December 9. Green River Formation

During Eocene time, about 50 million years ago, large lakes developed in the basins between the Laramide uplifts in the Central Rocky Mountains, in what are now Wyoming, Utah, and Colorado. The adjacent mountains, including what are now the Uinta and Wind River ranges, shed sediment into the lakes. No surprises there. 

The sedimentary layers in the Green River Formation that accumulated in these lakes are thin, mostly fine-grained materials, mostly silts and muds, but there were some sands and limy sediments too. The climate was sharply seasonal, with a wet growing season and an arid season, and this is reflected in the sediments that show annual deposits, very much analogous to tree rings. These thin layers are called varves, and they average about two-tenths of a millimeter in thickness. Pretty thin. But they add up to an almost continuous record of time spanning 6 million years, from about 53 to 47 million years ago.

While the lakes were accumulating sediment, there was an extensive volcanic area not too far away. The Absaroka Volcanics were erupting in what is now northwestern Wyoming. Although the Absaroka Volcanics extend into much of Yellowstone National Park, they have nothing directly to do with the Yellowstone geysers and supervolcano. 50 million years ago, when the Absaroka Volcanics were erupting, the Yellowstone Hot Spot probably did not even exist. Eruptions related to subduction went on for millions of years, adding up to a pile more than 10,000 feet thick in places. The fine ash from those volcanoes settled into the lakes where the Green River Formation was accumulating, and they give us minerals that provide accurate age dates on the sedimentation in the Eocene lakes. 

Stingray fossil photo by Didier Descouens,
used under Creative Commons license. 

The Green River Formation has a couple zones that are lagerstatten, outstanding assemblages of well-preserved fossils. The formation is famous for its fish fossils, and there are also delicately preserved fossils of stingrays, insects, leaves, and even birds, the oldest known bats, reptiles, and mammals. It’s really a remarkable place. Some of the fossil zones contain so many contemporaneous animals that it may be that they represent widespread, nearly instantaneous deaths as a result of a major volcanic event.

Besides the fossils, the Green River Formation contains a couple other economic resources. It is one of the world’s most voluminous oil shales. At times, the lakes became anoxic – another possible cause for some of the animal deaths – and the organic matter washing into the sediment did not decompose, but became entrapped in the rock. Oil shale is solid rock – no liquid at all, in contrast to a shale oil, like the Bakken formation, where there is liquid oil, but it is trapped in tiny tiny pores that are poorly interconnected if at all. Oil shale is like the tar sands we talked about last month, but even more solid rock. It’s very energy-intensive to get oil out of oil shale. Techniques are always improving, but I think the energy return on energy invested for oil shale is something like 2 or 3 to 1 – meaning, you spend one unit of energy to get back 2 or 3. For conventional oil resources, that return is 20 to 40. So, while there might be 3 trillion barrels of oil equivalent in place in the Green River Formation, it will be very expensive to get it out. There are no current plans that I know of to exploit this resource, although there is of course research going on.

The other economic resource in the Green River Formation is trona – the world’s largest deposit of trona, by far. What is trona? Sodium carbonate. So what? You make use of trona every day – it is a critical additive to glass, which makes the melting point of silica lower and more manageable, as well as cheaper in terms of the energy cost to produce glass. Virtually all common glass today uses sodium carbonate in its manufacture. About 80% of all the trona in the world is produced from mines in southwest Wyoming. It accumulated in one of the Eocene lakes during a time of extensive evaporation when the chemical conditions were right for its crystallization.

Trona is called soda ash in the industry, and the U.S. produces about 12 million tons of it a year, more than half of which is exported. It’s one of the few mineral products for which the United States is a net exporter, and it’s a $1.8-billion-dollar business. Turkey is a distant second in terms of world production, at about 12% of the total. I have a bit more about trona in my other book, What Things Are Made Of.

—Richard I. Gibson

Absaroka Volcanics
See also Roadside Geology of the Yellowstone Country, by William J. Fritz and Robert C. Thomas (Mountain Press, 2011)

Stingray fossil photo by Didier Descouens, used under Creative Commons license. 

November 20. Teleost fish

Photo of Xiphactinus from Kansas by Spacini, used under Creative Commons license.

