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salt water amphibians?

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If amphibians evolved from fish, which presumably lived in oceans, why are there no salt water amphibians? Or am I mistaken?

 

Or did amphibians evolve from freshwater fish because there were no oceans...?

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Interesting; I can't think of any examples of saltwater amphibians either. There were definitely oceans, though. Probably they evolved from freshwater fish for some other reason. Maybe freshwater habitats test their inhabitants more and thus are more likely to produce a lot of evolutionary changes of all sorts, or maybe it was just that the land was there waiting for something to take advantage, and it just ended up being a freshwater species that took advantage. I'm inclined to believe the former, though, because it isn't just amphibians; very few insects live in salt water either.

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You're correct that there are no saltwater amphibians, but the important thing to realize is that the modern amphibians are *very* different from the first tetrapods. Imagine how people would puzzle over mammals if rodents were the only remaining lineage; that's pretty much how it is with amphibians.

 

The very first amphibians probably lived in brackish water, but had scales and retained gills even as adults. Even once they became completely terrestrial, they likely retained scales for a long while, which is where reptile scales come from.

 

The loss of scales and the gain of poison glands happened only in one particular group of amphibians, and unfortunately, that's the only group which survives today.

 

So basically, there are no modern saltwater amphibians, because of their permeable skin, but their ancestors had scales and at least some are *definitely* known from brackish or salt water.

 

very few insects live in salt water either

 

Sort of. Arthropods, including crustaceans, originated in the ocean, and diversified *very* rapidly. Eventually, one lineage made its way onto land and became the insects. They haven't returned to the ocean much in part because their crustacean and other cousins would out-compete them, but also because it would 'waste' the adaptation behind most of their success, namely their wings.

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Thanks for your answers.

 

However, I cannot believe that all saltwater amphibians would have become extinct. Also, where would these brackish waters in the ocean exist ?

 

I am beginning to wonder if maybe there simply were no oceans, only huge freshwater (or, due to leaching, slightly brackish) lakes.

 

It seems a strange co-incidence that at this time there was only one continent - Pangea. Maybe the expanding Earth theory should be revisited. Can you see what I am thinking...?

 

I have become suspicious of the whole Wegener idea of plate tectonics - continents have to wiggle and rotate in freakish ways order for it to be correct. Using occams razor, the EET is far simpler - and works - exactly. The continents fit together exactly!

 

Few ancient fish fossils have been found on oceans floors - which are between 10 and 180 million years old. Fish fossils are almost exclusively found from contental rocks...

 

 

 

http://www.youtube.com/watch?v=VjgidAICoQI

 

http://www.youtube.com/watch?v=BvgJrDH2qXQ&feature=related

Edited by bombus

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I have become suspicious of the whole Wegener idea of plate tectonics - continents have to wiggle and rotate in freakish ways order for it to be correct. Using occams razor, the EET is far simpler - and works - exactly. The continents fit together exactly!

 

And by what means, exactly, do you propose that the Earth is expanding? :doh:

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you can't believe that a couple hundred species have died out but you do believe that the earth is magically gaining quadrillions(ridiculously low i know) of tonnes of material every century?

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However, I cannot believe that all saltwater amphibians would have become extinct.

 

99% of everything is extinct. All the proto-bats, all the proto-snakes, all the synapsid mammals, all the ostracoderm fish, all the euryapsid reptiles, all the toothed birds, all the amonites, all the trilobites, all the eurypterids, all the proto-whales, all the notoungulates, all the mesonychids, all the anapsid reptiles, all the acanthodian fish, all the belmenites, all the freshwater sharks, all the maadtsoiid snakes, and of course, all the non-avian dinosaurs.

 

What we have left today is a pale shadow of biodiversity, the tattered remnants of a tree of life that has lost most of its twigs and many of the large branches.

 

Also, where would these brackish waters in the ocean exist ? I am beginning to wonder if maybe there simply were no oceans, only huge freshwater (or, due to leaching, slightly brackish) lakes.

 

Most early tetrapods evolved in rivers or brackish regions where the river meets the sea, such a modern deltas and wetlands.

