History of the Earth

As we approach the end of March, we’re getting later and later in the Ordovician, and the collisions along the east coast of North America are getting more and more intense. We’re building up to the Taconic Orogeny, and yesterday we talked about one of the consequences of that mountain-building event, the eroded sediment that created the Queenston Delta. Rutland marble quarryCloser to the action, where blocks of continental crust, piles of volcanic rocks, slices of oceanic crust, and whatever else was out there were actually colliding to raise up those mountains, things got pretty hot and heavy. All those nice tropical shallow seas earlier in the Ordovician made nice limestones. But when you put a limestone under high pressure and add some heat, the calcite crystals in it recrystallize – it’s still calcite, but instead of gently interlocking and usually tiny crystals, they grow and intergrow more tightly, producing a metamorphic rock called marble. Some of the best marble in the United States was created during the Taconic Orogeny, cooking those nice Ordovician limestones. Especially in Vermont, but also in Tennessee and Georgia, Ordovician marbles have been mined for centuries as building stone and for monuments. The Amphitheater of Arlington National Cemetery is made of Ordovician Vermont marble – one of a great many structures that are. The quarries near Rutland, Vermont, yield white marble, but with plenty of variations in the original limestone, you can get all sorts of colors in marble, from red to black. The oldest marble quarry in Vermont, at Isle la Motte, was opened by Ichabod Fisk in 1664. Vermont quarries also produce verde antique, a metamorphic rock containing green serpentine and similar minerals, used for countertops, tiles, and facades. * * * Today, March 22, is the birthday of Adam Sedgwick. You must remember him, the geologist who defined the Cambrian and lost a friend over the details of Cambrian-Silurian stratigraphy. He was born March 22, 1785, in Dent’s Town, Yorkshire, England. —Richard I. Gibson Photo by C.W. Nichols, Rutland, VT marble quarry c. 1870. Public domain, via NY Public Library and Wikipedia. Reference Vermont Marble Industry

Red rocks of the Queenston Formation in OntarioYou recall from our discussion of the Bighorn Dolomite a few days ago that much of western North America was a tropical shallow sea into the Late Ordovician. To the east, in Illinois, Michigan, Indiana, Ohio, and Kentucky, things were changing because of the onset of a mountain-building event, the Taconic Orogeny, even further east. A string of volcanic islands – an island arc, probably with other sorts of things in it, something like Indonesia today – began to collide with the ancestral core of North America, and a mountain range formed in what is now central New England and points to the north and south. What happens as soon as you lift up a mountain range? It starts to erode. The stuff eroded off this mountain range was carried by large river systems to the west, especially into what is now eastern Ontario and western New York and Pennsylvania. The sediments were dumped into the shallow sea that was there, forming a huge delta, probably quite a bit larger in area than today’s Mississippi Delta. The pile of sediment is called a clastic wedge, because it contains clastics – broken pieces of rock, worn down to gravel, sand, silt and mud, and it’s a wedge because it’s shaped like a doorstop – thickest toward the mountains, and thinning off to the far west. It’s called the Queenston Delta and the rocks in it are called the Queenston Formation. Yellow=sandy sediments of Queenston Delta. Mud across Ohio is part of the system, too.,The Queenston Formation – it’s called the Juniata Formation and other names in some places –  was laid down over a period of several million years, about 451 to 446 million years ago. In places it’s close to 1,000 feet, 300 meters, thick. A lot of it contains iron oxide cement, making the rock reddish in places, and suggesting the deltaic nature of the sediments – sometimes underwater, sometimes exposed to air. The Queenston Delta system was so vast, and contained so much sediment, that it may have contributed to a reduction in atmospheric CO2 which could have reacted with all that exposed sediment. Maybe even enough to reduce the ongoing Ordovician greenhouse situation and to contribute to the glaciation that’s coming at the end of the period. —Richard I. Gibson Photo by Ian Muttoo licensed under the terms of the cc-by-sa-2.0    Map from Ohio Geological Survey, Ohio Geology, Fall 1997 (PDF)

