Trilobite Enroll

The right eye of Phacops latifrons (Bronn), from the Devonian of Germany. (Loaned by E. N. K. Clarkson.) (xl4)

Left eye of Phacops rana crassituberculata Stumm, from the Devonian Silica Shale at Sylvania, Ohio (xl3). (RLS coll.; now at FMNH.) Specimen whitened with magnesium oxide. The lenses at the top of the visual surface appear incompletely developed.

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Right eye of another specimen of Phacops rana crassituberculata Stumm (x!3). (RLS coll.; now at FMNH.) Specimen whitened with magnesium oxide. The lenses are deeply encased in the scleral surface.

The right eye of Phacops rana milleri

Stewart, from the Devonian Silica Shale of Sylvania, Ohio (xl3.5). (RLS coll.; now at FMNH.) This trilobite, familiar to many collectors, differs from the subspecies shown in plates 31 and 32, mostly in the larger number of eye lenses and in the less tuberculate surface of the palpebral lobe.

Well known to the trilobite collector are the giant Devonian Phacops (Drotops) megalomanicus, (aff. Phacops rana crassituberculata), that are extracted in staggering number from the Anti-Atlas mountains of Morocco. Its eye, as shown in this example, is about twice as large as that of the typical Phacops rana shown in the preceding plates, yet its lenses, deeply set, are smaller. Prominent scleral tubercula occupy the increased spacing between the lenses in a regular hexagonal pattern that mimics the lensar arrangement. (RLS coll.) (x7.7)

SEM view of the eye of Denkmannites voborthi (Barrande). Devonian of Bohemia (x65). (Negative loaned by E. N. K. Clarkson.) This type of schizochroal eye is among those containing the smallest number of lenses.

In fact, this optical doublet is a device so typically associated with human invention that its discovery in trilobites comes as something of a shock. The realization that trilobites developed and used such devices half a billion years ago makes the shock even greater. And a final discovery—that the refracting interface between the two lens elements in a trilobite's eye was designed in accordance with optical constructions worked out by Descartes and Huygens in the mid-seventeenth century—borders on sheer science fiction.

Of course, the laws of physics existed prior to their discovery by man. And we shouldn't perhaps be too surprised that the drive to optimize biological function—one of the fundamental evolutionary forces in all biological organisms—caused trilobites to follow physical laws to the fullest possible extent in their development of visual systems. The real surprise should not be that they did construct eyes that work according to the laws of physics, but that they did it with such ingenuity. The basic lens designs recognized in the original studies by Clarkson are reproduced in figures 10b and lib. These drawings schematize the eyes of two dalmanitid trilobites: on the left, Crozonaspis struvei (Henry), from the Middle Ordovician of Brittany; and on the right, Dalmanitina socialis (Barrande), a Middle Ordovician species from Bohemia. The thick biconvex lens shape of both results from the matching of two basic parts: an upper lens unit and an intralensar bowl. A wavy interface separates (and unites) the two components.

Long before trilobites were even recognized as ancient inhabitants of our planet, Descartes in his La Geometrie (1637) and Huygens in his Traite de la Lumiere (1690) both had derived the general shape that the second refracting surface of a lens should have to have in order to eliminate spherical aberration—a defect of simple lenses which causes a point object to be imaged as a blurred disk. To recreate an exciting moment of my literature search, it is still worthwhile to reproduce in figures 10a and 11a the pages of these treatises that contained such derivations. Since the two authors assumed somewhat different shapes for the first refracting surface, they reached somewhat different conclusions about the shape of the second (correcting) surfpce.

(a) Construction of a lens free of spherical aberration, from C. Huygens' Traite' de la Lumiere. (b) Cross-sectional view of the lenses in the eye o f Cro%onaspis struvei Henry, an Ordovician trilobite from Brittany (Clarkson 1968). The intermediate surface is shaped accordingly to the prescription by Huygens.

(a) Construction, similar to that of figure 10, after Descartes in La Geometric Here again, the shape of the second surface makes the lens free of spherical aberration.

(b) Cross-sectional view of the lens structure in the eye o f Dalmanitina socialis (Barrande) (Clarkson 1968). The intermediate surface is shaped in remarkable accord with the design by Descartes.

However, the physics involved is the same in both cases and consists of an application of Fermat's principle of least time to achieve the so-called stigmatic condition for an optical system: to obtain a point image of a point object, parallel beams of light must traverse the path between the object and the image in the same amount of time—either by traveling along a straight line connecting the two points (the shortest path and therefore the fastest route), or by traveling along any other off-axis path through the lens. In more modern language: all rays from a point source will converge to a point image if they traverse minimal and identical optical paths. A lens that satisfies this condition is free of spherical aberration and is called aplanatic. Its exit surface will depart from a spherical profile being one of a family of mathematical curves called Cartesian Ovals (described by a fourth-order equation in Cartesian coordinates).

