Post-dinosaurian mammals 1
Although they were already in existence perhaps three hundred million
years ago – a time that considerably predates the appearance of the first dinosaurs
(which did not evolve until about 240 million years ago) – until relatively recently (at
least on a time scale of hundreds of millions of years), for the most part, in a way,
mammals played only minor roles in the drama of life, though one has to be cautious
about how to interpret this statement. It is true that, for well over five sixths of the
time they’ve been on the planet thus far, they were generally physically diminutive,
and in no sense could they be considered a dominant life-form, at least in terms of
their physical size.
Whether they were also functionally diminutive though is open to discussion.
It has often been argued that by and large, apart from a very brief interlude towards the
end of the Permian period (which ended around 245 million years ago), the mammals
occupied only those peripheral ecological niches which dinosaurs or other reptiles did
not exploit. This may be true, but there are problems with understanding what exactly
this means. They were smallish, but it is always hard to establish exactly how
important a role a species, be it small or large, plays in an ecosystem.
To some extent, the current mainstream view of early mammals as peripheral
animals, living in the shadows of the dinosaurs, is an overreaction to an earlier view
propounded by palaeontologists that mammals were always the superior form, the
acme and pinnacle, the telos of evolution, and that all else was just incidental, sideentertainment.
This swinging of theoretical perspectives, reminiscent of a Hegelian dialectical
process is often how theorising in the sciences in general works: first, Victorian (or
earlier) scientists produce a (generally conservative) thesis; this is followed by a
reaction formation which leads to the construction of an overstated antithesis; then,
out of this emerges a new, generally empirically grounded and improved synthesis
which in the fullness of time becomes a new mainstream thesis, ripe for amelioration
by this same process once again. The process however is not Sisyphean in essence;
rather than being purely repetitive, each iteration does produce a better, by which I
mean more accurate (i.e. corresponds more closely to the way observable reality
works), picture of how things are (or were in the case of palaeontology).
Source: (dialectic) http://www.kheper.net/topics/philosophy/hegels_dialectic.gif
Sisyphus Pushing His Stone up a Mountain, 1731 by Bernard Picart, from:
http://www.reversespins.com/ancientmysteries.html
Anyhow, before we go any further, it might be useful to speculate on why
exactly dinosaurs superseded the mammals, which after all were around before the
dinosaurs were, and had the opportunity to evolve earlier. An intriguing suggestion
has been offered recently: the dinosaurs’ ancestors may have had bird-like / crocodilelike lungs, rather than mammal-like ones. In alligators for example, air follows a oneway loop, a design also found in birds, which allows them to fly at high-altitude where
oxygen levels are low. This similarity between bird and alligator lungs probably
indicates that one-way airflow emerged before the two groups split over 246 million
years ago.
What’s the significance of this? Oxygen levels dropped after the PermianTriassic extinction (around 250 million years ago), a period during which most landbased life died off, so, lungs with one-way air flow may have been the factor which
gave dinosaurs’ ancestors the competitive edge during this lung-trying time.
Source: http://www.wired.com/wiredscience/2010/01/alligator-lungs/
We know that before the Permian-Triassic extinction, synapsids, the ancestors
to modern mammals, were the dominant group, but after the extinction, the archosaurs
dwarfed the synapsids. Prestosuchids (early archosaurs), for example, could reach 7
metres in length, while mammals’ ancestors were at most just a metre or so long.
A prestosuchid, Source:
http://4.bp.blogspot.com/_OCgYxcoyLC8/S33lGRbflNI/AAAAAAAAAH4/l9H1hOdyqx
M/s320/800px-Batrachotomus1DB.jpg
Without this one-way lung system, mammals may just have been unable to
compete in important niches. As C. G. Farmer puts it, ‘if you can’t run you better hide,
and [so,] you better be small enough to hide.’
