The Crato Formation outcrops on the northern flanks of the Chapada
do Araripe, a plateaux on the border between Ceará, Pernambuco andPiauí States
in northern Brazil. In is noted for its exceptionally well preserved fossils,
which include Dinosaurs, Crocodiles, Fish, Pterosaurs, Crustaceans, Arachnids,
Plants and most notably Insects, which are present in large numbers, often
showing exceptional preservation. These Insects are of particular interest as
they date from a time in the Early Cretaceous when Flowering Plants were
rapidly diversifying, and relationships between Insect and Plant groups that
would come to dominate the Earth’s terrestrial biology were being formed.
Despite the importance of these fossils, the preservation process
that led to their formation is not well understood. The fossils occur only in
the Nova Olinda Member at the base of the Crato Formation, a laminated
limestone made up of alternating light and dark layers about 1 mm thick,
interpreted as representing a seasonal cycle. Preservation of insects in
limestone is exceptionally rare (most high quality Insect fossils are found in
amber), which suggests that the method of preservation in the Crato Formation
must have been exceptional.
In a paper published in the journal Cretaceous Research on 7 June
2014, Nathan Barling and David Martill of the School of Earth and Environmental Sciences at the University of Portsmouth, Sam Heads of the Illinois NaturalHistory Survey at the University of Illinois at Urbana-Champaign and Florence
Gallien, also of the School of Earth and Environmental Sciences at the
University of Portsmouth, examine the preservation of Insects in the Crato
Formation in an attempt to understand the processes that led to the high levels
of preservation seen.
Photomicrographs of Crato Formation Cockroaches. (A)
Specimen showing remarkable completeness. (B) Specimen with arrow highlighting
extremely fragile yet completely preserved antenna. Scale bars are 1 mm. Barling et al. (2014).
Exceptional preservation is a little hard to quantify in Insects; as
a rule a fossil is considered to show exceptional preservation if it shows any
preservation of non-mineralized tissue, however Insects are comprised entirely
of such tissue, and while describing the fossilization of any Insect as
exceptional might be accurate, this is not useful when comparing the relative
preservation of different Insect fossils. Many specimens from the Crato show
exceptional preservation of surface features, such as cuticular scales, setae
(‘hairs’ – though not formed from the same material as Mammalian hair), muscle
fibres and eye structures. This is often true of even fragmentary remains
(which would otherwise not be considered exceptional), suggesting that these
remains were fragmentary before reaching the site of preservation, possibly as
a result of predation or similar damage. However other specimens are less well
preserved, showing only the gross morphology of the Insect, not any fine
detail. Unusually these less well preserved fossils appear to be the exception
in the Crato (i.e. the better preserved fossils are more common), and seem to be
limited to certain horizons.
Damselfly Parahemiphlebia mickoleiti showing
structural iridescence; this is achieved through fine structure of the cuticle
rather than the presence of pigments, and therefore requires exceptional
preservational detail to be retained. Scale bar is 1 mm. Barlinget al. (2014).
All of the Crato Formation Insects are replacement fossils; that is
to say no original tissue has been preserved, everything seen is minerals that
have replaced these tissues, albeit with remarkably high fidelity. On exposed
limestone surfaces, which are buff in colour, the cuticletissue is preserved as
friable orange or brown amorphous goethite (limonite). In unexposed limestones,
which are blue-grey in colour, this tissue is preserved in a more delicate
black mineral, probably a form of iron sulphide. The exposed fossils (and
limestone matrix) have apparently turned brown as a result of recent oxidation,
probably over no more than a few thousand years. Other tissues appear to be
preserves as forms of apatite. Some fossils are surrounded by silica halos,
which may indicate the presence of former biofilms.
Cuticle of a Cockroach, showing spined cuticular scales
with a dimpled texture. Scale bar is 10 µm. Barling et al. (2014).
The preservation seen in the Crato fossils is even more remarkable
when it is understood that many are preserved as oxidised iron minerals
(essentially rust). Many of these specimens show preservation of fine
structures of the cuticle and other tissues, with little loss of detail
occurring during the mineral replacement process. Some specimens do show
structures that may be mineralogical rather than biological in origin, such as
spherical bodies resembling pyrite framboids (‘raspberry-shapes’). However
these do not appear to be pyrite, as they do not appear to contain much sulphur
(pyrite is a form of iron sulphide), leading Barlinget al. to hypothesise that these may
also be goethite replacements, this time of minerals formed at an earlier stage
in the preservation process.
Tubular structureswithin ?testes of an indeterminate
Fly with preserved genitalia. Scale bar is 10 μm. Barling et al. (2014).
Examination of the Crato Insect fossils under very high
magnification electron microscopy suggests that the mineral forming most of the
replacement material is a form of iron hydroxide, arranged in small
crypto-crystals (expand) about 100 nm in diameter, but arranged into larger spherical
or cylindrical clusters up to 20 mm in diameter.
These larger clusters superficially resemble Bacterial cells, and have been
previously interpreted as a sign of replacement of tissues by Bacteria, which
have subsequently auto-mineralized (i.e. mineralized as a consequence of their
own biological activity). However the microstructure of these structures is
more reminiscent of mineral replacement.
Hollow spherical structure composed of needle-like crystals
in an indeterminate Cockroach. Scale bar is 2 µm. Barlinget al. (2014).
The Crato Insects show a range of chemical phases, and it is not
possible to state with absolute confidence what is responsible for the
exceptional preservation seen at the site. Similar exceptional preservation is
known to occur at other locations where sulphate reducing Bacteria have coated
soft tissues under anoxic conditions, then auto-mineralized forming pyrite.
However it is not possible to prove that the first phase of replacement in the
Crato was pyrite, and nor have any bacterial cells been recovered from the
Insects, making it impossible to either prove or disprove this hypothesis.
Traces of bacterial mats have been found in the Crato formation – but not at
the horizons which have produced Insects. Exceptional preservation of Insects
is also known from the Early Cretaceous Jehol Biota of northern China, and in
this case is believed to be of geochemical origin. However the Jehol fossils do
not preserve the exceptional cuticle preservation seen in the Crato fossils,
suggesting that the fossilization process may have been different.
Finally Barling et al. note that while many studies of Crato Insects have been
carried out, relatively few have used scanning electron microscopy to study the
fine details of the surface of fossils. They note that this is a somewhat
destructive process, as it involves coating the samples with conductive
material, but recommend that this tool is more widely used when examining these
fossils, due to the high quality of results achieved.
See also…
A Chalcid Wasp from the Early Cretaceous of Brazil. Chalcid Wasps are among the most widespread and numerous of all Insects,
with over 22 000 described species to date, and estimates of up to half
a million species in total. Chalcids are found in almost every habitat
on every continent except Antarctica, though they are often overlooked
due to their small size. Most Chalcids are parasitoids, laying their
eggs in other invertebrates, with...
An early Woodwasp from the Early Cretaceous of Brazil. The Hymenoptera are one of the largest groups of insects, comprising
Sawflies, Wasps, Ants and Bees. The earliest members of the group were
Sawflies, which appeared around the beginning of the Late Triassic.
Sawflies have caterpillar-like larvae that...
A new Pterosaur from the Early Cretaceous of Brazil. The Crato Formation is an Aptian (Early Cretaceous, about 180 million
years old) fossil Lagerstätte (site where numerous well preserved
fossils have been...
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