Teleost fish are known as far back as the Triassic, but they diversified mightily in the Cretaceous. This is the group that includes about 96% of all modern fishes, many of which began in the Cretaceous. More than 26,000 living species of fish are teleosts.  

Teleosts have some skeletal differences that distinguish them from earlier fish, including a movable jaw and a spine that ends before the tail fin. 

While the Cretaceous saw the expansion of many groups of fish alive today, including salmon, bass, and cod, on an individual species level there were plenty of comings and goings – new species appearing, while others went extinct, which is always happening. On the whole you’d probably be hard pressed to see any big differences between many Cretaceous teleosts and modern varieties. Xiphactinus, whose name means sword-fin, was one huge teleost that reached 20 feet in length. They were fanged predators who lived in the late Cretaceous seas of Kansas, where they were first discovered in the 1850s, and at other locations in North America as well as Europe, Australia, and South America. Many fossil specimens include the skeletons of large prey in the stomachs of Xiphactinus.

The name teleost is from the Greek meaning “complete” and “bone” – a nicely, completely, boned fish.

—Richard I. Gibson

Photo of Xiphactinus by Spacini, used under Creative Commons license.

June 29. Mississippian fish

Goologongia

Rhizodonts were fish that grew to as much as 7 meters long, around 22 feet, the largest freshwater fish known. They got their start during the Devonian, but really proliferated during the Mississippian, only to go extinct by the end of the Pennsylvanian. 

The rhizodonts were a fish group whose limbs – fins – showed close affinities to the tetrapods, the primitive amphibians that were invading the land during the Mississippian. The fins, especially the front fins, were strong, with strong supporting tissues, and the rays of the fins were very much like fingers.

They had highly maneuverable jaws, and they were the greatest predators in the Mississippian lakes and rivers where they could have attacked things like lungfish and early amphibians. Some species had large tusks, and the name of the group, rhizodont, means “root-tooth” because their long teeth, some as long as 15 centimeters, or 6 inches, had distinctive root systems, another trait that connects them to the tetrapods and later animals.

Study of rhizodonts is an active area of paleontology today because of their relatively close relationship to the tetrapods, the ancestors of amphibians, reptiles, birds, and mammals. The fossil record of tetrapods, which began during the Devonian, still contains many gaps even millions of years later in the Mississippian.

—Richard I. Gibson

Image from Palaeos.

Reference: Gaining Ground: The Origin and Evolution of Tetrapods, by Jennifer A. Clack, Indiana University Press, 2012, p. 75-78.

May 26. Fish scales

In a group as diverse as fishes, it’s no surprise that they developed differing kinds of scales. You recall the placoderms and ostracoderms, which had bony plates covering parts of their bodies. That model was unsuccessful – but flexible bodies, covered in scales, not only let fish survive for 400 million years, but might have led to the development of a distinct neck – an important anatomical feature for living on land.

Devonian ganoid fish (Osteolepis)

The ganoid fishes where bony fishes abundant during the Devonian, and they have many modern descendents including sturgeons, paddlefish, and gars. If you’ve ever gone fishing in the waters of Arkansas and caught a gar, you know how bony they are. One of my few clear memories from the time I was around 7 years old is fishing with my grandfather. I caught a gar that seemed to be as long as the boat – in reality it was probably 3 feet long – with a sharp nose, silvery scales, and lots of teeth. My grandfather fought with it for what seemed like an hour – probably 2 minutes – until he broke it in half. Fishermen in the backwaters of Arkansas in the 1950s didn’t like gars.

Scales are similar to human hair and fingernails. They’re composed of collagen, the structural protein that makes up lots of connective tissues in animals, combined with calcium phosphate, the mineral apatite, which is found in bones and teeth. Some scales also include calcium carbonate. In ganoid fishes, much of the mineral matter in the scales is ganoine, glassy, rod-like calcium phosphate. That’s what makes gars so shiny and silvery.

Other types of fish scales are somewhat more bony in the case of primitive fish like coelacanths. Most common modern fish have the overlapping flexible scales that you’re probably familiar with.

—Richard I. Gibson

Ganoid fish drawing from an old textbook (public domain)

May 6. The Age of Fishes

The Devonian Period is called the Age of Fishes because fish were abundant then, and they also diversified dramatically. While some of the major groups got started in the late Silurian, it was during the Devonian that they came into their own. 

