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What is a tetrapod?
Topic Started: Jun 13 2017, 07:27 PM (1,742 Views)
IIGSY
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On another thread, the subject of the boundaries of "tetrapoda" was brought up.

There is two conflicting views. Anything with recognizable feet is a tetrapod, including the basal ichthyosteglians and other assorted animals.

The second view (and the one wikipedia goes with) is that tetrapoda is only the crown group, with the wider group being "Stegocephalia"

What do you think?
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Tri-devonian: A devonian like ecosystem with holocene species on three different continents.

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LittleLazyLass
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I've never really heard of the view that stuff like Ichtyostega doesn't count, and it sounds really stupid to me. Crown group definitions always really bothered me.
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IIGSY
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Little
Jun 13 2017, 07:49 PM
I've never really heard of the view that stuff like Ichtyostega doesn't count, and it sounds really stupid to me. Crown group definitions always really bothered me.
https://en.wikipedia.org/wiki/Tetrapodomorpha#Relationships

Take a look
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Punga: A terraformed world with no vertebrates
Last one crawling: The last arthropod

ARTH-6810: A world without vertebrates (It's ded, but you can still read I guess)

Potential ideas-
Swamp world: A world covered in lakes, with the largest being caspian sized.
Nematozoic: After a mass extinction of ultimate proportions, a single species of nematode is the only surviving animal.
Tri-devonian: A devonian like ecosystem with holocene species on three different continents.

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Mitosis
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According to the crown group definition, what is a Tetrapod is defined not by morphology, but by ancestry. Only the common ancestor of Lissamphibia+Amniota (or frogs and princes) and all of its descendants are members of the group Tetrapoda, while every other prehistoric four-legged creature (no matter how large degree of a terrestial adaptation it had) is classified as a Tetrapodomorph.

All in all, the phylogeny of non-Lissamphibian, non-Amniote terrestrial vertebrates is a mess, because we don't exactly know where the extant groups came from. Lissamphibia could have come from either Temnospondyls (crocodile-like creatures) or Lepospondyls (salamander or eel-like creatures). Frogs and Salamanders could have come from Temnospondyls, and Caecilians could have come from Lepospondyls, making Lissamphibia polyphyletic. Reptiliomorphs (the ancestors of Amniotes) could have been a sister group of early Tetrapodomorphs, evolving towards a more terrestrial condition in parallel with them, or they could be a sister group to Lepospondyls.

What we know for sure is that lots of lizard, salamander, and crocodile-like creatures were roaming around the Carboniferous, one group eventually spawned modern Lissamphibians, the other modern Tetrapods.


It is interesting to note that modern Tuataras (the most basal living amniotes) have a nervous system more similar to that of modern Lissamphibians, than to other living Tetrapods, which may imply that Lissamphibians and Reptiliomorphs share a close common ancestry, and both developed from a line more closely related to the less successful, more basal, and more aquatic Lepospondyls, rather than the more terrestrial Temnospondyls, which must have evolved a more Amniote-like nervous system convergently to cope with their more terrestrial habitat.

The amniotic egg likely evolved the amniotic membranes gradually, therefore a small, frail creature with small eggs (where gas diffusion could work better while the amnion and chorion were still in a more basal state) is a better candidate for the origin of Amniotes than a large crocodile-like animal, supporting the theory of a close ancestry of Lissamphibians and Reptiliomorphs and a general Lepospondyl affinity for both groups.
Edited by Mitosis, Jun 14 2017, 04:46 PM.
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IIGSY
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Mitosis
Jun 14 2017, 04:34 PM
The amniotic egg likely evolved the amniotic membranes gradually, therefore a small, frail creature with small eggs (where gas diffusion could work better while the amnion and chorion were still in a more basal state) is a better candidate for the origin of Amniotes than a large crocodile-like animal, supporting the theory of a close ancestry of Lissamphibians and Reptiliomorphs and a general Lepospondyl affinity for both groups.
That would place temnospondyls outside tetrapoda, and that would destroy the purpose of even having a "tetrapoda". Trying to figure out what falls in and out of the crown group is a real pain in the ass, so going by this definition is counterproductive as it would pointlessly exclude animals that would otherwise definitely be tetrapods. Tetrapoda should be equivalent to "Stegocephalia", because it includes everything more crownward than Tiktaalik*
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*Well, it depends on the author. In some places (Including wikipedia) it does include tiktaalik. I'v even seen include panderichthys one time. But anything beyond that is a no-no. I personally think we should draw the line between tiktaalik and elginerpeton.
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Punga: A terraformed world with no vertebrates
Last one crawling: The last arthropod

