September 30. End Triassic extinction

It’s the end of the Triassic, and it’s another mass extinction. It was one of the 5 largest extinction events, and some measures make it the second worst, after the event at the end of the Permian. About half the species living at the time disappeared, including a third of the marine genera and many land animals, from amphibians and reptiles to the ancestors of mammals. The conodonts, the little eel-like animals whose teeth are found throughout the geologic record from the Cambrian onward, survived the Permian extinction, but the group was totally eradicated by this one at the end of the Triassic. 

Unlike some mass extinctions, which seem to have spanned quite a period of time, a million years or more, this one appears to have had a sudden onset, and to have lasted for as few as 10,000 years – just an instant in geologic time.

And it’s an event that has no really clear cause. Over the course of these podcasts you’ve heard many causes for extinctions, from voluminous volcanism to glaciation and other climate change, changing sea levels and atmospheric chemistry to meteorite impacts. All those are possibilities for the end-Triassic event, but except for an impact, they are gradual processes, generally, and are challenging to use to explain the suddenness and short duration of the extinction.

There’s no evidence for a large impact crater at this time, and there’s no geochemical evidence such as an iridium layer, either.

Pangaea showing areas of volcanism near the end of the Triassic.
Map by Williamborg.

There was a period of massive volcanic eruptions at this time, just about 201 million years ago. As Pangaea began to break up and the Atlantic Ocean began to form between eastern North America and northwest Africa, the crust broke. You heard about the precursors to that break-up when we talked about the Newark Grabens a few days ago. As the rifting progressed, vast lava flows erupted to produce the Central Atlantic Magmatic Province (CAMP). Today the remnants of those flows and related intrusions are found in Morocco and throughout West Africa, in younger parts of the Newark Supergroup in eastern United States, from Newfoundland to Georgia, as well as Spain and Brazil.

There isn’t any doubt that they are related to the break-up of Pangaea, but there are some issues with the timing. It’s pretty close to the end-Triassic extinction, and might have been a factor. But why the very sudden onset of extinction?  These igneous rocks span about 600,000 years over a million or more years total. One idea is that one specific volcanic event created a tipping point, a volume of CO2 or acidic aerosols that was devastating to life on a global scale. Another is that greenhouse conditions that might have resulted from volcanic eruptions could have warmed the earth so that methane hydrates, methane frozen into water ice within the sea floor, could have melted. If it were released catastrophically, that might account for the suddenness of the extinctions. There are problems with that, too, in that the volcanic rocks closest in age to the extinctions are often separated from the extinction layers by sedimentary rocks indicating many hundreds of thousands of years. In many ways, this all comes down to the accuracy of age dating, which does have error bars for dates 200 million years ago.

I think this extinction is the least understood of all the Big Five extinctions. The Atlantic volcanism seems to be the best candidate as the cause, one way or another, but the jury is still out. One thing is clear, though. While dinosaurs were definitely impacted by the event, it was the dinosaurs that survived that were able to exploit the niches vacated by other forms of life. Tomorrow, when the Jurassic begins, we will really begin to see the Age of Dinosaurs.

* * *

Robert Folk was born September 30, 1925, in Cleveland, Ohio. He is a professor emeritus at the University of Texas where he taught for many years. He’s probably most noted for establishing a detailed classification system for carbonate rocks, but he’s also worked in the diverse fields of archaeological geology and on bacteria in sediments, research that has had implications for the study of structures in Martian rocks.

—Richard I. Gibson

CAMP website

Map by Williamborg, used under Creative Commons license. 

September 29. Wingate Sandstone and phytosaurs

It isn’t directly related to today’s post, but I wanted to call your attention to a recent blog post by Brian Switek who blogs at phenomena.nationalgeographic.com as Laelaps. His post on September 23 is a great one about carnivores of the Triassic. If you are fascinated by dinosaurs, Brian’s blog is for you.

Now, it’s back to the southwestern United States for today’s episode.

Wingate Sandstone photo by Greg Willis from Denver, CO,
used under Creative Commons license.

The Wingate Sandstone is a light red to buff cross-bedded sandstone of Late Triassic age. Although it looks a lot like the later Jurassic Navajo and Entrada sandstones, it’s actually at least 30 million years older, deposited in wind-blown dunes in an extensive desert. It forms many of the massive cliff faces in Capitol Reef National Park, Utah, as well as many of the sculpted towers in Colorado National Monument, near Grand Junction, Colorado. Parts of the Wingate were river-laid, so its deposition included several environments from deserts to riparian wetlands. 

The Wingate actually spans the time from late Triassic into the early Jurassic, about 210 to 190 million years ago, similar to the late part of the Karoo rocks that we talked about yesterday. The Triassic part of the Wingate was dated on the basis of a phytosaur skull. Phytosaurs were widespread Triassic reptiles whose fossils have been found on every continent except Australia and Antarctica. Phytosaurs first came up in our calendar on September 18 when we talked about the Chinle Formation, which underlies the Wingate Sandstone in many places in the southwestern United States. Phytosaurs were crocodile-like animals that appeared in the Carnian age, about 228 million years ago, more or less at the start of the Carnian Pluvial Event that we discussed a few days ago.

Phytosaurs were predators, and seem to have occupied ecological niches similar to those where crocodiles live today. But despite a similar appearance, with long toothy snouts, they were probably not crocodile ancestors, but were rather a failed early reptilian branch that evolved before the primary split between crocodilians and the archosaurs that led to dinosaurs and birds. Their exact relations and ancestry are not clear, but they spanned only a relatively brief period of time, about 28 million years during the late Triassic. They appear to have gone extinct in the event at the end of the Triassic, although there are some disputed phytosaur fossils that are very early Jurassic in age.

Smilosuchus drawing by Dr. Jeff Martz/NPS via Wikimedia, used under Creative Commons license.

There were lots of varieties of phytosaur in the late Triassic world – at least 24 genera encompassing more than 50 species.  Some, like Smilosuchus, grew to lengths of 15 feet or more.

—Richard I. Gibson

Drawing by Dr. Jeff Martz/NPS via Wikimedia, used under Creative Commons license.
Wingate Sandstone photo by Greg Willis from Denver, CO, used under Creative Commons license.

September 28. Karoo Supergroup

Map of the Karoo Supergroup in South Africa by
Oggmus, used under Creative Commons license.
Most of the Beaufort and Stormberg Groups are Triassic in age. 

The Karoo Supergroup is an extensive package of rocks in South Africa whose age extends from the Carboniferous into the Jurassic. As you probably recall, South Africa during most of that time was well inside the supercontinent of Gondwana and later Pangaea, distant from the sea, so it is no surprise that most of the rocks of the Karoo are non-marine, deposited in river systems, flood plains, lakes, deserts, and alluvial fans in uplands. The Karoo has some outstanding Triassic fossils.

Much of the late Triassic Stormberg group was deposited by a vast braided stream system that was home to abundant life by late Triassic time. Cycads and other gymnosperms including confers created diverse woodland habitats, from riparian along the rivers to marshes and meadows. The Molteno Formation in the Stormberg group is the primary coal producer in the eastern part of South Africa’s Cape Province – so we had clearly left the early Triassic coal gap behind.  

