 |
| Shonisaurus from the Triassic of Nevada. Maximum length, 49 feet. Drawing by Nobu Tamura http://spinops.blogspot.com used under Creative Commons license. |
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
No comments:
Post a Comment