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Ictonyx — Shonisaurus
#ichthyosaur #marine #mesozoic #reptile #triassic #shonisaurus #shastasaurus
Published: 2019-04-12 22:54:50 +0000 UTC; Views: 2373; Favourites: 65; Downloads: 8
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Description
Shonisaurus and Shastasaurus were whale-sized Triassic ichthyosaurs, with at least one species probably reaching or exceeding 20 metres in length. They are the largest known marine reptiles, and some of the largest animals of all time; only certain whales and perhaps sauropod dinosaurs would have been heavier. This Shonisaurus is accompanied by a pair of hybodontid sharks.Related content
Comments: 7
While I commend you for giving an ichthyosaur a bulky caudal peduncle, I'm afraid you gave it to the wrong ichthyosaur. This kind of tail would've been found in thunnosaurians, which as the name implies had a thunniform locomotion style. Shonisaurus is not a thunnosaurian and would not have had this kind of tail.
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I agree that the keels and bulky peduncle are highly speculative, and indeed the outline of the caudal fin itself is speculative. However, we do not have enough information from the fossil record to state confidently that Shonisaurus would not have had this kind of tail.
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Additionally, Shonisaurus had an extremely low tail bend that wouldn't be able to support a lunate tail fin.
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There is actually enough evidence that it would not have had this tail type. Shonisaurus was an anguilliform swimmer and not a thunniform swimmer. Only thunniform swimmers have tails with prominent keels like this.
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I'm afraid I think this is an over-confident stance. The main reference for my reconstruction was a skeletal by Scott Hartman, which shows a bilobed caudal fin with a reasonable degree of tail bend, roughly comparable to that of a tiger shark (though inverted of course, with the tail spine bending down rather than up). I increased the amount of soft tissue in the caudal fin compared to Hartman's skeletal, but a) the true extent of the tail fin is unknown, allowing for my speculaiton here, and b) as Hartman himself will tell you, he typically errs on the tighter soft-tissue outline side of things due to the diagrammatic nature of his skeletals. Also - do you realise how many living animals have large, fan or crescent shaped tails without any spinal support whatsoever? Billfish, tuna, salmonids, not to mention whales. The huge fluked tails of sperm and baleen whales have no internal bony support at all. And while you claim that only thunniform swimmers have prominent tail keels, this is not really true even for living animals - whale sharks, for instance, have pretty prominent keels despite being more carangiform than thunniform.
In addition, the depth and robusticity of the body makes true anguilliform locomotion highly unlikely for Shonisaurus - typical anguilliform taxa today are shaped like eels, lampreys and so on. If I had to guess - though I try never to make over-confident assumptions about extinct taxa - I would suggest sub-carangiform or carangiform locomotion for Shonisaurus.
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The bend of the tail in Shonisaurus is not nearly as high as the bend seen in thunnosaurian ichthyosaurs with lunate tails. For example, here's a comparison between Scott Hartman's Shonisaurus, Ophthalmosaurus from McGowan & Motani (2003), Stenopterygius from Motani et al. (2014), and Rebecca Groom's Ichthyosaurus.
Animals without any internal support for the caudal fin are irrelevant to this discussion because ichthyosaurs do have internal support. Thus based on animals with internal support for the caudal fin (sharks and other ichthyosaurs), Shonisaurus would certainly not have had a lunate tail fin.
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I'm uncertain on exactly what you mean by lunate fins; my fin in the drawing is clearly not a high aspect, close to symmetrical fin like those of the Opthalmosaurus, Ichthyosaurus and Stenopterygius illustrations you include; it is in fact much closer to that of Hartman's Shonisaurus, or a whale or tiger shark. The angle of my drawing means that it doesn't look exactly as it would in a pure lateral view, but nonetheless, it is a possibility consistent with the skeletal information available. So I wouldn't say I have drawn a 'lunate' fin, simply a larger fin than the one in Hartman's skeletal. Animals without internal support are not irrelevant; modern whales prove that truly enormous fins may be without direct fossil evidence. The size and shape of a sperm whale's tail, or a blue whale's tail, or a bowhead whale's tail could not be accurately determined if only fossil skeletons were known; I was simply making the point that there are large error bars with this stuff, and it is the privilege of artists to explore those error bars.
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