The coal-bearing sands of the Molteno Formation are overlain by younger red mudstones of the Eliot Formation, which spans the time from very late Triassic into the very early Jurassic, about 210 to 190 million years ago, with the Triassic-Jurassic boundary at 199 million years ago. The Eliot strata contain fossils of more than 40 species of dinosaurs, ranging from theropods like coelophysis which we mentioned a few days ago, to the multi-ton bipedal herbivore, plateosaurus.

The Stormberg group of the Karoo Supergroup also contains fossils of some of those almost-mammals that we discussed the other day, along with a remarkable array of fossil insects.

* * *

Triassic Lystrosaurus  drawing by Dmitry Bogdanov,
used under Creative Commons license

Edwin Harris Colbert was born September 28, 1905, in Clarinda, Iowa. He was a vertebrate paleontologist who discovered and described the coelophysis dinosaurs in New Mexico, and his discovery of Lystrosaurus in Antarctica in 1969 contributed to the acceptance of the theory of continental drift. The dicynodont Lystrosaurus, a mammalian ancestor, was the most common terrestrial vertebrate of the early Triassic. It’s abundant in the early Triassic part of the Karoo sediments.

—Richard I. Gibson

Image sources:
Map of the Karoo Supergroup in South Africa by Oggmus, used under Creative Commons license. Most of the Beaufort and Stormberg Groups are Triassic in age. 

Triassic Lystrosaurus  drawing by Dmitry Bogdanov, used under Creative Commons license. 

September 27. West Siberian Rift

Today I’m calling on your memory again, to think back on the vast flood basalts that erupted in Siberia just about at the end of the Permian. They’ve been implicated in the extinction event then, the most devastating extinction the earth has seen. 

Today, the basalt flows are exposed over extensive areas of Siberia, but west of those exposures, in the relative lowlands occupied by two huge river systems, the Yenisey and the Ob, the basalt flows are found in the subsurface of what’s called the West Siberian Basin. 

W. Siberian Rifts (green) from USGS (Ulmishek)

A few million years after the start of the Triassic and probably continuing for millions of years into the Triassic, a rift began to form in this West Siberian area. It was perhaps something like the rifts, the grabens, that were forming in eastern United States, but it was unlike them in that the whole system ultimately failed – no ocean was created by the rifting here, whereas the Atlantic formed from the rifting between North America and Africa. The West Siberian rifts were pretty big, though – one extends continuously for at least 1,800 kilometers.

Now visualize something like Nevada’s basin and range – long linear rift valleys adjacent to long linear mountain ranges. I’ve mentioned repeatedly that when you have such a situation, the erosion begins to fill those valleys – but this time, let’s focus on the mountains that are being eroded. The Permian and early Triassic flood basalts covered this region in vast, continuous sheets. When the rifting began, the sheets were broken – some segments were dropped down into the grabens, while other segments of the sheets were left on top of the mountain ranges, which are technically called horsts, a German word for heap or pile, something standing higher than the surrounding land.

The basalt on the tops of the mountain ranges got eroded away, leaving the basalt in the down-dropped basins. An alternative interpretation is that the basalts were erupting as the basins and ranges were forming, so the lavas flowed into the low-lying basins and were never deposited on the mountain tops, but I think the preferred interpretation is that the basalt flows were eroded off the mountains. In any case, what’s left, deep in the subsurface, is linear basins full of basalt and linear uplifts barren of basalt. The whole thing subsided even more, millions of years later, so all that complex structure, basins and ranges, is completely buried by later sediments, and the surface of the West Siberian Basin today is fundamentally a flat marshy plain.

Cross-section by Richard Gibson - source

So how do we know the basalt flows are down there? One way of course is to drill into the subsurface and get samples. But that’s expensive. Another way is to recognize that basalt is a dark, iron-rich rock, and a good bit of that iron is in the form of magnetite – an iron oxide mineral that is highly magnetic. We can measure the earth’s magnetic field at a distance from the magnetic rocks, from an airborne magnetometer, so that we can infer the distribution of magnetite-rich rocks in the subsurface. When Soviet geophysicists did that in the 1950s and 1960s, they revealed long linear magnetic highs – representing zones where there was a lot of magnetite – alternating with long linear magnetic lows, where magnetite was absent or less abundant. You’ve probably guessed that what they were defining were the grabens filled with basalt and the buried uplifts where the basalt had been eroded away.

So what? Well, those uplifts are buried beneath Jurassic and Cretaceous rocks that include some rich hydrocarbon source rocks and excellent reservoirs. The sedimentary rocks draped over the deep Triassic uplifts contain some of the largest natural gas fields in the world. And the gas fields coincide almost perfectly with the low values in the magnetic data, the uplifts where basalt is absent. Many trillions of cubic feet of natural gas from the West Siberian Basin have heated homes in Russia and much of Europe for decades. Back in 1990 I did an extensive analysis of the magnetic map of the Soviet Union for oil exploration, and in the process discovered this correlation between magnetic lows and gas fields. The Soviets had known it for decades, of course, but it was pretty satisfying to unravel the details of the relationships myself.

I have a link below to a nice paper from the USGS if you are interested in more about the oil and gas in the West Siberian Basin. Or email me.

—Richard I. Gibson

Links:
Siberian basalt flows 
West Siberian Basin Oil  (PDF - source of Map)
Cross-section by Richard I. Gibson
Urengoy gas field

September 26. Palisade disturbance

I hope you remember the grabens, those fault-bounded down-dropped basins that were forming in what is now eastern United States toward the end of the Triassic. They were the response to the extension, the rifting that was beginning to pull Africa away from North America to form the modern Atlantic Ocean. The sediments and igneous rocks that filled those basins are called the Newark Supergroup, which we talked about on September 19.

Image source: National Park Service - Newark Basin

After those sediments were laid down, and after they were injected with molten rocks that became sills and lava flows, the whole thing was tilted. The basins, which were already fault-bounded, broke even more. This probably represents a continuation of the extension that formed the basins in the first place, but now the basin rocks were being faulted as well. The newer faulting, near the end of the Triassic Period, produced asymmetrical grabens because typically one side was faulted more strongly than the other – or in some cases, one side was faulted and the other not at all. This resulted in tilting of the formerly horizontal layers of the Newark Supergroup rocks. 

These basins were set into the much older, highly deformed and metamorphosed rocks that were the result of the earlier collisions dating all the way back to the Ordovician – the Taconic, the Caledonian, and the Alleghenian-Appalachian Orogenies. So the whole works is tilted, but it’s most evident in the relatively flat layers of the earlier Triassic sedimentary rocks that filled the earlier basins.

It’s possible that the Palisades Sill, an igneous intrusion in the Newark beds, was formed at about the same time as the tilting of the rocks. It creates the Palisades of the Hudson River in New Jersey and New York, and its age is typically given as about 200 million years, which would put it just about at the end of the Triassic. But there are some more recent age dates that put it a little later, about 190 million years, the early Jurassic, so we’ll talk more about it next month. This tectonic activity, normal faulting and gentle tilting of the older Triassic rocks, is called the Palisade Disturbance. Not really an orogeny, but not a passive time, either.

