Post-dinosaurian mammals 2
In my last, rather meandering post, I started talking about how mammals
became more dominant after the dinosaurs died out. Today, I want to continue to explore that
theme. As I noted earlier, the mammals (or at the least mammal-like forms) were more
dominant a bit before the start of the ‘age of dinosaurs’ – which began around 240 million
years ago. At that juncture, something happened to suppress the proto-mammals, and allow
the proto-dinosaurs to surge to the fore. Whether this surging to the fore of the protodinosaurs was encouraged and triggered by the same event / set of circumstances which
suppressed the proto-mammals, or whether they surged forwards simply because there was no
competition, is still an open question though. What that event was, also no one knows for
certain, however intense volcanic activity may have played a key role in it. There is evidence
of this from certain coal deposits, for example certain ones found in China today which were
laid down around 250 million years ago. These ‘nasty coal’ deposits burn in a particularly
dirty way, and emit a lot of silica dust – so much dust in fact that their use constitutes a health
hazard, with masses of people regularly dying from lung disorders induced by the burning of
this coal (the rates are around 200 deaths per million, as compared to around 50 deaths per
million or less in areas where this dirty coal is not being used, and in China, 200 per million
adds up to a lot of people because of the enormous population of the country).
Could a massive volcano have
triggered the greatest
extinction event in the history
of life on the planet, The
greatest extinction event in the
history of the planet, with the
exception of the one we have
now triggered by our selfish
foolishness in overtaxing the
resources of the planet that is.
Source: http://palaeo.gly.bris.ac.uk/Palaeofiles/Permian/volc.gif
The hypothesized volcanic event most likely released massive amounts of gasses,
including carbon dioxide, as well as enormous amounts of silica-laden ash, which radically
altered the geochemistry of the planet. In fact, it is the silica in this ‘nasty coal’ which renders
its smoke so deadly; I would like you to just pause for a moment and think about this: the
same characteristics which were responsible for wiping life out at the end of the Permian, may
be at work today killing people who burn this 250 million year old material! A remarkable
thought I find.
In addition, there is also evidence that oxygen levels dropped dramatically around this
time, so this combination of factors may have been what was responsible for the dinosaurs’
ancestors taking over from those of the mammals. In effect, the mammals major scene was
shifted to later in the play of life, and it is this scene to which I now wish to turn.
The net result of all this may have been the production of masses of acid rain, which
would have denuded life on the surface of the planet, and acidified the rivers, lakes and
oceans, and there is evidence of this intense acidification in the geological record. Together
with the volcanic activity, this acid rain may have been so intense as to dissolve surface rocks,
and thus mobilise even more silica, which found its way into the atmosphere too. This acid
rain could also have eventually supersaturated the oceans with calcium carbonate, and again,
there is evidence that calcium carbonate beds were heavily eroded around the time of the endPermian extinction. There is a sort of caveat emptor here, as this may be what our massive
releasing of carbon dioxide through fossil-fuel burning will end up doing to the global
ecology all over again!
Sounds odd does it not? The greatest mass extinction in the historical record of the
planet caused by silicosis – variously also known as potter’s rot, or miner’s disease – and acid
rain, but it may, none-the-less, be true.
(For more on out-gassing during the Permian –Triassic boundary, see:
http://www.sciencedaily.com/releases/2007/08/070809104722.htm for more on ‘nasty coal’,
see: http://www.wired.com/wiredscience/2010/01/nasty-coal/#ixzz0n3FdWe9L)
Anyway, after the dinosaurs had their day (or rather their 180 million-odd years), the
mammals came back to the fore, and that is the subject of today’s post.
It was the best of times, it was the worst of times…, it was the Cenozoic (aka
Cainozoic) era, which began sixty five million years ago, and in which we are still living
today. It started out warm, so warm that there were no ice sheets on the poles, and it evolved
into experiencing a series of ice-ages. It began with very high carbon dioxide concentration in
the atmosphere of ca. 2000 parts per million (ppm), but this level has dropped to around 300
ppm today.
Source: http://hoopermuseum.earthsci.carleton.ca/evolution/equidae/cenozoic.jpg
Source<. http://static.howstuffworks.com/gif/willow/mammal-info0.gif
Some Cenozoic mammals
Source: http://jan.ucc.nau.edu/~rcb7/50moll.jpg
Cainozoic earth – notice that the modern continents are more or less recognisable , though
many bits still need to join up (India and Asia, Western Europe and Asia, Africa and Asia,
North- and South-America etc).
