February 19. Potsdam Sandstone

Potsdam, New York, is about 1800 miles from the Grand Canyon, but because of the extent of the Cambrian transgression by the sea, the lowest, oldest Cambrian rocks in both places are quite similar. Both the Potsdam in New York and the Tapeats in the Grand Canyon are sandstones, lithified from sediments laid down in a near-shore marine environment. The Potsdam sand was probably eroded off the high-standing Adirondacks, which may have almost been an island in the Cambrian sea. And both the Potsdam and the Tapeats lie above a profound unconformity, above Precambrian rocks that are hundreds of millions of years older.

The Potsdam in New York is mostly a clean quartz sandstone like you might expect from a beach, with some hematite (iron oxide) cement that makes it pinkish. It makes a good building stone and Canada’s House of Parliament, in Ottawa, is made from it.

One difference between the Potsdam and the Tapeats in the Grand Canyon is that the Potsdam is younger – probably late Cambrian in age, even though it’s the oldest Cambrian layer present, rather than early to middle Cambrian for the Tapeats. That reflects the millions of years that it took for the sea to encroach, to transgress, across much of North America. There’s a lot of that sand though. The Potsdam is as thick as 1,500 feet around Lake Champlain.

Potsdam near Chippewa Bay, New York, above the unconformity (Precambrian below). Photo by Michael C. Rygel via Wikimedia Commons, under Creative Commons Share Alike Unported license.

We used to give similar Cambrian sandstones in Ontario, Michigan, Indiana, Virginia, and as far west as Iowa and Wisconsin and even Wyoming the same name, Potsdam – but while the origin is practically the same, and the sandstones may be stratigraphically equivalent, it’s probably not correct to think of the sand as a continuous sheet of sand, at least not at the same time. The sea in which the sand was laid down varied in space as well as time. So these sandstones have different names today.

Today, February 19, in 1792, was the birth date of Roderick Impey Murchison, in Tarradale, Scotland. Together with Adam Sedgwick, Murchison became one of the great early British geologists who helped define many of the Paleozoic time intervals. In a few days, we’ll talk about the feud between Sedgwick and Murchison over the position of the top of the Cambrian in Britain.

Also on this day, February 19, 1600, the volcano Huaynaputina erupted in southern Peru. It was the largest volcanic eruption in South America in historic times. The years 1600-1602 were the coldest in at least 600 years in Russia, and many people starved. The wine harvest in France and Germany was negatively impacted, and climatic effects were noted in Japan and China as well. Ten villages were buried under ash in Peru, where at least 1500 died.

—Richard I. Gibson

Photo by Michael C. Rygel via Wikimedia Commons, under Creative Commons Share Alike Unported license.  

February 17. What's a Brachiopod?

Today, let’s talk about brachiopods. Possibly you’ve never heard about brachiopods – they are not widely distributed today, and when you do see their shells you might dismiss them as just another odd bivalve, or two-shelled mollusk like a clam. But brachiopods are not clams. They are their own phylum, not closely related to mollusks at all.

The main visible difference may seem subtle – brachiopods are symmetrical through a plane that divides the two shells vertically, while mollusks, if they have any symmetry at all, are symmetrical between the shells. There are other differences as well, especially in internal organs. Brachiopods have things called lophophores, tentacles that they extend and wave around to create a current from which they can filter food particles out of the ocean water where they live.


Brachiopods first appeared in the early Cambrian, part of the Cambrian explosion. They were incredibly prolific and successful during much of the Paleozoic Era, and in fact there are 12,000 fossil species known, in 5,000 genera, that is, groups of related species, in contrast to only 100 genera known today. Brachiopods suffered a lot in the Permian-Triassic extinction, at the end of the Paleozoic Era 250 million years ago, and their decreasing diversity after that time may also reflect the growing success and diversity of the bivalves that occupy some of the same ecological niches as brachs.

Although a few species reached nearly eight inches across, most brachiopods are an inch or two across, a perfect size for preservation and easy for collectors to find. There are two main types, articulate and inarticulate. Articulated brachs have two shells that are attached to each other along a hinge line, so the critter opens in a way similar to a clam. Inarticulate varieties have two shells that were not attached, but were held together by the animal’s muscular system. One of the most famous inarticulate brachs is one called Lingula – also called a living fossil, because the types that exist today are hardly changed from Lingulas that lived in the Cambrian period. They are also interesting because their shells are not made of calcium carbonate, like most clams, scallops, snails, and other shelly creatures, but they’re made of calcium phosphate, the mineral apatite, the same mineral that makes your bones and teeth.

