Physiological Adaptations of the Plesiosaur for 'holding Its Breath'

Physiological Adaptations of the Plesiosaur for 'Holding its Breath'

By 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 analogues, and the prospect of similar adaptations in the former will be speculated upon.

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 minute, 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:

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