Caloosahatchee Subsea
Florida during the
Calabrian glaciation
of 2.5 million
years ago.
- Period:
Pliocene
to
Pleistocene
.
- Geologic stage: Late
Piacenzian
~2.8 Ma. to late
Calabrian
~780,000, approximately
1.8
million years.
- Animal age:
Blancan
to
Irvingtonian
~4.9 Ma.—240,000 years ago.
The Caloosahatchee Subsea was named for the
Caloosahatchee Formation
. During this period,
the Tamiami Psedoatoll feature of islands and reefs became wider and the marine
environment was shallower and more of a connected lagoon system. The west and
south of the Okeechobean was covered with mangrove.
During the late Calabrian, ~1.8 Ma.—780,000 years ago, temperatures plunged
to that or possibly colder than the
Messinian
–Zanclean
with the Caloosahatchee basin rising and filling with rock and mineral deposits.
All of the paleoseas experienced mass extinction of endemic species.
Fossils (from Latin
fossus, literally “having been dug
up”) are the preserved remains or
traces
of animals (also known as zoolites),
plants, and other organisms from the remote past. The totality of fossils, both
discovered and undiscovered, and their placement in fossiliferous
(fossil-containing)
rock
formations and
sedimentary
layers (strata)
is known as the fossil record. The study of fossils across
geological time
, how they were formed, and the
evolutionary
relationships between
taxa
(phylogeny)
are some of the most important functions of the science of
paleontology
. Such a preserved specimen is
called a “fossil” if it is older than some minimum age, most often the arbitrary
date of 10,000 years ago.
Hence, fossils range in age from the youngest at the
start of the Holocene
Epoch to the oldest from the
Archean
Eon several
billion
years old. The observations that
certain fossils were associated with certain rock
strata
led early geologists to recognize a
geological timescale in the 19th century. The development of
radiometric dating
techniques in the early 20th
century allowed geologists to determine the numerical or “absolute” age
of the various strata and thereby the included fossils.
Like extant
organisms, fossils vary in size from
microscopic
, such as single bacterial cells
only one
micrometerr
in diameter, to gigantic, such as
dinosaurs
and trees many meters long and
weighing many tons. A fossil normally preserves only a portion of the deceased
organism, usually that portion that was partially
mineralized
during life, such as the
bones and teeth of
vertebrates
, or the
chitinous
or
calcareous
exoskeletons
of
invertebrates
. Preservation of soft tissues is
rare in the fossil record. Fossils may also consist of the marks left behind by
the organism while it was alive, such as the footprint or
feces
(coprolites)
of a reptile
. These types of fossil are called
trace fossils
(or ichnofossils), as
opposed to body fossils. Finally,
past life
leaves some markers that cannot be
seen but can be detected in the form of
biochemical
signals; these are known as
chemofossils or biomarkers.
Places of
exceptional fossilization
Fossil sites with exceptional preservation—sometimes including preserved soft
tissues—are known as
Lagerstätten
. These formations may have
resulted from carcass burial in an
anoxic
environment with minimal bacteria, thus
delaying decomposition. Lagerstätten span
geological
time from the
Cambrian
period to the
present
. Worldwide, some of the best examples
of near-perfect fossilization are the
Cambrian
Maotianshan shales
and
Burgess Shale
, the
Devonian
Hunsrück Slates
, the
Jurassic
Solnhofen limestone
, and the
Carboniferous
Mazon Creek
localities.
Earliest
fossiliferous sites
Lower
Proterozoicc
Stromatolites
from
Bolivia
, South America
Earth’s oldest fossils are the
stromatolites
consisting of rock built from
layer upon layer of
sediment
blue-green bacteriaa
), the growth of fossil
stromatolitic structures was biogenetically mediated by mats of
microorganisms
through their entrapment of
sediments. However,
abiotic
mechanisms for stromatolitic growth are
also known, leading to a decades-long and sometimes-contentious scientific
debate regarding biogenesis of certain formations, especially those from the
lower to middle Archean
eon.
