Magnesiumrich Intralensar Structures In

[Palaeontology, Vol. 50, Part 5, 2007, pp. 1031–1037]

MAGNESIUM-RICH INTRALENSAR STRUCTURES IN SCHIZOCHROAL TRILOBITE EYES by MARTIN R. LEE, CLARE TORNEY and ALAN W. OWEN Department of Geographical and Earth Sciences, University of Glasgow, Gregory Building, Lilybank Gardens, Glasgow G12 8QQ, UK; e-mail: [email protected]

Typescript received 14 March 2007; accepted in revised form 25 May 2007

Abstract: The interpretation of the lenses of schizochroal ucts reflect original differences in mineral chemistry between trilobite eyes as aplanatic doublets by Clarkson and Levi-Setti the upper lens unit and lower intralensar bowl. The turbidity over 30 years ago has been widely accepted. However, the of the bowl and of the core within the upper part of the lens means of achieving a difference in refractive index across the are the result of their greater microporosity and abundance interface between the two parts of each lens to overcome of microdolomite inclusions, both of which were products of spherical aberration has remained a matter of speculation diagenetic replacement of original magnesian calcite in these and lately it has been argued that the doublet structure itself areas. Such a difference in magnesium concentration in the is no more than a diagenetic artefact. Recent advances in original calcite has long been postulated as one of the ways technologies for imaging, chemical analysis and crystallo- by which the interface between these lens units could have graphic characterization of minerals at high spatial resolu- produced an aberration-free image and the present study tions have enabled a re-examination of the structure of provides the first direct evidence of such a chemical contrast, calcite lenses at an unprecedented level of detail. The lenses thus confirming the doublet hypothesis. in the eyes of the specimen of Dalmanites sp. used in the original formulation of the aplanatic doublet hypothesis are Key words: trilobite, schizochroal eyes, magnesian calcite, shown to have undergone diagenetic alteration, but its prod- microdolomite.

The schizochroal eyes of the Lower Ordovician–Upper matter of speculation and, most recently, the doublet Devonian phacopine trilobites are unique amongst the structure itself has been interpreted as a diagenetic artefact Arthropoda (Horva´th et al. 1997). They are characterized (Bruton and Haas 2003). by a relatively small number of highly biconvex calcite Clarkson and Levi-Setti (1975, p. 665) speculated that lenses separated by cuticular material, the interlensar sclera the intralensar bowl was composed of calcite containing (see Clarkson et al. 2006 for review). The internal structure organic material, possibly chitin, whereas the upper unit of these lenses and the mechanism by which they focused was pure calcite. Horva´th (1989) argued that the lower light have been the subject of considerable debate. Drawing lens unit was wholly organic in composition whereas on their own observations and those of others (e.g. Towe Campbell (1975) and Miller and Clarkson (1980) sugges- 1973), Clarkson and Levi-Setti (1975) argued that each ted that differences in magnesium concentrations between lens was a doublet with the curved junction between the the two parts of the doublet might have provided the upper lens unit and the underlying bowl acting as a cor- necessary contrast in refractive index but they lacked the recting surface with a change in refractive index across it evidence to support this interpretation using the tech- bringing light into sharp focus. This elegant explanation niques then available. Nonetheless, such a difference in drew analogies between the upper lens units and the apla- magnesium content has been assumed to be the case (e.g. natic lenses for correcting spherical aberration postulated Fortey and Chatterton 2003). Magnesium enrichment has by the Renaissance scientists Descartes and Huygens (see also been invoked for an enigmatic structure termed the also Levi-Setti 1993). The doublet structure, seen in several core, reported in the upper lens units in some schizo- species in both transmitted light (e.g. Campbell 1975; Mil- chroal eyes (see Clarkson et al. 2006). ler and Clarkson 1980) and in etched surfaces by scanning More fundamentally, Bruton and Haas (2003) disputed electron microscopy (SEM) (Miller and Clarkson 1980), the doublet model and argued that the intralensar struc- and Clarkson and Levi-Setti’s explanation for it have tures described by Clarkson and Levi-Setti were diagenetic become widely accepted. However, the means of achieving artefacts. They proposed that focusing of light by lenses the necessary difference in refractive index across the inter- of the Devonian phacopine Geesops sparsinodosus was face of the two components of each lens has remained a achieved by grading the refractive index of lens calcite by