Ordovician trilobiteAre you familiar with pill bugs? Roll-up bugs? I haven’t seen one in decades – maybe they don’t live here in the arid west, or heaven forbid, maybe I’ve lost my curiosity at what might be under a rock. If that’s it I’ll have to remedy that. But I recall well as a child in Michigan finding these little gray multi-legged critters in the garden. If you touched them, they’d roll up into a tight little ball. Their soft underbellies were protected by their relatively hard carapaces. Pill bugs are arthropods like insects, centipedes, and spiders, but they aren’t closely related to them. They are actually crustaceans, isopods, more closely related to shrimp and lobsters. And of course, trilobites were arthropods too, and they shared with pill bugs the ability to roll themselves up into a defensive posture. Flexicalymene, enrolledTrilobites may have actually peaked during the Cambrian Period, but they certainly participated in the Ordovician diversification, with lots of new species appearing. They seem to get a little fancier in their ornamentation and development of spines and they clearly became adept at enrolling. Trilobites could definitely roll themselves into tight defensive balls early in the Cambrian, but for some time it was thought that the earliest trilobites couldn’t do it. But in 2013, a team from the University of Cambridge described some olenellids – early trilobites – from about 510 or so million years ago that could and did enroll themselves. So this is not strictly an Ordovician trilobite thing. But some Ordovician trilobites, such as Felexicalymene from the Ordovician of Ohio, Kentucky, and Indiana, made themselves famous by doing it. —Richard I. Gibson Ordovician trilobite photo by Vassil, under GNU free documentation license. Flexicalymene photo by Steve Henderson, used by permission. Further reading: http://www.trilobites.info/enrollment.htm http://www.sci-news.com/paleontology/science-enrollment-trilobites-01420.html http://www.livescience.com/39920-trilobites-curled-like-pill-bugs.html

Much of what is now North America – especially the western part – was under a warm, shallow sea during most of the Ordovician. The continent was near the equator, and all that warm, shallow water undoubtedly contributed to the proliferation of life that we see in the Ordovician. In the west, toward the end of the Ordovician, a wide carbonate bank developed, similar in some ways to the shallow waters off the west side of Florida today. We call the rocks that lithified from those sediments the Bighorn Dolomite for prominent outcrops in the Big Horn Mountains of Wyoming. The carbonate platform where the Bighorn and equivalent rocks were deposited was much larger than the Florida coast – it extended from what is now Yukon Territory in Canada to northern Mexico. All of that area was pretty much tropical to sub-tropical during the Ordovician. Wide, flat, shallow water zones are sensitive to subtle changes in sea level. As planet earth approached the end of the Ordovician, polar ice caps were growing, on the way to a major glacial epoch that probably contributed to the mass extinction at the end of the period. Details of cycles in the sediments of the Bighorn Dolomite coincide quite well with the sea level changes related to the initiation of this ice age. The Bighorn Dolomite is pretty pure dolomite. Dolomite, you recall, is calcium magnesium carbonate – almost the same as calcite, calcium carbonate, which is the mineral that makes limestone. Dolomite has that added magnesium, which can be added during lithification, the process that turns soft sediment into hard stone. Adding magnesium to the molecular structure makes the rock more porous, and the Bighorn is a useful aquifer in places. Out here in the arid west, limestones and dolomites are resistant rocks. In rainy country, like the Midwest and eastern parts of the United States, such rocks tend to dissolve in the weakly acidic rain water – and that’s not really modern acid rain, but a very weak acid, carbonic acid, created when rain falls through the atmosphere and reacts with carbon dioxide. Acid, even weak acid, dissolves carbonates eventually – and that takes a lot longer where there isn’t much rain. So here in Montana and Wyoming, carbonates make prominent ridges, and the Bighorn Dolomite is no exception. In places like the Tensleep Canyon on the west flank of the Big Horn Mountains along highway 16, the Bighorn is a near vertical cliff that adds to the scenic beauty of that drive – a drive that I recommend highly. In some places, the Bighorn is as much as 400 feet thick – but in many locations, the Bighorn Dolomite is not present – not because it was not deposited, but more likely because it was eroded away during later times when the land was uplifted above sea level by mountain-building events. * * *  The mineral dolomite, calcium magnesium carbonate, which comprises the rock dolomite or dolostone, was named for Déodat Gui Sylvain Tancrède Gratet de Dolomieu, who lived from 1750 to 1801. He was a Knight of Malta, Professor of Mineralogy, and geologist to Napoleon. He wasn’t quite the first to recognize the mineral dolomite, but he got the credit and the mineral was named for him during his lifetime. He found his specimens in the Dolomites, a part of the Alps also named for him. In politics, he helped engineer the surrender of the island of Malta to Napoleon, which pissed off the Grand Master of the Knights of Malta. Dolomieu was imprisoned for 21 months in solitary confinement. He was freed, but in broken health, died at the young age of 51. —Richard I. Gibson Further reading http://jsedres.geoscienceworld.org/content/82/8/599.abstract

