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If birds are dinosaurs then what are dinosaurs?

novaphile said:
And some of the fluffy bunnies are bad ass killers.

If you don't know what I'm talking about, search for Bun-bun and prepare for a lot of web-comics. (Sluggy Freelance)
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I'm very familiar with Sluggy, having been a follower since very early on. I don't like the site redesign, though.
 
JoeMorgue said:
Categories describe reality, they don't create it,
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Categories are helpful when trying to understand the complexity of reality, but they are imperfect descriptions of it. Nature is a lot messier than most people would like to believe. At some point the insistence on putting everything into categories begins to reduce, rather than help, our understanding.
 
Pixel42 said:
Categories are helpful when trying to understand the complexity of reality, but they are imperfect descriptions of it. Nature is a lot messier than most people would like to believe. At some point the insistence on putting everything into categories begins to reduce, rather than help, our understanding.
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Yes, it gets rather complicated when you start to consider the edge cases for the definition of "species".
 
Ziggurat said:
Tetrapods are all descended from ancestors that had four limbs, but as a classification, they don't have to have four limbs anymore. I'm not aware of any tetrapods that have gained limbs, but plenty have lost them.
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South American monkeys such as the spider and woolly monkeys can be said to have gained a limb.
 
sphenisc said:
South American monkeys such as the spider and woolly monkeys can be said to have gained a limb.
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Skeletally it’s still very much a tail, which the first tetrapods also had. An elephant might be closer with its nose, though that lacks any skeletal component.
 
novaphile said:
And some of the fluffy bunnies are bad ass killers.

If you don't know what I'm talking about, search for Bun-bun and prepare for a lot of web-comics. (Sluggy Freelance)
Click to expand...
Not to mention the bunnies in Monty Python and the Holy Grail.
 
Ziggurat said:
Skeletally it’s still very much a tail, which the first tetrapods also had. An elephant might be closer with its nose, though that lacks any skeletal component.
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The primary function of limbs is locomotion though, and elephants don't walk on their trunks. Monkeys do use their tails for climbing. So I'd say tails are closer, but, evolutionarily neither are limbs.
 
hecd2 said:
The primary function of limbs is locomotion though
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The original function may have been. But plenty of limbs don’t serve that function now, so I don’t think that’s a good defining feature. The lack of any skeletal parts is probably better grounds for excluding a trunk as a limb than lack of locomotive use.
 
Ziggurat said:
Skeletally it’s still very much a tail, which the first tetrapods also had. An elephant might be closer with its nose, though that lacks any skeletal component.
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And it's that skeletal component that appears to be the roadblock. Googling this a couple days I could not find any examples of gaining a limb among tetrapods. I did find multiple statements that there are no known examples but none were authoritative enough to post individually. There are a lot of examples of cartilage adapting to produce new limb like thing, especially fins. And single bones adapting in odd ways (panda's thumb but that's not even a limb).
 
RecoveringYuppy said:
And it's that skeletal component that appears to be the roadblock. Googling this a couple days I could not find any examples of gaining a limb among tetrapods. I did find multiple statements that there are no known examples but none were authoritative enough to post individually. There are a lot of examples of cartilage adapting to produce new limb like thing, especially fins. And single bones adapting in odd ways (panda's thumb but that's not even a limb).
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The origin of tetrapod limbs are the four fins in lobe-finned fish from which the limbs developed. The body patterning of tetrapods is extremely conserved. The number of digits in early tetrapods varied from 5 to 9 (more than five known as polydactyly), but soon settled to five (and some or all of the five are lost in extant tetrapods). After very earlyy tetrapods there are no instances of more than four limbs or more than five digits per limb. Check out Gaining Ground by Jennifer Clack about the evolution of early tetrapods. Excellent book.
 
Taking the "ped" in "quadruped" literally as "feet/legs" rather than "limbs", a mantis is a quadruped, but still isn't a tetrapod.
 
Delvo said:
Taking the "ped" in "quadruped" literally as "feet/legs" rather than "limbs", a mantis is a quadruped, but still isn't a tetrapod.
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Yes, in the formal description of tetrapods the equivalent of the foot and hand are called pes and manus respectively.
 
hecd2 said:
The primary function of limbs is locomotion though, and elephants don't walk on their trunks. Monkeys do use their tails for climbing. So I'd say tails are closer, but, evolutionarily neither are limbs.
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Then bipeds have lost limbs. When did the arms of a theropod dinosaur evolving into a bird stop being limbs and then start being limbs again?
 
ponderingturtle said:
Then bipeds have lost limbs. When did the arms of a theropod dinosaur evolving into a bird stop being limbs and then start being limbs again?
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What I meant was that limbs originally evolved for locomotion. Limbs can add function, change function or be lost altogether. They were always limbs.
 
hecd2 said:
What I meant was that limbs originally evolved for locomotion. Limbs can add function, change function or be lost altogether. They were always limbs.
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I'm not seeing it. Our arms aren't used for locomotion (to the extent that they are, if we stopped using them that way, we'd still consider them limbs), but they are still limbs. So far, I understand, you agree. But you say "Ah, but they originally evolved for locomotion".

