[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
trilobite eyes as aplanatic doublets by Clarkson and Levi-Setti
over 30 years ago has been widely accepted. However, the
means of achieving a difference in refractive index across the
interface between the two parts of each lens to overcome
spherical aberration has remained a matter of speculation
and lately it has been argued that the doublet structure itself
is no more than a diagenetic artefact. Recent advances in
technologies for imaging, chemical analysis and crystallographic characterization of minerals at high spatial resolutions have enabled a re-examination of the structure of
calcite lenses at an unprecedented level of detail. The lenses
in the eyes of the specimen of Dalmanites sp. used in the original formulation of the aplanatic doublet hypothesis are
shown to have undergone diagenetic alteration, but its prod-
The schizochroal eyes of the Lower Ordovician–Upper
Devonian phacopine trilobites are unique amongst the
Arthropoda (Horváth et al. 1997). They are characterized
by a relatively small number of highly biconvex calcite
lenses separated by cuticular material, the interlensar sclera
(see Clarkson et al. 2006 for review). The internal structure
of these lenses and the mechanism by which they focused
light have been the subject of considerable debate. Drawing
on their own observations and those of others (e.g. Towe
1973), Clarkson and Levi-Setti (1975) argued that each
lens was a doublet with the curved junction between the
upper lens unit and the underlying bowl acting as a correcting surface with a change in refractive index across it
bringing light into sharp focus. This elegant explanation
drew analogies between the upper lens units and the aplanatic lenses for correcting spherical aberration postulated
by the Renaissance scientists Descartes and Huygens (see
also Levi-Setti 1993). The doublet structure, seen in several
species in both transmitted light (e.g. Campbell 1975; Miller and Clarkson 1980) and in etched surfaces by scanning
electron microscopy (SEM) (Miller and Clarkson 1980),
and Clarkson and Levi-Setti’s explanation for it have
become widely accepted. However, the means of achieving
the necessary difference in refractive index across the interface of the two components of each lens has remained a
ª The Palaeontological Association
ucts reflect original differences in mineral chemistry between
the upper lens unit and lower intralensar bowl. The turbidity
of the bowl and of the core within the upper part of the lens
are the result of their greater microporosity and abundance
of microdolomite inclusions, both of which were products of
diagenetic replacement of original magnesian calcite in these
areas. Such a difference in magnesium concentration in the
original calcite has long been postulated as one of the ways
by which the interface between these lens units could have
produced an aberration-free image and the present study
provides the first direct evidence of such a chemical contrast,
thus confirming the doublet hypothesis.
Key words: trilobite, schizochroal eyes, magnesian calcite,
microdolomite.
matter of speculation and, most recently, the doublet
structure itself has been interpreted as a diagenetic artefact
(Bruton and Haas 2003).
Clarkson and Levi-Setti (1975, p. 665) speculated that
the intralensar bowl was composed of calcite containing
organic material, possibly chitin, whereas the upper unit
was pure calcite. Horváth (1989) argued that the lower
lens unit was wholly organic in composition whereas
Campbell (1975) and Miller and Clarkson (1980) suggested that differences in magnesium concentrations between
the two parts of the doublet might have provided the
necessary contrast in refractive index but they lacked the
evidence to support this interpretation using the techniques then available. Nonetheless, such a difference in
magnesium content has been assumed to be the case (e.g.
Fortey and Chatterton 2003). Magnesium enrichment has
also been invoked for an enigmatic structure termed the
core, reported in the upper lens units in some schizochroal eyes (see Clarkson et al. 2006).
More fundamentally, Bruton and Haas (2003) disputed
the doublet model and argued that the intralensar structures described by Clarkson and Levi-Setti were diagenetic
artefacts. They proposed that focusing of light by lenses
of the Devonian phacopine Geesops sparsinodosus was
achieved by grading the refractive index of lens calcite by
doi: 10.1111/j.1475-4983.2007.00710.x
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PALAEONTOLOGY, VOLUME 50
an increase in organic material in the lateral parts of each
lens, thereby overcoming spherical aberration.
