October 31. India takes off

The end of the Jurassic is marked by a relatively minor extinction, perhaps only regional in extent rather than global. I talked a little about it a few days ago. It did significantly impact the distribution and numbers of Sauropods and stegosaurids as well as ammonites and other marine animals, but no major groups were wiped out completely as far as I can tell.

Tectonically, Pangaea’s break-up was increasing. By the end of the Jurassic, East Gondwana, one of the two big pieces of Gondwana that had stayed pretty much intact for hundreds of millions of years, was beginning to fragment. We talked earlier this month, on October 5, about the Karoo Volcanics that resulted from the initial rifting though parts of East Gondwana. The rifts, cracks, that allowed the volcanics to erupt were followed by a period of crustal stretching throughout much of the Jurassic period. By about 150 to 155 million years ago, within 10 million years of the end of the Jurassic, East and West Gondwana were probably fully separated from each other, albeit by a narrow, strait-like ocean. And at the same time, India plus Madagascar, which was part of East Gondwana rather than West Gondwana and Africa, began to separate from Australia.

Gondwana when it was intact

Also in the late Jurassic, the eastern side of what is now Australia was also fragmenting, with continental blocks now known as the Lord Howe Rise and New Caledonia pulling apart from the Australia portion of Gondwana.

Pretty close to the end of the Jurassic – it might have been a bit into the Cretaceous, but this whole process is millions of years, so it’s a little silly to point to an instant of geologic time as the time when continents separated – so close enough, at the end of the Jurassic, rifting between India and Antarctica and Australia and Antarctica were also well underway. Those separations continue into the Cretaceous (and in fact to the present day) so we may discuss them again, but they got started about the end of the Jurassic.

The maps of the world at the start and end of the Jurassic Period were remarkably different from each other. From Pangaea – admittedly with some cracks, but still pretty much one supercontinent – to a global distribution of continents that was becoming almost recognizable as the continents of today.

Tomorrow, the Cretaceous begins.

—Richard I. Gibson

Australia separates from Antarctica


Gondwana map based on original by Petter Bockman, public domain, via Wikipedia. 

October 30. A big predator

Where there are herbivores, there are carnivores. The gigantic Sauropods of the Jurassic must have been a feast for the gigantic carnivorous dinosaurs – whether they killed them or scavenged on dead bodies. Just this year, 2014, a new species of predatory dinosaur was described from Europe, and it may be the largest Jurassic predator known.  

Torvosaurus in Museum of Ancient Life in Lehi, Utah, U.S.A.
Photo by leon7 used under Creative Commons license 

Portugal has late Jurassic rocks containing many of the same dinosaur species found in the Morrison Formation of the western United States. The Jurassic Lourinhã Formation was laid down in a flat coastal plain along the shore of the newly formed Atlantic Ocean. Rifting here began in the Triassic, as Iberia began to pull away from Newfoundland. The rift that ultimately became the Atlantic Ocean was established a bit to the west, so this area remained a low-lying zone near the margin of the ocean. The setting was much like that of the Morrison Formation, although the Morrison was along the shores of an inland sea, while the Portuguese rocks formed along a real ocean coastline. 

In both the Morrison and in Portugal, the top predator was a dinosaur of the genus Torvosaurus – a carnivore that at a glance looks a lot like Tyrannosaurus, which is a Cretaceous dinosaur. Torvosaurus was about 10 meters, 33 feet, long, and bipedal. Despite all the exploration in the Morrison formation, it wasn’t until 1971 that the first Torvosaurus specimens were found in Colorado. The Portuguese specimens, discovered in 2000, have just this year been described as the second species within the genus Torvosaurus. The differences with the Morrison species are not huge, and the species are differentiated on the basis of the number of teeth and the shape of the mouth.

The Portuguese rocks have also yielded eggs and embryos that are ascribed to Torvosaurus. The specimens are the oldest theropod dinosaur eggs with a single layer to the shell.

The Portuguese specimen is the largest land carnivore known in Europe, and among the largest Jurassic carnivores anywhere. Other carnivores shared its ecosystem, including Allosaurus and Ceratosaurus.

—Richard I. Gibson

LINKS:
Largest predator in Europe 
Torvosaurus 

Lourinhã Formation 

Torvosaurus in Museum of Ancient Life at Thanksgiving Point in Lehi, Utah, U.S.A. Photo by leon7 used under Creative Commons license 

October 29. Cycads and Gingkoes

We’ve touched on the prolific plant life of the Jurassic several times. It resulted at least to some extent from the greenhouse conditions that prevailed over much of the globe for much of the period. And I mentioned yesterday that there were pretty much no flowering plants. The Jurassic forests were still dominated by conifers, ferns, rushes, and cycads. Not too different from the Carboniferous.

Cycads have cones with exposed seeds, and they first appear in the fossil record during the Permian, with some possible Carboniferous examples. They flourished during the Jurassic, so that they were characteristic of many Jurassic forests, to the point that the Jurassic is sometimes called the Age of Cycads. They are the primary constituents of many Jurassic coal beds. 

Jurassic gingkoes from Oregon.
USGS Monograph XLVIII, by Lester Ward, 1905.

Cycads are gymnosperms, with cones containing exposed seeds. They range from tiny plants only a few centimeters tall to large trees, and while many of them looked like palms, they are not closely related. Cycads today live mostly in tropical environments, but some varieties are found in desert conditions. Modern cycads have been called “living fossils” because they seem to be pretty much unchanged from their Jurassic ancestors, but a recent study has suggested that the diversity of modern cycads only dates to the past 10 million years or so – a second wave of cycad diversification. According to that research based on DNA analysis, cycads declined at and after the end of the Cretaceous but then radiated again during the Miocene, about 10 million years ago. (Note: text above modified thanks to the comment, below)

The giant herbivorous dinosaurs, Sauropods and others, undoubtedly munched on cycads as part of their diet. 

Gingkoes were also common in Jurassic forests. They are fairly closely related to cycads and conifers, although their precise relationship is not certain. The earliest fossils of the Gingko genus come from the early Jurassic, and while their abundance and diversity peaked during the Jurassic, gingkoes declined during the Cretaceous and later, so that today, there is only one living species, Gingko biloba, native to China.

Jurassic gingkoes lived across the northern continents, what are now North America, Europe, Siberia, and China. Today’s gingko trees can be well over 100 feet tall, and Jurassic varieties were probably of similar size. Gingko leaves dominate some fossil assemblages.

* * *

It’s appropriate that today’s birthday is Othniel Charles Marsh, one of the most prominent students of Jurassic vertebrate fossils in the United States. He was born October 29, 1831, near Lockport, New York, and he spent most of his career with Yale University and the U.S. Geological Survey. O.C. Marsh’s competition with Edward Cope in the rush to find and identify dinosaur fossils is known as the “bone wars,” but we’d probably say Marsh won the wars – he named at least 43 orders, families, and genera of dinosaur, and described 80 new species to Cope’s 56. The legacy of both Marsh and Cope is dinosaur fossils in museums around the United States.

—Richard I. Gibson

LINK
Modern cycads not so old?  
Cycads 

October 28. Bloodsucking parasites and other Jurassic insects

Something like 1,000 species of Jurassic insects have been described. Jurassic dragonflies were smaller than the giants of the Carboniferous, but still large, with wingspans on the order of 7 inches and bodies 5 inches long. They’ve been found in the Solnhofen limestone, and specimens are often offered for sale in the $1,200 to $2,000 range. The basic body plan of dragonflies hasn’t changed much over the history of the group, more than 300,000,000 years. 

