Biology

Because the Permian Extinction depopulated the world’s oceans, some reptiles took to the water to become the first air breathing hunters in the oceans. They took on a variety of forms and some of which, including the ichthyosaurs and plesiosaurs, rose to the top of the food chain.

Ichthyosaurs

The ichthyosaurs were a group of air breathing, marine reptiles that resembles present day dolphins or tuna. They first appeared 245 million years ago and survived and thrived through the Mesozoic era. Ichthyosaurs averaged between 2-4m in length, with some species smaller and others growing much larger.

Ichthyosaurs ranged widely in size, and survived for so long, that they are likely to have had a wide range of prey but it is suggested that most commonly they fed on small, fast moving prey. Typical ichthyosaurs had very large eyes which were protected inside a bony ring, suggesting that sight was a predominant sense. They may have hunted at night or at great depths where visibility was reduced and pressure was increased.

They had a small head, long, slender jaws, which were filled with numerous, pointed, closely set teeth. The jaws were fast snapping and once prey was caught between the mesh like teeth, it was swallowed whole.

Ichthyosaurs had fin like limbs which they used, possibly, for directional control. Their propulsion would have come from their powerful, shark-like tail.

Ichthyosaurs were viviparous (gave birth to live young). This is evident as some fossils were preserved with young inside the abdominal cavity.  Although this lead to a debate among scientists as to whether ichthyosaurs were cannibals, it was widely regarded that the smaller ichthyosaurs were embryos.

 

Shastasaurus

Shastasaurus was a genus of Ichthyosaur that lived in the Triassic period. One of the largest marine reptiles ever found, it differed greatly from its cousins. Shastasaurus had a slender body, a very short toothless snout, and so it was proposed that it was a suction feeder, mainly of soft bodied cephalopods although current research may discredit this hypothesis.

Plesiosaurs

The plesiosaurs were a group of marine reptiles that were air breathing, viviparous and had a worldwide ocean distribution. They evolved from the nothasaurs in the Triassic and survived up until the K/T extinction 66 million years ago.

Plesiosaurs are divided into two groups: Plesiosauroids (plesiosaurs), with long, snake-like necks, tiny heads, and wide bodies, and the Pliosauroids (pliosaurs), with large heads, short necks and very strong jaws.

 

 

Plesiosauroids

For a long time it was speculated that the very long necks of these reptiles were used as an underwater fishing apparatus. It was believed that the animal would swim along near the surface holding its head and neck out of the water and then use them to spear prey underwater. However it has become evident that the head and neck of the plesiosaurs were not strong or flexible enough to have been used in this way. Even the largest plesiosaurs, reaching up to 12m in length, were quite benign fish feeders. Despite their sleek body plan, the plesiosaurs were not, by far, the fastest of the marine reptiles and most would have been out swam by ichthyosaurs.

Pliosauroids

As a general rule the pliosaurs of the late Jurassic and cretaceous were larger, stronger and more aggressive than their plesiosaur cousins. Similar to their underwater neighbours, the prehistoric sharks, it is believed their diet consisted of fish, squid and other marine reptiles.

Leipleurodon was a pliosaur that reached lengths of 12m. An ambush predator, four strong paddle-like limbs suggest it was a strong swimmer. It was the apex predator of the Middle to Late Jurassic seas that covered Europe.

References:

Dinosaurs: a concise natural history, Fastovsky, D. E., Weishampel, D.B., Cambridge; New York : Cambridge University Press, 2009. ISBN; 9780521889964.

Dinosaurs, spitfires and sea dragons, McGowan, C., Cambridge, Mass: Harvard University Press, 1992. ISBN: 067420770X.

Motani, R., 2009. The Evolution of Marine Reptiles, Evolution Education Outreach, 2, 224–235.

Marine reptiles: Adaptions, Taxonomy, Distribution and Life Cycles, Marine Ecology- Bertolero, A., Donoyan, J., Weitzmann, B., Encyclopedia of Life Support Systems.

