Panoramic view of an entire eye section of Asaphus raniceps Dalman, from a thin polished section observed under the optical microscope (xl6, section loaned by E. N. K. Clarkson, Gr. I. 5512).
Greatly magnified detail of the Asaphus eye section of the preceding plate, observed in dark field illumination. The calcite crystal nature and orientation of each element is clearly revealed by the array of cleavage planes, which can be seen crossing the prisms at a constant angle to the axis of each element. The optic axis of the calcite crystals is aligned in all cases with the lens element axis. Along such an axis, the ordinarily birefringent calcite crystal behaves as an isotropic transparent material, thus avoiding double vision (x460).
This type of eye, which probably evolved from the holochroal eye, is a visual system quite different from any other eye that has ever appeared in the animal kingdom. In the structure of the schizochroal eye, we see trilobites at the peak of their functional creativity, taking advantage of the fundamental laws of geometrical optics in a direct and most efficient way.
The lenses in the schizochroal eye are generally larger (0.2 to 0.7 millimeters), and less numerous (from a few to a few hundred) than the lenses in the holochroal eye. Schizochroal eyes are turretlike, in the shape of truncated cones, and the lens arrangement is most orderly. Logarithmic spirals can still be seen, but the dominant theme is the dorso-ventral file—a rectilinear array of lenses running from the top to the bottom of the visual surface (see plates 24— 33). The evolution of the schizochroal eye has been attributed by Clarkson (1971, 1975) to a phenomenon called paedomorphosis, by which some characteristics that were present in juvenile forms of some trilobites with holochroal eyes became the dominant characteristics in adults of descendant species. We find this type of eye in only one group of trilobites: the Suborder Phacopina, which contains the Superfamilies Dalmanitacea and Phacopacea—the dalmanitids and phacopids. This group lived for a period of about 150 million years during the Ordovician, Silurian, and Devonian periods. Similarities with the schizochroal eye are found in the eyes of the Cambrian Eodiscina (Jell 1975; Zhang and Clarkson 1990).
Plate 24 shows one of the earlier forms of the schiz-ochroal eye and the trilobite that carried it: Pterygometopus brongniarti (Reed), a dalmanitid trilobite from the Ordovi-cian of Scotland. In this particular specimen, the visual surface of the prominent eyes is made up of empty alveoli in which the lenses were once encased. A mold taken from the external impression of the same eye, which was left in the limestone matrix when the trilobite was removed, shows that the lenses were quite convex and closely packed (plate 25). A later dalmanitid form is that of Odontochile hausmanni (Brongniart), shown in plate 26. The lenses here are well separated, and the growth mechanism of the visual surface is plainly illustrated by lenses which are in the process of being released from the generative zone at the base of the eye. In the phacopid line, one of the earliest forms of schizochroal eye is seen in Acernaspis sufferta (Lamont), from the Lower Silurian of Scotland, shown in plate 27. The lenses here are more compressed together, giving them an almost hexagonal outline. This eye form eventually led to the perfected eye of the Silurian Eophacops trapeziceps (Barrande), illustrated in plate 28, and of the Devonian Phacops latifrons (Bronn), seen in plate 29. In these late species, the characteristics of the schizochroal eye are exaggerated: the eye contains relatively few lens elements that are rather widely spaced, and each element is admirably set in a thick, sometimes swollen sclera. It is hard to refrain from presenting additional examples of these superbly architectured eyes. The eyes of Phacops rana crassituberculata Stumm from the Devonian Silica Shale of Ohio are shown in plates 30 and 31. These eyes show clearly a band of "sensory fossettes" below the visual surface. The lenses are quite deeply set inside the sclera. A variant of this trilobite, also from the Silica Shale, is Phacops rana milleri Stewart, whose eye is shown in plate 32. It contains more lenses than the previous one (see, for example, Eldredge 1972). The large trilobite Phacops (Drotops) megalomanicus, from the Devonian of Morocco has in recent years pervaded the shelves of every rock shop throughout the world. Its morphology is quite similar to that of, for example, Phacops rana crassituberculata, although in much expanded form, reaching a length of up to 15 cm, and its carapace more prominently tuberculated. Its eye is proportionally magnified, yet the lens size is not, as can be appreciated from plate 33. The increased spacing between the lenses left room for the sclera to develop a prominent hexagonal set of tubercula. In most phacopid eyes, the field of view of each eye could span 180 degrees longitudinally and a strip of ten to twenty degrees latitu-dinally. The schizochroal eye with the smallest number of lenses is shown in the SEM photograph on plate 34. It is the eye of Denckmannites volborthi (Barrande) of the Devonian of Bohemia. In the face of such a layout it becomes difficult to accept the notion that the optical interface of the schizochroal eye operated by fragmenting images of the outer environment into a mosaic of relatively few point elements. If each one of the relatively small number of lenses provided just one point element of the picture, such a mosaic would be very crude indeed. There is evidence that the schizochroal eye was much more efficient than that, and the most compelling part of this evidence is the astonishing structure of the schizochroal eye lens itself.
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