It is most widely accepted that stromatolites from the late Archean and
through the middle
Proterozoic
eon were mostly formed by massive
colonies
of
cyanobacteria
(formerly known as blue-green
“algae”), and that the
oxygen
byproduct of their
photosynthetic
metabolism
first resulted in earth’s massive
banded iron formations
and subsequently
oxygenated earth’s atmosphere.
Even though it is extremely rare, microstructures resembling
cells
are sometimes found within stromatolites;
but these are also the source of scientific contention. The
Gunflint Chert
contains abundant
microfossils
widely accepted as a diverse
consortium of 2.0
Ga
Microorganisms
In contrast, putative fossil cyanobacteria cells from the 3.4 Ga
Warrawoona Group
early lifee
diversified, pushing important
evolutionary
milestones further back in time.
The continued study of these oldest fossils is paramount to calibrate
complementary
molecular
phylogenetics
models.
Developments in interpretation of the fossil record
See also:
History of paleontology
Silurian
Orthoceras
Fossil
Ever since recorded
history
began, and probably before, people have
noticed and gathered fossils, including pieces of
rock
and
minerals
that have replaced the remains of
biologic organisms, or preserved their external form. Fossils themselves, and
the totality of their occurrence within the sequence of Earth’s rock
strata
, is referred to as the fossil record.
The fossil record was one of the early sources of data relevant to the study
of evolution
and continues to be relevant to the
history of life on Earth
.
Paleontologists
examine the fossil record in
order to understand the process of evolution and the way particular
species
have evolved.
Explanations
Fossil shrimp
(Cretaceous)
A fossil
gastropod
from the
Pliocene
of
Cyprus
. A
serpulid worm
is attached.
Various explanations have been put forth throughout history to explain what
fossils are and how they came to be where they were found. Many of these
explanations relied on folktales or mythologies. In China the fossil bones of
ancient mammals including
Homo erectus
were often mistaken for “dragon
bones” and used as medicine and aphrodisiacs. In the West the presence of
fossilized sea creatures high up on mountainsides was seen as proof of the
biblical deluge
.
Greek
scholar
Aristotle
realized that fossil seashells from
rocks were similar to those found on the beach, indicating the fossils were once
living animals.
Leonardo da Vinci
concurred with Aristotle’s
view that fossils were the remains of ancient life.[6]
In 1027, the
Persian geologist
, Avicenna explained how the
stoniness
of fossils was caused in
The Book of Healing
. However, he rejected
the explanation of fossils as organic remains.[7]
Aristotle
previously explained it in terms of
vaporous
exhalations
, which Avicenna modified into the
theory of
petrifying
fluids
(succus lapidificatus), which was
elaborated on by
Albert of Saxony
in the 14th century and
accepted in some form by most
naturalists
by the 16th century.[8]
Avicenna gave the following explanation for the origin of fossils from the
petrifaction
of plants and animals:
If what is said concerning the petrifaction of animals and plants is
true, the cause of this (phenomenon) is a powerful mineralizing and
petrifying virtue which arises in certain stony spots, or emanates
suddenly from the earth during earthquake and subsidences, and petrifies
whatever comes into contact with it. As a matter of fact, the
petrifaction of the bodies of plants and animals is not more
extraordinary than the transformation of waters.[9]
More scientific views of fossils emerged during the
Renaissance
. For example,
Leonardo Da Vinci
noticed discrepancies with
the use of the biblical flood narrative as an explanation for fossil origins:
“If the Deluge had carried the shells for distances of three and four
hundred miles from the sea it would have carried them mixed with various
other natural objects all heaped up together; but even at such distances
from the sea we see the oysters all together and also the shellfish and
the cuttlefish and all the other shells which congregate together, found
all together dead; and the solitary shells are found apart from one
another as we see them every day on the sea-shores.
