‘Geology Rocks!’
Guide to Palaeontology
an
A2 GEOLOGY
guide
(2007-08)
As taught by
Mr. Dr. JR Aggett
[Module 2834]
A2
Geology: Module 2834
PALAEONTOLOGY
“This A2 module
provides and in-depth knowledge and understanding of the major invertebrate phyla.
IT covers the preservation of fossils, invertebrate fossil morphology,
evolution and mode of life of specific fossil groups and the dating of Rocks.
Skills from both modules 2831 and 2832 [The Rock Cycle; see Geology Rocks AS
guide] are necessary. The module introduces the recognition of fossils from
drawings or photographs and the use of morphological terms.”
--from the A2 Geology Specification
Some things that may appear in the
exam as long written answer with marks
for diagrams questions are marked in red
5.4.1:
Preservation of Fossils
The
Principles of Evolution are that many individuals fail to survive or reproduce
( a ‘struggle for existence’), and that in this struggle individuals showing
natural variation best adapted to their environment have a reproductive
advantage and produce more living and reproducing offspring than less well
suited organisms. This is Natural Selection.
Since some species are made extinct
through natural selection, and only around 10 million species of the 5 billion
species ever to exist are still around today (only 1 million of which are
named), this means that we only know about species that lived on our planet in
the distant past through fossils. Only about 1 in 20000 species becomes
fossilized, and therefore only 250000 fossil species are known of.
Organisms are classified by taxonomy.
For A2 geology you will only need to know about the bottom two ‘taxons’ or
organisational ‘groups’: Phylum and Species.
We will be looking at the Phyla
Cnidaria, Brachiopoda, Mollusca (groups ammonoids, belemnoids, bivalves,
gastropods), arthropoda (trilobites), echinodermata (groups echinoids and
crinoids) and Hemichordata (Graptolites) which are the most significant for the
course.
a) Amber, Tar, and the Burgess Shales: Exceptional Preservation
The Burgess Shales
The burgess shale is a rock
formation near the
“Examples include Opabinia
with five eyes and a snout like a
vacuum cleaner; Aysheaia
which bears an extraordinary
resemblance to a minor modern phylum—the Onychophora;
Nectocaris
which is apparently either a crustacean
with fins or a vertebrate
with a shell; and Hallucigenia
which was originally reconstructed as walking on
bilaterally symmetrical spines…[but which is now reconstructed as] another onychophoran,
with the spines on its back.”
The variety
of the fossils has been suggested to imply that life during the Middle
Cambrian was much more diverse than today. These three factors make the site of
significant geological interest.
The Burgess Shale deposits
appear to represent small areas of
anoxic deep ocean mud at the
base of 100m high near-vertical Limestone cliffs. Mud-slides over the cliff
into the ocean (the shale deposits were located near the equator at the time of
deposition, on the continental margin of North America) or ‘submarine slumps’
carried fauna and flora not normally
found at the ocean floor into the anoxic
muds, where they were preserved in the low
oxygen fine sediment which was non-conducive to the micro-organisms which would otherwise destroy the remains, at a
depth where plankton and other small
marine creatures doing this were not
present either.The remains were therefore preserved at an exceptional
level.
The Burgess Shale fossils are preserved as
highly-compressed films on the surface of the rock, with some 3-dimensional
structure preserved.
FOSSILS PRESERVED INCLUDED:
Marrella
splendens (left) is a
small "arthropod” a bit like a trilobite, but with several distinctive
features. The contents of the body were squeezed out after burial and show up
as a dark blob in many fossils (seen here near the centre). Marrella is
one of the most common fossils in the Burgess Shale, and was probably the first
soft-bodied organism discovered there.
The Trilobite shown right is one of the largest species discovered in the
burgess Shale. This specimen is nearly complete, including many soft
appendages.
Amber
Preservation in amber occurs when animals become trapped in tree sap, which is hardened and lithified, preserving in great detail the hard and soft tissues inside, since there is little oxygen inside for decomposers, and the hard amber prevents scavengers attacking the animal. If the animal is not preserved intact, a detailed external mould will be left.
Tar
Again, there are few decomposers
or scavengers since the mud is anoxic and anything wanting to scavenge the
animal’s body would meet the same fate. Animals become trapped in the tar by
mistaking it for drinking water, and predators then follow them in and also
become stuck. The tar will seep into bones giving them a brownish colour and preserving
them.
(in a long answer question on common methods of preservation, only one of either amber or tar may
be used as an example)
b) Preservation
of hard skeletal Tissues
The
soft parts (everything except the shell, bones, or teeth) of animals are
preserved only in exceptional conditions because they easily rot away or are
eaten by scavengers after death. Even in anaerobic conditions, such as in the
Burgess Shales, bacteria destroy soft tissues to some extent. The hard parts
are most likely to be preserved if they are buried quickly, for instance in the
sea by sediment. Land areas are usually not sites of deposition but erosion, so
the chances of preservation are poor; the shell or bones of the animal will
remain on the surface and will weather away, or will be broken, scattered, or
eaten by scavengers.
The environment where most fossils are
preserved is in soft sediment at the bottom of lakes or the seabed, especially
in anoxic water or muds (where there
is no oxygen). Animals in the upper layers of oxygenated water sink to the
bottom after they die, and since there are few scavengers to eat or scatter the
remains, few decomposers, and sediment to cover them, they are likely to be
fossilised.