ª The Palaeontological Association doi: 10.1111/j.1475-4983.2007.00710.x 1031 1032 PALAEONTOLOGY, VOLUME 50 an increase in organic material in the lateral parts of each quality map whereby contrast represents differences in the lens, thereby overcoming spherical aberration. quality of electron backscatter (Kikuchi) patterns which Recent technological advances in imaging and chemical reflect variations in crystallographic orientation of the and crystallographic analysis at high spatial resolutions calcite and the presence of subgrain boundaries and have enabled the structure of calcite lenses to be elucida- pores; the latter give very poor patterns. The precise ted at an unprecedented level of detail. The various hypo- orientation of the pole to a specified crystal plane for theses for lens structure and function outlined above can each point in the EBSD maps can also be plotted as a now be tested and here we report results of a re-examina- pole figure. Quantitative chemical analyses were acquired tion of the lenses of the specimen of an un-named species using a Cameca SX50 electron probe operated at of Dalmanites (horizon and locality not known) that 15 kV ⁄ 10 nA and with a 10-lm defocused spot. Stan- formed a crucial part of the doublet hypothesis of Clark- dardization used wollastonite (Ca), periclase (Mg), Mn son and Levi-Setti (1975; also Horva´th 1989; Levi-Setti metal (Mn) and Fe metal (Fe). Count times were typically 1993). If the lenses were doublets, the two parts of each 30 s on peak and 10 s on background, and detection lens must have differed significantly in refractive index limits were 0Æ06 weight per cent MnCO3 and 0Æ07 weight and therefore in composition. The new technologies for per cent FeCO3. mineral characterization now provide an opportunity to assess whether such compositional differences did originally exist at the described interface. The absence of a con- RESULTS trast between the upper lens unit and bowl sufficient to yield the necessary difference in refractive index (e.g. cal- The trilobite exoskeleton studied is contained in a skeletal cite vs. chitin, magnesium-poor calcite vs. magnesium- packstone together with articulated and disarticulated rich calcite) would lend support to the contention by microfossils, including ostracodes, and fragments of trilo- Bruton and Haas that the doublet structure is a diagenetic bites and echinoderms. Small angular grains of quartz artefact, or at the very least that all of the original differ- also occur. The limestone is orange in optical-CL and has ences within the structure have been completely overprin- a greater luminescence intensity than the trilobite cuticles. ted by recrystallization during burial. In plane polarized transmitted light the lenses are defined clearly and have abrupt boundaries with the interlensar sclera and limestone (Text-fig. 1). Many of the lenses METHODS contain a bowl and core, both of which are turbid and pseudopleochroic in plane polarized transmitted light and The internal structure of the lenses and adjacent exoskel- so are distinguished clearly from the enclosing optically eton on polished thin sections of the indeterminate spe- clear lens calcite (Text-fig. 1). A small proportion of the cies of Dalmanites was observed initially by transmitted lenses are turbid throughout, although the bowl can still light and optical cathodoluminescence (optical-CL) micro- be recognized by a greater opacity. BSE imaging shows scopy. Higher resolution imaging and chemical and crys- that the turbidity of the bowl and core is due mainly to tallographic analysis of the lenses used an FEI Quanta 200F field-emission environmental scanning electron microscope equipped with an EDAX ⁄ TSL X-ray microanalysis and electron backscatter diffraction (EBSD) system. Conventional backscattered electron (BSE) imaging of lenses in thin section was used in conjunction with a new technique of charge contrast (CC) microscopy that utilizes secondary electrons emitted from uncoated samples with the microscope operated in environmental mode. Contrast within CC images reflects variations in the accumulation and dissipation of electrons on the sample surface (Watt et al. 2000) and may be comparable with that formed by optical-CL, but the CC images can be acquired at much higher magnifications and from non-luminescent minerals (Cuthbert and Buckman 2005). EBSD was used to determine the crystallographic orien- TEXT-FIG. 1. Plane polarized transmitted light image of a tation of lens calcite, and sample