An incomplete history of the fossil record
People have been finding fossils for a very long time, and by the 1950’s,
by using some new techniques and instruments, scientists began finding what are probably the
oldest fossils possible. They are about 3,850,000,000 years old. To discover why these are the
oldest fossils possible, please read the text which accompanies the images below.
Source:
(left) http://www.fossilmuseum.net/Tree_of_Life/Stromatolites.htm
(right) http://www.blackwellpublishing.com/ridley/image_gallery/Precambrian_jellyfish.asp
Stromatolites are the oldest fossils we have discovered. It must be remembered that because
of plate tectonics (i.e. continental drift – the subject of another post), very old fossils are often
destroyed as old bits of the earth’s crust (from the Archaean era) are more likely to have been
subducted by the movement of the earth’s plates, and to have melted in the heat, in addition,
there are doubtless countless fossils which have been eroded into nothingness by wind and
water, or otherwise lost to us, so beyond a certain point, the older they are, the rarer the
fossils which survive become. Also, very old life-forms were all simple single-celled or multicelled organisms, and these, because they have no hard parts, only fossilize under
exceptionally ideal conditions.
So, what does the fossil evidence tell us? All of the oldest fossils we have found
(between about 3,850,000,000 years old, and about 2,500,000,000 years old) are fossils of
simple single-celled organisms, rather like bacteria, called prokaryotes (these are mostly
single-celled organisms without a nucleus). From this, we can conclude that for a bit over
1,000,000,000 years, the only life-forms on the planet were these simple organisms. (Now, I
would just like to put that in perspective: from the Cambrian revolution, when life diversified
like mad to now is only half that time; from the start of the dinosaurs to now is only a quarter
of that time, and the from the very first proto-towns to now is one hundred thousandth of that
time.)
In other words, the simple single-celled forms had our earth all to themselves for an
enormously long time indeed. This has some interesting implications for areas like
exobiology, and SETI (the search for extraterrestrial intelligence). Presuming, as I do, that life
is probably fairly commonplace in the universe, and that there are loads of large lumps of
rock (planets, and perhaps moons etc) ‘out there’ with life on them, the chances are quite high
that these are going to be simple single-celled forms, with whom we will not even be having
involved conversations – let alone alien sex (tabloid revelations to the contrary
notwithstanding)!
Source: http://library.thinkquest.org/C004535/media/prokaryote.gif
A prokaryote. Below are some photographs of simple bacteria:
Source: http://www.biology4kids.com/extras/show_kingdoms/03.jpg
Around 3,500,000,000 years ago, some of these life-forms began producing a
chemical called chlorophyll (which as you know, is green in colour, and is found in most
plants today). This chemical allowed these organisms to make their own food, and this was a
very important ability, as we shall shortly see.
Source: http://kentsimmons.uwinnipeg.ca/16cm05/16lab05/lb1pg2_files/image007.jpg
Cyanobacteria structure.
Source: http://universe-review.ca/I11-30-cyanobacteria.jpg
Some examples of chlorophyll-containing bacteria, the cyanobacteria.
At some point, these simple single-celled organisms also clumped together, and
eventually became multi-cellular organisms. This however is a complicated story. First of all,
we have to define what exactly we mean by a multi-celled organism: At the most basic level,
we have the eukaryotes. Eukaryotes differ from prokaryotes (see above) in that their cells
contain membranous sacs called organelles, including mitochondria, chloroplasts, and the
nucleus. They look rather like they were assembled from a variety of different cells, and as a
result, many scientists think these organelles are descended from formerly free-living
prokaryotic organisms which banded together. Thus, many important functions of eukaryotic
cells, such as photosynthesis, and respiration (the process by which organisms use oxygen to
metabolize organic compounds to produce energy, giving off carbon dioxide) may well have
been acquired through a symbiosis of previously independent forms of life.
Source: http://www.fas.org/irp/imint/docs/rst/Sect20/celltypes.jpg
If this scenario is true, then Eukaryotes were the first multi-cellular organisms because
they were originally two (or more) organisms which somehow joined together to become one
symbiotic living entity (though we are not yet sure how exactly this may have happened).
Anyway, though we do not know enough about this, we do know that eukaryotes flourished
as the environment became richer in oxygen (which was generated by photosynthesizing
organisms). Perhaps this was in part due their more complex intracellular function.
Source: http://cosmology.net/images/AcritarchFossil9.jpg
An Acritarch fossil, one of the earliest multi-cellular organisms.
