Quadrupedal amphibians invade the land
Last post I wrote about how for the first time, life – beginning with plants, though
followed closely by arthropods – invaded the dry land, probably from freshwater lakes and rivers,
around 410 million years ago, i.e. towards the end of the Silurian period (438 - 408 million years
ago). As I noted there, none of the early terrestrial fauna which were around, at least up through the
Devonian period (408 - 360 million years ago), were not even vertebral (possessed of a spinal
column), let alone quadrupedal (having four legs). Although there were several thousand species of
arthropods – assorted six-legged insects as well as a whole host of other millipede-like forms, like
Arthropleura and Acanthenpestes (both six foot-long) for example, there wasn’t anything with four
feet and a backbone on the dry land, anywhere.
Devonian terrestrial plants.
Source:
http://www.geology.wisc.edu/homepages/g100s2/public_html/Geologic_Time/L11_Devonian_Life_M
ore_B.jpg
Devonian fauna.
Source: http://web.rollins.edu/~jsiry/devonian.gif
Life in the Devonian sea was quite diverse including giant eurypterids or 'sea scorpions' pursued early jawed
fishes, including acanthodians (sometimes called 'spiny sharks', though not related to true sharks) and shieldheaded fishes called placoderms (which probably shared a common ancestor with the sharks). Rugose and
halysite corals built great reefs, providing food and living spaces for many different kinds of creature. The sea
floor supported a rich variety of crinoid 'sea lilies', stalked clam-like animals called brachiopods, bizarre colonial
critters known as graptolites, and early versions of mollusks - such as chitons, tusk shells, and straight-shelled
cephalopods related to modern-day nautiloids. The first plants - small, leafy pioneers known as rhinophytes colonized the land, followed shortly thereafter by joint-limbed creatures such as scorpions, centipedes, and
millipedes.
Source: http://web.rollins.edu/~jsiry/CREATION.html
In the late Devonian seas though, the situation was different. There, as I noted in my last
post,, all manner of utterly unfamiliar forms were thriving, from squid-like animals to armour-plated
fishes, to creatures which defy modern-day analogous classification (as the figure above illustrates).
Soon however (in evolutionary terms at least), some of these aquatic vertebrates were going to start
invading the dry land.
Although there is much debate over when vertebrates first ventured out of the water, and
great uncertainty about why exactly they did this, there is no question that by the end of the Devonian
a new set of forms, had emerged, the amphibians. These amphibians were spending reasonable
amounts of their lives out of the water, albeit that they could never stray too far away from it: They
needed to keep their skins moist at all times; they needed it to lay their eggs in (which also would
have dried out in the air), and in many instances, they needed it for copulation.
Labyrinthodonts, early amphibian forms.
Source: http://library.thinkquest.org/20886/media/labyrinthodonts.gif
Artist’s impression of a Hynerpeton (an early amphibian) fleeing a predatory Hyneria fish.
Source: http://www.palaeos.com/Paleozoic/Devonian/Images/Hyneria.jpg
Acanthostega gunnari
Ichthyostega stensiöi:
Source: http://www.bluelight.ru/vb/showthread.php?t=187600
The details of what the route was through which the amphibians evolved is also a puzzle, but
fortunately one to which we have a sufficiency of suggestive pieces for us to take a reasoned guess at
the most likely scenario: for example, we know for certain that the progenitor of all vertebrates must
have been a fish (all of which are vertebrates).
Before proceeding with this though, we need to answer another question, a functional one
that in the typical manner of things palaeontological, will lead us to a possible answer to our initial
question: Why move onto land? One theory is that having first moved to freshwater habitats – which,
importantly, necessitated evolving a range of compensatory and regulative mechanisms to deal with
problems of osmosis (remember that the salinity of a fish’s innards is considerably higher than the
salinity of freshwater, hence, because their skins are semi-permeable membranes, water molecules
will migrate across this barrier, causing the fish to swell up which is what happens if most sea fish are
put into freshwater even today) – they then had to develop an ability to weather the seasonal droughts
that plagued the Devonian period.
According to one version of the classical theory, these freshwater creatures could survive the
dry spells by doing one of two things: estivating (burying themselves in the mud and hibernating until
the rains came again, much as modern lung-fish do today), or somehow moving across land in search
of other watery habitats. It is this latter option, according to A. S. Romer, who originated this idea in
the 1950s, that led, eventually to amphibians. This theory seems so far-fetched, that upon hind-sight,
it is a wonder that it was ever considered seriously.
A more likely course of events is that some freshwater lobe-finned fish species (lobe-finned
fish are fish with thick, limb-like fins), one which had probably already developed the ability to crawl
around lake or river beds on its well muscled fins, gradually moved onto dry land, in order to tap new
resources, especially food (both in the form of plants and arthropods) and oxygen, that were not being
exploited sufficiently by any other animal.