By comparing the shape of the aspheric lens exit surfaces constructed by Huygens and Descartes with the two lens structures identified by Clarkson (figures 10b and 1 lb), little doubt remains that trilobites utilized the properties of Cartesian Ovals more than 400 million years before the seventeenth-century masters discovered the principle. This is not all, however. A difference exists between the media for which the theoretical profiles were derived (a glass lens in air) and those involved in the trilobite's lens operation. We know in fact that the upper lens units of a trilobite's lenses were made of calcite oriented along the oaxis (refractive index n = 1.66), much as previously seen for the holochroal eye, and confirmed by Towe (1973) for the phacopid eye lenses, and that they operated in water (n = 1.33). If we attempt to trace light rays through such a system, of weaker converging power than the glass-air system, we find that the lens would not focus all rays to a point as expected. In fact, the more peripheral rays would diverge. Here the role of the intralensar bowl comes into play to restore the focusing property of the lens and the correcting function of the interface. The unknown refractive index of the intralensar bowl can be inferred from the construction shown in figure 12a. This illustrates the optical function of the doublet structure and also shows (on the right side of the lens axis) what would happen if the lens were a single solid unit. In the latter case, spherical aberration would prevent the formation of sharp images and would also dilute the amount of light collected along the axis of the lens. An experiment with a large scale reconstruction of the doublet with an upper unit made of calcite with its r-axis oriented along the lens axis, combined with an intralensar bowl made of transparent plastic (« = 1.63) that is shaped as in Crozonaspis, does indeed produce the results that were envisioned by Descartes and Huygens (and the trilobite), as shown in figure 12b. One can speculate that a perfusion of a small amount of soft organic tissue into oriented calcite, the materials available to the trilobite, could have easily reduced the refractive index of the calcite to the value inferred for the intralensar bowl.

The realization that trilobites made recourse to a doublet lens structure to achieve the goal of improving their vision left me with the uncanny feeling that, if needed, nature could have equally well developed other multi-element optical instruments that are touted as unique creations of human ingenuity. In our case, a doublet structure is added to the already sophisticated aspheric correcting interface. The design of the trilobite's eye lens could well qualify for a patent disclosure. Prior art would mention the Schmidt plate of modern telescopes, a Cartesian surface performing function similar to that of the wavy interface of the trilobite's eye lens. As I will discuss further below, there is a follow-up on the trilobite's eye story, dealing with the eye lenses of several modern arthropods, that makes my views much less facetious than they may seem at this point.

There is even more than meets the eye in the trilobite's feat. Lenses such as described are clearly optimized in more than one way. The fact that the upper unit is made of calcite, with its r-axis lined up along the axis of the lens, accomplishes two things. First, the refractive index of calcite is highest for this orientation (n = 1.66, which maximizes the light gathering ability of the lens). Second, the double refraction effect of calcite is effectively eliminated by this configuration, at least for paraxial rays. As a result, such lenses succeed in concentrating all the collected light in a

(a) Ray tracing through the lens of Crozonaspis struvei Henry. On the right of the optical axis, the internal lens structure is ignored. Very large spherical aberration ensues for any choice of the refractive index of the lens (» = 1.66 for the construction indicated). When the internal structure is taken into account, on the left-hand side of the axis, correction of spherical aberration obtains for the combination of refractive indices indicated in the text. This suggests that the two lens elements were made of oriented calcite and protein-rich material respectively, (b) Experimental verification of the operation of a large-scale doublet lens structure modeled after the lens of Crozonaspis struvei Henry (from Clarkson and Levi-Setti 1975, reprinted by permission from Nature 254: 663-67; Copyright © 1975 Macmillan Magazines Limited). The upper lens unit is made of calcite oriented with the c-axis along the lens axis, the intralensar bowl of transparent plastic with refractive index n = 1.63. The lens is immersed in milky water. A beam of parallel light incident on the lens from above is brought to a narrow focus by the corrected lens. The full width at half maximum of the light intensity depth distribution would be five times wider for a single uncorrected lens of the same overall external profile.

thin layer (approximately one lens-thickness below the vertex of the lens) where good images of the surroundings would be formed. The f: number (inverse of the relative aperture) of the lens of figure 12, operating in water, is - f: 1.1. Not bad at all, even by modern standards. And because of their size and short focal length, our wonder lenses give to the trilobite's eye a remarkable depth of field with no need for accommodation. Further analysis of the possible effects of the birefringence of calcite suggests that they were probably unimportant (Clarkson and Levi-Setti 1975). As to lens defects due to chromatic aberration, these also were considered unimportant, since even at moderate depths in sea water, the environment is essentially monochromatic.