ARTH-6810: A world without vertebrates (It's ded, but you can still read I guess)

Potential ideas-
Swamp world: A world covered in lakes, with the largest being caspian sized.
Nematozoic: After a mass extinction of ultimate proportions, a single species of nematode is the only surviving animal.
Tri-devonian: A devonian like ecosystem with holocene species on three different continents.

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Mitosis
Jun 14 2017, 04:34 PM
Frogs and Salamanders could have come from Temnospondyls, and Caecilians could have come from Lepospondyls, making Lissamphibia polyphyletic.
What? So it has actually been suggested that Lissamphibia is not a valid clade?


On crown groups I agree somewhat with Little's view on the matter. It does seem somewhat arbitrary to base the definitions of these groups on which creatures survive today. One good example is how things like morganucodonts and docodonts are seen as not being mammals, as they are outside the crown group, yet if they lived today they would almost certainly be seen as mammals.
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IIGSY
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Tartarus
Jun 14 2017, 07:36 PM
On crown groups I agree somewhat with Little's view on the matter. It does seem somewhat arbitrary to base the definitions of these groups on which creatures survive today. One good example is how things like morganucodonts and docodonts are seen as not being mammals, as they are outside the crown group, yet if they lived today they would almost certainly be seen as mammals.
Exactly. Or excluding fuxianhuids and megacheirans from arthropods because they are not in the crown group (well, megacheirans may be closer to chelicerates, but that's a whole nother can of worms)
Projects
Punga: A terraformed world with no vertebrates
Last one crawling: The last arthropod

ARTH-6810: A world without vertebrates (It's ded, but you can still read I guess)

Potential ideas-
Swamp world: A world covered in lakes, with the largest being caspian sized.
Nematozoic: After a mass extinction of ultimate proportions, a single species of nematode is the only surviving animal.
Tri-devonian: A devonian like ecosystem with holocene species on three different continents.

Quotes


Phylogeny of the arthropods and some related groups


In honor of the greatest clade of all time


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All African countries can fit into Brazil
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LittleLazyLass
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Paraphyletic Lissamphibia has been suggested, by my impression is that it's a fairly dead theory at this point. To my knowledge a temnospondyl identity has been favored recently, but it's not my area of expertise.
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Reptiliomorphs (the ancestors of Amniotes) could have been a sister group of early Tetrapodomorphs, evolving towards a more terrestrial condition in parallel with them
As far as I know this idea never held much water.
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Tuataras (the most basal living amniotes)
This is flat wrong, they're the sister group to lizards. No more "basal" than any given snake, archosaur, or mammal.
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Only the common ancestor of Lissamphibia+Amniota (or frogs and princes) and all of its descendants are members of the group Tetrapoda
I swear I never saw this be the consensus before, but wikipedia has indeed switched to this interpretation. Either way, it's a blatantly unhelpful way to classify things: by any reasonable definition something like Cassigyrinus is a tetrapod. Now everyone has to start using "stem-tetrapod" all the time.
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I don't even really like this song that much but the title is pretty relatable sometimes, I guess.
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IIGSY
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Little
Jun 14 2017, 07:48 PM
Quote:
 
Only the common ancestor of Lissamphibia+Amniota (or frogs and princes) and all of its descendants are members of the group Tetrapoda
I swear I never saw this be the consensus before, but wikipedia has indeed switched to this interpretation. Either way, it's a blatantly unhelpful way to classify things: by any reasonable definition something like Cassigyrinus is a tetrapod. Now everyone has to start using "stem-tetrapod" all the time.
How do you view tiktaalik in this regard?
Projects
Punga: A terraformed world with no vertebrates
Last one crawling: The last arthropod

ARTH-6810: A world without vertebrates (It's ded, but you can still read I guess)

Potential ideas-
Swamp world: A world covered in lakes, with the largest being caspian sized.
Nematozoic: After a mass extinction of ultimate proportions, a single species of nematode is the only surviving animal.
Tri-devonian: A devonian like ecosystem with holocene species on three different continents.

