While reading a BBC article regarding chickens and T Rex, a bad mood crept up on me and I really need to have the dinosaur extinction theory explained for me once and for all:
Is it supposed to have killed off every dinosaur, or only some species? Where the surviving types eventually turned into birds…?
Naah…
Or…?
Cladistically speaking, Aves are “Dinosaurs”. But that is only because the dudes that made the deliniations put them there. No one doubts birds are closely realted to some dinos and in fact sometimes it is hard to tell whether you have an ancient bird fossil or a dino.There is extremely strong evidence that Birds evolved from theropod dinosaurs. That does not nessesarily make them dinosaurs, but remember- there are no “dinosaurs” as such- in the classic "Kingdom, Phylum, Class, Order, Family, Genus, Species" , “dinosaurs” do not enter in. In actuality we have two orders- Ornithischia (bird-hipped) and Saurischia (lizard-hipped). They are part of the Class Sauropsida, which we’d usually call “reptiles”, and the Subclass Diapsida. The Dinosauria are not a order nor a Family, they are a “Superorder”. For various reasons, Aves has been assigned to this super order under most modern systems. The debate rages on.
However, “surviving Dinosaurs” did not* become *Birds. Birds had already come into existance millions (about 100 million) of years before the Cretaceous Period- The earliest known bird is Archaeopteryx , from the Late Jurassic period.
Right now, dudes are classifying Birds as Dinosaurs. That may change. But whether or not “birds are really dinosaurs” doesn’t help your theory as Birds split off from what we’d call the “classic dinosaur” some 100 million years before “the dinosaurs became extinct”.
Wow. Thank you.
As to why “birds are dinosaurs,” remember that “dinosaur” doesn’t mean “big, scary, and extinct” as it would to a 3rd grade student. Rather, it’s the name given to a group of archosaurs with specific adaptations that permitted them a couple hundred million years of dominance of the ecosystem – of which most of the typical well-known examples are in fact “big, scary, and extinct.”
But, just as bats and whales are mammals because they descended from “typical” quadrupedal furry mammiferous mammal stock, anything that descended from the common ancestor of dinosaurs has itself got to be included among the dinosaurs. And that includes the birds, which (probably) evolved from small arboreal or cursorial insectivorous dinosaurs that developed feathery scales for warmth and then discovered their advantages in insect-catching.
How far had birds diverged by the last days of the “classic dinos”? Songbirds? Gulls? Penguins? Ostriches?
I don’t follow this reasoning. Didn’t both mammals and reptiles descend from some amphibian? They therefore descended from the common ancestor of all amphibians. So by the reasoning given, reptiles and mammals have got to be included as amphibians?
In layman’s terms: yes! Explore the Tree of Life. One essential point of evolution is the nested hierarchies of life.
Genetic evidence suggests that the ancestors of many modern groups had already diverged from one another many millions of years before the end of the Cretaceous. That doesn’t mean that these ancestors closely resembled the members of modern groups. Songbirds are a relatively recent radiation, and I believe diverged post Cretaceous.
There were a number of highly modified flightless aquatic and land birds in the Cretaceous, as well as gull-like species, but most of these were not related to modern groups.
According to modern “cladistic” classification schemes, a named group must include all descendent groups.
According to modern classification, all land vertebrates belong to a group called “tetrapods” (“four-footed” animals, as opposed to fish). Amphibians represent the first branch off this group.
The next branch is known as the “amniotes” (having a hard-shelled egg capable of surviving on land, or descended from ancestors that had one). This group includes the mammals, the traditional “reptiles”, and birds.
The first branch off the amniotes was the ancestors of the mammals, the synapsids (formerly known as “mammal-like reptiles”, but no longer recognized as reptiles).
The other main group of the amniotes is still called the “Reptilia” by some. Successive branches off this group include the turtles; tuatara, lizards and snakes; crocodilians; dinosaurs; and birds. (I am simplifying greatly here and not identifying all the branching nodes or exactly how they are nested.) Because birds are nested within this group they are now regarded as Reptilia; since they are nested within one group of dinosaurs they must be regarded as dinosaurs.
The amniotes are not nested within the modern Amphibia, and so are not amphibians, although their ancestors may have had an amphibious life-style. The mammals are not nested within the group Reptilia in the above sense, and are therefore not reptiles, although their ancestors were reptile-like in the informal sense.
