tag:blogger.com,1999:blog-5821098719340852065.post1221245635971659957..comments2024-03-25T09:31:36.926+01:00Comments on Furahan Biology and Allied Matters: Alternate Future Evolution in JapanSigmund Nastrazzurrohttp://www.blogger.com/profile/16449461215427527447noreply@blogger.comBlogger6125tag:blogger.com,1999:blog-5821098719340852065.post-6298845116267821882010-04-09T02:24:16.611+02:002010-04-09T02:24:16.611+02:00Ah, thank you for this clarification. Really thoug...Ah, thank you for this clarification. Really though, I was only contradicting the statement that there have never been any vertebrates with only two limbs. The whole concept of limb mutability is extremely interesting, however.Waqualbushttps://www.blogger.com/profile/16043726517524474146noreply@blogger.comtag:blogger.com,1999:blog-5821098719340852065.post-10049676594950438762010-04-02T14:03:44.136+02:002010-04-02T14:03:44.136+02:00Hello Luke,
There are more examples than just moa...Hello Luke,<br /><br />There are more examples than just moa's: as Anonymous wrote, there are whales, snakes, legless lizards, etc. <br /><br />The issue is not whether or not the number of legs is mutable -it is- but how you get there. Whales and snakes got to where they are by adaptation to very different environments. Changing the limb number was part of that adaptation. The run of the mill mammal or reptile did not need to adapt to extremely different circumstances, so its original number of limbs remained as it was. <br /><br />Back to land-living cephalopods; adapting to a life on dry land would qualify as the kind of special circumstance in which changing the number of life might prove useful. You could have an octopus keep 6 arms/legs for locomotion and two for manipulation. Once that major redesign has been done, I do not think there would be much need to change the number again, provided the animals more or less keep to their lifestyle.<br /><br />What I find less likely is that the number should change without any major changes in life style. Exactly what constitues 'major' remains open to discussion.Sigmund Nastrazzurrohttps://www.blogger.com/profile/16449461215427527447noreply@blogger.comtag:blogger.com,1999:blog-5821098719340852065.post-85762674495582469412010-04-01T00:20:52.392+02:002010-04-01T00:20:52.392+02:00Actually, at least one two-limbed animal has evolv...Actually, at least one two-limbed animal has evolved, that being the giant flightless bird known as the moa. in the course of evolution it lost all trace of its front limbs, including vestigial bones, as the picture in this link shows. http://upload.wikimedia.org/wikipedia/commons/0/0a/Moa_skeleton.JPGWaqualbushttps://www.blogger.com/profile/16043726517524474146noreply@blogger.comtag:blogger.com,1999:blog-5821098719340852065.post-35021932397401930972010-03-27T04:35:17.395+01:002010-03-27T04:35:17.395+01:00The number of limbs is pretty mutable in vertebrat...The number of limbs is pretty mutable in vertebrates too. Most of the "agnathan" fish have no fins or two fins, some of the amphisbaenids have no legs or two legs, snakes have none, whales have virtually two legs (hind legs being vestigial), most tetrapods have four, fish have a potential fifth limb in the anal fin, and acanthodians could have more than three pairs of fins. It just seems like that the lineage that lead to tetrapods somehow got locked into four limbs (or else four limbs was chosen over three or six limbs by sheer luck, and the limbs remained generally conservative in number because in general the benefits of six limbs don't outweigh the liabilities of having to mess with your development to get said limbs).<br /><br />Perhaps the reason why we don't really see any two-limbed vertebrates (like Dixon's wakka as opposed to the amphisbaenids of Bipedidae) is that having a pair of grabbity appendages is just too good to pass up. I mean even those bipeds that seemed to be on the way to losing their arms, like abelisaurids and tyrannosaurids, evolved from longer-armed ancestors.<br /><br />If by some chance cephalopods did manage to invade land and get a toehold there, my money would be on the cuttlefish, because they have the beginnings of an endoskeleton that could be modified (albeit greatly) to support the animal's weight on land. From there, it seems likely that any land-squid would have but seven tentacles, because one of these tentacles is used in reproduction. Not only is it a good idea to protect your ability to reproduce, but internal fertilization is a big necessity for any wanna-be land colonizer (that actually wants to succeed). Perhaps four tentacles could go on to become the legs (or else four tentacles and derivatives of the side fins or something), and some of the remaining ones would either go to form tentacle-arms or trunk-like appendages. And it goes without saying that the former of these (the legs) would have some sort of bony support.Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-5821098719340852065.post-45009783659006722142010-03-25T23:08:57.066+01:002010-03-25T23:08:57.066+01:00Hi Anonymous,
That might be so. I tried to look u...Hi Anonymous,<br /><br />That might be so. I tried to look up what is known about the evolution of the number of arms/tentacles in cephalopods. Except for finding that the 'original' number is presumably ten, nothing much seems to be known about that. I had assumed that the number of legs might be fairly fixed in a specific body plan. In vertebrates, it is four, which holds for all the animals you mentioned except for the snake. But, seeing that existing gropus of cephalopods have different numbers anyway, perhaps the number is more mutable in them.Sigmund Nastrazzurrohttps://www.blogger.com/profile/16449461215427527447noreply@blogger.comtag:blogger.com,1999:blog-5821098719340852065.post-85974915380316836552010-03-22T00:07:51.909+01:002010-03-22T00:07:51.909+01:00"Instead, their resemblance suggests that var..."Instead, their resemblance suggests that various groups of cephalopods all made it onto land, bringing their different body plans with them. Not impossible, but what would drive such a tendency?"<br /><br />Or, the ancestral terrasquid had some basic body plan (perhaps similar to a more terrestrially adapted 'Purototerasukuido'), and then the body plan has changed radically as the squids radiate into different niches like cursorial runners, branchiating climbers, etc. I mean look how much variation you see in tetrapods. Compare a turtle to a snake/amphisbaenid to a bird to a gibbon to a horse. Once a particular body plan is established, there's a lot of wiggle room to alter the body's structure and occupy different niches.Anonymousnoreply@blogger.com