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Phylogeny of the arthropods and some related groups


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LittleLazyLass
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I always remember that being considered a tetrapodomorph.
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I don't even really like this song that much but the title is pretty relatable sometimes, I guess.
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IIGSY
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Little
Jun 14 2017, 08:22 PM
I always remember that being considered a tetrapodomorph.
Well, I'v heard some people call it a tetrapod. For me, and probably you, recognizable feet=tetrapod
Projects
Punga: A terraformed world with no vertebrates
Last one crawling: The last arthropod

ARTH-6810: A world without vertebrates (It's ded, but you can still read I guess)

Potential ideas-
Swamp world: A world covered in lakes, with the largest being caspian sized.
Nematozoic: After a mass extinction of ultimate proportions, a single species of nematode is the only surviving animal.
Tri-devonian: A devonian like ecosystem with holocene species on three different continents.

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Phylogeny of the arthropods and some related groups


In honor of the greatest clade of all time


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Mitosis
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I know Tuataras are not basal in a phylogenetic sense (they are firmly within Diapsids and Lepidosauria), but they are the ones that preserved the largest number of plesiomorphic traits from the common ancestor of all Amniotes.

Mammals and Diapsids all have nervous systems that are more complex than that of Lissamphibians, even lizards, which as you noted, are very close to Tuataras, but the fact that there is little difference between the Tuatara and the Lissamphibian nervous system suggests that all Amniotes evolved a more complex nervous system in parallel with eachother only way after the Synapsid-Sauropsid split.

Another trait that could suggest a close Lissamphibian-Amniote affinity is the presence of the Allantois in both groups. The Allantois is an amniotic membrane that collects nitrogenous waste in the developing embryo. Fish (even Lungfish) don't have this structure, but Lissamphibians have a small Allantois while lacking the other amniotic membranes, which suggests that Lissamphibians split from a line that was already evolving towards the Amniotic condition.

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Well, I'v heard some people call it a tetrapod. For me, and probably you, recognizable feet=tetrapod


Strict crown-group definitions rely completely on ancestry, while disregarding morphology. For example, the first crown-group Amniote is not the first creature that laid an Amniotic egg, but the last common ancestor of Mammals and Birds.

Similarily, the first Tetrapod is not the first Vertebrate that walked around on four legs, but the last common ancestor of Amniotes and Lissamphibians. If Amniotes and Lissamphibians share a close, Lepospondyl affinity, then no matter how terrestrial and crocodile-like Temnospondyls were, they are not Tetrapods.
Edited by Mitosis, Jun 15 2017, 11:16 AM.
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IIGSY
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Mitosis
Jun 15 2017, 10:45 AM
Similarily, the first Tetrapod is not the first Vertebrate that walked around on four legs, but the last common ancestor of Amniotes and Lissamphibians. If Amniotes and Lissamphibians share a close, Lepospondyl affinity, then no matter how terrestrial and crocodile-like Temnospondyls were, they are not Tetrapods.
This is why a crown based definition is extremely counter-productive. So many things that would otherwise be tetrapods are excluded for this dumb reason. Classifying organisms based on which ones are alive today is stupid. Hell, is monotremones where extinct, they wouldn't be classified as mammals.


Plus, how do we know that amniotes are closer to lepospondyls than temnospondyls?
Projects
Punga: A terraformed world with no vertebrates
Last one crawling: The last arthropod

ARTH-6810: A world without vertebrates (It's ded, but you can still read I guess)

Potential ideas-
Swamp world: A world covered in lakes, with the largest being caspian sized.
Nematozoic: After a mass extinction of ultimate proportions, a single species of nematode is the only surviving animal.
Tri-devonian: A devonian like ecosystem with holocene species on three different continents.

Quotes


Phylogeny of the arthropods and some related groups


In honor of the greatest clade of all time


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Mitosis
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Introducing morphological characteristics to define clades also introduces ambiguity in what constitutes a clade and where do we draw the line between two clades.