Despite this, the most fossiliferous rocks are those deposited
in shallow warm seas where there is much life, above the carbonate compensation depth where calcite begins to dissolve.
Figure 5.4.1.1 shows different depositional environments.
Hard parts made of calcite are more
likely to be fossilised than those made of aragonite, since it is a stronger
more stable form of calcium carbonate and so less easily dissolved. Sandstones
are permeable and tend to have acidic water percolating through which dissolves
shells. Clays are better preserving rocks since they have little acidic water.
The hard parts are often changed, chemically altered by minerals in percolating
waters or by metamorphism.
These elements combine to make up the Preservation Potential for an organism
and its environment of deposition.
Replacement
Aragonite
is replaced with the more stable calcite; other materials replaced by different
minerals e.g calcite by iron pyrites, silica, dolomite etc.
Preservation
by Alteration of Aragonite to Calcite
Aragonite
will turn into calcite over about 107 108 years.
Pyritisation
The
reducing (anoxic)
conditions in the sea-floor sediments, (and sometimes in the lower water column
near the sea-floor), were produced by the large supply of organic matter from
dead sea creatures. Sulphate-reducing
bacteria reduced the sulphate ions in seawater to sulphide,
producing the unpleasant gas hydrogen
sulphide (rotten eggs) in solution. This would then have reacted
with any iron available to produce the brassy-looking ferrous (Iron) Sulphide,
known as Iron Pyrite
or Fool's Gold. Pyrite is easily formed by decomposing organic matter when in
the presence of iron.
Carbonisation
Heat
due to deep burial changes plant
cellulose or the chitin skeletons
of arthropods and graptolites to carbon.
In anoxic environments organic soft parts become preserved as carbon films. Volatiles (H2,
N2, O2, and S) are lost.
Silicification
The
replacement of organic matter with quartz.
Cast and
Mould preservation
Mould Formation: depressions left by soft parts or when hard parts
are dissolved away. Moulds of outsides of shells are called external moulds,
and obviously inside ones are internal moulds.
Cast Formation: casts tend to stick out of the rock when the mould is
removed. Casts may retain great detail of the outer surface of the specimen but
all internal detail is lost.
c) Trace Fossils: Tracks, Trails and
Burrows
These
are preserved as moulds or casts detailing how the animal moved. Burrows in
which soft or hard bodied organisms lived are preserved; in soft sediment they
form layers. Coprolites (fossil poo) detail the diet, and sometimes the
structure of the gut. Boring is into hard rock and cuts through layers. Animals
bore to make holes in which to live or avoid predation. Tooth marks in these
rocks indicate predation. You may also see tracks from walking or distinctive
impressions left by resting. 
5.4.2: Morphology
a) Arthropods (Trilobites)
Cambrian-Devonian (peak time Cambrian)
Cephalon: The head of the Trilobite
Thorax: The middle section
Pygidium: the tail section
Glabella: a bulbous knob on the head, used for buoyancy
Compound Eyes: multi-cellular eyes
Cheeks: free and fixed sections for eating
Genal Angle: the back of the cephalon, may extend into genal spines for swimming
Thoracic segment (Pleuron): the plura have legs with gills attached underneath. The more legs, the more gills.
PHYLUM Arthropoda
GROUP Trilobites
Arthropods are segmented animals with a Chitinous skeleton. The skeleton has articulated segments, each with a pair of legs. The skeleton is rigid so the animal needs to moult so each one can leave multiple fossils. The nervous system heart and eyes are highly developed.
Trilobites are longitudinally divided into an axial and 2 lateral regions and transversely into the Cephalon thorax and pygidium. In many cases the trilobite could roll up. The Cephalon is typically semicircular with an axial area called the labella separated by axial furrows from the cheeks. Each cheek is crossed by a facial suture separating the free and fixed cheeks. Along the suture are well developed eyes. Genal spines may be present. The mouth is on the underside. The area around the eye has a line of weakness, the facial suture so that when it moults its case comes straight off the eye so it can still see predators. The thorax is divided into segments each of which had a pair of limbs, each limb having a gill. Pleural spines may be present.
b) Brachiopods
Cambrian-recent (peak time Devonian)
Brachiopods are
sessile (attached to a hard surface)
and have a calcite shell (occasionally Chitinous) made of two inequivalve equilateral shells.
Brachiopods are attached to a substrate via a pedicle
which enters the shell viaa foramen. They feed through a lophophore
or filtering system which may be supported in the shell by a brachidium. This
may be in the form of prongs, spiral ribbons, or loops. Some brachiopods have a
fold and a suchus
in the shell that separates incoming and outgoing water currents. Brachiopods are divided into
the articulates (held together with
a hinge) and inarticulates (held together by a system of muscle). They live between 100 and 200m down
in the ocean.
Brachiopods may be strophic (long hinged) or non-strophic (short-hinged).