This makes me ask, if some other animal evolved an analogous structure that functioned exactly like our arms, would you not consider them to be limbs?

It seems to me that limbs are defined by structure and function, not origin. I mean, a humanoid robot has limbs.
 
hecd2 said:
What I meant was that limbs originally evolved for locomotion. Limbs can add function, change function or be lost altogether. They were always limbs.
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This seems to be creating a definition of limb in no small part to exclude things like prehensile tails or trunks. It isn't what the state of the limb is now or what it can or can not do but you have too look into evolutionary deep time to determine if something is a limb?

If we found an alien could we determine if its appendages were or were not limbs without studying their whole evolutionary past.
 
ponderingturtle said:
This seems to be creating a definition of limb in no small part to exclude things like prehensile tails or trunks. It isn't what the state of the limb is now or what it can or can not do but you have too look into evolutionary deep time to determine if something is a limb?

If we found an alien could we determine if its appendages were or were not limbs without studying their whole evolutionary past.
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Roboramma said:
It seems to me that limbs are defined by structure and function, not origin.
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We need a definition of limbs. What's yours?
 
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hecd2 said:
We need a definition of limbs. What's yours?
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Not sure we really need one. The question we see to be addressing is why is it so hard to add to the tetrapod skeletal plan. Does that sum up the question.

Naively I think it's easy to think that the hard parts are more difficult to adjust than the soft parts, but during the embryo stage everything starts out soft and development is driven by a large repertoire of signaling mechanisms.

Various other species seem to have body plans that aren't fixed in some major aspect such as the number of segments on centipedes, etc. Our own skeleton isn't completely fixed, 24 ribs is the norm but it varies for a small percentage of the population.

Whatever signal happens at our hips and legs (I'm making some big assumptions here) that causes arms and legs to sprout, why can't it occur somewhere else along the spine? Other features, like mammary glands, move and repeat along certain "signal lines" and appear at different places and different numbers in different mammalian species.
 
RecoveringYuppy said:
Not sure we really need one. The question we see to be addressing is why is it so hard to add to the tetrapod skeletal plan. Does that sum up the question.

Naively I think it's easy to think that the hard parts are more difficult to adjust than the soft parts, but during the embryo stage everything starts out soft and development is driven by a large repertoire of signaling mechanisms.

Various other species seem to have body plans that aren't fixed in some major aspect such as the number of segments on centipedes, etc. Our own skeleton isn't completely fixed, 24 ribs is the norm but it varies for a small percentage of the population.

Whatever signal happens at our hips and legs (I'm making some big assumptions here) that causes arms and legs to sprout, why can't it occur somewhere else along the spine? Other features, like mammary glands, move and repeat along certain "signal lines" and appear at different places and different numbers in different mammalian species.
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Seems like segmented creatures can just keep producing standard subassemblies ad infinitum. Everything on modern tetrapods, at least, seems to be bespoke. Each phalangial bone is unique in the hand. The hip structure only works the once in a given body. All the bones in the torso have attachment points on which to hang a specific array of musculature to manipulate exactly those many extremities in exactly that arrangement.

Popping out another set of legs, or a pair of wings, would require a ground-up redesign of the entire system. Not that such bespoke designs are impossible, of course. Random mutation plus natural selection got us this far, after all.

But I suspect that this far along this particular evolutionary branch, ad hoc mutations are gonna get naturally selected out of the gene pool tens of thousands of generations before they yield any useful new anatomical designs.

And really I think the real problem with any modern conversation about evolution is that everyone tends to really really underestimate just how long evolution takes to do its work. Pretty much every example of natural selection we've observed in human timeframes has been a selection of phenotypes already extant in the genotype of the population. Truly novel, viable genotypes take a lot longer to emerge from the background noise of random mutation.

It's not enough to wonder when a tetrapod is going to sprout another pair of legs. You have to wonder when it's going to sprout a whole new hip structure and musculature to go with the new legs.
 