In the actual fossilized eye of phacopids, the evidence for the extraordinary doublet structure is occasionally obvious in polished sections of the lens surface. An example is provided by the series of images in plate 35, obtained from a polished thin section through the large eye of a Silurian Dalmanites specimen, prepared by Dr. Clarkson. Here dark field microscope illumination enhances the visibility of the doublet structure, and the shape of the correcting surface is intermediate between the two solutions previously mentioned. The characteristic calcite cleavage planes are clearly outlined in the upper unit of several of the lenses, again providing evidence of their single crystal nature and f-axis orientation. The intralensar bowl appears as a dark, cloudy region, suggesting that its chemical composition was different originally from that of the upper unit. Even when diagenetic alteration has rendered the upper unit opaque, the intralensar bowl can still be identified as a distinct region of the lens. In other cases, due to differential mineralization, the intralensar bowl splits from the upper lens unit. For example, the doughnut shape of the upper surface of the intralensar bowl is plainly visible in the SEM image shown in plate 36, where the upper lens units were not preserved in the fossilization process. The visual surface is, in this case, that of Zeliskella Lipeyrei (Bureau), from the Ordovician of the Crozon peninsula, France, which has a structure similar to that of Crozonaspis. Some remnants of the doublet structures are present in the holotype of Dalmanitespratteni Roy

(Roy 1933), a spectacular phacopid trilobite from the Devonian of Illinois. Front views of the right and left eye of this rare trilobite are shown in plates 37 and 38 respectively. Each eye contains more than 770 lenses, an absolute record for schizochroal eyes. Several of the lenses are split and show the intralensar bowl. In other cases, the upper lens unit is still in place, and in still others the entire doublet is missing and the cavity left is the lensar pit. The Huygens' type lens substructure has been established by Clarkson (1969) for the eye of Reedops sternbergi (Hawle and Corda), represented in plate 39. The evidence, however, is accessible only through sectioning. Two SEM views of the visual surface of this trilobite are shown in plates 40a and 40b.

Why did the phacopid trilobite develop such a sophisticated optical system? Were the perfected images produced by the corrected lenses exploited in any way? Were there other advantages that favored the evolution and retention of these optimal lens structures? What we would like to hear, to appease our Darwinian upbringing, is that new visual structures were evolved in response to new environmental pressures as a means of survival. A factor that may be considered is that the correction of spherical aberration increases significantly the level of light intensity in the focal plane of the lens relative to that concentrated by a solid lens. From the experiment of figure 12b, we did estimate (Clarkson and Levi-Setti 1975) that the full width at half maximum of the light intensity distribution along the axis would be contained, for the trilobite's lens, within a layer - 20 Lim thick. This is to be compared with a width of - 100 Llm for an uncorrected, solid lens of the same profile, due to the spreading of the collected light along the lens axis caused by spherical aberration. The fivefold increase in the level of illumination at the focal plane could conceivably have exceeded the threshold level of neural response in a dimly lit environment, allowing the trilobite to see at some depth in the sea, at dusk, or in turbid water. And yet the lens arrangement and shape of the schizochroal eye raises doubts that a useful mosaic image could have been formed by this type of eye. The number of lenses is generally too small and the angular coverage of their fields of view too discontinuous

plate 35

(Facingpage) The doublet structure of the dalmanitid lens is clearly visible in these thin sections from one specimen of a Silurian Dalmanites. The upper lens unit consists of oriented calcite crystals, while the intralensar bowls are composed of calcite mixed with remnants of organic material. Various stages of diagenetic alteration of the calcite are visible, but none of these obliterate the primary structure. Much as for the prisms of the Asaphus' visual surface in plate 23, the trace of the cleavage planes tell that the optic axis of the calcite crystals is along the lens axis. The Cartesian surface separating the two lens elements is recognizable in all cases shown, as well as in some sixteen additional lenslets from the same eye, not shown here. (Polished section loaned by E. N. K. Clarkson.) (xl08)

plate 36

The doughnut shape of the intralensar bowl is clearly shown in this scanning electron micrograph of the eye of Zeliskella lapejrei (Bureau), an early dalmanitid trilobite from the Ordovician of Rennes, France. In this specimen, only the lower lens units have been preserved. (Gr. I. 40192, loaned by E. N. K. Clarkson, SEM micrograph by the author.) (xl 13)

Frontal view of the left eye o f Dalmanites pratteni Roy, a

Devonian trilobite from Illinois (xl3). (Specimen loaned by Field Museum of Natural History, Chicago.)

