Cricoconarids from Arkona

Cricoconarids are fossil tubes that look like tiny squid shells or pencil points. They are mysterious fossils that are hard to categorize and considered problematic. Right now they are considered to be worm tubes or possibly related to jellyfish or corals. I've found two types of these enigmatic fossils at Arkona.

The first is Styliolina fissurella which are usually so tiny that they are often overlooked. They are small (1-2mm) smooth shells that look like miniature Belemnites. The fossil below represents hundreds, possibly thousands of individual shells that were washed together, by currents likely, into a small shoal which then fossilized. It was found washed out from the Arkona formation.

The other is Tentaculites which is well known from the lower Paleozoic. I wrote about some chunks of matrix that I found a few months ago that were packed full of Tentaculite shells here. Tentaculites are larger than Styliolina fissurella and have regularly spaced, raised growth rings along the length of the shells. I found this paper on Tentaculites from Poland, while researching Styliolina, that even breaks them, down into species and shows a cross setion through the shell. This first picture shows two examples of individuals that were found in the Arkona formation (top) and Hungry Hollow member of the Widder formation (bottom).

It's easiest to find and recognize Tentaculites within the Arkona formation. Here they can be found commonly associated with Mucrospirifer arkonensis, Devonochonetes sp. and individual loose columnal sections from crinoids.

Tentaculite mass grave

While exploring an exposure of the Upper Silurian/Lower Devonian aged Keyser formation in central Pennsylvania, I came across these chunks of rock that seem to record a mass die off. What I found are thousands of Tentaculite fossils all pointing in the same direction.




Tentaculite fossils are a mystery to Paleontology. Some think they may be worm tubes, others say they are related to jelly fish or corals. Either way, this rock has a story to tell.

Here is a side view of the above rock. The thickness of the layer is approximately 1 inch.



From what I can see, this is a relatively thick layer of these fossils and all of the shells are pointed in the same direction. I interpret the shell orientation as indicative of the current on the bottom of the sea floor. As the water swept over the shells it oriented them with the narrow end pointing in the direction of the current.

As to why the layer is so thick and dense is another matter. It could record a mass die off of the Tentaculites due to a red tide or some other sudden change in their environment. It could also represent years of accumulation of the shells such as at a wave base or a sheltered area.

Here is another piece of rock from the same locality.




Here is the side view of the above rock. The Tentaculite layer is thinner, only a few millimeters at the very top, but you can see some nice lamination in the rest of the limestone. Alternating layers of dark and light limestone may record years or seasons.



I wasn't able to locate the layer these pieces of rock came from so I don't know the full extent or thickness of the Tentaculite grave. The next time I visit the site I will try to find out more.