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.

Archaeopteryx has arrived in Sugar Land!

Last week a version of the show Archaeopteryx: Icon of Evolution‘ arrived at the Houston Museum of Natural Science at Sugar Land.  There are around 70 authentic fossils on display including the Geosaurs, Guitar Fish, several fossilized corals, insects, fish, plants and a cast of the Thermopolis Archaeoptertyx.

Check out the HMNS exhibit in Focus On: The Thermopolis Archaeopteryx [Pete Larson] from HMNS on Vimeo.

There have been lots of posts in the past year on the Archaeopteryx show here on the blog so take some time to refresh your memory on all there is to know!  And then come down the HMNS at Sugar Land to see these amazing and one of a kind pieces for yourself!

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.

Archaeopteryx – The Fossil that Proved Darwin was Right

1859: Charles Darwin published “On the Origin of Species.” Other scientists had proposed evolutionary theories before but Darwin was the first to work up a detailed case of how natural processes could transform one species into another.

Darwin claimed that even Classes could change – for example, the Class Reptilia could evolve into the Bird Class Aves.

“Where is the fossil proof??” exclaimed doubters. “Where is a transitional fossil that links one Class with another?”

The absence of missing links between Classes bothered Darwin.
………………………

Class to Class evolution would have to bridge immense gaps in anatomy and physiology:

  • Reptiles are a “Low Class.” They’re cold-blooded and can’t raise their body temperature much without basking in the sun. Birds are hot blooded and have so much metabolic heat that they can keep warm even in the snow.
  • Reptiles have scaly skin. Bird skin is clothed in feathers.
  • Reptiles have small, weak hearts and lungs. Birds have huge hearts and extremely efficient lungs.
  • Reptiles have small brains. Bird brains are gigantic, compared to their body mass.
  • Reptiles usually don’t spend much time in caring for their young. Birds lavish parental care on their babies.

Before 1861, it was hard to imagine how evolution could remake a reptile and make it into a bird.

……………..

Archaeopteryx changed all that. It was a bird because it had the complex flight feathers clearly preserved.  Feathers implied hot-bloodedness. And the brain appeared to be bigger than what a typical reptile had. Plus – the hind legs had long, narrow ankles, like a bird’s, not the flat-footed feet of a reptile. Archaeopteryx had three main hind toes pointing forward and a smaller toe pointed inward – the bird pattern, not the five-toed hind paw of a typical reptile.

The Archaeopteryx wing had three fingers arranged like a bird’s, not five as in most reptiles.

But Archaeopteryx  possessed extraordinarily primitive, reptilian features too. The tail had a long line of bony vertebrae. Modern birds have only a short, stubby vertebral column in the tail. Archaeopteryx had the three fingers of the hand separate instead of having the outer two fingers fused together.

Archaeopteryx had big, sharp claws on each of the three fingers instead of the blunt-tipped fingers of typical birds.

And Archaeopteryx had a mouthful of teeth instead of a modern bird’s beak.

More evidence of how birds evolved came in 1868. Professor Cope in New Jersey and Professor Huxley and Phillips in Oxford showed that meat-eating dinosaurs had been put together all wrong. Dinosaur legs weren’t flat-footed and five-toed. Carnivorous dinosaurs, in fact, had long, slim legs with ankles held high off the ground, and the hind foot had three main toes pointing forward. So these dinosaurs had bird-style legs.

Dinosaurs bridged most of the gap between primitive reptiles and Archaeopteryx. Most progressive paleontologists accepted the theory that Archaeopteryx evolved from a dinosaur.

The case became iron-clad in the 1880’s to early 1900’s. Excavations in the American West uncovered small meat-eating dinosaurs, like Ornitholestes, that had very long arms that matching those of Archaeopteryx closely.  The missing links were no longer missing. A primitive reptile had evolved into a primitive dinosaur which evolved into an advanced meat-eating dinosaur. And that dinosaur had evolved into Archaeopteryx, which in turn evolved into modern birds.

This is your last chance to see Archaeopteryx at HMNS. The exhibit is closing after labor day weekend. Don’t miss your chance to see the only Archaeopteryx on display in the Western Hemisphere.