Recent technological advances in imaging and chemical
and crystallographic analysis at high spatial resolutions
have enabled the structure of calcite lenses to be elucidated at an unprecedented level of detail. The various hypotheses for lens structure and function outlined above can
now be tested and here we report results of a re-examination of the lenses of the specimen of an un-named species
of Dalmanites (horizon and locality not known) that
formed a crucial part of the doublet hypothesis of Clarkson and Levi-Setti (1975; also Horváth 1989; Levi-Setti
1993). If the lenses were doublets, the two parts of each
lens must have differed significantly in refractive index
and therefore in composition. The new technologies for
mineral characterization now provide an opportunity to
assess whether such compositional differences did originally exist at the described interface. The absence of a contrast between the upper lens unit and bowl sufficient to
yield the necessary difference in refractive index (e.g. calcite vs. chitin, magnesium-poor calcite vs. magnesiumrich calcite) would lend support to the contention by
Bruton and Haas that the doublet structure is a diagenetic
artefact, or at the very least that all of the original differences within the structure have been completely overprinted by recrystallization during burial.
METHODS
The internal structure of the lenses and adjacent exoskeleton on polished thin sections of the indeterminate species of Dalmanites was observed initially by transmitted
light and optical cathodoluminescence (optical-CL) microscopy. Higher resolution imaging and chemical and crystallographic 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 orientation of lens calcite, and sample preparation and instrument operating conditions are described in Dalbeck et al.
(2006). Here the EBSD data are presented as an image
quality map whereby contrast represents differences in the
quality of electron backscatter (Kikuchi) patterns which
reflect variations in crystallographic orientation of the
calcite and the presence of subgrain boundaries and
pores; the latter give very poor patterns. The precise
orientation of the pole to a specified crystal plane for
each point in the EBSD maps can also be plotted as a
pole figure. Quantitative chemical analyses were acquired
using a Cameca SX50 electron probe operated at
15 kV ⁄ 10 nA and with a 10-lm defocused spot. Standardization used wollastonite (Ca), periclase (Mg), Mn
metal (Mn) and Fe metal (Fe). Count times were typically
30 s on peak and 10 s on background, and detection
limits were 0Æ06 weight per cent MnCO3 and 0Æ07 weight
per cent FeCO3.
RESULTS
The trilobite exoskeleton studied is contained in a skeletal
packstone together with articulated and disarticulated
microfossils, including ostracodes, and fragments of trilobites and echinoderms. Small angular grains of quartz
also occur. The limestone is orange in optical-CL and has
a greater luminescence intensity than the trilobite cuticles.
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
contain a bowl and core, both of which are turbid and
pseudopleochroic in plane polarized transmitted light and
so are distinguished clearly from the enclosing optically
clear lens calcite (Text-fig. 1). A small proportion of the
lenses are turbid throughout, although the bowl can still
be recognized by a greater opacity. BSE imaging shows
that the turbidity of the bowl and core is due mainly to
T E X T - F I G . 1 . Plane polarized transmitted light image of a
single lens. Faint trabeculae that fan out downwards can be
identified within the core. The subhorizontal black line at the
top of the lens is a fracture.
LEE ET AL.: INTRALENSAR STRUCTURES IN SCHIZOCHROAL TRILOBITE EYES
abundant micropores (Text-fig. 2A). The core has a
microporosity of c. 1Æ2 vol. per cent (determined from
computer analysis of BSE images) and the pores range in
size from c. 0Æ5–3Æ5 lm (mean c. 1Æ8 lm) whereas the
bowl has a microporosity of c. 0Æ6 vol. per cent and
the pores range in size from c. 0Æ7 to 1Æ8 lm (mean
c. 1Æ3 lm). The bowl and core both have a considerably
greater intensity of orange luminescence than the optically
clear lens calcite, the interlensar sclera and the limestone
matrix.
X-ray microanalyses demonstrate that the bowl and
core are enriched significantly in magnesium relative to
the optically clear lens calcite, sclera and limestone
(Table 1). Individual analyses of the bowl and core show
a considerable range in magnesium concentrations, with
maximum values of 7Æ0 mol per cent and 31Æ6 mol per
cent, respectively. Concentrations of manganese are low
and close to detection limits in many analyses, but iron is
present in significant concentrations and shows a good
positive correlation with magnesium, especially in analyses of the core where magnesium values are greatest. The
relatively high but wide-ranging magnesium concentrations of the bowl and core are due to the presence within
lens calcite of micrometre-sized euhedral crystals of dolomite (hereafter termed ‘microdolomite’) (Text-fig. 2A–B).
