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Arctic Sea Monster’s Hips Don’t Lie

By Lene Liebe Delsett

Last year we packed all of the fossil Spitsbergen marine reptiles in boxes and gave them museum numbers, and realized that we did not have 30 or 40, as we once thought. The number of Late Jurassic- Early Cretaceous specimens that we excavated between 2004 and 2012 is officially 60. These ichthyosaurs and plesiosaurs were found in a hillside representing somewhere between 10 and 15 million years of sea bottom deposition, and we think they can tell us something about body evolution. Through our most recent work, we studied ichthyosaur hips.

Excavation of Keilhauia nui at Spitsbergen. From the left: Linn Novis, Bjørn Lund and Tommy Wensås.
Excavation of Keilhauia nui at Spitsbergen. From the left: Linn Novis, Bjørn Lund and Tommy Wensås. Photo courtesy Lene Liebe Delsett

All the way back in 2010, we did a dig located approximately at the border between the Jurassic and the Cretaceous. It was hard to be certain about anything at that point in time, because tons of wet fog made the sight rather poor, but in the Spitsbergen Mesozoic Research Group spirits were high and the dig looked promising. Which was right, it turned out to be an almost complete ichthyosaur. There were only two disappointments: Almost the entire skull of the animal was missing. We were lucky to find parts of the snout, which was lying almost 30 cm lower than the rest, but the remaining skull parts were gone. In addition, the skeleton looked more like mud than bones, even when we got it to the lab. Only through an impressive effort by lab heroes May-Liss, Victoria and Aubrey, a nice ichthyosaur emerged. Something was still puzzling us, as it did in the field: What was happening with the hip and hind limb? Did it have a normal shape and size for these bones?

A hip in a box. Keilhauia nui has tiny and strange hip bones.
A hip in a box. Keilhauia nui has tiny and strange hip bones. Photo courtesy Lene Liebe Delsett

This was the start of the description process of new genus and species Keilhauia nui, published in PLOS ONE January 25th this year. After careful preparation and thorough study of the bones, we have concluded that this is something completely new. It does not belong to the most numerous (however very poorly defined) Cretaceous ichthyosaur Platypterygius, nor to the taxa we have previously described from our site. And the hip IS strange. Taken together, these things define the new taxa, named after the first Norwegian geologist to go to Svalbard for field work (Baltazar Mathias Keilhau), and the environmental organization Young Friends of the Earth Norway (NU in Norwegian).

Artist reconstruction of earliest Cretaceous ichthyosaur Keilhauia nui. Artist: Esther van Hulsen.
Artist reconstruction of earliest Cretaceous ichthyosaur Keilhauia nui. Art by Esther van Hulsen

When turning to the literature and other museum collections for comparisons, we found that Cretaceous hips are rare, and that our specimen turned out to possess the most complete hip from this time period so far. We also realized that that we actually had quite a lot of Late Jurassic hip material in our collection. The previously described taxa Cryopterygius and Janusaurus both preserve their hips, as do Keilhauia and three additional, more incomplete specimens. This is how we ended up discussing ichthyosaur hip evolution from the Triassic through to the Cretaceous, and compared length measurements for as many Jurassic and Cretaceous specimens as possible. From these comparisons, we could confirm that Keilhauia’s hip bones have a unique shape and size relationship. Janusaurus and two other specimens (in the new paper) turned out to have another, also unique size configuration. Why? We do not know, but it might be related to either reproduction of movement. Ichthyosaurs gave birth to living offspring, but few details about reproductive organs, courtship and birth are known. We do know they swam by using their tail as an outboard motor, but not to which degree the fore- and hind-fins were used for maneuvering.

When you have a lot, you get greedy and want more. We realized that we have 60 marine reptiles and that we can tell a little part of an evolutionary story of ichthyosaur hips. Now all we want is to go back to Spitsbergen to answer all of the new questions that have arisen. But first, we have a huge job in front of us finding all the other new cool evolutionary stories among our undescribed material.

Read the article in PLOS ONE here