In studying the evolution of Cenozoic mammals, three trends stand out: First, the
mammals diversified into a far greater variety of body-forms in this era than at any previous
time. Now, by the Cenozoic, the dry land no longer formed one contiguous land-mass, but
instead was divided up into a variety of biologically isolated continents. Consequently,
evolutionary trends in different areas tended to diverge from each other, thus further adding to
the diversity of the mammals.
Source: http://outline-of-history.mindvessel.net/figures/0041.png
Source: http://laelaps.files.wordpress.com/2007/06/sabreteeth.jpg
In case you were wondering, the answer is yes, the sabre-toothed cats were all from
this period, although they were not amongst the early mammalian forms to evolve once the
dinosaurs made their exit. The earliest probably appeared around 42 million years ago, and
the last died out about 9 thousand years ago, which means that at least one person in your
family, possibly even a direct ancestor, was probably eaten by one! The sabre-toothed
morphology has independently appeared many times during the history of mammals during
this period, and there were many forms, as you can deduce from the 4 drawings of skulls
depicted above: there were even 2 marsupial sabre-toothed animals. Since these different
species did not all evolve from a common ancestor, it is evident that at one point in the history
of life on the planet, having this type of dental equipment was a recipe for success. The most
well known is probably Smilodon. Some had relatively smaller (though still impressive)
protruding canines, whilst others had canines which were so long (ca. 50 cm), that it is
sometimes hard for us to understand how they were not a disadvantage to the cats which
owned them, though clearly these enormous teeth did not pose a problem to their survival, as
sabre-toothed animals were around for over 40 million years.
Source: (right) http://macombhistory.us/localanimals/Animals.html
(left) http://i.telegraph.co.uk/telegraph/multimedia/archive/01080/earth-graphics200_1080665a.jpg
Their most distinct feature of course are their maxillary canines (these are the canines
which grow from the skull / upper jaw downwards, as opposed to the mandibular canines,
which grow from the lower jaw upwards). The other thing to note is that these animals were,
for the most part, robustly built – having bodies which were stocky and almost bear-like,
rather than sleek, and cheetah-like. They were excellent hunters, and probably specialised in
large prey possibly including sloths, indricotheria and mammoths. The agglomerations of
their remains suggests that they were social carnivores, which hunted and lived
cooperatively, rather like modern day lions do today.
Smilodon fossils from the La Brea tar pits include bones which show evidence of
serious crushing or fracture injuries, or crippling arthritis and other degenerative diseases.
Such problems would have been debilitating for the wounded animals, yet many of these
bones show extensive healing and regrowth indicating that even crippled animals survived for
some time after their injuries. How did they survive? It seems most likely that they were cared
for, or at least allowed to feed, by other sabre-toothed cats. Solitary hunters with crippling
injuries would not be expected to live long enough for the bones to heal.
Thus, smilodon appears to have lived in packs and to have had a social structure like
modern lions. They were unlike tigers and all other living cats, which are solitary hunters.
Occasional finds of sabre-tooth-sized holes in Smilodon bones though suggest the social life
of Smilodon was not always peaceful! The cats may have fought over food or mates as lions
do today. Such fights were probably accompanied by loud roaring. From the structure of the
hyoid bones in the throat of smilodon, we know it could roar.
Why the enormous teeth? Certainly they were used in hunting, but opinions vary as to
exactly how they were used. Some palaeontologists have suggested that they were used to
grab and hold onto prey. However, attacking a large herbivore this way could easily break
the sabre-teeth and sabre-teeth that were demonstrably broken during an animal's lifetime
are rare in fossil deposits. A more plausible hypothesis suggests that sabre-teeth were used to
deliver a fatal ripping wound to the belly or throat of a prey animal. Sabre-toothed carnivores
may not have tried to grapple with prey. More likely, delivered one crippling stab wound and
then waited for the prey to die
It has also been suggested that the impressive teeth were used to attract mates, but
evidence for this hypothesis has not yet been found. It may not be that unlikely as a hypothesis
though: both elephant tusks, and walrus tusks are used not only as tools, but also for display
purposes after all, and tusks are just really long teeth – in elephants, they are the second
molars of the upper jaw (or maxillary), and in walruses they are the maxillary canines.