This however is only one version of multi-cellularity. Organisms also began grouping
together, first into simple agglomerations, and then into interdependent ones, and finally in
coordinated forms, and these were all different ways of being multi-cellular.
The oldest known possible multi-cellular eukaryote is Grypania spiralis (see
immediately below), a coiled, ribbon-like fossil two millimetres wide and over ten
centimetres long. It looks very much like a coiled multi-cellular alga and has been identified
in banded iron formations found in what is today Michigan which date to 2,1000,000,000
years ago.
Source: http://www.fas.org/irp/imint/docs/rst/Sect20/pmg31.jpg
Grypania may, or may not, be a eukaryote, but another, unrelated colonial eukaryote,
Horodyskia (see below), is known from sedimentary rocks which are 1,500,000,000 years old,
and which were found western North America as well as in rocks more than 1,000,000,000
years old, which were found in Western Australia.
Source: http://macroevolution.narod.ru/fedonk1.jpg
Some of earliest known eukaryote fossils are those of acritarch, shown directly above,
which date from around 2,100,000,000 years ago. In fact, acritarchs are the most common
fossils of the late Proterozoic.
The earliest known extended multi-cellular animals are to be found amongst the
Ediacaran fauna (see directly below), which are named for the Ediacaran hills of South
Australia: Some of these Ediacaran animals resemble modern jellyfish and segmented worms,
found in great numbers in the seas today. Others are unlike any known organisms and cannot
be classified with any certainty. One feature they have in common is that without exception,
they all lack the rigid, supporting skeletons and protective shells that characterize the first
fossils of the Cambrian Period (see further below).
Source:
http://arjournals.annualreviews.org/na101/home/literatum/publisher/ar/journals/production/
earth/2005/33/1/annurev.earth.33.092203.122519/images/medium/ea330421.f2.gif
Anyway, getting back to the story of the development of photosynthesis, perhaps
around 2,000,000,000 years ago, the ability to synthesize their own food from sunlight and
carbon dioxide (along with some minerals) allowed certain organisms to eventually leave the
water and to live (albeit initially only for short periods, but eventually permanently) whilst
exposed to the air (in other words, to live on the dry land), which up to this time had been
totally uninhabited; there were neither animals nor plants until this colonisation of the land by
early organisms (which initially were algae and possibly bacteria) began.
Source: http://www.uic.edu/classes/bios/bios104/mike/mycorr.gif
Fossil of an early terrestrial algae. These plants probably resembled the rather slippery algae
which one sees today growing on rocks which are periodically exposed by the tides.
Although I write that the incorporation of chlorophyll allowed some plants to leave the
water, the story, in fact, is somewhat more complex than I am letting on: A likely mechanism
by which plants moved onto dry land is that some of these early plants which grew
underwater, but near the water’s edge would have been exposed by tides or by the drying out
of the pond / pool in which they were growing, or possibly by changes in water-levels due to
some other factor. This would have forced them to eventually evolve structures which made it
possible for them to adapt to full terrestrial living – and this is exactly how natural selection
works.
Growing in water is a very different proposition from growing in air, and is much
easier, at least in physiological terms: In water, you do not have to worry about being
dehydrated (although growing in salt water can bring on its own special problems, especially
if your salinity balances are not right), and you do not need to worry about physically
supporting your own weight. Once out of the water, you need thicker, less permeable and
stronger cell-walls to address both of these problems; Also, once you are out of the water,
there are not likely to be many nutrients freely floating around, and available for you to
consume, thus the ability to synthesize your own food out of carbon dioxide, water, minerals
and sunshine, via the intermediation of chlorophyll would have been of incalculable use on
land (for more on this, see boxed section below).
The Silurian land was populated by early land plants as well as a variety of insects. Both
plants and animals had a number of challenges when they moved from the water to land.
1. Drying out. Once removed from water and exposed to air, organisms must deal with the
need to conserve water. A number of approaches have developed, such as the
development of waterproof skin (in animals), living in very moist environments
(amphibians, bryophytes), and production of a waterproof surface (the cuticle in
plants, cork layers and bark in woody trees).