Jiang, D., Motani, R., Hao, W., Schmitz, L., Rieppel, O., Sun, Y., Sun, Z., 2008. New primitive ichthyosaurian (Reptilia, Diapsida) from the Middle Triassic of Panxian, Guizhou, southwestern China and its position in the Triassic biotic recovery, Progress in Natural Science , 18, 1315-1319.

Vertebrate Palaentology, Benton, M.J., HarperCollins Academic, London, W68JB, UK, 1990. ISBN: 0-00-302092-4.

Images:

http://www.prehistoric-wildlife.com/

http://savalli.us/BIO113/Labs/09.Reptiles.html

http://carnivoraforum.com/topic/9762542/1/

http://alreadyanswered.org/q/sea-dinos/plio/

Video Clip:

Taphonomy

Mesozoic Marine Reptiles – ichthyosaur Taphonomy

Only a small part of what once existed was buried in the ground: only a small part of what was buried has escaped the destroying hand of time: of this part all not yet come to light again: and we all know only too little of what has come to light has been of service for our science. (O. Montelius 1888:5)

Taphonomy is the science of the laws of burial or embedding. This includes decomposition, post-mortem transport, burial, compaction, and other chemical, biological or physical activity which affects the remains of the organism.

Vertebrate Preservation

Being able to recognize taphonomic processes that have taken place can often lead to a better understanding of paleoenvironments and even life-history of the once-living organism. Taphonomy is used to explore how and why organisms are preserved

Exceptional Preservation:  Lagerstätten Deposits Posidinienscheifer
Formation Germany

Example of a female Stenopterygius. Embyros are within the skeleton. Disarticulation of foetus bones outside the parent body suggests possible implosion.

Conditions for burial:

The Formation comprises of Black Shale of up to 150m thick, laminated and show poor bioturbation. These shales attributed to a stagnant basin forming deposits. The environment would have been formed from oxygen poor bottom waters which were anoxic and inhibited bacterial decay allowing for preservation through diagenic purposes.

Anoxia:

The anoxic or disoxic water conditions of the time would help explain why there is so many ichthyosaur and marine vertebrate found among this sequence. Conditions would have been hostile preventing the opportunity for scavengers to disturb the embedding of the ichthyosaur within the soft sediment.

Rapid Burial:

The soft or “soupy” sediment was fundamental in the preservation of vertebrate fossils within the shale formation. This allowed the organism to sink fully articulated. It is thought that the Sternopterygius was only partially covering sediment. Lending for no preservation and decay of soft tissues (ligaments, cartilage) holding the bones. The remains would have remained this way for some time.

Soft Tissue Preservation:

The Posidonian deposits have also been important because of the rare soft tissue deposits among the Ichthyosaurus fossils. These soft tissue preservation indicated they possessed pectoral fins, dorsal fins and broad tails.

Articulation and Disarticulation:

This female Stenopterygius fossil has puzzled scientists. Assuming that the fossil fell to the sea floor postmortem, how is it that a fossil could become preserved nearly fully articulated, and almost in its anatomical position while the embryos have become disarticulated and dispersed around the parent body?

Posodian Shale Formation

Theories for exceptional preservation:

1. Bottom current caused smaller bones to be moved. The disparity with this theory is that smaller parent bones would also have been moved by bottom current i.e. phalanges (finger bones) on paddles.
2. Carcass explosion (Reisdorf et al. 2012). The gases that built up from decomposition could have caused the Stenopterygius to explode thus expelling the embyros disarticulated. The disparity with this theory is that the environment in which the ichthyosaur died in would have made it near impossible to explode. Assuming that the remains sink to death (150m), explosion at this depth would defy the laws of physics as the pressure surrounding the remains would have been much greater than the pressure within the remains. It is also thought that putrification (decomposition) slows with the drop in sea temperature.
3. Carcass Implosion (Van Loon 2013) Loon agreed with Reisdorf that the corpse would sink and decompose. However, he proposed that the putrification gases would build up and that a wall that was damaged or scavenged would erupt. Water rushed in due to the pressure difference inside and outside the carcass. Turbulent water removed some of the embryos but not strong enough to affect parent body. Small embryo remains were moved outside of the body.

