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.
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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.

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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.

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.

The Man Who Made Fossil Fish Famous

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 Louis Agassiz and his theories.

Louis Agassiz (1807-1873)

Paris and the Lure of Fish, 1836
Agassiz grew up in Switzerland where he excelled as a student in  chemistry and natural history. He went to Paris to study fish fossils under the Father of Paleontology, Baron Georges Cuvier. The geological history of fish seemed muddled at the time. Agassiz brought order to the fins and scales.

“There’s order in the way fish changed through the ages…” Agassiz concluded. He was the first to map out the long history of fish armor, fish jaws and fish tails.

1) The earliest time periods, the Paleozoic Era, most bony fish carried heavy armor in the form of thick scales covered with dense, shiny bone.

2) In the middle Periods, the Mesozoic, the armored fish became rarer and were replaced by fish with thin, flexible scales.

3) In the later Periods, the Cenozoic, thin-scaled fish took over in nearly all habitats.

4) Today, the old-fashioned thick scales persist only in a few fresh-water fish like the gar.

5) Tails changed too. The oldest bony fish had shark-like tails, with the vertebral column bending upwards to support the top of the fin. Later fish had more complicated tail bones, braced by special flanges, and the base of the tail was more symmetrical.

6) Jaws in the earliest bony fish were stiff, like the jaws of crocodiles. Later fish developed jaw bones that could swing outwards and forwards.

Discovery of the Ice Age
As he traveled across Europe, Agassiz saw evidence of giant ice sheets that had covered the mountains and plains. According to Agassiz’s theory, New England too had been invaded by mile-high ice layers. Giant hairy elephants – woolly mammoths – had frolicked in the frigid habitats. At first,  scholars harrumphed at Agassiz’s idea of a Glacial Period.  But by the mid 1840’s the theory was proven beyond a reasonable doubt.

Boston 1846: Toast of the Town & the New Museum
Fish and glaciers made Agassiz the most famous scientist of his time. When he came to Boston in the 1846, his lectures were so successful that the New England intellectuals wouldn’t let him leave. Poets and politicians, rich merchants and artists all helped raise funds to get Agassiz a professorship at Harvard. He repaid the support by working tirelessly to build a grand laboratory of science and education at Harvard – the Museum of Comparative Zoology. Opened in the 1859,  the MCZ has been a leader in fossil studies ever since.

Design in Nature
Agassiz’s interests spread beyond fish and glaciers. He sought the Plan of Creation, the key to understanding all of Nature. Was it  Evolution? No. Agassiz rejected any notion that natural processes somehow had transformed one species into another. He was a fierce exponent of the theory of Serial Creation: every species of fossil creature was created to fill its ecological role in its special geological time zone.

Darwin and Agassiz
Though he fought Darwin’s theories for his whole life, Agassiz’s work in fact provided support for the new views of evolution. The long trends in fish fins and scales were best explained by Natural Selection. Agassiz’s best students at Harvard went on to become strong supporters of Darwinism.  Endowed faculty positions were established in Agassiz’s name.  Agassiz Professorships were given to Alfred Sherwood Romer, the greatest Darwinian  paleontologist of the 20 century, and to Stephen Jay Gould, the most eloquent defender of Darwin in the last thirty years.

Don’t miss Archaeopteryx: Icon of Evolution, currently on display at HMNS. To read more about Agassiz and Darwin, check out my earlier blog.

The Animals of Solnhofen – Geosaurus

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 Geosaurus.

Geosaurus – Shark-Tailed Sea Croc
Speediest of the ocean-going crocodilians

Some creatures of the Late Jurassic lagoon were up & coming evolutionary clans – the teleosts, for example, were just beginning their takeover of the marine ecosystem. Other groups were Darwinian ultra-conservatives, living fossils in the Jurassic, changing slowly or not at all. The Chimaeras are a fine example.

And then there were a few very special cases – Late Jurassic critters that had reached the apogee of Natural Selection, the highest development of their race. Best representative of this phenomenon:

The Super-Swimmer Croc, Geosaurus.