2. Gas exchange. Organisms that live in water are often able to exchange carbon dioxide
and oxygen gases through their surfaces. These exchange surfaces are moist, thin
layers across which diffusion can occur. Organismal response to the challenge of
drying out tends to make these surfaces thicker, waterproof, and to retard gas
exchange. Consequently, another method of gas exchange must be modified or
developed. Many fish already had gills and swim bladders, so when some of them
began moving between ponds, the swim bladder (a gas retention structure helping
buoyancy in the fish) began to act as a gas exchange surface, ultimately evolving
into the terrestrial lung. Many arthropods had gills or other internal respiratory
surfaces that were modified to facilitate gas exchange on land. Plants are thought
to share common ancestry with algae. The plant solution to gas exchange is a new
structure, the guard cells that flank openings (stomata) in the above ground parts of
the plant. By opening these guard cells the plant is able to allow gas exchange by
diffusion through the open stomata.
3. Support. Organisms living in water are supported by the dense liquid they live in. Once
on land, the organisms had to deal with the less dense air, which could not support
their weight. Adaptations to this include animal skeletons and specialized plant
cells/tissues that support the plant.
4. Conduction. Single celled organisms only have to move materials in, out, and within
their cells. A multi-cellular creature must do this at each cell in the body, plus
move material in, out, and within the organism. Adaptations to this include the
circulatory systems of animals, and the specialized conducting tissues xylem and
phloem in plants. Some multi-cellular algae and bryophytes also have specialized
conducting cells.
5. Reproduction. Organisms in water can release their gametes into the water, where the
gametes will swim by flagella until they encounter each other and fertilization
happens. On land, such a scenario is not possible. Land animals have had to
develop specialized reproductive systems involving fertilization when they return
to water (amphibians), or internal fertilization and an amniotic egg (reptiles, birds,
and mammals). Insects developed similar mechanisms. Plants have also had to deal
with this, either by living in moist environments like the ferns and bryophytes do,
or by developing specialized delivery systems like pollen tubes to get the sperm
cells to the egg.
Plants divide into two large groups: vascular plants that contain lignified conducting cells,
and the nonvascular plants, which do not. Some Silurian plant fossils might be algae or
nonvascular plants. Vascular plants developed during the Silurian period, 400 million years
ago. The earliest vascular plants had no roots, leaves, fruits, or flowers.
From: http://www.emc.maricopa.edu/faculty/farabee/biobk/BioBookPaleo3.html
Before chlorophyll came along, organisms presumably had to depend for their food on
the amino acids which were dissolved in the water they were living in. This was a very
marginal, iffy kind of existence, and so it is unlikely that there was a very high density of
living things present on the planet until chlorophyll made its appearance by introducing the
food-synthesizing blue-green organisms onto the scene.
These new blue-green organisms, because they could make their own food wherever
there was sunlight, were much more independent than their forerunners, and so suddenly life-
forms were free to go forth and multiply far more quickly than they had been in the past (for
some images of blue-green organisms, please refer to the pictures towards the beginning of
this post). Remember, without food, it is very difficult to reproduce, and to live, so life had to
solve the problem of provisioning itself efficiently and reliably, before it could move on to
conquer the planet to the extent to which it has done so today.
Source: http://www.emc.maricopa.edu/faculty/farabee/biobk/BioBookPaleo3.html
Would it still be called a family tree even though it’s plants we’re talking about?! A very
general sketch of the ancestry of plants. If you’re not sure what the difference between
vascular and non-vascular plants is, please look at the boxed text above.
This ability to make your own food however was a double-edged sword, and was not
wholly and unequivocally a good thing. It certainly allowed one to live in new zones, but it
also stimulated the evolution of predators: Since there were now a bunch of these
concentrated food-packages (i.e. organisms which made their own food) floating around,
other organisms evolved the ability to get their food by simply eating these food producing
life-forms. These new organism-eating organisms were the first predators –which eventually
became the first animals (all animals sustain themselves by eating other organisms – be they
plants or other animals).
You may not know
this, but ladybirds are
actually aggressive
predators.
Source: http://www.eoearth.org/article/Predation
Source: http://australianmuseum.net.au/Uploads/Images/9148/herbivory_021_big.jpg
A stick insect eating a leaf.
Source: http://www.zoology.ubc.ca/courses/bio332/Labs/CiliateProject/DidEatpara.jpg
An organism in the process of consuming its fellow organism – it’s a dog-eat-dog (or would
that be blob-eat-blob?) world under the microscope too!
Anyway, from the beginnings of life on earth, until about 570,000,000 years ago,
things were pretty slow: There were a bunch of tiny blobs floating around synthesizing their
own food out of air, water, minerals and sunlight; there were some other blobs which
developed the predatory habit of swallowing their fellow blobs; and a bit later, there were also
some larger blobs composed of agglomerations of little blobs in various stages of mutual
integration, and that was about it.