Symmetry: equilateral (through valves)
Pedicle valve: the longer valve, to which the pedicle is attached
Brachial valve: the shorter valve
Ornament: the lines running down the shell
Foramen/Umbone: the original and oldest part of the shell
Cardinal process:
Adductor and diductor muscle scars:
Lophophore support system (brachidium): loops or ribbons covered in cilia
Pedicle: the small foot which keeps the brachiopod attached to the sediment.
c) Cnidarians (Corals)
PHYLUM: Cnidaria
Group: Corals
Corals
are coelenterates, which are simple multi-celled animals like jellyfish. They
have a simple structure with a central cavity, and may have stinging tentacles.
The corals comprise individual polyps which may exist in solitary or in a
colony (compound).There are three groups of Coral:
Group: Rugose
Corals
Ordovician-Permian (peak time Devonian)
These are solitary or
compound Palaeozoic corals with a calcite skeleton (corallum). This is divided radially by septae (6 primary, secondary septae in groups of 4). The structure
is reinforced by concave plates between the septae called dissepiments and
flattish horizontal plates called tabulae. The uppermost cup where the animal
sits is called the callice. In many
forms a supporting columella is present. They have a distinctive rough outer
surface which gives them their name. They live in clear shallow water, or, if
solitary, in deeper water. They have a symbiotic relationship with algae which
require them to be in the photic zone (light
penetration depth) to photosynthesise. They are often hexagonal when in
compound form, and horn shaped when solitary.
Group: Scleractinian
Corals
Triassic-recent (peak time Cretaceous)
These are similar to rugose except the septae are the
dominant skeletal feature. They are inserted in groups of 6 not 4. Dissepiments
and columella may be present. The corallum is fibrous aragonite.
The modern Goniopora
forms reefs between 35 degrees north and 32 degrees south. They are extremely
intolerant of environmental change, requiring constant specific salinity of 36%o (parts per thousand), a constant specific temperature of 25-29degrees Celsius, and a depth of no more than 90m (the extent
of the photic zone in tropical waters). These factors require the corals to be
living in clear tropical seas which may
indicate that rugose and tabulate corals inhabited a similar environment,
giving clues as to the past climates of areas where they are found.
Group: Tabulate
Corals
Ordovician-Permian (peak time Silurian)
Tabulate corals are similar to Rugose except with prominent tabulae. Septae, if present,
are small. Axial structures are rare. The callice is typically a few mm across. Tubes may connect
corallite walls, as tabulate corals are found only as compound groups. The
corallum is made of calcite.
Differences
between Coral Types
|
TABULATE (i.e Michelina ) |
RUGOSE (i.e Caninia ) |
SCLERACTINIAN (i.e Goniopora) |
|
-Small (mm) size -Connected by tubes -Compound only -No septae -no dissepiments -Many tabulae |
-Variable size -Hexagonal compound forms -Solitary or compound -6 prominent septae, with groups of four inserter between -dissepiments -some tabulae |
-Variable size -Variable compound form -Solitary or compound -Prominent septae in groups of 6 -dissepiments -some tabulae |
Sample Long-answer Question
Describe the morphological features of rugose corals. How do tabulate
and scleractinian corals differ from rugose corals? Explain why reefbuilding
corals are of palaeoenvironmental significance.
Rugose corals are solitary (Zaphrentites) or compound (Lithostrotion) corals, either separated or touching in hexagons (fig. 1). They commonly have columella (a supporting column down the centre of each individual) and prominent septae, 6 major and several minor inserted in fours. Like Scleractinians they have dissepiments (reinforced concave plates) around the outsides between the septae, and most prominently have a rough outer surface. The corallum, or skeleton, was made of calcite which has a high preservation potential. Like modern scleractinians, rugose corals had a symbiotic relationship with algae meaning they inhabited the photic zone where light penetrated the water, solitary ones living at greater depths than compound ones.
Tabulate
corals can be told easily from Rugose corals by their small size (each individual of a colony being only a few mm across)
their lack of dissepiments, their chain structure, their many prominent tabulae, and the fact they
are only found in colonies (compound).
Scleractinian corals are the only group left today, and although they too have
dissepiments and tabulae, and are found both as colonies and individually,
their septae are inserted in cycles of 6
not 4 and they are not so
distinctively rough as rugose corals. Also, they commonly have an axial complex.
Reef building corals are of palaeoenvironmental significance because colonial, compound corals only abound in clear, shallow tropical seas, and are very sensitive to environmental change: they will only grow in a 36% salinity (approx.) in a temperature range from 25-29 C. The depth and clarity of the water, meaning the corals must inhabit the photic zone (<90m) is due to the aforementioned symbiotic relationship with photosynthesising algae. This means that where coral reefs appear in the fossil record, the corals must have been deposited between 35 North and 32 South, in tropical regions near the equator, assuming the needs of corals have remained unchanged since the deposition. Therefore, any fossil not in that area must have been moved after deposition, providing evidence for continental drift. (mark received: 14/20)
d) Echinoderms
Group:
Echinoids (Sea urchins)
Ordovician-recent (peak time tertiary)
Echinoids have an internal
calcite skeleton of fused plates called a test.
The animal has a gut, mouth and anus, but lacks specialist organs. It does,
however, have a complex water-vascular system for operating tube feet: when
water is pumped into them they extend like long balloons. The test is
hemispherical, and disc or heart shaped, depending on whether it is regular or
irregular. The test is divided up into ten double columns running from the anus
at the top to the mouth underneath, five ambulacra
(one ambulacrum) covered in tube feet and five interambulacra covered in spines.