Van Loon A. J 2013: Ichthyosaur embryos outside the mother body: not due to carcass explosion but carcass implosion.

References

Books:

Paleoecology: Ecosystems, environments and Evolution. Patrick J. Brenchley and David A.T. Harper

Vertebrate Taphonomy: R.Lee Lyman

Introduction to Paleobiology and the Fossil Record: Michael J, Benton and David A.T Harpers:

Scientific Papers:

Van Loon A. J 2013: Ichthyosaur embryos outside the mother body: not due to carcass explosion but carcass implosion.

Reisdorf et. al 2012: Float explode or sink: post-mortem fate of lung breathing marine vertebrate.

Images:

Taphonomy Diagram – Adopted from A.K Verhrensmeyer & S, M Kidwell 1985

Stenopterygius – Van Loon A. J 2013: Ichthyosaur embryos outside the mother body: not due to carcass explosion but carcass implosion.

Podisonian Shale – Universitat Tubingen Manuela Aiglstorfer

Fossil Locations

Uniformitarianism “the present is the key to the past, James Hutton observed the processes of the earth, in 1785 

pic courtesy of discovering fossils.uk image of  Lyme regis

 1811 the first complete skeleton of a Mesozoic marine reptile  ichthyosaurs was found by Mary and  Joseph Manning , the rock type found in  Lyme Regis UK, found within  laid down shale  of sediments roughly 200 million years , fast forward to 1982 fossils of the Ichthyosaur  found in north Japan within black slate, Holzmaden in Germany  fossils   found in a location called lagerstatten ,trend among all three locations is that all the species were found in similar shale rock of layered sediments that were once part of the sea, uplifted a now  inland. On February 12^th 2014 a fossil 248 million years old was discovered in Anhui China the fossil revealed it was that of an Ichthyosaur carrying two unborn inside her the fossils are from the Mesozoic era

image courtesy of Plusone, oldest recorded Icthyosaur fossils location China

 The Mesozoic world  

Roughly 190 million years ago  Pangaea divided into two main land mass areas, Gowanda to the south  and to the North Laurasia German scientist Alfred Wegner had a theory that the continents were joined into a super continent ,Wegner read about identical plant and animal fossils found on opposite sides to the Atlantic ocean,  the fossils he reckoned were too similar to be dismissed, plant fossils from the fern Glossepters were found in south America, India, Australia, Africa, and Antarctica the countries that largely made up Gowanda.   the  seeds were too large to of traveled by wind, in which the plant grew must  also of have been of similar climatic temperatures, his  hypothesis is the continents drifted apart and we now know that plate tectonics and convection currents  allowed this mass drift to occur. Rock formations of America and Ireland, Scotland and  Norway  as were the marks left by glaciations showing the unique direction added to the evidence.

pic courtesy of International Appalachian trail, the break up of the super continent pangea.

Jurassic period  was undergoing a period of transgression  the sea was rising and resulting in marine strata over  terrestrial strata,  chains of mountains along  France and the Armorican massif of Brittany were formed and  the Vosges of the black forest in Germany, there was some folding thrusting and metamorphism in the chains, along the Alps trenches were formed, from the Liassic seas plate convergence uplifting stratified layers of sedimentary rock give us  fossils from the black carbon rich shale’s of the Holzmaden in Germany, and that of plesiosaurs and crocodiles, In Asia high pressure low temperature metamorphism  terrains along subduction zones   around Japan,,the Kumikotan    the cretaceous Yezo grouping of sedimentary  rock   and it unconformity is in a fault contact with the Kamuikotan, the  limestone and shale location of  fossils found in 1982 .End of the Cretaceous  only in the Danian strata between Denmark to Sweden is the crocodilian Thorasosauras still swimming the Mesozoic waters, the reptiles  time had come to an end, the fossil of the Ichthyosaurs recently found in china a reminder of what once was.

 
small Island Japan, courtesy of Scientific American

References

  Bjorn, Kurten (1968). The Age of the Dinosaurs. London: Weidenfeld and Nickolson. p1-222.