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Frontal view of the right eye of Dalmanites pratteni Roy, as in plate 37. The lens structure described in the text is exposed in several instances, where the front lens-unit has split away. In some elements, the entire lens appears intact, encased in the cylindrical sclera. Among schizochroal eyes, the eyes of this trilobite exhibit the largest number of lenses ever recorded.

plate 39

The eye of Reedops

(Hawle and Corda), a Devonian trilobite from Bohemia (x22). (Negative loaned by E. N. K. Clarkson; Clarkson 1969.) Sectioning of the lenses of similar specimens by Clarkson has revealed an internal structure similar to that described in figure 10 (b).

(a) SEM view of portion of the eye of Reedops Sternberg!, as in plate 39 (xl08). (Negative loaned by E. N. K. Clarkson.) (b) Another SEM view, at a glancing angle, of the same visual surface. (x216)

to form a detailed mosaic similar to that that we presume formed by the schizochroal eye and that of insects and crustaceans. Further questions come to mind: was the schizochroal eye a regressive trait? Was the improved light-gathering ability the only factor promoting the correction of lens aberrations? It is somewhat disappointing to conceive that the sharp and detailed images produced by each corrected lens were simply used as a trigger by the photoreceptors and their structure ignored by the "brain" of the trilobite.

Questions such as these obviously stem from our ignorance of the visual receptor structure of the trilobite's eye. Claims of having detected crystalline fibers in the pyritized eye of phacopid trilobites, preserved in the Hunsruck Slate, have been advanced by W. Sturmer, based on his radiographic observations. If true, this knowledge would set to rest any notion that the schizochroal eye was anything more than what is found in the compound eye of groups of extants arthropods. Dr. Sturmer tried hard to convince his dissenting collaborator Jan Bergstrom, and then myself, of his interpretation. In the process, he provided me with the radiograph shown in plate 41, which shows a pattern of parallel filaments seemingly emerging from the visual surface and leading to points located in the posterior part of the cephalon. Additional examples of a similar nature (Sturmer and Bergstrom 1973) have been presented and interpreted as crystalline fiber lightguides by W. Sturmer, against J. Bergstrom's better judgment. As explained to me by the latter author, who had occasion to examine Sturmer's material, the observed structures do not belong to the eyes at all, but are gill branch setae, seen as a single file extending from the eye region to the pygidium. In some sense, this rejection is welcome. In fact, fiber optics coupled to phacopid lenses would seem to defeat the purpose of the sophisticated design of the latter, as already remarked above. Furthermore (Clarkson and Levi-Setti 1975), the hypothesis is advanced that schizochroal eyes may be regarded as an aggregate of individual eyes rather than the mosaic-forming device common to other arthropods. It is tempting to speculate that the schizochroal eye was a transition from the truly compound eye scheme of the holochroal eye to the cameratype eye of more advanced life forms. Perhaps minuscule retinas in these schizochroal eyes could already analyze the images supplied by the state-of-the-art lenses they possessed. If this were so, one can speculate that image contrast may have been the factor prompting selection of ever better corrected lens systems to secure survival. In fact, the improved contrast resulting from the correction of spherical aberration could well have provided the advantage of a prompter recognition and response to impending dangers. Perhaps, in addition, mating may have proven more efficient with sharper images. The schizochroal eye disappeared toward the end of the Devonian, with the extinction of the phacopid trilobites. Was the evolutionary wonder represented by their eye lenses an unchallenged occurrence?

3.3.6 A Treasure Not Lost

Even if the genetic information of the perfected visual apparatus in the phacopid trilobite's eye became lost to further evolution within the phylum, the fundamental principles of physics that guided its development obviously survived. And, indeed, they guided other unrelated creatures to reproduce the mastery. From recent studies of vision in modern invertebrates, it has become apparent that the correction of spherical aberration following the precepts of Descartes and Huygens, as well as the concept of adopting a doublet structure for the dioptric apparatus, have not been the unique prerogative of the trilobites. The corneal thick lenses in the compound eye of the backswimmer Notonecta glauca, a predatory aquatic insect, have been shown (Schwind 1980) to consist of a doublet structure with an unmistakable bell-shaped optical interface. Much as for the phacopid lens, the lower unit has a refractive index slightly lower than the upper unit, except that no calcite is involved in the lens composition, only organic material. Theoretical calculations and experimental determinations of the focusing properties of these lenses have confirmed that they are well corrected for spherical aberration (Horvath, 1989). Although we believe that the structures observed in the trilobite's lenses are real

