History of the Earth

Today we take a break from earth history per se to mention an important anniversary. On this date, March 7, 1785, in some ways, the modern science of geology was born. On this day the first of two papers by James Hutton was read before the Royal Society of Edinburgh, Scotland. The pair of papers was entitled Concerning the Systems of the Earth, its Duration, and Stability. These papers, with little change, formed the core of Hutton’s Theory of the Earth, published in 1795. Many of the ideas first clearly expressed there serve as the basis for modern geology. James HuttonHutton was born June 3, 1726, son of a Scottish merchant who was Treasurer of the City of Edinburgh. Hutton’s early interest in chemistry led him into medicine – he was a practicing physician – and ultimately into geology. He studied unconformities – breaks in the rock record – cross-cutting relationships, and the tilting of formerly horizontal strata to come to his famous conclusion, expressed later by Charles Lyell as “the present is the key to the past,” the idea that processes operating today must have operated in the past, and that they produced the features we see now. Hutton actually said "from what has actually been, we have data for concluding with regard to that which is to happen thereafter." An important aspect of his conclusion was that while there was much change in earth history, the processes were fundamentally the same, and operated continually. This led to his famous concluding statement, "The result, therefore, of our present enquiry is, that we find no vestige of a beginning,–no prospect of an end." Today we call this uniformitarianism, a big word that just says the processes affecting things on earth happened uniformly over geologic time. At the time there was plenty of opposition to Hutton’s ideas. Prevailing thought included the catastrophists, who believed that short intense events created the things we see in the rocks, rather than the gradual effects of erosion, deposition, and multiple relatively small tectonic events such as earthquakes. In fact uniformitarianism was sometimes called gradualism. Hutton was also opposed by the Neptunists, who were in their own fight against the plutonists. Neptunism suggested that everything was formed in the sea, and plutonists said everything came from molten, volcanic origins. Hutton’s ideas allowed for a nice compromise between those two fairly rigid and extreme views, and gave room for both processes and gave both considerable importance. The ultimate adoption of the concept of gradual change, uniformitarianism, became so well entrenched in geological thought that even into the middle part of the 20th century, the idea of any kind of catastrophe was ridiculed. Consequently when Luis and Walter Alvarez suggested, in 1980, that a catastrophic impact caused the extinction at the end of the Cretaceous Period that wiped out the dinosaurs, there was considerable laughter at the idea. The good news is that the idea was tested scientifically, and the evidence mounted, culminating in the discovery of the “smoking gun” -- a huge crater in the subsurface of Yucatan that has been dated to just the right time. Today, I would say that geologists firmly believe that uniformitarianism has been the dominant factor in the evolution of planet earth – but there have indeed been some changes in the processes affecting the planet over its 4.6-billion-year life, and there have indeed been some catastrophes too. But James Hutton first clearly expressed the basic concepts that govern modern geologic thought, on this day back in 1785. —Richard I. Gibson Image from Wikipedia (public domain)

As I discussed yesterday, all of the modern phyla of animals except one were established during or before the Cambrian Explosion. Today let’s talk a bit about some of the variations that came about during the Ordovician diversification. CystoidsEchinoderms are known today in starfish and sea urchins, as well as sand dollars and sea cucumbers. They often, but not always, have a five-fold symmetry that gives them the common star shapes. There are more than 7,000 living echinoderm species. And today, at least, they only live in marine waters – there are no freshwater or land-dwelling echinoderms. Pleurocystis filitextus, Ordovician cystoidThe fossil record of echinoderms includes a great many varieties, some of which are extinct. One of those extinct branches of the echinoderms is the cystoids, which only lived from the Middle Ordovician, about 465 million years ago, until the extinction event at the end of the Devonian, about 360 million years ago. Cystoids look like crinoids – which are nearly extinct but probably more familiar. Both look like plants, rooted to the sea floor, with a clear stalk and feathery arms like long petals at the top, so that crinoids especially are sometimes called sea lilies, but they are actually animals. Cystoids differ from crinoids in having triangular pores in their rigid calcareous skeletons – they are like sponges in that way – and their body forms often have a cruder expression of that five-fold symmetry than crinoids, but there are exceptions to that on both sides. The name “cystoid” means sack-like, and that’s what their bodies typically were. Cystoids are sometimes called Lazarus animals because they seem to come and go over the time they were around. It isn’t likely that they became completely extinct and then miraculously reappeared, but it does seem that they were decimated severely by things like the end Ordovician extinction, only to reappear in good numbers in the Silurian Period. But they were definitely done for at the end of the Devonian. —Richard I. Gibson Left image from an old geology textbook (public domain); right image from Ernst Haeckel's Kunstformen der Natur (1904; public domain, via Wikipedia)

