Trilobite Eyes: Calcified Lenses in vivo

Towe, Kenneth M.

doi:10.1126/science.179.4077.1007

Cited by 71 publications

(59 citation statements)

References 11 publications

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“…Trilobites, an extinct class of marine arthropods, had calcitic lenses as part of their visual perception system. [9] Calcitic lenses are found in the skeletal part of the arms of certain species of brittlestars, marine echinoderms. [10] Some sponge spicules form fibers with unique optical properties, [11,12] and regularly spaced platy guanine crystals are known to constitute the basis of structural colors in fish and spiders.…”

Section: Introductionmentioning

confidence: 99%

Certain Biominerals in Leaves Function as Light Scatterers

et al. 2012

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Cystoliths are amorphous calcium carbonate bodies that form in the leaves of some plant families. Cystoliths are regularly distributed in the epidermis and protrude into the photosynthetic tissue, the mesophyll. The photosynthetic pigments generate a steep light gradient in the leaf. Under most illumination regimes the outer mesophyll is light saturated, thus the photosynthetic apparatus is kinetically unable to use the excess light for photochemistry. Here we use micro‐scale modulated fluorometry to demonstrate that light scattered by the cystoliths is distributed from the photosynthetically inefficient upper tissue to the efficient, but light deprived, lower tissue. The results prove that the presence of light scatterers reduces the steep light gradient, thus enabling the leaf to use the incoming light flux more efficiently. MicroCT and electron microscopy confirm that the spatial distribution of the minerals is compatible with their optical function. During the study we encountered large calcium oxalate druses in the same anatomical location as the cystoliths. These druses proved to have similar light scattering functions as the cystoliths. This study shows that certain minerals in the leaves of different plants distribute the light flux more evenly inside the leaf.

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Section: Introductionmentioning

confidence: 99%

Certain Biominerals in Leaves Function as Light Scatterers

et al. 2012

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“…Actually, there are three main questions which it seems worthwhile to answer: The fact that the material of trilobite lenses was primary calcite, as proposed by Towe (1973), 175 has been unequivocally confirmed, the lenses of all known species were originally calcitic, 176 independently of how they have been preserved (Clarkson 1975, 1979, 1997, Clarkson et al 177 2006. This understanding has been strengthened by the use of mineralogical methods and 178 particularly by the use of Electron Backscattered Diffraction (EBSD) technology (Lee et al 2007(Lee et al , 179 2012.…”

Section: Peerj Preprintsmentioning

confidence: 99%

Why did the UV-A-induced photoluminescent blue-green glow in trilobite eyes and exoskeletons not cause problems for trilobites?

Schoenemann

Clarkson

Horváth

2015

Preprint

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The calcitic lenses in the eyes of Palaeozoic trilobites are unique in the animal kingdom, although the use of calcite would have conveyed great advantages for vision in aquatic systems. Calcite lenses are transparent, and due to their high refractive index they would facilitate the focusing of light. In some respects, however, calcite lenses bear evident disadvantages. Birefringence would cause double images at different depths, but this is not a problem for trilobites since the difference in the paths of the ordinary and extraordinary rays is less than the diameter of the receptor cells. Another point, not discussed hitherto, is that calcite fluoresces when illuminated with UV-A. Here we show experimentally that calcite lenses fluoresce, and we discuss why fluorescence does not diminish the optical quality of these lenses and the image formed by them. In the environments in which the trilobites lived, UV-A would not have been a relevant factor, and thus fluorescence would not have disturbed or confused their visual system. We also argue that whatever the reason was that calcite was never again used successfully in the visual systems of aquatic arthropods, it was not fluorescence. 14 Calcite lenses are transparent, and due to their high refractive index they would facilitate the 15 focusing of light. In some respects, however, calcite lenses bear evident disadvantages. 16 Birefringence would cause double images at different depths, but this is not a problem for 17 trilobites since the difference in the paths of the ordinary and extraordinary rays is less than the 18 diameter of the receptor cells. Another point, not discussed hitherto, is that calcite fluoresces 19 when illuminated with UV-A. Here we show experimentally that calcite lenses fluoresce, and we 20 discuss why fluorescence does not diminish the optical quality of these lenses and the image 21 formed by them. In the environments in which the trilobites lived, UV-A would not have been a 22 relevant factor, and thus fluorescence would not have disturbed or confused their visual 23 system. We also argue that whatever the reason was that calcite was never again used 24 successfully in the visual systems of aquatic arthropods, it was not fluorescence. 35 n ω =1.640-1.660, n ε =1.486) by contrast with that of chitin, the lens material of most other 36 arthropods (n=1.46, rarely up to 1.56 (Land & Nilsson 2012)), increases the difference in 37 refractive indices between the visual system of the arthropod and water (n=1.334, seawater), 38 and thus facilitates focusing due to strong refraction. Of special interest has been the visual 39 system of a suborder of trilobites, the Phacopina, because their large lenses (diameters of up to 40 2 mm and more in e.g. Drotops megalomanicus Struve 1990) have an elegant internal 41 substructure, which probably corrected lens aberrations (especially spherical aberration), which 42 would otherwise be produced by the thick lenses that phacopid trilobites possess (Clarkson & 43 Levi-Setti 1975). Although nothing is usuall...