 

Also, we know for a fact that the oceans were salty at the time via geological evidence, as well as where rivers and deltas were and how salty they were.

 

It seems a strange co-incidence that at this time there was only one continent - Pangea. Maybe the expanding Earth theory should be revisited. Can you see what I am thinking...? I have become suspicious of the whole Wegener idea of plate tectonics - continents have to wiggle and rotate in freakish ways order for it to be correct. Using occams razor, the EET is far simpler - and works - exactly. The continents fit together exactly!

 

EET is also flat-out wrong - how can Earth expand without additional mass? And the fact that we've *directly observed* continental drift is rather a big issue. This is drifting off topic and into pseudoscience.

 

Few ancient fish fossils have been found on oceans floors - which are between 10 and 180 million years old. Fish fossils are almost exclusively found from contental rocks...

 

Because people live and breath on continents, not on ocean floors.

 

In fossil hunting, you traverse *vast* areas prospecting for bones - dozens of miles a day. If you're lucky, after a few weeks or months of this, you find one fossil. If you're unlucky, you can spend an accumulated time of several years without finding anything. On land, this can be done for free with volunteers. Underwater, it simply cannot be done, and that's ignoring the silt, sand and detritus covering the ocean floor.

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What would happen if an amphibian were to live in salt water for any period of time? Would they absorb too much salt through their skin and suffer health problems?

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They'd both dehydrate and absorb salts, probably leading to death within a minutes.

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What would happen if an amphibian were to live in salt water for any period of time? Would they absorb too much salt through their skin and suffer health problems?

 

I'm not sure how salty amphibians are, but I imagine they would lose water through osmosis to their surroundings faster than they would take up too much salt.

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The original amphibians evolved from fresh water fishes. It appears that they were 'preadapted' because the fish ancestors lived in pools and streams with little dissolved oxygen, requiring them to obtain oxygen from (initially) the richer film of water at the surface, and (later) from air itself. They adapted by developing highly vascular tissue at the roof of the mouth, where air would be held. This respiratory structure later evolved into lungs. They also preadapted by evolving fins that could hold them in shallow water so they could raise their heads to gulp air. Once those fins were strong enough, they could support the fish completely out of water - the road to becoming true amphibians.

 

The classic intermediate form is Tiktaalik, which meets those criteria for a fish on the way to becoming an amphibian.

http://en.wikipedia.org/wiki/Tiktaalik

 

A step further along in evolution was Acanthostega.

http://en.wikipedia.org/wiki/Acanthostega

 

However, the evolutionary path was via fresh water - not salt - which explains why there are no salt water amphibians.

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Um, sorry SL, but that's almost entirely wrong.

 

While many of the early tetrapods are from freshwater deposits, some are known from marine deposits, including some of the pre-tetrapod sarcopterygians such as Panderichthys. The geology of Tiktaalik may have been freshwater or a brackish water. In all likelyhood, they inhabited a wide range of salinities, mostly in coastal areas like modern river deltas.

 

Second, lungs are a basal feature of bony fish - all bony fish started out with lungs, and those that lack them now either lost them secondarily or later turned the lungs into swim bladders. This was to address the issue of nourishing the heart. In fish without lungs, the heart only gets deoxygenated blood, greatly reducing endurance. Chondrichthyes like sharks have independently evolved coronary arteries (blood vessels after the gills that return a small portion of oxygenated blood to the heart), while bony fish have lungs which feed oxygenated blood to the heart (though by dumping it in the main heart vein, not by coronary arteries).

 

Also, lungs did not evolve from the roof of the mouth, but rather from an outpocketing in the gut, and this can be seen in modern lung development in all tetrapods (which is controlled by the same genes for all of them).

 

The use of fins probably was for lifting the head up, though not completely out of the water until later species - early tetrapods still had spiracles, a pair of valved holes behind the eye which connect the outside to the interior of the mouth. (These spiracles are the vestige of the first gill slit, which was lost after the evolution of jaws, and is retained even in modern mammals as the Eustachian tube.) They probably also played an important role is moving tetrapods back into the water after launching themselves after insects on the shore, much like modern crocodiles do (please note that these insects included six-foot millipedes and spiders the size of your head).