If the Cambrian Explosion marked the evolution of almost all the modern phyla of animals, and in most of those phyla also marked their development of hard parts, the Ordovician Biodiversity event is more a matter of all those groups filling out and expanding into as many niches as they could. It’s the development of details and, as the name says, diversity. After the Ordovician diversification, there were close to three times as many families of animals as there were in the Cambrian – 400 versus 150, but only one new phylum. It took place during the early and especially the middle Ordovician, about 485 to 460 million years ago, a span of just 25 million years. Like the time frame of the Cambrian explosion, this is a remarkably short period of time, geologically speaking, for so much to happen. Some scientists see the Ordovician diversification as a natural outgrowth of the Cambrian explosion, a predictable expansion but one that perhaps was episodic, with the Ordovician event being the greatest pulse. Factors such as the evolution of jawed animals, like the conodonts, and increases in predation might be part of that way of thinking. And it could be as simple as the idea that once animals began to become complex, and to change, it was an ongoing process and the diversification event was a natural part of it. It could be a 100-million-year-long “event,” but one that had two relatively short pulses. Others point to external causes, and many of those possible factors are the same or similar as those that are often called on as causes of the Cambrian explosion. More niches – more places for life to exploit – that’s a common possible explanation. During the early Ordovician, glaciation had just ended, tectonic activity was spreading continents apart more and more, and there was considerable active volcanism. All these things would have put more nutrients and useful trace elements into the oceans, and would have created more coastal areas with diverse environments – tantalizing empty spaces where ecosystems might thrive. The climate was largely warm, and likely greenhouse conditions prevailed, with high CO2 values that were perhaps initiated by the active volcanism and perpetuated by an ongoing greenhouse effect. The Ordovician saw some of the most intense volcanic activity in the past 600 million years. Sea levels were generally high. One suggestion is that there was an explosion in phytoplankton, the base of the food chain, taking advantage of all those volcanic-derived nutrients. In turn, that might have stimulated the diversification of animals, especially the filter feeders that were expanding dramatically during the Ordovician. And then there’s the Great Meteorite Shower idea that we discussed yesterday. So, as with the Cambrian Explosion, it seems likely that the Great Ordovician Biodiversification probably had many causes, and it may be incorrect to think of one, or even multiple specific triggers. I kinda like the idea that the various factors contributed to the changes, and that what we perceive as sharp events might be thought of as life arriving at some kind of threshold that allowed for dramatic expansion. Research continues. If you have suggestions for this podcast, please leave a review on iTunes, or add a comment on the blog. —Richard I. GibsonReferences and links for further reading: http://en.wikipedia.org/wiki/Great_Ordovician_Biodiversification_Event Paleocast http://icb.oxfordjournals.org/content/43/1/178.full http://www.webpages.uidaho.edu/bionet/biol116/o3/presentations/t4_cambrian-ordovician_overview_slide_notes.pdf http://www.geosociety.org/gsatoday/archive/19/4/pdf/i1052-5173-19-4-4.pdf http://onlinelibrary.wiley.com/doi/10.1111/j.1502-3931.2008.00115.x/abstract;jsessionid=82A42FF821D9026C586CA8EB3C671F2A.f04t02   Photo by Ryan Somma under Attribution-ShareAlike 2.0 Generic (CC BY-SA 2.0) 