Then, for some as yet unknown reason, in a matter of a few million years, a whole
bunch of different, and rather strange looking life-forms evolved. This explosion of life is
known as the Cambrian revolution. It was at this time that vaguely familiar ancient animals
like trilobites, and the ancestors of prawns, crabs and lobsters (as well as those of humans and
most other things which are around today) first emerged; it was also the time when many
forms which were utterly bizarre, utterly unfamiliar, and which have left no descendants
today – organisms with five eyes, and so on – also first made their appearance on the scene.
As an aside, I want to note that some religious fundamentalists have tried to argue that
the Cambrian explosion is evidence of the hand of god at work, but I find this a bit of an odd
argument for a fundamentalist to make: did a supposedly omnipotent, omniscient,
omnipresent and benevolent god make a mistake, not think things through properly, or
perhaps change its mind, in deciding to suddenly cause life to radiate out in new and really
rather radical directions? This does not seem to fit well with the 4 classical properties
normally ascribed to god. Doubtless though, the response of the fundamentalists (might I say
irrationalists) would be that the ways of god are inscrutable to the intelligence of man: a
convenient verbal-conceptual trick which allows one to dodge difficult questions.
Source: http://www.emc.maricopa.edu/faculty/farabee/biobk/BioBookPaleo3.html
The thickness of the lines indicates relative diversities of the various forms. Notice that the
chordates (which includes us, or rather our distant direct ancestors) did not clamber onto the
stage until relatively late in the story, whilst molluscs and arthropods (this includes clams,
squid, snails and slugs) are far more blue-blooded than any peer of the realm, since they
really can trace their ancestry back to the dim past.
Source: http://webecoist.com/2009/09/22/gone-wild-7-extinct-wonders-of-the-animalkingdom/ Opabinia – a five-eyed predator with an unusual feeding appendage from
510,000,000 years ago. There are no descendants of this guy around today: how do we know?
Do you see any 5-eyed animals with one feeding-appendage around?
Source: http://www.emc.maricopa.edu/faculty/farabee/biobk/BioBookPaleo3.html
Meet anomalocaris (whose name means anomalous shrimp), the biggest (ca. 1 metre long),
baddest predator of the Cambrian period – it was in fact the first great predator of the
Cambrian period, and its prey included trilobites. It had an unusual, disk-like mouth which
was composed of 32 overlapping plates, four large and 28 small, resembling pineapple rings,
with their centres replaced by a series of serrated prongs. The mouth could constrict to crush
prey, but never completely close, and the tooth-like prongs continued down the walls of the
gullet. Truly an animal fit for a starring role in an alien horror-movie.
Source: http://paleobiology.si.edu/burgess/imgBurgess/anamalocaris1.gif
A photograph of the fossilised mouth of anomalocaris.
The Cambrian was also the time – with the first appearance of hard, more easily
fossilised parts – that the fossil record picks up the pace.
Source: http://www.didmancreategod.com/Chapter_excerpts/images/C3%20Marrella.jpg
Above is marrella, a delicate looking arthropod often referred to as a ‘lace-crab’.
Source: http://paleobiology.si.edu/burgess/imgBurgess/hallucigenia.gif
The perhaps aptly named hallucigenia. When it was first discovered, some scientists thought
it walked on the long spines, rather like a stilt-walker does today. It has since been discovered
that the spines were on its dorsal (back) side, not on its ventral (stomach) side.
Source: http://digsfossils.com/fossils/pics/trilobites/morocco-trilobite001a.jpeg
There are about 5,000 genera, encompassing over 17,000 species of trilobites (the name
means 3 lobes). They first appeared 526,000,000 years ago, and finally died out about
250,000,000 years ago (that’s a longer span of time than that which separates us from the
earliest proto-dinosaurs by the way). They had a variety of life-styles: some swam, some
crawled; some were predators, some were scavengers; some were spiny, some were smooth;
some were tiny (ca. 1 mm), some were quite large (ca. 720 mm); but none of them played jazz,
and none of them sang the blue; and though there is some debate about it, the general
consensus is that there are no direct trilobites descendants around today. Why they
disappeared is a bit of a mystery, but their numbers seem to have diminished drastically once
predatory fish made their big entrance, so there may b a connection here.
Source: http://en.wikipedia.org/wiki/Trilobite
Some more examples showing the enormous diversity of trilobites.