—Richard I. Gibson

Image source: National Park Service - Newark Basin

September 25. Ichthyosaurs

Shonisaurus from the Triassic of Nevada. Maximum length, 49 feet. Drawing by Nobu Tamura http://spinops.blogspot.com used under Creative Commons license.

UPDATE January 2015: An early Triassic ancestral ichthyosaur has been discovered that shows likely amphibian lifestyle characteristics. Here's the link.

We mentioned plesiosaurs earlier this month when we talked about reptiles returning to the sea. Plesiosaurs, which appeared in the late Triassic, were long-necked, seal-like reptiles with paddle fins. Another large group of reptiles that returned to the ocean in the Triassic were the ichthyosaurs. Ichthyosaurs, whose name means “fish lizard,” were completely aquatic carnivores that got their start in the early Triassic, about 246 million years ago. Their fossils have been found in China, Canada, and Spitsbergen in the Norwegian Arctic, so they were widespread early in their evolution and the earliest known varieties were already quite diverse, suggesting older ancestors whose fossils have not been found. It is not at all clear how the ichthyosaurs evolved – they certainly descended from some branch of reptilian tetrapods, but the details of that evolution are unknown. The earliest varieties are a bit more like finned lizards than fish, but not that much more. So their ancestry and descent is unclear.

By the middle Triassic and especially in the late Triassic, ichthyosaurs had developed a characteristic dolphin- or tuna-like shape. They also reached their peak of diversity during the late Triassic when they grew to huge sizes – some Triassic ichthyosaurs are known whose length is estimated at 20 or more meters, close to 70 feet. As carnivores that ate ammonites, squid, and fish, they would have been at the top of the food chain in Triassic seas. It’s possible that some scavenged bottom-dwelling animals such as mollusks.  

There’s a bit of a decline in ichthyosaurs toward the end of the Triassic, perhaps in part because of competition from the plesiosaurs that had just appeared on the scene, but the ichthyosaurs did survive the end-Triassic extinction to expand again during the Jurassic – but they never reached the sizes and diversity that they had in late Triassic time. 

One of the largest late Triassic varieties, Shonisaurus, named for the Shoshone Mountains of Nevada, is the state fossil of Nevada. Its fossils can be seen at the Berlin-Ichthyosaur State Park near Gabbs, Nevada. It lived about 215 million years ago.

* * *

Two geological birthdays today: T.C. Chamberlain was born September 25, 1843, at Matoon, Illinois. He founded the Journal of Geology and was its editor for many years. He’s most closely identified with the geology of Wisconsin, where he was on the state geological survey, as well as with the U.S. Geological Survey, the University of Wisconsin, and the University of Chicago. He’s also known for a theory of the formation of the Solar System which included the idea that planetary bodies were built up by accretion of smaller pieces. 

Abraham Gottlob Werner was born this day in 1749 in Wehrau, Germany. He’s been called the Father of German Geology, but he is best known as a proponent of the theory of Neptunism, the concept that all rocks, including igneous and metamorphic rocks, were deposited from water in the originally all-encompassing sea that had gradually retreated to its present position. He was among the most influential geologists of his day, but the Neptunist idea fell to Plutonism, which held, correctly, that igneous rocks derived from molten magma. In retrospect it may seem easy to go to a volcano and see lava erupting and then solidifying into basalt, but this was not a given in the late 18th century.

—Richard I. Gibson

Shonisaurus from the Triassic of Nevada. Maximum length, 49 feet. Drawing by Nobu Tamura http://spinops.blogspot.com used under Creative Commons license.

Berlin-Ichthyosaur State Park

September 24. The Carnian Pluvial Event

You have probably gotten the impression that the Triassic was a pretty awful time, hot and dry on land and with tropical seas that might have been too hot for life. But I hope I have also made it clear that things were not uniform, and not necessarily so bad, all over the entire globe, and also not through time during the Triassic. During the late Triassic, during the Carnian stage, at about 230 million years ago, there was a big-time disturbance in the carbon cycle that among other things suggests that the climate was dramatically more humid and rainy, at least for a while. It’s called the Carnian Pluvial Event, and like most geological “events,” we’re probably talking about a million years or more. 

The evidence comes mostly from changes in carbon isotope ratios. Different carbon isotopes are taken up at different rates by organisms including plants and calcite-secreting animals such as clams and corals. These rates of uptake are proportional to temperature and can be used to infer temperatures in past geologic times. 

Geological evidence for this humid interval includes the presence of paleosols, ancient soil horizons that would require increased weathering and water for them to form. Fossil spores suggest an increase in more humid-adapted plants, and there is also a world-wide pulse in sediment such as sand and silt, implying that there was greater runoff and erosion.

Map from The Accreted Wrangellia Oceanic Plateau in Alaska, Yukon, and British Columbia

There are two possible causes suggested for this event. First, the carbon cycle could have been disrupted by a massive eruption of volcanic rocks, which would put lots of CO2 into the atmosphere. There is indeed a possible smoking gun for this scenario. A complex block of material called the Wrangellia Terrane, which is now in Alaska, Yukon, and British Columbia, in the Triassic was hanging around offshore western North America. It was a mess, a combination of island arcs, oceanic crust, and other stuff, but it also contains an extensive pile of basaltic lava flows. The flows were flood basalts, similar to but less extensive than the Siberian Flood Basalts that were extruded near the end of the Permian. The Wrangellia Flood Basalts are dated to 230 to 225 million years ago, virtually coincident with the start of the Carnian Pluvial Event. The CO2 released by the volcanism could have brought on a period of global warming, more favorable to evaporation and an active hydrologic cycle, which is a fancy way of saying it would have rained more. Enough CO2 could have impacted the ocean’s waters to change the depth where calcium carbonate can precipitate out – and that could have resulted in the changes we see at this time in the way carbonate platforms, reefs and such, changed to low-relief ramps with less calcium carbonate available.

The changes in the ocean led to extinctions in some groups, including crinoids, bryozoa, and ammonites, but after the Carnian event, some groups including corals diversified dramatically. And on land, it is impossible to ignore the fact that the dinosaurs seem to have begun at just about this time as well. And they also diversified quite rapidly.

In addition to the Wrangellia basalt flows, the Cimmerian continental blocks were beginning to collide with Asia. You recall Cimmeria, a long narrow disjointed strip that rifted off the northeast edge of Pangaea in late Permian time. Since then, it had been chugging across the ocean, closing the Paleo-Tethys ahead of it and opening the Neo-Tethys behind it. Well, it was finally colliding with something big – Eurasia – by late Triassic time. It’s kind of like the collision of India with Asia, in miniature, but high mountains were uplifted. The mountains could have served as a barrier to moisture-laden winds, resulting in a prominent monsoon with lots of rain, which in turn would have produced active erosion and the thick piles of erosional debris, sand and silt and mud, that we see in the rock record.