Sometime in the early Ordovician, around 470 million years ago, it appears that there was a massive collision between some objects out in interplanetary space. The result on earth was a rain of meteorites, and we know about it because some of the meteorites are preserved. They fell into Ordovician sediments, so we can date the impacts quite well; in Sweden, at one locality, there are about 90 such meteorite fragments, adding up to about 8 kilograms of material. Wait, you say, how can 90 little chunks and 8 kilograms make for a “Great” meteor shower? Well, such things are incredibly rare, because they represent such instantaneous events. These little pieces didn’t make craters – although there is a probable Late Ordovician crater in Sweden, the Lockne Crater. But there are little grains of the mineral chromite, iron-chromium oxide, that are known to come from extraterrestrial sources. They and a few other fossil meteorites have been found in Ordovician rocks from Scotland and Argentina as well as the big find in Sweden. It probably does represent a global event. So what? Interesting, but what’s the big deal? Some scientists have linked the meteorite impact event with a couple of other global events. The Buttermere formation in England’s Lake District contains evidence for a massive landslide along what was then a continental margin. John Parnell, at Aberdeen University, suggests that it and 13 similar deposits of similar Ordovician age around the world could have been a response to a rain of meteorites over a relatively short period of time. It may well be possible, but there are many other possible causes of massive landslides in earth history – falling sea level, tectonic activity, and volcanism, to name a few. So the jury is still out on that one. More intriguingly, Birger Schmitz at the University of Lund, Sweden, and colleagues identified a close correlation in time between the meteorite falls and the onset of the Great Ordovician Biodiversity Event, a surge in life comparable in many ways to the Cambrian Explosion. Why would a disaster, a global-scale set of meteorite impacts, stimulate the diversification of life? The idea is that there was a quick, short, intense period of extinctions, and maybe “sterilization” of habitats, which was followed by a very opportunistic expansion by the survivors into those empty niches, by proliferating and adapting types of animals. There’s nothing like a disaster, at least up to a point, and room to grow to stimulate life into experiments and diversification. The close timing is interesting, but really doesn’t prove anything. There are many other potential factors in the Ordovician Biodiversity Event, which we’ll talk about tomorrow. If you have questions about this or any topic, or suggestions for how we can improve the podcasts, please let me know! You can post a review with ideas on the iTunes page for History of the Earth, or add a comment on the blog. —Richard I. Gibson Simon Wellings’ blog - primary resource for this post Article by Schmitz et al. Image is a drawing of the 1833 Leonid meteor shower

For decades, no one knew what conodonts were. They are little fossils, at most 3 millimeters long, typically with little projections like teeth on them. They first appear in the very late Cambrian or earlier, but they survived the end-Cambrian extinction to really proliferate in the Ordovician. And they lasted 200 million years, but perished in the mass extinction at the end of the Triassic Period. They’ve been known since at least 1856 when a Russian scientist, C.H. Pander, named them for the conical tooth-like projections they have. He thought they were fish teeth. But it was more than 120 years before we learned for sure what they are. Even without knowing what they were, they were useful little critters. The tiny bits are made of calcium phosphate, the mineral apatite, like human bones and teeth and kidney stones, and when they are heated up in nature, they change color in predictable ways. This change has been used in petroleum exploration for years to estimate temperatures in the subsurface, which in turn is valuable in figuring out things about generation of oil and natural gas. And because there are clearly defined species of conodonts, they also serve as biomarkers in studies aimed at determining the details of stratigraphy. It was presumed that the small phosphatic fossils were parts of some larger animal, and the search for the “conodont animal” continued for decades. One good candidate, a large fossil associated with many of the small tooth-like elements, was ultimately shown to be a critter that ate the conodont animals – whatever it was. Reconstructed animal (right) and teeth (left). The animal is a couple centimeters long.Eventually, in the early 1980s, good fossils of soft-bodied eel-like animals were discovered with the tooth-like pieces clearly displayed in their mouth area. The first confirmed discovery was in a museum, in old specimens from Carboniferous rocks of Scotland, and they’ve also been found in Ordovician rocks of South Africa. And there’s another locality in Iowa with excellently preserved conodont animals. The whole critter was only one or two centimeters long, and there are only a handful of good examples of the entire animal.  Conodonts are thought to be chordates, the same group that includes vertebrates, but that’s not absolutely certain, and many scientists classify them in their own phylum. If they are chordates, then their calcium phosphate “teeth” may be the first hard parts created by chordates, rather than a backbone, as suggested by Neil Shubin in his excellent book, Your Inner Fish, which I recommend if you are interested in the evolution of the human skeleton. —Richard I. Gibson Reconstructed conodont image by Philippe Janvier under CC-A 3.0 Further reading: http://en.wikipedia.org/wiki/Conodont What are conodonts?  Derek Brooks paper on the conodont animal good pictures Iowa locality Excellent overview and photos

