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!

Thursday, October 16, 2014

October 16. Pentacrinus

Although crinoids had been declining in diversity from their peak back in the Mississippian, in some places during the Jurassic they were locally quite abundant. One group, called Pentacrinus, was common during the Jurassic, and some species of Pentacrinus are distinctive enough that they serve as index fossils for particular strata of Jurassic age. The Rierdon limestone that we discussed yesterday has them – and they can be a great geological “friend” when you are out mapping rocks. Except where it was within a few miles of the Belt Island that was shedding chert pebbles into it, the Rierdon Limestone looks a lot like the other limestones of western Montana, ranging in age from Cambrian to Cretaceous. The best way to determine which limestone you’re walking on is to identify the sequence or rocks, which like a fingerprint is truly distinctive. But if you can find a pentacrinus in the limestone, you know you’re in the Jurassic. 

Pentacrinus fossils are usually just the columnals – the stem-like sections that connected the lower part, which is also called the hold-fast keeping the animal attached to the sea floor, the columnals connected that to the main body of the crinoid animal, the tentacles and feeding structures. Pentacrinus columnals are – wait for it – pentagonal in shape, in contract to the circular ones so common in Mississippian rocks. They might be only four or five millimeters across – pretty small – but the species that lived in the Rierdon Sea had columnals that were perfect 5-point stars in cross-section. Really distinctive if you can find one – but it’s harder than you might think. Many are the times that I’d start looking for one in some unknown limestone, only to give up and fall back on the more reliable sequence approach to determining if it was the Rierdon or not.

—Richard I. Gibson

No comments:

Post a Comment