And we find oysters together in very large families, among which some
may be seen with their shells still joined together, indicating that
they were left there by the sea and that they were still living when the
strait of Gibraltar was cut through. In the mountains of Parma and
Piacenza multitudes of shells and corals with holes may be seen still
sticking to the rocks….”[10]
William Smith (1769–1839)
, an English canal
engineer, observed that rocks of different ages (based on the
law of superposition
) preserved different
assemblages of fossils, and that these assemblages succeeded one another in a
regular and determinable order. He observed that rocks from distant locations
could be correlated based on the fossils they contained. He termed this the
principle of faunal succession.
Smith, who preceded
Charles Darwin
, was unaware of biological
evolution and did not know why faunal succession occurred. Biological evolution
explains why faunal succession exists: as different organisms evolve, change and
go extinct, they leave behind fossils. Faunal succession was one of the chief
pieces of evidence cited by Darwin that biological evolution had occurred.
Georges Cuvier
came to believe that most if not
all the animal fossils he examined were remains of species that were now
extinct. This led Cuvier to become an active proponent of the geological school
of thought called
catastrophism
. Near the end of his 1796 paper
on living and fossil elephants he said:
- All of these facts, consistent among themselves, and not opposed by
any report, seem to me to prove the existence of a world previous to ours,
destroyed by some kind of catastrophe.[11]
Biological
explanations
Early
naturalists
well understood the similarities
and differences of living species leading
Linnaeus
to develop a hierarchical
classification system still in use today. It was Darwin and his contemporaries
who first linked the hierarchical structure of the great tree of life in living
organisms with the then very sparse fossil record. Darwin eloquently described a
process of descent with modification, or evolution, whereby organisms either
adapt to natural and changing environmental pressures, or they perish.
Petrified cone of
Araucaria
sp. from
Patagonia
,
Argentina
dating from the
Jurassic Period
(approx. 210
Ma
)
When Charles Darwin wrote
On the Origin of Species by Means of Natural Selection, or
the Preservation of Favoured Races in the Struggle for Life
, the
oldest animal fossils were those from the
Cambrian
Period, now known to be about 540
million years old. The absence of older fossils worried Darwin about the
implications for the validity of his theories, but he expressed hope that such
fossils would be found, noting that: “only a small portion of the world is known
with accuracy.” Darwin also pondered the sudden appearance of many groups (i.e.
phyla
) in the oldest known Cambrian
fossiliferous strata.[12]
Further discoveries
Since Darwin’s time, the fossil record has been pushed back to between 2.3
and 3.5 billion years before the present.[13]
Most of these Precambrian fossils are microscopic bacteria or
microfossils
. However, macroscopic fossils are
now known from the late
Proterozoic
. The
Ediacara biota
(also called Vendian biota)
dating from 575 million years ago collectively constitutes a richly diverse
assembly of early multicellular
eukaryotes
.
The fossil record and faunal succession form the basis of the science of
biostratigraphy
or determining the age of rocks
based on the fossils they contain. For the first 150 years of
geology
, biostratigraphy and superposition were
the only means for determining the
relative age
of rocks. The
geologic time scale
was developed based on the
relative ages of rock strata as determined by the early paleontologists and
stratigraphers
.
Since the early years of the twentieth century,
absolute dating
methods, such as
radiometric dating
(including
potassium/argon
,
argon/argon
,
uranium series
, and, for very recent fossils,
radiocarbon dating
) have been used to verify
the relative ages obtained by fossils and to provide absolute ages for many
fossils. Radiometric dating has shown that the earliest known
stromatolites
are over 3.4 billion years old.
Various dating methods have been used and are used today depending on local
geology and context, and while there is some variance in the results from these
dating methods
, nearly all of them provide
evidence for a
very old Earth
, approximately 4.6 billion
years.
