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

June 19. Crinoids

Actinocrinites gibsoni, Montgomery Co., Indiana

Crinoids really thrived in the warm shallow Mississippian carbonate seas of central North America. At Crawfordsville, Indiana, there’s a lagerstatten – an assemblage of fossils that is remarkable in the completeness and preservation of the fossils. It has lots of crinoids, with the fine details of their arms and other structures remarkably well preserved. They lived about 340 million years ago, a time called the “Age of Crinoids,” at least in Indiana and much of the rest of the region covered by those seas. Crawfordsville crinoids are found in museums of paleontology around the world.  

A picture is worth a thousand words - check the links below. 

—Richard I. Gibson

Photo galleries:
Virtual Fossil Museum 

Indiana Fossils website 

April 10. Crinoids

First, an update. This week which is early April 2014, scientists from China, England, and the United States announced a new discovery from the Chengjiang faunal of China. Those are the remarkably well preserved Cambrian fossils that we talked about on February 8. 

The report details an arthropod that’s so well preserved, they have been able to describe the animal’s cardiovascular system. I’ve put a link to this paper on the February 8 episode.

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Crinoids are echinoderms whose plant-like appearance gives then the name “sea lilies,” but they are animals related to starfish. And the name itself comes from the Greek word for a lily. Crinoids originated during the Ordovician biodiversification, but they expanded dramatically during the Silurian. As fossils, their disk-like segments, from their stems, are really common. Their more fragile, cup-shaped calyxes, where the animal’s feeding and digestive organs were located, are more rarely preserved. But when they are, they are some of the coolest fossils around.

Most crinoids were attached to the sea floor, though a few were free-floating, and most of the modern species are free-floating as well. The sea-floor attachments are called "holdfasts," but they really do look like the root system of a plant.

—Richard I. Gibson

Crinoid drawn by James Hall (1881). Natural height about 5 inches.

Link:

Earliest crinoids 

Blog extra: Today, April 10, in 1815, Tambora erupted in Indonesia. The eruption was violent, and put so much dust and ash in the atmosphere that it caused the "year without a summer" when it snowed in Washington D.C. in July 1816.