October 3. Jurassic plesiosaurs and ichthyosaurs

We talked about marine reptiles, including plesiosaurs and ichthyosaurs last month, since they began during the Triassic. But they really began to grow and to dominate the seas during the Jurassic.

Plesiosaur drawing by Adam Stuart Smith  via Wikimedia, used under Creative Commons license

Ichthyosaurs, “fish lizards,” were shaped like dolphins or tunas and dominated the early Jurassic. The first complete skull of an ichthyosaur was found on the Jurassic Coast of England, a World Heritage Site along the English Channel in Dorset and Devonshire in southwest England. Mary Anning made many fossil discoveries there in the 1820s and 1830s, including the first correctly identified ichthyosaur, which she and her brother found when she was 12 years old. She found and described the first plesiosaurs as well.

Jurassic ichthyosaurs were one to ten meters long, 3 to 30 feet. Ichthyosaurs were completely adapted to aquatic life, and never came onshore. They bore their young alive in the water, and they clearly were predators. One specimen has been found with more than 200 belemnites, squid-like cephalopods, in its stomach.

As ichthyosaurs begin to decline in diversity by middle Jurassic time, they seem to have been replaced by long-necked, seal-like plesiosaurs. There are more than 100 species of plesiosaur known, and they were distributed in seas worldwide. They grew to lengths of 17 meters, more than 50 feet, and they were certainly at the top of the food chain.

Ichthyosaurs survived into the Cretaceous Period but were extinct before the end-Cretaceous extinction. While we don’t know for sure why ichthyosaurs went extinct, competition from plesiosaurs could have been a factor. Plesiosaurs survived until the end of the Cretaceous.

—Richard I. Gibson

Jurassic Coast World Heritage Site

Plesiosaur drawing by Adam Stuart Smith  via Wikimedia, 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 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 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 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

September 11. Rhynchocephalians

There were plenty of other reptile lineages evolving during the Triassic besides the archosaurs and their descendents, the dinosaurs. One group, the rhynchocephalians, became quite diverse soon after they emerged in middle Triassic time. They were reptiles covered with overlapping scales, in contrast to the other large group of reptiles, the archosaurs, which had more flexible skin and other coverings, including feathers in birds.  

Homoeosaurus, a rhynchocephalian. Photo by Haplochromis under GFDL 

The parent group to the rhynchocephalians also includes snakes and most lizards, but despite their abundance and diversity during the Triassic and later Mesozoic, only two species of rhynchocephalian survive today – two varieties of the tuatara of New Zealand, the only reptile native to New Zealand. It’s a large, 3-foot-long lizard-like reptile. As the only survivor of the rhynchocephalian order, its heritage goes back a long way and information about the tuatara may be significant to our understanding the evolution of snakes, dinosaurs, and birds.

The name rhynchocephalia means “beak-head” for various bony plates found on the skulls of the animal.

—Richard I. Gibson

Photo by Haplochromis under GFDL 

August 18. Permian reptiles

Bradysaurus, a Permian reptile

Reptiles were better suited to the dry, arid climates that were common during Permian time than amphibians, and reptiles proliferated and diversified during this time. The Pareiasaurs were large herbivores, two to nine feet long, with bony plates armoring their bodies. Reconstructions look rather like big horned toads. In some varieties the bony plates have grown together, suggesting that this group may be ancestors of modern turtles, but the group was extinct at the end of the Permian and it is not certain that they gave rise to descendents that became turtles. It may be that the coalescing bony plates, somewhat like turtles’ shells, might have developed independently in both lineages.  

Mesosaurs, of early Permian age, were among the first reptiles to return to the water. They were clearly adapted to an aquatic life, with webbed feet and a long, streamlined body. Its leg joints – wrists and ankles – were designed in a way that would have made it impossible for them to walk on land, but they might have waddled ashore to lay eggs as modern sea turtles do, but embryo fossils of mesosaurs are not associated with egg shells, so an alternative interpretation is that they bore their young alive. If so, they are among the first animals to do so.

