DINOSAURS AND THE ORIGIN OF BIRDS

Dinosaurs-1 (theropods/carnivores and sarupods/brontosaurs), dinosaurs-2 (duckbills/crested-duckbills/boneheads, stegosars/ankylosaurs, ceratopsians), crocogators (crocodiles, alligators, caymans, and so forth), and pterodactyls (both short-tails and long-tails) are all grouped into the category Archosaurs ("ancient lizards").

If Protoavis (about 230mya) is really a bird (most think it's some random bones from two or more creatures) then one would have to make the birds equal in stature to the dinosaurs, rather than descended from them.

It's not really clear if all of these disparate archosaur-creatures descended from one single ancestral species yet to be found in the fossil record, or if they arose simultaneously from various species of Thecodont (the earlier group that ran the planet before 240mya) or Reptiles.

Of course, scientists are just not at all sure how a new species arises (although they know it does; DNA tests of polar bears has proven they are mutated brown bears). It is possible that one day a bit of cosmic radiation hit the DNA of some pre-embryo and caused a mutation which happened to turn out to be beneficial and meant that descendants of that one critter became in ten-fifty thosuand years the new species. It's also possible that some environmental effect caused a fairly common and harmless variation that runs rampant through a species to become advantageous (but then again, did this fairly common and harmless mutation come from a single unique ancestor?). For what it's worth, we do know that every single person on the planet descends from one specific woman who crossed the Red Sea from Africa into Yemen, and that every single person on the planet descends from a single male who lived in some different time and place, so maybe...

[BTW: That sailback Dimetrodon in your toy dinosaur set is actually a pre-dinosaur (Permian age) critter which is the distant ancestor of mammals including humans.]

The most widely accepted theory is that birds are descended from some creature that looks pretty much like Archaeopteryx (154mya), and that the dramaeosaurs (which mostly date after Archy) are just the closest dino relatives to birds (cousins), not ancestors of birds (birds split away from the dino family tree about 160mya). Virtually all dinosaur paleontologists accept this as more or less proven fact (admitting that a lot of details remain unknown).

This is part of the generally-accepted theory that there was a huge gene pool of many unrelated small theropod species (and a few mediums and one really big species) at any given time, and that when the environment changed the top predator went extinct and one of the medium-sized ones grew up into the six-ton/forty-foot niche and one of the really small dinos grew up to take the opening in the medium-size racket. We do know that the Allosaurus bloodline (which ends more or less with Acrocanthosaurus) died out (after evolving into something halfway between Allosaurus and T-Rex) and the Coelurosaurs (which include T-Rex) grew up and took over.

I should mention radical Olshevsky Theory which nobody but George Olshevsky accepts. Under this theory, there is always a number of even smaller species living in the trees (birds!) and that whenever the big six-ton dino dies, one of the one-ton dino species grows up, one of the 50/500-pound dino species grows up to take that job, and one of the 1-pound birds grows up to take THAT job. George thinks that dinosaurs are in fact the descendants of ancient birds (never found in the fossil record since bird bones are too small to make nice fossils), not the other way around. If George wasn't such a gadfly then maybe his theory would be taken seriously.

The other theory (which for some reason is a favorite of Biblical Paleontologists and, less curiously, of a whole lot of Bird Paleontologists) is that whether Protoavis is real or not, there just must have been real birds not related to dinos around for a long time prior to Archy (but birds don't fossilize very well so we have never found these). Except for the questionable Protoavis, there are no birds earlier than Archaeopteryx, but there are a lot of small feathered dinosaurs after him. Some of those could be descendants of Archy who evolved back into dinosaurs rather than continuing into birds.

Of course, I'm an engineer, not a Paleontologist, but I have read a lot of dinosaur books.