Practical 3: Corals and Brachiopods
The aims of this practical are as follows:
1.
2.
3.
4.
To teach you how to identify and describe corals and brachiopods.
To enable you to understand these groups on the context of their mode of life.
To help you to think about faunal replacement and functional similarity.
To think about the process of fossilisation, and what data is lost and gained between a living and
fossil assemblage (the effects of taphonomy).
5. To enable you to assess what constitutes an appropriate sample to assay a fossil fauna.
At the end of it you should be able to:
1.
2.
3.
4.
Draw and describe any coral or brachiopod, identifying any unusual features.
Comment on taphonomic effects on any fossil or set of fossils.
Discuss reef faunas through time.
Draw up an appropriate sampling strategy for a fossil locality.
Part 1: Corals
Hard corals, with a skeleton of calcium carbonate, have been important reef builders for much of the
Phanerozoic. As such they have taken part in forming an ecosystem, with a raised an complicated
topography above the sea bed. There are three major types of corals, each of which evolved
separately from a soft-bodied ancestor.
The three have a similar morphology, because their skeleton had to perform a similar function.
However, their detailed morphology is distinct. A brief key below will enable you to identify any
coral to this level.
Tabulate corals range from the Ordovician to Permian, but are mainly Lower
Palaeozoic in age.
Rugose corals have the same range, but were most common in the Silurian and
Carboniferous.
Scleractinian corals range from the Jurassic to the Recent.
You are provided with three coral specimens. Using the key, you should identify them as
rugose, tabulate or scleractinian corals. Make a representative drawing, or set of drawings for
each, making especially sure that you illustrate the key features that enable you to classify them
in the key. Give a brief description of each coral.
In your drawings and descriptions remember scales, or measurements of appropriate features, and
remember to label the view from which the fossil is drawn.
Coral
specimen
Solitary
coral
Is the centre of the
corallite empty or
full of septae?
Empty:
Tabulate
coral
Mirror plane:
Rugose coral
Colonial
coral
Is the centre of the
corallite empty or
full of septae?
Full: Are the septae
symmetrical or is
there a mirror plane
of symmetry?
Symmetrical:
Scleractinian
coral.
Empty:
Tabulate
coral
Full: Are the septae
radially symmetrical
or is there a mirror
plane of symmetry ?
Mirror plane:
Rugose coral
Figure 1. Identification key for the three main groups of hard corals.
Hint: the
central
structure may
show this most
clearly
Symmetrical:
Scleractinian
coral.
Coral 1:
Coral 2:
Coral 3:
A.
B.
Corallites – individual skeletal
elements occupied by one polyp.
These tend to be small in tabulate
corals, and to lack complicated
internal structures.
Coenenchyme – shared calcareous
tissue that conjoins corallites in
highly interconnected colonies.
Septa – small or absent
in tabulate corals. This
example shows small
septa that grew a short
distance from the
corallite wall.
Individual corallites are linked into
a corallum shaped like a chain
(cateniform). The shape of the
corallite and the corallum are
highly variable in corals
Tabulae – horizontal
plates which cut the
corallite into a series of
chambers. These
represent the base of the
section of the calice
occupied by the polyp at
different times during its
development.
Corallum is built of calcite
and is a solid structure.
Figure 2. Main features of the hard part morphology of tabulate corals. A. Halysites, B. Heliolites.
Central structure – frequently
developed from a range of
other skeletal elements.
Septa – everted over the rim
of the calice in most
scleractinian corals. Inserted
regularly around the corallite.
Corallum – the shape of the
colony is highly dependent on
environmental factors. Lightly
constructed from porous
aragonite.
Figure 3. Major elements of the hard part morphology of scleractinian corals. This example is
Confusastraea.
Calice – convex surface at the top of
the corallite, occupied by the polyp.
Septa project into the calice,
providing a secure attachment surface
for the animal.
Dissepiments –
small,
upwardly
convex plates
which are often
developed in a
marginal zone
at the edge of
the corallite.
A.
Septa – radially
arranged,
vertical plates,
added within
the corallite in
a characteristic
pattern (see
opposite).
Central column
(columella) –
common in
rugose corals
and developed
from the
modification of
a range of
other structures
including
tabulae and
septa.
Corallite –
often horn
shaped in
solitary corals.
Usually
constructed
from calcite.
C.
B.
Tabulae –
horizontal
plates
dissecting the
corallite.
Mature septa
with fossulae
Septa are added
on the
‘cardinal’ sides
of the alar and
counter lateral
septa.
Counter-lateral
septa added.
Alar septa
added
Coral
grows
Cardinal and
countercardinal septa
develop.
Corallite shape – controlled by the overall shape of
the colony in many rugose corals. In this example,
close contact between adjacent corallites causes
them to grow in a polyhedral shape.
Corallum – the whole coral colony.
Most commonly massive and dome
shaped in rugose corals. Its shape was
modified by the environment.