In the upper surface is the anus opening from a membrane called the periproct. The opening to the vascular system is here. The mouth is on the lower surface of the peristome membrane. The animal has
simple ‘jaws’ (the ‘Aristotle’s Lantern’). Around the peristome is the perignathic girdle. Each ambulacrum is
made of two plates, each plate with two rows of holes, one ‘in’ and one ‘out’.
In irregular echinoids
the anus is not central, and lies
outside the apical system. The mouth
may be either central or, more usually, towards the anterior margin. In the
latter the posterior interambulacrum forms a broad ridge called the plastron, which may be a lip projecting
over the mouth called the labrum.
Mode of Life of burrowing
echinoids:
Echinoids like echinocardium
live in burrows in the sediment. Living below the surface gives it protection
from predators, but it must keep a connection with the surface of the sediment
to take in food and to respire, and it must also deal with its faeces (like a
burrowing bivalve in fact). This led to the reshaping of the test and
modification of tube feet. The flattened heart shaped echinocardium is covered in short spines and has a perfectly adapted shape for digging. The
funnel connecting the echinoid to the surface is built and maintained with
elongated tube feet, and a similar set build the sanitary tube behind it. The fascioles create currents wafting food
into the mouth and waste into the sanitary
tube by waving cilia. When the
sanitary tube is full the echinoid moves forwards. The funnel spines are withdrawn,
the anterior spines are erected, and scrape away at the front wall of the
burrow whilst the paddle like spines of the plastron move the echinoid forwards. A new funnel is created while
the old one collapses behind it.
Group: Crinoids (sea lilies)
(cambrian-present) (peak time
Ordovician)
Crinoids are sessile
echinodermata (attached to a surface —non-moving). They live on the seabed in
deep water, as high energy environments destroy the structure. Like echinoids
they have tube feet, an anus on top, 5-fold symmetry and a water
vascular system. Often all that is found of the fragile animal are the
ossicles. Many lithified crinoids form crinoidal
limestone. The ossicles make up
the stem (also called columnals), and the animal is held in a calyx or calice. It has tegmen, brachia, pinnules, and holdfasts.
|
ECHINOIDS |
CRINOIDS |
|
Vagrant No arms No stem No roots Mouth opposite anus |
Sessile Arms Stem Roots Mouth on top |
e) Graptoloids
Cambrian-silurian (peak time Ordovician)
Graptolites
are extinct colonial marine organisms
confined to the Palaeozoic. They
secreted a proteinaceous skeleton (rhabsodome) from the sicula, with branches or stipes with short thecae in which an
individual zooid of the colony was
housed. There are two varieties of graptolites, the ‘true’ graptolites and the
dendroid graptolites with branches
and crossbars. The thecae may be simple, hooked, sigmoid, or lobate. The
rhabsodome is described based on the position and direction of the thecae
relative to the stipes.
The fact that graptolites are found in anoxic black shales suggests that,
since they would both have required oxygen this environment lacked, they
floated in the oxygenated layers above, floating around or perhaps attached to
seaweed or flotation devices. They were planktonic, probably pelagic.
f) Molluscs
Bivalves
Cambrian-recent (peak time Today)
These are soft invertebrates with a hard external
shell or valve, also known as lamellibrachs and pelecypods. They are filter feeders via
two siphons. The siphons extract
oxygen and food from water. They have a foot
for manoeuvre in or on sediment, and two shells connected on one side by a ligament. The ligament springs the
shells open, so the bivalve also has muscles
to close them. Most bivalves are
bilaterally symmetrical with the plane
of symmetry where the shells meet. They are equivalve (have two identical sized shells) and inequilateral. Some bivalves have
complex tooth-and-socket arrangements to align shells when closing and to hold
them shut. Surface ornament includes growth lines, ridges and furrows, and
sometimes spines.
Gastropods
Cambrian-recent (peak time Today)
Gastropods are soft invertebrates with a hard external
shell which is usually coiled. They are both marine and terrestrial. Marine
examples have gills or a siphon, and a foot similar to bivalves. The shell is
CaCo3 (calcium carbonate), usually aragonite, which has a poor preservation
potential. Unlike the ammonoids, the gastropod has no internal divisions,
although planar (flat) gastropods may otherwise look very similar.
The opening or aperture
may have a siphonal canal or
external slit which would have housed the siphons. Most are coiled in a clockwise direction
(‘dextral coiling’—aperture on the right).
Gastropods are mobile (vagrant)
and generally feed on algae. Thicker
shells or ribs indicate a higher
energy environment, while spines
could be used for anchoring in soft
sediment or for protection. An ‘entire’ aperture (with no slit or canal)
often indicates a herbivore on hard sediment whereas a siphonal canal indicates
soft sediment. Freshwater gastropods have the thinnest shells.
Cephalopods
Cambrian-recent
Cephalopods have in common a hard shell with a number
of prehensile ‘arms’ or tentacles. The shell is normally coiled. They are
divided further into three smaller groups:
-Nautiloids (cambrian-recent) (peak time
Ordovician)
Nautiloids’
shells are divided into chambers by saucer shaped septa. They are principally made of aragonite, and may be conical (orthocone) or coiled. The chambered part
is the phragmocone which held gas
for buoyancy, regulated by a porous tube called the siphuncle. The animal lived in the body chamber. The septal necks point backwards.