If you listened to the end of the Cambrian, you know that it was marked by a major mass extinction event dated to 488 to 485 million years ago, so that’s the official start of the Ordovician Period. You will see other times given, depending on the source. Many say 490 million years, reflecting a time when the date of the mass extinction wasn’t so accurately known, and others may round it off to about 500 million years which is a convenient number. But officially it’s 488 to 485 million years, at least for now. Like the Cambrian the Ordovician is subdivided into three major sections, or epochs. They’re simply the early, middle, and late Ordovician, but each of those epochs is further broken down into two or three ages. Ages of time correspond to stages when we talk about the rocks themselves, just as early time equates with lower rocks. When we say Lower Ordovician, we’re talking about the rocks dating to Early Ordovician time. Ages and stages tend to be regional in extent, so they don’t necessarily have the same names all over the globe. There’s a real attempt at standardization, and the International Commission on Stratigraphy approved a global system in 2008. Some of the names, like the Tremadocian, the oldest part of the early Ordovician, have a long history. That name, from Wales, was first used by Adam Sedgwick in 1846 when he thought it was part of the Cambrian. But the Sandbian age of the Late Ordovician, named for a place in Sweden, wasn’t introduced until 2006. The Ordovician lasted about 45 million years, and ended with another major extinction event about 443 million years ago. —Richard I. Gibson Time scale from Wikipedia.

Nerds in a bar, volume 4. Dick Gibson sits down with geologist Katie McDonald to talk about some of the interesting details of the Cambrian rocks of Montana, and the unresolved problems they pose for the geologic history of western North America. This discussion builds on the outline in the podcast on February 22.

As we discussed a couple days ago, for about 50 years in Britain, where the work was done that laid the basis for much of modern geology, the early part of the Paleozoic era was the Cambrian and the Silurian Periods. Not until 1879 was the controversy about the boundary between them settled, by establishing another period, the Ordovician. Charles Lapworth named it for the Ordovices, an ancient Celtic tribe that lived in North Wales. They were conquered by Roman Governor Julius Agricola in AD 78-79. The Ordovician was the last of the major periods of geologic time to be named. There was an analogous controversy brewing in the United States over much of the same time as the Sedgwick-Murchison squabble in England, beginning in 1842 when Ebenezer Emmons described the Taconic System. It’s been called the greatest controversy in American geology, and we’ll talk more about that toward the end of March, but one consequence in the United States was that the U.S. Geological Survey didn’t fully accept the designation “Ordovician” until 1903. —Richard I. Gibson