A couple weeks ago we talked about cystoids, an extinct class of the echinoderms. Today let’s focus on some echinoderms that you’ll be more familiar with: starfish. The problem with studying ancient starfish is that they tend to fall apart – they don’t fossilize well. Professor Tony Martin’s blog, which I linked in the post on trace fossils on March 11, has a photo of a great trace fossil, the resting mark of an Ordovician starfish, so we know they were around at least that long ago. Stenaster huxleyi, from the Ordovician of Newfoundland, drawn by Elkanah Billings in 1865. Animal is about 4” across. I checked quite a bit and so far as I can tell, there are no known Cambrian starfish. So the echinoderms form another group, a phylum, that was established during the Cambrian explosion but diversified greatly during the Ordovician. Starfish were part of that diversification. Starfish were described from the Tremadocian, the lower Ordovician, by W.K. Spencer in 1951, published in the transactions of the British Royal Society, and I think no older starfish have been found so far. It appears to me that specialists in echinoderms really don’t know the ancestry of starfish. Maybe it was a soft-bodied critter originally, and evolved to produce a skeleton of sorts during the Ordovician, but it doesn’t seem clear to me that we know what starfish evolved from. Perhaps it was a quick evolution, something like the development of the trilobite eye, which seems to be almost instantaneous in the fossil record. However they came about, starfish have survived for almost 500 million years. They’ve changed some – the Silurian Period, which we’ll take on next month, saw the development of starfish with more than five arms – and those multi-armed sea stars survive to this day as well. Will they continue to survive? Who knows? Last fall, 2013, the popular science press was full of a mystery story of millions of starfish dying. Along parts of the west coast of North America, as much as 95% of the starfish population has vanished. It appears to be some disease that makes a starfish turn to goo – quoting one news story. It affects a dozen different species – that’s unusual – and as of a month ago, February 2014, scientists were still trying to figure it out. It’s a big deal, because starfish are the main thing keeping opportunistic organisms like mussels in check. Stay tuned. And for the record, while radiation from Japan’s Fukushima nuclear plant hasn’t been ruled out entirely as a cause, it’s considered to be quite unlikely. See the links below. —Richard I. Gibson Starfish deaths  Starfish epidemic  Oldest multi-armed starfish (and lots more)

Cephalopods – the name means head-foot, because their heads typically have tentacles, which seem like feet – cephalopods today are represented by octopuses, squids, and cuttlefish, plus the chambered nautilus. nautiloid TrilacinocerasLike so many other groups, cephalopods saw a huge diversification and even a period of dominance during the Ordovician. The most common type from that time is the nautiloids. Nautiloids lived in a shell, but unlike a snail or clam, where the animal lives in essentially a single hollow space, even if it is complex, nautiloids’ shells had multiple chambers in which the animal lived, with the segments interconnected by a thin tissue called a siphuncle. As the animal grows, more segments are added, and each one is separated by a distinct layer called a septum. The boundary layers, the septa, eventually became incredibly complex, with a fractal-like appearance. Ordovician nautiloid from KentuckyThey grew to be pretty big – Endoceras, an Ordovician type, has been measured at 11 feet long – you can imagine that with a mass of grasping tentacles at the front –  and it’s possible that there were even longer nautiloids. Earlier specimens tended to be straight, but some from the Ordovician are curved as well. They were predators, and probably amounted to the terror of the seas during much of the Paleozoic Era. Nautiloids survived multiple mass extinctions until the Late Cenozoic, only about 10 or 20 million years ago. They’ve declined to the point that there are only six species today, compared to 2,500 fossil species known. —Richard I. Gibson Photo by Mark Wilson, public domain. Ordovician of Kentucky; an internal mold showing siphuncle and half-filled camerae, both encrusted. Photograph of the fossil nautiloid Trilacinoceras taken by Dlloyd, used under GNU free documentation license.