Image of scorpionfly Jurassipanorpa sticta holotype. (Scale bar: 1 mm)
from He Ding, Chungkun Shih, Alexei Bashkuev, Yunyun Zhao,
and Dong Ren used under CCA-4.0-International

In 2014, researchers described a strange new insect from the Daohugou beds in northeastern China – an aquatic fly larva that is interpreted to be a salamander parasite, sucking the blood of its host to which it was attached somewhat like a remora on a shark, but only about an inch long. Those rocks in China have yielded more than 300,000 insect fossils, so that’s the source of a lot of our knowledge of Jurassic insects. Some show the color patterns in wings. And because the China locality represents just one tiny little ecosystem, it means that while we know a lot about it, there’s plenty more to know about Jurassic insects. 

One of the earliest examples of mimicry in the insect world is from a Jurassic fly from the Daohuguo beds whose wings look so much like gingko leaves that they were missed by early workers and discarded as “just another gingko leaf.” Such an adaptation would have certainly been advantageous to a bug in a world full of insectivores, from salamanders to early gliding mammals to small reptiles to, probably, early birds.

Beetles, crickets, caddis flies, moths, stoneflies, flea-like bugs, and more have been identified from Jurassic rocks around the world. Clearly, insects had diversified significantly early in their history, well before their explosive radiation that’s tied to the development of flowering plants, which we’ll get to next month.

—Richard I. Gibson

Image of scorpionfly Jurassipanorpa sticta holotype. (Scale bar: 1 mm) from He Ding, Chungkun Shih, Alexei Bashkuev, Yunyun Zhao, and Dong Ren used under CCA-4.0-International


News report:
Bloodsucking parasite

October 27. Late Jurassic oil source rocks

On October 22, we talked about the lush vegetation of the Jurassic that thrived in greenhouse conditions. The volume of that plant matter is likely a factor in the astonishing hydrocarbon source rocks that are found in the upper Jurassic. By some estimates, a quarter of all the discovered oil and gas on earth was sourced by Jurassic rocks.  

We’ve mentioned Saudi Arabian oil already, back in the Silurian (April 18). But a much larger source of Arabian oil lies in the Jurassic section. During the late Jurassic, as Pangaea broke up, new mid-ocean ridges were displacing more and more water, so the sea transgressed over the land in many places. The northeastern part of Africa – the Arabian Peninsula today – was just about on the equator and was along the southern margin of the Tethys Ocean. As sea level rose, a wide, shallow, tropical marine shelf formed – a perfect place for life. Meanwhile the land contained all that plant matter, and as it washed into the oceans, it would typically be dispersed and added to the ocean’s store of nutrients to support marine life. But that shallow marine shelf in what is now Saudi Arabia wasn’t flat.

The shelf had deeper basins in the sea floor where oceanic circulation was restricted, so when plant matter washed into them, they tended to accumulate without being dispersed. It was probably something like the modern Bahamas, where shallow shelves give way to deep troughs. Another analog would be the basin where the Solnhofen Limestone formed, but in this case, with a lot more organic matter. You know what’s coming – the organic matter, most of which was actually algae, which piled up in those troughs didn’t decompose as it would in the open ocean, so it became part of a limy, organic-rich mudrock. The Tuwaiq Mountain and Hanifa Formations contain as much as 5% total organic carbon, a huge value, making for a world-class oil source rock.

Don’t visualize instant oil formation, though – it took at least 50 million years, and probably closer to 100 million years, for the source rocks to be buried sufficiently for the oil to be cooked out of the source rocks and to begin to migrate into overlying reservoir rocks. By that time, the ocean was closing, and Arabia was beginning to impinge on what is now Iran and adjacent areas that were part of the Cimmerian Continent that took off from the margin of Gondwana back in the Permian. The collision produced folds, like a carpet caught between two pieces of furniture being pushed toward each other. Those folds made excellent traps, and the oil migrated up into porous rocks that formed in the Tethys Sea before the collision began. As the collision proceeded, the seaway between what is now Arabia and Iran became restricted – just as it is today, in the Persian Gulf, but more so – and thick evaporites formed. So you had everything needed for a great oil province – world-class Jurassic source rocks, magnificent reservoirs crunched into big, broad upwarps, and covered by evaporites to seal the oil into the reservoir. There are variations, of course, but this is pretty much the scenario for most of the oil in the Middle East.

Oil & Gas Fields of West Siberian Basin

Similar situations developed in other areas, which were less tropical and more temperate, but recall that the temperate zones during the Jurassic were wider and warmer than they are today. One is the present-day northwest shelf of Australia, another margin of the Tethys Ocean. And another was in the West Siberian Basin, where a restricted, narrow sea developed between the remnants of the Ural Mountains, formed in the collision between Europe and Siberia, and the high-standing block of the Siberian craton itself.

You may recall from the episode on September 27 that the West Siberian Basin was initiated by Triassic rifting, rifting that never went to completion to make an ocean basin like the rifting in the Atlantic did. But the region sagged, so that during the late Jurassic a deep, restricted basin formed in which organic-rich shale was deposited. The Bazhenov Suite of rocks contains an estimated oil-in-place of around two trillion barrels, of which anywhere from 75 to 360 billion barrels may be recoverable. For comparison, Prudhoe Bay, the largest oil field in North America, has produced about 13 billion barrels, and it is largely depleted.

Most of the oil and gas fields of the West Siberian Basin are sourced in the Bazhenov Suite, a package of rocks that covers something like a million square kilometers, but it’s typically only about 150 meters thick. Some estimates suggest that in the long run, the Bazhenov may be a more prolific source of oil and gas than the rocks in the Arabian-Iranian basin. Much of the Bazhenov oil is probably tightly locked in the rock, so it will have to be produced using horizontal drilling and hydrofracturing techniques, like the Bakken formation in North Dakota.

As I mentioned, virtually all of this oil and gas, and in fact almost all oil and gas, of every age, everywhere on earth, comes from the organic matter in plants, especially marine plants like algae. For all the dinosaurs we’re talking about this month, their contribution to oil and natural gas accumulations is practically zero.

—Richard I. Gibson

Why so much oil in the Middle East?

Petroleum geology of the West Siberian Basin  (source of map)

October 26. Rise of the Rudists

Today is episode 300 of the History of the Earth Calendar. The blog has reached about 23,000 total page views and as near as I can tell, the podcast is getting about 900 downloads a day. I have no idea whether that’s a little or a lot in the grand scheme of things, but I’m very happy that more than two or three people find this stuff interesting. I really do appreciate the support and the feedback you’ve given. Thanks!

* * *

Today’s Jurassic topic is the Rise of the Rudists. As much as I’d like that to be an episode of Dr. Who, it’s a little more mundane. 

During the early part of the late Jurassic, a new form of marine invertebrate appeared. The rudists were mollusks, part of the class of bivalves, and related to clams and oysters. But their shell forms were distinctive, and quite dramatically NOT like clams and oysters. 

One of the shells became highly elongate – a tall tube, ranging from a few centimeters to more than a meter high. The other shell, or valve, sat like a hinged lid on top of that tube, in which the animal lived. These tall tubes, growing near each other, trapped sediment between them, and by the Cretaceous, rudists were really important reef-builders. The porosity in those reefs makes Cretaceous rudist reefs excellent reservoirs for oil. 

During the Jurassic they were proliferating but had generally not become so large and abundant yet that they formed reefs. There’s a wide diversity among rudists, which are grouped into at least 11 different families and more than 1000 species. Some Jurassic forms were coiled with snake-like shells while others were more boxy in shape. All of this variety in morphology led Lamarck to give them their name, a reference to their unusual or “rude” appearance.