Nautiloids are pelagic, floating in the water column, rising and falling by
adjusting their buoyancy, extracting gas from the water and ejecting water via
a funnel for propulsion. Tentacles are used for catching prey and for
propulsion horizontally. The modern
nautilus lives in warm shallow water.
-ammonoids (Devonian-cretaceous) (Peak
time Triassic)
Ammonoids
are similar to nautiloids but with folded suture lines. The siphuncle is
typically towards the outer (ventral) margin.
The septal necks point forwards. Some
had a pair of plates, aptychi, used
as doors to protect the opening. The shell is usually aragonite, but the aptychi are calcite and so preserve better than the shells.
Ammonoids are divided into Goniatites, Ceratites, and
Ammonites (see 5.4.3 d). 
-belemnoids(carboniferous-cretaceous)(peak
time Jurassic)
Cambrian-present
Spotting the difference
between a bivalve and a brachiopod
|
BRACHIOPODS |
BIVALVES |
|
-Two inequivalve equilateral shells -Sessile -Feed through a lophophore -Equilateral symmetry -Attached by pedicle -Calcareous AND Chitinous -2 teeth and sockets -No pallial line |
-Two equivalve inequilateral shells -Burrowers, swimmers vagrant or attached by a bissus -Feed through two siphons -Inequilateral Symmetry -Attached by foot -Calcareous -Many teeth and sockets -Pallial line |
5.4.3: Evolution and Extinction
a) The uses of the first appearance and
extinction of the main invertebrate groups to establish a relative time scale
for the phanerozoic (eras and systems, stages and zones)
Zone
fossils are very useful for establishing relative timescales. Their first
appearance in the fossil record is very important, since it can be used to mark
the beginning of a period. The final appearance is less good for this since
fossils can be reworked into younger beds, so it could appear the period ended
after it actually did. Since fossils can’t be reworked into older strata, you
can be certain that the stratum a fossil first appears in was formed within the
time period in question. If the zone fossil is found world-wide, it can be used
to correlate strata globally, to show which were laid down at the same times.
b) Possible
reasons for the PT and KT extinctions
The Permian Triassic
Extinction (251 Ma)
The Permian-Triassic Extinction event was
the most catastrophic Extinction.
“Ninety to
ninety-five percent of marine species were eliminated as a result of this
Permian event. The primary marine and terrestrial victims included the fusulinid
foraminifera, trilobites,
rugose and
tabulate corals, blastoids,
acanthodians,
placoderms, and pelycosaurs,
which did not survive beyond the Permian boundary. Other groups that were substantially
reduced included the bryozoans,
brachiopods,
ammonoids,
sharks, bony fish, crinoids,
eurypterids,
ostracodes, and echinoderms.”
Theories as to the cause of the Permian
Triassic Extinction
It has been
speculated that the Permian Triassic extinction event could have been caused,
like those previous, by glaciation of Gondwana, and consequent climate change
and loss of continental shelves. However, other hypotheses abound, and it has
been suggested that continental shelves could have been reduced by the
formation of Pangaea, leading to similar competition for living space as the
ice-age theory: this idea is thrown into doubt by the fact that mass extinction
did not occur until the late Permian, whereas Pangaea was constructed during
the early and middle Permian.
Volcanic Eruptions
have also been blamed for the event, as there was much explosive volcanic
activity at the time, as shown by silica rich volcanic deposits in
Other events suggested to explain at
least part of the extinction are meteorite
impacts, supernova explosion, release of methane from the ocean causing global
warming, and volcanism producing flood-basalt events; immediately at the
end of the Permian 200,000 square kilometres were flooded with basic volcanic lava, and despite its
runniness the eruptions which produced the lava traps were probably
pyroclastic. These Traps ran into coal beds, and along with the pyroclastic dust the smoke from these fires
would have clouded the sky considerably.
“The direct effects of the Emeishan and Siberian Traps eruptions would have been:
- Dust clouds and acid aerosols
which would have disrupted photosynthesis both on land and in the upper
layers of the seas, causing food chains to collapse.
- a cooling of the climate because dust
clouds and aerosols blocked the sun.
- For the Siberian Traps eruptions,
possibly immediate warming because of the carbon dioxide emitted as the
lava heated the Siberian coal beds.
- Acid rain when the aerosols washed out
of the atmosphere. This would have killed land plants and mollusks and planktonic
organisms which build calcium
carbonate shells.
- Further warming when all of the dust
clouds and aerosols washed out of the atmosphere but the excess carbon
dioxide remained.
