History of the Earth

You don’t expect much oil and gas in the Cambrian. Partly that’s because it’s so old and deep, the rocks that might hold oil or gas may have been buried so deeply that the hydrocarbons, the oil and gas, may have volatilized, turned to gas and seeped out. Or the pressure could have reduced the porosity to not much. And since oil and gas come mostly from decaying plants, you have to wonder if there was enough life around to accumulate to be cooked into oil. But we do find some oil in Cambrian rocks. Oil wells in back yards in Cardington, Ohio, 1964. Photo from Ohio Geological Survey.In Morrow County, Ohio, just north of Columbus, in the 1960s there was quite an oil boom for a while. It discovered a bunch of little fields in a special kind of oil trap. The upper part of the Cambrian in Ohio includes carbonates – limestones and dolomites, the kinds of rocks that flowing water can dissolve to make caves. At some point not too long after they were deposited, parts of those layers were eroded into hills, and the hills, standing above water line were dissolved – not really into caves, but little dissolution cavities developed in the rocks, excellent spaces to have an oil reservoir. Ordovician rocks – we’ll talk about the Ordovician next month – were deposited over those rocky hills and served as a tight, impermeable seal to keep fluids from escaping. Oil migrated into those little hills, and stayed trapped until the 1960s, when thousands of wells tapped hundreds of fields. They have produced about 38 million barrels of oil and 35 billion cubic feet of natural gas over time – not that much in the grand scheme of things, but not too shabby, either. To put it in perspective, the United States today consumes almost 20 million barrels of oil every day, so all of the 38 million barrels produced by those wells in the 50 years since the 1960s amounts to about 2 days’ consumption. Where did the oil come from? Good question. It’s in Cambrian rocks now, but did it start there? Oil reservoirs are not usually where the oil originates. It starts in a rock with lots of organic material, a source rock. Heat, from burial, cooks that solid organic matter over sometimes millions of years, and oil is generated. Then it migrates until it reaches a suitable place to accumulate, a reservoir. The oil in Cambrian reservoirs in Ohio is probably from organic-rich black shales of Ordovician age – younger than the reservoir. How do you push light oil DOWN into older formations? Well, you don’t, with some unusual exceptions. The deep Appalachian basin where the Ordovician shales were heated up – oil people call it the oil kitchen – was deeper than the reservoirs up in central Ohio. The oil did migrate up, as it pretty much must – but it got into older rocks that were physically above the younger source beds. Seems counterintuitive, but it happens more often than you might think. —Richard I. Gibson Photo: Oil wells in back yards in Cardington, Ohio, 1964. Photo from Ohio Geological Survey. http://pubs.er.usgs.gov/publication/70020638

We’ve talked about the Cambrian explosion and some of the cool critters that evolved during it, like trilobites and brachiopods. But the less obvious life was still around, if not thriving. That included blue-green algae, or cyanobacteria. Those little guys way back in the Precambrian were the primary builders of our oxygen-rich atmosphere. But during the Cambrian, calcareous algae, that is those that could create fine layers of calcite, calcium carbonate, were still abundant enough to make small reefs in the Cambrian rocks near Saratoga, New York, at Jackson, Wyoming, and elsewhere. Cambrian stromatolites near Saratoga Springs, NY. Photo by Michael C. Rygel, via Wikipedia under Creative Commons Attribution Share Alike unported licenseIt’s fair to call these things stromatolites, just as we did during the Precambrian. Stromatolites have survived to the present, but they have declined from their peak in the Precambrian. Some scientists think they suffered from the Cambrian proliferation of new grazing animals like trilobites, which might have roamed the surfaces of stromatolites, scraping the living algae off as food. This seems reasonable, and there is also a well-documented example from the Ordovician of stromatolites increasing in abundance during extinction events that killed off marine animals. Conversely, stromatolites decreased as animal life recovered from the extinctions. Should we care about ancient algae? Well, ancient algae and other plants are the biggest sources of organic matter that becomes oil and natural gas. You decide whether or not to care about them. Two noteworthy geologists were born on this day. Nathaniel Southgate Shaler was born February 20, 1841, in Newport, Kentucky. He became a fixture in the paleontology and geology departments at Harvard University. Ray C. Moore was also born today, in 1892, in Roslyn, Washington. He worked for the US Geological Survey and the University of Kansas, and he initiated the massive Treatise on Invertebrate Paleontology, 50 volumes, still in progress, and the definitive encyclopedia on invertebrate fossils. And on this day in 1962, John Glenn orbited planet earth. —Richard I. Gibson Photo by Michael C. Rygel, via Wikipedia under Creative Commons Attribution Share Alike unported license.