Rudists lived mostly in shallow, tropical waters bordering the Tethys Ocean and its remnants, including the Gulf of Mexico.

They only existed from late Jurassic to the end of the Cretaceous. They were wiped out by the extinction event at the end of the Cretaceous. Nonetheless, they had a run of close to 100 million years. 

—Richard I. Gibson

Rudists
Rudists at UCBerkeley
Cretaceous Rudists

October 25. Sauropods

Diplodocus painting by C.R. Knight (1911)

Sauropods are another well-known group of dinosaurs that became gigantic during the Jurassic. Sauropods include such famous dinosaurs as diplodocus, brachiosaurus, and Apatosaurus. For many years, one of the most popular Sauropods was the brontosaurus, but that name was a result of the “bone wars” between paleontologists Marsh and Cope, in the 1870s. It turns out the bones used to define brontosaurus were from a juvenile variety of Apatosaurus, and the Apatosaurus name had seniority. Marsh’s huge reconstruction of a brontosaurus at the Yale Peabody Museum popularized the animal in the public consciousness, but that reconstruction used a Camarasaurus head. So today, there’s no such thing as a brontosaurus. 

But the real Sauropods were huge, with long necks and long tails balancing a big, oval body. They are almost certainly the largest animals ever to live on land, and their only competition in terms of size are aquatic mammals like the blue whale. Diplodocus, a common Jurassic sauropod, was up to 170 feet long, and some brachiosaurs were as much as 60 feet tall, four times the height of a modern giraffe. The smallest Sauropods were around 20 feet long.  Many of the most famous specimens of Sauropods came from the Jurassic Morrison Formation of western United States, especially in Colorado, Wyoming, and Utah, where Marsh and Cope worked in the 1870s and 1880s. 

In contrast to early depictions that indicated these animals were so big they had to live in water to be supported, the modern interpretation is that Sauropods certainly walked on dry land. Or not-so-dry land: the environment of the Morrison Formation was wet mud flats and river flood plains, and sauropod footprints show that they lived in such areas as well as wet, coastal environments.

Although Sauropods appeared in the late Triassic, they diversified and grew to gigantic sizes throughout the Jurassic and into the Cretaceous. By Jurassic time they were widespread. Even though few complete, or even reasonably complete fossils are known, their bones are common enough that we know Sauropods were present on every continent, including Antarctica.

Sauropods appear to have been herd animals that traveled in groups segregated according to age. That fact can be interpreted to suggest that they did not exhibit much parental behavior, and that juveniles quickly began to flock together, but some fossil assemblages include mixtures of individuals of different ages, which could mean there was some parental care for the young. Some of the oldest fossil eggs with well preserved embryos, from the early Jurassic of South Africa, are from prosauropods that are probably an ancestral or sister group to the Sauropods. They’ve been interpreted to suggest that the animals crawled on four feet before learning to walk and rear up on two legs. Sauropods were not bipedal, but they probably could lift themselves on their hind legs to reach high into trees to get at leafy vegetation there. 

You might expect sauropods’ feet to be something like those of modern elephants, but that’s not correct. Their front feet did not splay, like elephants’, but in many species were almost a stiff bony column with the digits reduced to near-invisibility. Big stumps, more or less. The hind feed did have claws, and I think that’s the basis for the name, which means “lizard-foot.”

As you can imagine, there’s a vast literature on sauropod dinosaurs and a lot of it is readily available online. What I’ve given here is just a basic outline.

—Richard I. Gibson

Brontosaurus story
Prosauropod eggs and embryos 
Painting of diplodocus rearing, by Charles R. Knight, 1911 (public domain via Wikipedia)

October 24. Jurassic life of China

Today, let’s talk about another lagerstätte – not as well known as the Solnhofen Limestone, but pretty cool nonetheless. It’s called the Daohugou Bed or Daohuguo biota, and it’s found in northeastern China. That part of the world was warm and wet in late Jurassic time, about 160 million years ago or a bit older – but there is some controversy as to the age. Rocks deposited in this setting actually extend from the Jurassic up into the Cretaceous, where the fossils are called the Jehol biota. Whatever the age, and I think most researchers accept a late Jurassic age, a lush forest ecology developed. 

Preservation is outstanding because the area was occasionally covered by thick, fine ash falls that likely killed the animals and preserved them intact, including exceptional detail of soft parts. For example, there’s a bee whose proboscis is preserved. 

Pencil drawing of gliding mammal Volaticotherium
by ArthurWeasley; head based on skull image published by
Meng in Nature, Dec 2006. Used under Creative Commons license 

The Jurassic rocks contains the fossil of Juramaia – not the bullfrog, but the early mammal we talked about October 13.  Other life in that forest included undisputed feathered dinosaurs – the first one was found there in 1996. The first gliding mammal, which looked an awful lot like a modern flying squirrel, was described from these beds in 2006. Like Juramaia, it was an insectivore. 

Castorocauda was an aquatic mammal that lived in the lakes of the region. At 2 pounds and 17 inches, it was probably the largest Jurassic mammal, and shows that mammals were adapting to various niches including the water world early in their evolution. Castorocauda probably occupied ecological niches similar to those occupied by beavers and platypuses today. Its scientific name actually means “beaver tail,” and it did have a wide tail similar to those of modern beavers. It also had webbed feet. Castorocauda is another of those critters that may or may not have quite been true mammals. It depends on your definition and on the interpretation of fine details in the fossils.

Along with the small feathered dinosaurs, some of which appear to have been adapted to climbing trees, the fossils include several different types of pterosaur, the flying reptiles that were not closely related to the dinosaurs or their descendents, birds. Some of the pterosaurs had wingspans close to three feet, making them some of the largest animals in the Daohugou beds. The feathered dinosaurs, including Eosinopteryx, were generally small, perhaps a foot in length – but reconstructions of some of them look an awful lot like birds. No actual birds have been found in the rocks, but they’re still looking.

The forest where these animals lived was dominated by conifers, horsetail rushes, cycads and ferns, and gingko-like trees. No flowering plants of the modern sort. The insect life included lots of flies, various spiders including an orb-weaver, mayflies, and water beetles. Plenty of prey for the insectivorous mammals, salamanders, and small dinosaurs that lived there.

* * *

J. Tuzo Wilson was born October 24, 1908, in Ottawa, Canada. In many ways, he was the architect of the concept of plate tectonics, which grew out of the idea of continental drift and sea-floor spreading. He synthesized many of the disparate ideas into an overarching theory. He also recognized the nature of transform faults, like the San Andreas, and their role in plate interactions.

Tuzo Wilson explaining transform faults (video)

—Richard I. Gibson

Chinese lagerstatte

Pencil drawing of gliding mammal Volaticotherium by ArthurWeasley; head based on skull image published by Meng in Nature, Dec 2006. Used under Creative Commons license 

October 23. Morrison Formation

The Morrison Formation, named for Morrison, Colorado, west of Denver, has been the most prolific source of Jurassic dinosaur fossils in North America. The original stegosaurus and many others came from the Morrison. 

Camarasaurus skull in wall of Dinosaur Quarry,
Dinosaur National Monument, Utah. USGS photo.

