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|Physiological Adaptations of the Plesiosaur for ‘

Holding||its Breath’|
|Nathan Eaton|
One hundred and fifty million years ago, large aquatic species of reptile
such as the Plesiosaur dominated the ocean, and were pre-eminent predators
of the sea. The branch of now extinct Plesiosaurs, or ‘near lizards’,
evolved into variant closely related species specialised to take different
niches in the food chain. Such species of Plesiosaur include the
phenotypically similar Plesiosauroid and Pliosauroid. The physiological
adaptations of the long necked variant, the Plesiosauroid, as it relates to
deep sea diving, will be addressed in depth.

Oxygen breathing lungs are a universal trait of class reptilia. As such, it
would have been necessary for the Plesiosauroid – a marine reptile, to
return to the ocean surface to inhale air. Oxygen expenditure in reptiles
is proportional to strenuosity of locomotion (Frappell, Schultz ;
Christian, 2002). Therefore the Plesiosauroid must have held physiological
traits that enabled the species to avoid oxygen deficit while hunting deep-
sea dwelling prey. This essay will outline the hypothesised respiratory,
circulatory, pulmonary and sensory attributes of the Plesiosauroid as they
relate to diving. These hypotheses will be supported by investigating the
physiological adaptations of the Plesiosaur’s biological analogues1, and
the prospect of similar adaptations in the former will be speculated upon.

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Reptiles have a low metabolic rate: they consume energy, and therefore
oxygen, slowly. According to Robinson (1975), Plesiosauroids were enduring
swimmers with lower flipper aspect ratios and drag-causing long necks.

Massare (1988) made the same conclusion, since the hydrodynamic properties
of the Plesiosauroids indicate the species moved no faster than 2.3 metres
every second. Therefore, the species was confronted by a conundrum: it
sought to dive hundreds of metres to hunt its prey yet was constrained, by
virtue of its body shape, to travel at slow speed. Invariably, the animal
would have been required to forgo oxygen for periods of more than a
minute2, while keeping the presence of mind to hunt.

Fortunately, when making its descent of hundreds of metres, the
Plesiosauroid would have been able to exploit traits possessed by many of
the reptile class. Many reptiles hold the ability to temporarily slow their
heart rate to reduce their oxygen consumption, via bradycardia. This effect
may be caused by low temperatures, such as is found deeper in the ocean, or
may be voluntarily triggered by the animal.

There would be no need for the Plesiosauroid to retain all of its oxygen-
consuming faculties during the long descent. The body processes required
would have not extended beyond locomotion (the tail) and limited
consciousness. When a small garden lizard loses its tail, it is able to
prevent fluid loss by engaging in peripheral vasoconstriction around the
site of the severed appendage. Conversely the Plesiosauroid, also a
reptile, may constrain blood (and oxygen) flow to the propelling tail,
neglecting unnecessary and oxygen consuming processes unrelated to descent,
and entered a state of semi or un-consciousness.

Once at the depth frequented by it prey, the animal would need to engage an
appropriately developed sensory system to quickly catch prey in a low-light
environment. The Plesiosaur, constrained by time, cannot afford to be a
‘trial and error’ strategist. An odour detection, or olfaction organ is an
adaptation in the plesiosaur (Brown ; Cruickshank, 1994). Two nostrils
which channelled water would have enabled the creature to detect molecules
at extremely low concentration. It is possible that the creature used odour
‘triangulation’: by comparing minute differences in odour entering each
nostril; to compensate for an absence of light at great depth when hunting,
and effectively catch prey when time was limited.

The best adapted seals can dive to depths of 1600 metres. An analogue of
the Plesiosaur, in terms of dive depth, diet, and body shape, is the modern
Sea Lion. The sea lion has been known to dive up to two-hundred metres.

Additionally, Plesiosaurs ‘used their hyperphalangic paddles for subaqueous
flight in the manner of modern sea lions’ (Chatterjee ; Small, 1989).

Sea lions and whales, Plesiosaur analogues, can endure environments which
would kill a human. Humans are at risk of illness or death when returning
from a great depth to the ocean’s surface. Under pressure, nitrogen
liquefies, or dissolves into the bloodstream, and an abrupt reduction in
pressure can cause it to want to escape the body in the same way gases in a
pressurised can wish to. Whales overcome this problem, because their rib
cage and lungs collapse and compress under higher pressure: forcing the air
into non-absorptive areas of the lung, and blood flow is reduced to the
lung, reducing the intake of air and importantly, nitrogen:
(The whale’s adaptations allow) …the heartbeat to slow, peripheral
arteries to constrict, and shunting of oxygenated blood to vital
organs. During a whale’s dive, the metabolic rate drops, causing a
reduction in heart rate, or bradycardia. A bradycardia state in an
animal allows the animal to restrict movement of blood to only regions
of the heart, brain, and lungs. This redistribution of arterial blood
and vasoconstriction keeps blood away from sensitive tissues, which
require less oxygen supply in cold water. (Carlson, Schuler ; Smith,
All air breathing mammals are constrained by the fact that air is only 20%
oxygen by mass. Every time their lungs expand to accept air, invariably the
majority of the air in comprised of useless nitrogen. During a dive, far
more gaseous nitrogen will be carried in the lung cavity than oxygen. This
nitrogen may be merely useless for terrestrial animals, but for aquatic
animals which experience rapid changes in sea pressure, nitrogen threatens
harm. Myoglobin reserves in muscles serve as an oxygen ‘buffer’; an oxygen
storage mechanism which allows these animals to saturate their bodies with
oxygen without the usual nitrogen burden (Oxford Dictionary of Biochemistry
and Molecular Biology, 1997). By undergoing a period of ‘loading’ of
oxygen, or rapid breathing which saturates their muscles with oxygen rich
myoglobin, their bodies absorb vital oxygen without additional nitrogen.

Once at the depth frequented by prey, the lumbering plesiosaur needed the
ability to rapidly engage its muscular and nervous systems. Ordinarily,
respiration (and thus muscular contraction) requires rigorous circulatory
blood flow to facilitate the diffusion of oxygen from red blood cells.

Myoglobin with its oxygen cargo would be valuable if it were concentrated
in the animal’s muscles. The whale is a deep diving mammal which makes full
use of myoglobin, specifically within its muscles.

|Human vs. Whale Dive: O2|
|usage |
| |Human|Whale|
|Oxygen in|34% |9%|
|lungs |||
|Oxygen in|41% |41% |
|blood |||
|Oxygen in|13% |41% |
|Oxygen in|12% |9%|
|Adapted from Carlson,|
|Schuler ; Smith|
Whales have a large volume of blood, and a high lung surface area for
maximum oxygen transfer to blood cells. Whales are so oxygenated that their
muscles are black, rather than red. Vinogradov (1998) explains that the
black pigmentation is due to high concentration of myoglobin, an
evolutionary adaptation common to aquatic diving animals. Carlson, Schuler
; Smith (1998) explain that 41% of a whale’s oxygen is stored in its
muscles during a dive, compared to 13% for when a human dives (see: Table).

There is no doubt that the myoglobin presence endows its muscle fibres with
abundant oxygen.

The air stored in a whale’s lungs are adapted so ‘they can exchange up to
85-90% of the air, as compared to humans who exchange only 15%’ (NOAA
n.d.), likely through a higher concentration of oxygen carrying red blood
cells. Such efficient use of oxygen: its efficient extraction (;85%),
metabolism and allocation (


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