But there is doubt about whether these
eruptions were enough to cause directly a mass extinction as severe as the
end-Permian:
- For dust and aerosols to affect life
worldwide, the eruptions should be near the equator. But the much larger
Siberian Traps eruption was near the
- The carbon dioxide emissions would have
been more dangerous. If the Siberian Traps eruptions mostly occurred
within a period of 200,000 years, they would have approximately doubled
the atmosphere's carbon dioxide content, and recent climate models suggest
that would have raised global temperatures by 1.5 to 4.5 °C. 200,000 years
is near the short end of the range of estimates, and the warming would
have been less if the eruptions were spread over a longer period. “2
The Cretaceous-Tertiary
Extinction (65Ma)
The Cretaceous
Tertiary Extinction was the second largest extinction event in geological
history, where 85% of species perished. Aside from the dinosaurs, the
pterosaurs, belemnoids, many species of plants (except amongst the ferns and
seed-producing plants), ammonoids, marine reptiles, and rudist bivalves were
made extinct whilst many planktic foraminifera, calcareous nannoplankton,
diatoms, dinoflagellates, brachiopods, molluscs, echinoids, and fish were
severely affected. Most mammals, birds, turtles, crocodiles, lizards, snakes,
and amphibians were for the most part unaffected, which has led to some
controversy over the exact cause of the event.
Causes of the K-T Extinction Event
Whilst older
hypotheses concerning the Cretaceous extinction concerned volcanism and
glaciation as probable major causes, later hypotheses have used the evidence
presented by 
the layer of iridium, containing droplets of
basalt (volcanic glass), at the K-T boundary to suggest that more dramatic
events such as meteorite impact or comet showers could have wiped out the
dinosaurs and their contemporaries. Iridium suggests extra-terrestrial impact
or interference, as it is a substance found only in the earth’s mantle and in
concentrated quantities on meteorites. The iridium layer (left) has been found
consistently at the same position in nearly all sediments around the world,
suggesting an impact of the size required to wipe out so many species, whilst
the basalt supposedly represents rock that was melted on impact and flung into
the atmosphere. An early problem with this theory was the absence of an obvious
impact site, but the Yucatan Peninsula Crater (right) is now generally accepted
as being large enough, and in the right place, to match the impact.
Alternatively, the layer of Iridium has been
attributed to volcanism, its source therefore being the Earth’s own mantle. It
is speculated that the world-wide layer could be caused by the eruption of a
super-volcano, such as that evidenced by the
c) The morphological changes and evolution
of graptoloids in the lower Palaeozoic
d) The morphological changes
and evolution of ammonoids and nautiloids in the Palaeozoic and Mesozoic 
e) The morphological changes
and evolution of Micraster in the
cretaceous
|
Morphological change |
explanation |
|
Broader
higher test, |
Strengthens
test against deep burrowing pressure. Sediment falls off test more easily.
More spines for protection. |
|
Mouth
moves forwards |
Better
for feeding: closer to anterior groove
where food is brought by currents |
|
Labrum
develops |
Directs
water into mouth better |
|
Fasciole
broadened |
More
cilia for waste removal |
|
Anus
moves to rear |
Directs
waste away from mouth |
|
Deeper
anterior groove |
Improves
water flow to mouth |
|
Larger
plastron tubercles |
Spoon
shaped spines more efficient for digging. Support animal better in soft
sediment. |
|
Petals
longer with granular texture |
More
cilia improve water flow, more respiratory feet to extract oxygen from water. |
f) Factors making a good zone
fossil
Good zone fossils must be easily
identified so they are not mistaken for other zone fossils; well preserved,
abundant and widely (i.e. globally) distributed (pelagic or nektonic oceanic
creatures are best as they can reach anywhere in the world) to ensure they will
be found in most places for correlation purposes. They must have evolved
rapidly and therefore have a short range to define shorter, more exact, time periods.
5.4.4: Palaeoenvironments and mode of
life
Key physical factors controlling environments are
· Light intensity
· Oxygen concentration
· Salinity
· Water availability
· Temperature/climate
· Pressure (water)
· Flora/fauna
· Tectonics
· Minerals
· pH
· energy
Salinity
This is normally around 35%0 (parts per thousand). Freshwater is around 1 ppt, hypersaline around 40 ppt. It is therefore harder to make shells in freshwater. Cephalopods, brachiopods, echinoderms, trilobites, and corals are intolerant of different salinities, while some bivalves and gastropods are not.
Sunlight
Light only penetrates (ha!) the photic zone (c.150m in clear water, down to 10-30m where turbulent)
Corals need light for their symbiotic relationship with algae.
Oxygen
(Eh= Oxidation potential). Most organisms can’t live without oxygen. Its availability is controlled by depth. The higher energy an area is, the better oxygenated. Anoxic conditions are difficult for scavengers and microbes, so preservation potential is good. Some anoxic bacteria take O2 from dissolved SO42- . The sulphur reacts with iron to make iron pyrites, which replaces some organisms.
Other Factors
Temperature: -89o to 58oC
pH: 8.1-8.3 in oceans. Slightly alkaline.
Range of around 5 (lateritic soil) or 10 (more evaporates)
Palaeoenvironmental Analysis!
