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As ichthyosaurs begin to decline in diversity by middle Jurassic time, they seem to have been replaced by long-necked, seal-like plesiosaurs.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   

October 2. Jurassic time

The Jurassic began with the end of the Triassic, at about 201 million years ago, and lasted about 56 million years, until about 145 million years ago. Its three main subdivisions, or epochs, are relatively straightforward as early, middle, and late Jurassic, which span about 27, 11, and 18 million years, respectively. In Europe, the epochs were historically referred to as Lias, Dogger and Malm from the names of packages of rocks, but these informal designations are disappearing because more accurate information shows that, for example, the rocks referred to the Lias were actually partly late Triassic in age as well as early Jurassic. Also, generally, today there’s an attempt to clearly distinguish between periods of time and rock units that can span multiple time periods, so different names are preferred. The epochs of the Jurassic are further subdivided into ages based on particular fossil assemblages, and the ages range from about two to eight million years in length.   

The subdivisions of Jurassic time are pretty widely used around the world in part because of the distinctive ammonite fossils that typify each time period. That only works in marine rocks since ammonites lived in the oceans.

—Richard I. Gibson

October 1. The Jurassic begins

The middle period of the Mesozoic Era, the Jurassic, was named for the Jura Mountains in the western Alps, near where France, Germany, and Switzerland come together. In the late 1790s, Alexander von Humboldt was examining rocks in the Jura Mountains, where the strata are mostly limestone in contrast to the abundant red beds of the underlying Triassic. Von Humboldt called the sequence the Jura-kalkstein, or Jura Limestone, in 1799, and by the 1820s French geologists were calling them the Terrains Jurassiques. The formal name Jurassic came from geologist Leopold von Buch in 1839.

In Britain, rocks of Jurassic age were being studied by William “Strata” Smith in the late 1790s. The Jurassic section in Britain was called the Lias and Oolitic Series for many years, following the usage of William Buckland and William Smith.   

The name Jura is from a Celtic root word for “forest.” 

While the Jurassic is known for its dinosaurs, we’ll talk about a lot of things besides dinosaurs during October. I hope you enjoy the month!

—Richard I. Gibson