How To Evolve a Wing


August 23, 2010
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Our Archaeopteryx show has bedazzling fossils – the only Archaeopteryx skeleton in the New World, complete with clear impressions of feathers. Plus frog-mouthed pterodactyls, fast-swimming Sea Crocs, and slinky land lizards. Today we learn the different ways in which wings evoloved on various prehistoric creatures.

Solnhofen show us three ways for Darwinian processes to construct a wing from a normal arm

Dactyls:
Dactyls evolved from very close relatives of early dinosaurs. The dinosaurs and their crocodilian kin are archosaurs. Archosaurs developed a unique asymmetry in the hand. Primitive reptiles, like today’s lizards, have five fingers, each with a strong claw. In archosaurs the outer two fingers are weak and have no claw at all.

Crocodilians and many dinosaurs kept this arrangement –  for example, stegosaurs and Triceratops had five fingers and three claws on the inner fingers. Meat-eating dinosaurs usually evolved three-fingered hands, doing away with those outer two claw-less fingers.

‘Dactyls evolved their archosaur hand in a different manner: they lost the pinky (the outermost finger). The claws on the inner three fingers were strong – useful for climbing trees and the sides of cliffs. The fourth finger evolved into an organ we see in no other creature: Finger four became immense, as thick as the thigh or thicker. The finger could be folded back where it joined the wrist for walking on the ground. When flying, the giant finger four was stretched outwards.

 Schematic of a generic pterosaur wing, pencil drawing, digital coloring
Creative Commons License photo credit: Arthurweasley

Solnhofen fossils showed that the wing surface was attached to the finger four and to the sides of the body and the inner edges of the hind leg. So ‘dactyls could flap like a bat – using up and down strokes of both arm and leg to make the power stroke.

Dinosaurs and Birds:

 Archaeopteryx

Birds evolved their wing by another wonderfully unique method. Their hand bones were 99% identical to those in small meat-eating dinosaurs. Only the three inner fingers were retained. Darwinian processes had clipped off the pinky and fourth finger. Solnhofen fossils prove that specialized wing feathers were attached to the second finger. So Archaeopteryx flew with the feathered arm.

Raptor-type dinosaurs, like Velociraptor and Microraptor, had evolved feathers very like those of birds. But these small dinosaurs evolved hind-leg wings to assist the arms. Flight feathers were attached to knee and shin as well as to the forelimb. When a tiny raptor-like dinosaur evolved into Archaeopteryx, the feathers were lost from the hind-legs, leaving just the arm to do the work of flying.

Bats:

Bats are specialized mammals and no bats had evolved in the Jurassic. The first bats appear much later, about 55 million years ago.

Bats use strong skin to make the wing. But unlike ‘dactyls, who evolved just one finger to support the wing surface, bats use three or four fingers to spread the wing and control the wing in flight.

Don’t miss Archaeopteryx: Icon of Evolution, currently on display at HMNS. Want to learn more? Check out our previous blogs on Archaeopteryx.

Bob
Authored By Bob Bakker

The Museum’s Curator of Paleontology, world-renowned Dr. Robert T. Bakker (or, as some call him, Bob) is the leader of the handful of iconoclastic paleontologists who rewrote the book on dinosaurs three decades ago. Along with other noted paleontologists, Bakker has changed the image of dinosaurs from slow-moving, slow-witted, cold-blooded creatures to — at least in some cases — warm-blooded giants well-equipped to dominate the Earth for 200 million years. Dr. Bakker can be found all over the globe, notably leading the Museum’s paleontology field program.

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