Plate tectonics, the process driven by heat convection down in the mantle, has operated pretty much continuously at least since the late Precambrian, and probably even longer than that. During the Ordovician, it seems that there may have been a little more “action” than at some other times – probably the normal variation in the intensity of tectonic activity. Most of the supercontinent of Gondwana, which we first discussed on February 9, was finally assembled during the Ordovician, and it would remain together for the next 200 million years or more. At least by the late part of the Ordovician, Gondwana was situated near the south pole. But apart from Gondwana, several important continental blocks, including Laurentia, which is the core of North America, Baltica, the heart of northern Europe, and Siberia, were still moving around pretty much independently. And moving continents means subduction zones, volcanic island arcs, and small continental blocks fragmenting and colliding. Those three major continental blocks, Laurentia, Baltica, and Siberia, were relatively close to each other, but there were also some complex island chains not too far away. One of the most important of those is called Avalonia, named for the Avalon Peninsula of Newfoundland. The long, linear Avalon terrane today is found in Massachusetts, Maritime Canada, Newfoundland, southern Ireland and Britain, and maybe some bits of northwestern Europe. But during the Ordovician, it was separate from the major continental blocks. I think of Avalonia as something like the western Pacific today – from the Kamchatka Peninsula to Japan to the Philippines. A discontinuous string of continental fragments, volcanic islands, oceanic crust, and more – a real mess.  This string began to collide with North America during the Ordovician Period, probably causing the buckling of the crust that formed the Cincinnati Arch and Michigan Basin, which we talked about earlier this month. We’ll talk about the culmination of this collision, called the Taconic Orogeny, at the end of March. You can imagine that such a complex strip of diverse geological settings wasn’t necessarily just sitting there idly, even before it collided. We’ll talk about some of the consequences of the plate tectonic events related to Avalonia in a few days. —Richard I. Gibson Ordovician 450 million years ago, map by Ron Blakey, via Wikipedia, public domain. See also http://jan.ucc.nau.edu/~rcb7/global_history.html

The first critters that you’d recognize as primitive fish appeared during the Ordovician. They were armored with bony plates, and their general informal name, ostracoderms, means shell-skinned. For the earliest varieties, we only know them from fossils of these individual scaly plates. They didn’t have a rigid internal skeleton, so they generally fell apart when the animal died. The Ordovician ostracoderms, thelodonts, which means “nipple teeth,” didn’t have jaws, but they did have scales much like fish today – but the scales were tiny, only a millimeter or two long. They appeared probably during the Middle Ordovician, around 470 million years ago or a bit earlier. Since they were without jaws, and their mouths were on the bottom of the head, we think that these early fish were probably sediment bottom feeders, sucking stuff into their mouths and filtering out food. I talked about conodonts on March 3, and indicated that once we finally found the conodont animal, it was seen to be a small, eel-like animal. Eels are fish, and the conodont animal was indeed a primitive fish. But ostracoderms were the first ones that really had a fishy look to them. And they were widespread – they’ve been found in Ordovician rocks all over the world. Neil Shubin, in Your Inner Fish, a book I’ve recommended previously, tells us that the bony plates on ostracoderms’ heads were made of material – calcium phosphate, the mineral apatite – and have structures that are essentially teeth – teeth fused together and on the outside of the animal, but teeth nonetheless, in evolutionary terms. So, Shubin argues, the first hard parts in chordates, the group that includes us and the other vertebrates, were teeth in conodonts, the better to eat you with, and the second hard parts were teeth that evolved into armor – protection from those other gnashing tooth-filled mouths. It’s really a cool story that hangs together quite well, and if you’re interested in this sort of thing, I recommend – again – Shubin’s book, Your Inner Fish. The entire body of ostracodems was covered in scales, like modern fish, but the head area was more strongly armored by the fused-together plates into a more bony shield.  If you have comments about the podcast, please leave a review on iTunes or a comment on the blog. * * * Today, March 15, is the birthday of Wallace Pratt, born in 1885 in Phillipsburg, Kansas. Pratt was a pioneer in petroleum exploration geology. In 1918 he became the first geologist hired by Humble Oil & Refining – a company that would eventually evolve into the giant corporation we know as Exxon today. One of his major contributions was fostering the use of geophysical instruments in oil exploration, and he was also a founding member of the American Association of Petroleum Geologists. He donated 23 square miles of land in West Texas, where he had a ranch, to the National Park Service, forming the core of what today is Guadalupe Mountains National Park. He died in 1981. —Richard I. Gibson Drawing of reconstructed ostracoderms by Philippe Janvier under CC-by-A license. The black and white drawing is from an old textbook.

