Ancestors and Feathered Dinosaurs

Archaeopteryx

Confuciusornithes

Birds are dinosaurs?

Paleontologists have determined that the ancestors of birds can be found among the dinosaurs so it is no surprise that members of the two groups share a handful of basic characteristics:

The diapsid skull.Both birds and dinosaurs are descended from primitive vertebrates in which muscles from the top of the skull pass through two passages to operate the lower jaw. Mammals have a synapsid skull with one passage way and turtles have an anapsid skull with no openings.
Bipedal gait. Although some dinosaurs walked on all fours, they appear to have descended from earlier forms that were more-or-less bipedal. All of the meat-eating dinosaurs, including the apparent ancestors of birds were bipedal and all birds are bipedal.
Digitigrade stance. Birds and dinosaurs walk on their toes while mammals walk on the flat of their foot and are plantigrade.
Suspension from the hips. The bodies of birds and bipedal dinosaurs hang down from a vertebral column that balances, more or less horizontally like a teeter-totter, across the hip joint.
Food ingestion. Both birds and dinosaurs are designed to ingest food in relatively large pieces that are processed internally. Mechanical reduction in the mouth (chewing) is very rare in birds. The best examples occur in cuckoos, hoatzins, and turacos. The teeth of dinosaurs suggest that chewing was limited to a few groups of plant-eaters. Both dinosaurs and birds ingest rocks or grit to assist with mechanical reduction. The saliva of birds lacks digestive enzymes and chemical reduction (digestion) is wholly internal. The properties of dinosaur saliva are unknown.

Primitive birds also share some very specific features with their closest relatives among the dinosaurs. Archaeopteryx, the earliest known bird, shared so many of these characteristics that in many ways, it was just another feathered dinosaur:

Feathers and scales
The long, reptilian tail
Pubic bones that are fused at the tips (a pubic spoon or symphysis)
A wishbone or furcula instead of separate collar bones (clavicles)
The four-toed foot that apparently included the erectile claw of the dromaeosaurs
A long, curved shoulder blade (scapula)
A semi-lunate carpal in the wrist that facilitates the peculiar sideways folding (pronation) of the forelimb or wing
Simple conical teeth of the type characteristic of dromaeosaurs.

Only fine details of its skull architecture and the rather large brain distinguish Archaeopteryx from other dinosaurs

Birds are not dinosaurs anymore!

Since the days of Archaeopteryx, birds have abandoned or modified all of their most dinosaur-like features:

The simple reptilian tail has been replaced by a shorter but more specialized structure that facilitates control in the air. About half of the original tail is now fused to the hipbones.
The tips of the pubic bones are no longer fused. Because they do not form a bony ring, there is no longer a skeletal constraint on egg size.
The wishbone has undergone a great many changes. It has been abandoned in some species but is a large and varied bone in others.
The bird’s foot may have two, three, or four toes and has been adapted to a variety of special purposes beyond walking.
A small bump on a wing bone may be all that is left of the semi-lunate carpal.
The teeth have completely disappeared and their function has been taken over by a keratinous beak.

Birds have also developed novel skeletal structures that facilitate advanced forms of flight and a broad abdominal vault with room for the large egg that makes intense parental investment a successful reproductive strategy. They have used those adaptations and others to invade the oceans, a habitat apparently closed to dinosaurs.

For information on current trends in the study of fossil birds go to the website of SAPE (Society for Avian Paleontology and Evolution):

http://www2.nrm.se/ve/birds/sape/litt001.html.en

What are the differences between Dinosaur-like Birds and Bird-like Dinosaurs?

Some answers are proposed in the following pages.