Disparate compound eyes of Cambrian radiodonts reveal their developmental growth mode and diverse visual ecology

Abstract

Radiodonts are nektonic stem-group euarthropods that played various trophic roles in Paleozoic marine ecosystems, but information on their vision is limited. Optical details exist only in one species from the Cambrian Emu Bay Shale of Australia, here assigned to Anomalocaris aff. canadensis We identify another type of radiodont compound eye from this deposit, belonging to 'Anomalocaris' briggsi This ≤4-cm sessile eye has >13,000 lenses and a dorsally oriented acute zone. In both taxa, lenses were added marginally and increased in size and number throughout development, as in many crown-group euarthropods. Both species' eyes conform to their inferred lifestyles: The macrophagous predator A. aff. canadensis has acute stalked eyes (>24,000 lenses each) adapted for hunting in well-lit waters, whereas the suspension-feeding 'A.' briggsi could detect plankton in dim down-welling light. Radiodont eyes further demonstrate the group's anatomical and ecological diversity and reinforce the crucial role of vision in early animal ecosystems.

Fig. 1. Acute zone–type eye of ‘A.’ briggsi.

(A to C) SAM P54853; inset in (A) shows the position of (B), and inset in (B) shows the position of (C); arrowheads in (A) indicate contact between eye sclerite and visual surface. (D) SAM P57421. (E and F) SAM P48377a,b, part (E) and counterpart (F). Scale bars, 5 mm (A and D to F), 1 mm (B), and 0.5 mm (C). es, eye sclerite; mr, marginal rim. Photo credit: J. Paterson, University of New England (A to F).

Fig. 2. Acute zone–type eye of ‘A.’ briggsi.

(A to D) SAM P54248. (E and F) SAM P52901a,b, part (E) and counterpart (F). (G) SAM P55428. Scale bars, 5 mm (A), 3 mm (B and E to G), 2 mm (C), and 1 mm (D). es, eye sclerite; mr, marginal rim. Photo credit: J. Paterson, University of New England (A to G).

Fig. 3. Eye sizes, lens counts, and lens diameters for the best-preserved Emu Bay Shale radiodont eyes and select euarthropods known for their extreme eye morphology.

(A) Total number of lenses and (B) maximum lens diameter (μm) plotted against maximum eye diameter across long axis (mm). Data sources: Acutiramus cummingsi (34), Anax junius (39), ‘Anomalocaris’ briggsi and Anomalocaris aff. canadensis (tables S1 and S2), Carolinites genacinaca (21), Cystisoma sp. (42), Dalmanitina yichangensis (21), Dollocaris ingens (37, 38), Eurypterus sp. (34), Jaekelopterus rhenaniae (35), and Limulus polyphemus (34).

Fig. 4. Compound eye of A. aff. canadensis.

(A and B) SAM P49070; inset in (A) shows the position of (B). (C) SAM P52893. Scale bars, 5 mm (A and C) and 1 mm (B). Photo credit: J. Paterson, University of New England (A to C).

Fig. 5. Head reconstructions of Emu Bay Shale radiodonts, with the visual surface of the eyes shown in gray.

(A and B) ‘A.’ briggsi showing sessile (nonstalked) eyes in lateral and anterior views, respectively, with the acute zone depicted by lighter shading; the dorsal head sclerite and oral cone are conjectural. (C and D) A. aff. canadensis showing stalked eyes in lateral and anterior views, respectively; the position and orientation of the eyes and the presence of a dorsal head sclerite are based on specimens of A. canadensis from the Burgess Shale [e.g., figures 1 and 5 in (16); supp. figure 8 in (10)] and Anomalocaris saron from the Chengjiang biota [e.g., figures 1B and C and 2 in (2)], in addition to the eye pair from the Emu Bay Shale [figure 1a and b in (20)].