Anatomical and Physiological adaptations of the Plesiosaur in respects to extended high pressure diving

The Saurian family Plesiosauria included both Plesiosaurs and Pliosaurs, the largest marine reptiles to inhabit the planet between the early jurassic and late cretaceous periods. Ranging in size from 2 to 15, and weighing up to 25 tonnes. The Plesiosauria family is divided into 2 superfamiles; Plesiosauriodea and Pliosauroidea, having important differences as well as similar features.
Fossils of the jurassic Plesiosaurs (Plesiosaurus and Cryptoclidus) and Pliosaurs (Liopleurodon) have been found in Europe, while remains of the cretaceous Plesiosaur (Elasmosaurus) and Pliosaur (Kronosaurus) have been discovered in Asia/North America and Queensland, Australia respectively.

Members of the Plesiosauriodea superfamily are described as generally having long necks and small heads, with forelimbs generally larger than the hind limbs. The Pliosauroidea superfamily had shorter necks but much larger heads, with extremely powerful jaws. Plesiosaurs were efficient swimmers, but it is believed that their hunting styles were varied in accordance with their shape. It is believed that the Plesiosaurus gracefully "flew" through the water in search of prey, laying in wait, highly manoeuvrable and able to turn on the spot in ambush of prey (1), while Cryptoclidus were very agile long distance swimmers and would pursue much more active prey, hunting by the chase. (2)

As Plesiosaurs hunted at depth for long periods of time, they would have had to develop specialised anatomical and physiological traits to withstand the immense pressure and energy debt that such diving put on their bodies.
Several factors are known to impede the ability of any animal to dive to great depth, be they mammalian or reptilian. The problems posed by deep sea diving consist of maintenance of homeostasis through immense pressure, decompression, lack of oxygen, and thermoregulation.

This essay will explain the possible adaptations Plesiosaurs developed to survive in their environment with respect to the mechanisms that marine animals use to maintain homeostasis in a deep diving environment.




The pressure exerted upon the torso of a Plesiosaur at 200 meters would have been roughly equivalent to 21 atmospheres. It is known for Green turtles (Chelonia mydas) to dive to much larger depths than this.
It is shown through the fossil record that Plesiosaurs had long, wide bodies. For such deep dives to be possible without harm to the animal, a sort of protective "cage" would need to surround the internal organs to avoid being crushed by immense pressure, yet the cage itself would need to be flexible and collapse to a degree to keep internal pressure equal to the pressure of the sea. The Plesiosaur overcame this hurdle with a series of dense, closely packed gastralia, or “belly ribs”. These ribs made the torso rigid and able to withstand the pressures of the deep sea, yet flexible enough to almost totally collapse. These ribs also protected the internal organs of the plesiosaur when the animal left the ocean to lay its eggs on the beach. The external shells of modern turtles have the basic same function, providing rigidity at great depths, and protection of the internal tissues when surfacing to lay eggs. It is possible that smaller Plesiosaurs used their gastralia as protection from predation. (3)

When diving to depths of greater than 30 meters and resurfacing, some humans experience a condition known commonly as "the bends" or decompression sickness. It is basically caused by nitrogen which has come out of solution in the divers blood and formed small bubbles of gas in the surrounding tissues. Decompression sickness is a serious and potentially fatal side effect of deep dives, but many marine reptiles and mammals frequently dive and resurface without any ill effect.
There are 2 possible mechanisms the Plesiosaur could have employed to overcome decompression sickness during frequent dives.

The first mechanism would be similar to the one employed by the Leatherback turtle (Dermochelys coriacea) , in that the skeleton and skin are suffused with a large amount of oily polyunsaturated fatty acids. The excess nitrogen bubbles are believed to enter this oily insulating layer where they can be stored until such time as they can be excreted by the animal with no detrimental effect to the animal or its tissues. (4)

Secondly, and most importantly, the Plesiosaur would not have suffered the bends during its long and frequent dives for the same reason that marine animals today do not suffer the bends during dives; that is, they only have the percentage of gas mixtures in the breath they take with them at the surface for the entire dive. The small amount of nitrogen that would be present in that one breath would hardly be enough to saturate the blood with nitrogen, would easily decompress to the same volume upon resurfacing and would leave the bloodstream and fill the lungs to be exhaled at the surface. (5)

The plesiosaur must have had the facility for storage and sustained utilisation of oxygen to allow it to be such an active hunter at great depths. To accomplish this, it must have had the ability to regulate its heart rate and metabolism of oxygen by its tissues.

