Wednesday, June 16, 2010

Common Spirifers of the Middle Devonian

When collecting in Devonian aged rocks you will find a lot of brachiopods from the Spiriferida order. This prolific order is abundant in the middle to late Paleozoic but is mostly decimated by the Permian mass extinction until finally disappearing from the fossil record by the early Jurassic. The most recognizable and well known member of this order is Mucrospirifer but there are several other common genera in the Middle Devonian who look very similar and are often mislabeled.

Below are four different genera who's names are often used interchangeably by some dealers and collectors who have not yet learned the differences.

This is how I tell the different genera apart in the field and in my collection.

The easiest to separate out is Spinocyrtina Mediospirifer (edit 02/02/14) which has a flat brachial valve and a deeply curved pedicle valve on the front. The back of the pedicle valve is very straight and flat so that when viewing the Brachiopod from the back it has a triangle shape.

The next commonly confused shell is that of Paraspirifer. Paraspirifer has a large, walnut shape and a very deep sulcus in the middle of the shell. when you turn the specimen around you'll notice that the Brachial and Pedicle valve curve in towards each other and nearly touch.

In the middle between Paraspirifer and Spinocyrtina is Mediospirifer Spinocyrtina (edit 02/02/14) which has the long interarea but it's sulcus is much less exaggerated. When looking at the back of the shell where the pedicle and brachial value meet, you will notice that the interarea is more elongated and has a slight curve.

Lastly we have Platyrachella which has a similar body plan and shape to Mediospirifer, but Platyrachella's hinge line is wider than Mediospirifer's and it's interarea has a more pronounced curve.

Hopefully this makes sense and I'm not too far off on my logic. There are many other species that have similar looks to the above four examples but I'm not going to get into that much detail. Have a different opinion? Let me know in the comments.

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