Geoscience Reference
In-Depth Information
Stromatolites
A number of horizons in the Bertie
Waterlime are characterized by hummocky,
finely laminated structures known as
stromatolites. One horizon particularly rich
in stromatolite beds is the Ellicott Creek
Breccia at the top of the Fiddlers Green
Formation (
85
). Stromatolites are mats of
lime mud precipitated by the action of
photosynthesis by algae or cyanobacteria
('blue-green algae'). They can be found in
rocks from as old as Precambrian to the
present day because cyanobacteria and
algae have been on Earth for such a long
time (see Gunflint Chert, Chapter 1).
evidence of a tubular structure (i.e. the gut)
in the metasoma. Whether these specimens
really represent dead animals is debatable,
however, because: (a) arthropods molt all
ectodermal structures, which includes the
fore- and hind-gut; (b) judging from the
photographs, the 'tubular structures' could
just as easily be external carinae; and
(c) dead animals are usually scavenged and
so rarely turn up as fossils unless they have
been quickly buried. Even if these three
specimens do represent dead animals, the
ratio of them to molts is clearly so small that we
should consider that molts are the norm.
While collectors favor complete specimens,
most eurypterid remains in the rock are
disarticulated parts that have been torn apart
during transportation. Most eurypterid
fragments occur in current-oriented debris
accumulations known as windrows (
86
).
Commonly they comprise the more resistant
body parts such as telsons, and often there is
taxonomic segregation so that, for example,
pterygotids, cephalopods, or gastropods
occur in discrete windrows.
Cooksonia
was derived from the land,
where it might have been growing in
boggy, riverside habitats, yet it occurs
together with aquatic animals in current-
oriented deposits. So, many people
consider that the eurypterids lived in
adjacent rivers and their molted remains
were washed in during occasional storms
together with other nonmarine organisms.
The hostile hypersaline conditions might
have been conducive to the preservation
of organic matter by decreasing the
numbers of detritivores and bacteria. We
should note, however, that the Bertie
Waterlime sequence, as shown in (
67)
is
complex, and many different micro-
palaeoenvironments are represented,
some of which contained good marine
organisms (e.g. orthocone nautiloids)
while others were washed in from
elsewhere (e.g.
Cooksonia
).
P
ALEOECOLOGY OF THE
B
ERTIE
W
ATERLIME
The Bertie Waterlime eurypterid
horizons are fine-grained dolostones
containing a mixture of clay and quartz
and which break with a conchoidal
fracture. Sedimentary features such as
ripple marks, cross bedding, fine
lamination, lunate scours, halite
pseudomorphs, and trace fossils indicate
that their deposition was in shallow water,
between coastal stromatolite shoals and
deep-water sponge reefs. The common
halite psuedomorphs indicate that most -
possibly all - waterlimes were formed
under hypersaline conditions, and in
cyclic sequences in an evaporating basin
(Ciurca, 1973). The depositional
environment is thus interpreted as a
hypersaline lagoon. The question is: did
the eurypterids live in this salty lagoon or
were their remains washed in after death?
In other places, the presence of
eurypterids is considered to indicate
freshwater or brackish waters. Most (all?) of
the eurypterids are molts, although
Heubusch (1962) searched the collections
of
Eurypterus lacustris
in the Buffalo
Museum of Sciences and found two
specimens, plus another in a private
collection, which she considered showed
Search WWH ::
Custom Search