The Morrison is mostly reddish and greenish mudstones and shales reflecting different oxidation states of iron, together with some channel sandstones, thin lake beds that include limestones, and swamps in some places. Taken together, the rocks portray a vast, relatively flat flood plain of a complex river system. The Morrison strata contain the sediments that were being eroded from the uplifts in the west – the mountains resulting from the Nevadan Orogeny in eastern California, where subduction was creating a magmatic arc. We talked about it October 10 – the Sierra Nevada Batholith today is essentially the roots of the volcanoes that developed above that subduction zone. 

Rivers flowing to the east from the mountains spread sediments across a zone from Alberta to New Mexico. Deposition spanned about 9 million years, from 156 to 147 million years ago, in the late Jurassic. The rocks include volcanic ash – no surprise, since the mountains providing the sediment were volcanoes. The situation must have been somewhat like Patagonia today, a wide sloping plain east of a high volcanic mountain chain. But unlike the Andes and Patagonia today, the Morrison was laid down in a warm, wet setting. North America in late Jurassic time was subtropical, or at least in the warmer parts of the wide temperate zone.

The remnants of the Sundance Sea were still around too, so parts of the Morrison include possible marine sediments, but for the most part it was a low-relief terrestrial flood plain. Dinosaurs apparently loved it. 

One of the best places to see Jurassic dinosaurs in their original position is the Dinosaur Quarry at Dinosaur National Monument, Utah. A steeply tilted rock face exposes hundreds of fossils within a coarse sandstone bed of the Morrison Formation. The interpretation is that the bodies of many dinosaurs were washed onto a sandbank in a large river, perhaps during a flood. They collected there and were entombed by later deposits. The titling that helped expose the layer took place about 80 million years later, during the Laramide Orogeny, which we’ll get to late next month.

In addition to its famous dinosaur fossils, the Morrison Formation has been one of the primary sources of uranium in the United States. The uranium minerals occur in lenses and layers a few feet thick, concentrated in the sandstones of the Morrison. Their shape in cross-section gives these ore bodies the name C-roll deposits, which probably form when mineral-bearing waters flowed through the porous rock. The Uravan area – for uranium and vanadium – lies in western Colorado and eastern Utah. This mineral belt supplied half the world’s supply of radium in the 1910s, and uranium mining continued at Uravan, Colorado, until 2009 when low prices closed the last operating mine. In 2008, the United States imported 85% of the uranium it consumed for nuclear power, mostly from Canada, Australia, Russia, Kazakhstan, Namibia, Uzbekistan, and South Africa.

The geologic map symbol for the Morrison is Jm – capital J for Jurassic, little m for Morrison. Legions of geology students learning about the stratigraphic section in the Rockies have referred to it as the Jim Morrison formation.

* * *

Today is the day the earth was created, according to Bishop James Ussher of Dublin, Ireland. His analysis of the Old Testament was published in 1654. In that work, he determined that creation took place on October 23, 4004 B.C. The exact time of day is somewhat debated.

—Richard I. Gibson

October 22. Climate and extinctions

You really can’t fully characterize the climate of an entire 50-million-year period like the Jurassic with a few sentences, but we can make some generalities. With the break-up of Pangaea dominating the planet’s tectonics during Jurassic time, there was more volcanism associated with the rifting, and because the supercontinent was fragmenting, there were much longer coastlines. As sea levels rose because new mid-ocean ridges displaced large volumes of water, there were more shallow seas spreading over low-lying areas of the continents. This created many ecological niches for marine and near-shore life.  And with smaller continents, more regions were relatively close to the sea and its moisture, so the desert conditions of the Triassic were much less widespread during the Jurassic.  

That’s the general picture, but it’s punctuated by variations from that general trend at least a few times during the Jurassic. 

There’s no evidence of glaciation anywhere on earth during the Jurassic. It was a warm time, if not so exceedingly hot as the Triassic appears to have been. The atmosphere contained both more oxygen and more carbon dioxide than today, leading more or less to greenhouse conditions – Jurassic plants flourished, and eventually became coal and oil. Many plants were distributed worldwide, suggesting that the climate was probably more uniform than today, both in terms of geographic changes from the poles to the equator and perhaps in terms of seasonal changes as well. With an average temperature as much as 3°C above the present, winters would have been mild and summers hot. There may have been winter snow in the polar regions, but probably nothing like today. Temperate conditions and vegetation extended much further north and south than they do today.  

There were a few relatively minor extinction events during the Jurassic. One, during the Toarcian age toward the end of the Early Jurassic epoch, impacted diversity of brachiopods, crinoids, bivalves, ammonites, and other groups. We talked about this extinction on October 5 in connection with the eruption of the Karoo Volcanics about 183 million years ago.

The Tithonian is the last stage of the Jurassic, with the end of the period at about 145 million years ago. It is marked by an extinction event that appears to have killed off at least 7 of the 11 ammonite families living at the time. About a quarter of the mollusk families in Europe died in this event. There was a regression of the sea at the same time, at least in much of Europe, which could be an important factor in the extinctions. This wasn’t a world-wide sea-level change, and in South America the sea actually transgressed over the land in places. There is actually no evidence for a mass extinction among bivalves in South America at this time, quite a contrast to the situation in Europe. So I think at best, we have to see the end of the Jurassic as a time when regional extinctions happened, but there was nothing huge, nothing of global extent.

—Richard I. Gibson

LINKS:
Smithsonian
Declining Oxygen Levels and Jurassic Extinction
Toarcian biological crises 
Minor extinctions of the Jurassic
Jurassic climate

October 21. Stegosaurus

Drawing by Marsh (USGS, public domain). This is an inaccurate depiction – a single row of back plates (rather than two) and too many tail spikes (should be two pairs).

We really can’t go through the Jurassic without talking about stegosaurus, arguably one of the most recognizable dinosaurs. The first specimen of stegosaurus was found near Morrison, Colorado, in 1877, by O.C. Marsh during his “bone war” with Edward Cope. Its remarkably small brain was noticed immediately – something like 2½ ounces for a 10-ton body, and modern reconstructions depict a creature that may have moved slowly but that probably carried its spiked tail high and might have wielded it like a weapon. 

Stegosaurus, whose name means “roofed” or “covered lizard,” in reference to the bony plates on its back, was a herbivore that grew to 30 feet long. It couldn’t lift its head very high, so it probably grazed on ground cover or short bushes.

For all its fame, only about 80 individuals are represented in fossils. Most are from the Morrison Formation of western United States, but in 2006 one was found in Portugal. Ancestral stegosaurids – not the genus Stegosaurus – are known from the United States, China, England, Germany, and France, but Stegosaurus itself appears to have had a limited distribution in both time and space. There are some Cretaceous specimens that some researchers have attributed to Stegosaurus, but this is not generally accepted, and as far as we can tell with certainty, Stegosaurus was only on the scene for about 5 million years, from about 150 to 155 million years ago.

If you see an image of Stegosaurus juxtaposed with Triceratops or Tyrannosaurus rex, be very suspicious. That image is off by about 80 million years.

—Richard I. Gibson

Kung-fu Stegosaur

Drawing by Marsh (USGS, public domain). This is an inaccurate depiction – a single row of back plates (rather than two) and too many tail spikes (should be two pairs).

October 20. The Solnhofen Limestone

In several of this month’s episodes I’ve mentioned the Solnhofen Limestone, the rocks in Bavaria where spectacular fossils are found – from the lobster-like Eryon to the flying pterosaur Rhamphorhynchus to Archaeopteryx that we talked about yesterday. Why is this rock so special?