Deep diving animals have very similar physiological adaptations to their environment. An animal that submerges for extended periods of time must be able to efficiently store large amounts oxygen in its tissues, and be able to utilise it through anaerobic mechanisms.
Turtles and Seals, such as the Weddell seal (Leptonychotes weddelli) use identical physiological mechanisms to store and utilise the oxygen during a deep sea dive, so it is very likely that Plesiosaurs also utilised similar mechanisms.
Deep diving animals have a tremendous ability to store oxygen- up to twice as much per kilogram of body weight compared to a human- and do this by storing the oxygen in tissues which are not used for respiration. Animals such as the Weddell seal store only 5% of their oxygen in the lungs, and around 70% in their blood (6) and about twice as much in their muscles compared to a human. Storage of oxygen in non-respirative tissues is vastly more efficient than storage in the lungs as it allows for a greater capacity to do anaerobic activity. The two oxygen storing proteins responsible for this increased capacity to store oxygen are haemoglobin (in the blood) and myoglobin (in the muscles), and diving animals have large amounts of these proteins present in their tissues.
Haemoglobin present in the blood of the Plesiosaur would serve much the same purpose as haemoglobin in any animal, to transport the oxygen which is absorbed from the lungs and bonded to haemoglobin to the tissues of the body. It is possible that the plesiosaur had a comparatively large spleen, able to store large amounts of blood, which contracted during a dive, suffusing the blood stream with oxygen-enriched blood and providing more oxygen for use by tissues. (6)
The myoglobin contained in the muscle fibers of the Plesiosaur would have provided the greatest oxygen stockpile for use by this active deep diver. during periods of oxygen deprivation when bursts of activity were needed, such as chasing prey or evading predators, the myoglobin (oxymyoglobin in its oxygen-bound state) would release its oxygen and generate ATP via glycolysis. The generation of ATP via glycolysis is preferable for quick release of energy, as it requires fewer steps than oxidative phosphorylation (aerobic ATP production), and is able to proceed in the absence of oxygen altogether. (7) The disadvantage to glycolysis for energy production is the generation of the waste product lactic acid, which cannot be broken down into any useable molecules, and accumulates in the muscles causing fatigue and acidifying of the blood. The Plesiosaur must have had a high tolerance to muscle fatigue and blood acidosis to be able to sustain anaerobic activity for long periods of time, much in the same way that crocodiles (Crocodylus porosus) do. (8)

Even the large amounts of oxygen available to the Plesiosaur can be expended in a very short time without the ability to maintain and conserve the oxygen stored. The Plesiosaur would have accomplished this by decreasing its heart rate and therefore its oxygen consumption. An example of this is the ability of the green turtle to decrease its heart rate to one beat every 9 minutes during the deepest part of its dive. The Plesiosaur must have had receptors called Baroreceptors, which are sensitive to changes in blood pressure, present in its heart and carotid artery. The baroreceptors would constantly provide information about blood pressure and adjust cardiac output to sustain normal blood pressure. Therefore, as depth increases and the pressure on the body and blood vessels increases, so does the blood pressure. Baroreceptors would trigger an autonomic reflex and slow the heart rate down to normalise blood pressure. (9)

