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Andrew Alden

A Close Look at the Irvingtonian

By May 20, 2013

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I'm spending most of the week in Fresno, California, at the Cordilleran Section meeting of the Geological Society of America. Yesterday, in one of the preliminary field trips for the event, I joined a party of 25 to visit the fossil collection from the Fairmead Landfill project a few miles north of town. For 20 years, paleontologists from Fresno have been picking over the ground excavated by the landfill operators. The fossils represent a community of grazers: 60 percent of them are horse bones, followed in abundance by camel, mammoth, and elk. There are no bison bones, which puts this assemblage firmly in the Irvingtonian section of the North American Land Mammal Age sequence. If you've gotten used to the Pleistocene, Miocene and other time terms, the NALMA units are strange and awkward. But they need to be, as I explain in my new article on provincial time terms. When I get back I'll put up some photos from a remarkable afternoon.


May 20, 2013 at 2:38 pm
(1) Hollis says:

Any chance that dating of rocks or sediments themselves will replace species-based “time”? Seems like it would be a lot more straight-forward …

May 20, 2013 at 7:06 pm
(2) Geology Guide says:

Hollis, these deposits are hard to date. The gold standard is isotopic dates from volcanic ash beds, and the only one around is beneath the fossils. There are other dating methods, like optical luminescence and amino-acid racemization and paleomagnetism, that can help a little, but they aren’t always possible. Fossil populations are the best time framework we have for now. These are not nice clean seafloor clays, but sand and gravel from the foot of the Sierra Nevada strewn (and restrewn) along the side of the Great Valley with occasional disconnected, water-worn bones scattered in it. On top of that, the Fairmead deposits have almost no mice in them, which might give us a more accurate date (because their species evolve so quickly).

May 22, 2013 at 4:31 am
(3) Michael says:

Some here still use the “archaic” Senonian..

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