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What is a tetrapod?
Topic Started: Jun 13 2017, 07:27 PM (1,746 Views)
Inceptis
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Part of the whole point of crown groups is that cladistics should ideally be as monophyletic as possible. In real life, that's impossible because we like to associate scientific names with informal names, and the longer we've associated them, the harder it is to unassociate them. You can find examples of such paraphyletic clades from Tetrapoda and Reptilia, to numerous arthropod clades, and all the way back to Porifera. In such cases of popular naming, we're not going to suddenly change our view of tetrapods from Tetrapoda to Stegocephalia. It simply makes it harder for the majority to grasp, which is the whole point of communication and naming things.

Think of it this way: If all cladistics were changed based on crown grouping and monophyly, it would make it easier for scientists to get a grasp of but harder for civilians to understand. If it's based on morphological analysis, it's easier for non-scientists to understand, but more of a pain for scientists. So, the cladistics community does their best to compromise, resulting in some groupings that are less than satisfactory for scientists.
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Tartarus
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On what is and isn't a tetrapod I do find it odd to exclude such creatures as Icthyostega and Acanthostega as I reckon they would be considered very early stage tetrapods if one is looking at their morphology and physiology rather than just at whether they fit a crown group. I can agree with excluding Tiktaalik though as it still had fins, albeit fins that could be used for moving on land as well as in water. Some have used the term "fishapod" to describe it. This is not a cladistic term mind you, but it does emphasise how this creature had a blend of fish and tetrapod features (and yes I know "fish" has no cladistic meaning either).
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IIGSY
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Inceptis
Jun 15 2017, 06:31 PM
Part of the whole point of crown groups is that cladistics should ideally be as monophyletic as possible. In real life, that's impossible because we like to associate scientific names with informal names, and the longer we've associated them, the harder it is to unassociate them. You can find examples of such paraphyletic clades from Tetrapoda and Reptilia, to numerous arthropod clades, and all the way back to Porifera. In such cases of popular naming, we're not going to suddenly change our view of tetrapods from Tetrapoda to Stegocephalia. It simply makes it harder for the majority to grasp, which is the whole point of communication and naming things.

Think of it this way: If all cladistics were changed based on crown grouping and monophyly, it would make it easier for scientists to get a grasp of but harder for civilians to understand. If it's based on morphological analysis, it's easier for non-scientists to understand, but more of a pain for scientists. So, the cladistics community does their best to compromise, resulting in some groupings that are less than satisfactory for scientists.
Just because it's not a crown based definition doesn't mean it still can't be monophyletic. I mean, if we discovered an amphibian-like creature that was descended from lungfish or rhizodonts, then it's not a tetrapod. But all the non-crown tetrapods are very close to crown tetrapods with no "fish" in between, so all logic points to them being tetrapods.
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Nematozoic: After a mass extinction of ultimate proportions, a single species of nematode is the only surviving animal.
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LittleLazyLass
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Hell, dinosaurs are defined based on morphology.

This is false, they're generally defined as the most recent common ancestor of a handful of species (it varies which ones), it just so happens only one if any of these are ever living taxa (Passer domesticus). Either way, I agree with the sentiment: anything laying amniotic eggs clearly makes sense to classify with everything else that does. Just because one group happened to survive to the present doesn't mean the others should be left out.

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Part of the whole point of crown groups is that cladistics should ideally be as monophyletic as possible. In real life, that's impossible because we like to associate scientific names with informal names, and the longer we've associated them, the harder it is to unassociate them. You can find examples of such paraphyletic clades from Tetrapoda and Reptilia, to numerous arthropod clades, and all the way back to Porifera. In such cases of popular naming, we're not going to suddenly change our view of tetrapods from Tetrapoda to Stegocephalia. It simply makes it harder for the majority to grasp, which is the whole point of communication and naming things.

Think of it this way: If all cladistics were changed based on crown grouping and monophyly, it would make it easier for scientists to get a grasp of but harder for civilians to understand. If it's based on morphological analysis, it's easier for non-scientists to understand, but more of a pain for scientists. So, the cladistics community does their best to compromise, resulting in some groupings that are less than satisfactory for scientists.