 

Remember, the hyper-permeable skin of modern amphibians is a recent evolutionary trait. Tiktaalik has scales, and while other tetrapods lost some or all of these scales, a scale-less hide does not prevent tolerance of brackish or even marine waters, as the existence of entire families of marine catfish (which lack scales) testifies.


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I can't believe I forgot about this guy, Rana cancrivora, the only amphibian whose salt-water tolerance is known (apparently a species of toad also can tolerate brackish water, but hasn't been really studied well).

 

None can tolerate fully marine conditions for extended periods, but apparently have hypertrophied excretory systems that can deal with brackish water for extended periods.

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And by what means, exactly, do you propose that the Earth is expanding? :doh:

 

How about a diminishing gravitational constant?? I dunno!

 

However, it seems to me that the biggest obstacle to the EET is that there does not appear to be a mechanism for planetary expansion. However, all the other facts seem to fit very well, and in some cases, better than the plate tectonics theory. Just becasue we can't come up with a mechanism shouldn't mean the whole theory is dismissed out of hand.

 

I'm not saying I believe in the EET, but I really don't think it should mocked. Remember, Wegeners theory was mocked by the most eminent of scientists at first, and yet became the paradigm.


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EET is also flat-out wrong - how can Earth expand without additional mass?

 

Well, that is a very good question. But maybe rather than dismiss the theory we should figure this out first.

 

And the fact that we've *directly observed* continental drift is rather a big issue. This is drifting off topic and into pseudoscience.

 

Actually, evidence is still pretty scant. It's still very much a theory.

 

 

Because people live and breath on continents, not on ocean floors.

 

Ocean floors are at most only 180 million years old.


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you can't believe that a couple hundred species have died out but you do believe that the earth is magically gaining quadrillions(ridiculously low i know) of tonnes of material every century?

 

Mmmmm. Well, it does seem strange that EVERY saltwater amphibian died out - considering how many freshwater ones didn't, and how many other marine species from all vertebrate groups are still around.

 

The thing with the EET, everyone seems totally freaked by it simply because we know of no mechanism for planetary expansion. I am just not freaked out by it. It is a very big problem to explain, but so is the double slit expt.

 

The thing is that it would seem that the plates fit together EXACTLY on a smaller globe. EXACTLY! If this is true could it really be just coincidence?

Edited by bombus
Consecutive posts merged.

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Ok, this is *way* off topic. If you want to discuss this theory, start a thread on it.

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I'm fairly certain that the ocean(s) was not nearly as salty hundreds of millions of years ago as it is today.

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I'm fairly certain that the ocean(s) was not nearly as salty hundreds of millions of years ago as it is today.

 

Actually, it was saltier. See here. I got the full text of the article, and there is indeed a general decline in ocean salinity since the Cambrian. At the time of the origin of tetrapods, seawater was at roughly 4.5%, now it's at 3.5%.

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They haven't returned to the ocean much in part because their crustacean and other cousins would out-compete them, but also because it would 'waste' the adaptation behind most of their success, namely their wings.

I'm having trouble understanding what you mean by "waste the adaptation". Why is it a problem, from an evolutionary perspective?

 

Surely adaptations are wasted all the time, otherwise we'd never have vestigial anythings?

 

Couldn't insect wings also potentially re-adapt in the same way that penguin wings, for example, have?

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Well, consider where you see flightlessness evolve: usually in situations where there's few predators or competitors. Insects returning to the ocean, on the other hand, would face intense competition, both from crustaceans and from larvae of other insects.

 

Perhaps "waste" was the wrong word. Without their wings, insects would have no special advantage over other arthropods and would face increased competition. Only a few lineages have lost their wings, and those usually only do so because they have some other advantage, like eusocial ants. That doesn't preclude a return to the water, but it makes it more difficult. Only a few species have made that switch, largely because of competition.