Figure 4. Major features of the hard part morphology of rugose corals. A, Generalised solitary coral;
B, . The sequential development of septa within a rugose coral. C, Generalised colonial coral.
Part 2: Reef taphonomy
Reefs, sometimes built by corals, sometimes by other groups of organisms, have been a feature of the
marine realm since the Cambrian. In some ways they appear to be ideal fossils. They are solid
structures, built of durable material, large in scale, and carrying useful environmental information.
As such they are ideal for thinking about taphonomy.
Taphonomy is the study of information loss and gain between a living and a fossil assemblage. It
covers information loss or bias due to sorting and decay, and the loss and gain inherent in the
chemical processes of fossilisation.
You are provided with two photographs, one of a modern reef, the other of a fossil reef, the
Wenlock Limestone for the Silurian of Wales. Use these to help you to think about taphonomic
processes that affect reefs. Try to answer the questions set out below. You should do this in a
group of between two and four.
More useful information is available on the side bench.
1. Can you identify bias that would be likely to occur in faunas between a living and a fossil reef?
a. Bias by trophic level (eg producers, consumers, carnivores)?
Most primary producers in the marine realm lack skeletons – they are either algae or
phytoplankton. They have a low chance or preservation. Some corals have symbiotic
primary producers, and some in the past might also have had this, but the only way to infer
this will be indirectly, by assaying the geographical distribution of fossils, or perhaps by
assessing how easy it was for them to produce calcium carbonate
Carnivores tend to be rare and active. They also tend to be vertebrates. This means their
chances of preservation are low, and if they do preserve they are likely to be in bits and may
be difficult to identify.
b. Bias by type of skeleton?
Hard parts are more likely to fossilise than organisms that don’t have them. Of these,
agglutinated skeletons tend to fall apart. Some calcareous skeletons dissolve, either due to
environment or because they are aragonitic. Articulated skeletons fall to bits, especially
echinoids and vertebrates. Thick, one piece skeletons fossilise much better than thin,
skeletons with several pieces.
c. Bias by mode of life, for example, sessile, swimming, burrowing?
d. Bias by position on the reef (eg forereef, backreef, reef front)?
2. What elements of information are lost as a result of fossilisation, regardless of whether the
organisms is preserved in some form? For example, colour is not usually preserved in fossils.
3. Can you think of any information that might be gained during fossilisation?
Part 3: Brachiopods
You are provided with two brachiopods. Produce detailed anatomical drawings of these. You
should concentrate particularly on the elements of symmetry shown by the shell. This is because
brachiopods are superficially like bivalves (which you will see in practical 4) and can be confused
with them.
Brachiopods have a mirror plane of symmetry that bisects the shell.
Bivalves have a mirror plane of symmetry between the shell (usually).
Brachiopod 1:
Brachiopod 2:
A
Pedicle foramen –
opening for pedicle
Delthyrium - closed
by deltidal plates
Commissure – line
where valves meet
Ventral valve
Umbo – rounded
area which marks
point of valve
growth
Rib
Growth line
B
Interarea - ventral
valve
Dorsal valve
Notothyrium
Delthyrium
Interarea - dorsal
valve
Figure 5. Brachiopod external morphology A. Astrophic B. Strophic
Mode of life
Infaunal
Burrowing
Semi-infaunal (partial
burrowing)
Epifaunal
Attached by pedicle
Encrusting
Cementing
Unattached
Shell form
Substrate
Example
Smooth
Spines on ventral valve
Soft-bottom
Soft sediment overlying
hard-bottom
Lingula
Konchiproductus
Pedicle opening
Closed pedicle opening,
irregular ventral valve
Closed pedicle opening,
ventral spines, umbo scar
Closed pedicle, saucershaped
Hard-bottom
Hard-bottom
Megellania
Crania
Hard-bottom
Chonosteges
Hard or soft-bottom
Rafinesquina
Table 1. Table relating mode of life to morphological characters in brachiopods.
Part 4: Sampling a brachiopod assemblage.
One of the main problems encountered by palaeontologists is knowing how many
specimens need to be collected to form a representative sample of a given locality. If
the sample is too small, then critical species may be missed. Too large, and the work
involved in processing the data is partly redundant.
Cumulative number of species
One of the commonest ways to resolve this is with a rarefaction curve. This is a plot
of the number of specimens collected against the cumulative number of species
identified. Initially, new species are encountered frequently. Slowly, the number of
new species found declines and the curve levels off. When it reaches a more or less
flat line, then the locality is considered to be adequately sampled.
Adequate sample
number
Specimens collected (batches of ten)
(log scale)
Figure 6: Sketch of a rarefaction curve.
You are provided with a set of 10 brachiopods collected from Shadwell Quarry
in Shropshire. The assemblage is from a Silurian muddy sea floor. Identify your
brachiopods using the key below, and add these data to the rarefaction curve on
the computer screen. Please follow the instructions next to the computer
carefully.
At the end of the practical note the number of specimens that adequately samples this
assemblage.