I'm not sure what you're trying to say here. It's perfectly possible to use a stem- or node-based definition of Tetrapoda that's not a crown-group and still be perfectly monophyletic, easy to understand (it have four legs, be tetrapod), and convenient for scientists. A stem-based definition would include some stuff most people would prefer to call tetrapodomorphs, but a node-based one (like Dinosauria) could easily be roughly equivalent to a non-paraphyletic Stegocephalia (which includes stuff traditionally considered tetrapods, contra what you say about shifting our views; we also did completely change our definition of Reptilia, even though I still think we should use Sauropsida).
Edited by LittleLazyLass, Jun 16 2017, 10:00 AM.
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Mitosis
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The whole reason for using crown-, rather than stem-based clades arises from the fact that we are incapable of time travel, and thus unable to observe all the morphological characteristics of the extinct creatures we study.

Anchoring Tetrapoda, Amniota, or Placentalia to extant species, thus making crown-groups may leave out basal animals that happened to have respectively four terrestrial legs, an amniotic egg, and a true placenta, but at least it can be a safeguard that we don't include animals, which happened to lack these structures inside the group.

Yes, crown-groups are also -at least partly- based on morphology. But morphology based on crown groups follows Occam's Razor:

It is more likely for temporal fenestra to have arisen only once, rather than multiple times, therefore, a reptile-like creature that happens to have no fenestra, but has remaining fossil features that are similar to both Sauropsids and Synapsids is classified to be outside crown-group Amniota.

There were lots of Eutherian Mammals in the Mesozoic whose skeletal configuration is more similar to those of extant Eutherians than to those of extant Metatheres and Monotremes. However, if they can't be clearly assigned to either Boreutheria or Atlantogenata (the two clades of extant Eutherians) then they are not a member of Placentalia, but occupy a more basal position among Eutheria, especially if we find remains of the Epipubic Bones, whose presence would prevent a placental pregnancy.
Edited by Mitosis, Jun 16 2017, 01:34 PM.
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LittleLazyLass
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Anchoring Tetrapoda, Amniota, or Placentalia to extant species, thus making crown-groups may leave out basal animals that happened to have respectively four terrestrial legs, an amniotic egg, and a true placenta, but at least it can be a safeguard that we don't include animals, which happened to lack these structures outside the group.
What? What are you trying to say here? On paper it makes sense, saying that non-crown definitions of something like Tetrapoda or Amniota could lead to inclusion of organisms lacking the defining traits. But applying this it makes no sense at all in this case. What you're essentially saying it that we should leave stegocephalians out of Tetrapoda on the off-chance they all evolved four legs independently and the grade contained some members who at no point in their evolution had the four-legged anatomy associated with tetrapods (unless snakes don't count either), even though every stegocephalian known does have this anatomy and everything we know about tetrapod evolutionl suggests this was ancestral to the grade?
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Mitosis
Jun 16 2017, 11:32 AM
find remains of the Epipubic Bones, whose presence would prevent a placental pregnancy.
If that's the case, then yeah it wouldn't be a placental.

Plus, is we found out the ichthyostega actually came from rhizodonts (which is astronomically unlikely) than yeah, it's not a tetrapod. But as far as we know, the tetrapod like features in basal stegocephalians are homologous with those in crown tetrapods, so they're tetrapods.

A stem based definition of tetrapod isn't very good either, as that would include things like rhizodonts and kenichthys, which is way too much of a stretch. So a node based definition is best. It should go like this.
Tetrapodamorpha
-Tetrapoda
--Eutetrapoda (crown group)

A crown or stem based definition would mean that either birds are the only dinosaurs, or all avemetatsaralians are dinosaurs.
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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Inceptis
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Yeah, I'm not sure I understood the discussion now. I guess I was saying that rearranging the cladistics would cause some initial issues with recognization, but I had so much tunnel vision I didn't even think about altering the names of groups, which seems to be what you're suggesting. Correct me if I'm wrong.
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IIGSY
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Inceptis
Jun 16 2017, 02:24 PM
Yeah, I'm not sure I understood the discussion now. I guess I was saying that rearranging the cladistics would cause some initial issues with recognization, but I had so much tunnel vision I didn't even think about altering the names of groups, which seems to be what you're suggesting. Correct me if I'm wrong.
Exactly.
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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Jun 16 2017, 12:56 PM
What? What are you trying to say here? On paper it makes sense, saying that non-crown definitions of something like Tetrapoda or Amniota could lead to inclusion of organisms lacking the defining traits. But applying this it makes no sense at all in this case. What you're essentially saying it that we should leave stegocephalians out of Tetrapoda on the off-chance they all evolved four legs independently and the grade contained some members who at no point in their evolution had the four-legged anatomy associated with tetrapods (unless snakes don't count either), even though every stegocephalian known does have this anatomy and everything we know about tetrapod evolutionl suggests this was ancestral to the grade?
The "problem" with Tetrapods is that only descendants of relatively derived clades (Lissamphibia and Amniota) survive to the present day, while the very numerous earlier groups that were definitely Amphibians and not Fish in layman's terms (the various -stegas and -erpetons) have left no living descendants. Crown groups depend on an anchor point in living clades, that is why a strict crown group definition excludes so many animals that were obviously capable of terrestrial quadrupedal locomotion from the clade Tetrapoda.

