The Ghost Sharks of The Jurassic

Fish Pieced Together by Committee

Our HMNS at Sugar land exhibition on Archaeopteryx is festooned with splendid finny fossils, the ichthyological gems of the Late Jurassic. Some of the Jurassic fish were new comers, recently evolved clans that were poised to conquer the watery ecosystem. Teleosts are one such progressive group.  But there are Jurassic living fossils. These are old, old clans that had evolved a hundred million years before the Jurassic, or more. And these groups had stalled out, in evolutionary terms, changing little.

Chimaeras, the “ghost sharks,” are the most exotic of the Jurassic living fossils. The label “Chimaera” evokes the mythological critter that was put together with spare parts from many species. The real chimaera fish do seem to be constructed that way. The huge eyes and  nibbling snout look like a rabbit’s, inspiring the label “Rabbit Fish.” The front fins are huge, resembling those of a butterfly-fish. The tail is long and thin, like a rodent’s – hence another nickname “Rat-Fish.”

In fact, Chimaeras are distant cousins of sharks and rays.

Chimaeras were already an antique group by the Jurassic, with an origin going back almost to 400 million years ago. The body form had been standardized by the Coal Age, 330 million years ago. The key feeding feature was the solid, strong, stiff heads. Teleost fish gained success by loosening up face and jaw bones, so the mouth could expand. Chimaera evolution went the other way. Their skulls were solidly knit together, with jaws, face and braincase units braced against each other.

Global paleogeographic reconstruction of the Earth in the late Carboniferous (“Pennsylvanian”)
period 300 million years ago.
Creative Commons License photo credit: Dr. Ron Blakey

The solid chimaera skull was co-evolved with flat, thick teeth that could crunch and crack crabs, clams and all sorts of other hard-shelled teeth. In other words, the chimaera was the fishy equivalent of a sea-otter. For protection, chimaeras were outfitted with a sharp, tall spine at the front of their dorsal fin.

Even though they hadn’t upgraded their adaptive equipment for a long time, Jurassic chimaeras continued to succeed as  bottom-hunting predators of shellfish. And they kept on going, and going, and going….

….surviving through Period after Period. They’re still around today. In their anatomy, the modern chimaeras are barely different from the Jurassic species. But habitats have changed dramatically. Jurassic chimeras thrived in shallow water of the Solnhofen lagoons and reefs. And the earlier chimaeras too were mostly inhabitants of the shallows. Most species alive now are  deep-water specialists. The same slide from shallow to deep water happened to one group of bony fish, the coelacanths.

That’s a general evolutionary principle: fish that ranged through upper waters in the past tend to get restricted to the deep today.  Why? Maybe new predators and competitors tend to evolve first in shallow water. And thus shallow water becomes the most dangerous place for old clans who don’t evolve fast. Going deep may free the old-style fish from many of the exuberant new clans.

Maybe….

Learn more about evolution by visiting our unique collection of fossils in Archaeopteryx: Icon of Evolution, now on display at HMNS at Sugar Land.

Edward Hitchcock

Archaeopteryx: Icon of Evolution is coming back, a week from tomorrow! Don’t miss the exhibit as it flies into the Houston Museum of Natural Science in Sugar Land, opening Oct. 22, 2010. Famed paleontologist Bob Bakker provides more insight into this unique fossil.

The Reverend Edward Hitchcock -
The Man Who Predicted Archaeopteryx – a Quarter Century before the Discovery.

Who discovered Archaeopteryx?  In 1861 Herman von Meyer recognized a Solnhofen fossil as a bird.

But another scientist had predicted that Archaeopteryx HAD to exist, back in 1836.

Edward Hitchcock was the State Geologist of Massachusetts, and a leading Biblical scholar of the time. He was a serial creationist who believed that Creation was accomplished through many events spread across geological ages. Hitchcock was a Jurassic specialist who dug hundreds of fossil footprints from the Connecticut Valley. From this evidence, he figured out that there must have been  birds in the Jurassic. Some of Hitchcock’s Jurassic birds were as tiny as sandpipers. Others were as large as rhinos.

Hitchcock didn’t have any good fossil skeletons from his Jurassic digs. But still he  reconstructed the toe bones from the imprints in the rock.  First he ran all sorts of living animals on muddy fields, from turkeys to frogs to raccoons to barefoot farm boys. He diagrammed where the toes fit in the footprint. Soon the Reverend Hitchcock knew more about the animal sole than any other scholar.

Here are the clues he gathered from scrutinizing feet:

  • The dominant beasts who left their tracks in Jurassic rocks walked on their hind-feet – like birds.
  • The Jurassic creatures walked with their ankles high off the ground in long strides – like birds.
  • These animals had three long hind toes spread out, with the longest toe in the middle – like birds.
  • There was a little toe on the inside of the foot, pointing inwards and backwards – like birds.

Then Hitchcock reconstructed the details of the foot skeleton of the Jurassic track-makers.  He discovered that joints where two toe bones came together usually were supported by a thick pad of skin. So, using the pad marks in fossil tracks, Hitchcock worked out the toe bone geometry. The first toe had two bones, the second had three, the third had four, and the fifth toe was absent. The toe-bone design was exactly like a bird’s!

Hitchcock never suspected that his track-makers were dinosaurs, because at the time all dinosaurs were reconstructed with lizard-like feet, five-toed and flat-footed.

One discovery bothered Hitchcock – occasionally he found imprints of the forefeet where his animals had rested. The fingers had sharp claws, things that normal birds didn’t carry.

By 1840, Hitchcock was sure that a special subclass of birds had ruled the Jurassic – birds with clawed hands.

But where were the fossil bones?

In 1861 the first skeleton of Archaeopteryx was discovered. The bones testified that Hitchcock had been right. Here was a bird with feathers and toes arranged perfectly to make Hitchcock’s tracks. Plus, the hands had sharp claws.

In 1868, Professors Thomas Henry Huxley and John Phillips added more proof that the Jurassic had been ruled by birds. They restudied bones of meat-eating dinosaurs and exposed the mistakes in previous reconstructions. Dinosaurs were NOT flat-footed at all. Instead, carnivorous dinosaurs had bird-style hind feet that fit Hitchcock’s tracks. Hitchcock’s Subclass of Jurassic birds turned out to be dinosaurs!

Archaeopteryx was both a bird and a dinosaur.

In the 1990’s, Chinese discoveries showed that many carnivorous dinosaurs had feathers.

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.