Crinoid from Jurassic Solnhofen limestone (source)

During Late Jurassic time, about 150 million years ago, central Europe was a string of islands, the high-standing areas along seaways defined by rifts that formed as Pangaea began to break apart. The area was close to the Jurassic equator. The array of islands made for some restricted lagoons between them, and in some locations, arid conditions together with restricted circulation made for anoxic conditions where life could not survive. No scavengers, no oxygen to decompose bodies. We’ve heard this story before.

But wait, you say – when we’ve heard this in previous episodes, those stagnant lagoons accumulated organic-rich mud that became black shale. You said this was limestone. What’s the deal? We have to infer that the islands were low-lying, and not shedding much in the way of clastic sediment into the lagoons. Clastics – sand, silt, and mud – are deposited typically in settings where the topographic relief is at least moderately high, so erosion can remove those materials from outcopping rocks on land and dump them into adjacent marine settings. Here in Bavaria during the Jurassic, I think we have to see the setting as something like the modern Bahamas – low islands, and even there the rocks on the surface were probably limestone, not granite or other rocks that would yield quartz sand, silt, and mud. The lagoons were carbonate-rich, so it was fine grained calcite, calcium carbonate, that precipitated out.

The resulting extremely fine-grained rock was also remarkably uniform, so much so that the Solnhofen is called a lithographic limestone – ideal for making lithographic plates – stones carved in fine detail to use in printing illustrations, including multi-color lithographs. It was the quarrying operation in the 19th century for lithographic uses that revealed the spectacular fossils of the Solnhofen.

Fossils are actually not all that common in the Solnhofen limestone, but when they are found, they are preserved in exquisite detail, some of the finest fossils ever found anywhere. In addition to the animals we have talked about, the fauna includes jellyfish with soft parts preserved, free-floating crinoids, beetles, cephalopods, horseshoe crabs, turtles, fish, dragonflies, crocodiles, and more. It is truly a world-class lagerstätte, one of those rare natural collections of spectacularly preserved fossils.

—Richard I. Gibson

Link:
UC Berkeley on Solnhofen 

Photo by Ushakaron, used under Creative Commons license. 

October 19. The first bird

Photo of the Berlin Specimen by
H. Raab (User:Vesta) under creative commons license. 

In 1861, the first complete skeleton of a bird was found in the Solnhofen Limestone of Bavaria, in southern Germany. It was a strange bird, with teeth, a long second claw, and a reptilian tail – but it had something displayed by no other reptile fossil discovered to that time – Feathers. The animal was named for a fossil of a single feather found the year before in the same rocks. It was called Archaeopteryx, meaning “ancient wing” or “ancient feather.” 

Since 1861 eleven more specimens have been found. While the animal is covered with feathers so that it is instantly reminiscent of a raven-sized bird, the differences were enough that almost immediately it was seen as a transitional fossil between reptiles and birds. As much as Archaeopteryx is ingrained in our imaginations as the first bird, just as we saw with the mammals, there’s a degree of disagreement as to exactly what constituted a bird back in the Jurassic. But I don’t think it’s anything like as complicated as the story of mammals.  

Archaeopteryx was certainly an early bird (or at least an extremely bird-like reptile), but whether or not it was truly ancestral to modern birds is debated. Discoveries of other avians in Jurassic rocks of China may be as much as 10 million years older than the Bavarian specimens, which are dated to about 150 million years ago. Other candidates have been suggested as early bird ancestors as well. Their existence leads to the idea that some other lineage might be the one that led more directly to modern birds.

Studies of the feathers have suggested that Archaeopteryx was incapable of true flight, but I think that’s a minority view. It lacks a backward-pointing toe, which modern perching birds have, so it might have been more of a ground-dweller, like a chicken. With only 12 specimens, some represented by only a few bones, it’s challenging to reconstruct the daily life of Archaeopteryx.

Many fossils have been found of dinosaurs that are definitely not birds, but which have feathers. So you can’t use the presence or absence of feathers as a simple criterion to draw the line between reptiles and birds. There must have been transitional species, and Archaeopteryx was probably one of them. If a scientist could go back to the Jurassic and examine a live one, he or she might well classify the creature as neither a reptile nor a bird, but as something else – like some of those early mammal ancestors we’ve heard about.

One evolutionary trend that seems clear among bird-like dinosaurs and birds themselves is their shrinking size and evolution of a lightweight skeleton, presumably to reduce weight for flight and to increase maneuverability generally. I have a couple links below to some recent papers on the topic of size in dinosaurs and early birds. Obviously there are some pretty big birds today, including ostriches, but the increase in size of birds may be a development of relatively more recent times, the past 60 million years or so as mammals and birds began to fill niches vacated by dinosaurs killed in the extinction event 65 million years ago.

Today, I think there is no doubt whatsoever that birds are descended from theropod dinosaurs. In some ways of looking at it, as I’m sure you have heard, birds ARE dinosaurs – evolved and modified, and it really becomes a semantic issue of definition – exactly where IS the line between them? It may be clear today, but it definitely was not clear back in the Jurassic.

And even though technically Archaeopteryx is generally seen as not really the ancestor to modern birds, if you want to think of it as the first bird, I’d say you’re only going to get in trouble calling it that if you get into a debate with a real specialist in this field.

And whichever particular group might be the ultimate ancestor to modern birds, there is no doubt that by the late Jurassic, we had animals that a modern time traveler would have called birds. Strange birds, maybe, but birds nonetheless.

—Richard I. Gibson

Links:

Shrinking dinosaurs

bird bones

Photo of the Berlin Specimen by H. Raab (User:Vesta) under creative commons license. 

October 18. Ammonites

While we are in the Jurassic, I wanted to mention that most of today’s oceanic crust was formed during the Jurassic, or more recently. The oceanic crust at a spreading center, like the Mid-Atlantic Ridge, is essentially brand new – new crust is forming in Iceland right now, and in a gradual way all along the mid-ocean ridges. Because oceanic crust is consumed by subduction, the oldest areas of crust out there today are the first crust formed in the opening of the Atlantic, which began in Jurassic time, about 180 or so million years ago. There’s a patch of oceanic crust in the western Pacific that’s about the same age. The only exceptions are pieces of older crust that get stranded by complex collision and subduction processes. Some parts of the oceanic crust in the Mediterranean offshore Greece are probably as old as late Permian. But all the older crust has been subducted. 

Today’s episode brings us back to the ammonites. 

Ammonites, you recall, were shelled relatives of squids and octopuses. 

Photo by Ghedoghedo, used under Creative Commons license.

Jurassic ammonites represent a spectacular recovery from the extinction event at the end of the Triassic. Ammonites were nearly wiped out by that poorly understood extinction, so that only one single family of ammonites is known to have survived. But that one family seems to have been extremely adaptable, because during the Jurassic it expanded into more than 200 genera and many hundreds of species. As we’ve seen before, extinction events, while they spell doom for some species, they also open up ecological niches where opportunistic survivors can proliferate. This process is called adaptive radiation, a result of environmental pressures driving evolution, resulting in diversity.

By later Jurassic time, one genus, Titanites, had species that attained sizes of more than 1.3 meters (6 feet). But some Jurassic ammonites were the size of a small coin.

Ammonites evolved rapidly, with individual species appearing and disappearing over periods of a few million years or less. Consequently they serve as excellent index fossils that define particular intervals of Jurassic time. They lived in oceans all over the world, and were free-swimming carnivores. Because they often swam in the shallows above abyssal ocean plains, when they died they often sank into poorly oxygenated waters where their chemical decomposition was limited and scavengers were less abundant, which explains the fact that they are common in the fossil record.

—Richard I. Gibson

Photo by Ghedoghedo, used under Creative Commons license.