There’s actually a lot of work going on about the Carnian Pluvial Event and its possible causes and consequences. I doubt quite a bit if this is fully understood yet.

* * *

Philip B. King was born September 24, 1903, in Chester, Indiana. His career with the U.S. Geological Survey was marked by extensive work in West Texas, especially the Marathon region, where he worked out much of the stratigraphy and tectonics. He became a synthesizer of geologic data, and compiled the Tectonic Map of the United States, Tectonic Map of North America, and the 1974 Geologic Map of the United States. These maps were and still are of great importance to our broad understanding of how continents are constructed and evolve.

—Richard I. Gibson

Links:
Carnian event and Wrangellia basalts 

Wrangellia oceanic plateau 

Map from The Accreted Wrangellia Oceanic Plateau in Alaska, Yukon, and British Columbia [Excellent resource]

September 23. Early mammals?

You remember the therapsids – that’s the group that includes all mammals, together with their pre-mammalian ancestors. They got their start in the Permian, and only a few pre-mammals, cynodonts and dicynodonts, survived the extinction at the end of the Permian. And most of them were gone by the Late Triassic. The only group that survived that long is called the eucynodonts, meaning “true dog teeth.” They were still probably not true mammals, but they were becoming more and more like modern mammals. One variety, called Tritylodonts, lived in what is now South Africa in late Triassic time. They were small warm-blooded rodent-like critters and were initially considered to be the first mammals, but since the 1980s they’ve been seen as close relatives to the mammals. There is no good consensus as to whether that group, or one of several others, was the direct ancestor to modern mammals. 

All of the descendents of the cynodonts and dicynodonts that survived into the late Triassic were probably insectivores or herbivores, an interpretation based on their teeth and jaw structures. Some were burrowing animals and some may have been arboreal and nocturnal. They were only a few inches long, so the traditional comparison of early mammals and their ancestors to modern shrews is a good one. 

Eozostrodon was one of these animals, found in the very late Triassic rocks of England. It dates to about 201 to 205 million years ago, the last 4 million years of the Triassic. It may be a true mammal, but there is ongoing discussion about that question. Its teeth were very much mammalian. Bones of some of its close relatives, called Morganucodon, have been found around the world, from China to South Africa and in North America. Whether Morganucodon is a mammal or a pre-mammal almost seems to me to be a matter of semantics, and exactly how one defines a true mammal versus a close ancestor that wasn’t quite a mammal. It has some teeth that are absolutely mammalian, as well as hair, and some skeletal features that are not so mammalian. Its ear bones are closer to those of reptiles than to mammals, but are clearly evolved from the original reptilian anatomy. So it’s a little subjective, but I guess maybe I’d call it one of the earliest mammals. But I’m not an evolutionary paleontologist, so take that with a grain of salt.

Morganucodon, a possible early mammal from the Late Triassic. Length about four inches.
Drawing by FunkMonk (Michael B. H.) used under Creative Commons license.

In any case, we were getting very, very close to having true mammals on the scene by the very late Triassic. But the Triassic mammals or pre-mammals were certainly not as dominant as their ancestors, the cynodonts and their kin, had been several million years earlier. The Triassic favored the development of the archosaurs and their descendants, the dinosaurs, perhaps because they had better ways of managing water – mammals excrete urea, which includes a lot of water, while archosaurs excreted a paste containing little water. But the dominance of archosaurs might have been a blessing in disguise. If the only surviving pre-mammals in the late Triassic were small insectivores and herbivores, their small size might have pushed them into better modes of regulating temperature, and their likely nocturnal lifestyles might have pushed the development of senses like hearing, sight, and smell, as I suggested in the post on the Antarctic a couple days ago. Those developments in turn might have helped these animals develop larger brains.

So although the ancestors to mammals barely survived the Permian extinction, and then barely survived the rigors of the Triassic world, enough did survive to fill some specific ecological niches so that they could expand when the time was ripe – even if that time wasn’t for another 135 million years.

—Richard I. Gibson

New blog post about Morganucodon

Morganucodon, a possible early mammal from the Late Triassic. Length about four inches.
Drawing by FunkMonk (Michael B. H.) used under Creative Commons license.

September 22. Triassic plants

Plants during the Triassic were much like those of the preceding periods, although much diminished in extent by the Permian extinction. I mentioned the coal gap a few days ago, the early Triassic time from which no coal beds are known, with the implication that large coal-making forests were decimated by the extinction event, but compare that to the forests known in Antarctica, that we talked about yesterday. The earth is wildly varied, and was in the Triassic, too. 

The ferns, conifers, and other plants of the Triassic were largely gymnosperms, plants that make seeds without fruiting bodies to enclose them. The name gymnosperm means “naked seed.” Angiosperms’ seeds are enclosed in fruits and other protective matter, and they also have flowers. The name means “seeds enclosed in a protective vessel.” Angiosperms are today’s flowering plants. But there were no flowers during the Triassic – the oldest likely possible angiosperms are from the Jurassic, about 160 million years ago, and they really aren’t abundant and confidently identifiable until the Cretaceous, 125 to 130 million years ago. But the ancestors of the angiosperms were probably beginning to diverge from the gymnosperms by Late Triassic time. 

To an extent, flowering plants burst on the scene in the fossil record quite suddenly. Charles Darwin saw this as a problem for his theory of evolution, and called it an “abominable mystery.” But more recent discoveries have extended the story so that we can see much of the step-wise evolution of flowering plants, though much of the story remains obscure.

The genus Sanmiguelia, ferns found in late Triassic rocks of northwest Texas, has sometimes been suggested as the first angiosperm. The fossils, from about 230 million years ago, preserve the plants in growth position, allowing for more accurate interpretations of their lives. It was probably a herbaceous plant that grew in swampy wetlands that had angiosperm-like flowers. If it was not a primitive angiosperm, it seems likely that it was a close ancestor to them. There are also some spores that seem to be angiosperm-type pollen grains, dated to the middle Triassic, about 245 million years ago in Switzerland.

The question remains, if they existed, why did angiosperms not diversify and expand for almost 150 million years more, until the mid-Cretaceous, when they underwent a huge radiation. That question still stumps researchers, and we can’t be entirely certain when flowering plants, angiosperms, began. If they are as old as Triassic, or even Jurassic, they certainly were not common then.

—Richard I. Gibson

Links:
Sanmiguelia 
Purported Triassic angiosperms

Image from R.W. Brown, USGS Prof. Paper 274-H (public domain)

September 21. Triassic Antarctica

We haven’t talked much about Antarctica, for the obvious reasons that it’s remote and largely covered with ice and snow. The big-picture aspects of Antarctica are well known, including its position within the ancient Gondwana continent and its rifting history, which separated it from Australia, India, Africa, and South America as Pangaea was dismembered. But there’s plenty of detail to be filled in.

Despite the ice and snow, there are good outcrops of rock across the continent, especially in the Transantarctic Mountains, a 3500-km chain that was uplifted in the Cretaceous, which we will get to in November. The mountains expose older rocks, including a thick sequence laid down during the Triassic Period. 