There was quite a wide radiation of bird types in the Cretaceous. Using just the list in Wikipedia, here are some examples, with “the closest parallel among modern-day birds” in parentheses:
[ul][li]Avisaurus (marsh hawk, or small eagle)[/li][li]Baptornis (grebe, or possibly diving duck)[/li][li]Confuciusornis (crow, bird of paradise)[/li][li]Enantiornis (eagle, or possibly vulture)[/li][li]Gansus (loon or diving duck)[/li][li]Hesperornis (flightless bird with lifestyle resembling loon or grebe)[/li][li]Iberomesornis (finch)[/li][li]Ichthyornis (gull, tern, or petrel)[/li][li]Lectavis (curlew, rail, bittern, or other shoreline or wading bird)[/li][li]Nanantius (small gull or similar aquatic bird)[/li][li]Omnivoropteryx (not totally clear, but apparently similar to a crow or other opportunistic omnivore)[/li][li]Paleocursornis (if a bird – the issue is still unresolved – ostrich, emu, or similar ratite lifestyle)[/li][li]Patagopteryx (flightless fowl of gamebird-like lifestyle)[/li][li]Sapeornis (see Omnivoropteryx above)[/li][li]Shenzhouraptor or Jeholornis (hunter? seedeater? your guess is as good as anyone’s, at this point)[/li][li]Vegavis (a true duck or goose)[/li][li]Yanornis (gull or other fisheating bird)[/li]Yungavolucris (evidently some type of perching bird, known as yet only from fragmentary remains)[/ul]
To further drive home the relationship: A T. rex is more closely related to a parrot (or to an eagle, or a crow, or a finch, or any other modern bird) than it is to a triceratops. So if you’re going to talk about a group which includes both T. rex and triceratops, then logically you ought to include the birds in that group, too.
Well put. And since crocodiles are much more closely related to birds than they are to turtles, if you want to call both crocodiles and turtles “reptiles” in the technical sense you have to include birds in that group too.
However, I think it is OK to continue to refer to turtles, lizards, snakes, and crocodilians as “reptiles,” as a group of animals separate from birds, in the informal sense. Likewise “fish” includes a variety of different groups that are not a coherent taxonomic group; lungfish are much more closely related to elephants than they are to sharks or trout.
The analogy of bats:mammals::birds:dinosaurs is very apt. Birds are a specialized subset of dinosaurs that survived when all the other dinosaurs went extinct. If all mammals except bats went extinct, future cephalopod paleontologists might have a hard time explaining to larval cephalopods why bats should be considered mammals, since they are obviously so different than those extinct horses, elephants and humans. And the cephalopod paleontologists will show the larvae some insectivore fossils, and show how similar they are to bats, and say that although they aren’t sure exactly that bats evolved from insectivores, they must have evolved from a mammal very much like an insectivore. And the larvae will say “Ooooooh, COOL!”. And then larval Calvin will give a school report on bats claiming that bats are bugs.
So early birds are very very similar to small meat eating dinosaurs, in fact we have a much better record of early bird evolution than we do for bat evolution.
And also note that not just dinosaurs went extinct at the end of the Cretaceous. Marine reptiles, ammonites, pterosaurs, toothed birds, all kinds of animals from all over. And of course, the KT extinction which wiped out the dinosaurs so mammals could take over was just a pipsqueak compared to the earlier Permian extinction, which wiped out the mammals so dinosaurs could take over.
How about a “yahbut” on this?
Yes, there needs to be a clade that includes the common ancestors of mammals, modern reptiles, and birds, and it’s referred to as Amniota – all those vertebrates that produce a fluid filled protective membrane containing liquid, within which the embryo can grow, usually accompanied by a yolk or other food source, and hence can be laid as eggs that do not have to develop into tadpoles in the water. (Mammals, of course, retain their “eggs” within the body and, in lieu of yolk, develop the placenta to nourish the developing embryo from food assimilated by the mother – but we retain the amnion, the “bag of waters” within which the baby develops.) Amniotes and living and extinct amphibians share a common ancestry and set of characteristics – four limbs, lungs for breathing air, etc., in the clade Tetrapoda. Another clade unites Tetrapods with lungfish and coelacanths and the extinct rhipidistians in having two sets of paired appendages containing bone and muscular flesh for use in locomotion, as well as several other adaptations.
The point here is that there are a shitload of kinds of bats, adapted for a wide range of aviational lifestyles: insectivores, fruit-eaters, pollinators, haemovores, etc. They have a common ancestor but have radiated into a number of roles.
Nobody has an issue with bats being mammals. But you can identify a bunch of characteristics that unite shrews, rabbits, oxen, bears, elephants, monkeys, and armadillos that do not describe bats – they’ve specialized for flight.
Likewise, dinosaurs radiated over a wide range of characteristics, including small carnivorous, insectivorous, herbivorous, and fructivorous roles as well as the ones better known because they were huge and intriguing. Like the chiropteran mammals, the aviform dinosaurs conquered the air and radiated widely.
But, unlike the present day, where mammals are widespread, the only surviving archosaurs are the crocodiles, distant cousinsof the dinosaurs, and the birds, the group of dinosaurs which adapted for flight-based lifestyles.
Imagine a mass extinction event that kills off almost all warm-blooded tetrapods. Lizards and snakes survive. And so do some bats, on remote islands protected from the mass extinction by distance and isolation. The didelphids (opossum family) live through this one the same way as they did through the K-T one, by conservatively hanging to an opportunistic omnivorous lifestyle.
Lizards radiate to fill most land-animal niches. Possum descendants take a few niches, but remember that they’re conservative in evolutionary terms. And bats radiate widely to fill the niches once occupied by birds. It’s easy to see the relationships between the great-hunter eagle-bat, the white-furred long-legged egret-bat, the conservative acrobatic swift-bat, the diving mallard-bat, etc. But it is very difficult to conceive that the great panoply of Chiropterans were ever closely related to those strange extinct creatures like the rhinoceros, the elephant, the Kodiak bear, etc. – they seem nothing alike.