The beginnings and endings of the subdivisions of geologic time are usually well recorded in the rocks. Many of them are major changes in the life of the time, as indicated by fossils. And many of those changes are mass extinction events. Extinctions occur probably almost continuously, but there’s clear evidence in the fossil record for relatively short time spans when the rate of extinctions ramped up dramatically, killing many more species than usual. These mass extinctions punctuate the geologic record. You’re probably familiar with the mass extinction at the end of the Cretaceous period, 65 million years ago, when most of the dinosaurs died, and the even greater event at the end of the Permian period when about 96% of all marine species vanished. But there were two or maybe four mass extinctions during the Cambrian period that were probably worse than any later events except the Permian one. The last of these events was about 488 million years ago, and is taken to mark the end of the Cambrian. Many of the marine animals that we described in the Cambrian explosion of life died. Brachiopods and trilobites especially saw a serious reduction in the number of species, and this is clearly recorded in the fossil record. What caused it? Until quite recently we haven’t been able to point to smoking guns, explicit causes for mass extinctions. You’re undoubtedly familiar with the idea of an asteroid impact causing the end Cretaceous extinction. Other extinctions are not so clear cut. There is evidence for increased glaciation at about the start of Ordovician time, and that’s been cited as a possible cause or factor in the mass extinction. Besides colder temperatures, glaciation lowers sea level by locking water up in ice, so there would have been fewer of the popular shallow water niches for trilobites and such to live in. Cooler water is also less able to hold oxygen, so oxygen depletion is also cited as a possible factor in the end-Cambrian extinction. Bottom line: we have some reasonable well thought-out ideas for causes of the Cambrian mass extinctions. But we really don’t know. * * * Today, February 28, 1743, is the birth date of René-Just Haüy, at St-Just in Picardy, France. Haüy was a mineralogist, often called the Father of Crystallography. He studied the regular way minerals break apart, a property called the mineral’s cleavage, and applied mathematical approaches to crystal forms, anticipating the much later understanding of molecular crystal structure. He was imprisoned during the French Revolution, but survived, and under Napoleon became a professor of mineralogy. He died in 1822.  —Richard I. Gibson Image from Wikipedia under GNU free documentation license

Adam SedgwickAdam Sedgwick was born in Yorkshire, England, son of a not-so-well-to-do preacher. He was an unruly student, but made it to Cambridge at age 20. With poor-man’s clothes and a hinterland accent, he didn’t fit in too well with his wealthy classmates, but he was near the head of his class until he was felled by a bout of typhoid, which would leave him sickly for years. Of necessity – it was required by the Church, which controlled Cambridge University – he studied theology, which he detested so much that he applied for a professorship in geology, about which he knew nothing. But he was elected to the job, which paid a measly hundred pounds a term. As an infant science, geology left him plenty of room for invention, and maybe for objectivity as well. His self-taught geology and enjoyment of a free life led him eventually to ramble around Wales, where he described the lowest, oldest series of sedimentary rocks he could find, and called them Cambrian. Roderick MurchisonSedgwick of course encountered other geologists, including Roderick Murchison, whose birthday we celebrated a few days ago. They became friends, though they were from different walks of life. Murchison was rich, a son of landed gentry in Scotland. In contrast to Sedgwick’s solitary camping expeditions into the wilds of North Wales, Murchison took with him his “wife and maid, two good gray nags and a little carriage, saddles being strapped on behind for occasional equestrian use.” He smoked expensive cigars with colleagues in a salon-like atmosphere even if it was in a carriage. Where Sedgwick focused on the physical nature of the rocks – we’d call that petrology and lithology today – Murchison focused more on the fossils in them. Sedgwick defined the Cambrian from its position low in the section and from its rock types, while Murchison defined strata in South Wales based on their fossils. He called that package of rock the Silurian, for an ancient Celtic tribe who lived in South Wales, the Silures. All well and good. Murchison and Sedgwick teamed up to work in Devonshire and Cornwall, jointly announcing the Devonian Period in 1839. This was a controversy of its own, which we’ll talk about at the appropriate time… but as the friends continued to extend their work on their other units, the Cambrian and Silurian, problems developed. Sedgwick was increasingly plagued by health problems while Murchison actively extended his Silurian System. It became evident to Murchison that some of Sedgwick’s Cambrian rocks actually contained fossils that should be classified as Silurian, so he extended his Silurian formation lower and lower in the section, taking up more and more of the Cambrian. This upset Sedgwick, although he had tacitly—or, he said later, inadvertently—approved the extension, and sometimes he denied the whole thing in harsh terms. The friendship was at an end, and the controversy pervaded British geology for the next 40 years. Everyone chose one side or the other, but on the whole Murchison’s later career was far more successful than Sedgwick’s. Murchison was knighted, and he became director of the British Geological Survey. Sedgwick, in declining health, kept a professorship, but seems to have been relegated to a by-way in British geology. Both Sedgwick and Murchison died before their controversy was settled. It fell to English geologist Charles Lapworth to study the Cambrian and Silurian strata and to propose that it was necessary to include another time period there, embracing parts of Sedwick’s Cambrian and parts of Murchison’s Silurian. He called it the Ordovician, and we’ll be there in a couple days. —Richard I. Gibson Images are public domain.