There is another thing about them: Smilodon was about a foot shorter than living lions
but was nearly twice as heavy. Also, unlike cheetahs and lions (which have long tails that help
provide balance when these animals run) Smilodon had a bobtail. These two facts: their
weight and the bobtail, suggest that Smilodon did not chase down prey animals over long
distances as lions, leopards, and cheetahs do. Instead, it may have charged from ambush,
waiting for its prey to come close before attacking.
By the way, even though they are sometimes called sabre-toothed tigers, this is
misleading, since they are not at all closely related to tigers.
For more, see: http://www.ucmp.berkeley.edu/mammal/carnivora/sabretooth.html
Source: http://www.cryptomundo.com/cryptozoo-news/munns-art/
More reconstructions of sabre-toothed cats. There are some wishful thinkers – cryptozoologists by name – who believe that there still are sabre-toothed cats alive in parts of Africa
today, particularly the Central African Republic, and Chad. Whilst I suppose this is always
possible, and there have been recent cases of large animals being found in the forests of
Vietnam for example, I find it not very probable. This is a large, presumedly aggressive
animal, and I find it odd that no one has captured one on film, or shot one yet. I’d be inclined
to put this up there with the yeti, the sasquatch, the mokele-mbembe, nessie, and all that
motley crew.
Source: (right) http://exurbanpedestrian.files.wordpress.com/2009/02/ground-sloth.jpg
(left) http://www.prehistoria.piwko.pl/Obrazki/Indricotherium.jpg
The giant ground sloth right was a huge animal, weighing perhaps 5-6 tonnes, whilst
the indricotherium (aka paraceratherium or baluchitherium) was a gargantuan species of
giant hornless rhinoceros. It was so big, at ca. 20 tonnes, that it was the size of medium-sized
dinosaurs. They were both herbivores, and the carnivorous, group-hunting sabre-toothed cats
may well have hunted these impressive beasts, using a coordinated strategy to bring their
gigantic prey down.
Source: http://tanystropheus.wordpress.com/page/3/
The animals depicted in the image above are as follows:
1=Moeritherium trigodon 2=Numidotherium koholense 3=Barytherium grave 4=Prodeinotherium
bavaricum
5=Palaeomastodon beadnelli
6= Mammut americanum
7= Gomphotherium
angustidens 8= Platybelodon grangeri 9= Rhynchotherium tlascalae 10= Cuvieronius hyodon
11= Tetralophodon longirostris 12= Anancus arvernensis 13= Stegolophodon cautleyi 14=
Stegodon ganesa 15= Primelephas gomphotheroides 16= Loxodonta africana 17= Elephas
maxiums 18= Mammuthus primigenius
This is just to show you the enormous diversity of forms which evolved during this
period. Above is an illustration of the diversity of elephants which evolved over this period.
Of all these forms, there are only 4 elephant species and sub-species left alive today, the
African elephant, the African forest or pygmy elephant, the Asian elephant, and the Asian
pygmy elephant.
Yet another very good example of this trend to diversify, which is familiar to us today,
is the speciation of marsupial mammals in Australia, which is paralleled by that of placental
mammals elsewhere. So, there are marsupial grazers (kangaroos), browsers (koalas), and until
rather recently, predators (thylacines, also known rather misleadingly as Tasmanian tigers, or
wolves). These marsupials have functions which more or less mirror those of their placental
counterparts, animals like say deer, leaf monkeys, and lions respectively.
Source: http://www.txtwriter.com/backgrounders/graphics/evolution/page14.jpg
An illustration comparing placental- and marsupial-mammals which shows both
convergent evolution (how different species evolve to look rather like each other when they
occupy similar ecological niches), and the diversity of marsupial forms.
A second trend is the development of large body sizes. As noted in a previous post,
throughout most of the Mesozoic (age of dinosaurs), mammals were forced to compete with a
variety of swift and voracious carnivorous dinosaurs, many of which would doubtless have
viewed the average mammal as a tasty snack-sized creature. Nor could the mammals attempt
to become larger as a means of self-protection – those niches were already filled with all
manner of dinosaurs. After the extinction of the dinosaurs however, the mammals were free to
evolve into much bigger forms.