Brenton,J,Micheal (2004). Vertibrate Paleontology. New Jersey: Wiley-Blackwell. p1-472

Briggs,E.G.Derek & Crowther,R.Peter (2001,2003). Paleobiology 2. Oxford: Blackwell Science Ltd.. p440-444.

Okada,H. & Mateer,N.J. (2000). Creataceous Enviornments of Asia. Tokyo: Elesevier. p155-167.  

Journals Montani,Ryosahe.&da-young,Jiang.&Tintori,Andre&Rioppel Olivier. (2014). Terrestrial Origin of Viviparity in Mesozoic Marine Reptiles Indicated by Early Triassic Embryonic Fossils. plosone. 1 (1), 1

Websites

Bbc nature prehistoric life.

Wikipedia, the Mesozoic

 

Evolutionary Trends

What is an evolutionary trend? It can simply be described as a directional change within a single or multiple lineages that either allows that taxa to survive or adapt to changing surroundings. An early major trend is the return to the oceans of all the below groups and how they adapted from walking land creatures to powerful ocean predators at the top of their niches.

There are 4 main groups of reptiles that returned to the oceans during the Mesozoic. The Mosasaurs (similar to modern day monitor lizards in shape), Sea Turtles (relatives to our current turtles), Ichthyosaurs (reptiles with a similar form to dolphins) & Plesiosaurs (from the super order Sauropterygia – like the loch ness monster). All of which had to adapt to the seas high salt content. Other general adaptations would have been the ability to dive and streamlining for speed to catch prey. Below is a brief outline of the 4 main groups and their adaptations they evolved during the Mesozoic (Triassic 252MA to the end of the Cretaceous 66MA) also commonly referred to as ‘the age of reptiles’.

The Mosasaurs

As Mosasaurs ancestors returned to the sea like all marine reptiles they needed to advance their locomotary skills. This came in the evolved form of webbing between their toes that would eventually become flippers/paddles for steering and a slender body assisted with the long tail with its fluke shape to create more whip for propulsion. This use of a tail for movement in water most likely evolved from the ‘waddle’ seen in nowadays lizards. A clip of their form and shape changes can be seen below.

Ichthyosaurs

Ichthyosaurs spanned the Triassic to the beginning of the Cretaceous. Over this timescale they evolved along two major trends. 1) A streamlined shape like a fish/dolphin, a perfect example of convergent evolution (where species of different ancestry have evolved similar traits to adapt to the same environment) The below image shows the evolution of the fusiform. 2) The largest eyes relative to body size of any animal that has lived. This has been hypothesized to be linked to their ability to dive at depth and therefore lack of light would require a larger receptical to take in as much light as possible for these predators.

Sauropterygia – Plesiosaurs

The first thing you may notice about Plesiosaurs are their necks and these come in a range of lengths from short to long and are a case of divergent evolution thought to be dependent on their feeding strategy.

But Plesiosaur’s main advancement that we can see in the fossil record are its flippers which become steadily larger and stronger for increased propulsion giving rise to shortening of the tail that becomes more of a rudder or for balance. The below image shows how structure of the limb evolved from the beginning of Triassic to the end of the Jurassic for the Sauropterygia super-group showing a more rigid/sturdier paddle for increased power.

Sea turtles

The formation of a shell by turtles for defensive purposes was an important adaptation to predators and one that lasts today in their modern-day relatives. The thickening of the ribs to eventually form plates can be seen in the below clip. Another trend we see in turtles is the loss of teeth from their early ancestors being replaced by the beak we see today.