I’ve never seen a modern brachiopod, alive or dead. But they were so prolific in Paleozoic seas that most any fossil collector will have some in his or her collection. We’ll talk about some of them later in the Paleozoic.

—Richard I. Gibson

Images from USGS.

February 16. Cambrian rocks of the Grand Canyon

White line marks Great Unconformity,
with Tapeats Sandstone above.

The Grand Canyon of the Colorado River is a geologist’s dream. The rocks scream out their relationships, and as you descend into the canyon, the rocks are older and older. In the inner gorge, the dark-colored rocks are Precambrian in age. They are metamorphic rocks, altered during their long lives by heat and pressure. And the top of the Precambrian rocks is a surface called an unconformity. That means a break in the rock record – a gap in time when sediments were not laid down, or they were eroded away, or sometimes a combination of both. The unconformity in the Grand Canyon is called an angular unconformity, because the layers below it are at an angle to the layers above it – a clear violation of the rule of original horizontality that we talked about a few days ago. Not only were the lower rocks cooked and changed, they were tilted – all before they were eroded off to create that unconformity surface.

The Great Unconformity in the Grand Canyon is part of a nearly continent-wide break. The amount of time it represents varies, even within the Grand Canyon area, from as little as 175 million years to possibly as much as a billion years or more, depending on the age of the rocks beneath the erosion surface.

In the Tapeats Sandstone

But it’s February, and we’re in the Cambrian now. Let’s talk about the rocks above the unconformity – the Cambrian strata. There are three distinct packages of rocks, called formations, in the Cambrian of the Grand Canyon. The lowest, the oldest, is called the Tapeats sandstone. When you look into the Canyon, if you can see the inner gorge, the Tapeats is the relatively thin, resistant lip on the rim of the gorge. It’s probably around 525 million years old, which puts it in the Middle Cambrian, and it averages something like 200 feet thick, pretty thin for the Grand Canyon.

Above, and younger than the Tapeats we find the Bright Angel Shale. Shale is a fine-grained rock that solidified from mud, and it often has really thin beds, sometimes microscopic. All of that adds up to a rock unit that may be a lot less resistant to erosion than something like sandstone, and that’s the case in the Grand Canyon. Consequently, the top of the Tapeats Sandstone is marked by a wide, flattish expanse called the Tonto Platform. It’s the place where the Bright Angel Shale would have been but it’s been eroded away – at least eroded back, pretty far from the rim of the inner gorge. When it’s still present, it tends to form slopes rather than cliffs because it’s more easily eroded. The Bright Angel is reddish and greenish in color because of variable iron content, and it contributes to the beautiful colors deep in the canyon. It’s around 500 feet thick, which gives plenty of room for lots of erosional variety and interesting landforms.

The upper, youngest part of the Cambrian in the Grand Canyon is the Muav Formation. It’s a multi-colored limestone interbedded with mudstone and some other rocks. It’s as much as 600 feet thick, and it’s a resistant cliff-former, making some of the first steep cliffs above the inner gorge and the Tapeats Sandstone.

Traditionally, geologists interpreted a change in rock type from sandstone that might have been deposited on a beach, to shale, which would be the finer sediment carried out into deeper water, to limestone, which could form in very deep water – all that would have been seen as evidence of the Cambrian Transgression that we talked about on February 5, with the seas encroaching and getting deeper and deeper across North America. That’s generally the way it worked, but it’s also possible for things like limestone to form in fairly shallow water – think of the calcareous white sand beaches on the west coast of Florida – so don’t look at it as entirely smooth and continuous. Stuff happened.

Geologists name rock formations, like they name periods of geologic time, to make it easier to refer to them, but it’s not arbitrary – there are distinct characteristics in each formation that make each one relatively easy to identify. Names come from a lot of sources, but all the Cambrian formations, Tapeats, Bright Angel, and Muav, were named for creeks and canyons in the Grand Canyon area.

—Richard I. Gibson

February 15. Plate Tectonics 1

Nerds in a Bar, volume 3. Colleen Elliott and Dick Gibson discuss the evolution of thought on continental drift. This episode isn’t specific to the Cambrian, but it will help provide general background for future presentations.

Image from USGS (public domain).

February 14. Fossils and Preservation

Nerds in a Bar, volume 2. Colleen Elliott and Dick Gibson discuss what a fossil is, and how fossils are preserved.