October 17. Ceratosaurus

Ceratosaurs were late Jurassic carnivorous dinosaurs that lived at least in western North America and in Portugal, two places where its fossils have been found. It was around 20 to 25 feet long and had a pretty typical dinosaurian shape, but with several horns on its snout and head. Its name means “horned lizard.”

The first specimens were discovered in 1884 and described by O.C. Marsh, a paleontologist who explored the western United States. He was prolific in his fossil collecting. His discovery of Ceratosaurus came in the midst of the Bone Wars – the intense competition between Marsh and Edward Drinker Cope, a personal animosity that spilled over into their professional lives. Many books address this battle, which was also a time when vast numbers of dinosaur fossils were discovered.  

Technically, Ceratosaurus is a genus of the larger group of ceratosauria, whose fossils have been found in North and South America, Europe, Africa, and Madagascar. There may be three species of Ceratosaurus, but it’s not completely agreed that that’s the case. It’s possible that some of the fossils assigned to different species, such as the ones found in Portugal, may be variations in a single species due to age or simply to individual variation.

The first known ceratosaurs date to about 225 million years ago, the late Triassic, but it was during the Jurassic that they diversified significantly. Ceratosaurus nasicornis – meaning “horned lizard with a nose horn” – was the one described by Marsh from Late Jurassic sediments, and it’s the largest ceratosaur known at 20 or more feet long.

Modern Ceratosaurus reconstruction drawing by DiBgd at en.wikipedia used under CC-BY-2.5

Ceratosaurs were theropods, a large diverse group of dinosaurs that were bipedal, though not necessarily with the upright stance that has traditionally been depicted. More likely, the body was carried more close to a horizontal position with respect to the ground. Marsh’s drawings of dozens of dinosaurs, many of them published in USGS Annual Report Volume 16 part 2, Dinosaurs of North America, showed the upright orientation, leading to reconstructions in museums that have continued that tradition. Today, better understanding of the ways the hip and leg bones worked has led to the view that they carried themselves on two legs, but more level with the ground.

Theropods include dinosaurs that had feathers, as well as all birds. Ceratosaurus was in a different branch of theropods – closer to birds than crocodilians, but not all that close.

—Richard I. Gibson

Modern reconstruction drawing by DiBgd at en.wikipedia used under CC-BY-2.5 

Ceratosaurus as a tattoo  (YouTube)

October 16. Pentacrinus

Although crinoids had been declining in diversity from their peak back in the Mississippian, in some places during the Jurassic they were locally quite abundant. One group, called Pentacrinus, was common during the Jurassic, and some species of Pentacrinus are distinctive enough that they serve as index fossils for particular strata of Jurassic age. The Rierdon limestone that we discussed yesterday has them – and they can be a great geological “friend” when you are out mapping rocks. Except where it was within a few miles of the Belt Island that was shedding chert pebbles into it, the Rierdon Limestone looks a lot like the other limestones of western Montana, ranging in age from Cambrian to Cretaceous. The best way to determine which limestone you’re walking on is to identify the sequence or rocks, which like a fingerprint is truly distinctive. But if you can find a pentacrinus in the limestone, you know you’re in the Jurassic. 

Pentacrinus fossils are usually just the columnals – the stem-like sections that connected the lower part, which is also called the hold-fast keeping the animal attached to the sea floor, the columnals connected that to the main body of the crinoid animal, the tentacles and feeding structures. Pentacrinus columnals are – wait for it – pentagonal in shape, in contract to the circular ones so common in Mississippian rocks. They might be only four or five millimeters across – pretty small – but the species that lived in the Rierdon Sea had columnals that were perfect 5-point stars in cross-section. Really distinctive if you can find one – but it’s harder than you might think. Many are the times that I’d start looking for one in some unknown limestone, only to give up and fall back on the more reliable sequence approach to determining if it was the Rierdon or not.

—Richard I. Gibson

October 15. The Sundance Sea and an Island in Montana

I’ve had some inquiries about assembling the podcasts into bigger packages. Assuming I manage to get all the year’s daily episodes done, the podcast will continue next year but not on a daily basis – I’m not sure if it will continue to follow a chronological format or not, but there will be more posts – just not every day. And yes, I do plan to edit the existing episodes into collections, probably one for each month as well as a complete collection of all episodes on DVD or whatever is appropriate. Thanks for your interest and support – I appreciate it very much.

So now back to the Jurassic.

Although much of Pangaea was breaking apart during much of the Jurassic, there were of course places where compression was happening, too. We talked about the Jurassic collision in western North America that produced the Sierra Nevada Batholith and began a long series of accretions that added to the western margin of North America. As the mountains were lifted up in what is now California and points north, they began to constrain a seaway to the east, in what is now much of the Rocky Mountains in Wyoming, South Dakota, eastern Montana and adjacent areas.

The Sundance Sea, named for the Sundance Formation and the town of Sundance, Wyoming, in the northern Black Hills, transgressed across an unconformity surface that represents either non-deposition or erosion or both, so that the Sundance Sea’s sediments were laid down typically on top of the Triassic Chugwater Formation that we talked about on September 4.

Map of Jurassic sedimentation patterns (USGS map I-175). Purple is the Belt Island where Phosphoria chert was exposed and eroding into the Rierdon Limestone sea.

The Sundance Formation in the Black Hills area is mostly marine sandstones and shales, sediment eroded from the land off to the southeast in what is now central South Dakota and Nebraska. The Hulett sandstone member of the Sundance is a 60-foot-thick resistant bed that makes ridges in the Black Hills area, including a ridge that nearly encircles Devil’s Tower. In many places it contains excellent ripple marks, a testament to the shallow water in which it was deposited. Both the Hulett member and other parts of the Sundance Formation contain abundant belemnites (see the episode for October 6) that date it to late Jurassic time. Parts of the Sundance are also noted for pterosaur tracks.

While this relatively quiescent deposition was going on in the Black Hills and adjacent areas, further west, the first impacts of the tectonic activity, the collision, in the Cordillera were beginning to be felt. In central western Montana, the Jurassic sea was interrupted by a rising island, called Belt Island because it is more or less centered on the town of Belt, Montana.

Evidence for this island can be seen clearly in the sedimentation patterns. The sandy rocks of the Sundance Formation become limestones, called the Rierdon Formation, as you head into central and western Montana. The Rierdon rocks north of the Tobacco Root Mountains contain grains, pebbles, and cobbles of chert within the limestone, increasing in size as you go north within the Rierdon. This indicates you’re getting closer and closer to an erosional source area for those chert pebbles – and eventually, the Rierdon rests upon not the underlying Triassic, but the Permian Phosphoria formation, which we talked about August 10. The Phosphoria contains abundant, thick bedded chert, and it’s clearly the source for the chert pebbles deposited in the Rierdon limestone. How can that happen? The Phosphoria had to be exposed, above sea level, for this erosion to happen. The Phosphoria was in the core of the rising Belt Island during Jurassic time, with its debris, the chert pebbles, washing into the Jurassic sea where the Rierdon limestone was forming.

Other Jurassic strata record the initial rise of Belt Island, and as the Jurassic progressed, later sediments nearly overtopped the low-lying island. We’ll come back to this in about 10 days when we talk about the Morrison Formation. For now, just take the Belt Island as early evidence of increasing tectonic activity in the west. The Rockies are starting to form.

