Shark Progressives in the Jurassic Period

Maybe you have heard this Shark Myth:
The Great White is a Living Fossil, a man-eating hold-over from the dim reaches of time.

No.  It is true that the oldest sharks do go back to 400 million years ago. However, an enormous amount of evolutionary change separates these primordial sharks from a modern-day Great White Shark. The final episode of shark modernization didn’t really get going until the Jurassic about 150 million years ago. The vertebral columns became far firmer and stronger, so the body and tail muscles could contract with greater power – and that meant faster speeds. The Great White and many other sharks alive today inherited their firm vertebrae from Jurassic ancestors. Plus – this  Darwinian revolution gave some sharks, including the ancestors of Great Whites, bigger brains and more complex behavior.

Guitar Fish Rhinobatis

The modernized sharks that evolved in the Jurassic included the “normal” shaped sharks and the flat-bodied skate/ray group. The skates and rays of the Late Jurassic at Solnhofen had already evolved a modern stage of vertebral column, fins and jaws. Guitar-fish are rays with long, elegant, pointed snouts. The Solnhofen Guitar Fish Rhinobatis is so thoroughly modern-looking that it would look at home today in Bermuda or Sumatra. Wide, flat pectoral fins and a flat underside let the guitar-fish hug the sand while searching for shrimp buried beneath. Special sensory organs could detect the electrical impulses from living bodies hidden from view. They had tightly packed rows of small, blunt teeth worked to crush clams, snails and other hard-shelled prey.

For some mysterious reason, people the world over look at these flat sharks and think of folk music and rock ‘n roll. Hence the nickname “Guitar Fish.” Because of a difference in the culture of stringed instruments, the guitar fish are called by a different name in Russia: here they are known as “Balalaika Fish.”

Poison Spine Sharks – Hybodonts

Solnhofen hosted both the most modern, advanced sharks and some hold-overs, families that were already a hundred million years old at the time. Hybodonts are an excellent example of ancient shark technology in body form. The vertebral column was still  constructed from soft connective tissue, not the tough cartilage of modern sharks. Hybodont teeth were dual function. Up front were sharply pointed nippers for grabbing prey. In the back were many flat teeth for crushing lobsters, crabs, snails, clams and sea urchins.

Hybodonts had the ancient way of protecting themselves from predators. They carried  poison-spines at the leading edge of the two dorsal fins. A gland at the base of the spine secreted venom that oozed up along a groove. Any predator that attacked would run the risk of getting a painful jab that would leave a throbbing wound.

How do we know about poison from hundreds of millions of years ago? Because there are some sharks today with this type of weaponry. The Horned Shark, Heterodontus, is a smallish species with a slow-swimming style. Both dorsal fins have spines, and venom is produced at the base of the spines. Horned Sharks are small enough to be swallowed whole by some bottom-living predators. But when the attacker feels the spines poking into the roof of its mouth, the Horned Shark is spit out with no harm done. The anatomy of Horned Shark spines is close enough to what we seen in fossilized hybodonts that we are safe to conclude that the hybodonts too were poisonous.

Another common shark alive today, the Spiny Dogfish, is also outfitted with hard spines and poison glands, though the toxic properties of the apparatus isn’t as extreme as that of Horned Sharks.

Poison spines go way back in shark evolution, and both fresh-water and salt-water species were equipped with the venom-delivery system in the Coal Age, some 340 million years ago. It’s intriguing that the modernization process in the Jurassic caused a shift – most of the new clans of sharks would give up the bigs spines. Great Whites are not protected by spines, nor are Tiger Sharks, Blue Sharks, Bull Sharks and most other “normal” sharks.

Why did so many progressive sharks discard the venom-delivering spines? The loss of spines occurred as the most advanced bony fish were shifting from thick, armored scales to thinner, more flexible body coverings. It looks like evolution favored speed and agility over passive defensive structures.

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


Which raptor turned into the first bird?

We get so many great questions through our blog, and every now and then we can turn those responses into a blog post. One our readers favorite posts is “What would YOU ask a paleontologist?”