Cambrian trace fossils, public domain photo.

February 13. The Burgess Shale, again

Today’s podcast is a brief report on my personal favorite Burgess Shale animals: Anomalocaris and Opabinia.

Anomalocaris - up to 6 feet long

Opabinia - less than 3 inches long

And here is a link to the February 2014 news report mentioned in the podcast (thanks to Colleen Elliott for finding this). And another link, with photos.

UPDATE: from predator to filter feeder

Artist’s reconstructions both by Nobu Tamura, via Wikipedia under GNU Free Documentation License: Anomalocaris  •  Opabinia.

February 12. The Burgess Shale and Charles Walcott

So much has been written about the Burgess Shale I’m not sure there’s much I can add, given how accessible that information is today. I’ll just say that the soft-bodied fossils found in the Burgess Shale, in the Canadian Rockies near the town of Field, British Columbia, were some of the most weird and wonderful fossils ever found. They are the subject of Stephen Jay Gould’s 1989 book, Wonderful Life, which I recommend highly. It’s a wonderful book, and a great starting point for exploring the Cambrian explosion through the explosion in scientific investigation that has taken place in the past 25 years.

Walcott and his son and daughter working in the Burgess quarry, c. 1913.

But maybe I can talk about Charles Doolittle Walcott, the man who discovered the Burgess fauna. Walcott was born in New York in 1850. He never finished High School, but he became a knowledgeable expert on New York’s fossils, and when he was 29, he joined the new United States Geological Survey as a geological assistant. 15 years later, he was the director of the Geological Survey.

His middle name, Doolittle, certainly seems like a misnomer, because he was always a doer, and he did a lot.

He became the Secretary, which is to say the head, of the Smithsonian Institution in 1907 and continued in that job until he died in 1927. In those days, the head of the Smithsonian was anything but a desk bound bureaucrat, and it was in that job that he led fossil explorations to the Canadian Rockies, where he discovered the Burgess fauna in 1909. Over the next 15 years, his digs uncovered more than 65,000 specimens and brought them back to the Smithsonian.

Today, February 12, is the birthday of Charles Darwin in 1809, the same day Abraham Lincoln was born. Charles Walcott received an honorary doctorate from the University of Cambridge in 1909 as part of the centennial celebration of Darwin’s birth. February 12, 1813, was the birth date of James Dwight Dana. He devised the system of mineralogy, and wrote the textbooks still in use with revisions, today, that have educated tens of thousands of geology students over the past 150 years or more. Yet another prominent geologist was born on this day, in 1850. William Morris Davis came up with some of the earliest theories of landscape formation and erosion. Many of those ideas have been superseded, but Morris laid the groundwork for the modern science of geomorphology.    

—Richard I. Gibson

Photo (public domain) from Smithsonian via Wikipedia.

February 11. The concept of stratigraphy

Today I thought we’d talk a bit about the concept of stratigraphy, some ideas that will help with understanding of the geologic events we’re talking about.

Steno

Stratigraphy is the study of strata, or layers within the earth. Stratum, the singular of strata, comes from Latin for a bed, and ultimately from a word meaning to spread out—and that's what geological strata do: they spread out over wide areas. The science focuses mostly on sedimentary layers, beds of sandstone, shale, limestone and so on. One key aspect of stratigraphy is the law of superposition – an fancy way of saying that lower layers are older than higher layers. This may seem obvious – if it doesn’t, think about throwing some red sand into a pail on Wednesday, then on Thursday come back and throw in some lime. The sand is older than the lime. It was not obvious to early scientists, and it was Nicholas Steno, a Danish Catholic Bishop, who lived in the 1600s and pioneered and promoted this and other basic aspects of geology. He also conceived the principle of original horizontality, which says that layers of sediment – sand, silt, mud – were laid down in horizontal layers under the action of gravity. There are some obvious exceptions to this, such as deposits on mountain or undersea slopes, but it’s a general principle that matters greatly when we look at rocks that have been deformed by faulting or folding.

Smith

If you’re interested in Steno’s story, I can recommend a book by Alan Cutler called The Seashell on the Mountaintop.

While in many ways Steno was the father of stratigraphy, the one who really implemented stratigraphic ideas in a modern way was a British surveyor, William Smith. To this day I still think of him as William “Strata” Smith, as he was called when I first took physical and historical geology classes back in the 1960s. He recognized that different layers or strata of rocks had distinct fossil assemblages, and that he could recognize those characteristic fossils to help him identify the rock packages elsewhere, even if they were distant and disconnected from the original rocks. And even if the kind of rock changed. That meant that the same kinds of fossils, in a sandstone here, but in a limestone there, meant those diverse rocks were of the same age.