You recall that the Triassic was a time of global high temperatures and arid conditions, but don’t think of that as something uniform across the planet. It’s inconceivable that the poles were not at least somewhat cooler than the tropics, but there is no evidence for any glaciation anywhere, even at the poles, during the Triassic. That’s a dramatic shift from the extensive glaciation during Permo-Carboniferous time just 50 or 60 million years before the start of the Triassic.

Within the supercontinent of Pangaea during the Triassic, Antarctica was pretty close to the South Pole, if not quite on top of it. Much of Triassic Antarctica was within the Antarctic Circle, though, and some reliable paleolatitude measurements put parts of the continent as close to the pole as 75° S Latitude. But there were still no glaciers.

Triassic Globe (Antarctica near south pole) by Ron Blakey, NAU Geology, under Creative Commons license (notes by Gibson)

Much of the Triassic that’s exposed in Antarctica is called the Fremouw Formation, a thick package of river-laid sediments including channel gravels, sands and silts, and floodplain mud deposits. The age is estimated to span many millions of years, probably beginning in the latest Permian and continuing well into the Triassic. That’s an unusually long period for a more or less unchanging depositional setting, and perhaps it represents episodic deposition over the long time span rather than continuous deposition, but the package is more than 3,000 feet thick, so it probably represents a long period, no matter how you slice it.

The Fremouw contains lots of amphibian and reptile bones, as well as synapsids such as cynodonts and dicynodonts that are more closely related to mammals than to reptiles. Because the deposit spans the Permian-Triassic extinction event, it’s a valuable source of information about the responses to the extinction. The fact that the same lineages exist in Permian and Middle Triassic rocks means that they must have existed during the early Triassic, right after the extinction event, even if their numbers were reduced. The fossils in the Fremouw Formation include extensive early Triassic species that are absent or poorly represented in most of the rest of the world.

Tree logs, including an intact fossil forest, have also been found in the Fremouw. They lived along the rivers and on the floodplains, together with many other plants including cycads, horsetails, and ferns.

Taking the early Triassic animal fossils together with the evidence for extensive forests leads to the speculation that Triassic Antarctica was a refuge ecosystem where survivors of the Permian extinction rode out the event and the challenging environments of the early Triassic that prevailed in many other parts of the globe. You recall the other day I mentioned a paper that suggested that the tropical Triassic Earth might have been “lethally hot” and that ocean temperatures in the Triassic might have been so hot that life could not survive. The polar regions might have been enough cooler that they provided this refuge for life, which would have expanded across the planet as conditions became more favorable later in the Triassic and Jurassic. A 2010 research report, linked below, focused on one of the dicynodont survivors of the extinction whose fossils are found in the early Triassic rocks of Antarctica. I also have a link below to the paper describing the intact fossil forest.

One thing to keep in mind when you think about the life in Antarctica during the Triassic: even though the polar areas were much, much warmer than they are today, the seasonal change in length of the day would have been about the same. The poles would have experienced a season of 24-hour sunshine as well as a season of 24-hour darkness. It’s conceivable that the need to forage in continuous darkness might have provided evolutionary pressure that helped animals develop their hearing and vision, and that could be an important step for the ancestors to the mammals.

The name Fremouw comes from Fremouw Peak in the Transantarctic Mountains, which in turn was named for Edward J. Fremouw, an American Antarctic aurora researcher.

* * *

Today we mark the birthday of Douglass Houghton, born September 21, 1809 in Fredonia, New York. He’s best known for his investigations of the copper deposits in the Keweenaw Peninsula of Upper Michigan, work that stimulated the first copper boom in the United States – really the first really big mining boom of any sort – in the early 1840s. He died in a boating accident in a storm on Lake Superior in 1845. He was only 36 years old. Houghton, Michigan, in the copper country, is named for him.

—Richard I. Gibson

Links:
In situ fossil forest 
An Antarctic Refuge? 
Triassic Globe by Ron Blakey, NAU Geology, under Creative Commons license (notes by Gibson)

September 20. Late Triassic cratering

About 214 million years ago, during the Norian stage of the late Triassic, a large astronomical body broke apart above the earth. Fragments of it that impacted the surface may – or may not - have left at least five significant craters across the globe.  

Manicouagan Crater, Quebec (NASA photo)

The five craters are Rochechouart (France), Manicouagan and Saint Martin (Canada), Obolon (Ukraine) and Red Wing (North Dakota, USA). See the links below for a map.

The craters are widely separated today, but when the continents are reconstructed in terms of their plate tectonic motion, the crater positions are consistent with multiple impacts that occurred within hours of each other, ending up in a straight line across the earth. That’s the interpretation of researchers who published the idea in Nature in 1998. There are some problems with the interpretation, including more recent age determinations that indicate that there’s millions of years between some of the impacts. For example, Rochechouart is now dated to about 201 million years ago, Saint Martin is estimated at 227 million years, and Obolon may be as recent as 169 million years, in the Jurassic. None of the age dates can be considered absolutely definitive, and most of them have pretty wide error bars, so I don’t think they completely destroy the multiple-impact hypothesis.

Manicouagan, in northern Quebec, is one of the oldest impact structures whose expression is still visible on the surface. It’s well dated to 214 million years ago, plus or minus 1 million years. A circular lake, a reservoir, occupies the 70-kilometer-diameter inner ring of the crater. The entire crater is about 100 kilometers across, making it the 6th largest known impact feature on earth. St. Martin, in Manitoba, is about 40 km in diameter, and the craters in France and Ukraine are about 20 to 23 km across. The crater at Red Wing, North Dakota, is buried beneath thousands of feet of later sediments, but seismic data define it nicely at about 9 km across, and oil well samples show the shock metamorphism that is typical of impacts.

The Manicouagan impact was clearly a big one. But at about 214 million years ago, it’s about 13 million years before the end Triassic extinction event, and not even obviously associated with smaller global extinction in the Late Triassic, although there’s a regional extinction in North America at about 215 million years ago that might be connected to Manicouagan. The linear alignment of these craters is pretty much inarguable in a reconstructed earth globe, but the age discrepancies are real problems for a multiple impact scenario. Put this in the category of intriguing, but unproven.

—Richard I. Gibson

Links:
Multiple Impact hypothesis

Map of multiple impact hypothesis 

Photo from NASA (public domain)

September 19. Triassic Grabens: the Newark Group

A few days ago we started talking about the extension that was pulling Pangaea apart. The rift zone was following, more or less, the old collision zone between Europe and North America and Africa and North America – the Caledonian and Appalachian Mountains.

Map of Eastern North America Rift Basins, via Wikipedia

The extension that would ultimately become the Atlantic Ocean was not along a single, sharp, linear fault zone, however. The pulling apart affected a wide area before the final snap that separated what is now Africa from the eastern United States. Parts of the eastern U.S. must have looked somewhat like today’s Nevada, with long mountain ranges alternating with deep troughs or basins between them. The troughs are called grabens, a German word for trench. The grabens were fault-bounded down-dropped valleys that resulted from the pull-apart stress that was imposed in Triassic time on a vast region, from at least Nova Scotia south to Georgia.

