Preliminary elemental analysis of fossil insects from the Middle Jurassic of Daohugou, Inner Mongolia and its taphonomic implications

Abstract

Fossil insects from the Middle Jurassic of Daohugou, Inner Mongolia were investigated using Scanning Electron Microscope (SEM)
for the first time, and portions and distribution of some elements in compression and pyrited fossils were also revealed by
X-ray Energy Dispersive Spectroscopy (EDS) attached to SEM. Most of compression fossil insects from the Daohugou Biota are
preserved in organic remains (diagenetic products of the original organic components). A small part of compression fossils
retain a comparatively high Fe concentration which probably resulted from the absorption of Fe by biopolymers during the decaying
period. Pyritized insect fossils suggest that the “fossil envelop” model found in the Early Cretaceous Jehol Biota probably
also occurs in the Daohugou Biota. Different preservation modes show various mechanisms of fossilization, and also suggest
that several different microenvironments are present in Daohugou palaeolakes.

Full text

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ARTICLES
GEOLOGY
Preliminary elemental analysis of fossil insects from
the Middle J urassic of Daohugou, Inner Mongolia and
its taphonomic implications
WANG Bo1,3†, LI JianFeng2,3, FANG Yan1 & ZHANG HaiChun1
1 State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of
Sciences, Nanjing 210008, China;
2 Key Laboratory of Marginal Sea Geology, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou
510640, China;
3 Graduate University of the Chinese Academy of Sciences, Beijing 100049, China
Fossil insects from the Middle Jurassic of Daohugou, Inner Mongolia were investigated using Scanning
Electron Microscope (SEM) for the first time, and portions and distribution of some elements in com-
pression and pyrited fossils were also revealed by X-ray Energy Dispersive Spectroscopy (EDS) at-
tached to SEM. Most of compression fossil insects from the Daohugou Biota are preserved in organic
remains (diagenetic products of the original organic components). A small part of compression fossils
retain a comparatively high Fe concentration which probably resulted from the absorption of Fe by
biopolymers during the decaying period. Pyritized insect fossils suggest that the “fossil envelop”
model found in the Early Cretaceous Jehol Biota probably also occurs in the Daohugou Biota. Different
preservation modes show various mechanisms of fossilization, and also suggest that several different
microenvironments are present in Daohugou palaeolakes.
Inner Mongolia, Middle Jurassic, Daohugou, fossil insects, taphonomy
Fossil insects have been studied for over 200 years, but
their fossilization is still far from clear[1,2]. In recent 10
years, some considerable progress has been made in
studying the insect taphonomy[3―13], and major studies
refer to the preburial processes before the fossiliza-
tion[3―6]. Analyses of the chemical composition of fossil
insects focused mainly on those in amber[7,8] and some
Cenozoic specimens[9―13], but much less on Mesozoic
ones[4]. So far, the taphonomy of some early arthropods
in some famous lagerstatten has been studied in
detail[14―17], and these methods and results could also be
applied to the study of Mesozoic fossil insects. Although
abundant well-preserved fossil insects have been dis-
covered from the Mesozoic of China, their taphonomy is
completely unknown. Very recently, abundant plant and
animal fossils have been reported from the Middle Ju-
rassic of Daohugou, Ningcheng County, Chifeng City,
Inner Mongolia, China[18―21]. The rich insect fauna from
this locality provides some important clues to under-
standing the evolution of Mesozoic insects. These fossil
insects are probably similar to those from the Jehol Bi-
ota in the taphonomic process, and both faunas are re-
lated to the volcanic activities. Therefore, fossil insects
from Daohugou provide an approach to understanding
the taphonomy of fossil insects from the Mesozoic of
northeastern China.
1 Material and methods
The four orders, Diptera, Hemiptera, Orthoptera and
Coleoptera, are common in fossil insect assemblages.

Received September 16, 2008; accepted October 21, 2008
doi: 10.1007/s11434-008-0561-5
†Corresponding author (email: savantwang@gmail.com)
Supported by National Basic Research Program of China (Grant No. 2006CB806400),
National Natural Science Foundation of China (Grant Nos. 40872015, 40523004,
40632010), and Special Fund for Graduates in the Graduate University of the Chi-
nese Academy of Sciences

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2 WANG Bo et al. Chinese Science Bulletin | Ja??? 2009 | vol. 54 | no. ? | ?-?