Modern view
“The fossil record is life’s evolutionary epic that unfolded over four
billion years as environmental conditions and genetic potential interacted in
accordance with natural selection.”[14]
The earth’s climate, tectonics, atmosphere, oceans, and periodic disasters
invoked the primary selective pressures on all organisms, which they either
adapted to, or they perished with or without leaving descendants. Modern
paleontology has joined with evolutionary biology to share the interdisciplinary
task of unfolding the tree of life, which inevitably leads backwards in time to
the microscopic life of the Precambrian when cell structure and functions
evolved. Earth’s deep time in the Proterozoic and deeper still in the Archean is
only “recounted by microscopic fossils and subtle chemical signals.”[15]
Molecular biologists, using phylogenetics, can compare protein amino acid or
nucleotide sequence homology (i.e., similarity) to infer taxonomy and
evolutionary distances among organisms, but with limited statistical confidence.
The study of fossils, on the other hand, can more specifically pinpoint when and
in what organism branching occurred in the tree of life. Modern phylogenetics
and paleontology work together in the clarification of science’s still dim view
of the appearance of life and its evolution during deep time on earth.[16]
Phacopid
trilobite
Eldredgeops rana
crassituberculata, the genus is named after
Niles Eldredge
Crinoid
columnals (Isocrinus
nicoleti) from the Middle
Jurassic
Carmel Formation
at Mount Carmel
Junction, Utah
; scale in mm
Niles Eldredge’s
study of the Phacops
trilobite genus supported the hypothesis that modifications to the arrangement
of the trilobite’s eye lenses proceeded by fits and starts over millions of
years during the
Devonian
.[17]
Eldredge’s interpretation of the Phacops fossil record was that the
aftermaths of the lens changes, but not the rapidly occurring evolutionary
process, were fossilized. This and other data led
Stephen Jay Gould
and
Niles Eldredge
to publish the seminal paper on
punctuated equilibrium
in 1971.
Example of
modern development
An example of modern paleontological progress is the application of
synchrotron
X-ray
tomographic
techniques to early Cambrian
bilaterian embryonic
microfossils that has recently
yielded new insights of
metazoan
evolution at its earliest stages. The
tomography technique provides previously unattainable three-dimensional
resolution at the limits of fossilization. Fossils of two enigmatic bilaterians,
the worm-like
Markuelia
and a putative, primitive
protostome
,
Pseudooides
, provide a peek at
germ layer
embryonic development. These
543-million-year-old embryos support the emergence of some aspects of
arthropod
development earlier than previously
thought in the late
Proterozoic
. The preserved embryos from
China
and
Siberia
underwent rapid
diagenetic
phosphatization resulting in
exquisite preservation, including cell structures. This research is a notable
example of how knowledge encoded by the fossil record continues to contribute
otherwise unattainable information on the emergence and development of life on
Earth. For example, the research suggests Markuelia has closest affinity
to priapulid worms, and is adjacent to the evolutionary branching of
Priapulida
,
Nematoda
and
Arthropoda
.[18]
Rarity of fossils
Megalodon
and
Carcharodontosaurus
teeth. The
latter was found in the
Sahara Desert
.
Eocene fossil flower, collected August 2010 from Clare family fossil
quarry, Florissant, Colorado
Fossilization is an exceptionally rare occurrence, because most components of
formerly-living things tend to decompose relatively quickly following death. In
order for an organism to be fossilized, the remains normally need to be covered
by sediment
as soon as possible. However there are
exceptions to this, such as if an organism becomes frozen,
desiccated
, or comes to rest in an
anoxic
(oxygen-free)
environment. There are several different types of fossils and fossilization
processes.
Due to the combined effect of
taphonomic processes
and simple mathematical
chance, fossilization tends to favor organisms with hard body parts, those that
were widespread, and those that existed for a long time before going extinct. On
the other hand, it is very unusual to find fossils of small, soft bodied,
geographically restricted and geologically ephemeral organisms, because of their
relative rarity and low likelihood of preservation.
Larger specimens (macrofossils)
are more often observed, dug up and displayed, although microscopic remains (microfossils)
are actually far more common in the fossil record.
Some casual observers have been perplexed by the rarity of
transitional species
within the fossil record.