That's why the people born with extra limbs have no use of them. It does no real good to have extra arms or legs if they don't come with full attachment to the rest of the skeleton, ligaments, muscles, nerves, and a section of the brain to control them. At least these days you have better options than spending your life with the circus.
 
theprestige said:
Seems like segmented creatures can just keep producing standard subassemblies ad infinitum.
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There's a general pattern in evolution of repeated parts, from the earliest & most straightforward executions of the concept to later, more derived ones:
1. At first, they're all identical and it doesn't seem to matter much how many there are.
2. Later, some of them differentiate from each other (an easy way to add a new thing without adding a new thing).
3. The more differentiated they become, the less freedom they have to vary their number.

This progression is easier to see in invertebrates.
  • First, consider segmented worms (annelids): not only no segment differentiation and usually pretty free variation in number, but also not really a brain or any hard parts around the mouth for cutting/poking/grinding/chewing; only muscles for pulling in something that already happens to be small enough.
  • The next step from there is velvet worms (onychophorans): now the sides of the body wave in & out, with each "out" wave being visibly a precursor to a leg, but nothing else happens in the big picture except the beginnings of some slight modification at the front end.
  • Then we get centipedes. Now there's suddenly a very distinct head, with knives/forks near the mouth-hole! Where did that come from‽ It's the first few segments, fused together. The first few segmental ganglia (nerve clusters at the nerve branching points) have been assembled into a tiny brain. The first few pairs of legs have been reshaped into cutlery (and antennas).
  • All other arthropod groups had a stage like this too, but they didn't all make their heads out of the same number of segments, so, when paleontologists find a clear enough fossil of an early and not-very-thoroughly-distinguished arthropod, counting the number of legs that have been drafted by the head is the only way to see which modern lineage it's most likely related to. Since then, though, they've all stuck with a particular number; their heads all settled into one particular format for each lineage, which only has a particular number of segments in it and can't be made properly out of the wrong number of segments.
  • Since then, other differentiations of segment & limb numbers have come along in the groups that aren't still essentially lightly-armored velvet worms with heads. In some cases, some legs near the front get especially big and oriented forward for grabbing things, especially to tear/cut bits off of them and put them in the mouth; in the most drastic and specialized forms, we call them "claws", but, in cases of less drastic modification, we call them something like "feeding legs" or "gnathopods" (jaw-feet). Crustaceans have that, but insects don't, but both do have the next category back behind that, the walking legs. Behind that, we get more disagreement again; crustaceans have legs modified for swimming, which are absent in insects and seem to be modified into the "book lungs" of chelicerates (arachnids & their relatives). And each of those specializations or eliminations came with its own specification of number. Presumably, unlike in the head, a mutation changing one of these numbers would be less than disastrous, but, for the most part, the numbers get stuck when they differentiate & specialize.
  • Remember that different kinds of arthropods also don't have the same numbers, locations, or types of eyes. This follows the same pattern as the legs & ganglia: they must have had more freedom to vary in some more basic version found on the ancestor's head, and then gotten fixed after some later differentiation & specialization into different sizes, shapes, & functions kicked in. (Weirder yet, on some modern arthropods, I think only crustaceans, you can find simple light-sensitive spots creatively named "light organs" on some of the legs, so one could infer that eyes, like hard mouth parts, might be another thing that got included in the arthropod head because they were part of the legs on those segments.