As with any valley alongside a mountain range, these valleys began to accumulate sediments. The Triassic sediments that filled these fault-bounded valleys are called the Newark Supergroup, from exposures near Newark, New Jersey. The rocks are mostly terrestrial, derived from rivers, floodplains, alluvial fans, lakes, and swamps. Many of the sandstones and shales are red, indicating periodic exposure to air to oxidize the iron, and the rocks contain mud cracks, ripple marks, raindrop impressions, and dinosaur footprints – all indicative of wet flood plains and similar environments.

The Newark Supergroup rocks are very thick, up to 6 kilometers or nearly 20,000 feet – almost four miles of sediment. This is typical of rift valleys, where relatively rapid uplift and subsidence can make such huge piles of sediment. The modern rift at Lake Baikal, in Russia, is close to 10 kilometers or more than 30,000 feet deep.

The extension that produced the grabens of eastern North America was active about 220 million years ago, during the late Triassic. All of the basins are technically “failed rifts” – the successful rift is the Atlantic Ocean. It’s fair to think of the whole thing a pulling apart, breaking things here and there, until at last an irregular, jagged break formed where the extensional forces were greatest, or the breaking rocks were weakest, or some combination of both of those factors.

There are at least 20 distinct, separate grabens or fault-bounded valleys of Triassic age along the east coast, from the Bay of Fundy south to North Carolina. There is at least one more large one buried in the subsurface of South Carolina and Georgia, along the Savannah River.

Source: National Park Service - Newark Basin 

Rifting thins the continental crust. Ultimately, it will thin to nothing, and what’s left is oceanic crust. The whole process results from upwelling heat and magma, forcing older material at the rift axis apart – remember the discussion we had on September 8, about rifting in general. That means that molten material is often associated with rifting, even with failed rifting. The Newark Supergroup rocks are injected with igneous rocks, mostly in thin but often laterally extensive sheets. If those sheets cut across the pre-existing rocks, they are called dikes, and if they force their way between the beds of the older sedimentary rocks, they are called sills. Some flows on the surface would end up as sill-like bodies after they were buried by later sediment. Dikes and sills are common in the Triassic grabens of eastern North America, especially in the Connecticut Graben and the Newark Graben.  One prominent sill, the Palisades Sill, will be our topic early next month.

The basalt lava flows in the Triassic grabens of eastern North America are called trap rocks, from the Swedish word trappa, meaning step. We encountered that word at the end of the Permian, when we talked about the Siberian traps. The alternating, step-like topography of trap rocks can be found in New Jersey, too. In northeastern New Jersey, the basaltic trap rocks contain interesting and rare minerals, including zeolites, franklinite, an iron-manganese-zinc oxide, and pectolite, a calcium-sodium silicate. Many of the rocks from Franklin, New Jersey are fluorescent in spectacular ways.

Body fossils of animals are not common in the Newark Supergroup rocks, although footprints are abundant. One of the most famous fossils is Icarosaurus, a small gliding reptile about 7 inches long including the tail, with a 10-inch wingspan. It was discovered in 1960 by a teenage collector in a quarry in North Bergen, New Jersey. There’s only one specimen that is definitely this animal. Thanks to Steve Henderson for pointing me to the information about Icarosaurus, whose name means “Icarus lizard” for the mythological flying man, Icarus.

—Richard I. Gibson

Links:
Map of Eastern North America Rift Basins, via Wikipedia


Newark Basin 

Mesozoic Basins 

September 18. The Petrified Forest: Chinle Formation

As we get later into the Triassic, environments were changing from the common hot arid desert settings to more complex systems. In what is now Arizona, the variegated Chinle Formation was laid down. It did include extensive wind-blown desert sands, but it also was a time for lakes, swamps, and river systems. The Chinle and its equivalent, the Dockum Group, extend from western Kansas west across the Colorado Plateau into Nevada, and south from Colorado and Utah into Arizona and New Mexico. Stratigraphically, the Chinle typically lies on top of the Moenkopi Formation that we talked about September 7, but there is usually an unconformity between the two packages of rock, indicating either a period of non-deposition, or deposition and erosion, or both. 

Fossil animals in the Chinle formation include various reptiles, including semi-aquatic crocodile-like phytosaurs, small dinosaurs including Coelophysis which we mentioned yesterday, amphibians, lungfish, and sharks. But probably the most famous life forms in the Chinle are trees.

Petrified wood, Arizona. Photo by Kumar Appaiah used under Creative Commons license

The Petrified Forest of Arizona is in the Chinle Formation. Fossil logs represent trees that grew up to 200 feet high and two feet in diameter. They were mostly conifers, like modern pines. The trees grew along river channels, and when they died and fell into the rivers, logjams sometimes developed. The region was also one where occasional volcanic ash falls occurred. Groundwater dissolved silica from the volcanic ash and carried it into buried tree trunks, where the wood was replaced by silica in the form of multicolored agate. The diverse colors reflect trace elements such as iron. In some cases, the replacement was so delicate, practically on a molecular scale, that bark and tree rings are preserved. Since silica is quartz, a resistant mineral, the petrified logs typically weather out of the soft shale that constitutes much of the Chinle formation.

The Petrified Forest contains at least 200 different plant species, making it the richest Triassic plant fossil locality in the world. The most common tree fossils are conifers.

The nearby Painted Desert is also in the Chinle Formation. The alternating shales, mudstones, sands, lakebeds, and volcanic ash are colored mostly by iron and manganese in various amounts.

—Richard I. Gibson

Photo by Kumar Appaiah used under Creative Commons license. 

September 17. Dinosaurs

A week ago we talked about the archosaurs, the reptilian ancestors of dinosaurs, and I mentioned the oldest known dinosaur, Eoraptor, dated to 231 million years ago, in the late middle Triassic. By late Triassic, dinosaurs were clearly expanding both in terms of diversity and geographically. Coelophysis was a 9-foot, 100-pound carnivore that lived in southwestern North America by about 225 million years ago. Fossils similar to Coelophysis have since been found in many parts of the world, indicating it was a successful and adaptable animal that took advantage of the more-or-less connected nature of the supercontinent of Pangaea.  

Coelophysis was a theropod, meaning “animal foot,” a branch of the dinosaurs that were bipedal, with an upright stance. This group also includes birds, which descended from a dinosaur lineage.

Herbivorous dinosaurs had also evolved by late Triassic time. Plateosaurus was an exclusively bipedal animal up to 30 or so feet long, much of that in its neck. This presumably made it able to reach higher into trees and other leafy vegetation.

Plateosaurus skeleton photo by FunkMonk, under creative commons license

Dinosaurs are defined taxonomically as archosaurs with distinctive bone structures that allowed them to carry their bodies more directly above their legs. This applies to both two-legged stances and four-legged stances. The non-dinosaur crocodilians continued a more sprawling stance, with the legs extended away from the body rather than beneath it. The upright stance allowed for a speedier gait. It’s possible that the earliest dinosaurs were bipedal, and that the later 4-legged varieties actually reverted to that anatomy somewhat later.

