Note: Bold black and red were discussed in class and should be studied. The orange is some material covered in fall 2001 (when we covered these topics more extensively) that is not part of the exam, but does complement the subject.
|
Information to know |
The basis for the information |
Explanatory comments |
|
The oldest known rocks are about 3. 9 Ga |
radiometric dating |
-- rocks much older than this
were probably re-melted -- these rocks are metamorphosed sedimentary rocks |
|
The oldest mineral grains
within rocks are about 4.1 Ga |
radiometric dating of zircon
crystals |
-- the mineral grains within a
sedimentary rock are older than the rock itself |
|
The oldest evidence for life is in the 3.9 Ga rocks |
radiometric dating |
-- carbon isotopes with
graphite in the rocks is poor in 13C[1] |
|
The oldest fossils known are about 3.5 Ga, of bacteria |
-- radiometric dating of
associated igneous rocks -- we know these are biological because of the form of
the stromatolites and from individual bacterial cells found in chert |
-- these fossils are stromatolites |
|
Life can be divided into
bacteria (prokaryotes) and everything else (eukaryotes) |
-- the cells of prokaryotes
and eukaryotes differ considerably in their structure |
-- (among other features)
eukaryotes (1) have “organelles” such as mitochondria, chloroplasts,
ribosomes, etc., (2) have their genetic material enclosed in a cell nucleus,
and (3) are typically much larger |
|
The earliest known protists appear in the Proterozoic
Eon, about 1.5 Ga |
-- relatively large cells
preserved in chert |
--
eukaryotes can be difficult to distinguish since organelles and nuclei do not
preserve; size is the best available criterion -- later eukaryotes may have
tough cyst walls, which are unique to eukaryotes -- the record of eukaryotes
has been pushed back to over 2 Ga just in the past decade |
|
Bacteria were common in shallow marine environments and
increased in diversity through the Precambrian |
Stromatolites and records of individual bacteria
increase through most of the Precambrian |
Stromatolites
have been recorded for many years in Precambrian rocks throughout the world; records of bacteria were discovered in cherts[2] in the 1950s and have since been widely studied |
|
Oxygen increased in the
atmosphere to about 1/100 present levels by about 2.2 Ga |
Banded iron formations from
this interval Disappearance of minerals such
as uraninite |
Banded
iron records oxidation of Fe in marine water, forming Fe3O4, which
precipitates from solution Minerals
such as uraninite do not form in environments with too much oxygen |
|
Oxygen is likely to have arisen from photosynthesis of
cyanobacteria (blue-green algae) |
The early atmosphere is not believed to have contained
much oxygen The only apparent source for significant amounts of
oxygen at this time is increasing numbers of photosynthetic bacteria |
Cyanobateria make
stromatolites, which increase in abundance through the Precambrian |
|
By the late Proterozoic, about
1.2 – 0.6 Ga, cyst-walled eukaryotes had become more abundant and diverse |
Organic-cysts increase in
abundance in sedimentary rocks of this age |
Some
kinds of organic cysts preserve readily in rocks; sedimentary rocks of this
age, if broken down into their component grains, often have such cysts; they
are also preserved in cherts |
Bacteria dominated the first 3 billion years of life in
Earth history. Bacteria continue to be a major force in ecology in every
environment. From such a perspective, the Earth is the planet of bacteria, and
other life forms are relatively strange late-comers.
The Precambrian is known in less
detail than any other interval in Earth history, because so much of the record
from that time has had a chance to be eroded away or metamorphosed.
The origin of life is not known to be approachable from
the fossil record. It is studied by chemists, who have attempted to make in a
laboratory increasingly complex, self-replicating molecules using conditions
thought to have existed early in Earth’s history.
Hot topic: Much of what is known about the Precambrian has been learned since the 1960s. This is because new techniques of looking for fossils have led to the discovery that more exist in Precambrian rocks than was previously recognized, and new techniques in geochemistry give us new insights into ancient ocean chemistry.
How old is it? This was
discussed in the first half of the semester, but here is another discussion.
There can’t be a
“History of Life” class if we can’t figure out how to order events in history.