The microdolomites range from c. 2Æ0–4Æ5 lm (mean
c. 3Æ5 lm) in well-defined cores to c. 2Æ5 lm in bowls,
and are more abundant in the core (c. 4 vol. per cent)
than the bowl. Within larger cores the microdolomites
can reach 25 lm and have a very fine-scale oscillatory
zoning which is also seen in the adjacent calcite subgrains
(Text-fig. 2C–D). Calcite within the bowl and core additionally contains submicrometre inclusions of calcium
phosphate (Text-fig. 2B) and iron sulphide, but in very
low abundances.
EBSD mapping shows that orientation of the calcite
c-axis is invariant throughout each lens but also that
elongate subgrains c. 150–160 lm in length by c. 10 lm
in width can be recognized (Text-fig. 3A–B). Sub-grain
boundaries are defined by a rotation of c. 3–6 degrees
about the c-axis and are orientated parallel to the c-axis
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in central and outer parts of the lenses, but fan outwards
towards the base of the lens where they are orientated at
angles of up to 40 degrees to the c-axis (Text-fig. 3A).
Sub-grain boundaries can be identified by transmitted
light microscopy but only within turbid parts of the
lenses (i.e. the bowl and core), where they are outlined by
discontinuous lines of micropores (Text-fig. 1); these
structures are the trabeculae described by Miller and
Clarkson (1980) and others.
INTERPRETATION
The lenses of Dalmanites sp. are interpreted as having
undergone considerable diagenetic modification because
the turbidity of the bowl and core would have rendered
them opaque to incoming light and so unusable in vivo.
A crucial question to answer in order to distinguish
between the competing hypotheses of lens function is
whether this post-mortem alteration has completely overprinted the primary structures or has preserved at least
some parts or aspects of them. The susceptibility of intralensar structures to recrystallization, or even wholesale
dissolution during diagenesis, has been noted in several
previous studies. Campbell (1975) found that lenses of
several phacopine species have an upper unit of radialfibrous calcite below which is an inclusion-rich bowl and
core. He observed that the bowl was especially prone to
diagenetic dissolution, which was also noted by Clarkson
and Levi-Setti (1975). Miller and Clarkson (1980) described neomorphism of the lenses of Phacops [now
Eldredgeops] rana milleri from the Devonian Silica Shale
Formation of Ohio whereby both the core and bowl had
been replaced by ferroan calcite. As the enclosing LMC
cuticle had undergone much less diagenetic alteration,
Miller and Clarkson (1980) speculated that the core and
bowl were originally composed of high magnesian calcite
(HMC).
Electron microscopy demonstrates that the turbidity of
the lenses of Dalmanites sp. described herein is mainly a
result of the presence of abundant micropores, many of
Mean compositions of Dalmanites sp. lens calcite and the adjacent cuticle and limestone determined by electron probe
microanalysis
TABLE 1.
Clear lens calcite
Sclera
Limestone
Bowl
Core
Mol. % MgCO3
Mol. % MnCO3
Mol. % FeCO3
mean
range
mean
range
mean
range
n
1Æ16
1Æ90
2Æ10
2Æ66
3Æ94
0Æ77–1Æ71
1Æ51–2Æ23
1Æ30–2Æ87
0Æ89–7Æ04
0Æ95–31Æ61
0Æ07
0Æ11
0Æ05
0Æ09
0Æ07
d.l.)0Æ17
0Æ05–0Æ15
d.l.)0Æ10
d.l.)0Æ23
d.l.)0Æ19
0Æ48
0Æ28
0Æ33
0Æ51
0Æ54
0Æ27–1Æ27
0Æ22–0Æ38
d.l.)0Æ85
d.l.)1Æ03
0Æ20–2Æ54
38
10
15
21
27
d.l. denotes present in concentrations below the limit of detection. n denotes number of analyses.
