How To Evolve a Wing

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.

Solnhofen – Birthplace of Pterodactyls

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 about the different types of Pterodactyls that have been found at Solnhofen.

The Jurassic Air Was Full of Wings!

The First ‘Dactyl Ever Discovered – 1784, Solnhofen
In that year, Cosimo Collini thoroughly scrutinized a perfect skeleton of a curious creature preserved in the Solnhofen lithographic stone.

Jaws: long, slender, with sharp teeth.
Arms: very long, with an extraordinary finger that could be stretched out and  folded up.
Tail: very short.
Inner construction: bones hollow, like a bird’s.

Verdict: Not a bird, not a bat, but a new life form, never seen before.

More studies produced the name: “Wing-Finger,” Pterodactylus.

Solnhofen gave science a dozen more ‘dactyl species, some as large as a big seagull. Other Jurassic salt-water deposits produced specimens in England and France. Solnhofen still gave the best preserved ‘dactys. Some had the clear impression of the wing skin, connected to the single long finger on each hand.

Science Marches….BACKWARDS!

 William Buckland

Pterodactyl science has made a giant U-Turn in the last decades. Way back in the 1820’s, the best minds of paleontology were convinced that ‘dactyls walked and flew like bats. The Reverend William Buckland – “Mr. Jurassic” in England – drew up cartoons of Pterodactylus hanging onto cliffs like a bat, and fluttering over the Solnhofen lagoon with bat-style wings. His German colleagues agreed:  in design of wings and hind feet, ‘dactyls were definitely batty.

Then came the New Wave of ‘dactyl research in the 1970’s. Young Ivy League scholars went “tsk, tsk….that silly old Buckland. He was wrong.  Dactyls were really built like birds. All dactyls walked upright on their hind legs. They didn’t scuttle about in a bat manner.”

And so textbooks were revised. ‘Dactyls stood up and walked like Jurassic pigeons.

Footprints Back Buckland
But in 1980, a new set of evidence emerged – fossil tracks.  If the New Wave was right, then ‘dactyl tracks should look like bird tracks. The left and right hind feet should have swung back and forth close together, and the hands should have been kept off the ground.

Surprise! Fossil tracks made by small dactyls and by big ‘dactyls showed hind feet that were held out sideways. And the hands were flat on the ground! “Dactyls walked like………BATS!

The case is closed. Hundreds of trackways and bio-mechanical studies of shoulder and hips say the same thing. ‘Dactyls moved on all fours when on the ground, like bats.

Pterodactylus – Sand-Piper ‘Dactyl
The original ‘dactyl, the first species discovered..

Pterodactylus was the commonest ‘dactyl at Solnhofen. No bigger than a tiny sand-piper, Pterodactylus had extra long arms and neck and a compact body with almost no tail.

The thin jaws could be used to snatch small fish. Or the snout might have been used to probe the sand flats, like a shore bird, searching for hidden worms and baby clams.

The long neck, arms and short tail are proof that Pterodactylus was a Jurassic cousin of the enormous ‘dactyls of the Cretaceous, including the Texas Quetzalcoatlus.

Rhamphorhynchus – The Devil-Tailed ‘Dactyl
Rhamphorhynchus was a flying Fish-Trap. The jaws carried long, sharp teeth that slanted forward, and the tips of the snout and lower jaw were pointed too. This ‘dactyl  could dive down on the water and use its strong neck to throw the muzzle at fish and impale them.

The short arms made the wingstroke slower but more powerful than in Pterodactylus, so maximum speed was less but maneuverability was greater.

The long, bony tail was tipped with a vertical rudder of skin, reinforced by stiff fibers.

Mysterious Extinction
Long-tailed ‘dactyls went extinct in the Early Cretaceous and all the common ‘dactyl species of the Cretaceous were short-tailed relatives of Pterodactylus.  Why did the long-tails die out? What gave the short-tails their superiority?

The final extinction of all ‘dactyls struck when the last dinosaurs died out 65 million years ago, at the very end of the Cretaceous.

Anurognathus - Frog-Mouthed Mini-‘Dactyl
Jewel of the Solnhofen Exhibit.

Rarest of the rare, with only two specimens known, is the famous “Frog-Mouthed ‘Dactyl,” Anurognathus.

The Frog-Mouth breaks all the ‘dactyl rules. It’s super-tiny, half the size of Pterodactylus and about as bulky as the average little brown bat that hovers over Texas meadows today. And unlike nearly all other ‘dactyls, the Frog-Mouth doesn’t have a long snout. Instead the mouth is short and very wide.

The Frog-Mouth design is what we’d expect from an aerial insect-hunter, a ‘dactyl that hunted the dragon-flies and beetles of the tropical Jurassic sky. The short tail and abbreviated wrist bones would make quick turns easy. The wide mouth would work like a bug-trap.

Night Flier?
Anurognathus had the biggest eyes for it’s head size of any Solnhofen ‘dactyl. The acute visual system is a clue to unusual habits. Perhaps the Frog-Mouth was an insect-eater who flew in the darkness. More insects get airborne in the evening than during the daylight, so there would be more targets. And the eyes would have to be enlarged to detect  prey in low-light environments.

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