Stratigraphy is the
science of figuring out the “relative” ages of rocks – which are older and
which are younger (as opposed to figuring out a numerical age).
Stratigraphy was
developed in Europe for the practical purposes of looking for mineral and
energy resources that were known to occur in certain layers and their
experience that the ordering of rock layers was the same in different places
locally. The first and most obvious technique was simply to characterize the
nature of the rocks – the composition, texture, and so on. It was eventually
found that over larger distances such layers can change from place to place, or
even disappear, and something more general was needed to figure out one’s
position in a vertical rock column. Early stratigraphers learned from practical
experience that the vertical sequence of types of fossils in the rocks was the
same everywhere they looked. This
ordering was recognized long before it was understood how old the rocks were
and before discussions of the evolutionary significance of organisms changing
through time.
Based on fossils, early
geologists worked out a “geological time scale” and, for convenience, gave the
different sequences of rocks names by which to refer to them. At this time,
geological ages in years were completely unknown and of no practical significance.
Interest grew, however, in mapping in all industrial countries and gradually
other parts of the world the relative ages of rocks found at the surface.
Eventually it was found that stratigraphy would be of tremendous use and
economic importance for petroleum companies coring through rock at any given
point, until they reached layers that were likely to contain oil or gas.
Fossils were found consistently to be the most practical value for determining
relative ages, creating the field of “biostratigraphy,” and it was quickly
found that some groups of fossils were more useful that others.
Characteristics of
fossils useful for biostratigraphy:
--
Abundance and likelihood of fossilization is critical. One must have
confidence that if the animal lived in the area that you will find it.
Organisms are likely to fossilize if they have a hard mineralized skeleton, as
do clams, snails, etc.
-- Obviously, to correlate
rocks in two different areas, the organisms must be sufficiently widespread to
live in both areas. Further, the
organisms should be tied too tightly to any particularly environment, or else
the absence of a species is more likely to represent local environment change
than global extinction of the species. Organisms that fit these characteristics
tend to be planktonic, floating across large expanses of ocean. However, some
bottom-dwelling organisms can be widespread too, especially if they have larvae
(babies) that live in the plankton for a little while before taking up
residence on the bottom.
-- Small size is useful.
Firstly, small organisms tend to be potentially more abundance. But in
addition, fossils that can be found in abundance in a sediment sample as small
as a drill core are extremely useful – such fossils are microscopic, collectively
called “microfossils.”
-- Finally, individual taxa
within the organisms should be reasonably easy to identify and geologically
short-lived, to provide precision in relative dating.
It is less easy to correlate
rocks – figure out the relative ages of rocks in different places – for rocks
of Precambrian age, because the fossils that exist do enable us to identify
specific taxa that existed for a short time interval. For example, the
cyanobateria present in Precambrian rocks look much like the cyanobacteria
present today.
Records of the diversification of animal life
|
Information to know |
The basis for the information |
Explanatory comments |
|
Late in the Precambrian the
Earth experienced significant glaciation – so much so that some believe the
Earth was covered in ice even in the tropics. |
Glaciers leave very
distinctive kinds of sedimentary deposits and other characteristics. These
have been found in many places in rocks of this age. Chemistry of the
sediments also suggest something strange happened at this point in time. |
The “snowball Earth”
hypothesis is just a few years old. Some have suggested that, since the event
came just before the diversification of animals, that maybe the event somehow
stimulated this evolution, but no convincing arguments have made for how
these events would be linked. |
|
The oldest known body fossils of something apparently
multicellular occurs shortly before the Cambrian Period, about 0.6 Ga[3] |
A couple dozen shapes of mystery “animals” appear nearly worldwide; none are skeletonized, yet they seem to preserve well in sandy environments; they are known as the “Ediacaran” or “Vendian fauna” |
Some people have tried to link all of these forms with
modern taxonomic groups; others have suggested that they are all some form of
organism that thrived briefly, but died out soon after; some think some
combination |
|
The oldest known trace[4]
fossils are about 0.6 Ga, but some have claimed to have found trace fossils
as far back as 1 Ga or more |
Trace fossils are found in
many rocks in the world. |
Trace fossils bigger than
microscopic are thought to have been formed by a multicellular organism. |
|
A few types of shell fossils
of unknown affinity occur before the Cambrian boundary, then a number of mysterious “small shelly fossils” occur in
the early Cambrian. |
These occur through the world
in sediments of this age. |
It is now believed that some
or all of the “small shelly fossils” are sclerites – each is just one piece
of a shelly armor that covered some kind of early Cambrian animals. |
|
Animal life after the Cambrian[5]
boundary is much more diverse, and/or the animals are better skeletonized.