X-ray radiograph of a pyritized specimen of Phacops sp. from the Hunsriick Slate of the Lower Devonian of Germany (WS 2617, x4.5). The presence of a planar structure of filaments superposed to the palpebral lobes was interpreted by W. Sttirmer as evidence of crystalline fibers belonging to the ommatidia of the compound eyes. A more likely interpretation, discussed in the text, assimilates the filamentary structures with those of the gill branches or exopods, commonly preserved in these pyritized trilobites and often seen underlying the carapace. Radiograph contributed by W. Sttirmer.

(not, for example, due to diagenetic alteration of the calcite crystals), and that our interpretation of their function is sufficiently supported by our model, it is gratifying to find confirmation of our conjectures in a living system that can be studied without the need of assumptions. Since no direct connection can be seen between trilobites and Notonecta, it must be inferred that the similarity in the solutions to the problem of vision optimization was the result of convergent evolution.

Another most intriguing two-component, corrected optical system is that found in the eyes of the scallop Pecten (Land 1965) and only recently brought to my attention (Horvath and Varju 1991). In this bivalve mollusc, fifty to sixty simple eyes are embedded in the pallium and appear as tiny, bright iridescent pearls. Their optical structure, sketched in figure 13, rivals in perfection and ingenuity that of the phacopid trilobite's lenses. They also consist of a compound, two-element structure. The upper unit is a soft lens, almost identical in shape to Huygens' solution, but mounted in inverted geometry, so that the bell-shaped interface is external. Facing the spherical, internal interface of this lens is a spherical mirror, the argentea, made out of thirty to forty layers of guanine crystals, interleaved with layers of cytoplasm. This multilayered structure acts as a highly reflecting, interferometric, quarter wavelength mirror. The image is formed on a retinal surface, located between the mirror and the correcting lens, that responds to a decrease in the level of illumination (the "off signal). Another retinal surface, located beneath the former, responds to "on" signals only. The ensuing neural response imparts to the eye remarkable sensitivity to dimming of light levels and angular movement of the light-dark stimulus. In other words, the eye takes advantage of the image contrast, as surmised in our previous discussion of possible evolutionary advantages of correcting eye lens defects. Astonishing as this may seem, the two-element structure of the scallop's eye corresponds to the structure of the catadioptric telescope or Schmidt optical system. This compound lens system has an amazingly large angular acceptance, expressed by an f: number equal to 0.6.1 should mention that the Cartesian Ovals connection was not recognized in the earlier studies of the Pecten 's eye, although the function of the aspherical lens in correcting spherical aberration was fully documented. I also became aware of this preexisting evidence, after having already formulated my reflections, expressed earlier, concerning the significance of such complex designs found in naturally evolved living systems. Whatever repetition this may involve, I felt compelled to narrate how my premonition became eclipsed by reality. Indeed, a wide-angle imaging lens, inspired by the design of the Pectens lens system, has been incorporated into a miniaturized fiber optics endoscope, named the "Tube Peeper" (Greguss 1985).

I indulged in a limited search, among living arthropods, for some evidence that the trilobite's visual treasure may not have been irretrievably lost. My quest dealt with an attempt to find whether any living marine invertebrates may have exploited calcite to construct their eye lenses. Although the search was unsuccessful from this standpoint, I enjoyed the opportunity of becoming aquainted with a group of marine

Crustacea that evolved compound eyes and body morphologies quite similar to those of trilobites. These are the antarctic isopod Crustacea of the genus Serolis. I was alerted to their existence by a letter from David K. Bernhardt of the State University of New York at Albany, following publication of the first edition of this book. Mr. Bernhardt had located, in the local archives of the Albany Institute of History and Art, a nineteenth-century report announcing the discovery of a trilobitelike crustacean that may have possessed calcific eye-lenses. This letter triggered immediate follow-up on my part. The report (Eights 1833), that I promptly obtained from the Albany Institute, was written by James Eights, "Naturalist in the Exploring (Antarctic) Expedition of 1830," and Corresponding Member of the Institute. It described the discovery of a new "Crustaceous Animal" found on the shores of the South Shetland Islands that he named Brongniartia trilobitoides from its resemblance to trilobites and in honor of Alexander Brongniart, eminent paleontologist of the time who first developed the systematics of trilobites. The reading of Eights's report was most rewarding: he mentions that the eyes of his new find were "elevated