The conventional explanation for this rarity was given by
Darwin
, who stated that “the extreme
imperfection of the geological record,” combined with the short duration and
narrow geographical range of transitional species, made it unlikely that many
such fossils would be found. Simply put, the conditions under which
fossilization takes place are quite rare; and it is highly unlikely that any
given organism will leave behind a fossil. Eldredge and Gould developed their
theory of
punctuated equilibrium
in part to explain the
pattern of stasis and sudden appearance in the fossil record. Furthermore, in
the strictest sense, nearly all fossils are “transitional,” due to the
improbability that any given fossil represents the absolute termination of an
evolutionary path.
Types of preservation
Permineralization
A permineralized
trilobite
,
Asaphus kowalewskii
Permineralization
occurs after burial, as the
empty spaces within an organism (spaces filled with liquid or gas during life)
become filled with mineral-rich groundwater and the minerals precipitate from
the groundwater, thus occupying the empty spaces. This process can occur in very
small spaces, such as within the
cell wall
of a
plant cell
. Small scale permineralization can
produce very detailed fossils. For permineralization to occur, the organism must
become covered by sediment soon after death or soon after the initial decaying
process. The degree to which the remains are decayed when covered determines the
later details of the fossil. Some fossils consist only of skeletal remains or
teeth; other fossils contain traces of
skin, feathers
or even soft tissues. This is a form
of diagenesis
.
External mold of a
bivalve
from the
Logan Formation
, Lower
Carboniferous
, Ohio
Recrystallized
scleractinian
coral (aragonite to
calcite) from the
Jurassic
of southern Israel
Casts and molds
In some cases the original remains of the organism have been completely
dissolved or otherwise destroyed. When all that is left is an organism-shaped
hole in the rock, it is called an external mold. If this hole is later
filled with other minerals, it is a cast. An
endocast
or internal mold is formed when
sediments or minerals fill the internal cavity of an organism, such as the
inside of a
bivalve
or
snail
or the hollow of a
skull
.
Authigenic
mineralisation
This is a special form of cast and mold formation. If the chemistry is right,
the organism (or fragment of organism) can act as a nucleus for the
precipitation of minerals such as siderite, resulting in a nodule forming around
it. If this happens rapidly before significant decay to the organic tissue, very
fine three-dimensional morphological detail can be preserved. Nodules from the
Carboniferous
Mazon Creek fossil beds
of Illinois, USA, are
among the best documented examples of authigenic mineralisation.
Replacement
and recrystallization
Replacement occurs when the shell, bone or other tissue is replaced
with another mineral. In some cases mineral replacement of the original shell
occurs so gradually and at such fine scales that microstructural features are
preserved despite the total loss of original material. A shell is said to be
recrystallized when the original skeletal compounds are still present but in
a different crystal form, as from
aragonite
to
calcite
.
Adpression (compression-impression) fossils
Compression fossils
, such as those of fossil
ferns, are the result of chemical reduction of the complex organic molecules
composing the organism’s tissues. In this case the fossil consists of original
material, albeit in a geochemically altered state. This chemical change is an
expression of
diagenesis
. Often what remains is a
carbonaceous film
known as a phytoleim, in
which case the fossil is known as a compression. Often, however, the phytoleim
is lost and all that remains is an impression of the organism in the rock—an
impression fossil. In many cases, however, compressions and impressions occur
together. For instance, when the rock is broken open, the phytoleim will often
be attached to one part (compression), whereas the counterpart will just be an
impression. For this reason, it has proved to convenient to have a combined term
for both modes of preservation: adpression.[19]
Bioimmuration
The star-shaped holes (Catellocaula vallata) in this Upper
Ordovician bryozoan represent a soft-bodied organism preserved by
bioimmuration in the bryozoan skeleton.[20]
Bioimmuration is a type of preservation in which a skeletal organism
overgrows or otherwise subsumes another organism, preserving the latter, or an
impression of it, within the skeleton.[21]
Usually it is a
sessile
skeletal organism, such as a bryozoan
or an oyster, which grows along a
substrate
, covering other sessile
encrusters
. Sometimes the bioimmured organism
is soft-bodied and is then preserved in negative relief as a kind of external
mold. There are also cases where an organism settles on top of a living skeletal
organism and grows upwards, preserving the settler in its skeleton.
