‘It’s head is as long as I am tall!’

       ‘What is it?’

‘Hmm…  A giant fish?  A lizard?  Don’t know.’

       ‘Well I know!  It’s a sea dragon!’ 

It is spring, 1811, the morning after a storm.  Mary is 12 years old, and fearless.  She edges across the cliff to where her brother is already working to free the fossil bones.  Fragments of weathered mudstone clatter down onto the beach.  The eye sockets of the huge skull are wider than the span of her hand.

---

During the 200 million years that dinosaurs roamed the land, the oceans were ruled by formerly land-dwelling reptiles.  Of these, ichthyosaurs adopted dolphin-like forms, plesiosaurs became sea lion-like, and mosasaurs occupied crocodile-like ‘ambush’ predator roles.

Of these, ichthyosaurs were arguably the most successful.  Their multiple adaptations to a fully aquatic life included huge eyes, a stiffened, fluked tail, and the ability to mate and give birth to live young in water.  Much as killer whales do today, these predators structured the marine ecosystem.  Many came to resemble the modern whales, dolphins and tuna fish that now fulfil similar ecological roles.  The convergence of body forms between some ichthyosaur species and tuna are particularly astonishing because these reptiles were air breathers.

Why did icthyosaurs evolve to look like modern marine animals?

Life in water poses a specific set of challenges.  These marine reptiles fulfilled similar ecological roles  to whales, and in time evolved body forms similar to these modern mammals.  This is known as convergence.

Like whales and tuna, ichthyosaurs were adapted for long distance energy-efficient swimming.  The respective horizontal and vertical strokes of both ichthyosaur and dolphin tails give an equally powerful ‘lift’ in both directions, propelling these animals forward in a near straight line.

A further example of this convergence is seen in the modern otter.  Unlike the Ichthyosaurs, both whales and otters had land-based mammalian ancestors with a vertically moving spine, giving them their bounding gait.  The whale-like ichthyosaurs moved their tail flukes horizontally, like a modern lizard.

Like whales and tuna, ichthyosaurs were adapted for long distance energy-efficient swimming.  The respective horizontal and vertical strokes of both ichthyosaur and  , propelling these animals forward in a near straight line.

What can modern animals tell us about ichthyosaurs?

Modern tuna and lamnid sharks have converged into a similar ‘deep water sprint-predator’ ecological role.  These long distance migrants move constantly at a moderate speed, except for short fast bursts when chasing prey.  Chunky vertebrae stiffen their bodies at their core, reducing sideways movements except at the narrow ‘hinge’ before the tail.  The continuously active red ‘cruising’ muscles either side of the spine are warm, in marked contrast to most other fish.  In combination with large tendons, these muscles work like pulleys, flicking the fluked tail from side to side. The surrounding white muscles give extra power during short ‘sprints’.

The ichthyosaur Stenopterygius was tuna-shaped with chunky stacked vertebrae.  This stiffened body form suggests that these reptiles converged on the cruise-and-sprint deep water hunting role of modern tuna and lamnids.

Did ichthyosaurs have warm muscles like whales and tuna?

The isotopic proportion between the heavy ¹⁸O and the light ¹⁶O oxygen (the ratio is given as d¹⁸O) in the bones of living fish and marine animals decreases as body temperature increases.  In principle we can use cold-blooded fish fossils as a ‘thermometer’ to indicate the water temperature, and compare this against isotope-predicted body temperatures for other fossil animals from the same rocks.

Oxygen isotope data allows us to infer that Jurassic plesiosaurs and ichthyosaurs had body temperatures of around 35⁰C, much higher than that of their environment.  This indicates that they could generate heat, and may well regulated their body temperatures independently of their environment (homeothermy).  Modern warm bodied marine animals have to conserve their body heat.  This means it is reasonable to infer that ichthyosaurs used similar methods such as a counter-current blood circulation  system and/or heat-insulating blubber.

What happened to the ichthyosaurs?

Ichthyosaurs dominated the world’s oceans for around 150 million years, but then disappeared from the fossil record after the mid-Cretaceous (around 95Ma).  The cause of their sudden extinction remains a mystery.  The empty ecological roles that this created were later filled by mosasaurs; relatives of modern monitor lizards including the Komodo dragon.  In turn these reptiles died out during the Cretaceous-Tertiary mass extinction (65Ma), making way for the later evolution of modern whales, dolphins and tuna.