Today, by request, the podcast has some background information about me as a geologist, and the March 14 episode focuses on the rare-earth mineral deposit at Bayan Obo, which was formed at least partly during the Ordovician. The image below, of the Bayan Obo mine complex, is from the NASA earth observatory’s photo of the day. 

Three or four times now, I’ve mentioned that all the modern phyla of animals were established during the Cambrian, except one. It’s time to talk about the one, the bryozoans. They began during the Ordovician, so far as we know. Ordovician bryozoaIf you saw a bryozoan fossil, you might think it was a relatively delicate kind of coral. They have lots of diverse appearances, like corals, and their colonies are usually calcareous, made of calcium carbonate, like corals, though some bryozoans apparently had phosphatic colonies. Some bryozoans are stubby little pillars, some are lacy branches, and others are tubular, branched like staghorns, or form tiny thin hair-like crusts on other fossils such as brachiopods. Some bryozoans that grow into flat, fan-like branches with numerous small rectangular openings in the overall structure are called fenestrate bryozoans, for the window-like openings in the colony. Fenestrate means window-like.  Encrusting types are probably the most common, at least in living species. and there are plenty in the fossil record as well. They grow on other animals, on rocks, on modern seaweed, and one colony might have two million or more individual zooids cooperating to make a colonial structure half a meter long. These encrustations sometimes look like moss, and modern bryozoans are sometimes called “moss animals” for that reason. They were encrusters pretty much from their beginning in the Ordovician, and they are encrusters today – to the extent that they can become nuisances on ships’ hulls and dock pilings. What are they? They do still exist – more than 4,000 modern species are known, along with 15,000 fossil species – and they are colonial animals, like corals or graptolites, meaning that the individual zooids can’t survive on their own, away from the colony. The zooids are tiny, maybe a half millimeter long, but the whole colony can be many centimeters across, even a meter in some varieties. They are filter feeders, taking nutrients out of the water the live in, and they make that happen with the help of lophophores – the tentacle-like features that make brachiopods different from clams and other mollusks.  The oldest mineralized colonial bryozoan known is from the Lower Ordovician. Since everything else got started in the Cambrian or earlier, there’s been an intense search for Cambrian bryozoa. One candidate, described from the Upper Cambrian of Mexico in 2010, was thought to fill the bill, but it has since, just last year, 2013, been reclassified as a kind of coral. You can imagine that it’s a little challenging to be certain about these things, when the fossil remains – the structure that held the colony – might not include anything of the original animals, the zooids. It’s not as if lophophores, soft structures a tiny fraction of a millimeter long, are easy to preserve for 460 million years, but many of the modern orders of bryozoa were established by that time in the Ordovician. It is likely, though, that bryozoans did exist during the Cambrian – but that they were late to the game of secreting calcium carbonate to make a hard skeleton, the colony. Why were they the only phylum of animals that didn’t figure out how to do that during the Cambrian explosion? There’s an enigma waiting for someone to explore. Bryozoans contain interesting chemicals – some that cause serious skin diseases in fishermen, as well as some that show potential against Alzheimer’s disease. —Richard I. Gibson Photo by Mark Wilson via Wikipedia (public domain) Further Reading Cambrian bryozoans? Not yet.