Smith made the first geologic map of England and Wales, published in 1815. That was The Map that Changed the World, in the title of the book by Simon Winchester that recounts Smith’s story.

—Richard I. Gibson

The "map that changed the world"

February 10. Baikalian orogenies

Parts of Asia and Europe were also growing during the early Cambrian, if not quite on the scale of Gondwana that we talked about yesterday. The Baltic Craton, also known as the Russian or East European Platform, added a triangular block called the Timan-Pechora terrane, in what is now northwestern Russia and the adjacent Arctic Ocean. It was probably added in very late Precambrian time, possibly overlapping into the early Cambrian. The map shows this new addition outlined in green, and the red line with the cross marks is the zone where the two continents came together. This would have been a mountain range during the Cambrian, and even today, thanks to some rejuvenation, it is a range of hills.

At around the same time, very late Precambrian, the Baikalian Orogeny (named for Lake Baikal, in southern Siberia) added some small continental blocks and island arc terranes to the southern margin of Siberia – which was not at the time connected to the Baltic Shield and Europe. That’s a much later assembly, marked by the Ural Mountaina.

Most of the Baikalian “events” spanned at least 150 million years, and were largely accomplished by the time the Cambrian opened. They set the scene, provide the geography, for not just the Cambrian, but for a good bit of Paleozoic time that we’ll be discussing over the coming months.

—Richard I. Gibson

February 9. Pan-African Orogeny

You remember Rodinia, the supercontinent that assembled around a billion years ago and started to split apart again around 750 million years ago? Well, it’s time to put it back together again. At least some big pieces of it.

Yellow = West Gondwana, Lilac = East Gondwana

Near the end of the Proterozoic and into early Cambrian time, most of what we know as Africa today came together, along with some other important continental blocks. It wasn’t one big collision, but several collisions, which brought what is today central Africa, the Sahara, Congo, and Cape (or Kalahari) Cratons, together with east Africa, Arabia, Madagascar, India, and eventually Antarctica. That happened along a zone called the Mozambique Belt today.

At the same general time, South America and the West African craton were added on the other side of the Sahara-Congo-Cape continent. The final amalgamation, called the all-Africa, or Pan-African Orogeny, resulted in a supercontinent – not one involving all the continents, but pretty super nonetheless – that stayed pretty much intact for the next 350 million years. It’s named Gondwana.

You might have heard this called Gondwanaland – that’s how I learned it back in college – but Gondwana means “forest land of the Gonds,” so Gondwanaland is redundant. Who were the Gonds? They were – and still are – a native people of central India. Their homeland contains rocks that helped us understand the assembly and breakup of the supercontinent that now has the name Gondwana.

Don’t forget that while various continental pieces were coming together, in other parts of the world extension and pull-apart were happening. North America, for example, was pretty much going its own way during the Cambrian.

—Richard I. Gibson

Image from Wikipedia, public domain.


Further reading:
http://www.utdallas.edu/~rjstern/pdfs/PanAfricanOrogeny.pdf
Map

February 8. The Chengjiang fauna

Chengjiang, in Yunnan, southwest China, is one of the most celebrated and important fossil localities in the world. When well-preserved soft-bodied animals were found there in the 1980s, they were immediately compared to the famous Burgess Shale fossils that we’ll talk about in a few days. And at about 525 million years old, they were found to be about 10 million years older than the Burgess fauna.

Maotianshania cylindrica, a nematode worm.

This was a big deal – because the Burgess shale animals were practically unique in the world. With the discovery of Chengjiang, the record of early Cambrian life forms expanded in time as well as space. Many of the fossils in China are the same types as those in the Burgess Shale, and these critters are critical to our understanding of the Cambrian Explosion.

The life that lived in the Cambrian of China included trilobites, as well as sponges, jellyfish, lots of kinds of worms, and importantly, the oldest probable chordates. Chordates have notochords, a linear arrangement of nerves that in vertebrates like us has evolved into our backbone and spinal chord. This means that the ancestors of modern birds, reptiles, fish, amphibians, and mammals – and us – are pretty ancient, at least 525 million years old.

I would encourage listeners to check the links below to sources for more information and photos of the Chengjiang fauna.

Photo by SNP under GFDL. 

