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Did birds actually evolve from dinosaurs or from other reptiles?
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Dinosaurs.
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Depends on the bird. Some evolved from raptors, I believe a few evolved from Dactyl's. Most evolved from theropods and are actually just very advanced dinos.
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Evolution does not work that way. If a group is a true group, ie
monophyletic, they all descend from a single common ancestor. That chances of another species evolving not just in parallel (which is common) but into genetically the same taxa is so small that we'd expect all the statues in the world to wave due to atomic vibrations lining up in just the right way before it ever happened. The "raptors" we all know (Deinonychosauria) are not their ancestors either, they are all Cretaceous species and birds evolved in the Jurassic. Though they are indeed the most bird-like of known non-avian species, meaning either they continued evolving along many of the same lines, have a similarly bird-like Jurassic ancestor that we (or I) do not yet know of, or else those particular dinosaurs are in fact the earliest flightless birds.
The pterosaurs are completely different from birds, and indeed from anything else. I don't doubt that if they had survived the KT extinction they would have produced an entirely new and unique class of vertebrates. Probably flightless, given the way birds were taking over the skies and the way the last of the pterosaurs spent a good deal of time on the ground anyway. Example of their uniqueness: their "hairs" that made up the fur on their bodies was not hair at all, but a unique fibre called "pycnofibres".
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How do they evolve from different dinosaurs, since birds (or aves) are a class shouldn't they have a common bird ancestor?
And why are they even their own class shouldn't they be a part of reptiles? BM clear this up for me...
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You are right on both counts. As we increasingly classify organisms by their evolutionary relationships rather than appearance, the old Linnaean taxonomy is falling out favour for the new cladistics. Now, reptiles as a class is not complete without the mammals, birds and synapsids (a group now extinct except for the mammals, without which it is incomplete). I would say the same for fish, except "fish" does not correspond to an scientific group, being made of multiple classes more different from each other than we are from (some of) them.
Including every clade and taxon I can find and figure, birds (Aves) fits into the tree of life like this:
-- actually, this is going to be harder than I thought.
Superregnum:
Eukaryota (nucleated cells)
Supergroup:
Unikonta (single or no emergent flagellum)
Cladus:
Opisthokonta (posterior flagellum)
Cladus:
Holozoa (Fungi excluded here)
Cladus:
Filozoa (Animals and nearest unicellular relatives closer to animals than Fungi)
Regnum:
Animalia/
Metazoa (Animals)
Subregnum:
Eumetazoa (Sponges excluded here)
Cladus:
Bilateria (Bilateral symmetry, Cnidarians excluded here)
Cladus:
Nephrozoa (God knows)
Cladus:
Deuterostomia (the blastopore becomes anus. Protostomia excluded here)
Phyla:
Chordata (animals with a notochord)
Cladus:
Craniata (has skull, "fish" start here)
Subphyla:
Vertebrata (fish with spinal column, lampreys & hagfish excluded here))
Infraphylum:
Gnathostomata (jawed fish)
Microphylum:
Eugnathostomata (really jawed fish)
Classis:
Chondrichthyes (cartilaginous fish. Sharks etc)
Cladus:
Teleostomi (Osteichthyes and Acanthodii ("spiny sharks"). Sharks excluded here)
Superclassis:
Osteichthyes (bony fish, Acanthodians excluded here)
Classis:
Sarcopterygii (lobe-finned fish. Ray-fins (classic fish) excluded here)
Cladus:
Rhipstidia (coelacanths excluded here)
Subclassis:
Tetrapodomorpha (Lungfish excluded here)
Superordo:
Osteolepiformes (Rhizodonts excluded here)
Ordo:
Elpistostegalia (the fishapods)
Familia:
Panderichthyidae (also fishapods)
Superclassis:
Tetrapods (four legged vertebrates. All fish excluded here!)
Classis:
Amphibia
Subclassis:
Labyrinthodonta (ancestral to all land-living vertebrates)
Ordo:
Reptiliomorpha (reptile-like amphibians)
Cladus:
Amniota (terrestrially adapted eggs, essentially the reptiles)
Cladus:
Sauropsida (synapsids (including mammals) excluded here)
Classis:
Reptilia (all other birds and reptiles)
Cladus:
Eureptilia (Anapsids excluded here (meaning turtles)
Cladus:
Romeriida (Diapsids and Protorothyrididae)
Subclassis:
Diapsida (Reptiles with two holes (temporal fenestra) in each side of their skulls)
Infraclassis:
Archosauramorpha (ruling lizard forms)
Divisio:
Archosauria (ruling lizards)
Subsectio:
Avemetatarsalia/
Panaves (crocodiles excluded here)
Subsectio:
Ornithodira (Scleromochlids excluded here)
Subtaxon:
Dinosauromorpha (pterosaurs excluded here)
Series:
Dinosauriformes (dinosaurs + basal forms)
Superordo:
Dinosauria
Ordo:
Saurischia (lizard-hips, ornithischians excluded here)
Taxon:
Eusaurischia (Herrerasaurids excluded here)
Subordo:
Theropoda (Bipedal, mostly predators. Sauropods excluded here)
Subsectio:
Neotheropoda (basal theropods excluded here)
Infraordo:
Tetanurae (Ceratosauria excluded here)
Taxon:
Avetheropoda (Megalosaurids & Spinosaurids excluded here)
Taxon:
Coelurosauria (Carnosauria excluded here)
Taxon:
Maniraptoriformes (Compsognathids excluded here)
Taxon:
Maniraptora (Ornithomimosauria & Tyrannosauroidae excluded here)
Apomorphy:
Aviremigia (all dinosaurs with pennaceous feathers)
Taxon:
Paraves (Oviraptorsauria excluded here)
Taxon:
Avialae (Deinonychosauria excluded here)
Classis:
Aves (Birds. Scansoriopterygids & perhaps
Archaeopteryx excluded here)
Whew! You know, if I, or anyone, had the required data we could break every line here with another whole sequence (and break those lines too) such that we get down to the exact species in the whole lineage. As it is, I was a bit iffy about getting as specific as Panderichthyidae.
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birds aren't reptiles because after over 65 my of evolution they are distinctively different, that's like saying why aren't we all in the same class as giant sea scorpions for the Devonian?
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But as you can see, there's no way to count all reptiles and not birds without arbitrarily excluding them. They sit on the same branch of the tree of life, which is rented from the fish. Also, sea scorpions (Eurypterida) were arthropods, which are protostomes. They branched off my list at line nine. Humans branch off at twenty-nine.
Moxco has the abridged version.