Source: http://www.emc.maricopa.edu/faculty/farabee/biobk/BioBookPaleo3.html
Above is pikaia, it is the oldest known chordate – in so being, it turns out that it is likely also
your direct ancestor, as well as being the ancestor of every other vertebrate (lions and tigers
and bears – oh my!) and a bunch of other animals too.
Very soon after this came the first fishes, many of which, although very strange
looking, were still recognizable as fish, albeit that some had a rather terrifying aspect.
Source: http://www.fish.state.pa.us/pafish/prehstrm.jpg
This is one of a whole slew of armoured fish which for a while swam in the oceans of the
planet, before going extinct. The largest of these armoured fish was Dunkleosteus, at 10 m
long. It all brings to mind Omar Khayyam:
Think, in this batter'd Caravanserai
Whose Portals are alternate Night and Day,
How Sultan after Sultan with his Pomp and Glory,
Abode his destined Hour, then was sent his way. – The Rubaiyat of Omar Khayyam
Source: http://www.topnews.in/files/ancient-fish-fossils.jpg
It was also the beginning of the first arms race on the planet of which we have a
record: Some fish, in order to protect themselves from other, predatory, fish began developing
defensive heavy armour – bony plates which were presumably hard to bite through. Arms
races however can have no positive final outcome, and so this gambit was finally abandoned
by the forces of evolution, – though apparently not by human heads of state / government,
who really could learn some lessons from palaeontology!
Source: http://www.emc.maricopa.edu/faculty/farabee/biobk/1ostraco.jpg
A reconstruction of an ostracoderm.
Around this time (450,000,000 years ago), the first ‘proper’ plants (as opposed to
algae and such) also began growing on the land. Again, these first land plants were, by our
standards, rather odd looking.
Source: http://www.emc.maricopa.edu/faculty/farabee/biobk/cook.jp
This is a fossil of Cooksonia, member of an extinct group of early land plants.
Source: http://website.nbm-mnb.ca/Doliodus/Devonian%20mural.jpg
A Devonian scene as interpreted by an artist.
These plant, in turn, were followed by some early arthropods (centipede-like animals),
which could now live on land, as there was something there for them to eat.
This model is of a rather
large arthropod called
Arthropleura shows you
just how big they were.
Source:
http://www.icteach.nl/Pagina's/Uitgestorven/Inhoud/Phanerozoïcum/Paleozoïcum/Carboon/B
oven%20Carboon/Fauna/Geleedpotigen/arthropleura07.jpg
Source: http://ncse.com/files/images/Tetrapod_transition.preview.jpg
Then, about 400,000,000 years ago, the first amphibians evolved, probably out of
some sort of fish (which is why you, your dog / cat and your chicken dinner are all really just
somewhat evolved bony fish [osteichthyes] at heart). These animals, of which Ichthyostega is
perhaps the most well known by the public at large, were still highly dependent on water, and
like modern-day amphibians (e.g. frogs), they had to lay their soft, shell-less eggs in the
water, (or in a watery medium like foam), but they could breathe air, and they could walk
around on land. They were still on a fairly short, water-linked tether though, because their
skins could not tolerate extremely dry environments, and had to be kept moist.
Source: http://webspace.webring.com/people/cu/um_1925/eustenovsichthy.jpg
Sorry it’s not in English, but you can work out what all the bits are: yet another way in which
science is sometimes a universal language (just kidding!).
None-the-less, this ability to live, or at least spend lots of time on land was a great
boon, as it now permitted vertebrates to colonise a part of the earth where they were not
present before. These early amphibians eventually evolved into all sorts of terrestrial (and
some marine) vertebrates. They also inherited bilateral symmetry, and a basic body-plan from
their fish ancestors (which is why on top of being bony fish at heart, you, your dog / cat, and
your chicken-dinner all share so many skeletal features in common – 4 limbs, each of which
has a1-bone-then-2-bone-then-a-bunch-of-bones-layout for example).
As far as the story of fossils goes, the newer they are, the more common, or rather the
less rare they become, and this is well reflected in the fossil record, in the frequency with
which fossils from different periods are found. Thus, non-rational (irrational?) religious
objections to evolution notwithstanding, the facts are that the record of evolution from fish to
terrestrial animal is already quite well reflected in the fossil record (as you can clearly see
from the graphic above entitled: ‘Evidence of Macroevolution’), and with time, and the future
disinterring of more fossil-discoveries, this particular story is only going the get more and
more detailed.
