caz

wow. that's convergent evolution demonstrated perfectly. it must be that there simply is no better design for an amphibious swamp ambush predator. on crocs, there's a parallel of sorts with amphibians. australia's top end has two types, the large, dangerous "salties" and their smaller, relatively harmless cousins, the "freshies". while the saltwater crocs are equally at home in fresh or salt water, the "freshies" won't go near salt water if they can avoid it, presumably because they'll become prey. they've moved a long way inland in the monsoonal climate and inhabit rock pools and fresh water streams. so i guess something similar might have happened with "lissamphibia". they became specialists. sadly, the "freshies" are in danger of extinction because the introduced cane toads they have started eating are lethal to them. the indestructable "salties", however, can consume the toads with no ill effects.

thanks, i will definitely have a look at that.

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.

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?

wouldn't have a problem with resurrecting the thylacine (tasmanian tiger) or any of the australian marsupials we've condemned to extiction in the past few hundred years. there are ethical considerations, but i think you can make a compelling argument that we should do it if the means exist. their time wasn't up. nothing in their evolutionary history prepared them for man (particularly the europeon invasion) and the diseases and introduced species (many of which are now feral and should be culled) he bought with him. as a sidenote, it is appalling that you tend to hear more about the thylacine than the Tasmanian Aboriginals, who were also hunted and driven to extinction by the white man. would you bring them back? No, i think. For one thing they weren't a separate species and for another their culture (which is what made them unique) is largely forgotten to history. sadly I think we've just got to live with that one. the neanderthals is an interesting one. though the scientific curiosity of having another species of humans around would be immense, i think ethical considerations would prevail and we'd have to resist the temptation.

i also find it interesting that the marsupial embryo reportedly has a sort of a vestigial egg shell and an egg-tooth ... both fully functional features of baby monotremes. if that's true then, reproductively at least, it's not that big an evolutionary leap between an echidna (which lays its tiny eggs in its pouch) and a typical marsupial (which discards the egg shell within its body and guides it's tiny young into the pouch). monotremes and marsupials also share epipubic bones, which i've done a bit of reading up on. i found it interesting that the extinct thylacine (which had a pouch) had only the vestiges of them and certain pouchless marsupials still have the real deal. go figure! perhaps they are more related to the locomotor function.

echidnas have pouches but not nipples. so in their case pouches came first. if the pouch did evolve only once as you say, it would suggest that marsupials descended from something like the pouched monotreme echidna.

it's complicated. the earliest marsupial fossil (125 million year old) was found in china and the ones found in australia are much younger. others have been found in the americas and the only mammal fossil found in the antarctic was of a marsupial. http://www.sciteclibrary.ru/eng/catalog/pages/6763.html

as i understand it, marsupials such as kanagaroos have nipples but monotremes (platypus and echidna) do not. so, did the placentals descended directly from marsupials or from a monotreme-like common ancestor? current thinking seems to be that they developed at about the same time in different parts of the world. but, to me, the marsupials seem more ancient.

absolutely. both do, but the monotremes had not yet evolved nipples and that is another thing that suggests how ancient their line is.

hi. i'm a first-time poster on here and i'd like to point out straight away that i don't have a science background and am a layman who is fascinated by natural history. was wondering whether anyone could tell me what the best current thinking is on how and when the mammals split into the three groups we have today - the placentals, the marsupials and the monotremes. in particular, do the marsupials date back further than the placentals and, if so, did the placentals descend directly from them or, more likely, diverge later from a common ancestor. i asked a similar question on another forum and a couple of guys on there were quite helpful in telling me that the fossil evidence is ambiguous and, basically, no one knows. so it would seem that we know more about the dinosaurs than about our closer mammalian cousins. common sense tells me that the monotremes (platypus and echidna) are the most ancient of the groups that are still around. they lay eggs and have other features that place them closer to birds and reptiles. the same thing tells me that the marsupials pre-date the placentals and split from a common ancestor before mammals had the ability to develop a proper placenta. both the monotremes and marsupials suckle young that seem almost embryonic and it's interesting that the female echidna (the forgotten monotreme) develops a simple pouch in which to lay her egg. The "puggle" (as i believe a little echidna is known) is not much bigger than a jellybean when hatched and is carried around in her pouch for about three months. I do think marsupials are a more ancient lineage than placentals, but, the fossil record seems to be ambiguous. any thoughts on this or good, simple further reading would be appreciated.