Bioimmuration is known in the fossil record from the Ordovician[22]
to the Recent.[21]
To sum up, fossilization processes proceed differently for different kinds of
tissues and under different kinds of conditions.
Trace fossils
Main article:
Trace fossil
Trace fossils
are the remains of trackways,
burrows, bioerosion
,
eggs
and eggshells, nests, droppings and other
types of impressions. Fossilized droppings, called
coprolites
, can give insight into the feeding
behavior of animals and can therefore be of great importance.
Microfossils
Main article:
Micropaleontology
‘Microfossil’ is a descriptive term applied to fossilized plants and animals
whose size is just at or below the level at which the fossil can be analyzed by
the naked eye. A commonly applied cutoff point between “micro” and
“macro” fossils
is 1 mm, although this is only
an approximate guide. Microfossils may either be complete (or near-complete)
organisms in themselves (such as the marine plankters
foraminifera
and
coccolithophores
) or component parts (such as
small teeth or
spores
) of larger animals or plants.
Microfossils are of critical importance as a reservoir of
paleoclimate
information, and are also commonly
used by
biostratigraphers
to assist in the correlation
of rock units.
Resin fossils
Leptofoenus pittfieldae
trapped
in
Dominican amber
, from 20 to 16
million years ago
Fossil resin (colloquially called
amber
) is a natural
polymer
found in many types of strata
throughout the world, even the
Arctic
. The oldest fossil resin dates to the
Triassic
, though most dates to the
Tertiary
. The excretion of the resin by certain
plants is thought to be an evolutionary
adaptation
for protection from insects and to
seal wounds caused by damage elements. Fossil resin often contains other fossils
called inclusions that were captured by the sticky resin. These include
bacteria, fungi, other plants, and animals. Animal inclusions are usually small
invertebrates
, predominantly
arthropods
such as insects and spiders, and
only extremely rarely a
vertebrate
such as a small lizard. Preservation
of inclusions can be exquisite, including small fragments of
DNA.
Pseudofossils
Manganese dendrites on a limestone bedding plane from
Solnhofen
, Germany; scale in mm
Main article:
Pseudofossils
Pseudofossils
are visual patterns in rocks that
are produced by naturally occurring geologic processes rather than biologic
processes. They can easily be mistaken for real fossils. Some pseudofossils,
such as
dendrites
, are formed by naturally occurring
fissures in the rock that get filled up by percolating minerals. Other types of
pseudofossils are kidney ore (round shapes in iron ore) and
moss agates
, which look like moss or plant
leaves. Concretions
, spherical or ovoid-shaped nodules
found in some sedimentary strata, were once thought to be
dinosaur
eggs, and are often mistaken for
fossils as well.
Ginkgo biloba
Eocene fossil,
MacAbee,
B.C.
, Canada
Living fossils
Main article:
Living fossil
Living fossil is an informal term used for any
living
species
that is apparently identical or closely
resembles a species previously known only from fossils—that is, it is as if the
ancient fossil had “come to life.”
This can be (a) a species or
taxon
known only from fossils until living
representatives were discovered, such as the lobe-finned
coelacanth
, primitive
monoplacophoran
mollusk, and the
Chinese maidenhair
tree, or (b) a single living
species with no close relatives, such as the
New Caledonian
Kagu, or the
Sunbittern
, or (c) a small group of
closely-related species with no other close relatives, such as the
oxygen-producing, primordial
stromatolite
, inarticulate
lampshell
Lingula
, many-chambered pearly
Nautilus
, rootless
whisk fern
, armored
horseshoe crab
, and dinosaur-like
tuatara
that are the sole survivors of a once
large and widespread groups in the fossil record.
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