So, how does the mammalian lung work? In mammals, air flows into the lungs
through progressively smaller airways, stops in little sacs where oxygen is absorbed
into the blood and then reverses course and is exhaled by the same route. In birds, air
goes one way through small tubes called parabronchi that loop around to send the air
back out. This system allows modern birds to function in a low-oxygen atmosphere.
For instance, bar-headed geese can fly over the thin air above Mt. Everest. A similar
ability to squeeze out ample oxygen from thin air may well have given bird’s’
archosaur ancestors the edge over the synapsids.
Source: http://universe-review.ca/I10-32-Permian.jpg
Some early, mammal-like reptiles – at least we presume these were reptiles, though it
is a little difficult to say what exactly we mean by a reptile here. In fact, we have the
same problem with dinosaurs, which we used to think of as reptiles, but which we now
think of in more loose and fuzzy terms. For one, at least some of them had feathers,
and they may have been more ‘warm-blooded’ than we think. In addition, their
skeletal structures and postures are generally not very reptilian It is also possible that
the general category (biological-class) dinosaur, or mammal has no stability over the
tens to hundreds of millions of years of evolution experienced by these classes.
Perhaps early dinosaurs and mammals were more reptile-like, and not quite so
‘warm-blooded’ in our modern sense of the term, but later ones were more bird- or
classic mammal-like respectively.
Even today, there is a vast range of ‘states of bloodedness.’ Some mammals
are homeothermic (i.e. they maintain a constant body temperature). Some are
endothermic (i.e. generate their own heat), but not homeothermic (i.e. they cannot
maintain a constant body temperature). There are even mammals which have entirely
lost the ability to regulate their temperature endothermically (like the naked mole-rat
[Heterocephalus glaber] for example), and this is a very unmammal-like trait indeed.
Then, there is gigantothermy, whereby very large animals are able to maintain a more
constant body temperature by sheer virtue of their massive bulk (the icthyosaurs,
mosasaurs, used this form of thermoregulation, and certain large fish and turtles use
this method today. It is also why elephants, despite being relatively inhirsute, can
withstand the cold of austral and upland Africa, whereas even tropical mice need fur).
There are also animals which are heterothermic, having both ‘cold-blooded’ and
‘warm-blooded’ characteristics, and those which have a bradymetabolism (where
their metabolic rate can switch between very high states like a ‘warm-blood’, and
very low states, like a ‘cold-blood’).
So, given this variety of possibilities, you can see how it might be difficult to
decide if a particular animal is a mammal or a reptile. To confuse matters even
further, it’s not just mammals and possibly dinosaurs who have this potential
ambiguity in their evolutionary lineage: Crocodiles – though generally regarded as
reptiles – have some characteristics which are not very reptilian at all: their hearts,
and the way they fill their lungs for example are more mammalian than reptilian. So
all in all, I hope that you see how ambivalent these categories are. To me, it may well
make more sense to conceptualise matters in terms of a punctuated continuum, than of
distinct categories when talking of the thermoregulatory systems of animals.
Source:
http://www.nature.com/nature/journal/v450/n7172/fig_tab/nature06277_F2.html
a, Representation of the traditional assumption that Mesozoic mammals were generalized
small animals with generalized feeding and terrestrial habits, and had few of the diverse
ecomorphotypes of Cenozoic mammals; the hypothesis on decoupling of lineage splitting
from ecological diversification is based on this assumption 27, 31, 32, which is now
contradicted by recent discoveries of a great range of ecological specializations, such as: b,
swimming and fish-feeding in the docodont Castorocauda38, and semi-aquatic habits of
Haldanodon 39, 40; c, ambulatory carnivory or scavenging (predation or feeding on other
vertebrates) in large gobiconodontids 42 and large individuals of Sinoconodon 41; d,
scratch-digging and feeding on colonial insects in Fruitafossor 20; e, scansorial (climbing)
limb characteristics in basal eutherians and metatherians, and their near relatives 11, 47,
48; and f, volant (gliding) adaptation in Volaticotherium 49. The Jurassic and Cretaceous
mammals developed, iteratively, similar niche specializations to modern Australasian
monotremes and marsupials, and are no less diverse, ecologically, than the early-to-middle
Palaeocene mammals of similar body-size range. Splits of Mesozoic mammal groups were
accompanied by ecological diversification.
