Naraoia c.f. bertiensis Caron et al., 2004

Naraoia c.f. bertiensis Caron et al., 2004

Fig. 2 View Figure 2

Diagnosis.

Unchanged from Caron et al. (2004).

Occurrence.

Williamsville Member, Bertie Formation (Upper Silurian, Pridoli), Ridgemount, Ontario, Canada ( Caron et al. 2004; type locality; Fig. 2 D View Figure 2 ); Phelps Member, Fiddlers Green Formation, Bertie Group (Upper Silurian, Pridoli), Passage Gulf, Herkimer County, New York State, USA.

Description.

Specimen is preserved as a partly articulated cephalic shield and trunk in dorsal view showing no relief. Neither section shows morphological features. Cephalon-trunk articulation is linear, 11.9 mm long, showing limited overlap of cephalon above trunk. Cephalic shield is suboval with minute genal angles, 14.6 mm wide, and 11.4 mm long. No marginal cephalic rim or doublure are noted. Posterior margin of a possible hypostome noted. Trunk shield is suboval, slightly pointed posteriorly, 12.3 mm wide, and 9.49 mm long. Minute, radial ~ 2.5 mm long linear structures are observed on right side of trunk under low-angle light. Possible terminal axial trunk spine noted. No marginal trunk rim noted. No appendages or soft internal anatomy are preserved in either section.

Remarks.

Our comparisons are limited to the holotype specimen of Naraoia bertiensis ( Caron et al. 2004; fig. 3; Mayers et al. 2019; fig. 1 D; Fig. 2 D View Figure 2 ). The Royal Ontario Museum specimen ( ROM 56013 ; Fig. 2 D View Figure 2 ) from the Williamsville Member lacks external features with a smooth carapace ( Caron et al. 2004). We also demonstrate a naraoiid with an effaced cephalon. The Williamsville Member specimen shows a marginal rim and doublure. These are not documented in NYSM 19522 . However, the lunate / crescentic furrow in the centre of the cephalic shield may represent a posterior hypostome margin. This difference likely reflects the substantial dorsoventral compression compared to the Williamsville Member specimen. Size differences between ROM 56013 and NYSM 19522 (Table 1 View Table 1 ) indicate that NYSM 19522 is ~ 40 % smaller than ROM 56013 , suggesting it may be a more juvenile specimen.

Partial rotation of cephalon over the trunk indicates that NYSM 19522 is a moult. The specimen aligns with Type C naraoiid preservation of Lin (2006) — a specimen “ with articulated dorsal sclerites, but no preserved information regarding appendages and digestive organs ” ( Lin 2006, p. 17). Both Silurian Naraoia specimens are therefore moults ( Zhang et al. 2007).