Source: http://download.naturkundemuseumberlin.de/presse/Darwin/Stuttgart/Fossil_Sclerocephalus.jpg
Now, while this was going on, the plants were also evolving. They were growing
bigger, and more complex. By about 350, 000, 000 years ago, the land was covered with all
sorts of plants, mostly ferns and mosses, although there were also a number of different kinds
of trees to be found. Much of the earth’s surface at this time was swampy, so many of the
plant-forms were adapted to living in swampy conditions. These plants died, were buried, and
eventually became coal. So coal too is in fact a sort of trace of the history of life on the planet,
and can give us more indicators of how life on earth evolved.
Source: http://www.emc.maricopa.edu/faculty/farabee/biobk/pro_plfr.gif
The width of the blue or green sections is proportional to the number of species.
Source: http://www.sparththenandnow.org.uk/prehistoric/sparthfossils/528/
Fossilised tree-root
Also around this time, the first reptiles evolved, probably from one of the amphibians.
These new animals had a great advantage over their amphibian cousins: Because they had
developed hard-shelled eggs, the reptiles didn’t need to lay their eggs in water. Therefore, the
reptiles were able to live very far away from water (they needed it to drink of course, but they
didn’t need large pools of it in which to lay their eggs). Gradually, these reptiles grew in size
and in complexity, and some of them evolved into other things (which is why you, your dog /
cat, your chicken-dinner, and any lizard you come across all share the same basic skeletal
plan which I mentioned above, and which you can clearly see in the fossil below).
Source: http://www.tucsonshow.com/reports/tucson2002/p1.shtml
Discosauriscus pulcherrimus
After 100,000,000 years or so, evolution again produced some new types of animals:
mammals, and dinosaurs. These early mammals evolved out of some rather odd looking
mammal-like reptiles like good old moschops and thrinaxodon below (again note the skeletal
similarities between you, your dog / cat,…); the dinosaurs meanwhile evolved out of a
different group of reptiles.
Source: http://www.volkoff.net/Images/links/Moschops.gif
Source: http://upload.wikimedia.org/wikipedia/commons/d/d1/Thrinaxodon_Lionhinus.jpg
Thrinaxodon
The oldest dinosaur fossils we have found are those of Eoraptor and Herrerasaurus,
both of which are about 230,000,000 years old.
Source: http://maniraptora.com/dinosaurs_world/macn/eoraptor/eoraptor_final.gif
Eoraptor – dawn thief – one of the earliest dinosaurs of which we have fossil evidence.
Presumable, it was descended from some earlier form, but we have yet to find fossil evidence
of this ancestor. One thing to note, dinosaurs were initially bipedal, so somewhere along the
way they must have evolved from the standard reptilian quadrupedal form, but that story is as
yet incomplete.
Source: http://www.zum.de/Faecher/Materialien/beck/bilder/herrera.gif
Herrerasaurus.
Out of these animals (and perhaps a few others early dinosaurs, which may not have
left fossil remains) evolved all the other dinosaurs, many of which are familiar to you already.
One important thing to note here is that about 180,000,000 years separates these first
dinosaurs, from the last dinosaurs to have lived. Thus, a lot of evolving was going on in
between. The at-one-time-usual pictures, showing diplodocus fighting with t. rex whilst a
pterosaur flies overhead is quite simply false. These animals lived at vastly different points in
time, and each was already extinct before the next one had evolved.
There were also flying reptiles and a bit later birds, but fossils of these animals are
rather rare, as their bones were rather fine and delicate.
Source: http://media-2.web.britannica.com/eb-media/40/3640-004-D1583DEA.jpg
Pterosaur
Source: http://www.fossilmuseum.net/fossilpictures-wpd/Archaeopteryx/Archaeopteryx.jpg
Archaeopteryx
So as you can see, from this little survey post, the fossil record, though not complete
(hence the title of my post), is very good, and the more recent the date of fossilisation, the
better the fossils, and hence the better and more complete the fossil record becomes. By the
time one comes to the end of the Tertiary period (when the dinosaurs were around), and to the
Quaternary period, which was after the dinosaurs, the remains more often become not just
fossils, but also sub-fossils and such. These are remains which either because the soil
chemistry was not right, or simply because they are not old enough, have not fossilised
completely, as I noted in another post within this ‘History of life on the planet series’.
Furthermore, the remains change from fossils and sub-fossils to actual
remains, organic matter – bones, skin, teeth, hair etc, but that is another
story.