Devonian fish

The two main subdivisions of fishes that survive today, the cartilaginous fish which include sharks and their relatives, and the bony fish, which is pretty much everything else, became well established during the Devonian. The armored placoderms did well during the Devonian but became extinct at the end of the period. The more primitive jawless fish, which we referred to in March with the general name ostracoderms, declined a lot, presumably in part due to competition from the more efficient predators that had jaws. But jawless fish have survived, and are represented today by hagfish and lampreys, although the exact relationship between them and fossil jawless fish is not completely clear.

As we discussed the other day, we know for sure that fish had invaded fresh-water environments by the Devonian, since their fossils are found in the Old Red Sandstone, whose sediments were laid down by rivers.

The largest fish of the Devonian, the terrors of the seas, were the arthrodires, placoderms that grew to as much as nine meters long – close to 30 feet. Although they survived as a group for nearly 50 million years, they too died out in the extinction near the end of the Devonian Period.

The group of fish that would diversify the most was the lobe-finned or fleshy-finned fishes that we talked about on April 22. Their descendents would become land-dwellers, and we’ll get into that in more detail later in May.

* * *

May 6, 1843, was the birthday of Grove Karl Gilbert, in Rochester, New York. G.K. Gilbert was a geologist with the fledgling U.S. Geological Survey, and did a lot of work in the Rocky Mountains. He’s noted for his work in the Henry Mountains of Utah, and for his studies of Glacial Lake Bonneville, a huge lake of which the Great Salt Lake is a tiny remnant. He was also a pioneer in the study of craters, including Meteor Crater in Arizona, which he thought was of volcanic origin, and the moon, which he interpreted to represent impacts. While that may seem obvious today, the origin of lunar craters was debated well into the 20th century. 

—Richard I. Gibson

Further reading
National Geographic

Drawing by Joseph Smit (1836-1929), from Nebula to Man, 1905 (public domain)

April 22. Fish fins get strong

 


First off, I’m pleased to announce that you can now subscribe or listen to our podcasts in Stitcher. Stitcher is a free, mobile-optimized app that you can download, so if you prefer to listen that way, it’s now available. Thanks to listener Max for pointing me to Stitcher.

* * *

A couple days ago we talked about fish getting jaws. They were also getting other things during the late Silurian, including stronger fins. The crossopterygians – that means fringe-finned – and sarcopterygians – that one means fleshy fin – are most closely related to today’s lungfish and the famous “living fossil,” the coelacanth, as well as the tetrapods – four-limbed animals including amphibians, reptiles, birds, and mammals. “Living fossil” isn’t really a very useful term, but it’s out there – it mostly means that whatever we have today isn’t too much different from fossil versions.

Reconstruction of Guiyu oneiros, late Silurian of China

The earliest known sarcopterygians are from the very late Silurian, about 418 million years ago. The Silurian ended about 416 million years ago, and it’ll be next month, the Devonian, when the fish really took off and diversified.

The modern coelacanth, which was thought to have been extinct since the Cretaceous Period, was first caught off the coast of South Africa in 1938. Its four strong fins are the main connection to the Silurian and Devonian forms that also gave rise to tetrapods, land-walking animals. And obviously, to live on land animals had to be able to breathe air directly rather than taking it from water, and the closely related lungfish, which also probably originated in the very late Silurian but survived to the present, are probably examples of early stages of that evolution.

A related footnote – Sharks are in the news, fossil sharks. A report in the journal Nature last week described a fossil shark with significant differences from modern sharks – in the jaws and gill structures. It’s common to say that sharks haven’t really evolved all that much for millions of years, but this discovery reminds us that there has indeed been considerable change even in things that seem to remain the same for long periods of time. Sharks have evolved, too.

—Richard I. Gibson

Image by ArthurWeasley via Wikipedia under Creative Commons license.

April 19. Fish get jaws

Last month we talked about some early animals that you’d call fish if you saw them: the ostracoderms, a general name for several groups of armored, jawless critters. The Silurian Period was a time of considerable diversification among the fishes. Some of the structures called gill arches evolved into the lower jaw, a movable structure. It’s easy to imagine the advantages of a jaw that could open and close as compared to a relatively solid opening that could take in food but relied on whatever happened to be in front of the animal. Now, fish could bite.