Perhaps you recall on February 4 we talked about archaeocyathids, animals that most scientists believe were an early type of sponge. By the Late Cambrian, the archaeocyathids were extinct, and that might be because of the increase in numbers and diversity of more modern sponges. There were probably sponges in late Precambrian time. The Ediacara fauna includes probable sponges. But these primitive animals took part in the Cambrian explosion, specifically the explosion in development of hard parts – the same kinds of hard parts that sponges have today. Most modern kitchen sponges are plastics made from oil and natural gas, but natural sponges are still harvested for household sponges. Their roundish or cylindrical bodies consist of spongin, a protein similar to the collagen in humans that makes up things like tendons and skin. And sponges are full of holes – pores, which gives the name to their phylum, Porifera. Those holes are vital to their simple lives, necessary for circulating water to bring in nutrients and wash out wastes. Sponges don’t have nervous systems or circulatory systems. The water they live in does it all for them. Microscopic sponge spiculesSo what happened to sponges during the Cambrian explosion? They developed things called spicules – pointed structures, sometimes microscopic and sometimes macroscopic, that they used to help support their spongy bodies and that may have provided at least a bit of defense against predation. Although sponge spicules can be made of hardened spongin or calcite like most shells, many are siliceous – SiO2, the same as the mineral quartz. That’s the most common mineral in the earth’s crust and it’s the most common constituent of sand. Because silica is resistant, sometimes sponge spicules are all that survives in the fossil record from what may have been a great abundance of sponges. In addition to providing a support structure for sponges, spicules might have served as little fiber optic bars focusing light into a sponge. This might have helped attract algae or other organisms that sponges had symbiotic relationships with, but study of this aspect of sponges is pretty new. It does have implications for the fiber optic industry, because the cold-temperature secretion of silica by sponges would probably be cheaper than the high temperatures used in the industry today, and it might allow for more efficient introduction of impurities to improve optical characteristics. A common example of such impurities being used is the photo-sensitive chemicals introduced into eyeglasses to make them darken in ultraviolet light. Another is adding tiny amounts of the rare-earth element lanthanum to improve the refractive properties of camera lenses. I discuss some of these applications in my other book, What Things Are Made Of and if you’re interested, you can find information about it here. Today is Joseph Le Conte’s birthday. He was born in 1833 on Woodmanston Plantation, Georgia, and he became a prominent physician and geologist, the first professor of geology at the University of California at Berkeley. He was a friend of John Muir and was a founding member of the Sierra Club. —Richard I. Gibson Photo by NOAA (public domain).