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“…As the present-day crystallographic orientation of the lens calcite is ideal for minimising birefringence and focusing light (Towe, 1973;Clarkson and Levi-Setti, 1975), it is reasonable to assume that in vivo lens orientations and microstructures have been largely retained. Such epitaxial replacement is the norm for recrystallization of high-Mg calcite to low-Mg calcite plus dolomite, which takes place by dissolutionreprecipitation across a narrow fluid film (e.g.…”

Section: Evidence For Diagenetic Alteration Of Lensesmentioning

confidence: 99%

Biomineralisation in the Palaeozoic oceans: Evidence for simultaneous crystallisation of high and low magnesium calcite by phacopine trilobites

2012

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Lee, M., Torney, C. and Owen, A.W. (2012) ABSTRACTThe chemical composition and microstructure of the calcite cuticles of eleven species of phacopine trilobites has been investigated by electron beam imaging, diffraction, and microanalysis, and results reveal that the lenses of their schizochroal eyes differed significantly in chemical composition from the rest of the cuticle in vivo. Apart from the eye lenses, most cuticles are inferred to have escaped extensive recrystallization because their constituent crystals are sub-micrometre in size and have a preferred orientation that is consistent between species. Their current compositions of ~1.4 to 2.4 mole% MgCO 3 are likely to be close to original values, although as they commonly luminesce and contain detectable manganese and iron, some diagenetic alteration has taken place. The associated lenses have a microstructure that is suitable for focusing light, yet are optically turbid owing to the presence within calcite of micropores and crystals of microdolomite, apatite, celestite and pyrite. The microdolomite indicates that lenses recrystallized from an original high-Mg calcite composition and this is supported by the presence of nanometre-scale modulated microstructures in both the calcite and dolomite. These lenses currently contain ~1 to 6 mole% MgCO 3 , and by comparison with the proportion of magnesium lost from echinoderm stereom in the same thin sections, may have contained ~7.5 mole% MgCO 3 in vivo. In some samples, more extensive diagenetic alteration is evidenced by recrystallization of the cuticle including lenses to coarse equant calcite or enrichment of the cuticle, but not necessarily the lenses, in magnesium accompanying replacement by a Mg-Fe phyllosilicate. The phacopine trilobites had to modify partition coefficients for magnesium considerably in order to grow lenses with contrasting compositions to the rest of their cuticles, and such a strong vital effect on biomineralization suggests that incorporation of magnesium was essential for functioning of their calcite optical systems.Keywords: Trilobite, microdolomite, eye, high-Mg calcite, cuticle 2 1. Introduction Many Palaeozoic invertebrates used low-Mg calcite (Mg/Ca <0.04), high-Mg calcite (Mg/Ca >0.04) or aragonite to construct their shells and cuticles (e.g. Zhuravlev and Wood, 2008;Porter, 2010). Most previous workers have concluded that trilobite cuticles were made solely from low-Mg calcite given the good preservation of their finely crystalline microstructures (James and Klappa, 1983) and results of chemical and isotopic analyses that are inconsistent with the former presence of diagenetically unstable aragonite or high-Mg calcite (James and Klappa, 1983;Wilmot and Fallick, 1989; Brand, 2004). Despite this apparent consensus, there have been a number of suggestions that cuticles contained higher concentrations of magnesium. Teigler and Towe (1975) arrived at this conclusion by analogy with the ostracodes, and Lowestam (1963) recorded up to 5 mole% MgCO 3 from a Carboniferous (Lower P...

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Trilobite Eyes: Calcified Lenses in vivo

Cited by 71 publications

References 11 publications

Certain Biominerals in Leaves Function as Light Scatterers

Certain Biominerals in Leaves Function as Light Scatterers

Why did the UV-A-induced photoluminescent blue-green glow in trilobite eyes and exoskeletons not cause problems for trilobites?

Biomineralisation in the Palaeozoic oceans: Evidence for simultaneous crystallisation of high and low magnesium calcite by phacopine trilobites

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