Crow Shark

Squalicorax

Cretaceous formation of Alabama

Whorl-tooth

Helicoprion
National Museum of Natural History, Smithsonian Institution, Washington DC

Though it is not clear by looking at it, this is perhaps one of the most interesting vertebrate dentitions. It is actually the teeth of the lower jaw of an ancient shark that lived from 280 to 225 million years ago. The entire set of lower teeth is visible. How it captured prey with this unique structure is not understood. Here is what it would have looked like in life:

End of the Semester

Carcharodon megalodon

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Today was the day of my last final. I'll actually be heading to the Calvert Marine Museum on Monday to decide on my project. That means I'll be seeing this reconstruction of the extinct giant white shark on a daily basis. This is pretty much the pride and joy of the museum and Calvert Cliffs, given the abundance of teeth found. You probably would not have wanted to have approached this close when it was alive though.

The Little Placoderm Who Could

Bothriolepis

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When one considers the placoderms, the image of Dunkleosteus terrelli probably comes to mind. However, the class is much more diverse than the arthrodires, with a myriad of body forms arising over the group’s fifty million year run. Bothriolepis is a good example of this. Though they share armor plating, the similarities between these general seem to end there. Size, for instance, provides a stark contrast. Dunkleosteus grew to between twenty and thirty feet long, with a head up to four feet wide, whereas the diminutive Bothriolepis had a head only four inches wide (Waggoner, 2000).

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Bothriolepis lacks the craniovertebral joint of the arthrodires that allowed its chondrocranium to rise as its splanchnocranium drops. However, this may have allowed for a larger braincase in this benthic creature (Young, 1984). This genus, along with the rest of the placoderms, went extinct in the Mississippian Period with no modern relatives. The reason for placoderm extinction, especially when members of Class Chondrichthyes survive today, is mysterious.
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Young, G.C. (1984). “Reconstruction of the Jaws and Braincase in the Devonian Placoderm Fish, Bothriolepis .” Palaeontology, Vol. 27 (3), 635-661.
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Waggoner, B. (2000). “Introduction to the Placodermi.” Retrieved 29 Mar 2008 from http://www.ucmp.berkeley.edu/vertebrates/basalfish/placodermi.html.