References:

Dinosaurs, spitfires and sea dragons Christoper McGowan 1992 ISBN: 067420770X

Evolution of Marine Reptiles, Evolution Education Outreach.  Ryosuke Motani 2009

Marine Ecology – Marine reptile adaptations A Bertolero, J Donoyan, B Weitzmann – Extract chapter <http://www.eolss.net/sample-chapters&gt;

Correlated trends in the evolution of the plesiosaur locomotory system F. Robin O’Keefe & Matthew T. Carrano 2005

Images:
http://plesiosauria.com/evolution.php
http://www.karencarr.com/News/motani
http://www.prehistoric-wildlife.com/
http://www.arcadiastreet.com/cgvistas/earth

Anatomy

 

Anatomy of Mesozoic Marine Reptiles

Looking at 3 major groups of animals and some of the specific anatomical advantage s they possessed to exploit their habitat particularly regarding locomotion and feeding.

The Sauropterygia -‘winged lizard’ are a group of reptiles and among their groups include the plesiosaurs and the pliosaurs.

Fig. 1 -Artists impression of a  Pliosaur- Liopleurodon- attacking a plesiosaur
image courtesy of science photo library

Either long or short necked reptiles of varying size from 60cm up to 18m. Plesiosaurs being usually long necked where prey might be grasped as it swam past in a shoal of fish, whereas the shorter necked and much larger headed pliosaurs were a, larger, swifter, more aggressive creature and fed on larger prey most likely other plesiosaurs. The large head allowed it to develop bone crushing jaw muscles and combined with its long thick teeth it was a formidable predator. (Massar, 1988)

The fore limbs resembled great paddles that, it is believed, flapped similar to a modern birds wing where the motion in a figure of eight rotation  generates lift and thrust. By evolving a circular girdle, the huge pressures upon the flippers required for this type of manoeuvrability were equally distributed throughout the structure . (Fig 2)

Fig. 2 pectoral girdle development of the Sauropterygia.
Image courtesy of palaeos.com

 The Ichthyosaurs – fish reptile- were one of the earliest Mesozoic marine reptiles.

 

Fig. 3 Artist impression of an Ichthyosaur group.
image courtesy of BBC Nature

Varying in size and form, all possess a long rostrum usually packed with teeth, four fin like limbs, a fish like body and unmistakable large eyes. It is these, almost out of proportion, eyes that set the Ichthyosaurs apart from any other animal of the deep, presumably necessary for these creatures to be constantly alert for predators and prey. The more streamlined the body of the ichthyosaur -the larger the eye, and this has led to the conclusion that some of them were deep foragers (Perkins 2002).The limbs over evolutionary time have become more and more fin like but unlike the pliosaurs and plesiosaurs, these were not used for propulsion but for control similar to sharks and fish. Torpedo shaped bodies coupled with a spongy flexible bone structure, indicates that they were by and large fast and long ranging swimmers (De Buffrenil and Mazin, 1990) and may well have been endotherms (heat genearators). The vertebrae of the Ichthyosaurs are also very tightly packed only loosening up toward the tail indicating that they were powerful swimmers like modern day tuna.

The Mosasaurs-Meuse lizard, were probably the most lizard like of all the marine reptiles.

 

Fig. 5 Artist impression of a pair of Mossasaurs.
image courtesy of   Nobu Tamura, Wikipedia

The body is usually slimmer and proportionally longer indicating that these creatures were not as fast or nimble as the Ichthyosaurs or Sauropterygia and swam with an undulating motion similar to eels of today.  The limbs,generally fleshed webbing between elongated digits, performed like paddles and were  used for locomotion along with the tail which was fleshed towards the posterior.  (Masar,1988). Mosasaurs were squamates (snakes and lizards) possessing an intermandibular joint which, like snakes today, allowed  the lower jaw to dislocate from the cranium enabling consumption of large prey (Lee et al. 1999). New fossil evidence coming to light shows some that later mosasaurs stiffened the anterior portion of the body and the as those of ichthyosaurs or the girdle became larger and stronger and the limbs became like those of the pliosaurs, enabling sub-aqueous flight. (Lindgren, 2011).