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PALAEONTOLOGY, VOLUME 50
A
B
C
D
T E X T - F I G . 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 calcite. In a wide variety of mineral systems, microporosity
is a characteristic of the fluid-mediated replacement of
one mineral by another (Putnis 2002). The dissolutionreprecipitation reactions are mediated by a very thin film
of fluid, and in some minerals, such as the alkali feldspars, the alteration products may display oscillatory zoning in CL images that reflects temporal changes in fluid
compositions (Lee et al. 2007).
Microdolomite crystals comparable in size and shape
with those found in the Dalmanites sp. lenses are also a
characteristic product of fluid-mediated alteration, and
specifically of the replacement of biogenic and inorganically formed magnesian calcite by more diagenetically stable LMC (Lohmann and Meyers 1977; Taylor and Wilson
1999). Importantly, Lohmann and Meyers (1977) found
that the microdolomite-rich LMC contained c. 2Æ5 mol
per cent MgCO3, a value much lower than the inferred
magnesium concentration of the precursor marine cements,
indicating that much of the original magnesium must
have been lost during recystallization. The diagenetic
factors that determine the proportion of magnesium lost
from biogenic HMC have been investigated by Dickson
(2001, 2002, 2004) in a number of detailed studies of echinoderm stereom. Dickson (2001) found that tests that
had been enclosed within an early diagenetic magnesian
and ferroan calcite cement had recrystallized to micropore-rich LMC containing calcian microdolomites less
than 1–3 lm in size with pyrite and celestite. The early
diagenetic cement provided an effective seal so that during recrystallization ions were redistributed on the scale
of a few micrometres and the stereom retained its original
LEE ET AL.: INTRALENSAR STRUCTURES IN SCHIZOCHROAL TRILOBITE EYES
A
1035
B
T E X T - F I G . 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
a proxy for the chemical composition of ancient seawater
(Dickson 2002, 2004). Dickson found that recrystallization in a more open diagenetic system forms larger
microdolomites (1–20 lm) with accessory barite, celestite
and siderite. The grain size of these microdolomites indicates redistribution of ions on the scale of tens of micrometres and the host sediment-derived iron and barium in
the accessory minerals must have been transported over
the millimetre scale.
By analogy with the work on diagenetically altered
magnesian calcites described above, we conclude that
parts of the lenses (the bowl and core) of Dalmanites sp.
originally had greater concentrations of magnesium than
the enclosing clear lens calcite. Magnesium concentrations
in vivo are likely to have been greater than the mean values determined by electron probe microanalysis (i.e. 2Æ66
and 3Æ94 mol per cent MgCO3 for the bowl and core,
respectively) as the diagenetic system is likely to have
been relatively open so that only a proportion of the
magnesium was retained, most of which subsequently
formed the microdolomite. Even constraining the original
chemical composition is difficult because the lower limit
of magnesium in calcite required to drive recrystallization
is poorly known and will be highly dependent on the nature of the ambient diagenetic environment. The greater
average size of microdolomites within the core than in
the bowl could suggest that the core originally had greater
concentrations of magnesium, which is also supported by
electron probe data, or that the core was altered in a
more open diagenetic system that enabled larger crystals
to grow. The much greater luminescence intensities of the
bowl and core than their enclosing clear lens calcite and
cuticle suggest that manganese, which is the main activator of CL in calcite, was acquired during diagenesis, presumably from the enclosing limestone. Iron is also
inferred to have been derived from outside of the lenses
and its good correlation with magnesium in electron
probe analyses of the bowl and core indicates that most
of the iron is contained within the microdolomite such
that an analysis solely of the dolomite would yield
c. 4Æ5 mol per cent FeCO3. The zoning seen in CC images
of coarse microdolomite crystals may represent temporal
variations in iron concentrations during crystal growth.
The presence of manganese in lens calcite and iron in
microdolomite is therefore good evidence for exchange of
ions during diagenesis of the lens over the millimetrescale and through a complex three-dimensional matrix of
dissolving magnesian calcite and precipitating LMC and
microdolomite.
