely preserved on the frontal area (fig. 2B). Terrace lines are evident on the border of the holotype (fig. 2A) and some better-preserved specimens (fig. 2B). Fortey (1974) differentiated Balnibarbi erugata and Balnibarbi sombrero based on the length (sag.) of the preglabellar field relative to the glabella. The type and figured specimens of B. sombrero do have a larger preglabellar field (fig. 3), and there is no granulation visible on the external surface where preserved. However, some specimens that have fine granulation

Figures

FIGURE 2. Balnibarbi erugata Fortey, 1974, close-ups. A. Balnibarbi erugata erugata, holotype cranidium, PMO-NF-3016. Note exfoliation of frontal area and glabella. Arrow points to terrace lines on anterior border that are expressed on both the external surface and internal mold. B. Anterior border of Balnibarbi erugata erugata, CAMSM F3976, showing faint caeca and very fine granulation on frontal area near anterior border furrow and terrace lines on border. C. Posterior border furrow on Balnibarbi erugata erugata, CAMSM F3976, showing very fine granulation. D. Occipital ring of Balnibarbi erugata erugata, PMO-NF-695, showing very fine granulation on glabella. E. Balnibarbi erugata erugata PMO-NF-772. Arrows point to select pits in anterior border furrow; specimen is exfoliated. F. Balnibarbi erugata sombrero, PMO-NF-2054. Arrows point to select pits in anterior border furrow on exfoliated surface. Scale bars = 1 mm.