Saturday, 21 July 2012

Describing a cryptoclidid forelimb from the Lower Oxford Clay Formation


I have still been receiving a lot of interest in the plesiosaur we recovered early last year and I did promise some more detail for those of you who have asked for it. So what follows is a description of the right forelimb in its entirety. That will be it for a while until preparation of the specimen restarts which may be later this year but is more likely to be early 2013. So enjoy for now!


In April 2011, a partial cryptoclidid skeleton was found by the writer in an undisclosed quarry near Peterborough in Cambridgeshire, UK (exact location available). The specimen OCWG 160411.01 was extracted soon after by members of a specialist conservation group and other volunteers with permission of the quarry owners.
The specimen was recovered from the Peterborough Member of the Lower Oxford Clay Formation (Middle Callovian) in Bed 10 – the Kosmoceras jason zone (Callomon 1968) .  Bed 10 is usually described as a finely laminated organic-rich mudstone and is exposed at the base of working brick quarries where excavation of the clay ceases due to the presence of abundant septarian concretions (Hudson 1978).
OCWG160411.01 represents a partially articulated skeleton comprising of a fairly complete right forelimb, both humeri, a femur, various vertebrae, odd pelvic elements, and many other limb elements as well as multiple rib fragments. The right forelimb is described here.
Unusually for skeletal elements from the clay, this specimen is remarkably uncrushed and there appears to be very little other taphonomic distortion. As is the norm for plesiosaurs found in the clay, the specimen was found belly-up. The right forelimb is very well preserved and is missing only the phalanges.
Preparation of the forelimb was carried out by the writer and, with no pyrite contamination, was fairly straightforward. Any encrusting sediment, shell debris and matrix was removed by mechanical means including air pen and dental picks whilst finer detail was preserved by utilising pin vices and scalpels. The elements were consolidated by a weak solution of Acryloid B72.
The humerus is considerably larger than the femur which is a characteristic of cryptoclidid plesiosaurs. It measures 290mm in proximodistal length. The proximal articular surface is large and slightly convex and displays numerous tubercles and rugosities. The deltopectoral crest is enlarged and tapers directly off of the capitulum.
The shaft ventral to the proximal head is straight for a short length and has a circumference of 196mm. However, the shaft compresses down as it tapers distally to the distal expansion of the humerus. As is often the case with plesiosaurs, there is a heavily rugose patch of bone on the ventral surface of the humerus situated proximally above mid-shaft and extends from the preaxial to the postaxial margins. The shaft on the preaxial margin displays no foramina but there are a few on the postaxial margin with the largest oval-shaped foramen situated in isolation more or less on the cusp of the dorsal surface just as the humerus expands (eg see Benson et al 2011).
The distal expansion of the humerus is dorsoventrally flattened and expands to 196mm at the widest point between the entepicondyle and ectepicondyle.  Articular facets for the radius and ulna are readily apparent and distinct and the humerus is 44mm thick dorsoventrally at the distal-most point of the bone. The radial facet is bigger than the ulna facet but the size disparity is not as great as it is in other cryptoclidid plesiosaurs such as Cryptoclidus eurymerus (Phillips, 1871). The distal extremity is also rugose but there are no tubercles present.
The ventral surface of the expansion immediately below the proximal rugosity is generally smooth but soon displays striations and foramina that trend with the distal expansion of the humerus. Likewise, the dorsal surface displays similar tendencies but there is only a small rugose patch of bone situated ventral to the proximal head.
In dorsal aspect

The radius is 84mm in length proximodistally along the preaxial margin and is longer than it is wide. It is an unusually shaped element displaying numerous articulating facets. The largest of these is the articulation with the humerus. It is angled obliquely, concave, is 76.79mm in length and has an average dorsoventral depth of 33.82mm. The preaxial margin is concave. The distal most facet articulated with the radiale and is 51.89mm in length and gently concave. Postaxial to this articulation is the oblique intermedium facet and is 26.04mm in length.  The postaxial margin forms the large foramen with the ulna. The notch is 20.62mm in length proximodistally and 22.25mm dorsoventrally. Both the ventral and dorsal surfaces of the radius are covered in numerous scars and foramina.
The ulna is small when compared to the radius and is 75.37mm in length from the preaxial to the postaxial margin. Like the radius, the largest facet is the articulation for the humerus. It is 64.23mm in length and slightly convex. The preaxial margin forms the foramen with the radius although the notch is not so heavily sculpted as in the radius. Ventral to the notch and obliquely angled is the articulating facet for the intermedium. It is 48.59mm in length and the surface is flat. Postaxial to this facet and very obtuse is the articulation with the ulnare which is also strongly oblique but trends anterodorsally and is flat surfaced. This facet is 45.12mm in length. The postaxial margin is strongly convex.  The ventral and dorsal surfaces of the ulna are heavily pitted and rugose – much more so than the radius.

