Geoscience Reference
In-Depth Information
C
OMPARISON OF
B
EECHER
S
T
RILOBITE
B
ED WITH OTHER
L
OWER
P
ALEOZOIC BIOTAS
Beecher's Trilobite Bed is just the thin
(40 mm, 1.5 in) microturbidite bed; the
other Frankfort Shale layers above and
below, which also include microturbidites
but lack the extraordinary pyrite
preservation, contain similar faunas, and
are typical of Ordovician deep-water
faunas. There are few other Ordovician
Lagerstätten known. The Soom Shale in
South Africa (Selden and Nudds, 2004,
Chapter 3) preserves a unique fauna from
a high-latitude (60°S), cool, glacially
influenced marine habitat. The Soom
Shale contains trilobites, including a
trinucleid, but also has a strange naraoiid
similar to
Naraoia
of the Burgess Shale
(Chapter 3). Both the Soom Shale and
Beecher's bed yield have orthocone
nautiloids, and these large animals seem
to have been common members of the
nekton throughout the Ordovician
Period. Inarticulate brachiopods are also
found in both Lagerstätten. The Soom
Shale has the eurypterid
Onychopterella
,
which is not known from the Frankfort
Shale but occurs in Silurian beds such as
the Kokomo Limestone of Indiana. Both
the Soom Shale and Beecher's Bed yield
belong to the late Ordovician, Ashgill
Epoch. At this time, Earth experienced
global cooling, with a glaciation
centred on the part of Gondwana now
occupied by north and west Africa. Its
effects were wide-ranging, and in New
York there was a noticeable severe
decline in the diversity of graptolites,
plankton which are normally associated
with low, subtropical latitudes. This
end-Ordovician mass extinction event
wiped out nearly a quarter of animal
families living at the time. It was the
greatest extinction to affect trilobites
during their time on Earth; more than
half of all trilobite families, including
the olenids and trinucleids, did not
survive to the Silurian Period.
ming trilobites such as odontopleurids.
When bottom conditions were more
oxygenated, soft-bodied worms burrowed
into the sediment and left their traces
behind. So, it seems that the
microturbidite disturbed this happy scene
and transported the live animals in a single
event from their original feeding site just a
short distance to where they were
deposited entombed in mud. What killed
the animals could have been smothering
with sediment (i.e. lack of oxygen), or
temperature shock: while the bottom
waters where the animals lived was likely to
have been cold, it might have been colder
still where they landed (cf. the Burgess
Shale, Chapter 3).
Triarthrus
belongs to the trilobite
family Olenidae, characterized by thin
cuticles, wide pleural regions, and many
thoracic segments (hence many
appendages). Their abundance in dark
mudstones, rich in sulfides, suggests they
were tolerant of low-oxygen conditions.
This led Fortey (2000) to suggest that they
could utilize sulfur bacteria as chemo-
autotrophic symbionts. These bacteria live
in conditions of low oxygen, often just at
the interface between oxic and anoxic
environments, and use sulfur (commonly
as hydrogen sulfide) as a source of energy.
Fortey suggested that the peculiar
morphology of the later olenids
(
Triarthrus
is among the last of the family)
is consistent with them utilizing chemo-
autotrophic bacteria.
Triarthrus
has many
appendages with gill filaments which could
act as substrates for bacterial cultivation;
the lack of a hypostome in some olenids (it
is poorly known in
Triarthrus
) is consistent
with it being unnecessary if food was
absorbed directly through the gill
filaments. This method of feeding is known
from a wide variety of organisms which
inhabit hypoxic (low oxygen) environ-
ments today; examples are known from
crustaceans, worms. and bivalve mollusks,
which show varying degrees of adaptation
to bacterial cultivation. If Fortey's idea is
correct, then olenids are the oldest animals
known to have used this feeding method.
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