This is known as the “Cambrian explosion.” |
The fossil record becomes much more extensive after the
Cambrian boundary. |
There has been some debate about whether the key factor
at this boundary was the origin of most animal phyla, or the emergence of skeletons
among many of the phyla. |
|
Most animal phyla[6]
with a good fossil record seem to show up in the early-mid Cambrian Period,
or slightly before the Cambrian. |
The fossil records of most skeletonized phyla and a few
soft-bodied forms from exceptionally preserved fossils. |
- Very small soft-bodied phyla have little or no fossil
record. - This interval of time is known as the Cambrian
radiation or Cambrian revolution. It is considered by many to be the most
important event in the history of animal life. |
When you look at a book on the history of life, it will
usually gloss over the first 3 billion years of life in a couple paragraphs,
then begin in more detail in the Cambrian.
Many feel that it is interesting
that the Cambrian shows high disparity before showing high diversity. That
means that there are many different “body plans,” as you can see from the
number of phyla that show up in the Cambrian, but not necessarily lots of species
within each phylum (the opposite pattern would be, say, lots and lots of
species of beetles that all look fairly similar to one another – that would be
essentially no disparity and high diversity). One might have imagined that the
phyla would show up gradually over the course of the Phanerozoic eon, as the
number of species increased and colonized various habitats, but it turns out
that life did not evolve in this way.
Furthermore, (1), few or no new phyla show up after the
Cambrian and (2), in the Cambrian even within each phylum, we see a large
number of shapes, some of which became extinct shortly afterward. Paleontologists have wondered why new body
plans arose so quickly and “easily” in the Cambrian, but never again since.
This is a good example of a pattern in the evolution of life that would not be
able to understand without the fossil record.
The Precambrian-Cambrian
transition is considered one of the most important times in this history of
life, and is studied intensively. Within the past 10-20 years our knowledge of
the fossil record of this interval has grown enormously, as well as other clues
such as the chemistry of the ocean at this time. For many this research is
interesting not only because of its significance for understanding the history
of life, but because it is filled with interpretation of weird and wonderful
creatures that became extinct shortly after their origin.
Fossils are usually the
“hard parts” – the skeleton or shell – of past organisms, since the “soft
parts” – the organic tissues – almost always rot away long before they become
permanently buried in sediments. Therefore, much of what we think we know about
the history of life comes from the fossil record of organisms with hard parts.
We know little about the history of jelly fish, for example, even though they
have probably been common since early Cambrian or before.
Sometimes, however,
unusual circumstances lead to the preservation of soft parts. We saw this in
the Precambrian rocks, in which microscopic cells were preserved in chert. The
main way to preserve soft parts of animals is to create chemical conditions
that prevent the growth of bacteria or other organisms that would decay soft parts.
The main way to do this is to create a place with little water overturn, so
that oxygen is used up and not replaced.
These exceptionally preserved fossils
are called Lagerstaetten.
A famous locality in
Canada known as the Burgess Shale has preserved many soft-bodied organisms from
the Middle Cambrian. This fauna is incredibly important because it gives us a
window in on a critical interval in Earth history, when animals were first
evolving into the many groups that we find today. In
the early 1990s paleontologists started publishing on a newly described
exceptionally preserved fauna, from Chengjiang, China. This fauna is slightly
older than the Burgess Shale fauna, and has many of the same kinds of animals,
plus some new ones. There are also remarkable lagerstaetten in Sweden from the
late Cambrian, a site from the late Proterozoic of China, and a new Cambrian
site in Nevada, and others.