Source: http://universe-review.ca/I10-33-earlymammals2.jpg
This is just to give you some idea of the diversity of early mammalian forms. One
thing you will note, they were all relatively small animals, though there is now some
evidence that at least 150 million years ago, some of them did in fact hunt and eat
small dinosaurs.
How do we know this? Relatively recently, villagers digging in China's rich,
and until largely untapped fossil beds uncovered the preserved remains of a tiny
dinosaur in the belly of a mammal. This was a startling discovery for scientists some
of whom had long believed that early mammals couldn't possibly have attack and
eaten dinosaurs. The fossilized remains of the animal's last meal though is convincing
proof that some mammals did hunt and eat small dinosaurs some 130 million years
ago. It could, I suppose, be argued that the find contradicts conventional evolutionary
theory wherein early mammals have been assumed to be timid, chipmunk-sized
creatures which scurried about in the looming shadow of the giant reptiles, but in fact
there is no contradiction here, as a little reflection will show: Small mammals
probably did scurry away from large dinosaurs, but this does not mean they could not
have successfully attacked and eaten smaller ones.
In the case of the recent find in China, the mammal was about the size of a
large opossum, and the victim was a 5-inch baby Psittacosaurus ( the ‘parrot
dinosaur’).
It never ceases to amaze me how palaeontologists frequently reason in this
value-laden-assumptions-driven meta-theoretical manner, and how fraught the field
has been with dodgy reasoning leading to ludicrous conclusions: They’ve gotten it so
terribly wrong, so many times before (for example when they assumed that dinosaurs
were slow, dim-witted, lumbering brutes – a sort of incidental side-plot in the long
story of the evolution of life on the planet – who were just doing their bit whilst
waiting for the mammals to show up for the main act – which they weren’t!), but they
(the palaeontologists that is) still persist in applying a rather distorted form of
reasoning which seems to assume that things ‘back then’ were somehow
fundamentally different than they are now, or that different biological-evolutionary
rules applied then (in fact, it is almost as if some scientists think that the rules of
evolution themselves evolved, and became more streamlined (would that be a sort of
meta-evolution?), when in fact this is not the case at all .
The reality is that the same laws of Darwinian evolution applied ‘back then’,
as now, because the method by which evolution operates is based on the physical
reality of (biochemical) life interacting with a physical environment, and the forces
shaping and determining behaviours and physiologies have operated the same way
ever since ever since really. What I am saying, in other words, is that evolution too,
just like physics, chemistry, geology and so on operates on certain principles (what
one might call laws), and these are invariant over time.
Consequently, it is unreasonable, and unreasoning to simply assume that no
mammals ate dinosaurs. It is a point of view that one could come to only by placing
the cart of foregone conclusions before the horse of reasoning from known facts. A
little reflection would have indicated that it would have been far more reasonable to
assume the obverse: that some (possibly larger) early mammals did eat dinosaurs –
and quite possibly that they ate dinosaurs larger than themselves (though this has yet
to be shown in the fossil record). To give you a parallel argument, it would be like
saying, say, that no spiders eat anything but insects, because other biological classes
were too sophisticated, when in fact, the reality is that there are spiders which eat
animals from many other biological classes (there are spiders which eat [small] fish,
[small] amphibians, [small] reptiles, possibly [small] mammals, and [not quite so
small] birds!).
Source: (left) http://thepirata.com/wpcontent/uploads/2008/10/bird_eating_spider01.jpg (spider eating a bird)
(right) http://pixdaus.com/pics/clXdk3jcsw7Rxpdgnb.jpg (spider eating a lizard)
Source: (left) http://farm1.static.flickr.com/102/257007481_0d3273eed1.jpg?v=0
(spider eating a frog)
(right) http://farm3.static.flickr.com/2123/2447467154_f2caf13e39.jpg
(spider eating a fish)
Source:
http://msnbcmedia4.msn.com/j/msnbc/Components/Photos/050112/050112_superdog
_vmed_10a.widec.jpg
Artist’s reconstruction of Repenomamus giganticus, one of the two recently
discovered dinosaur-eating prehistoric mammals which lived in what is today China
about 130 million years ago.