preparation and instru- single lens. Faint trabeculae that fan out downwards can be ment operating conditions are described in Dalbeck et al. identified within the core. The subhorizontal black line at the (2006). Here the EBSD data are presented as an image top of the lens is a fracture. LEE ET AL.: INTRALENSAR STRUCTURES IN SCHIZOCHROAL TRILOBITE EYES 1033 abundant micropores (Text-fig. 2A). The core has a in central and outer parts of the lenses, but fan outwards microporosity of c. 1Æ2 vol. per cent (determined from towards the base of the lens where they are orientated at computer analysis of BSE images) and the pores range in angles of up to 40 degrees to the c-axis (Text-fig. 3A). size from c. 0Æ5–3Æ5 lm (mean c. 1Æ8 lm) whereas the Sub-grain boundaries can be identified by transmitted bowl has a microporosity of c. 0Æ6 vol. per cent and light microscopy but only within turbid parts of the the pores range in size from c. 0Æ7to1Æ8 lm (mean lenses (i.e. the bowl and core), where they are outlined by c. 1Æ3 lm). The bowl and core both have a considerably discontinuous lines of micropores (Text-fig. 1); these greater intensity of orange luminescence than the optically structures are the trabeculae described by Miller and clear lens calcite, the interlensar sclera and the limestone Clarkson (1980) and others. matrix. X-ray microanalyses demonstrate that the bowl and core are enriched significantly in magnesium relative to INTERPRETATION the optically clear lens calcite, sclera and limestone (Table 1). Individual analyses of the bowl and core show The lenses of Dalmanites sp. are interpreted as having a considerable range in magnesium concentrations, with undergone considerable diagenetic modification because maximum values of 7Æ0 mol per cent and 31Æ6 mol per the turbidity of the bowl and core would have rendered cent, respectively. Concentrations of manganese are low them opaque to incoming light and so unusable in vivo. and close to detection limits in many analyses, but iron is A crucial question to answer in order to distinguish present in significant concentrations and shows a good between the competing hypotheses of lens function is positive correlation with magnesium, especially in analy- whether this post-mortem alteration has completely over- ses of the core where magnesium values are greatest. The printed the primary structures or has preserved at least relatively high but wide-ranging magnesium concentra- some parts or aspects of them. The susceptibility of intra- tions of the bowl and core are due to the presence within lensar structures to recrystallization, or even wholesale lens calcite of micrometre-sized euhedral crystals of dolo- dissolution during diagenesis, has been noted in several mite (hereafter termed ‘microdolomite’) (Text-fig. 2A–B). previous studies. Campbell (1975) found that lenses of The microdolomites range from c. 2Æ0–4Æ5 lm (mean several phacopine species have an upper unit of radial- c. 3Æ5 lm) in well-defined cores to c. 2Æ5 lm in bowls, fibrous calcite below which is an inclusion-rich bowl and and are more abundant in the core (c. 4 vol. per cent) core. He observed that the bowl was especially prone to than the bowl. Within larger cores the microdolomites diagenetic dissolution, which was also noted by Clarkson can reach 25 lm and have a very fine-scale oscillatory and Levi-Setti (1975). Miller and Clarkson (1980) des- zoning which is also seen in the adjacent calcite subgrains cribed neomorphism of the lenses of Phacops [now (Text-fig. 2C–D). Calcite within the bowl and core addi- Eldredgeops] rana milleri from the Devonian Silica Shale tionally contains submicrometre inclusions of calcium Formation of Ohio whereby both the core and bowl had phosphate (Text-fig. 2B) and iron sulphide, but in very been replaced by ferroan calcite. As the enclosing LMC low abundances. cuticle had undergone much less diagenetic alteration, EBSD mapping shows that orientation of the calcite Miller and Clarkson (1980) speculated that the core and c-axis is invariant throughout each lens but also that bowl were originally composed of high magnesian calcite elongate subgrains c. 150–160 lm in length by c. 10 lm (HMC). in width can be recognized (Text-fig. 3A–B). Sub-grain Electron microscopy demonstrates that the turbidity of boundaries are defined by a rotation of c. 3–6 degrees the lenses of Dalmanites sp. described herein is mainly a about the c-axis and are orientated parallel to the c-axis result of the presence of abundant micropores, many of