Ordovician to Devonian Periods and the Paleozoic marine
fauna
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Information to know |
The basis for the information |
Explanatory comments |
|
Brachiopods, trilobites, sea lilies, horn (rugose) and colonial (tabulate) corals are
typical skeletonized marine invertebrates from the Ordovician to Devonian |
Central and western NY bedrock is Ordovician to
Devonian-age sedimentary rock from a shallow sea that contains millions of
these fossils; such fossils are common in many other parts of the world as
well |
Of course, not every kind of animal is found in every
kind of rock (shale, sandstone, limestone), because different animals have
different environmental preferences |
|
Ordovician to Devonian fish
are mostly “armored” |
Plates of armor from around
the heads of Ordovician to Devonian fish are sometimes common, but the rest
of their skeleton is was cartilagenous and is rarely preserved |
Cartilaginous fish are poorly
known from the fossil record. |
|
Many fish in the Orodovician
and Silurian are “jawless” |
Armored head shields had an
opening for a mouth, but no moveable jaw |
These fish presumably filtered
particles from the water (filter feeders) or digested organic matter from mud
(deposit feeders) |
|
In the Devonian, enormous
armored fish with jaws known as placoderms, are the first known great fish
predators |
Dunkleosteus[7] is the most famous example, known from head plates in
the midwest and northeast US |
Placoderms did not have
“teeth”, but instead used the sharp jagged edges of armored jawline |
|
The Devonian is know as the Age
of Fish, because of the great diversity of fish at that time |
Devonian fish are present
(though usually not common fossils) in central NY, but their records are
known from around the world |
The diversity was so great in
part because it combined the end of the days of the armored fish with the
early expansion of sharks and bony fish and heydey of the lobe-finned fish |
|
Land life is evolving, including simple plants and some
invertebrates – most arthropods |
Records from Ordovician to Devonian, in many cases microscopic remnants; a good record is
known from Gilboa, NY (Devonian) |
This fossil record has only
recently expanded as people have looked more carefully with special for tiny
parts of plants and arthropods. |
|
Early plants were very simple and small, with no leaves
and reproductive organs at their tips. |
Records around the world contain such plants. These
plants were probably near water and needed water for the reproductive part of
their life cycle. |
|
Central and Western New York were covered with a shallow
sea in the Ordovician to Devonian, and in this sea accumulated thick layers of
sediment that we can now find as sedimentary rocks. The rocks contain
information on life in this area, and the rocks have been well studied over the
past 150 years. At one time there was
discussion of naming the Devonian the New Yorkian.
The biggest part of the fossil record accessible on land
is from continental seas like the one that covered central NY and from coastal seas.
We hear a lot about dinosaurs, but far more paleontological problems (such as
under what conditions organisms are likely to form new species or go extinct)
are solved by studying abundant invertebrate fossils from marine (sea)
sediments and sedimentary rocks.
A fossil is any
evidence of past life. For a fossil to form, it usually has to be buried (there
are a few exceptions in which fossils that are not very old are mummified in
caves), or else forces at the surface will destroy it – if it is organic matter
it will rot, if it is skeletal matter is will weather, dissolve, or otherwise
be destroyed by physical processes.
The way nearly all
fossils get buried is in mud and sand. Mud and sand don’t accumulate just
anyway, as erosion ultimately carries such particles downhill until they get
trapped. The particles generally get trapped in aquatic bowl-shaped settings
geologists call “basins” – once the particles reach a water body without
flowing water to carry it further, it settles to bottom.
Such water bodies can
be lakes or large rivers, but in area the biggest water bodies are the coasts
of oceans and continental seas. During most of Earth history, sea level has
been high enough to flood parts of many continents, creating broad shallow
seas, and these accumulated large amounts of sediment. The fossils we find in
central New York are from such a sea. It just so happens that enough water is
tied up in glacial ice today that sea level is relatively low and extent of
continental seas is fairly small.
