Wednesday, 15 June 2011
Plesiosaur Sp. - The Most Common Taxon of All
Plesiosaurs were a derived group of sauropterygians that used their flippers to, what is known today as, fly underwater. These limbs were used as their principal source of propulsion although it seems likely that there was also an additional tail fin in some cryptocleidoids to provide additional thrust and agility as required (Wilhelm 2010). The limbs were a wonderful design being rigid, paddle-like and elongate.
Unfortunately, this is about as simple as it gets since, as plesiosaurs evolved into more efficient swimmers, modifications to the limb structure, especially to the bones, has made identification of genera problematic. This is amplified when you realise the amount of bone remodelling that occurs throughout plesiosauromorph ontogeny.
Basal plesiosaurs from the Lower Jurassic, such as Plesiosaurus hawkinsi, reveal that the femur and humerus were approximately of similar size. The radius and ulna, in the case of the forelimb, and the tibia and fibula, in the rear limb, were much reduced in size. Postaxially, both the ulna and fibula were remodelled from the basal sauropterygian bone shape and became concave.
These basal plesiosaurs also increased the amount of phalanges throughout their limbs - this is known as hyperphalangy - and they retained this condition throughout their long existence.
More derived plesiosaurs, such as our recent example, display a number of characteristics. The humerus and femur are always less than half the length of the entire limb. Both the front and rear limbs are extremely long due to hyperphalangy as previously mentioned and also, as in basal plesiosaurs, the radius, ulna, tibia and fibula are also significantly reduced in size. Carpal and tarsal identification is somewhat problematic because of remodelling in their morphology.
Collectively, the mesopodials of both the rear limbs and forelimbs are well ossified and are firmly concentrated together. Likewise, the carpals and tarsals are also tightly compacted together. This made for a limb that was rigid and particularly strong and made for very effective swimming throughout the warm Mesozoic seas.
In cryptocleidoid plesiosaurs the fore limbs appear to have provided the majority of thrust since they are bigger than the rear limbs – the humerus is notably bigger than the femur. In pliosaurs it is the reverse and the rear limb appears to have been the main provider of forward thrust.
Although ontogenetic limb ossification is the primary driver of morphological change of bone within the limbs of plesiosaurs, the difference between juvenile and adult plesiosaur limb bones is never the less astounding. Cryptoclidus is the best known of the Oxford Clay plesiosaurs and is represented by multiple skeletons and yet, as with all vertebrate remains, juveniles are scarce. The lack of a decent ontogenetic series of complete specimens causes so many issues when it comes to identification. But the specimens that are available for study do demonstrate the problems faced by palaeontologists.
The Cryptoclidus growth series below from Caldwell (1997) gives an idea of the variability within species throughout ontogeny.
The differences from juvenile to adult are quite extreme. There are enough morphological changes in the humerus and femur alone to cause doubt when trying to establish taxanomic identification – especially when dealing with isolated elements. The other elements such as the mesopodials and metapodials also demonstrate extreme remodelling.
So this particular foray into the world of plesiosaur ontogeny and limb morphology has passed its first phase and we move on forward. Next up is preparation and comparative study of the new material and, if all goes well, the identity of this particular specimen will become apparent.
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.
Wilhelm, B.C. 2010. Novel anatomy of cryptoclidid plesiosaurs with comments on axial locomotion. Ph.D thesis, Marshall University, Huntington, WV. USA