Corrections and additions

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.

Genus Level

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.

LYMPH 2006-72 Forefin with labelled anatomy.

Fig.1: LYMPH 2006-72 Left Forefin. (click for big)

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.


Species Level

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:

Corrected measurements and ratios for LYMPH 2006-72This 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.

REFERENCES

McGowan, C., Motani, R. 2003, Handbook of Paleoherpetology, Part 8. Ichthyopterygia. Verlag Dr Friedrich Pfeil, München, 175 pp.

A new beginning and a new experiment!

Hello and welcome one and all to a new wordpress blog!
Those of you who have arrived here from my personal blog at www.benjamindbrooks.co.uk are especially welcome, though if you were looking for more of the same eccentric subjects and idiosyncratic rants, you are likely to be rather disappointed!

‘The Research Notes…’ (this blog) is  a place for me to attempt some (ostensibly open-access) science after having graduated from Southampton University last year. The reasons or having it are twofold, one selfless, one selfish.

Firstly and selflessly, if I am going to do Science, I want it to be Open Access fo the simple reason that I believe in the disemination of knowledge to as many people as can get it, for free as far as is possible. Secondly and selfishly, as I am no longer attached to an institution as a student or researcher, and have no advisor to chase me up on things… posting the work (slow as it will necessarily be as it is presently an avocation) will allow anyone to call me out on not doing anything for a while, and so hopefully spur me to do more!

The First Project! Ichthyosaur Specimen LYMPH 2006-72
So then, on to the first project I’m going to be undertaking here on ‘The Research notes…’. The description of an Ichthyosaur specimen on public display at Lyme Regis Museum.
This is a suitably fascinating project to start with (as you’ll see) with but equally it’s a little daunting to someone with no previous descriptive experience – as a ‘generalist’ Geologist by training and a Palaeontologist by interest, I haven’t picked up as many of the requisite skills as I would like, but hopefully this will help change that.

The first thing to do then is to upload a couple of the photographs I have of the specimen to give a general impression of the task ahead, so here you go…

LYMPH 2006-72

Photo 1, An overview photograph of LYMPH 2006-72

this shot (Copyright: Chris Andrew - he has a better camera than me!) gives an excellent impression of how well preserved the intestinal contents are, you can almost follow the intestine as it wends its way through the body.

Vert. Column showing odd Neural spine taphonomy

this shows one of the interesting features of taphonomy, the specimen's neural spines have "rolled off" in a single block, remaining together even though not articulated with the vertebrae.

What have my researches taught me so far?

Sadly the most useful extremities of the animal are missing (Tip of snout and fore-paddles, loss of rear paddles and most of the tail including the tail bend.) which makes identication somewhat more tentative, but I’ve got to work with what there is¹. (Note; all diagnostic features/tests are taken from McGowan and Motani, 2003).

A fair estimate of  the missing jaw section can be made from the tapering of the mandible and dentary (lower and upper jaw-bones respectively) and it seems to me that an extra 15 cm of jaw is justifiable, this gives a Snout length (SL) of approx 28 cm and a Jaw length (JL) of approx 43 cm.

These two measurements produce a Snout ratio (SL/JL) of 0.65

The Orbit diameter (OD) of  the animal can be easily measured to 8 cm as it’s fairly intact (N.B.: orbit = eye socket opening)

This coupled with the Jaw length produce an Orbit ratio (OD/JL) of 0.18

These two skull based ratios alone knock out one of the Ichthyosaur species² (Ichthyosaurus breviceps) as the Snout ratio is far too large, and the Orbit ratio far too small!

I also see no evidence (in the photgraphs I’m using) of waisted teeth in the jaw, which knocks I. intermedius out as well.

Then we move on to other factors to determine between I. conybeari and I. communis. The major features normally used here are the Pre-sacral and Pre-flexural vertebrae counts, however as  this specimen has neither an intact Ilium (The thigh bone in you or I, used to identify the Pre-Post Sacral divide) and no tailbend (for the Pre-Post flexural divide) these features cannot be used.

The number of front paddle digits on this specimen is 6, the maximum for I. conybeari and minimum for Dorset specimens of I. communis so again this feature isn’t much help. We can however use the presence or absence of notching on the phalanges (digit bones) of the forelimb. in what little of the front paddles is evident, there are no notched phalanges which is consistent with this being a specimen of I. communis. The orbital ratio also helps as it is closer to the upper bound for I. communis (<0.26) than it is to the upper bound for I. conybeari (<0.28).

Anyway, that’s all for now, probably not going to get much if any more done before April (work commitments etc.) but any comments on this post are most welcome! Especially as it’s my first try at this open notebook thing, any tips, suggestions and such. Or corrections for that matter…. if I’ve said/done something stupid please call me on it.

FOOTNOTES

¹ As I’m in Yorkshire at the moment on a Museum Internship I’m working solely from photo’s and making measurements in a rather awkward fashion with CorelDRAW, so these measurements may change – slightly – when I return to Devon next month!

² The more astute observer will notice I don’t explain why I think it belongs to the Genus Ichthyosaurus sp. – Simply put because I cannot prove it using the photographs I have at my disposal presently. But rest assured I’ll remedy this with some close personal inspection of the specimen soon! Apologies for the cop-out 😦

REFERENCES

McGowan, C., Motani, R. 2003, Handbook of Paleoherpetology, Part 8. Ichthyopterygia. Verlag Dr Friedrich Pfeil, München, 175 pp.