The earliest crocs of the Triassic were land animals, roughly fox-sized with long legs. In the Early Jurassic, crocs went into rivers and lagoons.  That’s not a surprise. All living crocodilians swim well in freshwater, and a few – the Florida Croc and the Australian Salt Water Croc – will go out beyond the surf and navigate between oceanic islands.

But…..no modern-day croc is super-specialized for life in the high seas. None have the double-lobed tail of the sort we see in big, fast sharks, like the White Shark. Open-water sharks have a characteristic double-lobe tail. The vertebral column takes a sharp bend upwards to support the upper lobe of the tail. The lower lobe is made of tough skin and connective tissue. You can see the double-lobed tail configuration in our Archaeopteryx show in the hybodont sharks, a family common in the Jurassic.

To compete with such speedy sharks, a croc would have to evolve a double lobed tail. No crocs did – except one extraordinarily graceful clan, the geosaurs.

Our exhibit is graced with one of the finest geosaur specimens ever dug. This awesome Solnhofen skeleton demonstrates how evolution had transformed a “normal” river & lagoon crocodile into a reptilian torpedo, an open water predator that matches a shark in efficiency.

Geosaur evolution made a sacrifice unusual among the crocodilians – it traded in armor for velocity. All early crocs from the Triassic  and earliest Jurassic had thick bone plates over the back and neck and all over the throat and belly. All modern day crocs too carry extensive armor plate. This armor is useful when crocs are attacked by land predators or by other crocs. Most of the sea-going crocs of the Jurassic and Cretaceous kept some armor. Case in point: armor was carried by the teleosaurs, big  sea-crocs who were the apex predators at Solnhofen and most other sites in the Mid and Late Jurassic.  There is excellent evidence that large Jurassic dinosaur meat-eaters did indeed attack teleosaurs.

The geosaurs went a different way. They went skinny-dipping.

Geosaur skin was totally devoid of bone armor plates. They were naked. This development made the geosaur body lighter and more flexible.

Fast-swimming demands a specialized flipper for steering. The “normal” croc has long front legs and very long hind legs. The hind legs have wide webbed feet and assist the tail in propulsion underwater. All modern crocodilians and most fossil species keep this arrangement.

The geosaur limb equipment evolved in a unique way. Those long, strong hind legs were retained. But the fore-limbs were transformed into short flippers that worked like the diving planes of a submarine. No other croc clan did this with their front limb.

Impressive….but the outstanding geosaur specialization was the tail. “Normal crocs” have a deep, strong tail that bends down just a little bit at the end. The geosaurs went far beyond “normal” – they evolved a tail almost identical in profile to that of a modern tiger shark or a Jurassic hybodont. The geosaur tail possessed  two lobes, one bigger than the other in shark-fashion. 

Take a good look at our geosaur…..notice something strange?

The tail is upside down!  The vertebral column bends down, not up the way it does in sharks. Mummified geosaurs show that the upper lobe was made from tough skin and connective tissue, just like the lower lobe of sharks. The hydrodynamics of the upside-down tail worked just as well as the right-side-up shark tail.

Here’s a wonderful example of how evolution works: Natural Selection is opportunistic. It operates on what is already there. “Normal crocs” already had a slight down bend of the vertebral column.  For “normal crocs” to evolve a right-side-up version of a shark tail was almost impossible. But evolution took the simpler path by emphasizing the downward bend and then adding the upper lobe.

No croc of any age matched the swimming efficiency of geosaurs (although the Cretaceous Hyposaurus, from my home state of New Jersey came close). Most other croc groups are distant seconds. Therefore, the Late Jurassic was the high point of croc-natatory prowess (look it up;  “natatory”, a good adjective).

Why? Why didn’t some later croc group evolve upside-down shark tails as specialized as those of geosaurs?  We don’t know. My guess is that sharks evolved so fast in the Cretaceous that crocs were pushed out of the open-water/fast-swimming niches.

One more thought – geosaurs probably had to crawl onto sandy beaches to build nests and lay eggs. Their tiny flipper-like fore limbs would have been a big disadvantage – mom geosaurs must have been far more vulnerable to land predators than “normal crocs.”