A lot of the known Permian reptiles are lizard-like, several inches to a foot or so long, and many are presumed to have been insectivores, filling the ecological niche that similar reptiles do today. The basic body plan of these animals seems to have been well established by the Permian. One lizard-like Permian reptile called Eudibamus was described in 2000 from a fossil found in Germany. It is possibly the first bipedal reptile. And another, known from Madagascar, had a wide skin layer between its ribs that probably allowed it to glide like modern flying squirrels. All of these adaptations make it clear that the Permian was a time of experimentation and expansion for the reptiles.

* * *

On the night of August 17-18, 1959, the strongest earthquake recorded in the Northern Rocky Mountains struck the upper Madison River Valley near Hebgen Lake west of Yellowstone National Park. It measured about 7.4 on the moment magnitude scale and it triggered a huge landslide in Madison Canyon. The slide buried a campground, killing at least 26 people and damming the Madison River, which backed up to form a new lake, Quake Lake. The earthquake re-set the rhythms of geysers in Yellowstone Park and damaged buildings as far away as Butte and Bozeman. The fault scarps were as much as 19 feet high and can still be seen 50 years later. The faults were normal faults, dropping the Hebgen Lake basin down relative to the adjacent mountains.

—Richard I. Gibson

Drawing by Nobu Tamura (http://spinops.blogspot.com) used under GFDL

Reference:
Road Log for the Hebgen Lake Earthquake Area, by Michael Stickney, Tobacco Root Geological Society Guidebook (2012), p. 71

July 12. Proto-reptiles and proto-mammals

A couple days ago I mentioned the first reptile, a fossil found in a fossil tree in the Joggins formation of Nova Scotia. Before we get into that a little more, let’s define how a reptile differs from an amphibian. Amphibians, as you know, have their feet in both the world of water and the world of land; the name itself means “life of both kinds.” While some amphibians are mostly aquatic and some live largely terrestrial lives, they must return to water to breed. 

Reptiles have broken the tie to the water by evolving a way of containing the water world within a package kept on land – the reptilian egg. The word “reptile” is from Latin meaning “creeping.”

Reptiles evolved from amphibians that had some characteristics of reptiles, including strong legs and skins that resisted the loss of moisture. The oldest unquestionable reptile is an animal named Hylonomus, whose name means “forest dweller” or “forest mouse.” The first fossils were found at Joggins, Nova Scotia, with the animals found within fossilized tree stumps, where they had presumably gotten trapped and died.

They were small, maybe 20 centimeters or 8 inches long, and looked a lot like modern lizards. They lived about 312 to 315 million years ago, during the early part of the Pennsylvanian Period. It had a solid skull without the openings typical of amphibians and impressions and footprints that are probably from Hylonomus show it had protective scales rather than smooth, water-permeable skins. It was probably an insectivore.

Hylonomus - Drawing by Nobu Tamura (http://spinops.blogspot.com) – via Wikipedia under GFDL

The earliest synapsids, once considered to be “mammal-like reptiles,” also developed at about the same time as Hylonomus. They are now considered to be the ancestors of mammals, and it’s thought that reptiles on one hand and mammals on the other evolved at about the same time from an ancestor that laid its eggs on land. The first synapsids may have developed as long ago as 324 million years, putting them into the very late Mississippian Period, but they began to have more mammal-like adaptations in late Pennsylvanian time, about 306 to 312 million years ago, pretty much the same time the first true reptiles were evolving. So rather than thinking it was a smooth progression from amphibians to reptiles to mammals, as I was taught, it seems that the current thinking would have amphibians developing into animals that could lay eggs on land, and those animals fairly quickly branched into the varieties that would lead, respectively, to both reptiles and mammals. The earliest amniotes, animals that lay eggs on land, may be as old as 340 million years, middle Mississippian time. That interpretation is based on a single poor specimen called Casineria from Scotland, but it may represent an animal that was ancestral to all reptiles, mammals, and birds.

Conditions during the Pennsylvanian clearly must have favored the development of land life. The diversification of amphibians into both reptiles and mammals may have been fostered by the abundance of plants in the extensive swamps, a situation that would have created innumerable niches for both small and large animals to thrive. And there were also lots of bugs for those insect-eating early reptiles and mammals to eat.

—Richard I. Gibson

Drawing by Nobu Tamura (http://spinops.blogspot.com) – via Wikipedia under GFDL