Four Daohugou specimens within the above four groups
are picked to represent three different modes of preser-
vation. Specimen NIGP149368 is a compression of
Procercopidae within Hemiptera, and its body is brown
and its abdominal segments are clear. Specimen
NIGP149369 is a compression of Prophalangopsidae
within Orthoptera, and its body is grayish brown and
strongly deformed. Based on our collections, most of
Daohugou compressions (more than 99%) are preserved
in this mode. Specimen NIGP149370 is a compression
of Diptera without wings. Its body is brown and its ab-
dominal segments are clear. Just a small part of Dao-
hugou compressions (less than 1%) are preserved in this
mode, which is similar to the first mode under the light
microscope, except exhibiting yellowish. However, the
elemental analysis demonstrates some distinct differ-
ences between these two modes (see below). Specimen
NIGP149371 is a pyritized beetle retaining slightly
three-dimensional structures. The beetle is yellowish
brown, with appendages missing, tegmina broken and
granules clear. The third mode, pyritization, is rare in
the Daohugou Biota, and just occurs in several beetles.
All examined specimens are preserved on the surface of
gray or grayish-yellow thin-bedded tuffaceous siltstones.
Conchostracans (Euestheria) are usually preserved on
the same bedding plane. All specimens for this study are
deposited in the Nanjing Institute of Geology and Pa-
laeontology (NIGP), Chinese Academy of Sciences.
The SEM (scanning electron microscope) and EDS
(X-ray energy dispersive spectrum) analyses were per-
formed in the Key Laboratory of Marginal Sea Geology,
Guangzhou Institute of Geochemistry, Chinese Academy
of Sciences. Samples were cut to the appropriate size,
and their undersurfaces were polished (topside of sam-
ples should not be polished in order to avoid configura-
tion disturbing). Then these samples were cleaned with
distilled water, and dried in the desiccator. Additionally,
the samples were analyzed uncoated for subsequent
analyses. Micro-surface information was obtained under
the low vacuum mode (100 Pa in sample chamber) of
the Quanta 400 SEM with the accelerating voltage 20 kV.
The elemental analyses of points and areas were exam-
ined by the EDS produced by EDAX company (model:
Genesis 2000). In order to obtain more exact data, the
acquisition time for the SUTW-Sapphire EDS detector
was between 90 and 120 s.
2 Results
The concentration of both elements C and Ca in fossils
NIGP149368 and NIGP149369 is distinctly higher than
in matrix, while that of other elements is lower than in
matrix (Table 1). The backscattered electron map of
specimen NIGP149368 indicates that element C is not
equably distributed in the fossil: the darker in the BSE
map, the higher the carbon content (Figure 1(c)). Ele-
mental maps for C and Ca match the profile of the fossil.
Furthermore, the lighter areas in the C elemental map
agree with the darker areas in the BSE map (Figure
1(c)―(e)). The concentration of C and Fe in fossil
NIGP149370 is respectively twice than in matrix, while
that of other elements is lower than in matrix (Table 1).
Moreover, the Fe elemental map matches the profile of
the fossil very well (Figure 1(g)). Fossil NIGP149371
comprises pyrites, and thus its content of both elements
Fe and S is higher than in matrix. Partly oxidized pyrites
make the fossil reddish in colour. SEM analysis reveals
that pyritic crystals are mainly distributed on or around

Table 1 Relative abundance for elements in six spots shown in Figure 1a)

wt%
NIGP149368 (fossil)
at%
wt%
NIGP149368 (matrix)
at%
wt%
NIGP149369 (fossil)
at%
wt%
NIGP149369 (matrix)
at%
wt%
NIGP149370 (fossil)
at%
wt%
NIGP149370 (matrix)
at%
a) “−” represents non-detected; wt% and at% represent the percentage of atom weight and atom quantity, respectively.
C O Mg
0.27
0.19
0.42
0.40
0.15
0.10
0.29
0.24
0.73
0.56
1.00
0.83
Al
4.26
2.72
Si K Ca
3.48
1.50
Fe
−
−
−
−
0.75
0.21
1.50
0.53
4.34
1.46
2.84
1.02
28.75
41.28
−
−
46.58
60.94
6.51
10.72
14.64
22.82
7.66
12.83
35.08
37.82
43.38
58.38
25.54
25.08
46.24
57.15
44.29
51.82
42.40
53.30
23.30
14.31
37.77
28.95
13.75
7.69
34.07
23.99
25.33
16.88
31.70
22.70
4.60
2.03
8.59
6.86
3.46
2.01
6.33
4.64
6.36
4.41
8.36
6.23
9.83
5.41
5.94
2.39
4.59
2.32
3.42
1.64
4.94
2.54
−
−
3.02
1.18
−
−
0.89
0.41
1.11
0.56

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GEOLOGY

Figure 1 (a) Light photomicrograph of a procercopid (Hemiptera) (No: NIGP149368; Middle Jurassic, Daohugou, Inner Mongolia); (b) light
photomicrograph of a prophalangopsid (Orthoptera) (No. NIGP149369; Middle Jurassic, Daohugou, Inner Mongolia); (c)―(e) BSE photomi-
crograph, C elemental map, Ca elemental map for specimen NIGP149368 (area shown by black square in (a) respectively; (f) light photomi-
crograph of a dipteran (No. NIGP149370; Middle Jurassic, Daohugou, Inner Mongolia); (g) Fe elemental map for specimen NIGP149370.
Triangles showing the spots in fossils for EDS analysis, and squares showing the spots in matrix for EDS analysis.