—Richard I. Gibson

LINKS:
Sundance Formation 

Enigmatic tracks in the Sundance Formation

Belt Island   

Map modified from Paleotectonic maps, Jurassic system, 1956, by McKee, Edwin D.; Oriel, Steven S.; Swanson, Vernon E.; MacLachlan, Marjorie E.; MacLachlan, James C.; Ketner, Keith B.; Goldsmith, June Waterman; Bell, Ruth Young; Jameson, Dolores J.; Imlay, Ralph W. USGS IMAP: 175

October 14. The North Atlantic and North Sea

As we discussed on October 7, the North Atlantic Ocean was very definitely forming during the Jurassic. In late Triassic time, you could probably have walked on dry land from what is now New York City to Casablanca, Morocco. You’d have had to deal with lakes, rivers, swamps, and volcanic eruptions, but probably no ocean. By Late Jurassic time, there was an ocean 1000 kilometers wide between the coasts of New York and Morocco.

But today I want to go a little further north, where Europe and North America were breaking apart but were still pretty close together. The initial rifting between Greenland and Scandinavia may date back to the Permian, barely after Pangaea was finally assembled. But it didn’t take off and separate the continents in a big way for millions of years. Even by late Jurassic time, there was probably nothing but a relatively narrow oceanic strait between Greenland and Scandinavia. Further south, Europe was a complex of islands. Map from Ziegler 

Some of those islands were high-standing, continental blocks, like Iberia and Ireland, but most were segmented and separated from each other by some degree of tectonic activity, analogous to the Triassic grabens of eastern North America. By late Jurassic time, active faulting and rifting were producing oceanic troughs between what is now Iberia and Brittany in France – the modern Bay of Biscay – as well as between smaller blocks such as the Grand Banks, off Newfoundland, and the Flemish Cap, Orphan Knoll, and Rockhall-Hatton Bank, all under water today. A narrow island extended approximately from where London is today across the English Channel into central Germany and the Czech Republic today. Together with another block in northern England and Scotland, those areas were separated from Scandinavia by a developing rift system beneath what is now the North Sea.

Some of these rifts would fail more or less quickly – the one between the Grand Banks and Newfoundland, for example, more or less ceased to be active by sometime in the Cretaceous, and the Grand Banks essentially became an extension of Newfoundland, itself part of North America. The Avalon Peninsula of Newfoundland is another example, like Florida, of rocks that were originally part of another continent – Avalonia – that was accreted to North America during the construction of Pangaea but was left back on the North American continent when the rifting began in Jurassic time. The main rift that formed the Atlantic Ocean might have gone through the zone between the Grand Banks and Newfoundland – but it didn’t. It was just a little bit further east, between the Grand Banks-Flemish Cap blocks and Iberia. That’s where the Atlantic Ocean opened eventually.

North Sea Oil and Gas Fields (USGS)

Let’s go back up to the North Sea area. The rift that formed there was pretty aggressive, producing a deep trough between Britain and Norway. That rift also ultimately failed – if it had not, Scotland today might still be attached to Greenland, or it might have become a microcontinent, independent of either Europe or North America, something like Madagascar today. But it did stay attached to Europe.

The heritage of the North Sea rift probably dates back to the Permian, an extension of that early break between Greenland and Scandinavia. The Permian salt beneath the modern North Sea and across northern Germany – the Zechstein, which we talked about on August 22 - was probably related to tectonic sagging that became a real rift system by Jurassic time.

A low-lying sag accumulates a lot of sediment, and in a restricted basin, it can be starved of oxygen in the deep marine waters, so organic matter can build up without being dispersed. That happened in the Jurassic rifts under the modern North Sea. As faulting continued, high and low ridges and basins formed. If you bury all of that under thick piles of later sediment, you have an excellent oil province. Much of the oil in the North Sea today was sourced in Jurassic rocks, and about half of the oil is in Jurassic reservoir rocks.

Because the rift was active, but not extending to produce a wide open ocean as was happening between Africa and North America, abundant sources of sediment were available throughout most of the Jurassic. This provided material for source rocks, reservoirs, and seals – the elements necessary for hydrocarbon accumulations – and later burial provided the thermal maturation that completed the picture. The North Sea oil province was explored beginning in 1964, although the Groningen gas field was discovered just offshore the Netherlands in 1959. Numerous discoveries in the late 1960s and 1970s made the North Sea into a world-class oil province. The North Sea has produced at least 30 billion barrels of oil over time. It’s a mature oil province, meaning that it is in decline in terms of oil production. At its peak of production in the late 1990s the province produced about 6,000,000 barrels per day. Natural gas production appears to have not yet reached its peak.

By the end of the Jurassic, it was apparent that the far northern Atlantic Ocean was forming, but it wouldn’t be for 40 million years or more, well into the Cretaceous, that separation between Europe and North America was significant.

—Richard I. Gibson

References:
J. R. Underhill, Jurassic – Chapter 8 in Petroleum Geology of the North Sea (K.W. Glennie, ed.). Blackwell Science, 1998.

Evolution of the Arctic-North Atlantic and the Western Tethys, by Peter Ziegler (1988, AAPG Memoir 43) 

Map by D.L. Gautier (USGS, public domain) 

October 13. Real mammals, finally

We’ve been dancing all around the start of the mammals – mammal-like reptiles, synapsids, pelycosaurs, therapsids. All related, but, generally, not quite mammals. And depending somewhat on exactly how you define a mammal, and how you look at the fossils, we just might have had real mammals by late Triassic time – we talked about those critters on September 23, with a question mark because it’s not completely agreed that they really were mammals.  

By the Jurassic, the arguments are over. The eutherians, a group whose name means “true beasts,” include all the modern placental mammals – which means all mammals except the marsupials and their close relatives. Today, about 90% of all mammals, including humans, are placentals, and the rest are marsupials like opossums and kangaroos. The oldest eutherian is named Juramaia sinensis, a name meaning “Jurassic mother from China.” It was described and named in 2011 and dated to 160 million years ago, about the boundary between the middle and late Jurassic epochs.

Juramaia reconstruction drawing by Nobu Tamura (source)

The discovery and date of Juramaia pushed the age of mammals back 35 million years and also indicates that the placental and marsupial lines of mammals diverged at least that long ago.

Like so many early mammals, Juramaia was probably an insectivore and the bones in its feet indicate that it was probably arboreal, climbing through trees in search of prey. It was only four or five centimeters long – a couple inches – and it was still very much shrew-like in its basic plan. It appears that mammals maintained a pretty low profile – a small size – for many millions of years, and presumably that was a survival advantage at a time when the world was dominated by larger animals including dinosaurs. The image of little mammals scurrying through the underbrush and up in the trees, avoiding predators, is a pretty good one.

Another Chinese fossil from the Jurassic is a little more debatable as an early mammal. Haramiyids were herbivores that may be ancestral to extinct multituberculates, egg-laying mammals that in turn might be related to modern platypuses. Haramiyids are known from the late Triassic, and if it can be agreed that they were true mammals, that obviously pushes the origin of mammals back that far – but that agreement isn’t there, yet. Jurassic haramiyids from China were squirrel-like creatures with long, possibly prehensile tails like monkeys. Like Juramaia, they lived in trees. So it would appear that placental mammals and non-placentals were occupying the same ecological niches, at least in some places. Haramiyids were extinct by late Jurassic time, so far as we know.

You can imagine that preservation is challenging for mammal fossils. Generally, when an animal died, it might have been eaten by its predators or scavenged by other organisms, and even more likely, would have rotted away on a forest floor. So unraveling the early history of the mammals, during Triassic and Jurassic time, has been challenging.