Records of the expansion of life onto land
|
Information to know |
The basis for the information |
Comments |
|
The Late Devonian experienced
a mass extinction in marine animals. |
Paleontologists compiled information from scientific references globally to figure out how many different kinds of animals there have been at each interval in Earth history. |
The Late Devonian extinction
was one of five Phanerozoic mass extinctions. It seems to have been the most
gradual of the five, taking place over several million years. The cause
remains poorly known. |
|
Land vertebrates – amphibians arise at the end of the
Devonian. These amphibians still have many fish-like characteristics. |
Fossils in Greenland and
Pennsylvania, among others. |
The primary anatomical
differences include modification of limb bone structure from that of
lobe-finned fish, modification of skull, and modification of vertebrae so
animal could support itself and move on land. |
|
“Bony fish,” which today
include 99% of all fish species, diversify in the seas and become the
dominant kind of fish. |
Marine fossil record. |
|
|
The first forests occur in the Late Devonian, dominated worldwide by a plant known as Archaeopteris[8]. |
Archaeopteris had fern-like leaves and reproduced through spores
rather than seeds. |
Forests were extremely important, creating new habitats
and also influencing rates of weathering and erosion. |
|
Giant forests thrived in swampy areas worldwide during the Carboniferous. These swamps were dominated by lycopod trees (related to “club mosses” of today), but included other plants related to horsetails, “tree-ferns,” and other plants now extinct. |
There are excellent records of these forests because of the wet environment. So much organic matter accumulated that O2 was quickly used up by bacteria, causing enormous sedimentary accumulations. These formed coals. |
Ithaca’s electricity comes from coal from Pennsylvania
– we are burning wood from the Pennsylvanian Period. |
|
Most continents come together
in “supercontinent” known as “Pangea” during the Permian, creating a more
extreme and arid environment in the interior of Pangea. |
Coal swamps mostly disappear. Rocks from this interval
are characteristic of dry climates. |
Climates are moderated by
water bodies such as lakes and oceans, which maintain a relatively stable
temperature. The bigger the landmass and further from a major water body, the
more extreme and arid the interior of the landmass will be. |
|
Reptiles arise in the early Carboniferous, and are similar in many anatomical respects from their amphibian predecessors. They presumably have amniotic (hard-shelled) eggs. |
Bones have been found in early Carboniferous rocks as
close as Pennsylvania. Reptiles are defined in part on the basis of their
eggs, but no eggs have yet been found from this interval. |
Organisms have a wide variety of characteristics, some
of which are preserved in skeletons, but some of which involve soft parts,
eggs, or other features that are only rarely found in the fossil record. |
|
Reptiles are relatively
insignificant until later in the Carboniferous, and become the dominant land
animal in the Permian. |
Based on the fossil record of bones worldwide. |
The amniotic egg and reptilian characteristics such as
scaly skin enable reptiles to live in very dry habitats. This enabled
reptiles to move into new drier habitats, and to thrive when climates turned
drier. |
|
The dominant reptiles of the
late Carboniferous and Permian are synapsid or so-called “mammal-like”
reptiles that would eventually lead to mammals. |
There are especially rich records from South Africa,
but they are known worldwide. |
Other reptiles such as the
predecessors to dinosaurs and modern groups of reptiles were present, but not
as abundant. |
|
One mammal-like reptile
well-known to the public is Dimetrodon, popularly known as the
“sail-back reptile.” Dimetrodon is not a dinosaur, nor did it live at
the same time as dinosaurs. |
|
Many decades ago popular
authors put Dimetrodon in children’s dinosaur books, and the error has
been propogated ever since. |
|
The Permo-Triassic ended with the biggest mass
extinction ever known, wiping out most species of animals on both land and
sea. |
Fossil
records worldwide have been compiled to give us information about changes in
diversity. This extinction, however, was recognized by the mid-1800s,
however, since it was so obvious. |
The extinction is the basis for the boundary between
the Permian and the Triassic Periods. |
Concepts:
Science is the process of
figuring out how the world works by testing out ideas through watching nature
to see if it actually looks or behaves like we thought it would. We
employ such common sense thinking in our daily lives, but science takes the
method to its most rigorous, logical extreme. Laboratory sciences involve
testing ideas by setting up an experiment and watching what happens. Historical sciences (sciences that deal with natural
events from the past) like paleontology cannot literally replicate history, so
we have to look for telltale evidence left behind as our observations. The best
way of doing this with some confidence is to look for consistency among a wide
variety of independent of evidence – confluence of
evidence. This gathering of evidence to make a case for a particular
viewpoint is why doing paleontology is so often compared to gathering detective
evidence for a criminal case.