That hypothetical describes very well our problem with dinosaurs and birds. Say “dinosaur” and people think Brontosaurus, Tyrannosaurus, Triceratops – not, as a rule, Coelurus or Psittacosaurus. But there are some pretty clear transitional forms between the smaller raptors (Coelurosaurs) and the early birds – Shenzhouraptor and Yandangornis are typical of these “feathered raptor” sorts of creatures.
65 million years ago, a big chunk of rock slammed into mexico so hard that debris from the impact rained down as far north as Tennesee. That was a pretty big whump. I’ve also read that based on analysis of charcoal in the KT boundary, that pretty much most, if not all, of the earth ended up on fire as a result of this event. I believe the theory is that the impact threw up lots of pieces of molten rock, which floated through the upper atmosphere and then rained down all over the world, igniting everything they fell onto. This may or may not be an accurate portrayal of exactly what happened, but it hints at it being a rather unhappy time for pretty much anything that was alive then.
If you are a big animal that ate plants, suddenly there aren’t any plants left, and if you didn’t get cooked in the fire, now you don’t have any food to eat. Life sucks. If you’re a meat eater and you didn’t get cooked in the fire, WOOHOO it’s feast time! Lots of dead animals all over the place! But when those are gone, there aren’t any more plant eaters for you to eat because they’ve all starved to death, so life sucks for you too.
Add into this mix some rather nasty volcanic activity, some of which (like the Deccan Traps) started before the KT extinction event, and a bunch more activity which was likely set off by the huge shock waves from the Chicxulub impact.
It wasn’t a very happy time.
Within a very short amount of time, many, many species of animals, especially the larger ones, die off completely. The only things that made it through something like this were the smaller creatures that didn’t need a lot of food to get by. Some of these little guys were dinosaurs, some of them were birds (or the precursers for what we now call birds), some of them were mammals, and some were insects.
The big extinction event didn’t just target dinosaurs, but most of the big animals at the time happened to be dinosaurs and they weren’t built to survive an event like this. A lot of creatures did survive the extinction event, though. Alligators and crocodiles were around back then and obviouslly they live on today. Dragonflies, horseshoe crabs, and cockroaches (none of which are dinosaurs, but they were also around back then) also all managed to survive the big extinction event.
By the way, while the KT event gets a lot of press, it isn’t the only mass extinction event in history. The worst one (called the permian-triassic extinction) occured about 250 million years ago.
To further answer this question, no, all dinosaurs did not die out at the end of the Cretaceous (even discounting birds for the moment). Dinosaurs were a very diverse bunch, with some 500+ genera currently known (and possibly as many as an additional 1300 or so as yet unknown), spanning the bulk of the Mesozoic Era (~251 million years ago - ~65.5 milion years ago). During this time, there was a constant turnover of species. At the very end, there were approximately 213 genera still extant. So, in very rough terms, perhaps only about 11-12% of all dinosaurs that ever lived actually bought it at the end of the Cretaceous (though all of the then extant dinosaurs – with the exception of many avian ones – are presumed to have gone extinct).
And, as mentioned, the dinosaur-to-bird transition began well before the K-T extinction. Archaeopteryx, the first known bird (though already possessing many derived, bird-like features, so very likely not the first bird), is from the Jurassic, dating to about 150-155 million years ago. So, birds evolved alongside their dinosaurian brethren for a good 90 million years, at least, before the latter were snuffed.
It’s also worth noting that there were lesser extinction events that marked the transitions between the Triassic and Jurassic as well as the Jurassic-Cretaceous boundary (indeed, most, if not all, major geological boundaries mark a major extinction of some sort). A number of dinos bought it during these intra-Mesozoic extinctions, as well.
Also worth noting, I think is that the transition between the two standard epochs of the Cretaceous (usually referenced as Upper and Lower) was itself a major change far eclipsing most of the animal mass extinctions: the fern/cycad/gymnosperm floras that had dominated the Mesozoic to date were replaced by the angiosperms, the flowering plants, with only a few survivors from the previously dominant classes of plant retaining certain niches.
The scene is the American South at dusk. A grove of tall pines is in the background; the foreground is dominated by a few magnolia trees. A couple of opossums can be seen, busily hunting insects and unwary worms. Something that looks very much like a palmetto is growing over in the shelter of the pines.
The stillness of the scene is disturbed by a small band of coelurosaurs passing through in search of prey. Because the time is late in the Cretaceous, about 75 million years ago. But the flora and the opossums are true to life.
Thank you for this discussion (how do you know all this, I mean about dinosaurs?); very enlightening.
I you read the beeb article, with sentences like this
pehaps you understand why an uneducated person like myself has to post OP like this at the SDMB.
With the new understanding I got from your posts, the article makes sense.
See here for a nice layman’s summary of the evidence (and controversy) for the dinosaur-bird lineage.
Could the most recent common ancestor of modern birds fly? (Sorry if that seems like a silly question, but I haven’t been able to find the answer.)