theprestige said:
Everything on modern tetrapods, at least, seems to be bespoke.
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It looks that way because we're more derived & developed from the original forms in which various things would have been more obviously cases of repetition. But some aspects of the original repetition are still with us.
  • It's easiest to see in our vertebrae, ribs, and the associated ganglia & muscles, because they still tend to look like each other, although some of that series of muscles are now much bigger or smaller than other muscles in the same series or pulled in different directions by the limbs. Notice, though, that the relatively fixed numbers of different kinds of vertebrae we have now came with other structural distinctions in those vertebrae: some are fused with the pelvis and some aren't, and, of the remainder that aren't, the middle section attaches to ribs and the ones above & below don't. Also notice that the one lineage which changed its vertebral number the most since those kinds of distinctions arose, snakes, also ditched most of the distinctions among its vertebrae, reverting to a previous state of low specialization and high number variability. (Also, changing vertebral number seems easier for diapsides than for synapsids, although I don't know why, since that has nothing to do with how differentiated the vertebrae are.)
  • The fact that vertebrae are all variations on a single theme shows up even more clearly in some of the more common kinds of deformities/mutations they can have. It's often not a matter of a simple wrong number of a certain type of vertebra, but a matter of some vertebrae taking a shape that isn't quite either one of the two adjacent types, such as lower cervical (neck) vertebrae with tiny ribs, or lumbar (lower back, ribless) vertebrae that are fused to each other or seem to stretch outward & downward to the pelvis and/or drag part of the pelvis upward & inward so they can fuse, or the top sacral (pelvic) vertebra being fused into the sacrum (pelvis) only in its bottom half while its top half is shaped like the top of a lumbar vertebra.
  • Reptiles & amphibians & fish, outside of venomous snakes, generally don't do different kinds of teeth for different jobs in the same mouth, and they also don't specify exactly how many need to be there or in exactly what positions. There are genes active in the jaws that just pretty much say "grow teeth here wherever there's a gap to fill in; don't try it if there isn't". (Gaps for new ones get created when old ones fall out one at a time.) Mammals started that way but then started putting different kinds of teeth in the same mouth in a particular order, and then the numbers of each eventually started holding pretty steady, to such an extent that whole families/orders can be defined as having particular "dental formulas" (specific numbers for each kind of tooth, always in the same order). And even their positions within the mouth are so uncreative that if, like horses for example, you just eliminate a type of tooth, the neighboring types don't even grow there either; you just get a gap that happens to be perfect for humans to put a "bit" in (which means your dental formula gets a zero in it to represent where that gap is, instead of having fewer entries in it for the reduced number of kinds of teeth that are actually present). That's a pretty consistent arrangement that was once thoroughly haphazard.
  • Early in fetal development, we have a circulatory system like those of some of the simplest boneless fish, consisting of a major vessel running front to back and a series of identical loops/arches deviating off from that. Some parts of this setup close off & dissolve away while other parts twist around each other & sometimes fuse together to become the heart, aorta, and a few other major vessels of the modern circulatory system you eventually get born with. In species with the original series of repeated vessels, the numbers can vary, but ours can't and still survive because it wouldn't be able to form the proper specialized shapes later on out of a wrong number of constituent pieces.
  • Our fetal pharyngeal arches are famous for eventually developing into gill struts (and sometimes jaws) in fish but jaws, inner ear bones, hyoid bone, and trachea-supporting cartilage arches in us. But the connection can be seen not only in our fetal development and that of our relatives, but also in some of the defects that can happen there. For example, some birth defects result in extra-low ears, closer to the throat, but never extra-high ears, farther from the throat, because when a process of separation & differentiation is disrupted the result is something less separated/differentiated. It's also why some people are born with a small bit of indented skin in front of one ear or both, which, in some cases, sometimes discharges a bit of fluid. That discharge tells us that it's not just indented skin; it's actually an extra bit of the fetal pharyngeal arch system that failed to connect to, or got cut off from, the rest of the ear-nose-throat system. It was essentially trying at one time to become another ear (or gill opening), but couldn't because another copy of what started as the same thing soaked up all of the "turn this bit into a real ear" attention. All pharyngeal arches start out the same, and their numbers are variable in fish where they mostly remain the same, but if we get a wrong number of them, some of them near where the number went wrong can't tell which of a few different development paths they're supposed to take.
  • Your arms and legs are repetitions of each other, one single kind of thing in barely-different forms. The general bone pattern for both is the same (one bone at the base of the limb, two in the next segment, then a bit of a jumble (although still recognizably dividing into four along the way in there), then a clean set of five parallel lines of bones at the end, with the first layer stuck together and the outermost layers independent. (And the 5 was up to 8 in our ancestors, making the pattern 1-2-4-8 at one time.) Even the fact that one of those five lines is missing one segment and spaced a bit farther away from the other four is the same in both hands and feet; your biggest toe is your foot's thumb. That's because one template is used for both kinds of limb, with minor adjustments on top of that to make them any different from each other at all. The template is that the growth buds that start extending out from four places on the torso during fetal development sometimes split along the way, leaving bone-precursors behind on their path. The fact that it's two implementations of one template is also why individuals who have a birth defect in one kind of limb, like those associated with Thalidomide, tend to have similar defects in the other kind too; in cases like that, it's the template itself that got disrupted, not just one of its two kinds of implementation.
  • Even the original structure of a single limb might have been another case of segmentation before it became more recognizable (and if not then it became segmented from a non-segmented origin in some of our cousins, which at least makes this a potential direction that our ancestors would have once been capable of going instead of the way they did go). In some of our shallow-water fish ancestors or cousins that weren't quite ready to crawl on land yet, there was a series of roughly squarish blocks down the middle of each fin, with a pair of more conical spikey bones sticking out forward & backward from each central block. As can be told from a set of other fossils in various intermediate stages between that and something more like our current limb bone structure, the first squarish block of that central series of squarish blocks equates to our humerus, the second equates to our ulna, and the first forward spike equates to our radius, with the other blocks & spikes getting lost and the wrist & hand bones getting added on when the growth mechanism started extending farther out and dividing a few more times along the way. Changing the number back when it was just a series of blocks & spikes wouldn't have mattered; trying to change it now would be trying to change the number of humeri, ulnae, or radii in the same arm.
  • There's still more to the fact that our arms & legs are modified duplicates of each other again if we look at our ancestors & cousins again. Our limbs developed from the fins of lobe-finned fish, and the basic arrangement of fins in lobe-finned fish is two close to the front on the bottom, two more on the bottom farther back, one on the bottom of the tail, one on top of the tail, and one more on top of the torso. That's seven repetitions of the same template, of which we have lost three. (And if there's one that looks different from the others at this stage, it tends to be the forward dorsal one, not either of the pairs.) So clearly there was a time when the number & placement of precursors to our limbs relative to the rest of the body could be changed, back when their designs & functions were less specialized.
  • Some vertebrates, including not just fish but also some tetrapods such as turtles and Tyrannosaurs, even duplicate some aspects of the spine as a separate pseudo-spine well above or below the spine (in the form of bones and their affiliated joints & muscles but no extra major nerves). Here is a fish which happens to be a good example of that, but also...
  • Back to the vertebrae for something I held back earlier so you'd have time to get used to this stuff before I dropped one of the really bizarre-seeming ones: look at the back of that fish's head. Notice not only that the skull isn't closed in back, so it's mostly just the facial bones and some side plates leaving the brain case open to the torso, but also that the spine doesn't stop short of it. It continues right on ahead into and through that opening, ending behind/inside the face. If you were to enclose the back of that brain, you'd also be incorporating the first few vertebra as part of the head. And in fact, you already did that to yourself, before you were born. In the middle of your head now, at the bottom of the brain case so the brain is on top of them, is a line of three bones continuing right on from the cervical spine, with cores that are more chunky instead of plate-like, although distorted: in order the occipital, sphenoid, and ethmoid. Those are, or at least originally were, the real first three vertebrae, meaning your neck begins at the fourth. The tip of you spine forms parts of your eye sockets and nasal sinuses. The three big sections of your brain are your first three spinal ganglia. And the top, back, & sides of your cranium are essentially patches of skin that calcify. So even your brain and cranium aren't made of original unique parts. (You essentially assembled your head from your first few body segments just like an arthropod... only without the mouth-made-of-legs thing.)
  • Eyes again (saving the weirdest for last?): we have relatives called "tunicates" which are pretty easily recognizable as related to the simplest boneless fish in their larval stage, but then find a good rock to smack their faces into and metamorphose into an adult stage which sits still on that rock for life, like sponges and corals. And some tunicates have light-responsive "eyespots" scattered all over their bodies. This might not have ever been the case in our ancestors, but, if not, it's at least a condition that our ancestors had the potential to evolve into, back when the precursors of our eyes were just light-responsive patches of skin. Our eye placement & number is no longer quite so free anymore, because now eye development affects so much other tissue around the eyes (and creates a significant vulnerability).