Dinosaurs competed with other varieties of reptile and with therapsids like cynodonts. Two extinction events, one at about 215 million years ago during the late Triassic and the other at the end of the Triassic affected the primitive reptiles and therapsids more drastically than the newly evolved dinosaurs, and this may have given them more ecological niches to expand into. We’ll talk more about the extinction events at the end of the month. But that end-Triassic extinction seems to have ultimately proved beneficial to the dinosaurs, because they really began to take off after it happened.

—Richard I. Gibson

Plateosaurus skeleton photo by FunkMonk, under creative commons license.   

September 16. The First Pterosaurs

By late Triassic time, reptiles had diversified a lot, and were occupying more and more ecological niches. We’ve already talked about some of the aquatic reptiles, and today our topic is the first reptiles – in fact the first vertebrates – that learned to fly. Pterosaurs’ name means “winged lizard” and they first appear in the rock record in the first part of the late Triassic, around 220 to 225 million years ago. 

Eudimorphodon, Triassic flying reptile. Photo by Tommy from Arad, under Creative Commons license

The first fossils, discovered in Jurassic rocks in 1784, were thought to be aquatic, but in 1801 Georges Cuvier suggested that they were flying animals, and that idea has survived the test of time. In pterosaurs, generally the fourth finger of the front legs is tremendously extended to support the flap of skin that served as the wing. Triassic varieties mostly had teeth – in some cases lots of teeth, even as many as 110 in a jaw only 6 centimeters (2½ inches) long. They are generally seen as fish-eaters, but they might have eaten insects as well. There’s some evidence that their bodies were at least partially covered by hair or fur.

Most of the known Triassic pterosaurs are from Europe, with most species represented by just one or two specimens. The first Triassic pterosaur wasn’t described until 1973. Their wingspans were small compared to the giants that would evolve by Jurassic and Cretaceous times – Triassic pterosaurs had wingspans ranging from about 1½ to 4 feet. One possible pterosaur from Brazil was about the size of a sparrow.

Because of the paucity of Triassic pterosaur specimens, their early heritage remains uncertain. They are definitely not dinosaurs, nor are they related to birds, which descended from dinosaurs. It’s not even clear whether they are descended from the common archosaurs, perhaps from a gliding variety, or from some other reptile lineage. They do appear relatively suddenly in the fossil record, during the Norian stage of the upper Triassic, but whether this represents a true sudden appearance or is a reflection of the poor preservation isn’t certain.

* * *

Today’s birthday is my professor of geophysics at Indiana University, Judson Mead, born September 16, 1917, in Madison, Wisconsin. He was part of the team during World War II that developed airborne submarine detectors using magnetometers. He taught geophysics at Indiana from 1949 to 1983, and was the director of the Indiana University Geologic Field Station in Montana from 1960 to 1980.

—Richard I. Gibson

Links:
Triassic pterosaurs 

Triassic pterosaurs (U of Bristol)

Photo by Tommy from Arad, under Creative Commons license 

September 15. Sonoma Orogeny

While Pangaea was more or less intact during the Triassic, together with the initial rifting we discussed a few days ago, there were places where Pangaea was still growing. Western North America was one such place.

Several long linear island arcs and zones of oceanic crust, and the sediment associated with them, was amalgamated to what is now northwestern Nevada and adjacent areas over millions of years, culminating in the Sonoma Orogeny in early Triassic time. The mountain-building event gets its name from the Sonoma Mountains in that part of Nevada.  

The Sonoma Orogeny was the second round of accretion, or adding of tectonic terranes, in this part of North America. We talked about the Antler Orogeny, in central Nevada, back in May, during the Devonian.

I visualize this something like a modern island arc, say, Japan, colliding with the nearby Asian continent. Slices of oceanic crust, piles of sediment, volcanic piles, and perhaps small bits of continents all became welded to the older North American continent. There is some controversy as to exactly when all this happened, and it did happen over millions of years. Some lines of evidence favor earlier or later culmination, but I believe the consensus is that it was complete by Early Triassic time, around 230 million years ago. It’s complicated by overprints of later tectonic activity as well as intrusive igneous and extrusive volcanic rocks, and a 2008 paper by Keith Ketner with the USGS even suggests that there wasn’t really a specific Sonoma Orogeny at all. The geology out there is complex enough that questions like this can be raised legitimately even now.

Sonomia Terrane in green (including parts covered by Cenozoic). Strontium 706 line in red.

The set of terranes in northwestern Nevada are usually combined together and called the Sonomia Terrane. The eastern edge of the Sonomia Terrane coincides fairly well with a geochemical boundary called the strontium 706 line. This value, 0.706, is the ratio between two isotopes of the element strontium, and it forms the western boundary of igneous rocks of continental origin, the main mass of North America to the east of that line. West of the Sonomia Terrane, the strontium isotope ratio reaches 0.704, indicating sources of magma in oceanic crust. Thus the Sonomia Terrane lies between the strontium 706 and 704 lines, a position that would be predicted for intermediate crust such as an island arc. All of this supports the idea that the rocks of the Sonomia Terrane came in from a considerable distance to be accreted, or attached, to the North American continent.

—Richard I. Gibson

Links:
Island arcs and oceanic crust  USGS Bull. 1857-B


Ketner 2008 paper

Map from Ron Blakey 

September 14. The first frog

Update:  A week ago, on September 8, when I was talking about the initial breaks that were starting the dismemberment of Pangaea, I mentioned the possibility that mantle plumes, rising heat and magma that could produce hotspots like Yellowstone and Iceland, the possibility that mantle plumes might contribute to initiating continental breakup. There’s a link on the Sept. 8 episode to a 2014 article supporting that idea, which I said was controversial. Just last week, a new paper by researchers at CalTech came out suggesting just the opposite – that mantle plumes, at least as narrow features a few hundred kilometers across, like hotspots, may not exist at all. This idea is that heat and magma do rise, but not as narrow jets or plumes. Rather, broad upwellings of heat are pulled upward by cooling and subduction near the surface, more or less in the standard model of heat convection. Magma pools beneath the crust or within it, constrained by solid rocks. It erupts to the surface in volcanoes – both above subduction zones and above hotspots – through cracks and weak zones created by tectonic processes. This work was based on seismological studies that fail to find evidence of the narrow mantle plumes the older hypothesis requires. Here's a link to a summary of the new paper.  

* * *

Triadobatrachus fossil at
Paris Natural History Museum
photo by Ghedoghedo used under
Creative Commons license

Now back to the Triassic and a short episode on the first frog. It’s September 14. Despite the rigorous conditions in the early Triassic just after the devastating extinction at the end of the Permian, life found ways to survive. The first known frog dates to the early Triassic of Madagascar. This critter, named Triadobatrachus, meaning “triple frog,” wasn’t exactly a true frog in the modern sense. It had many more vertebrae – 14 compared to 4 to nine in modern frogs – and a short tail, lacking in modern frogs. In fact modern frogs and toads comprise the order Anura, which means “tailless.” The first true frogs in the modern sense did not evolve for 40 million years, in the early Jurassic.