Schematics of the two-element structure in the single eyes found in the pallium of the scallop Pecten (adapted from Land 1965). A lens of characteristic Huygensian shape corrects the otherwise aberrated image formed by a spherical mirror, the argentea. This type of structure corresponds closely to that of the catadioptric telescope or Schmidt optical system. In the scallop, two retinal surfaces, located where the image is formed between the lens and the argentea, respond respectively to "on" and " off light signals.

and prominent: cornea oblong, lunulate, reticulate, composed of an infinite number of facets, distinctly visible to the naked eye: color blue, the superior surface covered by an irregular calcareous incrustation." The anatomy of this isopod was meticulously described by detailed ink drawings. The report also recreates in vivid color the misty, frigid, and yet sublime athmosphere and scenary of these antarctic shores being seen for the first time. It did not take anything more to fire up my curiosity and the desire to examine firsthand these blue-eyed little beasts. I poured over the report (Beddard 1884) on the Isopoda collected by the Challenger Antarctic Expedition of 1873-76, where I learned that Eights's "trilobite" is now called Serolis trilobitoides (Eights) and identified as an Isopod crustacean. Another report (Sheppard 1933) on the serolids collected by the Discovery Expedition (1925-32) lists some thirty-seven species of Serolis, all found in antarctic waters, some at great depths in the ocean. I then proceeded to borrow several Serolis specimens from the U.S. National Museum and alerted Dr. Clarkson that we could perhaps learn something about the habits of trilobites if we could observe the behaviour of live, look-alike Serolis. My search for calcite in the eyes of Serolis, carried out with radiographs and X-ray diffractometry, failed to detect crystal lenses. Calcite was found, but only external to the eye, in the form of incrustations, as originally surmised by Eights. The internal structure of the eye of several serolids is described in the report by Beddard (1884). The dioptric apparatus contains a biconvex lens followed by a separate crystalline cone that in some species exhibits a wavy upper profile. A, by now, familiar finding.

Even if not made of calcite crystals, the eye of Serolis schythei (Liitken), shown in plate 42, nonetheless gave me a glimpse of what the holochroal eye of a trilobite must have looked like. Its shape and structure beautifully mimic what we had learned about it from the fossil record, but never had seen in its limpid glow. A specimen of Serolis trilobitoides (Eights) is shown in plate 43. Its turret-shaped eyes were not blue anymore, much to my dismay, possibly due to their denaturated state of preservation in ethanol. A radiograph of Serolis schythei is shown in plate 44. What gives away the fact that Serolis is a crustacean and not a trilobite are the double pair of antennae. Trilobites had only one. Most of the ventral appendages, furthermore, are uniramous instead of biramous, among several other differences. Alas, trilobites are indeed extinct. Nevertheless, the overall appearance of the carapace does indeed remind one of, for example, Arctinurus occidentalis Hall (see plate 45), with its falcate pleural spines.

The eye of a specimen of Serolis schythei Liitken, photographed while immersed in ethanol. The eye structure of this living antarctic isopod ctustacean strikingly resembles that of the holochroal eye of trilobites. However, the eye lenses are made of organic material in this case, not of calcite as the trilobite's. (x60) (Specimen loaned by the U.S. National Museum, Washington, D.C.)

Dorsal view of a specimen of the isopod crustacean Serolis trilobitoides (E i ght s), from the Antarctic Bransfield Strait, photographed while immersed in ethanol. The morphology of this antarctic isopod is reminiscent of that of some trilobites, hence the specific name, given by the nineteenth-century naturalist James Eights. (x4.4) (Specimen loaned by the U.S. National Museum, Washington, D.C.)

X-ray radiograph of a specimen of Serolis schythei Liitken, from the Straits of Magellan. (x4.5) (Specimen loaned by the U.S. National Museum, Washington, D.C.) Similar life habits and basic metameric body plan may have favored (convergent evolution) the selection of body morphologies similar to those developed independently and much earlier by trilobites.

A perfectly preserved example of Arctinurus occidentalis Hall, from the Silurian Rochester Shale of Lockport, New York (xl.49). This is the trilobite that James Eights alluded to when naming trilobitoides the isopod of the genus Serolis, which he discovered on the shores of the South Shetland Islands in 1830. (Photographed by the author through courtesy of Michael Thomas.)

In the meantime, Dr. Clarkson succeeded in interesting the British Antarctic Survey in the task of bringing home several live specimens of Serolis. Observations were made of these gentle creatures frolicking in a tank and staring at the observer with their multifaceted, translucent eyes. A movie was made, one which gave me the eerie feeling of watching a primordial pool crawling with trilobites.

