Locomotion

Plesiosaur locomotion

N.B. The animations from this page are currently missing, but will be added in due course.

There has been considerable debate over the locomotion of plesiosaurs (Robinson, 1975). Although today, the general consensus view of plesiosaur locomotion is a combination of predominantly subaqueous flying (table 1).

Table
1: Representation of a variety of possible plesiosaur swimming modes.

Proposed mode of limb movement Animations of proposed limb paths(re-click to play) (by Louisa King )

Rowing
– Backstroke provides the thrust, the recovery stroke is ‘feathered’ (rotated so that the flipper cuts through the water like a blade), this provides little thrust. The plane of movement is horizontal, or forwards-backwards. Verdict – Unlikely, limbs are not efficient paddles and the limbs poorly developed for horizontal movement.


Representation of a rowing limb path (animal swimming to the left)

 

 

Sub-aqueous flying
– Both strokes are powerful and provide thrust: this method provides constant thrust. The plane of movement is vertical (up and down). Verdict – limbs designed for flying, with tapered tips, but overall unlikely – the limbs could not be raised far above the horizontal.

Representation of a sub-aqueous flying limb path (animal swimming to the left).

 

Modified form of flying

– modified nflying combines principles of both rowing and flying. The downstroke provides thrust and lift as in flying and the more-or-less passive recovery stroke is feathered, as in rowing.Verdict – Most likely.


Representation of a modified flying limb path (animal swimming to the left).

 

But how did the four limbs move in relation to each other? An alternating
fore and hind limb motion (fig 1 and animation 4) was portrayed in the
BBC’s Walking with Dinosaurs television series (episode 3, Cruel
Sea).

Fig 1. Representation of the alternating plesiosaur limb movement possibly
employed by plesiosaurs.

Animation 4. 3-D animation of the alternating plesiosaur limb movement possibly employed by plesiosaurs.

The justification for this hypothesis was provided by Frey and Riess (1982) – the pectoral and pelvic girdles are engineered to enable a very powerful downstroke but only a weak upstroke or recovery stroke, i.e. they are only reinforced ventrally (Massare, 1988). To compensate for the lack of thrust during the weak upstroke of one pair of limbs, the other limb pair would provide the thrust with a powerful downstroke. This would hypothetically enable constant forward motion of the animal and result in efficient locomotion. Other researchers (Lingham-Soliar, 2000) have recently regarded the rear limbs as being of little use in locomotion (animation 5), and still others (Sanders et al. 2004) endorse a repertoire in which all four-flippers move in synchrony (animation 6) (moreabout this on Mike Everharts website ‘Oceans of Kansas’). The debate goes on…

Animation 5. 3-D animation of plesiosaur limb movement possibly employed by plesiosaurs, showing the rear fins with little or no contribution to forward motion

Animation 6. 3-D animation of the synchronous plesiosaur limb movement possibly employed by plesiosaurs.

The limbs of plesiosaurs (fig 2), with their large surface area would also
increase the level of thrust and act as hydrofoils to compensate for the
density of the organism. Robinson (1975) uses many lines of evidence to
dispel the notion that plesiosaurs were exclusively rowing animals, the
most convincing points being the hydrofoil shape and the joint between
propodials (humerus and femur) and the girdles (pelvic and pectoral) both
of which facilitate a flying method. Well-formed gastralia (belly ribs)
and a tightly articulated spine provide a sturdy base for limb movement.
Short bursts of speed could have been performed on occasions by combining the down stroke of both limb pairs simultaneously. This could be used in predation to make a speedy kill, taking the prey by surprise. Also,
the momentum of such a method, especially for large pliosaurs, could send
the prey into the gaping jaws killing it immediately, allowing the animal
to eat without a struggle.

Fig 2. The limbs of Muraenosaurus.

Whether plesiosaurs were able to move on land, as depicted below (fig 3) is a point of contention. The mechanics of the skeleton imply a completely aquatic existence: the limb girdles are not strongly connected to the backbone, inhibiting transfer of force from limbstrokes into movement of the body. However, small plesiosaurs may have been relatively uneffected by such constraints and it is not unreasonable to imagine them using their powerful limb downstrokes to propel them forward in short ‘hops’. In contrast, larger taxa were unlikely to survive out of water, presenting problems for reproduction: all extant reptiles lay eggs on land. Plesiosaurs overcome this problem in the same way is the contemporaneous Ichthyosaurs and evolved the ability to give birth to live young. Triassic sauropterygian embryos are known, and one pregnant plesiosaur is known.

Fig 3. A land-loving Cryptoclidus as seen in the BBC’s Walking with Dinosaurs television series (episode 3, Cruel Sea)