Right: Repenomamus robustus. The other
recently discovered dinosaur-eating
mammal. It was smaller than its R.
giganticus relative—about the size of an
opossum. Inside its rib cage were found a
wad of broken bones and teeth. These
remains also included a skull and other
pieces, just as if an animal had been eaten
in chunks. Analyses of the meal suggest
that Repenomamus had eaten a young
Psittacosaurus, a common type of planteating dinosaur. When it was eaten, this
dinosaur was roughly the length of a
human hand. Fully grown, it would have
been about 2 meters (6 feet) long.
Source: http://universe-review.ca/I10-33-earlymammals1.jpg
Very rough branching of the mammals from early forms to now: Monotremes are the
egg-laying mammals (platypuses and the 4 species of echidnas). Monotreme means
single orifice (like birds, the monotremes have a single opening – the cloaca – which
serves for both excretion and reproduction); Marsupials are the pouched mammals,
which includes kangaroos, koalas, opossums, bandicoots, and all manner of other
animals; Placentals includes all other current mammals, dogs, pigs, humans, cattle,
deer etc and many extinct ones; Eutriconodonts are early mammals, distinguished by
their unique dentition, this category includes the Chinese fossil finds I just wrote
about; Multituberculates were a group of rodent-like mammals which existed for
about a hundred million years – the longest fossil history of any mammalian lineage.
They were eventually out-competed by newly evolved small mammals, possibly
rodents, and went extinct in the early Oligocene (ca 30 million years ago); and the
Docodonts, which were a category of extinct proto-mammals which lived during the
mid- to late- Mesozoic era (the ‘age of the dinosaurs’). Their most distinguishing
physical features were their relatively sophisticated set of molars, from which the
order gets its name. The exact phylogenetic position of the docodonts depends on the
method one is using. From a cladistic point of view, docodonts are advanced
mammaliaforms just outside the Mammalia proper. From a 'traditional' point of view,
the docodonts are basal mammals.
As an aside: often, especially in palaeontology, teeth are one of the best
indicators of the life-style of the animal, and often are the best preserved of all
remains (because they are exceptionally hard), hence teeth play a big role in
classification.
Anyway, picking up the main thread of the story again, sixty five million years
ago something happened to radically alter the entire situation (remember, whatever the
situation with regard to the role of the mammals was, the dinosaurs were dominant
then, by which I mean there were many of them, and many were far, far bigger than
the mammals, and may well have occupied many more ecological niches than the
mammals too, though this again needs to be verified in the fossil record). What that
event was, we do not know for certain, though, as I noted in an earlier post, this
uncertainty does not stem from any lack of theories, the most accepted of them
currently being that of a large meteor strike of some sort (possibly a bolide impact: a
bolide is a large meteor which explodes in the atmosphere). Anyhow, whatever
happened at the K-T boundary, as the division between the Cretaceous and Tertiary
periods is called, the fossil record clearly indicates that the dinosaurs – incontestably
very significant life-forms for about one hundred and sixty-odd million years – all
became extinct (with the possible exception of birds, which as I wrote in another post,
some palaeontologists postulate are in fact the descendants of dinosaurs).