The Michigan Basin is a bull’s eye on the lower peninsula of Michigan – a nearly circular target painted on the geologic map of North America. It’s about 250 kilometers wide, and 5 kilometers deep. Basins like the Michigan Basin are important because they often contain important resources such as oil and natural gas, so understanding how they form helps us explore for such resources. In some of the Ordovician rocks, called the Prairie du Chien Group, porosities are great enough to serve as natural gas reservoirs, and more than 5 billion cubic feet of natural gas has been produced from that part of the section. Not too shabby, but not too much in the grand scheme – and in fact the United States today consumes almost 100 billion cubic feet of natural gas per day, so that total historic production of 5 billion cubic feet from the Prairie du Chien of Michigan amounts to about 80 minutes’ worth of natural gas consumption today.  We’ll talk more about the Michigan Basin next month in connection with its mineral resources. The problem is, we really aren’t sure how the Michigan Basin formed. It’s shaped like a big bowl, and clearly there was subsidence in the basin to allow for the 5 kilometers of sediment to fill it. And fill it they did – the layers of rock are thicker in the center than on the flanks. One possible mechanism for formation suggests that the earth’s crust or upper mantle was weaker, or thinner, or of different composition, so that broad stretching on a crustal scale might have allowed this area to sink more than other areas, becoming the bowl in which the sediments were deposited. It’s a fact that a branch of the Mid-Continent Rift, the pull-apart zone that affected this region about 1.1 billion years ago – we talked about it on January 26 — but that zone was clearly very linear, oriented north-south. I suppose it might have controlled the subsiding, and the Michigan Basin is somewhat oval shaped, with the longer axis north-south, but honestly this seems to me to be a stretch. Possible, or possibly some degree of affect to the whole process, but hard to see as the one and only cause. Some mechanisms call on thermal subsidence as the basis for the Michigan Basin. In this scenario, a relatively small portion of the upper mantle cools more than adjacent areas, and when it cools, it contracts, it shrinks, and that smaller volume is also a physically lower place, a basin in which sediments can be deposited. This is a reasonable theoretical idea, but I don’t know of any good solid evidence for it in Michigan.  You can also get subsidence of the crust when you have an upwelling of the mantle down below. It pretty much stretches the crust above the upwelling hot mantle, and the stretched crust forms a neck, like when you pull silly putty apart – or partly apart. This has almost certainly happened in the Mississippi Salt Basin, near the Gulf Coast, but there, we have good geophysical evidence for that process which we don’t find in Michigan. And because the basin is so symmetrical, so nearly circular, it’s been suggested that it represents a huge impact crater. But beyond the circularity – and it’s really oval, not circular – there’s no evidence for an impact. Maps is from Devonian time; Michigan Basin began to subside in Late Cambrian and Ordovician time.Back in 1990, geologist Paul Howell and his colleagues at the University of Michigan studied the sequences of sedimentation in the Michigan Basin in detail, and found a good correlation in time between the deposition and tectonic events along the east coast of North America. I mentioned this idea in connection with the Cincinnati Arch the other day. The really good coincidence of several different mountain building events – collisions – on the east coast with pulses of subsidence in Michigan suggests a causative relationship, and it does put the development of the Michigan Basin into a plate-tectonic context, rather than a simple, isolated basin subsiding a lot, bu

Today’s topic is ichnology – the study of ick?  Nope, it’s Greek ichnos for track, plus logia for study. It’s the scientific study of traces of life, from footprints and burrows to feeding and resting marks, coprolites which are fossilized feces, and more. Ordovician trace fossils (borings) from Kentucky.Sometimes the traces are all we have providing evidence of ancient life, and sometimes, even when we have good body fossils of such life, the traces give us a lot more information about the critter than the body alone. How did it move around? Was it a crawler, or a swimmer? What did it eat? Was it a grazer, or an attacker? How did it live? Buried in the sand, in a constructed burrow, or just hanging out on the ocean floor?  All these questions are useful in terms of understanding life throughout earth’s history. And especially as long ago as the Ordovician, trace fossils can be pretty informative. When I was in college, we learned about two trace fossils – Cruziana, which we were told were trilobite tracks, and teonuris, supposedly the marks in sediment caused by a plant or animal rooted to the sea floor, swirling around in the waves – or maybe the grazing trace of some bottom-dweller. You still find the term Cruziana, and I think it generally is thought to represent trilobite activity. As for teonouris, I don’t even know for sure how to spell it and I don’t think it’s a word that’s used any more. Today the study of these things is much more advanced, enhanced by careful comparisons between fossil marks and the traces of life that we can see today. There’s even an International Congress on Ichnology. Tony Martin, a professor at Emory University in Atlanta, is a specialist in ichnology, and among his particular interests is the trace fossils of the Ordovician. His blog (see the links below) has some great photos, including the resting mark of an Ordovician sea star. I can really understand the fascination with these kinds of fossils. Where a trilobite fossil is cool, and of course there are things to figure out about it, on the whole, most of the time, you know when you’ve got a trilobite, or a brachiopod, or whatever. With trace fossils, you have more of a mystery story, and a fun challenge to figure out what it means. I remember being on a field trip in West Texas, and seeing these weird depressions in the rock, maybe a couple feet across, with narrow things like tentacles all around the rim – maybe 20 or 30 of those narrow branches focusing into the depression. No one I was with had a clue what it was. A giant jellyfish seemed unreasonable, but that’s what it looked like. Several years later, on a different trip in a different place, along a modern river, we saw the same thing in modern sediment – and like a flash, it was obvious. It was essentially a sink, a sump, where the last little pool of water on a drying riverbank collected. The tentacle-like marks were the runnels that had taken the last flow of water, from a rainstorm or the last high stand of the river, draining into the depression, leaving 20 or 30 little drainage channels. Even though that one wasn’t due to life, to my mind, that’s ichnology – a challenging mystery story, trying to figure out the non-obvious cause of something that may be quite obvious in the rock. And that can be a lot of fun. I’m a little envious of Professor Martin. —Richard I. Gibson* * * Today, March 11, in 1902, was the birthday of Marland Pratt Billings, in Boston, Massachusetts. Billings was a structural geologist, focusing on things like the way folding and faulting work in rocks. He spent most of his career at Harvard, and in 1942 published his textbook on Structural Geology, which became the bible of structural geology for a couple generations of geology students. Photo by Mark Wilson via Wikipedia, public domain.  Further reading: Ediacaran ichnofossils on Tony Martin’s blog Cambrian ichnofossils on Tony Martin’s blog Ordovician ichnofossils and modern traces on Tony