In ventral aspect

The ventral surface of the ulna also displays a quite distinct fossa situated ventral to the intermedium facet that trends dorsomedially and is formed by a distinct ridge that also trends from the intermedial facet. The postaxial margin of the fossa is formed by a prominent ridge that projects from the intermedium facet and forms the proximodorsally orientated margin of the foramen notch that is formed by the ulna and radius.      
The proximal carpals include the radiale, the intermedium and ulnare – there are no ancillary bones present. The ulnare is another strange shaped element similar to the radius and is 41.56mm from the postaxial margin to the articulating facet with the intermedium.  Proximally there is a convex facet that trends dorsolaterally for the ulna and, distally, a rugose convex facet for articulating with metacarpal 5. A further post axial facet is present for articulating with distal carpal 3. The intermedium and distal carpal 3 facets are flat faced. The ulnare tapers and thins down toward the postaxial margin which is dorsoventrally compressed and both the dorsal and ventral surfaces are gently concave and similarly textured.
The intermedium (or centrale in some literature eg Caldwell 1997) is a dense heavily constructed bone that is hexagonal in shape with every facet articulating with another element. It articulates with the radius, ulna, radiale, ulnare and distal carpals 2 and 3. Again all articulating surfaces are flat and the dorsal and ventral surfaces are heavily pitted – particularly the ventral surface. Here there is a heavily sculpted fossa that extends distally from the ulna articulating facet to the facet of distal carpal 3. The centrale is 48.06mm in width at its widest point between the radial and distal carpal 2 articulating facets.
The radiale is almost oblong in shape and is 47.41mm in length from the preaxial to postaxial margin. There are three flat surfaced articulating facets for the radius, intermedium and distal carpal 2 whilst the facet for distal carpal 1 is slightly concave. Similarly to the ulnare, the radius tapers and thins down toward the preaxial margin where it becomes dorsoventrally compressed and both surfaces are gently concave and textured alike.
There are three distal carpals since metacarpal 5 articulates directly with the ulnare. Distal carpal 1 is small – only 32mm from the preaxial margin to where it articulates with distal carpal 2.  Distally it articulates only with metacarpal 1. Distal carpal 2 is another polygonal element and very similar to the intermedium in a much as it is a very thick bone that displays six articulating facets. In addition to the radiale, centrale and distal carpals 1 and 3, it also articulates with metacarpals 2 and 3.
Close up ventral aspect

Interestingly, distal carpal 2 ventrally displays an almost identical trending fossa as both the ulna and intermedium have and is considerably more rugose and pitted than the dorsal surface. These fossae appear to constitute an enlarged proximodistal depression that angles obliquely across the ventral surface of the forelimb. What the significance of such a depression may be is beyond the scope of the writer at this point in time.
The postaxial articulation of distal carpal 2 with distal carpal 3 is pinched forming a more pronounced concavity on both the dorsal and ventral surfaces and the element gently tapers toward this point.
Distal carpal 3 is the most complex element of the three and is also multi-faceted as it articulates with the intermedium, ulnare, distal carpal 2 and metacarpals 3, 4 and 5. It tapers preaxially to mirror the condition in distal carpal 2 and equally compresses down postaxially to form the articulation with metacarpal 5. This is another heavily rugose and pitted element which, unusually in this specimen, displays a dorsal surface more heavily pitted than the ventral side. Distal carpal 3 is 47.91mm in proximodistal aspect.
The five metacarpals are all present. Metacarpal 1 is dorsoventrally compressed preaxially. Metacarpals 2, 3 and 4 are cylindrical in shape and display both proximal and distally expanded articulating ends. The first four metacarpals also display lateral facing facets for articulation with each other (Andrews 1910).
As mentioned previously, metacarpal 5 articulates with the ulnare as a result of shifting proximally to become, in effect, an additional distal carpal. Despite being dorsoventrally compressed postaxially, metacarpal 5 also articulates with distal carpal 3, metacarpal 4 and the first phalange of digit 5. This phalange is the only one that was found in situ although there are others recovered from the quarry. It is broken approximately half way down the shaft and appears to be fairly typical in as much as it is also cylindrical in shape and compressed mid-shaft although this particular bone is also dorsoventrally compressed on the postaxial margin.