Based on this, it is easy to see that questions that fall
within the realm of science are “testable” – that means that you must
potentially be able to make some kind of observations to show an idea is wrong.
For example, the color of most ancient organisms seems to be permanently lost,
so cannot be studied scientifically, even though the issue is interesting.
Ideas that are not testable are not necessarily false, but they simply cannot
be investigated by science. Some ideas that cannot currently be studied in a
scientific way may be candidates for science at some later time, if we find new
forms of information that relate to the truth or falseness of an idea.
It is generally assumed that a scientific hypothesis is more likely to be true if it explains all the available observations. A hypothesis that explains the extinction of the dinosaurs through a disease fails to explain why so many other types of organisms, on land and in the sea, also disappeared at the same time. Simple intuition suggests that an ecological crisis that could affect many kinds of organisms all at once is more likely to be true. Famous examples of hypotheses that elegantly explained dozens of kinds of information were biological evolution and plate tectonics.
Emergence onto land was a long
process. The fossil record of this emergence is rather poor because fossils
tend to form where sediments accumulate, which is usually in aquatic settings. Many
organisms, such as amphibians and spore-bearing plants, were still tied to
aquatic environments during at least one stage of their lives. The key
innovations that allowed plants and animals to move onto land without requiring
wet habitats involved preservation of water throughout life, including
reproduction and growth of the empbryo.
Animals of the Devonian-Carboniferous
are a bit like animals of the Cambrian, in that early on significant disparity
developed, even before diversity itself was not extremely high. In this case,
moving into a new habitat, disparity increases before diversity.
Most of the animals that have ever lived are now extinct. Most of these lived a few million years and then went extinct, as part of what is known as “background” extinct in order to distinguish it from “mass extinction.” Mass extinction is an unusually high rate of extinction over a relatively short time and that may drive extinct a wide variety of organisms. There are five that really stand out: near the end of the Ordovician, near the end of the Devonian, at the Permian-Triassic boundary, near the end of the Triassic, and at the Cretaceous-Tertiary boundary.
The Cretaceous-Tertiary
boundary is most famous because it involved the extinction of the dinosaurs and
of other large aquatic and flying reptiles. It also involved, however, many
other organisms on land and in the sea, including plants, invertebrate animals,
and protests.
The Permian-Triassic
extinction, however, is the largest. Over
½ the families of marine animals went
extinct, and nearly ¾ the families of land vertebrates. It has been estimated
that if over ½ the marine families went extinct, then over 90% of the species
must have gone extinct. It included all species of skeletonized corals (today’s
corals seem to have arisen from a different group of coral-like animals), all
trilobites (they were most extinct by then anyway, however), most species of
brachiopods, and many other marine organisms; it included many of the
mammal-like reptiles as well.
Mass extinctions are
extremely important because it would seem that many organisms went extinct in
unusual ecological catastrophes. Often the organisms that are ecological
significant after a mass extinction are not those that were important
beforehand. Many people are aware that it wasn’t until dinosaurs disappeared
that mammals diversified into many habitats and became the ecologically
important animals they are today. Similarly, brachiopods (for example) were a
key part of marine environments of the Paleozoic, but have been fairly
insignificant since the Permian-Triassic boundary.
The
cause of the Permian-Triassic (P-Tr) extinction has been intensively studied in
the past decade, in part because it is such an important problem to solve to
understand the history of life, but fueled by new data, especially from places
such as China that were previously behind the “Iron Curtain.”
Traditional
ideas about the P-Tr involve changing climate and sea level. It was unknown
over how long a period the extinction may have taken place, because the
geological record is rather poor over the boundary because of low sea level
(remember that low sea level means less continental seas and continental shelf
for sediments to accumulate and fossils to form). Climate change, however, did
not seem tightly connected to extinction.