Throughout the Cenozoic, mammalian herbivores, and to a lesser extent mammalian
carnivores, experimented with size. In fact, some earlier Cenozoic mammals were truly
gargantuan. Indricotherium for example, whom you just met, was a sort of giraffe-rhinoceros,
which tipped the scale at possibly 20 tonnes, making it by far the largest land-living mammal
ever. It was probably a browser (tree-leaf eater), rather than a grazer (grass eater), which
stood about 6 metres high at the shoulders, and was 12 metres long overall. When it extended
its head and neck high, Indricotherium must have been able to reach leaves and shoots which
were perhaps 9 or 10 metres above the ground.
Source: http://isaacbickerstaff.wordpress.com/
Another image of indricotherium, just to give you an idea of it’s size. Given how much
dung an elephant can produce in one bowel-movement, personally, I would not be standing
where the person in the illustration is standing, nor would I be holding my hand out as if
expecting some manna from heaven in just that precise spot!
Source: http://isaacbickerstaff.wordpress.com/
Unitatherium, arsinotherium, and megacerops, were all very large herbivorous mammals
from this period. None of these animals have left present-day descendants.
Source: http://isaacbickerstaff.wordpress.com/
The entelodonts were huge and terrifying looking prehistoric pigs. Most of them were
the size of bears, and stood 2 metres tall at the shoulder. They had some very convincing teeth
indeed, and were not called hell-pigs for nothing. Like all pigs, they were omnivores, and with
those teeth, they could ‘vore’ pretty much any thing they wanted to.
Source: http://www.avph.com.br/jpg/archaeotherium1.jpg
Archotherium was another prehistoric pig. They may well have been hunters, rather
than scavengers. To put these animals in perspective, see the illustration below (these animals
actually lived on this earth at one time, and may have rutted where your driveway now is:
sweet dreams!):
Source: http://big_game.at.infoseek.co.jp/Pleistocene/Daeodon/Daeodon.html
Source: http://macroevolution.narod.ru/glebpics/Dinohyus_Battle.jpg
Dinohyus, another entlodont. These pigs were most likely very aggressive, and far
more dangerous than modern-day pigs.
On the carnivore side of things, the largest meat-eating land mammal that we know of
is Andrewsarchus, a five-metre-long animal, distantly related to whales. As carnivores go,
Andrewsarchus must have been a truly frightening sight. It had a head which was over a
metre long, and it was considerably heftier than a full-grown grizzly bear. Recent work
however indicates that Andrewsarchus might only have been the second largest carnivorous
terrestrial mammal. There was a giant, ground-dwelling sloth, a relative of the sloths of today,
that was about the same length as Andrewsarchus, but somewhat taller, and much bulkier than
it. Whether this sloth was a carnivore or not though is still a matter of contention.
Source: http://www.newanimal.org/andrewsarchus_mesonychid.jpg
Above is andrewsarchus, whom you have already met in a previous post.
Incidentally, nature’s experimentation with large sizes is still not complete. The
existence of elephants for example attesting to this fact. Even though they weigh only about
one third as much as Indricotherium did, a five-tonne animal is still a sizable beast.
The third trend that is evident amongst Cenozoic mammals is again one that is still in
the process of unfolding, and that is the development of large brains. While not all mammals
have notably large brains, a fair number, including primates, dolphins and orcas, certain
carnivores, elephants, and some ungulates, namely members of the horse family, do. This
incidentally accounts for the fact that the majority of trained animals come from one of these
groups. Only large-brained animals are capable of complex training. Hence though there are
for example trained crocodiles, they cannot do terribly complex things.
It has been suggested that the large brain size was occasioned primarily by the
changing climate of the Cenozoic, which was far more seasonal and unpredictable – at least in
the short term – than the Mesozoic as a whole. Seasonality and unpredictability lead to
uncertainty about feeding. Consequently, or so it is argued, animals have had to develop the
ability to keep track of more complex cycles of food availability. Thus, the development of
larger brains.
While this is an interesting idea, the best we can say about it is that it does not
encompass the entire picture, as not all mammals developed the large, complex brain that this
theory seems to necessitate.
Now, I notice that I’ve been writing as if the only things of evolutionary significance
in the Cenozoic were the mammals. However, this is very far from the truth. all sorts of other
interesting evolutionary developments were taking place – for instance, a whole slew of large
carnivorous birds were appearing on the scene at around this time, and there were also many
very large and interesting lizards around. That however is material for another post.