Cladogram Questions and Segments

Which came first, flight or bipedality?

 

Some scientists have said that birds did not become bipedal until after their forelimbs were committed to flight. That hypothesis, too, can now be rejected because fossils show that bipedal habits arose before flight.

Some scientists have also said that flight is less likely to evolve in two-legged (bipedal) running animals than in quadrupeds. On the contrary, the fossil evidence shows that flight arose twice in the bipedal, bird-line archosaurs, first in pterosaurs and then in flying dinosaurs. This evidence allows us to conclude that freeing the forelimbs from the demands of terrestrial locomotion may have fostered flight.

Why did feathers arise?

 

Some scientists have said that feathers arose for flight, only later to be used to hold in body heat. That hypothesis can now be rejected, because the fossil record demonstrates that downy plumes appeared before flight feathers in dinosaurs. This indicates that feathers arose for insulation or, perhaps, some other function, such as display.

 

1. Hair-like feather structures for insulation
Large pubic boot
2. Strong transition point in tail (stiff tail)
3. Origin of “flight” stroke for grasping prey
4. Enlarged feathers on hands and tail
5. Reversed pubis
6. Flight feathers
Reversed hallux (first toe)
7. Keeled sternum
8. Strut-like coracoid
Less than 18 vertebrae in tail

When did feathers arise in dinosaurs?

 

Unfortunately, since no skin impressions are known from early Mesozoic theropods, we still don’t know. Scaly skin was clearly present in early dinosaurs: fossilized skin impressions are known from extinct ornithischians and sauropods.

So dinosaurs started out with scaly skin and ended up feathered. That change occured somewhere between Sauropoda and Sinosauropteryx, a narrower zone than before, thanks to the Liaoning fossils. But we have yet to find out precisely when feathers first appeared.

 

1. Hair-like feather structures for insulation
Large pubic boot
2. Strong transition point in tail (stiff tail)
3. Origin of “flight” stroke for grasping prey
4. Enlarged feathers on hands and tail
5. Reversed pubis
6. Flight feathers
Reversed hallux (first toe)
7. Keeled sternum
8. Strut-like coracoid
Less than 18 vertebrae in tail

Did Tyrannosaurus have feathers?

 

So it seems.

The scientists who devise cladograms like the one above focus on the appearance of new characteristics in the fossil record to attempt to establish evolutionary relationships. In the cladogram, all of the dinosaurs appearing to the right of Sinosauropteryx, where feathers arise, including Tyrannosaurus, share that characteristic.

Thanks in part to the recent discoveries in China, we now know that nearly all the ways in which birds differ from other living animals appeared in dinosaurs long before birds originated. We are now able to answer definitively some of these specific questions—or nearly so.

 

1. Hair-like feather structures for insulation
Large pubic boot
2. Strong transition point in tail (stiff tail)
3. Origin of “flight” stroke for grasping prey

Were dinosaurs warm-blooded?

 

Some scientists have wondered if dinosaurs were warm-blooded (endothermic). There is no doubt that the living ones are: birds are warm in the palm of the hand. Because the only dinosaurs that we can directly test are demonstrably hot blooded, the question should be “when” not “if” they became hot blooded.


1. Hand moves in flat plane
2. Hair-like feather structures for insulation
Large pubic boot
3. Strong transition point in tail (stiff tail)
4. Origin of “flight” stroke for grasping prey
5. Enlarged feathers on hands and tail
6. Reversed pubis
7. Flight feathers
Reversed hallux (first toe)
8. Keeled sternum
9. Strut-like coracoid
Less than 18 vertebrae in tail
10. Alula
Heterocoelus neck vertebrae

Are birds really dinosaurs?

 

Some scientists classify birds separately from dinosaurs because birds seem so different. The Liaoning fossils have, however, erased most of those differences. They show, for instance, that there is not much difference between a non-flying dinosaur, such as Protarchaeopteryx, and one, like Archaeopteryx, that flew.

Some scientists argue that dinosaurs were too big to have given rise to birds. It is true that many Mesozoic dinosaurs were the most gigantic land dwellers that have ever lived. But not all dinosaurs were large—the largest Chinese dinosaur in this exhibit is not much bigger than a modern-day turkey. Flying dinosaurs are small, and they show several features related to size reduction. The teeth of baby velociraptors, for example, are similar to the adult teeth in flying dinosaurs such as Archaeopteryx.

Reconstruction of Archaeopteryx by S.F. Glaser. © YPM