Update (April 2014): New arthropod with cardiovascular system reported
http://en.wikipedia.org/wiki/Maotianshan_Shales
http://www.fossilmuseum.net/Fossil_Sites/Chengjiang.htm

February 7. Trilobites

Let’s talk today about some of the most common life forms that lived in Cambrian seas. Trilobites were arthropods, invertebrates with segmented bodies, jointed legs, and relatively hard exoskeletons on the outside of their bodies. The group includes insects, spiders, centipedes, and crustaceans like shrimp, lobsters, and crabs. And trilobites.

Trilobite is a simple name meaning three lobes, which reflects their basic body plan – a central lobe flanked by two more that cover the legs. They appeared during the Cambrian explosion, around 540 to 521 million years ago, and were incredibly diverse and successful – all told, there are more than 17,000 species of trilobites, and they survived for about 270 million years.

Trilobites are a kind of holy grail for geology students, at least those of us who lived in places where they were not common. Because of their many segments and legs and other parts, they tended to break apart when they died, so they are fragile and can present a challenge to collectors to find and retrieve, but when you do, it’s a treasure. Finding your first trilobite was a big deal – at least for me. I even have a trilobite etched on my mug down at Quarry Brewing.

They range in size from tiny – around a millimeter – to huge, at 70 centimeters or 28 inches long. Their exoskeletons were not hard calcium carbonate, like so many shells are, but were a material called chitin, more like humans’ fingernails, and containing calcium, phosphorous, and organic material. This adaptation certainly offered some protection compared to soft-bodied animals like jellyfish, and allowed them to be mobile as well.

Specific subdivisions of Cambrian time around the world are usually related to the kinds of trilobites found in a particular time interval. So you have the Olenellus Zone, or the Bathyuriscus Zone, or whatever. Trilobites are the yardsticks of Cambrian time.

Trilobites had compound eyes, much like many insects, with lenses made of calcite, calcium carbonate.   As we discussed yesterday, the development of eyes and light sensitivity may have created evolutionary pressure that drove, or even initiated, the Cambrian explosion. Trilobites seem to have hung out in mud a lot, and that’s good news, because the really fine grained sediment in mud lends itself to preserving details in fossils, even the eyes. And it might help that muddy sea floors might tend to be anoxic, stagnant areas, with less oxygen to attack and decompose the animals when they died.

We’ll talk about trilobites from time to time again over the next few months, to point out a particularly weird or interesting variety.

The Wikipedia page provides a reasonable overview about trilobites. If you’re geeky enough to like cool names for trilobite parts like the cephalon and pygidium, start there and move on to the Trilobites.info site. If not, just go with head and tail.

When Trilobites Ruled the World - NY Times, March 2014

Blog extra: Feb. 7, 1812, was the date of the fourth and strongest in the series of earthquakes that shook southern Missouri around the town of New Madrid. The Feb. 7 quake damaged buildings in St. Louis and rerouted the Mississippi River, creating Reelfoot Lake in what is now northwest Tennessee.

—Richard I. Gibson

February 6. In the blink of an eye

Today is a follow up to the discussion Colleen Elliot and I had about the Cambrian Explosion a few days ago. In 2004, A book by Oxford zoologist Andrew Parker was published entitled In the Blink of an Eye – how vision sparked the big bang of evolution. It offers arguments for the idea that the development of eyes led to increases in predatory behavior, and was the ultimate cause for the Cambrian explosion.

Trilobite eye (NASA photo)

First, Parker defines the explosion not as the proliferation of animals, but as the proliferation of calcareous and phosphatic shells and exoskeletons in animals. He makes the argument that with new discoveries, we see that most or all of the existing phyla of animals had already been present for some millions of years when the explosion happened. So what we see as an explosion is really just those organisms figuring out a new way of living, inside various types of armor.

The focus of In the Blink of an Eye is why – why did this change happen? Parker objects to most of the possible causes that Colleen and I discussed the other day because they would probably have generated a gradual change, not an abrupt one as is observed. Or they were too far removed in time, as we mentioned regarding the end of Snowball Earth. So he makes the case for the development of eyes as the diving force behind the change.

It’s not hard to imagine that if, suddenly, some animals became able to sense movement, or to recognize patterns that indicated other animals – FOOD! – that the “arms race” Colleen and I mentioned in passing could have developed. There is pretty good evidence that animals DID develop eyes at the time of the Cambrian diversification, about 542 to 530 million years ago. It’s reasonable, but in my opinion not absolutely conclusive that it might have driven the increase in diversity that we see happening then.