The final obstacle to any deep diving animal is the ability to regulate temperature. It is possible that the plesiosaur was a warm-blooded reptile, but only insofar as it used a thermoregulatory process known as giganothermy. leatherback turtles utilise this mechanism with great success both in the water and on land. The gigantothermic process involves low metabolic rate, a temperature control mechanism called counter-current heat exchange, a large amount of insulative tissues and sheer size to operate.
Gigantotherms use their large body size to keep warm. This is possible as a gigantotherms body core is much further away from the surface than a smaller reptile, and temperature fluctuations occuring on the surface do not affect a gigantotherms internal temperature to any great degree. It is this surface area to mass ratio, together with counter-current heat exchange which allows a gigantotherm to maintain a stable body temperature both in near freezing and warm conditions.
Counter-current heat exchange operates by shunting blood away from the surface tissues and to the animals core when in cold temperatures (such as hunting in the arctic seas) to maintain a stable internal temperature, and shunting blood to the external tissues when in warm temperatures (when laying eggs on a warm beach) to lessen the temperature gradient between its internal temperature and the external temperature. As the animal would have a large amount of insulative tissues surrounding its organs and underneath its surface tissues, keeping its core warm even in sub-artic temperatures would not be a great problem. The leatherback turtle is able to maintain a temperature between 18 and 25 degrees celcius regardless of the external environment, if the Plesiosaur utilised giantothermy it may have been able to keep a similar internal temperature. (10)

The Plesiosaur was an amazing marine reptile, which may have utilised some amazing physiological and anatomical adaptations refined in mammals and reptiles of today, with the ability to survive extremes of temperature, pressure and physical stress in the prehistoric ocean. The length of survival of the species would certainly have depended on their adaptability to both the aquatic and terrestrial environment, shown in their metabolic and thermoregulatory mechanisms and anatomy.




references:

(1) Worth, G. The Dinosaur encyclopaedia, Version 4, 1999. http://www.isgs.uiuc.edu/dinos/de_4/dino30.htm

(2) Randerson, J. February 2, 2002. Reptiles at four o’clock. New Scientist. volume 173, issue 2328; pg. 17

(3) Smith, A.S. The plesiosaur directory. 2004. http://www.geocities.com/sea_saur/locomotion.html

(4) harford, J The great leatherback. 2002 http://jrscience.wcp.muohio.edu/FieldCourses99/TropEcoCostaRicaArticles/FinalDraft.TheGreatLeathe.html

(5) Antonio DeGorordo, Federico Vallejo-Manzur, Katia Chanin and Joseph Varon, Diving emergencies, Resuscitation, Volume 59, Issue 2, November 2003, Pages 171-180.
(http://www.sciencedirect.com/science/article/B6T19-49SFK87-F/2/4d909d32dbf3efa5576a0b600b15a754)

(6) Campbell et. al. Biology, 1999. 5th ed. pg 836

(7) Sherwood, Human Physiology: from cells to systems, 2001. 4th ed. pg. 260

(8) J. Baldwin, R. S. Seymour and G. J. W. Webb, Scaling of anaerobic metabolism during exercise in the estuarine crocodile (Crocodylus porosus), Comparative Biochemistry and Physiology Part A: Physiology, Volume 112, Issue 2, October 1995, Pages 285-293.
(http://www.sciencedirect.com/science/article/B6T2P-3XWRV4C-4/2/a8c1541e57934fd65dc249645be36446)

(9) Sherwood, Human Physiology: from cells to systems. 2001. 4th ed. pg 355

(10) Crawford, R. http://www.bio.davidson.edu/Courses/anphys/2000/CrawfordR/CrawfordR.htm

part troi

the plesiosaur may have developed a system similar to the oils in a leatherback turtle, in that its skeleton and skin was suffused with a high concentration of polyunsaturated fatty acids. This oil would absorb the nitrogen bubbles more effectively than normal tissues, and keep the nitrogenous wastes suspended until a time when they could be excreted as waste products without harm to the animal. This would only be a secondary precautionary development,

Most importantly, the plesiosaur would only have one breath to take with it down to depth, and not constantly breathing compressed gasses like human divers do (a prehistoric scuba tank would no doubt raise more questions than answers at this point!!) the benefit of this is that the plesiosaur only takes down with it what nitrogen it got from that one breath. The 70-ish% of nitrogen it got from that one breath would hardly be enough to supersaturate its blood or tissues at any depth, and would be easily decompressed to the same volume when surfacing. The lack of a continuous supply of nitrogen and oxygen would diminish the chance of the plesiosaur developing the common diving artefacts of nitrogen narcosis, decompression sickness and more seriously, oxygen toxicity, that human divers can develop during and after a long dive.

part deux

Several factors are known to impede the ability of any animal to dive to great depth, be they mammalian or reptillian. An animal that breathes air and has definite structure is going to suffer simmilar ailments as another animal at great depths.
These factors consist maintaining homeostasis through immense pressure, decompression sickness and lack of oxygen.