We could go with the Total Group definition and define anything more closely related to Frogs than to Lungfish as Tetrapoda. The problem with this view is of course, that it would completely eliminate the clade Tetrapodomorpha and include fully aquatic basal forms in Tetrapoda.


The third route, that would include more morphological definitions would of course be the most accurate, but the problem is that we had many Tetrapodomorphs that had structures that were very leg-like, yet they never used these to walk around on land, only as holdfasts and flippers in shallow water.

We don't even know whether there was a steady march of non-branching basal clades that eventually produced an animal that is capable of walking around on land and this animal was the common ancestor of Temnospondyls, Lepospondyls and Reptiliomorphs, or whether the ancestors of these groups branched off early, among more aquatic basal forms, and all acquired the ability to walk around on land convergently.

For example, Temnospondyls appear to be the earliest and most successful group in reaching terrestriality, but we also have Reptiliomorphs which seemed to suddenly jump out with a myriad of aquatic basal groups and later with terrestrial forms which already had more gracile, Amniote-like leg joints that Temnospondyls never evolved. Next to these two we also have the Lepospondyls sneaking around in the shadows, who seem to be very primitive, but at the same time strangely allied to the increasingly terrestrial Reptiliomorphs. And while we are considering all of these animals, we also have the modern Lissamphibians whose ancestors were surely around by this time, but we don't exactly know where they came from.
Edited by Mitosis, Jun 16 2017, 04:27 PM.
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IIGSY
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Mitosis
Jun 16 2017, 04:23 PM
The third route, that would include more morphological definitions would of course be the most accurate, but the problem is that we had many Tetrapodomorphs that had structures that were very leg-like, yet they never used these to walk around on land, only as holdfasts and flippers in shallow water.
You forget something. Feet evolved before land living. I think the best modern analogue for the basal stegocephalians is the chinese giant salamander. It spent most of it's time walking at the bottom of the lake/river. Not walking on land or actively swimming, though it could probably do either for short periods of time if necessary.

Not terrestrial or pelagic, but an amphibious bottom walker.
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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LittleLazyLass
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You're ignoring the option of just using a non-crown node-based definitions. Say Tetrapoda is the last common ancestor of x early stegocephalian and x living amniote.
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Little
Jun 16 2017, 08:31 PM
You're ignoring the option of just using a non-crown node-based definitions. Say Tetrapoda is the last common ancestor of x early stegocephalian and x living amniote.
That's actually not a bad idea.
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
Jun 16 2017, 11:32 AM
It is more likely for temporal fenestra to have arisen only once, rather than multiple times, therefore, a reptile-like creature that happens to have no fenestra, but has remaining fossil features that are similar to both Sauropsids and Synapsids is classified to be outside crown-group Amniota.
The problem with using temporal fenestra for classification is that we now know they can be secondarily lost. Hence why things like Anapsida and Euryapsida are no longer considered valid clades. As an example of what I'm talking about, turtles were placed in the now invalid Anapsida because they had no temporal fenestra. But nowadays they are classified as diapsids who merely lost their fenestra at some point.
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Mitosis
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Next to temporal fenestra, there are loads of other skeletal features that ally a fossil with Amniotes or more accurately with either Synapsids or Sauropsids. I don't know each and every one of these features, but I know that many of these are scientifically agreed upon, since there is a whole clade of Parareptilia, animals which have no temporal fenestrae, yet, are Amniotes, and even more accurately are allied more closely to Sauropsids than to Synapsids, for example the mysterious aquatic Mesosaurus, which appears to be the most basal Sauropsid, yet, has a skull configuration more similar to that of Synapsids.

I have no problem with assigning no-crown-group-anchored, terrestrial, four-legged Vertebrates to Tetrapods. It is just my experience that crown-group based definitions tend to be more accurate in light of the limited amount of knowledge we can gain from fossils. It is obvious that morphological definitions that are not anchored to extant clades are sometimes needed, for example when working out the relationships of totally extinct clades like Enantiornithes or Dicynodonts.
Edited by Mitosis, Jun 17 2017, 01:04 PM.
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