The Madagascar ancestral frog was found in sedimentary rocks that indicate it lived in a near-shore environment, which you could expect for an amphibian. There is only one specimen of this animal, but it’s an excellently preserved fossil, with the skeleton still fully articulated. The animal was about 4 inches long, and its leg bones suggest that it probably could not hop like modern frogs, but this is not certain. A 2012 interpretation of the bone structure concluded that it wasn’t capable of long jumps, but might have been able to do short hops.

* * *

We have four geological birthdays today. Friedrich Wilhelm Heinrich Alexander von Humboldt was born September 14, 1769, in Berlin. He was a naturalist and explorer, probably best noted for botanical studies, but he also recognized the change in the strength of the earth’s magnetic field from the poles to the equator, and his studies of volcanoes contributed significantly to the opposition to the Neptunist view that igneous rocks were formed from water.  Victor Hugo Benioff was born this day in 1899, in Los Angeles. He worked as a seismologist at Cal Tech, characterizing the locations of deep earthquakes. He recognized that there is an inclined array of earthquakes along a subduction zone, strong support for the mechanisms of plate tectonics. A subduction zone is called a Wadati–Benioff zone, honoring him and Kiyoo Wadati, the Japanese seismologist who also independently discovered the seismic activity on subduction zones. Robert S. Dietz was born this day in 1914, in Westfield, New Jersey. His work in geophysics and oceanography pioneered the concept of sea-floor spreading, the basis for plate tectonics. And today is also the birthday of geologist Patricia Dickerson, in San Jose, California. She has worked extensively on the geology and tectonics of West Texas and the Big Bend area, as well as central South America.

—Richard I. Gibson

Triadobatrachus fossil at Paris Natural History Museum photo by Ghedoghedo used under Creative Commons license

Did Triadobatrachus jump? 

September 13. Monte San Giorgio lagerstätte

Update. During the Precambrian, last January, we talked about the origin of atmospheric oxygen, a development tied to the expansion of oxygen-excreting, photosynthesizing cyanobacteria. The point at which oxygen began to become plentiful in the atmosphere has been pegged at about 2.96 billion years ago, with levels becoming noticeably high by about 2.4 billion years ago or later. A new study by researchers at Trinity College Dublin, Ireland, and the Presidency University in Kolkata, India, pushes the onset of oxygenation back about 60 million years, to about 3.02 billion years ago. Here’s a link (and it's also on the January 19 post about the oxygen crisis). 

* * *

Now back to the Triassic. In southern Switzerland, Triassic rocks record the remarkable variety of marine vertebrate life that lived in that region by middle Triassic time. Monte San Giorgio, overlooking Lake Lugano, was designated a World Heritage Site in 2003 because "is the single best known record of marine life in the Triassic period, and records important remains of life on land as well," according to the World Heritage designation by UNESCO. It is a lagerstätte, a fossil locality remarkable for the diversity of specimens as well as their state of preservation.

Photo of fossil in matrix by Tommy from Arad under GFDL. 

The area was a quiet, stagnant lagoon during the mid-Triassic, around 232 to 237 million years ago. It was similar to the Permian Basin and North Caspian Basin, but on a smaller scale. Reefs surrounded the lagoon so that little circulation resulted in an anoxic setting. Lack of oxygen at depth, no scavengers, and quiet conditions allowed for outstanding preservation of the shallow-water life as well as any skeletons that might have been washed into the lagoon. The richest fossil zone is only 5 to 16 meters thick, made up of thin laminations of alternating organic-rich black shale and organic-rich dolomite. These alternations may reflect sea-level fluctuations, with the dolomite depositing during high stands and the shale accumulating during low stands when the lagoon would have been even more stagnant than at other times.

The fossils in the thin layers at San Giorgio include aquatic reptiles similar to the nothosaurs we talked about yesterday, together with various fish and even a few terrestrial animals that must have washed into the lagoon. Tanystropheus was a 20-foot-long reptile, whose neck was more than half the total body length. Its skeletal structure suggests that it might have been a near-shore land dweller, using its long neck to snag fish from the shallows, or it might have been aquatic. Skin impressions of this animal have been found, showing it to be covered by rectangular scales.

Tanystropheus skeleton reconstruction (see below for credit)

A few embryos from the ray-finned fish Saurichthys have been found in the rocks at San Giorgio, retaining some soft parts. Their occurrence indicates that those fish bore their young alive rather than leaving unattended eggs.

—Richard I. Gibson

References and links:

Tanystropheus fossil in Milan Natural History Museum. Photo of fossil in matrix by Tommy from Arad via Wikimedia Commons, under GFDL. 

Restoration photo by Ghedoghedo under Creative Commons license.

Monte San Giorgio (in book, Exceptional Fossil Preservation)


Fish embryos 

September 12. Reptiles return to the sea

The Permian extinction and the dramatic changes in environment in the early Triassic certainly stressed many animal groups. Reptiles, which had become the first fully terrestrial animals back in the Carboniferous Period, about 305 million years ago, returned to the sea and a largely aquatic life during the early Triassic soon after the start of the period, about 245 million years ago. The oldest known aquatic reptiles come from early Triassic rocks in China.  

Triassic nothosaur drawing by Nobu Tamura under GFDL

The sauropterigians, whose name means “lizard flippers,” were reptiles somewhat like seals. Nothosaurs, a Triassic group, grew up to 3 meters or 10 feet long and had webbed, paddle-like feet. The long neck supported a head full of teeth, and they probably ate fish and squid.

It isn’t completely clear whether the sauropterigians were more closely related to turtles or the crocodile branch of early reptiles, partly because their aquatic lifestyle resulted in changes that obscure their taxonomic connections. It looks like all the sauropterigians died out in the extinction at the end of the Triassic, except for one lineage, the plesiosaurs. Plesiosaurs had just gotten started in the very late Triassic, survived the extinction event, and went on to proliferate and dominate the seas during Jurassic time.

Plesiosaurs were among the first fossil reptiles recognized as reptiles. Fossils found in the 1600s were thought to be fish vertebrae, but by the early 1700s they were identified as reptiles. Numerous specimens were found in Great Britain, increasingly complete in their preservation. One specimen was described in 1832 as "a sea serpent run through a turtle," a reference to the long neck and tail and broader central body.

* * *

Today is James Hall’s birthday, September 12, 1811, in Hingham, Massachusetts. Hall studied the geology of much of the northeastern United States as well as the Midwest, but he’s probably most closely identified with New York, where he was the state geologist and author of the 4500-page Paleontology of New York, in 13 volumes. He helped found the National Academy of Sciences and was the first president of the Geological Society of America. There’s a residence hall at the Rensselaer Polytechnic Institute named for him. It’s called Hall Hall. 

—Richard I. Gibson

Triassic nothosaur drawing by Nobu Tamura under GFDL