Taxanomic Assignment

OCWG 160411.01 was identified as a cryptoclidid plesiosaur on the day of its discovery due to the larger proportional size of the humerus relevant to the femur but was difficult to assign to a specific genera. When the specimen was excavated there were, unfortunately, no cranial remains and no teeth which has proven to be a significant stumbling block.
To try and identify this particular specimen has involved many hours of looking through the literature as well comparing the specimen with those in other collections – particularly the Leeds Collection in the Natural History Museum (London).
The earliest indication of identification was when looking at the Leeds specimens in the Hunterian Museum in Glasgow. GLAHM V1809, labelled as Cryptoclidus richardsoni, was the first specimen that was actually similar to OCWG 160411.001 but it was also apparent that it could also be mistaken for Muraenosaurus. Indeed, Brown (1981) stated that the humerus of Muraenosaurus leedsii is almost identical to that of Cryptoclidus richardsoni and specimen GLAHM V1809 displays many morphologies similar to OCWG 160411.001.
C. richardsoni (Lydekker 1889) has a rather convoluted history and has been the subject of various analyses. The taxon has also been reassigned to C. eurymerus and, Muraenosaurus leedsii (Seeley 1874) over the years. If it was C. richardsoni, then this specimen would be important since this particular taxon is amongst the rarest of plesiosaurs known. And yet it was clear that more comparison was required and, with these thoughts in mind, the specimen was taken to London where we were granted access to the Leeds Collection in the NHM (London).
There were many specimens on our list to check out but chief amongst these was a specimen of Muraenosaurus leedsii BMNH R2864 (Andrews 1910). Although Andrew’s description of BMNH R2864 is very similar to OCWG 160411.01, comparative study of the specimens reveal only limited similarities. However, the new specimen is exceptionally well preserved whilst the Andrews specimen is dorsoventrally flattened and has suffered other significant taphonomic distortion so a true comparison was unrealistic.
In fact many of the specimens in this collection are preserved in a similar fashion and it would appear that OCWG 160411.01 is unusually well preserved and, as such, is an important specimen. Other specimens of Muraenosaurus and particularly Cryptoclidus were observed but it was apparent that there was no comprehensive comparison that could be made. The different degree of preservation, the extreme variance in bone morphology and ontogentic disparity made it virtually impossible and, without the skull, was problematic.
Eventually it appeared that OCWG 160411.01 was, in all probability, a specimen of Muraenosaurus and this was further supported after further study of the radius (S. Moore-Fay, pers.comm) and some of the vertebrae (R. Frost, pers.comm), who also noted that the neural arches of the vertebrae are not fused and supported earlier supposition that the specimen was a juvenile or sub-adult.
Conclusion
OCWG 160411.01 represents a juvenile or sub-adult specimen of Muraenosaurus sp. that, although largely incomplete, is very well preserved and will be useful for further taxanomic study of the taxon – especially when the rest of the specimen is prepared. The genus Muraenosaurus is likely due for revision at some point in the future (R. Frost, pers.comm) and this specimen will add further important information to the dataset. 

References

Andrews, C.W. 1910. A descriptive catalogue of the marine reptiles of the Oxford Clay. Vol.1, London, British Museum of Natural History. 

Roger B. J. Benson, Hilary F. Ketchum, Leslie F. Noè and Marcela Gómez-Pérez (2011). "New information on Hauffiosaurus (Reptilia, Plesiosauria) based on a new species from the Alum Shale Member (Lower Toarcian: Lower Jurassic) of Yorkshire, UK". Palaeontology 54 (3): 547–571. 

Bown, D.S. 1981. The English Upper Jurassic Plesiosauroidea (Reptilia) and a review of the phylogeny and classification of the Plesiosauria. Bulletin of the British Museum of Natural History 35: 253-347.

Caldwell, Michael W. (1997) 'Limb osteology and ossification patterns in Cryptoclidus (Reptilia:
Plesiosauroidea) with a review of sauropterygian limbs', Journal of Vertebrate Paleontology, 17: 2, 295-307.
Callomon, J. H. 1968. The Kellaways Beds and the Oxford Clay. 264–290. In Sylvester-Bradley, P. C. and Ford, T. D. (eds). The geology of the East Midlands. Leicester University Press, Leicester, xx + 400 pp. 

Hudson, J.D. 1978. Concretions, isotopes, and the diagenetic history of the Oxford Clay (Jurassic) of central England. Sedimentology, Volume 25 (3) 339-370.  

R. Lydekker. 1889. Catalogue of the Fossil Reptilia and Amphibia, Part II. 


Phillips, J. 1871. Geology of Oxford and the Valley of the Thames. Oxford.

H. G. Seeley. 1874. On Muraenosaurus leedsii, a plesiosaurian from the Oxford Clay, Part I. Quarterly Journal of the Geological Society of London 30:197-208.



 




3 comments:

Testiculator said...

Should that be R. Forrest, pers.comm not R. Frost, pers.comm?

Testiculator said...

It's worth adding that the holotype of Cryptoclidus richardsoni os probably a chimera, made up of elements of Cryptoclidus and Mureanosaurus.

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