3.4 Enrollment

The great majority of trilobites could roll themselves up so that only the hard carapace was then exposed. In this condition, various types of spines which may have adorned the exoskeleton became functional, protruding from the enrolled body in a defensive manner. There are many examples of this behavior in modern arthropods. A garden variety millipede (Sphaerotherium) is often seen rolled up. In the horseshoe crab, this function is only partially available, since the thorax is fused with the cephalon. The articulation of the abdominal region, then, allows only a flexure, sufficient to draw-up the powerful telson to make a right angle with the rest of the body. Earlier related forms, however, could double up completely.

The mechanism of enrollment in trilobites is characteristic of particular phylogenetically related groups, as was recently emphasized by Bergstrom (1973a). The thoracic tergites were clearly engineered to permit this function, and a variety of articulating joints have been identified. To give a most schematic description of the thorax, individual tergites were hinged at two points, often characterized by visible notches, proximal to the axial furrows. Rotation of the tergites around these pivot points would separate the axial rings, exposing the articulating half rings, and at the same time cause the pleural extremities to overlap each other. Special devices, called panderian organs, would, like a doorstop, limit the extent of enrollment. Other devices, the vincular furrows, would ensure a safe interlocking of the pygidial and cephalic margins, as if to discourage casual intruders.

A few major modes of enrollment have been recognized by Bergstrom (1973a). These differ from previous descriptions in that they reflect functional characteristics rather than purely morphological distinctions; these forms of enrollment are sketched in figure 14. Some trilobites were unable to roll up completely, and this mode is called incomplete enrollment. Of those which could roll up completely, a distinction is made between spheroidal and spiral enrollment. Each contains a sequence of subcases. In the spheroidal enrollment, the pygidium comes to rest with its ventral side in contact with the cephalic doublure, not inside it, and the pleurae close the exoskeletal basket laterally. When the pleurae do not wrap around to seal the basket laterally, the enrollment is called cylindrical. When the pygidial termination overlaps the cephalic margin, the mode is called inverted spiral enrollment but is still considered part of the spheroidal main group. The spiral enrollment series contains the case in which the dorsalpzn of the pygidium contacts the ventral side of the cephalon; the uncoiled spiral enrollment, characteristic of the calymenids, where the pygidium is visible even in the enrolled condition; and finally the so-called basket and ///Enrollment Both spheroidal and spiral enrollment seem to have evolved from the incomplete enrollment of early Cambrian trilobites. Although enrollment most likely represented a defense mechanism, in the long run it may have precipitated the disappearance of the trilobites. It is conceivable that, with the advent of fishes, an enrolled trilobite could have been swallowed more easily than an outstretched one. Fortunately for us, if not for them, enrolled trilobites made better fossils, since the hard carapace is often quite impervious to weathering agents.

The Middle Cambrian trilobite Elrathia kingii (Meek) from the Wheeler Formation of Utah offers one of the earliest examples of spheroidal enrollment. In plate 46 we see an enrolled specimen of this species from the cephalic side. The ventral side of the pygidium is seen to protrude beyond the cephalic border, possibly as a result of compression. The entire specimen is, in fact, considerably flattened. The picture shows how the genal spines protrude from the body when the trilobite is enrolled.

Schematization of the types of enrollment in trilobites.

(a) Incomplete enrollment, as in Kjerulfia.

(b) Cylindrical enrollment as in Fallotaspis.

(c) Spheroidal enrollment proper, as in Asaphus.

(d) Inverted spiral enrollment as in Placoparia.

(e) Spiral enrollment proper, as in FJlipsocephalus.

(f) Uncoiled spiral enrollment, as in Flexicalymene. (g) Basket and lid enrollment, as in Harpes. (Adapted from Bergstrom, 1973a.)

Another case of spheroidal enrollment, this time in full undistorted relief, is shown in plate 47 (Clarkson 1973a). The trilobite is Encrinurus variolaris (Brongniart), from the Silurian of Dudley, England. Here the closure of the basket is perfect, and one begins to understand why the outline of a trilobite's carapace is often so longitudinally symmetrical. The two halves have to match rather accurately to ensure a complete enclosure in the enrolled condition.

In plate 48, parts a, b, c, and d, we see four different moments in the enrollment process of Phacops rana. In the Devonian Silica Shale at Sylvania, Ohio, this trilobite is commonly found in various postures, from the outstretched to the completely enrolled one. This has nothing to do with incomplete enrollment. The mode describes the ultimate capacity and not the intermediate steps to reach it. Although the character in plate 48 did not quite perform according to the prescription, we are still dealing with a classic example of spheroidal enrollment.