TABLE 1. Mean compositions of Dalmanites sp. lens calcite and the adjacent cuticle and limestone determined by electron probe microanalysis

Mol. % MgCO3 Mol. % MnCO3 Mol. % FeCO3

mean range mean range mean range n Clear lens calcite 1Æ16 0Æ77–1Æ71 0Æ07 d.l.)0Æ17 0Æ48 0Æ27–1Æ27 38 Sclera 1Æ90 1Æ51–2Æ23 0Æ11 0Æ05–0Æ15 0Æ28 0Æ22–0Æ38 10 Limestone 2Æ10 1Æ30–2Æ87 0Æ05 d.l.)0Æ10 0Æ33 d.l.)0Æ85 15 Bowl 2Æ66 0Æ89–7Æ04 0Æ09 d.l.)0Æ23 0Æ51 d.l.)1Æ03 21 Core 3Æ94 0Æ95–31Æ61 0Æ07 d.l.)0Æ19 0Æ54 0Æ20–2Æ54 27 d.l. denotes present in concentrations below the limit of detection. n denotes number of analyses. 1034 PALAEONTOLOGY, VOLUME 50

A B

C D

TEXT-FIG. 2. A–B, back-scattered electron SEM images of the core. A, a typical area comprising calcite (medium grey) with micropores (black) and microdolomite inclusions, three of which are arrowed. B, euhedral microdolomite (Dol.), comprising 4Æ5 vol. per cent of the image, and micropores (black) within calcite; a small grain of calcium phosphate occurs between the microdolomite and calcite. C–D, images of calcite and microdolomite within the core. C, a large microdolomite crystal enclosed within calcite that also contains micropores (black), some of which help to delineate subgrains (SG). D, a charge contrast (CC) image of the same field of view as in C. The calcite has a faint oscillatory zoning whereas the dolomite crystal has an intricate fine-scale zoning, which suggests that it grew within a pore existing after calcite crystallization. The oblique lines crossing the image are scratches. which may be fluid-filled inclusions within the intact cal- that the microdolomite-rich LMC contained c. 2Æ5 mol cite. In a wide variety of mineral systems, microporosity per cent MgCO3, a value much lower than the inferred is a characteristic of the fluid-mediated replacement of magnesium concentration of the precursor marine cements, one mineral by another (Putnis 2002). The dissolution- indicating that much of the original magnesium must reprecipitation reactions are mediated by a very thin film have been lost during recystallization. The diagenetic of fluid, and in some minerals, such as the alkali feld- factors that determine the proportion of magnesium lost spars, the alteration products may display oscillatory zon- from biogenic HMC have been investigated by Dickson ing in CL images that reflects temporal changes in fluid (2001, 2002, 2004) in a number of detailed studies of ech- compositions (Lee et al. 2007). inoderm stereom. Dickson (2001) found that tests that Microdolomite crystals comparable in size and shape had been enclosed within an early diagenetic magnesian with those found in the Dalmanites sp. lenses are also a and ferroan calcite cement had recrystallized to micro- characteristic product of fluid-mediated alteration, and pore-rich LMC containing calcian microdolomites less specifically of the replacement of biogenic and inorganic- than 1–3 lm in size with pyrite and celestite. The early ally formed magnesian calcite by more diagenetically sta- diagenetic cement provided an effective seal so that dur- ble LMC (Lohmann and Meyers 1977; Taylor and Wilson ing recrystallization ions were redistributed on the scale 1999). Importantly, Lohmann and Meyers (1977) found of a few micrometres and the stereom retained its original LEE ET AL.: INTRALENSAR STRUCTURES IN SCHIZOCHROAL TRILOBITE EYES 1035