The pressure exerted upon the torso of a plesiosaur at 200 meters is roughly equivalent to 21 atmospheres, It is known for green turtles existing today to dive to depths much larger than this.
It is known that the plesiosaur family had wide, flattened bodies to which their flippers were attached. For such deep dives to be possible without harm to the animal, a sort of protective "cage" would need to surround the internal organs so as not to be crushed by the immense pressure, yet the cage itself would need to be flexible to have the ability to collapse and keep the pressure in the lungs equal to the pressure in the water. The plesiosaur family overcame this hurdle through a series of dense, closely packed belly ribs. these ribs made the torso rigid and able to withstand the pressures of the deep sea, yet flexible enough to almost totally collapse with the lungs to maintain equal pressure with the ocean, but also protected the belly of the plesiosaur when the animal left the ocean to lay its eggs on the beach. The external shells of modern turtles have the basic same function to provide rigidity at great depths, and protection of the delicate belly tissues when descending from the sea to lay eggs. Turtles also use their shells as protection from predators, and it is possible that smaller plesiosaurs used their closely packed ribs as protection from predation, though understandably not as successfully as if it had been an external shell.
perhaps the closest example we have to the belly ribs of the plesiosaur in a modern-day animal is the cartilaginous shell of the leatherback turtle. the cartilaginous shell provides rigidity under immense pressure, but also has "give" and is able to collapse and not snap under immense pressure, allowing for the turtles lungs to collapse during a deep dive. The leatherback is the deepest diving reptile known today.

When diving to depths of greater than 30 meters, some humans experience a condition known commonly as "the bends" or decompression sickness. It is basically caused by nitrogen which has come out of solution in the divers blood and formed small bubbles of gas in the surrounding tissues. Decompression sickness is a serious and potentially fatal side effect of deep dives, but many marine reptiles and mammals frequently dive and resurface without any ill effect.
There are 2 possible mechanisms the plesiosaur could have developed to overcome decompression sickness during frequent dives.

Anatomical and Physiological adaptations of the Plesiosaur in respects to extended high pressure diving

Question: How did Plesiosaurs hold their breath for long enough to hunt at great depths in the ocean?

*an attempt to explore the possible anatomical and physiological adaptations that the plesiosaur may have utilised to ensure homeostasis during extended deep sea dives to hunt.*

the saurian family Plesiosauria included both plesiosaurs and pliosaurs, included some of the largest marine reptiles to inhabit the planet between the early jurassic and late cretaceous periods. Ranging in size from 2 to 15, and weighing up to 25 tonnes.
The origin of the plesiosaur is uncertain, though it is believed that the plesiosaur evolved from an intermediate form of Nothosaur (a streamlined fish-eating land reptile that used its forelegs to swim) The plesiosauria family is divided into 2 superfamiles; plesiosauriodea and pliosauroidea, having important differences as well as similar features.
Fossils of the jurassic plesiosaurs (plesiosaurus and cryptoclidus) and pliosaurs (liopleurodon) have been found in Europe, while remains of the cretaceous plesiosaur (elasmosaurs) and pliosaur (kronosaurus) have been discovered in asia/north america and Queensland, Australia respectively.

Members of the plesiosauriodea superfamily are described as generally having long necks and small heads, with forelimbs generally larger than the hindlimbs. The last surviving plesiosaur- elasmosaurus- had a neck as long as both its body and tail combined.

The pliosauroidea superfamily are descibed as having shorter necks but much larger heads, with extremely powerful jaws. The pliosaur Kronosaurus was the largest of these, with jaws 3 meters long and more powerful than that of a tyrannosaurus.