theprestige said:
Each phalangial bone is unique in the hand.
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They're the same thing in 14 copies per hand or foot, with tiny adjustments.

theprestige said:
The hip structure only works the once in a given body.
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I haven't seen an authoritative source say this, but my inference is that the pelvis and the scapulae & clavicles are versions of one underlying scheme just like the legs and arms are.

theprestige said:
All the bones in the torso have attachment points on which to hang a specific array of musculature to manipulate exactly those many extremities in exactly that arrangement.
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The main issue with extra limbs appears to me to be that there just isn't room for them. At large sizes, large limbs relative to the body are favored, and large limbs just can't be put very close to each other without getting in each other's way, including their support structures on the torso. I don't believe there'd be much of a problem with growing more limbs if the proportions of limb size to everything-else size were more like it is in arthropods and Chinese parade dragon costumes, but they aren't.
 
Delvo said:
There's a general pattern in evolution of repeated parts, from the earliest & most straightforward executions of the concept to later, more derived ones
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Fantastic post. Thank you very much.
 
Delvo said:
...The main issue with extra limbs appears to me to be that there just isn't room for them. At large sizes, large limbs relative to the body are favored, and large limbs just can't be put very close to each other without getting in each other's way, including their support structures on the torso. I don't believe there'd be much of a problem with growing more limbs if the proportions of limb size to everything-else size were more like it is in arthropods and Chinese parade dragon costumes, but they aren't.
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On that basis the olm might be a good candidate for future hexapody.
 
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