For example, imagine that you visit the Carboniferous with a time machine. You start studying a small animal, which is rather similar to a modern lizard, but also different. It has keratinous claws, there is a little differentiation in its teeth, you can clearly see proto-canines and proto-molars, it has a skin that is thicker than that of modern lissamphibians, but somehow also soft and a bit slimy, not at all modern lizard like.

At this point, everything seems to spell out "The First Synapsid" or at least a creature that is definitely an Amniote.

However, ambiguity starts setting in quickly: You put it under an X-ray machine, and see that it has a box-like skull with no fenestra, it gulps down large amounts of water frequently, and it urinates a lot.

Eventually, you get to see how it reproduces: The male mounts the female with a "cloacal kiss".

The female finds a muddy puddle and lays a large amount of small eggs in it.

You put one of the eggs under a microscope and observe its development:
The embryo forms, it has an amnion, a chorion, and an allantois, but all three of them are rather underdeveloped and seem more like random mutations that stuck, than true, functioning amniotic membranes.

The mother sometimes returns to the eggs to urinate on them, when the puddle starts drying up.

The embryo forms into a tadpole that seems to be capable of free living, but before it hatches, it transforms from a tadpole into the adult form. The hatchling emerges from the egg, it is air-breathing, has no external gills, and it is ready to start life outside of water.


Now, what kind of animal have you seen? If we go with the morphological definition of an Amniote we are stuck with "It may be an Amniote or may be not", if we go with the crown-group definition, it is definitely not an Amniote.

Since evolution happens gradually and not overnight, morphological characteristics don't help us very much in clearly delimiting clades.

The animals we observed may have been the great-great-great-great-grandparents of Bob the Basal Amniote, but it could be a species belonging to a different clade that died out, maybe a basal clade on the line towards crown-group Amniotes.

Quote:
 
Plus, how do we know that amniotes are closer to lepospondyls than temnospondyls?


I remember reading a study (don't know where sorry) that Lepospondyls and Reptiliomorphs did a lot of experimentation on either becoming pedomorphic (retaining a modified tadpole stage into adult life) or peramorphic (direct development to an adult form while minimizing the time spent as a tadpole) and this could have given rise to a lot of evolutionary flexibility.

Temnospondyls on the other hand pretty much stuck with the tried-and-true method of tadpole young and semi-aquatic crocodilian adults.
Edited by Mitosis, Jun 16 2017, 01:52 AM.
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IIGSY
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If I saw such an animal, I'd classify it as a basal amniote. But still an amniote nonetheless.

Quote:
 
Introducing morphological characteristics to define clades also introduces ambiguity in what constitues a clade and where do we draw the line between two clades.
People really underestimate the helpfulness of morphological characteristics. Hell, dinosaurs are defined based on morphology. Crown group definitions are not exempt, as you still need characteristics to figure out weather or not they belong in the crown group.

In reality, all classifications are arbitrary and no one is perfect. As you said, a purely morphology based "tetrapoda" can be ambiguous at times, but a crown group definition for tetrapods is even worse. It pointlessly excludes things that would otherwise that would otherwise be tetrapods under any reasonable definition. Same for other groups as well.

As the relative position of chelicerates, mandibulates, and artiopods relative to one another is very uncertain, a strict crown based definition of arthropoda could possibly exclude trilobites, which is extremely pandenic and counterproductive. Plus, trying to figure out weather something falls in or out of the crown group is a real pain in the ass, as is the case with that whole lissamphibia mess.
Edited by IIGSY, Jun 15 2017, 03:07 PM.
Projects
Punga: A terraformed world with no vertebrates
Last one crawling: The last arthropod

ARTH-6810: A world without vertebrates (It's ded, but you can still read I guess)

Potential ideas-
Swamp world: A world covered in lakes, with the largest being caspian sized.
Nematozoic: After a mass extinction of ultimate proportions, a single species of nematode is the only surviving animal.
Tri-devonian: A devonian like ecosystem with holocene species on three different continents.

Quotes


Phylogeny of the arthropods and some related groups


In honor of the greatest clade of all time


More pictures


Other cool things


All African countries can fit into Brazil
Offline Profile Quote Post Goto Top
 
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