Locomotion


Plesiosaur locomotion

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load – thanks for your patience. 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 complimented by rowing (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
– 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
flying 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
BBCís 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 hypothesys was provided by Frey and Riess (1982) – the pectoral and pelvic girdles are engineered in such a way as 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 femor) 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. Whilst triassic sauropterygian
embryos are known, unfortunately, no known plesiosaur embryos, eggs or
pregnant parents are known.

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