Last week we got this question from kght2:

“Do all birds come from a specific raptor, or do they come from different species of raptor that are cousins and not ancestors. I wonder this because while all birds are similar, they don’t seem to be any more similar than different raptors I have seen, and while this isn’t great information, I have heard of many raptors likely having some from for feathers. Primarily i wonder if the consensus is that all birds came from a single species, or that they came from a family of species instead, and this answer would also have implications that people should know for any species or family of species?”

Dr. Bakker, curator of paleontology here at the museum wrote this in response:

Another darn good question.

Archaeopteryx was the first bird, back in the Late Jurassic. It’s got the complicated arrangement of feathers on its arm to fly like a pheasant today does. All other birds evolved from Archaeopteryx or something very like it

Deinonychus (read my blog about Deinonychus) is a famous raptor-dinosaurs who look very close in their bones to Archaeopteryx. The tiny Microraptor from China is closer still.  Thanks to the dinosaur specimens from Laoning, China, we know that all the raptor-type dinosaurs had feathers. (T. rex had feathers too – the tyrannosaur clan were clothed in a full pelt of fine kiwi-style plumage.) But Deinonychus and all the Laoning feathered dinosaurs are from the Early Cretaceous – that’s too late to be an Archaeopteryx ancestor.

We need a Jurassic raptor to be our Archaeopteryx ancestor.

We now have a few specimens from the end of the Jurassic. These are advanced raptor-like dinosaurs with long arms built like Archaeopteryx.

So….we’re getting close to discovering the one, single raptor-dinosaur who evolved into the first bird. It had to be in the Mid or Late Jurassic.

If you have any questions you would like to ask any of our bloggers or curators, send us an email at blogadmin@hmns.org.

Interested in learning more about dino-birds? Make sure to check out our next exhibition, Archaeopteryx: Icon of Evolution, opening April 23, 2010.

100 Years – 100 Objects: Diplodocus

The Houston Museum of Natural Science was founded in 1909 - meaning that the curators of the Houston Museum of Natural Science have been collecting and preserving natural and cultural treasures for a hundred years now. For this yearlong series, our current curators have chosen one hundred exceptional objects from the Museum’s immense storehouse of specimens and artifacts—one for each year of our history. Check back here frequently to learn more about this diverse selection of behind-the-scenes curiosities—we will post the image and description of a new object every few days.

This description is from Dr. Bob Bakker, the museum’s curator of paleontology. He’s chosen a selection of objects that represent the most fascinating fossils in the Museum’s collections, that we’ll be sharing here – and at 100.hmns.org/ – throughout the year.

Diplodocus
Jurassic Period
140 million years old

B13_0018She’s not the weightiest herbivore in her Jurassic world. Her close kin Apatosaurus would be twice as heavy. Her neighbor Brachiosaurus would be four times her bulk. But no other dinosaur can exceed our Diplodocus in the combination of length and delicacy of architecture.

The animal is labeled a “she” from an old tradition but, in fact, we don’t know the gender, yet.

Diplodocus is one of a trio of long-necked giants who together make up 90% or more of the large dinosaurs in the American West during the final stages of the Period. Usually, Camarasaurus is commonest. Its thin neck, of moderate length, boxy head and long front legs contrast with the attenuated neck, pointed muzzle and short forelimbs of Diplodocus and the Apatosaurs. Apatosaurus itself matches the Diplodocus proportions closely except that every bone is greater in girth.

B13_0010 The trio was unearthed in the two decades of the Great Jurassic Gold Rush, when eastern museums revealed the riches of the Jurassic fauna in Wyoming and Colorado. The first good Camarasaur skeleton was dug in 1877; the first good Apatosaur in 1879, the first good Diplodocus in 1896.

The HMNS Diplodocus was 78 feet long, 12 feet high at the hips, and probably weighed 10-15 tons.

Wander among prehistoric beasts in the Paleontology Hall, a permanent exhibition at the Houston Museum of Natural Science.

You can see more images of this fascinating artifact – as well as the others we’ve posted so far this year – in the 100 Objects section at 100.hmns.org.