Both superfamilies were efficient swimmers, but it is believed that their hunting styles were varied in accordance with their shape. it is believed that the plesiosauriodea gracefully "flew" through the water in search of prey, laying in wait, being highly maneouverable and able to turn on the spot instantaneously, in ambush of prey with their long necks. in contrast, pliosaurs were very agile long distance swimmers and would pursue much more active prey, hunting by the chase.

as plesiosaurs hunted at great depths, and for long periods of time, they would have had to develop specialised anatomical and physiological traits to withstand the immense pressure and energy debt that such diving put on their bodies.

just a few corrections.

a few corrections on my previous post.

its green turtles i was thinking about, and they can stay underwater for about 6 hours on one breath, their heart beats only once every nine minutes. thats some pretty hardcore yoga moves right there!!!

an interesting parallel between the plesiosaur's flipper structure and plane wings... brings up some valid points on how they hunted their prey
http://search.epnet.com.gateway.library.qut.edu.au/direct.asp?an=6340339&db=afh

going to be studying alot this weekend after space-tribe, preparing a rough draft for my final assignment... so more to come this weekend!

well, first off, some thoughts on breathing

my current assignment is to research the mechanisms of how a pleisiosaur holds its breath for such a long time during pre-historic deep sea dives.

well, ive come up with a few thoughts on the subject, and these thoughts have mainly to do with turtles.
now i hear you say a pleisiosaur is not a turtle, that may be the case but consider how similar their body structure and environment is.

they are most definitely related!

leatherback turtles, the deepest diving turtles of the sea, have some ingenious ways of holding their breath.

some basic background:- cells need oxygen and nutrients to survive, grow and regenerate. the nutrients come from the food we eat (in the case of both the pleisiosaur and leatherback, this comes from the fish they eat) and oxygen comes from the air we breathe. here comes the paradox: if cells need a constant supply of oxygen to keep alive, and hence catch the fish, how does one catch fish underwater when there is no air to breathe? and how does one utilise the oxygen from the single breath of air in the lung and keep the cells oxygenated for such a long time?

firstly, its got to do with the lungs (of course)
the deeper you dive, the higher the water pressure is on your body, and hence your lungs. if you cant keep your lungs inflated then they will collapse from the immense pressure of the sea. The leatherback turtle achieves this balance of pressures by pushing the air out of its lungs to non-respiratory parts of their respiratory system (i.e. the parts that arent involved in gas exchange processes) namely the bronchii.

secondly its got to do with the heart.
parallels with hibernation occur here. during a deep dive, the heart rate of a leatherback decreases dramatically, down to around 1 beat per 90 seconds (i could be wrong with the rate/time here, will find out) Now, when your heart rate decreases, so does your rate of breathing. Consider this... if i could somehow slow my heart rate down to 1 beat per 90 seconds, i could stay underwater for 10 minutes on 7 heartbeats. By taking a deep breath and holding it, and taking my pulse, its quite obvious to me that i can hold it for more than 7 beats (just counted 42 beats for 30 seconds)
so if i was to slow my heart rate down to 1 beat per 90 seconds, i would have just been able to hold my breath for 63 minutes. This would be an extremely useful ability to have if you were travelling through say, sydney or many parts of paris.

{now theres something about the heart to do with baroreceptors here (a bunch of nerves around the (?) aorta that detect pressures and adjust the heart rate accordingly) that i will elaborate on a bit later when ive researched its mechanisms a bit more.}

and thirdly, its got to do with your haemoglobin, baby!
leatherbacks have the highest amount of haemoglobin and myoglobin of any reptile. (which means that they have ALOT more than us inefficient humans do) and twice the blood-oxygen carrying capacity of every other turtle. The more oxygen you can carry, the less breathing you have to do, and the more time you can spend in the deep blue chasing fishies.

thats all for now...

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this is the area i will be using to post my thoughts and explorings on stuff that ive come across at uni, in books or online.

consider it a learning tool for me, and some good reading for yourself.