For this to happen across species, in fact across phyla, at about the same time, there must have been some common reason. Maybe the development of eyes drove the Cambrian explosion, but why did eyes develop in so many kinds of animals at the same time?

Parker analyzes this question carefully, and ultimately suggests that there could have been a dramatic, and sudden increase in the amount of light reaching those animals, so that sensitivity to light became a useful survival mechanism. What could cause a sudden increase in light on earth?

Well, the sun could have suddenly become more luminous, or the atmosphere and oceans could have become more transparent. While those might seem far-fetched, there’s actually good reason to think they might have been possible, especially the changes on earth. There’s some evidence that the atmosphere, even while it was becoming increasingly oxygen-rich, might have been fog-like during the Precambrian. If something happened – a threshold was reached, or conceivably, a nearby supernova event drenched the earth in enough radiation to change it – it might have become more transparent. Or the water could have become more transparent because of changes in mineral content. Such changes certainly DID occur – but were they enough to suddenly change the amount of light reaching primitive animals, enough to make them all, suddenly, develop sensitivity that led to arms and armor as the world became an eat or be eaten kind of place?

I don’t know. The Blink of an Eye is a fascinating idea, very much worth considering, and the book is a good read filled with lots of earth history. Personally I think the jury is still out on why the Cambrian Explosion happened, and it remains perhaps the most intriguing and poorly understood event in the history of life since it originated on earth.

—Richard I. Gibson

 Further reading:

In the Blink of an Eye (Amazon)
In the Blink of an Eye (B&N) 
Calcite in the oceans (NASA - source of trilobite eye photo)
Trilobite eyes

February 5. The Cambrian transgression

During the early Cambrian, sea levels were rising. This produced a near-global transgression – which is not a sin, but rather it just means that the seas were advancing, covering more land area than they had previously. The opposite is a regression, when the seas become relatively smaller.

So how does that happen? With minor exceptions, the volume of water on earth has been more or less constant, at least since pretty early in the earth’s history, back in early January sometime. But the volume of the ocean basins that hold the water can change and does.  One way that can happen is by rifting apart continents, as Rodinia was splitting into several smaller continents in late Proterozoic and early Cambrian time. The volume change comes about because of the mid-ocean ridges, the point where oceanic crust is pulling apart. Today, the oceanic ridge system is the longest mountain range on earth, and added together, it takes up a pretty notable volume of oceanic water. Enough that if there are a lot more oceanic ridges, it can result in sea level rise. Likewise, if there were a lot more oceanic trenches, very deep water, that could also accommodate at least a little more water than a flat ocean floor.

We know from concerns today about sea-level rise that melting and freezing ice caps can contribute to sea level changes. And a small effect might even come about because of water temperature. Warmer water expands, if only a bit, but when the entire ocean expands, it can make a difference in sea level, especially on very low, flat shores.

Paleogeographic map by Ron Blakey via Wikipedia under CC-BY-SA & GFDL

In North America during the Cambrian, shallow seas covered a vast amount of the continent, all except high area on the Superior Craton and a long narrow peninsula called the Transcontinental Arch, extending from Minnesota southwest to what is now Colorado and New Mexico. Abbreviations in the middle Cambrian (about 500 million years ago) paleogeographic map above: NA-North America. B-Baltica (Europe). S-Siberia. SA-South America. AF-Africa. AUS-Australia. CH-China. ANT-Antarctica. Here is another version of a map showing the arch, and another one.  

The sea took millions of years to transgress across North America during the Cambrian. As it progressed further and further, the shoreline beach also changed position. Consequently there was a lot of sand – sandstone today – that marks the base of the Cambrian across much of North America, but don’t think of it as one big beach – think of it more like a continuously migrating beach that, over millions of years, ended up depositing sand across many hundreds or even thousands of square miles of the continent. We’ll talk about some of those sandstones as we work our way through February.

—Richard I. Gibson

Paleogeographic map by Ron Blakey via Wikipedia under CC-BY-SA & GFDL.

February 4. Archaeocyathids

Part of the early Cambrian explosion included the development of the archaeocyathids, somewhat conical animals with calcareous skeletons rather like corals. But they are not corals. Their affiliations are not 100% certain, and some scientists put them in their own phylum, but I think most taxonomists are classifying them as early sponges.

Their name means “ancient cup” and some are long inverted cone shapes as much as 30 centimeters long, close to 12 inches, while others are like nested bowls. Some were solitary, but others grew together to make some of the earliest significant reefs on earth.

