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.

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.”

Jurassic Puckering – Kissing Fish and Salt Water Evolution

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.

But wait…….there’s more!

The exhibit has spectacular Jurassic fish!

By “spectacular” I don’t mean gigantic. Most of the Jurassic finny species were small to mid-sized, salmon to tuna in bulk. The extraordinary thing about our Jurassic fish is that the fossils capture the single most dramatic moment in all of fishy evolution….

….The Teleost Takeover!!
Teleosts are the unrivaled Rulers of Fish-Dom. Teleosts make up three-quarters of today’s fish species. When you eat fish, chances are you’re eating teleosts: tuna are teleosts, and so are mahi-mahi, swordfish, bluefish, salmon, red snappers, trout, bass, eels, herring and anchovies…

Gefilte Fish?  Yep. Teleost. So are white fish, smoked or non-smoked. Sail fish, tarpon, guppies…..

Teleosts belong to the “Bony Fish Clan,” one of the two huge branches on the fish family tree. The other big branch is for sharks, rays and skates

Pucker Faces
What makes teleosts so great? Pucker skulls. Go to an aquarium or a really big fish store. Watch the feeding. Teleosts protrude their faces, pushing out the jaw bones so the mouth cavity expands. Water rushes in, carrying food. Large mouth bass expand their mouths so wide they can suck in an entire duck.

Teleosts can use their pucker faces to reach out and nibble. Parrot fish do that when they gnaw away coral polyps. Catfish use their suction-mouths to vacuum worms from a muddy bottom. Remoras use their pucker-sucker mouths to get a free ride from a big shark.

Ok – all fish anatomists (people who dissect fish, not guppies who get PhD’s) agree: the pucker-swing-out, expanded face & jaws is the key innovation that opens the door to thousands of potential ecological niches.

Pucker Free Times
It wasn’t always so. The first bony fish comes in at about 400 million years ago. For the next 200 million years, there was no puckering. Jaw bones were firmly attached to the rest of the skull. Today, we have one Texas fish that still is no-pucker – the garfish. It grabs prey the way a gator does, with a simple snap.

First Tentative Pucker
In Triassic times, about 220 million years ago, some bony fish evolved a semi-moveable set of jaw bones that let the mouth expand sideways. Bowfins today are at this stage of evolution. Our Jurassic fish show has some nifty bowfin fossils.

Teleost Shock Troops
Bowfins didn’t go far enough. In the Late Triassic and Jurassic the first teleosts evolved. Face and jaw bones became mobile. Sucker-pucker faces started to diversify. That’s just the stage we catch in our Archaeopteryx display. The Late Jurassic lagoon beds from Solnhofen, Germany, abound with this first wave of teleosts. Don’t take my word for it. Come close to the glass cases with Thrissops, Pholidophorus and other early teleosts. Check out the jaws.

Teleosts would continue to expand into the next Period, the Cretaceous, and the through the following Age of Mammals.

so…..come to the Archaeopteryx show for the feathers…stay for the fish-faces.

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.