A B

TEXT-FIG. 3. Results from an EBSD scan of one lens. A, image quality map of a single lens, with the cornea uppermost, sclera and enclosing limestone. Elongate subgrains within the lens can be recognized by slight differences in contrast and the poor image quality of their boundaries, producing black lines. Sub-grain boundaries are orientated parallel to the calcite c-axis in upper and middle parts of the lens but fan out towards its base. Small black spots in the centre of the lens represent micropores and the irregular black area in the upper left of the lens is a hole in the thin section. B, a pole figure of lens calcite showing the orientations of the poles to calcite (0001) planes (i.e. the c-axis). This plot was constructed using approximately 100,000 indexed diffraction patterns and demonstrates that the c-axis is orientated north–south with respect to the image quality map and almost in the plane of the thin section. The tight clustering of data points shows that the degree of variation in crystallographic orientations of lens calcite is very limited. bulk chemical composition, thus allowing it to be used as average size of microdolomites within the core than in a proxy for the chemical composition of ancient seawater the bowl could suggest that the core originally had greater (Dickson 2002, 2004). Dickson found that recrystalliza- concentrations of magnesium, which is also supported by tion in a more open diagenetic system forms larger electron probe data, or that the core was altered in a microdolomites (1–20 lm) with accessory barite, celestite more open diagenetic system that enabled larger crystals and siderite. The grain size of these microdolomites indi- to grow. The much greater luminescence intensities of the cates redistribution of ions on the scale of tens of micro- bowl and core than their enclosing clear lens calcite and metres and the host sediment-derived iron and barium in cuticle suggest that manganese, which is the main activa- the accessory minerals must have been transported over tor of CL in calcite, was acquired during diagenesis, pre- the millimetre scale. sumably from the enclosing limestone. Iron is also By analogy with the work on diagenetically altered inferred to have been derived from outside of the lenses magnesian calcites described above, we conclude that and its good correlation with magnesium in electron parts of the lenses (the bowl and core) of Dalmanites sp. probe analyses of the bowl and core indicates that most originally had greater concentrations of magnesium than of the iron is contained within the microdolomite such the enclosing clear lens calcite. Magnesium concentrations that an analysis solely of the dolomite would yield in vivo are likely to have been greater than the mean val- c. 4Æ5 mol per cent FeCO3. The zoning seen in CC images ues determined by electron probe microanalysis (i.e. 2Æ66 of coarse microdolomite crystals may represent temporal and 3Æ94 mol per cent MgCO3 for the bowl and core, variations in iron concentrations during crystal growth. respectively) as the diagenetic system is likely to have The presence of manganese in lens calcite and iron in been relatively open so that only a proportion of the microdolomite is therefore good evidence for exchange of magnesium was retained, most of which subsequently ions during diagenesis of the lens over the millimetre- formed the microdolomite. Even constraining the original scale and through a complex three-dimensional matrix of chemical composition is difficult because the lower limit dissolving magnesian calcite and precipitating LMC and of magnesium in calcite required to drive recrystallization microdolomite. is poorly known and will be highly dependent on the nat- Chemical differences between units within schizochroal ure of the ambient diagenetic environment. The greater lenses have been suggested previously, but with little firm 1036 PALAEONTOLOGY, VOLUME 50 evidence. Campbell (1975) first hypothesized that incor- 1975). However, it is clear that the larger scale structure poration of magnesium, manganese or iron into calcite of of the lenses of schizochroal eyes varies greatly between different lens units would have been sufficient to produce taxa (see review by Clarkson et al. 2006) and so differ- the necessary intralensar contrasts in refractive indices to ences in the finer scale morphology can also reasonably focus light. However, using an electron probe he found be expected to have developed. maximum and mean MgCO3 concentrations of 1Æ8 weight The demonstration that magnesian calcite was used in per cent and 0Æ9 weight per cent, respectively, which he the construction of the lenses of Dalmanites may help to concluded were insufficient to have modified refractive account for the disagreement in the literature on the indices. By analogy with the present work, Campbell’s in vivo chemical composition of trilobite exoskeletons. results could be explained by loss of magnesian calcite Lowenstam (in Richter and Fu¨chtbauer 1978) described intralensar structures during diagenesis and subsequent up to 5 mol per cent MgCO3 in the exoskeletons of occlusion of the void space by an inorganic LMC cement. Carboniferous trilobites and Richter and Fu¨chtbauer Despite the lack of supporting analytical data, subsequent (1978) used the replacement of trilobite exoskeletons by studies have suggested that intralensar structures were ferroan calcite as an indicator of an original magnesian constructed using magnesian calcite (Miller and Clarkson composition. In contrast, Wilmot and Fallick (1989)