 

References:

Bardet, N., Jalil, N-E., De Lapparent de Broin, F., Germain, D.,Lambert, O., Amaghzaz, M. 2013. A Giant Chelonioid Turtle from the Late Cretaceous of Morocco with a Suction Feeding Apparatus Unique among Tetrapods. Open access.

Carpenter, K., Sanders, F., Reed, B., Reed, J. and Larson, P. 2010. Plesiosaur swimming as interpreted from skeletal analysis and experimental results. Transactions of the Kansas Academy of Science 113: 1/2, 1-34.

De Buffrenil, V. and Mazin, J-M. 1990. Bone Histology of the Ichthyosaurs: Comparative Data and Functional Interpretation: Paleobiology 16: 4, 435-447.

Lee, M. S. Y., Bell, G. L. Jr. and Caldwell, M. W. 1999. The origin of snake feeding, Nature 400: 655-659

Lindgren, J., Everhart, M.J., Caldwell, M. W. 2011.Three-Dimensionally Preserved Integument Reveals Hydrodynamic Adaptations in the Extinct Marine Lizard Ectenosaurus (Reptilia, Mosasauridae). Plos one :6 ,11

Massare, J.A. 1988.Swimming capabilities of Mesozoic marine reptiles: implications for method of predation Paleobiology, 14:2,  187-205.

Perkins, S. 2002. Sea Dragons. Science News, 162: 8, 122-124.

Sander, P.M., Chen, X., Cheng, L., Wang, X. 2011. Short-Snouted Toothless Ichthyosaur from China Suggests Late Triassic Diversification of Suction Feeding Ichthyosaur. Open access.

Fossil Record

 

What is the fossil record? The fossil record is a history of life preserved through fossilized remains or imprints of living organisms, usually found in sedimentary rock.
Although it is incomplete, the fossil record gives us an overview of the different types of organisms, and their placement within the earth can give us an idea of various things such as date of life, geological conditions and environments, migration patterns and keys to evolution itself.
In the Mesozoic Era there is a wide variety of fossils which have been found, some exceptionally preserved. Most people are quite familiar with the reconstructed Dinosaurs which can be found in museums all over the world and are an extremely unique insight into that Mesozoic environment.
In the marine setting, there is some examples of life preserved that are equally as impressive.

 

Just recently in Majiashan, Anhui, China, 80 new Ichthyosaur remains were located in a series of excavations. One of the individuals that was discovered, of the genera Chaohusaurus aged approximately 248 ma old, was discovered to have been preserved during the act of giving birth to its young.

This astonishing find may well readjust the way  we think about the environment where these reptiles gave birth, due to the way it was preserved. The baby Ichthyosaur was found to be exiting the parent in a head first direction which indicates a land based birth rather than a sea based one.

The age of the embryos found are older than any found previous to that by approx 10 million years which may mean that live birth evolved much sooner than was thought. This type of rare and exciting find can clear up big concepts within a species and provides missing links of the overall picture that the fossil record paints.

 

   Chaohusaurus, Majiashan, Anhui, China

Terrestrial Origin of Viviparity in Mesozoic Marine Reptiles Indicated by Early Triassic Embryonic Fossils

References

http://novataxa.blogspot.ie/2014/02/chaohusaurus-terrestrial-viviparity.html

Systematics and the Fossil Record: Documenting Evolutionary Patterns - Andrew B. Smith 2.Terrestrial Origin of Viviparity in Mesozoic Marine Reptiles Indicated by Early Triassic- Embryonic Fossils

Ryosuke Motani,Da-yong Jiang mail,Andrea Tintori,Olivier Rieppel,Guan-bao Chen

Dinosaurs,Spitfires and Seadragons- Christopher McGowan