the tegmina, and distinctly decrease in the matrix far
from the tegmina. Pyrites are composed of irregular
crystals, of which each comprises some minute micro-
crystallines of 0.5―1 μm in diameter (Figure 2).
3 Discussion
Insect cuticles are mainly composed of chitin, which
may survive for a long time under anoxic conditions[2].
Hence, the parts of insect with thick cuticles are more
easily fossilized, exemplified by fossil cockroach teg-
mina and beetle elytra widespread all over the world. All
fossil chitin, however, are discovered from Cenozoic
insects. The oldest chitin is known from an Late Oligo-
cene (25 Ma) weevil at Enspel, Germany[13], and the
chitin in much older insects has been chemically de-
graded[22]. During diagenesis, degraded biopolymers
could be transformed into more stable biopolymers by
the random intermolecular polymerisation and polycon-
densation[9]. Furthermore, chemical experiments also
revealed that labile lipids could be incorporated into a
resistant aliphatic macromolecule via in situ polymeriza-
tion[22―24]. Some Palaeozoic and Mesozoic arthropods
(including insects) are composed of biopolymers[22]. A
much higher concentration of element C in insects of
specimens NIGP149370, NIGP149368 and NIGP149369
than in matrix suggests that these compression fossils
comprise biopolymers. So far, some types of biopoly-
mers have been discovered from fossil insects, and dif-
ferent biopolymers occur in different groups[9,11].
Therefore, the original biomolecules (such as chitin) in
Daohugou insects most probably have been transformed
into more resistant biopolymers, of which the exact
compositions remain to be determined.

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4 WANG Bo et al. Chinese Science Bulletin | Ja??? 2009 | vol. 54 | no. ? | ?-?


Figure 2 (a) Light photomicrograph of fossil beetle (No. NIGP149371; Middle Jurassic, Daohugou, Inner Mongolia); (b), (c), (d) SEM mag-
nifications of areas showed by white squares in (a), (b), (c) respectively.

Various types of fossilization occur in insects under
different microenvironments. Therefore, diverse fossil
insects from the same bed or different strata probably
have different fossilization modes[2]. Clay minerals
played a key role in the preservation of soft tissues in
some famous lagerstatten[4,25]. However, only elements
C and Ca possess a high concentration in insects of
specimens NIGP149368 and NIGP149369, suggesting
that clay minerals are not important for the preservation
of Daohugou insects. The high concentration of Ca in
fossil insects probably resulted from calcium carbonate
minerals, which are normally associated with fossil in-
sects[4]. These minerals could not normally replicate the
structure of insects, but may infill voids left following
the decay of soft-tissues and promote rapid lithifica-
tion[4]. The distinct difference between NIGP149370 and
other two specimens is that the former preserves a high
concentration of Fe, which is also higher than that in
matrix. Petrovich suggested that the preservation in the
Burgess Shale could be attributed to decay inhibition as
a result of adsorption of Fe2+ ions (products of Fe(III)
reduction) onto structural biopolymers[26]. Similarly, the
high concentration of Fe in fossil NIGP149370 probably
resulted from the absorption of Fe by biopolymers.
Pyritization is among the most common fossilization
modes, and usually occurs in animal soft-tissues and plant
remains[15,25]. It has been found in some Cenozoic in-
sects[1]. However, only a few previous studies focused on
taphonomic implications of Mesozoic pyritized insects.
Leng and Yang found some in situ pyrite framboids and
microcrystallines of plant and feather fossils in the Jehol
Biota, and proposed that the “fossil envelop” microenvi-
ronment during the early fossilization process is a prereq-
uisite to the formation of pyrites[27]. The pyritized insect
suggests that the similar microenvironment and fossiliza-
tion mechanism found in the Early Cretaceous Jehol Biota
probably also occurs in the Daohugou Biota. Generally,
pyritization only preserve the outlines of fossil insects,
because pyrite crystals are coarse and form too late in the
taphonomic process to replicate the finest details[16,17].
The physical properties of pyrites can be used to define a
series of geochemical and depositional parameters[4,16,27].
However, some information concerning the palaeoen-
vironment remains to be analyzed because of the lack of
enough well-preserved pyritized insects.
In conclusion, fossil insects from the Middle Juras-

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sic of Daohugou have different preservation modes,
which show various mechanisms of fossilization, and
also suggest that several different microenvironments
are present in Daohugou palaeolakes. However, some
questions should be highlighted to require further re-
search. How do different modes of preservation corre-
spond to different microenvironments? How do micro-

environments affect the taphonomic processes of fossil
insects?
The authors sincerely thank Prof. Yuan Xunlai for reviewing the earlier
manuscript, Drs. Hu Jie (NIGPAS), Cai Yaoping (Northwest University)
and Meng Fanwei (Nanjing University) for their helpful suggestions.
Thanks also go to three anonymous reviewers and the editors who greatly
improved the manuscript.
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