Source: (left)
http://www.copyrightexpired.com/earlyimage/prehistoriclifebeforekt/anchisaurus_coo
d_1894_smit_1929.html
(right)
http://www.copyrightexpired.com/earlyimage/prehistoriclifebeforekt/compsognathus_
cood_1894_smit_1929.html
Source: (left)
http://www.theage.com.au/ffximage/2006/04/05/dinosaur_narrowweb__300x313,0.jp
g
(right) http://www.dinosaur-world.com/feathered_dinosaurs/insets/microraptor_guismall.gif
In the above four images, please note the remarkably different depictions of
these dinosaurs. In the early black and white images, they look remarkably reptilian,
whilst in the more current coloured images, they look remarkably bird like. I’ve made
this point before, but I think it is interesting to let the iconographic representations of
these animals adumbrate and reveal the revolutionary transformation which our view
of these animals has undergone. This study of iconography in science is, I think, an
extremely useful exercise in bringing into relief the hidden contours of our
interpretative paradigms about how the world works, and how it is supposed, or
presumed by us to work.
Like Stephen Jay. Gould, I have no doubt but that the images we have been
surrounded by, and accustomed to, in no small way colour our interpretative
representations of dinosaurs (and other things too for that matter). The reptilianlooking dinosaur-prints, murals and models of Victorian science indubitably retarded
the advancement of palaeontology, by informing budding (and not so budding)
palaeontologists conceptions and paradigmatic assumptions underlying their thinking
about dinosaurs. This process is insidious, and subliminal. It shapes and tints our
interpretative framework in ways we are often not even consciously aware of, and
thus leads us to conclusions which may only have a tenuous link to ‘reality’ whatever
that may mean.
Freudian psychology, symbolic interactionism, behaviourism, or any other
interpretative science shares this trait of being highly paradigmatic. Whether Physics,
chemistry and the like also share this trait, and if so how exactly, is the subject matter
for an entire book.
Picking up that slippery thread of the main argument yet again, until very
recently, it was also posited that this K-T mass extinction event, one of perhaps twenty
major extinction events which have punctuated the history of life on the planet,
cleared the slate as it were, making available huge ecological niches which had
previously been filled by dinosaurs of various descriptions, and that for a variety of
reasons, the mammals happened to be in a position to move into, and occupy those
niches. However, that view has now come into question, since the mammals did not
immediately radiate outwards and fill all these ecological niches which were
presumably left behind by the retreating (‘extincting’ if I may coin a gerund)
dinosaurs.
In fact, it took about 10-15 million years for them to do so. Whilst some
mammals seized the day and diversified after the putative asteroid crashed off the
Yucatan peninsula, causing a mass extinction, they were largely evolutionary deadends. Mammals from the major groups which are around today arose tens of millions
of years before the asteroid struck and survived the calamity, but they remained
secondary to now-extinct mammal forms and did not start diversifying and asserting
themselves until about 55 to 50 million years ago.
Why exactly this was the case is a bit of a mystery. As it happens, some
mammal groups did diversify quickly after the dinosaurs’ coup de grace, giving rise to
species like Andrewsarchus, a ferocious hoofed predator. But these animals too
eventually went extinct, and today have left no living descendants.
Source: (top) http://www.physics4u.gr/news/images7/Andrewsarchus.jpg
(bottom) http://www.paleocraft.com/images/Andrwscale.gif
These two images give you an idea of the size of Andrewsarchus, which was by all
measures a rather frightening-looking and ferocious predator
It is possible that these temporarily dominant groups, like the dinosaurs before
them, may have kept our own ancestors in the evolutionary shade. As Andy Purvis, in
a way echoing C. G. Farmer put it, ‘for the first 10 or 15 million years after the
dinosaurs were wiped out, present day mammals kept a very low profile, whilst these
other types of mammals were running the show.’
Perhaps they adapted to a still-changing climate only to be wiped out mere
geological moments later, whilst the forefathers of today’s mammals, by waiting until
conditions settled down before diversifying, fared better because they played it slow
and steady. I don’t know, but please do not misinterpret what I have written. I am not
suggesting that there was a conscious decision here, on the part of some mammalian
lines to wait it out. It’s just that, that was what, for some unknown
reason, happened. In the history of evolution, one trend seems to be
fairly clear, if you specialise, you’re more adapted to your niche, but
also more vulnerable to small variations in your niche. You win
some, you lose some.