The 366 daily episodes in 2014 were chronological snapshots of earth history, beginning with the Precambrian in January and on to the Cenozoic in December. You can find them all in the index in the right sidebar. In 2015, the daily episodes for each month were assembled into monthly packages, and a few new episodes were posted. Now, the blog/podcast is on a weekly schedule with diverse topics, and the Facebook Page showcases photos on Mineral Monday and Fossil Friday. Thanks for your interest!

Saturday, November 15, 2014

November 15. Shark’s teeth

You probably recall that sharks have been around since the Silurian, about 420 million years ago, or possibly even longer ago. They became abundant in the Devonian and expanded in diversity during the Carboniferous. It’s not true that sharks have remained unchanged in all that time, but the basic plan and structure of sharks has been generally about the same. The second major radiation of shark varieties, including stingrays and their kin, began during the Jurassic and continued in the Cretaceous, and most of the modern families of sharks date to the Cretaceous.

Photo of Cretaceous shark’s tooth from Morocco by
Parent Géry, used under Creative Commons license.  
Modern sharks and rays are in the group called neoselachians, which date to Permian time. They expanded in diversity during the late Mesozoic, possibly because of a contemporary expansion in small bony fishes which must have constituted much of the prey of Cretaceous sharks.  

Marine reptiles such as ichthyosaurs and plesiosaurs undoubtedly competed with sharks for the role of top predator in the Jurassic and Cretaceous oceans, but the marine reptiles went extinct at the end of the Cretaceous, while sharks survived. 

Because sharks have cartilaginous skeletons rather than the hard calcium phosphate of bones, body fossils are quite rare. Shark’s teeth are by far the most numerous fossils representing sharks, because they are harder and more resistant – the calcium phosphate mineral, apatite, like human bones and teeth. Some individual sharks grew hundreds or even thousands or tens of thousands of replaceable teeth over their lifetimes, so they are abundant in the fossil record in many places. In the United States, the deposits of the Cretaceous Interior Seaway in the west and Great Plains area, as well as the Cretaceous rocks of the Atlantic and Gulf Coastal Plains, contain many sharks’ teeth. A nearly complete, articulated specimen of a shark has been found in the Cretaceous of Kansas.   

—Richard I. Gibson

Shark evolution 

Rise of modern sharks 

Hammerheads at the optometrist 

Photo of Cretaceous shark’s tooth from Morocco by Parent Géry, used under Creative Commons license.  

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