Skeletons of a crossopterygian lobe-finned fish and an early amphibian.
Source: http://www.biologyjunction.com/images/verteb15.gif
The fossil record is sketchy as to the precise details, but it does appear that both Ichthyostega
and Acanthostega (both amongst the earliest amphibian forms to evolve and both of which are shown
above), and Eusthenopteron (a late Devonian lobe-finned fish which apparently could “walk” with
the help of its fore-fins, using them to help it crawl along lake-beds, and perhaps even over uncovered
mud-flats, much as mud-skippers do today in tropical mangrove swamps), must have shared a
common ancestor, whose evolution split into two branches sometime towards the end of the
Devonian. One of these branches led to the first vertebral amphibians, and perhaps, eventually, to all
terrestrial vertebral forms, including ourselves. This conclusion is a bit tentative though, as both
Ichthyostega and Acanthostega had more digits (fingers and toes) than any vertebrates do today:
seven in the former case, and eight in the latter. Hence, palaeontologists are dived over whether either
of these early amphibians were our forerunners who subsequently shed the extra fingers and toes, or
whether present-day mammals, reptiles, avians and amphibians, all of which have (at least in
vestigial form) five digits at the end of each limb, are descended from some other five-fingered and
five-toed ancestor which we all share in common.
An interesting graphic illustrating the fairly well documented evolution of the terrestrial
amphibian form Ichthyostega from a fish, Eusthenopteron.
Source: http://sciencenotes.files.wordpress.com/2008/04/tiktaalik.jpg
What some fish got up to during the Devonian, and where their offspring ended up.
Source: http://bss.sfsu.edu/holzman/courses/Fall%2003%20project/CTScladogram.jpg
Possible outline of the evolutionary ancestry of terrestrial vertebrates.
Source: http://www.igsb.uiowa.edu/Browse/amphibs/amphib1.gif
Now the process of emerging from the water that I have just described was not an irreversible
one. Just as cetaceans (whales, dolphins and porpoises) today come from erstwhile terrestrial forms
that have reverted to the water, so also, at least by the Carboniferous period (360 - 290 million years
ago), animals like Crassigyrinus had abandoned the land and taken up permanent residence in the
water again, lurking around reed-beds at the edges of ponds and lakes, and preying on any
unfortunate fish that strayed too close to its thick-set and probably well camouflaged body.
Crassigyrinus scoticus
Source: http://tolweb.org/tree/ToLimages/12_Crassigyrinus_rec.GIF
We speculate that this was the case because of the fossil evidence: Firstly, looking at
Crassigyrinus’ remains, we can tell that its puny little fore- and rear-limbs would have been
ineffectual in supporting its bulk out of the water, though they look to be about the right size for
clinging onto reeds or submerged roots; Then, based on what sorts of plants were fossilized along
with Crassigyrinus, we can get some idea of the environment in which it lived; Finally, studying the
sedimentary matrix in which Crassigyrinus fossils are found, we can get further clues about its
habitat. All this is what leads us to the above-going picture.
Why revert to the water? As we have noted earlier, although capable of living outside it,
amphibians at that time (no differently from amphibians today) were highly dependent on water. This
dependency translates, in many cases, to a necessity for physical proximity to water. In this
circumstance, if some insufficiently exploited ecological niche presented itself, physical proximity
might have meant that Crassigyrinus’ ancestor was uniquely poised to take advantage of it. Hence,
the reversion to water.
Be that as it may, the main current of our story lies elsewhere: Further inland, myriad
possibilities were, by the end of the Devonian, ripe for the plucking, and for whatever combination of
reasons, vertebrates were eminently suited to the task. How do we know? Because from this time
onwards, vertebrates go forth and multiply in an uncountable variety of ways, as both the fossil
record, and a quick visit to any decent zoo can show (don’t forget, all the vertebral, quadrupedal,
terrestrial forms you’re likely to see there came, ultimately, from this period). This is not to say that
vertebrates ever have been the dominant form either in these early times, or for that matter in our
present times. If we base our calculation of dominance either on bio-mass – the sheer / total mass of
animals of any particular type – or on the number of species that have evolved, then the invertebrates
win hands down. No matter how you measure it, we vertebrates are in the minority. However, ever
since they emerged, vertebrates have been as survivable a category of life forms as the invertebrate
have, even if they are in the absolute bio-mass minority, and by this standard constitute a successful
design. Before they could attain this measure of success however, vertebrates had to work out a way
of divorcing themselves from the close tie to water that made the vast and insufficiently exploited
inland reaches inaccessible to any amphibians. Next post, the reptilian solution.