Evolution results in plants and animals becoming well adapted to their environment. Within any environment a community forms that either does not compete directly or has a stable predator-prey relationship. A collection of fossils forms an assemblage. It will not truly reflect a past community because not all members are preserved, soft organs may disappear, and fossils may be moved around or reworked. The term association is therefore sometimes used. In any community we must be clear in differentiating between Abundance (number of individuals)
And Diversity (number
of species)
(all you need to
learn are deltaic community, low energy
continental shelf, high energy continental shelf, and deep ocean)
a) Palaeoenvironment Recognition
Coal
measures
Coal
measures such as those found in the
Corals
Corals
are only found between 35 degrees north and 32 degrees south. They are
extremely intolerant of environmental change, requiring constant specific salinity of 36%o (parts per thousand),
a constant specific temperature of
25-29degrees Celsius, and a depth of
no more than 90m (the extent of the photic zone in tropical waters). These
factors require the corals to be living in clear tropical seas which may indicate that rugose and tabulate
corals inhabited a similar environment, giving clues as to the past
climates of areas where they are found.
Robust
molluscs, brachiopods, echinoderms (i.e, crinoids), corals and trilobites
High
energy continental shelf: lots of suspended food, many suspended feeders and
bottom dwellers. Stronger shells to withstand force of water.
Trace
fossils, delicate molluscs, brachiopods, echinoderms, corals and trilobites
Low
energy continental shelf: little suspended food, it sinks forming nutrient rich
sediment. Many burrowers, bottom feeders, grazers. Thinner shells, do not have
to withstand waves.
Pelagic
Microfossils, graptolites, some trilobites
b) Fossil
evidence for palaeoclimatic changes during the northward movement of
the
cambrian, Ordovician, Silurian: shale (deep water)
Devonian: desert sandstones (rare reptiles and freshwater fish)
Carboniferous: limestone, sandstone, coal, corals, brachiopods (tropics)
Permian, Triassic: Red sandstones (rare or no fossils)
Jurassic, Cretaceous, Tertiary: sandstones, shales, limestones (many common fossils)
Quaternary: Glaciers
c) Definition of terms
Fossil Life Assemblage: an assemblage of fossils that has not been reworked. The fossils are in their original condition, stratum, and community.
Fossil Death Assemblage: an assemblage of fossils that has been reworked. The fossils are not in their original condition, stratum, or community.
Derived Fossil: A fossil that has been removed from its stratum by erosion, reworked, and redeposited in a forming sediment
layer.
d)
Trilobites: Adaptive Radiation
Modes of
Life
Swimming (nektonic/pelagic):
E.g. Deiphon, Agnostus
Spiny
thoracic segments to increase surface area, very large eyes, no eyes, or
downward looking eyes, inflated glabella for
buoyancy, and a very small size.
Burrowing:
E.g. Trinucleus
Long genal
spines, lack of eyes or stalked eyes, shovel shaped Cephalon, pitted sieve-like
fringe
Crawling/ bed swimmer (Benthonic):
E.g. Parodoxides
Well developed raised eyes,
thoracic spines for protection,
e) Bivalves:
Adaptive Radiation
Examples:
Swimming-pecten, surface-Cerastoderma, Shallow Burrow-venus, Deep
Burrow-Solen
Cemented-Ostrea, Byssus-Mytilus, Boring-teredo
f)
Regular and irregular echinoids: are they so very different?
|
Regular |
Irregular |
|
Radial Symmetry Surface Dwelling (benthonic) Central anus in the apical system Mouth on oral surface |
Bilateral Symmetry Burrowing Anus at the back Mouth on aboral surface |
5.4.5:
Dating Rocks
a) Division
of the Geological Column
The
geological column is divided into eras and periods (or systems, as the
specification would have you call them) by defining the start and end of each
one by an event (such as a particular chronostratigraphic marker or the
beginning of a fossil’s range) which can be correlated globally.
b) Radiometric Dating
What are the
principles of Radiometric Dating?
Minerals
are made of elements, and elements can exist as different isotopes (same atomic number, different mass number). Some
isotopes, called radio-isotopes are radioactive. They decay to form new
elements (via emission of α, β, and γ radiation). The rate of
decay can be measured in the laboratory and is ‘fixed’ for a given isotope. In
practice we use the half life, which is the time taken for half of an element’s
original radioactive atoms (parent
atoms) to decay into daughter atoms of
the new element. All radioactive isotopes decay by this curve, the only
difference being the length of the half life: 
If
we know how many atoms were originally
present, how many there are now, and the
half life, we can determine the age of any rock. The total atoms A+B (if A
decays into B) equal the original number assuming there was no B in it to start
with. There will always by ½n (1 over 2n) of
the original atoms after n half lives. If there are 3 times as many daughter as
parent atoms, there have been 3 half lives (1:3 ratio/ ¼ / 25%).
Uses of
Radiometric Dating
Radioactive atoms are included in minerals when they crystallise. They then decay into ‘daughter’ atoms. The ratio of parents to daughters allows the mineral to be dated. Thus you can only date the minerals, not the rocks, unless they are unbroken igneous. In dating by the ratio we assume there were no daughter atoms in the mineral to begin with, and no atoms have been added or removed. Zircon fills these assumptions.
Potassium-Argon (K-Ar) Dating:
The half life of Potassium is 1.31x109 years. This makes it useful for dating rocks over 100,000 years old. Potassium is very abundant, and Argon is rare. Micas and amphiboles show the best results, and K-feldspar can be used. However, argon is a gas, which can easily escape rocks, and its presence in the atmosphere allows for easy contamination of samples. Typically the potassium is determined by chemical methods, and the argon by mass-spectrometry.