 

As far as using insect wings underwater, it's doubtful. The flight of vertebrates like birds and bats relies on fairly simple laminar flow, like over an airplane wing, which makes the transition to a more viscous fluid easier. Insect flight, however, depends a lot more on vortices, turbulence, etc, which makes it hard to switch fluids. Plus, their wings are usually relatively light and fragile, which may prove problematic for dealing with heavy loads associated with water. The few insects that have returned to the water, such as diving beetles, actually retain their wings and flight to move between locations, but fold up their wings and use limbs only when moving underwater.

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The other possibility is to re-adapt the wings into another function. For example, they have the surface area for gills, or to act as an air trap - holding a bubble of air between wings and body.

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Perhaps "waste" was the wrong word.

I suppose I was being a bit pedantic. "Waste" seemed to imply deliberately evolved.

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You're correct that there are no saltwater amphibians, but the important thing to realize is that the modern amphibians are *very* different from the first tetrapods. Imagine how people would puzzle over mammals if rodents were the only remaining lineage; that's pretty much how it is with amphibians.

 

The very first amphibians probably lived in brackish water, but had scales and retained gills even as adults. Even once they became completely terrestrial, they likely retained scales for a long while, which is where reptile scales come from.

 

The loss of scales and the gain of poison glands happened only in one particular group of amphibians, and unfortunately, that's the only group which survives today.

 

So basically, there are no modern saltwater amphibians, because of their permeable skin, but their ancestors had scales and at least some are *definitely* known from brackish or salt water.

 

 

 

interesting topic. i'd wondered about this and once thought it possible that the oceans must have been a lot less salty.

 

So basically the earliest vertebrate land invaders were amphibious, but not amphibians as we know them today?

 

they had scales and a non-porous skin and retained gills throughout their life cycle ... not a lot there to link them to frogs apart from the fact that they could live in and out of water.

 

is it possible that today's frogs, toads, caecilians, newts and salamanders evolved separately from a freshwater ancestor and are only distant cousins to the first tetrapods?

Edited by caz

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is it possible that today's frogs, toads, caecilians, newts and salamanders evolved separately from a freshwater ancestor and are only distant cousins to the first tetrapods?

 

No, we have quite strong evidence that tetrapods only left the water once, and all modern tetrapods evolved from them. Modern amphibians (technically called "Lissamphibia") are just a highly specialized lineage who evolved a hyper-permeable skin loaded with toxin glands.

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No, we have quite strong evidence that tetrapods only left the water once, and all modern tetrapods evolved from them. Modern amphibians (technically called "Lissamphibia") are just a highly specialized lineage who evolved a hyper-permeable skin loaded with toxin glands.

 

thanks for that. never ceases to amaze me and i must do some reading on this subject.

 

i guess what might have happened is that those ancient amphibians were drawn to estuaries, tidal swamps and river mouths and over time evolved a tolerance for brackish/fresh water that way.

 

as an aside, though there are no saltwater amphibians today, there are quite a few amphibious saltwater creatures including crocodiles, turtles, penguins, mudhoppers and seals.

 

one wonders whether these have filled ecological niches that once belonged to amphibians.

Edited by caz

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thanks for that. never ceases to amaze me and i must do some reading on this subject.

 

I recommend Gaining Ground by Jennifer Clack, one the leading researchers on early tetrapods. She's very good, and her writing is very understandable.

 

as an aside, though there are no saltwater amphibians today, there are quite a few amphibious saltwater creatures including crocodiles, turtles, penguins, mudhoppers and seals.

 

one wonders whether these have filled ecological niches that once belonged to amphibians.

 

In the case of crocodiles, it seems very likely. Early tetrapods are often reconstructed as having a crocodile-like lifestyle (preying on the large insects that colonized land before them), and very crocodile-like forms evolved more than once, namely Prionosuchus, Archegosaurus, and Melosaurus.

 

After they died out, a group of non-crocodile reptiles occupied the niche, the Phytosaurs, and only once they vanished did modern crocodiles take their present form (previously, they looked like reptilian greyhounds).

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