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.  

You remember Rodinia, the supercontinent that came together during the Proterozoic, and then rifted apart toward the end of the Proterozoic and in the early Cambrian? Today I want to talk about one chapter in that rifting apart that failed. Oklahoma’s Wichita Mountains are all that’s left of that failed rift, called the Southern Oklahoma Rift (called an aulacogen on the map). In early Cambrian time, about 540 to 525 million years ago, this area was much like the Red Sea, or East Africa. The continent was pulling apart, extending, and the cracks allowed diverse kinds of magmas, from granitic to basaltic, to ascend from deeper in the earth. It pulled apart enough that there was even a general rise in the earth’s mantle below what is now southern Oklahoma. The break was trying to turn into a new oceanic basin, but although the mantle was involved, and lavas flowed out, it never completely rifted. It’s kinda like the Mid-Continent Rift of Kansas, Iowa, and Minnesota that we talked about a half billion years ago, although this break is almost perpendicular in orientation to that older one. The crust subsided along two major west-northwest trending fault zones, and lavas as well as thick sediments went into the basin that formed. These rocks are exposed today in the Wichita and Arbuckle Mountains. But wait, you say – that was half a billion years ago. I thought erosion would have leveled the mountains by now. Well, you’re right. It would, and it did. But the break in the crust created a weak zone, still weak millions of years later. Fast forward to this coming July and August, when we’ll talk about the Pennsylvanian time period when Africa was colliding with eastern North America. Those forces were great enough to affect what is now Oklahoma too, and the formerly downdropped zone became active again – and this time, because of the squeeze play driven by Africa’s collision, things popped up. We call this a rejuvenation of the old fault zone, and it was in the opposite sense to the original rift structure. So that’s 250 million years ago or so – and the Wichita and Arbuckle Mountains are the low, eroded remnants of that uplift. But the surface rocks, granites and their volcanic equivalents, rhyolite, and other igneous rocks, date back to the original rift in the Early Cambrian. This zone probably extended west of Oklahoma, across what is now the Texas Panhandle (which has some complex geology beneath its flat surface), and into northeastern New Mexico, southwestern Colorado, and even into present-day Utah and maybe beyond. It has been rejuvenated in various ways over time. It was (and is) a big-time weak zone in the North American crust. It’s also been studied a lot, because there’s oil and gas trapped in some of these rocks and structures. At about the same time, but perpendicular to the Southern Oklahoma Rift, and a few hundred miles to the northeast, another rift was trying to break the continent apart. This is called the Reelfoot Rift or Mississippi Embayment. It runs from northeastern Arkansas and western Tennessee up the Mississippi River to southwestern Indiana. This zone, old as it is, is still active. The famous New Madrid earthquakes of 1811 and 1812, some of the most powerful earthquakes known in North America, reflect the presence of this ancient rift. We’ll talk more about those earthquakes another day. —Richard I. Gibson Map from Van Schmus, W. R., Bickford, M. E., and Turek, A., 1996, Proterozoic geology of the east-central mid-continent basement; in, Basement and Basins of Eastern North America, B. A. van der Pluijm, and P. A. Catacosinos, eds.: Geological Society of America, Special Paper 308, p. 7-32. Technical Links: https://gsa.confex.com/gsa/2001NC/finalprogram/abstract_5657.htm http://www.ogs.ou.edu/MEETINGS/Presentations/OilGasMar2012/Keller_Southern_OK.pdf

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.

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 SandstoneBut 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

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).

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.

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

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.

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. SmithIf 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"

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

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 GondwanaNear 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

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.  http://en.wikipedia.org/wiki/Maotianshan_Shales http://www.fossilmuseum.net/Fossil_Sites/Chengjiang.htm

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. 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