It
has been suggested that limited area alone could have caused extinction of
marine faunas. Bigger areas can hold more species. As Pangea closed, some
coastline was lost where the continents sutured together. And as sea level
dropped, areas for living in continental seas decreased. Further, continental
slopes are steeper than continental shelves, so a sea level drop leaving
shelves dry meant organisms had to live on the steeper slope, further
decreasing the area they could live. These factors along may account for part
of the decline of marine families.
Further,
although land animals would have the same amount of area when Pangea formed,
the different animals on the formerly separated land masses could have
competed, losing further diversity. This loss of geographic distinction is
called a loss of “provinciality.”
Yet,
looking broadly at diversity through geological time, and across areas as big
as continents, it is not clear that sea level and habitat area are very tightly
connected to global diversity. Might
there be other reasons?
Ocean
circulation is deeply affected by continental configuration. Water bodies get
stagnant if the surface water doesn’t occasionally sink to the bottom with
fresh oxygen. If the global ocean grew stagnant, organic matter would rot to
the point that all the oxygen would be used up and carbon dioxide would
accumulate to lethal concentrations; thus the next time the ocean somehow did
overturn, a lethal injection of carbon dioxide would be burped into the
atmosphere and mixed with the shallow marine waters. This hypothesis was
invented to explain geochemical signals at this boundary.
In
about 1980 it was proposed that an extraterrestrial impact had caused the
Cretaceous-Tertiary extinction. Though sounding like a tabloid story, many
forms of evidence proved to be consistent with this idea, including the element
Iridium that first gave rise to the idea (high concentrations of iridium is
known only from meteors and the Earth’s interior), tektites (small spherules of
meteoritic material), shocked quartz, and other evidence. Just in the past few
years evidence seems to be mounting for an impact at the P-Tr boundary. This is
an active field of research and doubtless new articles will be published in the
coming months.
[1] Photosynthetic organisms “fractionate” carbon, which means they selectively use 12C over 13C; for this reason, organic carbon has less 12C than the natural environment. Graphite is a mineral made of carbon, and such carbon accumulations are typically organic.
[2] Chert is “microcrystalline” quartz and seems to have formed from supersaturation of marine water with SiO2 (silica), causing it to precipitate around living bacteria, preserving their shapes and sizes. Internal features of bacteria are not preserved. We don’t understand chert formation well, because it does not seem to be a process commonly occurring in marine environments today.
[3] Ga = Giga ans (French) = billions of years (and Ma = millions of years)
[4] Trace fossils are made by the movement of
organisms through sediment, leaving behind trails, tracks, burrows, and so on. The Precambrian trace fossils are simple traces of
worm-like movement through sediment. Many traces are interpreted through traces
made in modern environments.
[5] The Precambrian-Cambrian was originally defined to
correspond to the record of the first animals, but now we have found records of
animals before the boundary.
[6]
Phyla are the largest categories in the Linnean hierarchy for animals. This
hierarchy includes phyla, classes, orders, families, genera, and species. Evolutionary biologists are moving away from assigning
groups of organisms to a particular level in this hierarchy, but the levels are
still useful for some applications such as this one.
[7] Genus and
species names are italicized; in the case of hand-writing, they are
underlined. Technically, many of the
kinds of fossil animals you here about are genera: Tyrannosaurus, Triceratops,
Archaeopteryx. Tyrannosaurus rex is a particular species of
Tyrannosaurus, Homo sapiens is a particular species of hominids, Mammut
americanum (American mastodon) is a particular species of mastodon. Species
are “real” biological grouping, in that all individuals within a species can
mate and produce fertile offspring (dogs, for example, are one species in spite
of the variety of form); genera and other categories are groups of
evolutionarily related organisms, but opinions often vary on which species to
include in which genus, etc.
[8] Yes, the name Archaeopteris is very similar to the name of the primitive bird Archaeopteryx. “Archae” means ancient in Latin, so is a common prefix of words in paleontology. American English tends to drop to “e” -- note also the field of “archeology.”