Uranium-Lead (U-Pb) Dating:
Overall, 238U is converted to 206Pb+8α (half life 4.51x109 years) and 235U is converted to 207Pb+7α (half life 0.71x109 years). Most uranium minerals contain Thorium (Th), and 232Th is converted to 208Pb+6α. The complete analysis of uranium bearing materials can give 3 independent age definitions. It is used for dating rocks from 10ma to 4,600ma. Zircon is often used because it contains substantial U and Th, and excludes lead on crystallisation. The amounts are measured through mass spectrometry. There are ways of dealing with initial lead atoms, so errors can be overcome.
Problems and
Limitations
In Metamorphic Rocks
Metamorphism can involve heating
and melting leading to ‘re-setting’ of the geological clock. New minerals form.
In Sedimentary Rocks
Sedimentary rocks contain grains of pre-existing rock. An average of the ages of the different grains does not date the formation, nor does analysis of grains. Analysis of cements in the rock doesn’t help because they contain few or no radio-isotopes; however, the iron mineral glauconite may form during early diagenesis, and contains substantial potassium, which can be used for K-Ar dating. Glauconite is found mainly in ‘green sands’.
Percolation
Liquid may drain through some
rocks, dissolving or re-depositing minerals.
d) Relative Dating
Cross-Cutting relationships
Features which cut across other
features (i.e. igneous intrusions) are always younger than the features they
cut across.
Included fossils and
Fragments
Fragments of older rock may be eroded, reworked, and redeposited in younger rock, but not vice-versa.
Superposition
Younger strata are redeposited above
older strata. To determine which way up strata were deposited we use
Fossils
Fossils, especially zone fossils,
can be easily dated, and the rocks in which they were found (unless they are
derived) will be the same age usually.
e)
Biostratigraphic correlation and the problems with derived fossils
Biostratigraphic correlation is
joining up the fossils of different areas, usually by first appearance,
occasionally by last appearance.
f) Varves and Ash Layers
Varves, ash layers, and the residues of meteorite impacts can be used as chronostratigraphic markers to date absolutely and correlate different areas in a similar way to biostratigraphy. Varves are Glacial Lake Sediments which alternate between sandstone and shale, one varve being two layers. Fine sediment in suspension settles to the bottom slowly when winter ice reduces water energy from currents. Varves can thus be used to show severity of winters and ages of lakes. More water=thicker sandstone, less water=thicker shale.
Volcanic ash layers result from dust and ash erupted high into the atmosphere and circulating the earth until they settle out again over the whole surface. They can readily be dated as igneous rock. Meteorite impacts may leave, for instance, layers of iridium.
OTHER THINGS YOU NEED TO KNOW
(Miscellany)
Cyclothems:
Cyclothems (see left) are alternating stratigraphic sequences of marine and non-marine sediments, interbedded with coal seamsWhen coal forms in deltaic environments, it forms as part of what are known as cyclothems, which are cyclical repeated patterns of sedimentary layers. They are caused by repeated changes in sea-level. Plant roots and leaves are often found in the strata immediately below the coal seams.
Glossary:
Sessile permanently
attached
to a substrate;
not free
to move about; "an
attached oyster"
nektonic Swimming independantly of
currents
pelagic Living in the open sea rather than in coastal
or inland
waters.
Benthonic Pertaining to the benthos;
living on the seafloor,
as opposed to floating in the ocean.
Planktonic Floating in the open sea rather than living on the seafloor.
Phanerozoic (aeon)
from about 570 million
years ago to the present; comprises the Paleozoic,
Mesozoic
and Cenozoic
eras.
Paleozoic (era) comprises the Cambrian,
Ordovician,
Silurian,
Devonian,
Carboniferous
and Permian
periods
from about 542 to 250 million years ago, from the age of trilobites
to that of reptiles.
Mesozoic (era) comprises the Triassic,
Jurassic
and Cretaceous
periods
from about 230 to 65 million years ago when life on earth was dominated
by reptiles.
Cenozoic (era)
comprises the Paleogene
and Neogene
periods
from about 65 million years ago to the present, when the continents
moved to their current position and modern plants and animals
evolved.
Derived A derived
fossil is a fossil found in rock made later than when the fossilized
animal or plant
Fossil died: it happens when a hard fossil is
freed from a soft rock formation by erosion and redeposited
or reworked in a currently forming sediment deposit.
Stage a
faunal stage will consist of a series of rocks that contain similar fossils.
There will be one or more index fossils
that are usually common, easily recognized, and limited to a single, or at most a few, stages. Faunal
stages are regional. They often include many formations of differing rock
types that were being laid down in different environments at the same time. In recent years, regional and
global correlations of rock sequences have become relatively certain and there is less need for
faunal labels to refine the age of formations.
Zone a period of time defined by a zone fossil
Differences
between a Jurassic and a Silurian reef:
Silurian
Reef:
Here you will find some rugose and many tabulate corals, some nautiloids,
trilobites (benthonic and nektonic), crinoids, regular echinoids, delicate
bivalves (benthonic and nektonic, some boring and burrowing) gastropods,
brachiopods.
Jurassic
Reef:
Here you will find scleractinian corals, bivalves, gastropods,
belemnites, ammonites (specifically ammonitic suture lines), nautiloids,
brachiopods, crinoids, echinoids.