They appeared about 525 million years ago, part of a famous fossil assemblage called the Tommotian fauna for the specimens found along the Tommot River in Siberia. They’ve been found all over the world, and they are distinctive enough that they serve as index fossils for the early Cambrian. That means rocks can be correlated across large distances using the particular varieties of archaeocyathids that are found in them.

Just nine or ten million years after they appeared and proliferated, the archaeocyathids went into a sharp decline in numbers and diversity about 516 million years ago. Interestingly, this time is when more modern sponges began a rapid diversification, another pulse in the Cambrian explosion. The sponges that evolved then have survived with variations to this day, but the archaeocyathids were all extinct before the end of the Cambrian.

—Richard I. Gibson

Drawing of reconstructed archaeocyathids by Stanton F. Fink, via Wikipedia under GNU free documentation license.  

February 3. The Cambrian Explosion

O Wonder! How many goodly creatures are there here! —Shakespeare



The Cambrian Explosion was not a violent volcanic eruption, but rather the sudden appearance in the fossil record of abundant and diverse shelly fossils, the remains of relatively large animal life. Even Charles Darwin recognized it, and saw it as an important objection to his theory of evolution by natural selection.

Like most geologic events the Cambrian explosion wasn’t instantaneous, but it did occur over a geologically short time, around 542 to 530 million years ago, right at the start of the Cambrian period of the Paleozoic Era.

Today’s podcast is a discussion between me and geologist Dr. Colleen Elliot. Neither of us is an expert on paleontology, so this will be something of an exploration for all of us.

Here are some definitions of terms non-geologists may be unfamiliar with in the discussion. Phanerozoic is the name of the eon, the largest subdivision of geologic time, that starts with the Cambrian period of the Paleozoic era. We talked about that a bit the other day. And the Great Unconformity is a break in the rock record that is nearly global in extent, a time before the first rocks of the Cambrian were deposited. It represents a time of near-global erosion of the older Precambrian rocks, and it means that the continents were standing above sea level so that they could be eroded.

—Richard I. Gibson

Links for further reading:
McKenzie et al. on climate and evolution (January 2014)
http://www.sciencemag.org/content/341/6152/1355
Smith and Harper on summarizing Cambrian explosion causes (Sept. 2013)
http://www.astrobio.net/pressrelease/5758/oxygen-not-the-cause-of-the-cambrian-explosion
https://www.pbs.org/wgbh/evolution/library/03/4/l_034_02.html
http://ucrtoday.ucr.edu/9063

February 2. Cambrian time scale

Almost from the start, geologists have argued, sometimes passionately, about exactly where the divisions of geologic time should begin and end. In the early days, when all ages were relative, meaning this is older than that, but we didn’t know absolute age dates closer than tens of millions of years or more, it was challenging to assign a particular time to the start of a period like the Cambrian. When I was in college, the start of the Cambrian was put at 600 million years ago, and honestly, it was a fairly arbitrary number even though it was generally accepted. My historical geology textbook, published in 1960, has a question mark after 600 million years.

Much of geologic research is devoted to pinning down the details of ideas and concepts that were sketched out in the early 1800s. So with better age dates and much more information from the far corners of the world, the timing of the start of the Cambrian has been refined over time. When I wrote the history of the earth calendar in 1994, the generally accepted start of the Cambrian was about 575 million years ago. Today that date is 542 or 543 million years ago, but even now that’s not universally accepted and the USGS time scale here has slightly different dates. It isn’t that the time changes, but our way of looking at the data does change, and everyone doesn't quite agree on all the details.

The Cambrian Period today is considered to have lasted until about 488 million years ago, for a total length of 54 million years, just 1% of Earth’s history. Yet we’re spending all of February on the Cambrian. Stuff happened!

Like the months of the year, the periods of earth history are subdivided and given names to make it easier to refer to them. So we have Early, Middle, and Late Cambrian as general names, and in parts of North America those times have been related to packages of rocks called the Waucoban Series, Albertan Series, and Croixan Series. You can imagine that everything geologic didn’t happen simultaneously, or in the same way, all around the world, so there are dozens of regional variations in the nomenclature. The subdivisions are not arbitrary, but are often based on the fossils that can be found in each subset of the rocks. Fossils can be related to age, and can help correlate between rocks of different type that may be of the same age.