Enrolled specimen of

Elrathia kingii

(Meek), seen from the cephalic side. Middle Cambrian, Wheeler Formation, Utah (xlO). (RLS coll.; now at FMNH.) This is an early example of spheroidal enrollment.

plate 47

A perfect example ot spheroidal enrollment. The trilobite is

Encrinurus variolaris

(Brongniart), Silurian,

Dudley, England (x7.6). (Negative-loaned by E. N. K. Clarkson.)

In plates 49 and 50, we encounter the same Dalmanites pratteni Roy that, in section 3.3, was shown to have such special eyes. The specimen is perfectly enrolled and represents an example of spheroidal enrollment in which the pygidium extends quite a way beyond the cephalic border. It must be realized that, due to its construction, the trilobite could not do any better than this in rolling up. The seal was very tight, however, as can be seen in plate 50, in spite of the limitations.

An example of uncoiled spiral enrollment is shown in plate 51. The trilobite is Flexicalymene meeki (Foerste), Ordovician, from Waynesville, Ohio. Although the enrollment resembles the spheroidal type, it has been shown by Bergstrom that this particular form is at the end of the evolutionary line of spiral enrollment. The margin of the pygidium is safely interlocked in a groove beneath the frontal cephalic doublure. The lateral cephalic margin contributes to wrap the pleural basket. The expansion of the axial rings and exposure of the articulating half rings is clearly visible in this beautifully preserved trilobite. It is the author's experience that enrollment helped the trilobite survive even as a fossil. The little ball was found intact after having been washed away from its burial sediment and down a steep ravine by a trickle of water. The other trilobites, which were not tightly enrolled (and which were not molts), would irreparably disintegrate given the same exposure.

3.5 Life Habits

An accurate reconstruction of the mode of life of trilobites cannot, of course, be given, since we only know them as fossils. Putting together bits and pieces of evidence, however, as in a detective story, we can arrive at a fairly plausible picture of trilobite habits. First of all, we know that trilobites were exclusively marine animals. Their habitat must have varied over a wide range of conditions, as can be conjectured on the basis of their adaptation. The morphology of the trilobite itself is one of the primary criteria in deducing life habits. Further evidence at our disposal is represented by faunistic associations, type of bed sediments in which the fossil was located, and ultimately the trilobite's own footprints. The description of the ventral appendage apparatus in Triarthrus •was discussed in section 3.2 in connection with feeding habits. This is an example of the kind of inference that can be made in a particular case. Although this trilobite did not qualify as a predator but as a rather timid small-particle feeder, there are other trilobites provided with better jaws, and there is evidence that some trilobites actually hunted down their prey in soft-bottom sediments or in the water. In general, trilobites crawled on the sea floor, leaving well-known trail patterns. In a few cases, the fossil trail leads to the trilobite itself, nested in its own burrow. Because of their many telopodites, actual locomotion for trilobites was a byproduct of sifting and reworking the soft substratum in search for food. Some pulled their bodies sideways in this process; others would use their telopods to burrow more deeply to reach for located prey. In other instances, burrows indicate a good resting or hiding place, or an observation post from which to wait for approaching prey. Burrowing became a mode of life for certain groups of trilobites, which developed a morphology particularly adapted to this purpose, such as a smooth exterior and broad axial lobe. The smooth exterior has the clear implication of reducing friction, and the wide rachis must have housed powerful appendage muscles essential to efficient burrowing. Some illaenid and asaphid trilobites have been found in what is thought to correspond to their life posture. The cephalon would rest on the surface, while the rest of the body projected downwards, as shown in figure 15, following the description by Bergstrom (1973a).

Active swimming, on the other hand, must have been the main occupation of other groups of trilobites. In these, the enrollment capacity is reduced, the body is slender and lighter, the pygidium small, the cephalon built hydrody-namically to favor laminar flow. In general, the swimming trilobites had large eyes, with a field of view spanning a circular horizon. P. E. Raymond (1939) must have thought of these characters when he referred to trilobites as the butterflies of the seas. The extreme extension of the visual

The trilobite represented in enrolled position is the same specimen of Dalmanites pratteni Roy, whose eyes are shown in plates 37 and 38 (x2.3). (Loaned by Field Museum of Natural History.) In this photograph, the portion of matrix covering the cephalon and carrying the tip of the pygidium is shown removed from the enrolled trilobite. The cephalic margin is seen to fit tightly against the ventral side of the pygidium.

3.5 Life Habits

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