Chemical differences between units within schizochroal
lenses have been suggested previously, but with little firm
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PALAEONTOLOGY, VOLUME 50
evidence. Campbell (1975) first hypothesized that incorporation of magnesium, manganese or iron into calcite of
different lens units would have been sufficient to produce
the necessary intralensar contrasts in refractive indices to
focus light. However, using an electron probe he found
maximum and mean MgCO3 concentrations of 1Æ8 weight
per cent and 0Æ9 weight per cent, respectively, which he
concluded were insufficient to have modified refractive
indices. By analogy with the present work, Campbell’s
results could be explained by loss of magnesian calcite
intralensar structures during diagenesis and subsequent
occlusion of the void space by an inorganic LMC cement.
Despite the lack of supporting analytical data, subsequent
studies have suggested that intralensar structures were
constructed using magnesian calcite (Miller and Clarkson
1980; Fortey and Chatterton 2003) and these inferences
are now supported by the results of the present study.
Interestingly, the magnesium concentration of the clear
lens calcite of Dalmanites is lower than that of the enclosing interlensar sclera. As calcite in both structures is
inferred to have escaped diagenetic alteration, these compositional contrasts suggest that magnesium was either
actively excluded when the clear calcite was being precipitated in order to create a greater contrast in refractive
index with the magnesian calcite bowl and core, or simply
because most of the available magnesium was being partitioned into the specialist intralensar structures.
CONCLUSIONS AND IMPLICATIONS
Our analysis indicates that Bruton and Haas (2003) were
correct in arguing that the material studied by Clarkson
and Levi-Setti had been diagenetically altered, but we
contend that the bowl and core in the lenses of Dalmanites sp. were enriched in magnesium relative to the optically clear calcite in vivo. Both intralensar structures were
subsequently recrystallized to microporous LMC plus ferroan microdolomite with accessory calcium phosphate
and iron sulphide. Despite the evidence presented here
for a doublet with a magnesian calcite bowl and core and,
hence, a potential difference in refractive index across the
interface between the upper lens unit and the bowl, the
exact manner in which the lenses of Dalmanites sp.
focused light remains to be determined. The function of
the core is not clear and there is still much to be learned
from the detailed crystallography of the lenses. Regardless
of its origin and subsequent diagenetic history, all of the
calcite within each lens has its c-axis in the same orientation and this crystallography is also independent of the
fanning of constituent subgrains towards the bowl. No
evidence can be found in Dalmanites sp. for the radialfibrous structure that has been reported from the upper
lens unit of many other schizochroal lenses (Campbell
1975). However, it is clear that the larger scale structure
of the lenses of schizochroal eyes varies greatly between
taxa (see review by Clarkson et al. 2006) and so differences in the finer scale morphology can also reasonably
be expected to have developed.
The demonstration that magnesian calcite was used in
the construction of the lenses of Dalmanites may help to
account for the disagreement in the literature on the
in vivo chemical composition of trilobite exoskeletons.
Lowenstam (in Richter and Füchtbauer 1978) described
up to 5 mol per cent MgCO3 in the exoskeletons of
Carboniferous trilobites and Richter and Füchtbauer
(1978) used the replacement of trilobite exoskeletons by
ferroan calcite as an indicator of an original magnesian
composition. In contrast, Wilmot and Fallick (1989)
found an average of 3Æ6 mol per cent MgCO3 and
1Æ0 mol per cent FeCO3 from wet chemical analyses of
trilobite exoskeletons from the Much Wenlock Limestone
Formation (Silurian, UK). Furthermore, they stressed the
absence of microdolomites from trilobite cuticles,
although they did not mention having studied individual
lenses of schizochroal eyes. Our preliminary analyses of
the schizochroal eyes of other trilobites have shown that
the presence of microdolomites is a common feature and
it is noteworthy that the mean composition of trilobite
exoskeletons determined by Wilmott and Fallick is very
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
mixed with magnesian calcite or dolomite.
Acknowledgements. We thank Euan Clarkson for his advice and
encouragement and for the provision of the thin sections used
in this study. We also thank Philip Donoghue and two anonymous reviewers for their helpful comments and we are grateful
to Robert MacDonald for his assistance with the electron probe.
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