I have links below to a couple really good articles by Brian Switek about mammals, including one on when mammals got fur. Spoiler alert – they probably got fur before they diverged into the modern branches of placentals and marsupials.

So mammals, true mammals, are finally definitely on the scene. This might be a good time to remind you of the scale of our calendar – it’s not a proper scale, because I want to spend a month with each of the periods of the Paleozoic and Mesozoic eras. If it was at a true scale, this first Jurassic mammal, 160 million years old, wouldn’t appear until December 19. The Precambrian wouldn’t end until mid-November, and everything we’ve talked about since February first would be concentrated into the last 6 weeks of the calendar.  

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John William Dawson was born at Pictou, Nova Scotia, October 13, 1820. He was a prominent Canadian geologist, often considered to be one of the fathers of paleobotany because of his studies of Paleozoic plants in Canada. He was a professor at McGill University and was first President of the Royal Society of Canada.

—Richard I. Gibson

When did mammals get furry?

BBC report on Juramaia 

Haramiyids


Juramaia reconstruction drawing by Nobu Tamura http://paleoexhibit.blogspot.com/ http://spinops.blogspot.com/ http://www.palaeocritti.com , used under creative commons license. 

October 12. Jurassic pterosaurs

Pterosaurs, whose name means “winged lizards,” began during the Triassic but really took off during the Jurassic. They were the first vertebrates to attain true flight, and while they are often called flying dinosaurs, they are not closely related to either dinosaurs or birds. Their descent within the reptiles is ultimately from some basal archosaur, but because of poor and sporadic preservation, their ancestry is not well understood. 

The wing-flap in pterosaurs was stretched over a greatly extended fourth finger. Bats, in contrast, have all their digits extended. 

Rhamphorhynchus image source: Picture by M0tty or Antoine Motte dit Falisse, used under Creative Commons license.

Rhamphorhynchus was an exclusively Jurassic pterosaur that grew to have a wingspan of about two to three feet, with a diamond-shaped tail appendage that might have served as a rudder. The name means beak-snout, and that snout was full of teeth. Spectacular examples have been found in the Solenhofen Limestone of Germany that preserve the soft tissues including the wings. Although it might seem that an active flying animal like rhamphorhynchus would have been warm-blooded, this is not established, and some lines of reasoning, including a slow growth rate to adulthood, something like three years, argue for a cold-blooded metabolism. This may not have been true of all the pterosaurs, however.

There are more than 30 different genera of Jurassic pterosaurs, so they were quite diverse. Most were relatively small, with wingspans of a few feet. There’s been a lot of analysis to try to separate small pterosaurs from juvenile versions of larger species, and there probably were some pretty small pterosaurs, the size of small birds like sparrows.

Geographically, Jurassic pterosaurs have been found on every continent, although they are quite rare in Australia and Antarctica – but whether that is a measure of their distribution or lack of appropriate sediments to preserve their fossils is unclear.

Next month, the Cretaceous, we’ll revisit pterosaurs because that’s when they became gigantic, the largest flying animals in earth’s long history.

If you are in New York, there is an exhibit at the American Museum of Natural History on pterosaurs. It runs through January 4, 2015.

—Richard I. Gibson

LINKS:
Pterosaur skeleton
Rhamphorynchus life story
AMNH Exhibit 

Rhamphorhynchus image source: Picture by M0tty or Antoine Motte dit Falisse, used under Creative Commons license.

October 11. Gold in California

Piping four streams from monitors (giants), with aggregate discharge of 2,500 miner’s inches at a hydraulic mine. Material is washed through bedrock cuts to the sluices which are not visible. Previously published in Annual Report of the State Mineralogist, v. 10, p. 122. Nevada County, CA, 1890. SOURCE

The granitic rocks of the Sierra Nevada Batholith that we discussed yesterday contain gold – enough gold to be a driving force in American history. You all know about the Gold Rush to California in the late 1840s and 1850s. It put California on the map, almost literally, and had a seminal influence on the course of America’s story.

Much of the California gold rush gold was found in placers, pockets in stream beds where gold weathered out of the granite had been concentrated by running water, nature’s own sluice box or gold pan. Miners followed the gold-bearing streams into the mountains to discover the outcrops, or lodes from which the eroded gold came. The California Mother Lode is a long belt of gold-rich rocks more than 100 miles long and typically one to three miles wide. The narrow zone that contains the gold appears to be a suture, the join line between two chunks of crust.

In this case, the suture seems to be between two island arcs that were colliding with western North America as part of the subduction process that gave rise to the Sierra Nevada Batholith. The Smartville Terrane might have been something like the modern Caribbean volcanic island arc, or perhaps more complex, like Japan or parts of Indonesia, but as it interacted with the subduction zone along the western margin of North America, it was pushed down deep enough into the earth that hot water and perhaps some actual magma, or molten rock, was given off. As those hot materials rose, they dissolved and concentrated metals including gold from the surrounding rocks. The gold eventually solidified into veins that filled cracks in the country rock.

The east side of the Smartville Block is a complex fault zone, the sort of thing you might expect to find along a suture between two colliding blocks. Most of the gold is concentrated west of the fault zone. It appears that the Smartville volcanic island arc might have been broken by a rift even as it was colliding, or maybe just before it collided. You can have extension, rifting, in an overall collision, or compression zone, in several ways. Sometimes the area between the compressed arc and the continent, or whatever it’s colliding with, is actually under tension, and a thing called a back-arc basin forms. There are other ways to get extension within compression zones too – these are really pretty complex systems. Bottom line, there are some diverse rocks within the Smartville Igneous Complex that point to a pretty complicated history.

Why is there so much gold right there, right along the suture line? There must have been some considerable amount of pre-existing gold either in the crust or the rocks of the island arc, something for the hydrothermal waters to concentrate into the ore veins of the Mother Lode. Exactly why that pre-existing gold was there is a good question, and I don’t think we have a good answer for that. Possibly something as simple as an ancient concentration of gold in the early earth, a plum in a plum pudding where the plums were scattered through the earth in irregular fashion.

At its peak in 1852, California produced 121 tons of gold. Today, the United States produces about 230 tons of gold per year, second to China and Australia. The leading producer by far is Nevada, whose gold comes mostly from the Carlin Trend, where production began in 1965. We talked about that gold deposit back on April 29.

If I find any thing more definitive about the ultimate reason for so much gold in the Mother Lode, I’ll provide an update.

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October 11, 1737, is the date given for a devastating earthquake and resulting tsunami that destroyed 20,000 ships and killed perhaps 300,000 people in Calcutta, India. Contemporary accounts estimated only 3,000 deaths, and recent scholarship suggests that there was neither an earthquake nor a tsunami, but rather that the destruction was the result of a tidal surge pushed up the Hooghly branch of the Ganges River to Calcutta by a strong cyclone offshore in the Bay of Bengal. Historical accounts can support both ideas, but the geometry of the coast and the likely positions of tsunami-generating earthquakes make the earthquake idea less appealing. Calcutta is about 50 kilometers – 30 miles – inland, and while it’s not impossible for a tsunami to reach that far, it’s not likely. One might say that it would be a challenge for a hurricane-generated tidal bore to reach that far up a meandering stream, but something certainly devastated Calcutta back in 1737.

—Richard I. Gibson

Smartville Intrusive Complex

Photo: Piping four streams from monitors (giants), with aggregate discharge of 2,500 miner’s inches at a hydraulic mine. Material is washed through bedrock cuts to the sluices which are not visible. Previously published in Annual Report of the State Mineralogist, v. 10, p. 122. Nevada County, CA, 1890. SOURCE