The Cincinnati Arch is a broad, low geologic structure that brings Ordovician rocks to the surface near Cincinnati, Ohio, and Nashville, Tennessee, where it’s called the Nashville Dome. This was not a mountain range, nor even a land area all that often, but it did separate the low-lying basins to the east – the Appalachian Basin – from those to the west, the Michigan and Illinois Basins. Map is from Devonian time, but the Cincinnati Arch began during the Ordovician.The best way to look at the Cincinnati Arch probably isn’t the most straightforward. It seems like an anticline, an upward arching of the rocks that brings the older, Ordovician rocks to the surface. But in terms of earth history, it’s probably more of a long linear stable area that stayed where it was when the adjacent areas subsided. It might have been related to the Middle Ordovician mountain building further east – the Taconic Orogeny, which we’ll talk about later in March when it reaches its culmination. The entire crust might have buckled very gently as island arcs and small continental fragments collided with the eastern part of North America. You can think of it as their added mass pushing down the crust, and the crust further to the west buckled upwards in compensation. The arch began to form in Middle Ordovician time, around 470 million years ago. If and when the arch broke the surface of the sea, it meant that marine sediments typical of the Ordovician in this area would not have been deposited. It would be an unconformity in the making, and that gap in deposition continued up into the Silurian and Devonian Periods. How do we know the alternative isn’t the correct story – what if those rocks were deposited, but were later eroded off? That would give us the same appearance today. To figure that out, you have to look at the little picture, the details, as well as the overall picture of what looks like a broad uplift. When you study the rocks, it appears that it was a combination of both processes – non-deposition as well as erosion. When you think about it, that shouldn’t come as a surprise. Land areas today are subject to both of those processes at the same time, and of course there is some deposition of sediment on land as well – just not as extensive and continuous, usually, as the piles of sediment we find in ocean basins. Over time during the Paleozoic Era, the Cincinnati Arch seems to have gone up and down several times. Or, alternatively, sea level rose and fell several times. Or, most likely, some combination of both happened. It’s all relative. Back in the 1980s, I think it was, I did some work with the geological surveys of Kentucky, Ohio, and Indiana to try to figure out the details of the subsurface of the Cincinnati Arch. A well had encountered unexpected sedimentary rocks below the lowest Cambrian rocks, the Mount Simon Sandstone, and the question was, could those sedimentary rocks contain any oil and natural gas? Even though they would be early Cambrian or Precambrian in age, their presence high on the arch might have allowed younger, but deeper sourced, oil to migrate into them like the oil we talked about in Ohio on February 21.   My contribution was to look at maps of the earth’s gravity field, which tells you things about the densities of rocks, and the magnetic field, which can help you say some things about the rock types. The project ultimately described an old basin there on the Cincinnati Arch, probably dating to Precambrian time and possibly related to the Grenville Front, a structure that represents a collision between continents about a billion years ago. We talked about it on January 29. Bottom line, I’m pretty sure no oil or gas has been found in those rocks. But the bigger structure, the Cincinnati Arch itself, certainly has affected sedimentation patterns and structures, and a lot of the oil and gas in Ohio, Indiana, and Kentucky is related to it. —Richard I. Gibson Image by Ron Blakey, licensed under the Creative C

