Wooster’s Fossils of the Week: Belemnites (Jurassic of Wyoming)

November 20th, 2011

This week’s fossils are among the most recognizable. They certainly are popular in my paleontology courses because no one has ever misidentified one. Belemnites (from the Greek belemnon, meaning javelin or dart) were squid-like cephalopods that lived in the Jurassic and Cretaceous Periods. You would never guess their original appearance from the fossils above. These are guards or rostra, internal hard parts that look nothing like the external animal. They are often found in large accumulations called “belemnite battlefields” (Doyle and MacDonald, 1993).
The above image shows a remarkable fossil belemnite in the State Museum of Natural History, Stuttgart, Germany (courtesy of User Rai’ke on Wikimedia). It shows their squidy form and ten equal-sized arms studded with little hooks for holding prey. They probably ate small fish and invertebrates.
The guard or rostrum is solid calcite at its distal end with a phragmocone (chambered shell) at the other. This phragmocone is only rarely preserved. The rostrum above is from the Zohar Formation (Jurassic) of the Golan in northern Israel near Neve Atif.

Belemnites have played an important role recently in sorting out Mesozoic climate change. Their solid calcitic rostra are ideal for examining stable isotopes that fluctuated with water temperature. Dera et al. (2011) showed that the Jurassic had significant climate variations based on the isotopes in belemnite fossils.

Belemnites have a long history in folklore. The English called them “thunderbolts” because they thought they were formed by lightning strikes. The Scottish knew them as “botstones” that cured horses of various ailments. The Swedish thought they were “gnome candles”. The Chinese called them “sword stones”. Much more prosaically, the belemnite is the state fossil of Delaware.
An engraving of belemnite rostra by Captain Thomas Brown (1889).

References:

Brown, Captain T. 1889. An atlas of fossil conchology of Great Britain and Ireland. With descriptions of all the species. Swan Sonnenschein & Co.

Dera, G., Brigaud, B., Monna, F., Laffont, R., Pucéat, E., Deconinck, J-F., Pellenard, P., Joachimski, M.M., and Durlet, C. 2011. Climatic ups and downs in a disturbed Jurassic world. Geology 39: 215–218.

Doyle, P. and MacDonald, D.I.M. 1993. Belemnite battlefields. Lethaia 26: 65-80.

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