Submitted by [deleted] t3_ywuejq in askscience
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FillRevolutionary900 t1_iwlnnqp wrote
What I fail to understand is how birds can be said to have hips like those of lizards, and not like those of birds
rootofallworlds t1_iwlruo6 wrote
Birds are members of a group, saurischia, best known for having "reptile-like" hips. Even though not all members of the group have that characteristic, that doesn't affect the validity of the group. In cladistics, which is by the far the dominant approach to taxonomy nowadays, taxons should be monophyletic groups or "clades" comprising a common ancestor and all its descendants.
There is current debate and research as to whether the traditional saurischia is a monophyletic group or not, and some classifications put ornithiscians as closely related to theropods with saurischia either redefined without theropods included or not used at all. But bird anatomy does not by itself invalidate the traditional view.
PlaidBastard t1_iwlzre4 wrote
The anatomy was named before they understood the evolutionary relationships between living reptiles and birds and extinct non-avian dinosaur lineages. There were dinosaurs unrelated to those which evolved into birds which had hip anatomy which looked similar to the hips of modern, living birds, so they described them as the group with 'bird-like hips,' not knowing how confusing a hundred more years of paleontology research would cause the name to become.
So, birds have bird hips, not lizard hips, in a purely descriptive sense that ignores established terminology, but these 'bird hips' are not hips of the type that put them in that named and defined group of extinct dinosaurs. Those modern bird-like bird hips happen to be descended from the hips of dinosaurs which had hips that didn't look as much like modern birds' hips at the same time as there were dinosaurs which, coincidentally, happened to have hips grossly like modern birds', despite not being direct ancestors of birds.
Coincidence, lack of current knowledge when anatomy was named, and the classic problem of taking descriptive morphological terms to imply origins, which is really the terminology's fault. Is this hip business really any more confusing than that we use the 'saur' affix which means 'lizard,' literally, for non-lizard species? Lizards are a specific thing, and dinosaurs (pardon the term) aren't lizards....but nobody's complaining about that term, are they? Because they were named when nobody knew any better, and we all acknowledge and agree that the name means they're like lizards, but real big and scary-like, to keep the term useful, not forget the literal etymology, and finally not ignore the newer science.
[deleted] OP t1_iwn0umb wrote
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urzu_seven t1_iwlq3xe wrote
The separation of dinosaurs into two groups (saurischia and ornithischia) occurred long before the modern idea of birds being considered dinosaurs was contemplated. Current evidence suggests birds are descendants of a subset of therapods, which were saurischia. Obviously their characteristics changed over hundreds of millions of years of evolution.
Basically they aren’t saurischian because THEY have lizard like hips but because their distant ancestors did.
The fact they are so different from their ancestors is why I, frankly, think calling them dinosaurs is grossly misleading, and scientific pedantry. It leads to exactly the kind of confusion you are experiencing.
FillRevolutionary900 t1_iwlrvun wrote
I was with you till the last couple of paragraphs where I was like nope. Birds aren't just descendants of theropods. They are theropods. And even going by what you are saying, many of the animals that anyone in their right mind would unequivocally and definitely categorise as both dinosaurs and theropods (such as raptors, T-Rex etc) had hips that were definitely not like lizards, but very much resembling those of modern birds. So they shouldn't be called saurischians either, but according to you it would be scientific pedantry to call them dinosaurs as well, because it was really their distant ancestors that actually had lizard like hips. No, I'm pretty sure that's not the correct way to approach it.
I think your understanding of cladistics is a bit erroneous, and what I'm gathering is that the saurischian and ornithiscian distinction might be dated (or based on superficial similarities). But I have studied enough about the question of birds being dinosaurs or not (by "not", I mean if it's really pedantry to call them that), and I can tell you it's not pedantry in any way shape or form unless you want to bring into question the whole group of animals called dinosaurs in general. Birds aren't distant descendants of dinosaurs, nor is it misleading to call them dinosaurs. They are dinosaurs.
atomfullerene t1_iwnoyu1 wrote
First of all, what is a "bird hip" vs a "lizard hip"? It's a reference to whether the pubis bone points forward (as in lizard hips) or backwards (as in bird hips). Think of this as a general description of shape, like "square" vs "triangle"
Ancestrally, dinosaurs had lizard-like hips.
Early on dinosaurs split into two groups, "Bird hipped dinosaurs" and "lizard hipped dinosaurs". The "bird hipped dinosaurs" had birdlike hips, with the pubis pointing backwards. The "Lizard hipped dinosaurs" kept the original arrangement with the pubis bone pointing forward.
Later, a branch of the "lizard hipped dinosaurs" (the maniraptorians) independently transitioned from "lizard like hips" to "bird like hips". They were still in the "lizard hipped dinosaur" group because that is the ancestry they descended from. But they no longer had lizard like hips. One branch of this group went on to become birds.
LtMM_ t1_iwlcycq wrote
The terms have to do with whether the legs are positioned underneath the body (like a bird), or splayed out to the side (like a lizard). I don't think you're correct saying birds are saurischians, apologies if wrong.
FillRevolutionary900 t1_iwldib4 wrote
Leg position (under or beside the body) is definitely not what distinguishes saurischians from ornithiscians. It's to do with pelvic structure and the position of the pubis bone.
And birds are saurischians. This is from Wikipedia:
All carnivorous dinosaurs (certain types of theropods) are traditionally classified as saurischians, as are all of the birds and one of the two primary lineages of herbivorous dinosaurs, the sauropodomorphs. At the end of the Cretaceous Period, all saurischians except the birds became extinct in the course of the Cretaceous–Paleogene extinction event. Birds, as direct descendants of one group of theropod dinosaurs, are a sub-clade of saurischian dinosaurs in phylogenetic classification.
LtMM_ t1_iwlf6n3 wrote
My bad, but I think you've somewhat answered your own question here. In this case saurischians and ornithicians are being used as phylogenetic terms to represent groups of evolutionarily related animals based on a key morphological trait that was easily distinguishable by comparison to birds versus lizards. The fact that bird and bird-hipped dinosaurs have similar pelvic structure is likely a simple case of convergent evolution.
[deleted] OP t1_ix0hym8 wrote
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rootofallworlds t1_iwlncwp wrote
Because the hips of ornithiscians are similar to the hips of birds. Current theory is it's a case of convergent evolution with the "bird hipped" layout having evolved three or four times in the dinosaurs.
Archeopteryx was known when the saurischian-ornithisician division was proposed by Harry Seeley in 1888, but I don't know if archeopteryx's relation to other dinosaurs was really known back then. Even if it was, Seeley might not have cared that a bird was in saurischia.