1980; Fortey and Chatterton 2003) and these inferences found an average of 3Æ6 mol per cent MgCO3 and are now supported by the results of the present study. 1Æ0 mol per cent FeCO3 from wet chemical analyses of Interestingly, the magnesium concentration of the clear trilobite exoskeletons from the Much Wenlock Limestone lens calcite of Dalmanites is lower than that of the enclo- Formation (Silurian, UK). Furthermore, they stressed the sing interlensar sclera. As calcite in both structures is absence of microdolomites from trilobite cuticles, inferred to have escaped diagenetic alteration, these com- although they did not mention having studied individual positional contrasts suggest that magnesium was either lenses of schizochroal eyes. Our preliminary analyses of actively excluded when the clear calcite was being precipi- the schizochroal eyes of other trilobites have shown that tated in order to create a greater contrast in refractive the presence of microdolomites is a common feature and index with the magnesian calcite bowl and core, or simply it is noteworthy that the mean composition of trilobite because most of the available magnesium was being parti- exoskeletons determined by Wilmott and Fallick is very tioned into the specialist intralensar structures. close to that of the bowl and core in Dalmanites sp. and nearly twice that of the sclera (Table 1). This may suggest that the samples they analysed could have contained LMC CONCLUSIONS AND IMPLICATIONS mixed with magnesian calcite or dolomite.

Our analysis indicates that Bruton and Haas (2003) were Acknowledgements. We thank Euan Clarkson for his advice and correct in arguing that the material studied by Clarkson encouragement and for the provision of the thin sections used and Levi-Setti had been diagenetically altered, but we in this study. We also thank Philip Donoghue and two anony- contend that the bowl and core in the lenses of Dalma- mous reviewers for their helpful comments and we are grateful nites sp. were enriched in magnesium relative to the opti- to Robert MacDonald for his assistance with the electron probe. cally clear calcite in vivo. Both intralensar structures were subsequently recrystallized to microporous LMC plus ferroan microdolomite with accessory calcium phosphate REFERENCES and iron sulphide. Despite the evidence presented here BRUTON, D. L. and HAAS, W. 2003. The puzzling eye of for a doublet with a magnesian calcite bowl and core and, Phacops. Special Papers in Palaeontology, 70, 349–361. hence, a potential difference in refractive index across the CAMPBELL, K. S. W. 1975. The functional anatomy of trilo- interface between the upper lens unit and the bowl, the bites: musculature and eyes. Journal and Proceedings of the exact manner in which the lenses of Dalmanites sp. Royal Society of New South Wales, 108, 168–188. focused light remains to be determined. The function of CLARKSON, E. N. K. and LEVI-SETTI, R. 1975. Trilobite the core is not clear and there is still much to be learned eyes and the optics of Des Cartes and Huygens. Nature, 254, from the detailed crystallography of the lenses. Regardless 663–667. of its origin and subsequent diagenetic history, all of the —— —— and H O R V A´ TH, G. 2006. The eyes of trilobites: calcite within each lens has its c-axis in the same orienta- the oldest preserved visual system. Arthropod Structure and tion and this crystallography is also independent of the Development, 35, 247–259. fanning of constituent subgrains towards the bowl. No CUTHBERT, S. J. and BUCKMAN, J. O. 2005. Charge contrast imaging of ﬁne-scale microstructure and compositional evidence can be found in Dalmanites sp. for the radial- variation in garnet using the environmental scanning electron ﬁbrous structure that has been reported from the upper microscope. American Mineralogist, 90, 701–707. lens unit of many other schizochroal lenses (Campbell LEE ET AL.: INTRALENSAR STRUCTURES IN SCHIZOCHROAL TRILOBITE EYES 1037

