OK so it’s been a while… indeed an almost unforgivably long time if I’m perfectly honest, but I’ve been busy… so here we go again!
Once again, all diagnostic features/tests are taken from McGowan and Motani, 2003.
The one thing I didn’t do last time was to justify the genus identification I gave for specimen LYMPH 2006-72. The first clue to this is the size of the specimen, at 116 cm long (excluding missing snout and tail sections) the complete length of this animal would be somewhere between 2 and 2.5 metres.
The forefin anatomy provides the bulk of the remaining evidence for genus identification. The visible forefin of this specimen is the left hand of the two and exhibits features which are consistent with it being of the genus Ichthyosaurus. Firstly the fin includes 7 digits (including two accessory digits) while the genus Ichthyosaurus is diagnosed as having no fewer than 5 digits. Digital bifurcation is also visible in the anterior portion (in this case left-hand side) of the fin – another diagnostic feature.
The second diagnostic feature of the forefin is that the ulnare (u in fig.1) is larger than the intermedium (i in fig.1).
The humerus proves somewhat more troublesome, as the distal end is still partly buried in matrix on the posterior portion. The exposed measurements are a proximal width of 41mm and a distal width of 40mm. In Ichthyosaurus, the distal width is the wider, however as it is still partly buried this may not be a crucial problem.
As the entire pelvic girdle and tailbend are missing from this specimen, the diagnostic features that arise from these parts have to be ignored, all I can say is that the minimum number of preflexural vertebrae is 61, which doesn’t exceed the maximum of 80 placed on it by McGowan and Motani (2003).
The basioccipical is also not visible in the specimen as it is buried within the matrix of the skull, so whether this specimen has the “extensive extracondylar area and well developed basioccipital peg” I cannot say.
Relying solely on size and the features described above, this specimen can identified as a member of the genus Ichthyosaurus.
The first thing to say is that I didn’t quite realise how much my estimate of the length of the missing section of LYMPH 2006-72’s jaw would affect the ratios I obtained for the species diagnosis.
On re-visiting the specimen and actually taking reliable measurements in person, talking to the Museum’s geologist (who’s seen far more Ichthyosaurs than me) and looking at photographs of full specimens, it seems that my photo based estimate of 15cm was too long by about a third, which means I have to revise my ratios as follows:
This doesn’t change the identification I made in my previous post, though it does make the differentiation between I. conybeari and I. communis a closer call (though not by much). Thankfully however the discovery of a 7th paddle digit (the preaxial accessory digit: see fig.1) removes any possibility for this specimen to be an example of I. conybeari because it now has more than the maximum reported number of digits for this species.
Next Time… Soft parts and Gastric Contents… Yummy.
McGowan, C., Motani, R. 2003, Handbook of Paleoherpetology, Part 8. Ichthyopterygia. Verlag Dr Friedrich Pfeil, München, 175 pp.