Plesioteuthis

[Phil]

Here we have attached a photo of the Upper Jurassic 'squid' Plesioteuthis prisca. Although this specimen is not as spectacular as the Trachyteuthis fossil posted here yesterday by Steve O'Shea, it deserves a little more detail than was afforded in the Photo Gallery. It is nonetheless an extremely uncommon and unusual specimen as soft bodied cephalopods very rarely fossilise due, of course, to the lack of resiliant hard parts. The specimen measures just over 7 inches long though other examples have been found that are almost twice this length.

This example of Plesioteuthis dates from approximately 150 million years ago placing it at just over 50 million years older than the Middle Cretaceous Trachyteuthis specimen. The specimen comes from the famous Solnhofen lithographic limestones in Germany that are famous largely due to the discovery of the seven specimens of the contemporary Archaeopterix, the infamous dino-bird. In a hot, dry climate, this area of Germany was, as with much of Europe at the time, a warm shallow sea dotted with islands which were separated by extensive coral reefs. The gaps between the islands formed isolated lagoons, due to their isolation these are believed to have contained high salinity levels and would have been uninhabitable to all but some micro-organisms at their deepest parts due to low oxygen levels. The tops of these lagoons would have been of normal salinity but the bottoms highly poisonous.

Creatures that swum into these lagoons and died, or were washed in from the sea would have quickly sunk and been buried in the carbonate-rich sediment formed by the decay of the coral reefs. Due to the lack of animal life in the lagoons, bodies sinking to the bottom would have decayed very slowly and would not have suffered from predation, hence most fossils found at Solnhofen are complete and not disarticulated. In an interesting contrast to the Lebanon deposits, the Solnhofen limestones demonstrate a mix of terrestrial and marine life as they were coastal deposits. The predatory dinosaur Compsognathus and pterosaurs have been found in practically the same strata as land plants, crinoids, fish and rays. Hence the Solnhofen deposits are an interesting snapshot of biodiversity at that time and over 600 species have been identified.

This Plesioteuthis preserved in a slab of lithographic limestone is interesting as it is one of a series of 'squids' that have been discovered clearly living in this coastal, reef dwelling environment. Others included earlier species of Trachyteuthis, Leptoteuthis and Palaeololigo. This specimen of Plesioteuthis we see here is not an ancestor of modern squid but a vampyromorph and, like Trachyteuthis, probably shared a common ancestor with Vampyroteuthis. However it is not believed to be on the same branch as Trachyteuthis but on a independant split from the lineage, it is probable that Plesioteuthis left no descendants post Jurassic. The fossil demonstrates a long tapering guard with a complex internal structure, including it seems, an central internal tapering 'rod', this differs somewhat from the form visible in the Trachyteuthis specimen. The gladius has been broken on the left hand side of the specimen where the hole is visible, though from the comparisons with other specimens it seems that the gladius is almost complete. The whole animal would have been about 12 inches long when alive including head and arms. We are probably looking at the gladius from above, the asymmetric surrounding 'mantle' should not be taken as evidence that the animal was lying on its side due to possible displacement of body tissues following mortality (if this is the genuine mantle we are looking at).

The Plesioteuthididae are believed to be a short-lived group thriving just at the end of the Jurassic, whereas the Trachyteuthididae continued on until at least the mid-Cretaceous. Occasionally these fossils are found with ink-sacs and with complete head structures including arms, though these are unfortunately missing in this specimen.

The hypersalinity of the lagoons is demonstrated by the free swimming crinoid Saccocoma. As can be seen here the tips of the arms are coiled up. It is believed the arms coiled up as it lost water via osmosis as the animal sunk into ever more saline depths. The specimen measures slightly under an inch in diameter.

A larger picture is attached for more detail on the gladius. More details on Plesioteuthis will be forthcoming....