Up until now, all the life we’ve been talking about has been in the sea. OK, maybe some trilobites ran up onto a wet beach above the water’s edge, and maybe some of the shelly critters lived in the intertidal zone, just as such animals do today, but they were fundamentally dependent on ocean water for their existence. And algae and bacteria can live almost anywhere today, so maybe they were exploiting some of the niches on land at a pretty early date. Liverworts It probably won’t surprise you to hear that the first big life on land was plants – moss-like plants, to be specific. The earliest evidence for them is spores in the fossil record of the Middle Ordovician, about 470 million years ago or maybe a few million years older. The spores are similar to the spores of modern liverworts, which are mossy plants. Spores from more complex vascular plants are found in Upper Ordovician rocks. Vascular plants have tissues for shipping fluids and nutrients around their bodies, and include modern trees and flowering plants. Once plants were on land, they began the chemical weathering of rocks, which until then had largely been subjected mainly to physical weathering, breaking apart by freezing and thawing and such. Certainly there were some chemical reactions among rocks, water, and the atmosphere, but plants must have accelerated that chemical alteration considerably. Chemical weathering of rocks removes things like calcium and other nutrients that plants can use – and combined with photosynthesis, which converts carbon dioxide to carbon and oxygen – might have removed enough CO2 from the earth’s Ordovician atmosphere to lead to a decrease in average temperature. So much so, according to some speculations, that it might have contributed to the onset of the Late Ordovician ice age. We’ll talk about that glacial period towards the end of the Ordovician, later in March.  But never underestimate the power of plants acting over a long period of time! We don’t find actual body fossils of plants in the geologic record until the Silurian, but the fossil spores in Ordovician rocks are pretty conclusive evidence that there were plants on land by Middle Ordovician time. —Richard I. Gibson Liverworts image from Ernst Haeckel's Kunstformen der Natur, 1904 (public domain) Moss froze the planet?

Graptolites are another group of animals that got started during the Cambrian, and survived the end Cambrian extinction to expand dramatically during the Ordovician. Ordovician graptolites. Photo by Steve Henderson, used with permission.Graptolites are a class of the phylum Hemichordata, a fairly obscure phylum that includes modern acorn worms and pterobranchs, which are worm-like filter feeders. There are only a handful of these surviving members of the Hemichordata, but there are hundreds of graptolite species. Their fossils are distinctive, little linear branches that look like they have sawteeth along at least one side. Some are short single segments, some branch into V or Y shapes or more complex fan-like arrangements, and some are organized into spirals. Most of them are only a few centimeters long, but that may be because larger colonies broke apart after death. I just said colonies – and they were colonial, like corals. That means that the individual animal elements aren’t really independent, and can’t survive apart from the colony. The individuals, called zooids, were tiny, almost microscopic, connected by a thin nerve-like structure. The whole colony was probably planktonic, floating on the sea surface or in the upper waters – and that made them incredibly widespread, which in turn makes them excellent index fossils world wide for identifying precisely what part of the Ordovician you’re in. You might not know the time to an accuracy of a million years, but you can know with incredible accuracy that you’re in the Pendeograptus fruticosis zone – or whatever – and where that zone is in relation to the other parts of the Ordovician. We’d call that very precise relative dating of rock layers. They’re called graptolites – which means “writing on rock” – because the broken, flattened segments are often fossilized in ways that look like hieroglyphs or other bits of writing in the rock. Graptolites were incredibly abundant during the Ordovician, and were decimated by the end Ordovician extinction, but survived. They finally died out during the Early Carboniferous, called the Mississippian in the United States, around 315 million years ago. So they had a good run, about 180 million years or so. It isn’t clear exactly how the modern pterobranchs are related to graptolites – as descendents, or cousins or maybe even actually representing surviving graptolites. 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";} I’d also like to express my thanks for the shout-out by Helena, Montana, earth science teacher Rod Bensen. You can check out his blog with excellent resources for science teachers.  —Richard I. Gibson Photo: Ordovician graptolites from Womble Shale, Arkansas. Photo by Steve Henderson, used with permission. http://en.wikipedia.org/wiki/Graptolithinia http://www.princeton.edu/~achaney/tmve/wiki100k/docs/Graptolite.html http://discovermagazine.com/1993/jul/itsaliveanditsag249#.UxUKzYWKKAw http://www.ucmp.berkeley.edu/chordata/hemichordata.html http://museumvictoria.com.au/discoverycentre/infosheets/marine-fossils/graptolites/