Your questions, answered: Do we know when chimaeras shifted to deep-water habitats?

Earlier this month we received a question on one of our past posts, The Ghost Sharks of the Jurassic, asking:

“Do we know when these chimaeras shifted to deep-water habitats? If predation and, in particular, the evolution and diversification of predatory species prompted their geographic transition, at what point would a sort of critical level have been reached to drive them into the deep? How many predators are too many?”

Why do “living fossils of the deep sea” so often represent lingering survivors of groups that long ago flourished in shallow water?


Examples: Rabbitfish (aka chimaeras), Coelacanths, Goblin Sharks, Giant Squid.

Excellent question – one that keeps evolutionary biologists awake at 2 a.m.

First thing: we never know when a clan of species invades deep water. This is why:

Sediments deposited on top of oceanic crust in deep-water – thousands of feet deep – rarely come to the surface where the layers can be seen by fossil-hunting paleontologists. Mud does form at the bottom of deep seas and fossils do form here. But such deep specimens have a low chance of being found by us.

Deep sea bottom mud is raised above sea level when continents collide and abyssal sediment is squeezed up and thrust across slabs of continental crust. There are narrow zones of such squeezed sediments – for example, in the Taconic Mountains of New York State. Here are slices of deep crust and sediment with deep-water trilobites. However, very few vertebrate and squid fossils are known from squeezed deposits.

Medium-deep sediment, up to 200 meters deep,  do form in the bottom of “epi-continental seas” like the famous  “Cretaceous Ocean of Kansas” that covered much of the central areas of North America. Such epi-continental seas  do drain away, and the bottom sediment becomes lifted hundreds or thousands of feet, so wind and water can erode valleys into the rock layers, exposing fossils. Epi-continental sea bottoms have given us 90% of our marine vertebrate and cephalopod fossils.

Coelacanth fossils are common in shallow-water and medium-deep sediments beginning in the Early Devonian, over 400 million years ago. From then on, coelacanths remain widespread and often common. Were they in deep water too? We don’t know – we don’t have enough deep sediment exposed for study.

Abruptly, coelacanths disappear from epi-continental sea deposits in the Late Cretaceous. Naturally, we thought they were extinct. But then the fish show up alive and well, hanging around at 130 meters to 700 meters.

Ditto for the Goblin Shark: common as a fossil along New Jersey in shallow sediment but now restricted to much deeper waters. Ditto for the giant squid, who left their shells in the Cretaceous epi-continental sea sediment but now prefer deeper water.

Goblin Shark

Is there a common explanation for all the survivors in deep waters?

The most popular theory is: 1) Most new types of fish and cephalopods first evolve in shallow water. 2) It takes time for evolution to modify a fish or cephalopod so the beast can survive at 200m + depths. So the early coelacanths couldn’t colonize the great depths for tens of millions of years. As more and more clans of fish evolved in shallow water, some began their adaptive descent too – but the coelacanth had a head start. Being fully adapted to great depth already may have protected the fish from predators and competitors who are behind in the degree of their transition.

There are holes in the theory. Coelacanths do have predators – they show up in shark stomachs. They must have competitors too – teleost fish with more complex jaws.

Deep Sea Refuges continue to irritate our neat little hypotheses.

WAnt more? See the past post on ghost sharks and full comment.

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.


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

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.

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.