DALBECK, P., ENGLAND, J., CUSACK, M., LEE, M. R. LOHMANN, K. C. and MEYERS, W. J. 1977. Microdolomite and FALLICK, A. E. 2006. Crystallography and minor ele- inclusions in cloudy prismatic calcites: a proposed criterion ment composition of the calcite–aragonite interface in Mytilus for former high magnesian calcites. Journal of Sedimentary edulis. European Journal of Mineralogy, 18, 601–609. Petrology, 47, 1078–1088. DICKSON, J. A. D. 2001. Diagenesis and crystal caskets: MILLER, J. and CLARKSON, E. N. K. 1980. The post-ecdy- echinoderm Mg calcite transformation, Dry Canyon, New sial development of the cuticle and the eye of the Devonian Mexico, USA. Journal of Sedimentary Research, 71, 764–777. trilobite Phacops rana milleri Stewart 1927. Philosophical —— 2002. Fossil echinoderms as monitor of the Mg ⁄ Ca ratio Transactions of the Royal Society of London, Series B, 288, of Phanerozoic oceans. Science, 298, 1222–1224. 461–480. —— 2004. Echinoderm skeletal preservation: calcite-aragonite PUTNIS, A. 2002. Mineral replacement reactions: from macro- seas and the Mg ⁄ Ca ratio of Phanerozoic oceans. Journal of scopic observations to microscopic mechanisms. Mineralogical Sedimentary Research, 74, 355–365. Magazine, 66, 689–708. FORTEY, R. A. and CHATTERTON, B. D. E. 2003. A RICHTER, D. K. and FU¨ CHTBAUER, H. 1978. Ferroan cal- Devonian trilobite with an eyeshade. Science, 310, 1689. cite replacement indicates former magnesian calcite skeletons. HORVA´ TH, G. 1989. Geometric optics of trilobite eyes: a Sedimentology, 25, 843–860. theoretical study of the aspherical interface in the cornea of TAYLOR, P. D. and WILSON, M. A. 1999. Dianulites schizochroal eyes of phacopid trilobites. Mathematical Bio- Eichwald, 1829: an unusual Ordovician bryozoan with a high- sciences, 96, 79–94. magnesium calcite skeleton. Journal of Paleontology, 73, 38–48. —— CLARKSON, E. N. K. and PIX, W. 1997. Survey of TOWE, K. M. 1973. Trilobite eyes: calciﬁed lenses in vivo. modern counterparts of schizochroal trilobite eyes: structural Science, 179, 1007–1009. and functional similarities and differences. Historical Biology, WATT, G. R., GRIFFIN, B. J. and KINNY, P. D. 2000. 12, 229–263. Charge contrast imaging of geological materials in the envi- LEE, M. R., PARSONS, I., EDWARDS, P. R. and MARTIN, ronmental scanning electron microscope. American Mineral- R. W. 2007. Identiﬁcation of cathodoluminescence activators in ogist, 85, 1784–1794. zoned alkali feldspars by hyperspectral imaging and electron WILMOT, N. V. and FALLICK, A. E. 1989. Original miner- probe microanalysis. American Mineralogist, 92, 243–253. alogy of trilobite exoskeletons. Palaeontology, 32, 297–304. LEVI-SETTI, R. 1993. Trilobites. Second edition. University of Chicago Press, Chicago, IL, and London, 342 pp.