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How Dinosaurs Shrank and Became Birds
An anonymous reader writes: Discoveries have shown that bird-specific features like feathers began to emerge long before the evolution of birds, indicating that birds simply adapted a number of pre-existing features to a new use. And recent research suggests that a few simple changes — among them the adoption of a more babylike skull shape into adulthood — likely played essential roles in the final push to bird-hood. Not only are birds much smaller than their dinosaur ancestors, they closely resemble dinosaur embryos. Adaptations such as these may have paved the way for modern birds' distinguishing features, namely their ability to fly and their remarkably agile beaks. The work demonstrates how huge evolutionary changes can result from a series of small evolutionary steps.
The more I read, the more it looks like it should be possible to "backport" birds to a surprising degree even without any unobtanium "dinosaur DNA". Even without studying what specific genes do, we can already start by comparing different lineages to see what genetic shifts in birds occurred between their theropod ancestors and modern descendents (for example, if most other groups of animals, including alligators, have a certain gene but birds don't, then that change occurred at some point on the bird side of the branching point between birds and alligators). Looking at modern descendents won't give us an exact picture of their common ancestors, but it'll certainly let us role back a lot of the changes. Combining that with reasoning out and experimenting with what morphological changes in birds that differ from dinosaurs are the result of what genes... we should be able to come up with something rather close to their ancestors at different stages.
It's amazing how much detail they're starting to be able to determine about ancient species - even to the point of being able to determine the number of wing quill feathers in velociraptors. We're certainly constraining the reversal problem more and more.
"Who the **** put an emergency exit in the interrogation room?!" -- Police chief, "Jesus Christ Supercop"
Other things too, from thinking about modern birds: can we assume that theropods had a syrinx rather than a larynx? Then they would be able to have very tonally-complex sounds, including vocalizing multiple different frequencies at the same time.
I assume they had a similar lung layout? Birds have a really brilliant respiratory system. The lungs are rigid and more like tubes for the passage of air rather than storing it. On inhalation, half the air goes directly into one air sac and the other straight through the lung into a different air sac; then on exhalation the sacs reverse so that the "used" air goes straight out and the "unused" air goes through the lung on the way out. So they get fresh air moving through their lungs both on inhalation and exhalation, and they never mix fresh air with used air. This means that the oxygen content of air in their lungs is much higher, which means that the oxygen levels in their blood can be much higher. It helps sustain them during high metabolic activity such as flight; I'm sure their giant predatory ancestors made good use of that oxygen as well.
I wonder if their ancestors had a similar sort of relatively inefficient fast-through digestive system, or whether that's an adaptation their descendents have made for flight? It's known for a fact at the very least that some dinosaurs consumed rocks to aid in digestion (gizzard stones) in the same way birds consume grit. Hmm, so theropods would likely have some sort of a crop then? I mean, there is evidence that at least some theropods cared for their young. Picture a bunch of baby velociraptors reaching their heads into a parent's jaw to get a meal!
It takes no imagination to picture correspondence between the legs / feet, bird legs and feet already look positively dinosaurian.
Even the evidence of fossilized prints of rough scaly skin from some tyrranosaurids (in addition to evidence of feathers, and some completely feathered) shouldn't be a real shock because we see that in modern bird species. For example, look at the head of a bald ibis or turkey vulture.
"Who the **** put an emergency exit in the interrogation room?!" -- Police chief, "Jesus Christ Supercop"
Well, dinosaurs were divided into two main groups, the lizard-hipped and the bird-hipped dinosaurs. Birds evolved from the lizard-hipped dinosaurs, and the bird-hipped dinosaurs died out.
Reptile is a catch-all term that doesn't really mean anything. Crocodiles are much more closely related to birds and dinosaurs than they are to any other reptiles (so should probably be grouped with birds rather than lizards and snakes), and extinct "reptiles" like dimetrodon are more closely related to humans than they are to any extant reptiles.
My general sense is that pretty much old everything is big. But what if fossils expanded with time?
Not really true. The number of different species that have lived on the earth, it would be astonishing if the largest species to ever have lived had evolved recently... but this is exactly what has happened.
Also, there are some things (I hesitate to call them species) like the Coelacanth which have stayed very similar in form over hundreds of millions of years.
Large animals get more press. I'd guess they also fossilise better and are easier to find.
Regarding large insects and arachnids.... no one is completely certain, apart from the fact that there was a much higher oxygen content in the atmosphere during the Cambrian, allowing larger sizes. This does not explain everything though (but what does?).