When we talk about mineral deposits, we often don’t know accurately the geologic time when the minerals came in – it might be much, much later than the rocks in which the deposits are found. That’s changing, as we get better and better at dating techniques, but for most of these podcasts dealing with mineral deposits, we’ll probably focus on the age of the host rocks and talk about the time the minerals came in more speculatively. The lead belt of southeastern Missouri is concentrated in Cambrian rocks, especially the Bonneterre formation, which is mostly dolomite, calcium magnesium carbonate. It’s much like limestone, calcium carbonate, but the magnesium in there skews the crystal structure, so dolomite crystals contain more intermolecular space than calcite. That makes them good candidates for oil reservoirs, or as hosts for mineral deposits. The Bonneterre rocks originated in a warm, shallow sea during Late Cambrian time, about 495 million years ago. They might have been limestone originally, converted to dolomite by magnesium-rich water percolating through the rock at a later time. The whole process of dolomitization is complex and not thoroughly understood, at least not by me – I’d like to find someone who knows more about it to talk with, as a future podcast. Galena from Sweetwater Mine, Viburnum Trend District, Reynolds County, Missouri, USA. Photo by Rob Lavinsky, CC-by-SA. The rocks sat there for a long time – probably at least a hundred million years – until the Devonian, about 385 million years ago, or maybe until the Pennsylvanian, 280 million years ago. The jury is still out, as far as I can tell, on when the minerals were deposited in the rock. My eyes tend to glaze over when I read the phrase “hot mineral rich waters came in” – because that’s often a cop-out meaning, we don’t really know. But, research continues, and this kind of thing is getting to be more and more pinned down as more information comes in. Waters that were heated by mountain-building activity, volcanism and the physical collision of plates, must have collected a lot of lead. Those waters were most likely driven into the Bonneterre formation from the south, possibly from as much as several hundred miles away, until they found a suitable rock in which to crystallize. There’s more galena, lead sulfide, in southeast Missouri than anywhere else in the world. The ores in Missouri are part of a class of mineral deposits called Mississippi Valley Type, which occur in sedimentary rocks. Southeastern Missouri has produced lead since about 1721, when early French explorers began mining. They produced as much as 1,500 pounds of lead ore per day, which was shipped down the Mississippi and on to France. Production has been pretty much continuous since about 1802, when Moses Austin began smelting ore, a year before the territory became part of the United States in the Louisiana Purchase. Historic mine and mill buildings at the Federal Mine and Mill #3, now a part of Missouri Mines State Historic Site. Note the tailings dam in the background.There are three major sub-districts within the lead belt. The newest is called the Viburnum Trend, a long string of mines that began lead production in 1960. Missouri produces a lot of zinc, which commonly goes along with lead, and fair amounts of other metals including silver. Missouri produces about 70% of the lead in the United States, with Alaska the second-leading producer. Idaho is third, and that’s it – that’s all the lead production in the U.S.  Until 2011 the nearly 400 tons of lead that came from US mines was enough to make the United States a net exporter of lead, but in 2011 and 2012 the U.S. imported 2 to 4% of its lead needs. Eighty-six percent of U.S. lead consumption goes to make lead-acid batteries for cars and trucks, and thanks to recycling, we get about three times as much lead from old batteries as we do from mines in Missouri, Alaska, and Idaho. China, the world’s leader in lead production with

Jellyfish may not have changed a lot in hundreds of millions of years. I guess that’s one measure of success, or at least an ability to survive environmental changes. There are in fact plenty of varieties of jellyfish today, but we don’t know that much about their evolutionary history because as soft-bodied animals, they don’t leave much in the way of fossils. Middle Cambrian cnidarian jellyfish. Black bar is 5 mm (© 2007 Cartwright et al.; under Creative Commons license) Normal 0 MicrosoftInternetExplorer4 /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Table Normal"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-parent:""; mso-padding-alt:0in 5.4pt 0in 5.4pt; mso-para-margin:0in; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:10.0pt; font-family:"Times New Roman";} In 2007 scientists from the University of Kansas, the University of Utah, the Smithsonian, and the University of Sao Paulo described some Cambrian jellyfish fossils from Utah that are remarkable in their preservation. The authors were able to identify such fragile structures as tentacles and organs, suggesting that modern aspects of jellyfish were developed within a few million years of the Cambrian explosion. For most other phyla, especially the chordates, which include us, evolving modern characteristics was a long process. Here's the paper. (also source of photo, used under Creative Commons license) The rocks that hold these fossils are about 505 million years old, which puts them in the Middle Cambrian, just before the start of the Late Cambrian. —Richard I. Gibson

