A Devonian Trilobite with an Eyeshade

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Science  19 Sep 2003:
Vol. 301, Issue 5640, pp. 1689
DOI: 10.1126/science.1088713

Trilobites include the oldest species with preserved eyes for which detailed optical mechanisms have been suggested. They appear in the Cambrian, and in comparison to other species present then, trilobites already had unusually sophisticated eyes with lenses made out of the mineral calcite (1). Trilobite eyes are of two types, holochroal and schizochroal (2). The former are composed of minute lenses, often numbering more than a thousand, and include adaptations both to benthic and pelagic modes of life (3). Schizochroal eyes, by contrast, have relatively few, larger lenses, each one strongly biconvex, and in most species separated from their neighbors by an inter-lensar sclera. These specialized eyes are characteristic of a great group of trilobites, Order Phacopida, that flourished between Ordovician and Devonian times; they have been claimed as permitting binocular vision (4), and they contained internal structures of high magnesium calcite that helped the lenses to focus more precisely by eliminating sources of “fuzziness,” such as spherical aberration (5, 6).

We report here a discovery of the only known complete specimen of the phacopoid trilobite Erbenochile, from the Devonian of Morocco. The same locality has yielded many beautiful spiny trilobites. Erbenochile was originally described from Algeria (7) where it was known only from a pygidium; the head region is newly described. The eyes extend prominently upward and have an extraordinary visual surface, marked by spectacular straight-sided towers of lenses (some 560 in all) with 18 lenses in a vertical file. The eyes are strongly curved from front to back, and the lenses accordingly commanded a field of view that was entirely over the sediment surface on which the animal lived. The compass of the eyes shows that they commanded a 360-degree sweep in the horizontal plane. The high elevation of the eyes meant that the animal could even see backward over its thorax.

The eye is different from that of other trilobites in that the palpebral lobe above it extends outward over the whole of the visual surface as an eyeshade, or visor. Because the corneal surfaces are spherical, they are vulnerable to picking up stray light from directions other than the favored one normal to the surface of the eye. But the eye is so straight-sided that this hood effectively protects the visual surface from glare derived from surface light. We were able to demonstrate this by shining a parallel beam down from above the eye (Fig. 1E); the eyeshade efficiently cuts out incident light from this direction. A similar protection is described from living arthropods for which visual acuity is at a premium, but contemporary arthropods usually accomplished this by differential development of the lens systems or by tiering of photoreceptors (8). This trilobite developed its own solution to the problem of seeing over the Devonian sea floor without distraction. The vertical array of lenses is arranged in such a way that the c crystallographic axes of the calcite forming the lenses—the optical “sights”—are parallel in the horizontal plane. Properly shaded, this ensures that the trilobite can detect even small movements at a distance over the sediment surface. By contrast, on a normal, curved eye, the sensitivity of an individual lens decreases with distance. Interestingly enough, the same adaptation shows that these animals were diurnal rather than nocturnal, a detail of life habit which has previously been speculative. An eyeshade is of little use in the dark.

Fig. 1.

Erbenochile erbeni (Alberti). Devonian (Emsian) Timrahrhart Formation (Jebel Gara el Zguilma, near Foum Zquid), southern Morocco. (A) Posterior view showing overhanging eyeshades. (B) Lateral view. (C) Dorsal view. The headshield is 32 mm across. (D) Side view detail of right eye showing lenses under optimum illumination, and (E) how the eyeshade cuts out light from above, when directed as a parallel beam above the palpebral lobe.

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