Placoderm

Some of the earliest Silurian fish to develop jaws were primitive sharks, which are cartilaginous animals rather than bony. Their skeletons are made of tough fibers – cartilage – rather than solid bones. That model works, since we have sharks and their relatives today. Another group that developed jaws during the Silurian were the placoderms – they looked a lot like ostracoderms and their name means plate-like skin, but they also had jaws. Placoderms appeared in the late Silurian, and they are extinct today, dieing off at about the end of the Devonian. You really wouldn’t think twice about calling a placoderm a fish – possibly a somewhat weird-looking fish, but certainly a fish. Some of them, especially the group called arthrodires, grew to huge sizes during the Devonian, reaching 10 meters or more in length , more than 30 feet. And they were predators.

Bony fish

The very first bony fish – the group that includes most modern fish, with 28,000 modern species, got started in the late Silurian, about 420 million years ago. In addition to jaws, bony fish have swim bladders, organs they use to maintain their position in the water.  

Fish, of course, have fins. The evolution of fins into legs suitable for walking on land is a huge event in the story of life, which we’ll get into a bit more, but in the meantime I want to recommend, yet again, Your Inner Fish by Neil Shubin – it’s a book and a three-part program on PBS that tells not only this story, but connects that evolution to modern vertebrates including humans.

—Richard I. Gibson

http://en.wikipedia.org/wiki/Evolution_of_fish

Bony fish image by ArthurWeasley via Wikipedia underCreative Commons license 

Placoderm by Nobu Tamura under Creative Commons license 

 

March 15. Ostracoderms

The first critters that you’d recognize as primitive fish appeared during the Ordovician. They were armored with bony plates, and their general informal name, ostracoderms, means shell-skinned. For the earliest varieties, we only know them from fossils of these individual scaly plates. They didn’t have a rigid internal skeleton, so they generally fell apart when the animal died.

The Ordovician ostracoderms, thelodonts, which means “nipple teeth,” didn’t have jaws, but they did have scales much like fish today – but the scales were tiny, only a millimeter or two long. They appeared probably during the Middle Ordovician, around 470 million years ago or a bit earlier. Since they were without jaws, and their mouths were on the bottom of the head, we think that these early fish were probably sediment bottom feeders, sucking stuff into their mouths and filtering out food.

I talked about conodonts on March 3, and indicated that once we finally found the conodont animal, it was seen to be a small, eel-like animal. Eels are fish, and the conodont animal was indeed a primitive fish. But ostracoderms were the first ones that really had a fishy look to them. And they were widespread – they’ve been found in Ordovician rocks all over the world.

Neil Shubin, in Your Inner Fish, a book I’ve recommended previously, tells us that the bony plates on ostracoderms’ heads were made of material – calcium phosphate, the mineral apatite – and have structures that are essentially teeth – teeth fused together and on the outside of the animal, but teeth nonetheless, in evolutionary terms. So, Shubin argues, the first hard parts in chordates, the group that includes us and the other vertebrates, were teeth in conodonts, the better to eat you with, and the second hard parts were teeth that evolved into armor – protection from those other gnashing tooth-filled mouths. It’s really a cool story that hangs together quite well, and if you’re interested in this sort of thing, I recommend – again – Shubin’s book, Your Inner Fish.

The entire body of ostracodems was covered in scales, like modern fish, but the head area was more strongly armored by the fused-together plates into a more bony shield. 

If you have comments about the podcast, please leave a review on iTunes or a comment on the blog.

* * *

Today, March 15, is the birthday of Wallace Pratt, born in 1885 in Phillipsburg, Kansas. Pratt was a pioneer in petroleum exploration geology. In 1918 he became the first geologist hired by Humble Oil & Refining – a company that would eventually evolve into the giant corporation we know as Exxon today. One of his major contributions was fostering the use of geophysical instruments in oil exploration, and he was also a founding member of the American Association of Petroleum Geologists. He donated 23 square miles of land in West Texas, where he had a ranch, to the National Park Service, forming the core of what today is Guadalupe Mountains National Park. He died in 1981.

—Richard I. Gibson

Drawing of reconstructed ostracoderms by Philippe Janvier under CC-by-A license. The black and white drawing is from an old textbook.