There are ongoing attempts to create internationally acceptable time scales. So there is actually an International Subcommission on Cambrian Stratigraphy that works on this kind of thing, and at present the Cambrian is divided into four major subdivisions, based in part on the timing of major extinction events. I think we’ll settle for early, middle, and late.

—Richard Gibson

Further reading:

http://tapestry.usgs.gov/ages/ages.html
International Commission on Stratigraphy
http://en.wikipedia.org/wiki/Cambrian

February 1. The Cambrian begins.

The Proterozoic is done. We’ve started a new era, the Paleozoic, which means “ancient life” in Greek. It’s part of an eon, the Phanerozoic, that includes all the eras and all the time beginning with the Cambrian and continuing up to the present day. Phanerozoic means “visible life”, reflecting the former thought that there was no life before this time. We know now that there was earlier life, and some of it, like the Ediacarans, was certainly visible, but the name continues to be used. It works if you don’t worry too much about the literal Greek meaning.

The first subdivision of the Paleozoic is the Cambrian Period. Although there are older fossils, the Cambrian is the first time period when they are abundant, and this helped early geologists work out the sequence of rocks and the order in which they were laid down. A lot of that work was done in Great Britain.

Adam Sedgwick at the University of Cambridge is considered to be one of the founders of modern geology. He studied rocks in Wales, which was called Cambria by the Romans, from the Welsh name for their land, Cymru. In 1835, he gave the name Cambrian to the older series of rocks in the mountains of central Wales.

We’ll spend the month of February in the Cambrian.

January 31. The Ediacara – 560-600 million years ago

The Ediacara Hills of South Australia contain some remarkable fossils discovered in 1946, but largely ignored. They were thought to be of Cambrian age, and they were vague enough that it was challenging to determine what they were. With improvements in age dating, these fossils were eventually determined to be about 560 to 600 million years old – the Precambrian, a time when no one thought life existed, at least not as large animals.

Dickinsonia costata was as much as a meter across, or more.

They appear to be soft-bodied organisms, preserved as imprints in fine sand. There are disk-like shapes, and ovoids, with radiating textures. Some frond-like fossils have a quilted appearance, and there are many other varieties. We don’t know what they are.

Early interpretations suggested they might be jellyfish, or worms, or sea anemones. They have a wide range in size, from a few millimeters to more than a meter across. Some researchers say they aren’t animals at all, but are some kind of fungus. Or they might belong to a phylum, or even a kingdom, that has no modern representatives. 

Examples have been found in Newfoundland, England, Namibia, and elsewhere, so they appear to have been pretty common in the oceans that appeared after the glaciations and possible snowball earth ended 650 million years ago, perhaps 50 million years before the Ediacarans appeared. And interestingly, the Ediacaran life seems to disappear just before the Cambrian Explosion of life, about 530 million years ago.

Whatever the Ediacarans are, they represent an important step in the evolution of multicellular life on earth. Figuring out what they were, or what they are related to, is one of the ongoing challenges of geology and biology.

With the Ediacaran fossils, we reach the end of the Precambrian. The Precambrian represents 88% of Earth history, but we’ve compressed it into the month of January alone. The rest of the year will cover the most recent 500 million years of Earth history, because we know a lot more about it. And because it contains cool things like fossils and mountain ranges and sedimentary basins and coal swamps and dinosaurs. Tomorrow the Cambrian begins.

—Richard I. Gibson

UPDATE: Mary Droser (UC Riverside) and colleagues have described a new critter from the Ediacara. This weird one gets as big as 30 cm across and apparently grew as fan-like branches, flat on the sea floor or within the upper part of the sediment on the sea floor.


The Wikipedia article is a good overview, and has extensive links to scientific literature.

Photo by Verisimilius from Wikipedia under GNU Free Documentation license.

January 30. Snowball Earth? 650-775 million years ago

By Richard I. Gibson

Here’s the podcast:

Snowball earth is the controversial idea that the Earth was entirely, or almost entirely, covered with ice and snow during three or four periods during the late Proterozoic, from about 650 to 775 million years ago. While there is abundant evidence that can be interpreted to suggest the existence of a snowball earth, much of that evidence can be interpreted in other ways as well.

The podcast outlines some of those lines of evidence. Below are some links for further reading.

http://www.snowballearth.org
http://en.wikipedia.org/wiki/Snowball_Earth
http://www.amazon.com/Snowball-Earth-Maverick-Scientist-Catastrophe/dp/1400051258
http://www.sciencedaily.com/releases/2011/10/111012083450.htm

Modern glacier photo by Dirk Beyer, under GNU free documentation license.