With over 4,000 mineral species, you could overflow this calendar with beautiful pictures and words about minerals, but most minerals don’t have a lot of specific connection to particular time periods in earth history. Some mineral deposits do, and we’ll talk about them. Today’s mineral, glauconite, does have a connection to the Cambrian, at least to some degree. Glauconite is a complex potassium-iron alumino-silicate, Normal 0 MicrosoftInternetExplorer4 /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Table Normal"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-parent:""; mso-padding-alt:0in 5.4pt 0in 5.4pt; mso-para-margin:0in; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:10.0pt; font-family:"Times New Roman";} K2(Mg,Fe)2Al6(Si4O10)3(OH)12. It can be found in many kinds of sedimentary rocks, and in many ages right up to the present, but it’s pretty common in the Cambrian. It occurs as little green pellets, often intermixed with good quartz sand, or interbedded with limestone. What made these pellets? Cambrian Lion Mountain Sandstone (green in lower portion from abundant glauconite), central Texas. To put it bluntly, glauconite pellets are trilobite poop. OK, not just trilobites, and that’s not the only way glauconite forms. But the little round grains in marine rocks are thought to be an alteration from the original fecal pellets excreted by marine organisms. It can also precipitate directly, and it can form when some iron-bearing minerals are weathered, but the pellets in sandstones are generally accepted to represent fecal material. Some rocks contain enough glauconite to be called greensands, but more often, the sand-sized glauconite grains are scattered through the rock and aren’t obvious until you look at it under magnification. Then they practically pop out at you. The Lion Mountain Sandstone, in the Llano Region of central Texas, is a Cambrian formation rich in glauconite – and no real surprise, some parts of the rock are mostly broken up trilobite skeletons. It’s a cool rock, and it was probably laid down in a wide sandy tidal flat. With trilobites crawling all over the place and pooping left and right. Occasional storms must have broken up the trilobite shells and dumped them into the piles in which they are found today. Glauconite is green because the iron in it is in its reduced state, rather than oxidized which would lead to a rusty red color. That means relatively anoxic, low oxygen, conditions, such as might be found on a sea floor below wave base or in a stagnant mud, or the gut of a trilobite. The presence of glauconite pellets is taken to mean that the rock they are in was formed in marine conditions, and that’s a useful conclusion to draw from the presence of little green grains in a rock. —Richard I. Gibson Cambrian Lion Mountain Sandstone (green in lower portion from abundant glauconite), central Texas. Photo by Erimus via Wikipedia, public domain. 

When I was a student at Indiana University’s geology field course, out here in Montana, we learned the stratigraphic section. The Cambrian part is Flathead-Wolsey-Meagher-Park-Pilgrim. The Flathead is the oldest layer of the Cambrian out here, and I hope you aren’t surprised to learn that it’s a clean quartz sandstone like the Tapeats in the Grand Canyon and the Posdam back east. Like them, the Flathead sandstone sits above a profound unconformity, a break in the rock record, and the rocks below it are Precambrian in age, hundreds of millions of years older than the Flathead. It’s pinkish, like the Potsdam, because of some iron oxide cement, and it has little round green grains in it in places – we’ll talk about them tomorrow – but mostly, it’s just nice sandstone. Trilobite Bathyuriscus formosis, Cambrian Meagher formation, Montana. Photo by Stephen W. Henderson, used by permission.The stratigraphic section here in Montana is a lot like the Cambrian section in the Grand Canyon. Above the Tapeats sandstone in the Grand Canyon we have the Bright Angel Shale, followed by the Muav limestone. Here in Montana, the Flathead sandstone is followed by the Wolsey Shale, then the Meagher Limestone. Then the Park Shale, and then the Pilgrim formation, limestones and dolomites. The seas came in, the seas came out…. Alternating shale and limestone might mean that, but there are other ways to make it happen. I’m planning to have a conversation with an expert on Cambrian stratigraphy in a week or so – we might be in the Ordovician by then, but if we are we’ll just think back on the Cambrian when that conversation happens. From the point of view of someone mapping geologic layers, the importance of the sequence – Flathead, Wolsey, Meagher, Park, Pilgrim – is that it’s really the best way, sometimes the only way, to be sure which rock or rocks you might be looking at. In western Montana, there’s another pinkish quartz sandstone called the Quadrant – a chunk of it looks an awful lot like a chunk of the Flathead, to the point that it’s virtually impossible to tell them apart in the field. But the Quadrant is Pennsylvanian in age, around 280 million years old, rather than around 500 million years for the Flathead.  If you look at the rocks below the Quadrant, you won’t find the Precambrian unless there’s some complicated structural thing going on, like faulting. And if you look above, you won’t find the precise sequence of the Wolsey, a specific kind of shale, the Meagher, a limestone with distinctive characteristics, the Park shale, and then the Pilgrim formation. It’s that sequence that’s like a fingerprint that tells you you’re in the Cambrian, even if the individual chunks of rock can’t tell you that for sure. —Richard I. Gibson Trilobite Bathyuriscus formosis, Cambrian Meagher formation, Montana. Photo by Stephen W. Henderson, used by permission.