Physiological Adaptations of the Plesiosaur for Holding its Breath
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 Plesiosaurs biological analogues1, 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 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 oceans 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 whales 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, 1998.)
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 animals 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 lungs
34%
9%
Oxygen in blood
41%
41%
Oxygen in muscles
13%
41%
Oxygen in tissues
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 whales 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 whales 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 (<9% in the lungs); certainly endows the whale with its ability to pass time without surfacing. The Plesiosaur would have benefited greatly had it adopted a similar cardio-respiratory system. Though, to infer about the reptilian Plesiosaurs physiology, based upon mammalian analogues is at best tentative: a reptilian analogue is required.
The turtle, a marine reptile, combines the advantages of a lowered metabolism with anaerobic glycolysis4, when subjected to oxygen deficit. The Plesiosaur and turtle are of the same class (although far removed). Its tempting to think the Plesiosaur had similar capabilities, since it dived to greater depths:
The animals are able to tolerate such long periods without oxygen by lowering energy metabolism to a bare minimum, where brain energy needs can be fully met by anaerobic glycolysis. As a result, the turtle brain is able to maintain ATP levels and ionic gradients during anoxia and thus avoid the fatal consequences of energy failure. (Milton, S. 2003)
Thus as Milton (2003) describes, the anaerobic metabolism of carbohydrates keeps the turtle brains energy needs fully met by anaerobic glycolysis. The turtle retains residual consciousness, and is capable of receiving signals from the turtles nervous system. The Plesiosaur could have benefited from the ability to retain residual consciousness during the long descent while it voluntarily reduced oxygen consumption. The reptilian analogue of the Plesiosaur has been found, and it has the adaptations of marine mammals.
Therefore it is possible that the Plesiosaur and deep sea diving mammals alike, despite their differences, evolved convergently to adopt the same solution to a problem posed by their similar environmental niches. Since most reptiles are already capable of abruptly changing their metabolism, the Plesiosaur may have also. Its known that some species of reptile have collapsible lungs, and this adaptation in the Plesiosaur would have enabled it to overcome the physiological diseases and restrictions which impair humans at high pressure. Its been explained that the reptilian marine turtle engages in anaerobic glycolysis to reduce the necessity for oxygen. The Plesiosaur could have done the same thing. And its been explained that, as occurs in the dolphin and whale, peripheral vasoconstriction may have directed and restricted the Plesiosaurs blood flow to the tail, brain, and lungs: the only organs and peripheries necessary during the minute or more of diving.
The plesiosaur has been extinct for eighty million years. Fossil remnants, as they exist may allow researchers to make assertions about a species physiological frame, their diet, and their developmental stages. However, it would be speculation and poor science to claim with any certainty that fossil records or present technology can conclusively describe the Plesiosaurs circulatory and sensory systems. Judging by the characteristics of environmental analogues, such as the whale and turtle, it can be supposed that the Plesiosaur utilised similar systems, but that supposition only stands because we as yet cannot conceive a better explanation for their apparently great diving capability.
Bibliography:
(1) Frappell P, Schultz T and Christian K. 2002. Oxygen transfer during aerobic exercise in a varanid lizard Varanus mertensi is limited by the circulation. The Journal of Experimental Biology. Vol 205:2725-2736
(2) Pough, F. H. 1980. The advantages of ectothermy for tetrapods. The American Naturalist. Vol 115:92-112.
(3) Robinson, J. A. 1975. The Locomotion of Plesiosaurs. Neues Jahrbuch fr Geologie und Palontologie. Vol 149:286332.
(4) Massare, Judy A. 1988. Swimming capabilities of Mesozoic marine reptiles: implications for method of predation. Paleobiology. Vol 14:187-205
(5) Brown, D. S. and Cruickshank, A. R. I. 1994. The skull of the Callovian plesiosaur Cryptoclidus eurymerus and the sauropterygian cheek. Palaeontology. Vol 37: 941-953.
(6) Chatterjee, S. and B. J. Small. 1989. New plesiosaurs from the Upper Cretaceous of Antarctica; Origins and Evolution of the Antarctic Biota. Geological Society Special Publication. Vol 47: 197-215
(7) A. D. Vinogradov. Dec 1998. Myoglobin: What Is It for? Moscow State University School of Biology. Viewed August 2nd 2004 <http://www.protein.bio.msu.su>
(8) Oxford Dictionary of Biochemistry and Molecular Biology. 1998. Oxford University Press, Oxford.
(9) Milton, S. 2003. Dont mind the GAP. Journal of Experimental Biology. Vol 206:2529-253
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(10) National Oceanic and Atmospheric Administration (NOAA). n.d. Anatomy and Physiology: Skeleton, Muscles, and Locomotion.
(11) Carlson, L. Schuler, A. and Smith, V. 1998. The Physiology of the Deep Diving Adaptations of Whales. University of Wisconsin-River Falls.
Appendix:
1 Biological Analogue: present day animals which were subjected to comparable environmental niches to the Plesiosaur. etc: Some turtles, whales, seals.
2 Assuming a dive depth of 100m, it would take (1002.3) x 2 ≈ 87 seconds
3 P1V1 = P2V2 , P1 = 1atm, V1 = 1, P1 = 1atm, V1 = 1, P2 = 10atm
:. V2 = (P1V1) P2
:. V2 = 110 = one tenth
4 Anaerobic glycolysis: burning of energy without oxygen.