Text copyright © 2015 Mags Leighton. All rights reserved.

References

Benson, R.B.J. and Butler, R.J. (2011)  Uncovering the diversification history of marine tetrapods: ecology influences the effect of geological sampling biases In Comparing the Geological and Fossil Records: Implications for Biodiversity Studies ( A.J. McGowan and A.B.Smith, eds).  Geological Society of London Special Publications 358, 191-208.

Bernal, D. et al. (2005)  Mammal-like muscles power swimming in a cold-water shark.  Nature 437: 1349-1352.

Brill, R.W. (1996)  Selective advantages conferred by the high performance physiology of tunas, billfishes and dolphin fish.  Comparative Biochemistry and Physiology, A 113, 3-15.

Brill, R.W. et al.(2005)  Bigeye tuna (Thunnus obesus) behavior and physiology and their relevance to stock assessments and fishery biology.  Collective Volume of Scientific Papers ICCAT 57, 142-161.  Online;http://www.iccat.es/en/pubs_CVSP.htm

Dickson, K.A. and Graham, J.B. (2004)  Evolution and consequences of endothermy in fishes.  Physiological and Biochemical Zoology 77, 998-1018.

Donley, J.M. (2004)  Convergent evolution in mechanical design of lamnid sharks and tuna.  Nature 429, 61-65.

Fröbisch, N.B. et al. (2013)  Macropredatory ichthyosaur from the middle Triassic and the origin of modern trophic networks.  Proceedings of the National Academy of Sciences, USA 110, 1393-1397.

Graham, J.B. and Dickson, K.A. (2004)  Tuna comparative physiology. Journal of Experimental Biology 297, 4015-4024.

Houssaye, A. (2013)  Bone histology of aquatic reptiles: what does it tell us about secondary adaptation to an aquatic life?  Biological Journal of the Linnean Society 108, 3-21.

Korsmeyer, K.E. et al. (1996)  The aerobic capacity of tunas; adaptation for multiple metabolic demands.  Comparative Biochemistry and Physiology, A 113, 17-24.

Lindgren, J. et al. (2007) A fishy mosasaur: the axial skeleton of Plotosaurus (Reptilia, Squamata) reassessed. Lethaia 40,153–160.

Lindgren, J. et al. (2011)  Landlubbers to leviathans: evolution of swimming in mosasaurine mosasaurs.  Paleobiology 37, 445-469.

Lingham-Soliar, T. and Plodowski, G. (2007) Taphonomic evidence for high-speed adapted fins in thunniform ichthyosaurs. Naturwissenschaften 94, 65-70.

Malte, H. et al. (2007) Differential heating and cooling rates in bigeye tuna (Thunnus obesus Lowe); a model of non-steady heat exchange. Journal of Experimental Biology 210, 2618-2626.

Motani, R. et al. (1998) Ichthyosaurian relationships illuminated by new primitive skeletons from Japan.  Nature 393, 255-257.

Motani, R. (2002) Scaling effects in caudal fin propulsion and the speed of ichthyosaurs. Nature 415, 309-312.

Motani, R. (2002) Swimming speed estimation of extinct marine reptiles: energetic approach revisited. Palaeobiology 28, 251-262.

Motani, R. (2010) Warm-blooded “Sea Dragons”? Science 328, 1361-1362.

Rothschild, B. M. et al. (2012)  Adaptations for marine habitat and the effect of Triassic and Jurassic predator pressure on development of decompression syndrome in ichthyosaurs.  Naturwissenschaften 99, 443-448.

Syme, D.A. and Shadwick, R.E. (2011)  Red muscle function in stiff bodied swimmers: there and almost back again.  Philosophical Transactions of the Royal Society, B 366, 1507-1515.

Thewissen, J.G.M. and Bajpai, S. (2009)  New skeletal material of Andrewsiphius and Kutchicetus, two Eocene cetaceans from India.   Journal of Paleontology 83, 635-663.

Pausing on the bank, the lead female gathers her herd, ensuring that she is followed before she plunges in. 

The smaller elephants swim, heads fully submerged.  Others walk at first.  All are snorkel-breathing through their trunks. 

A lone male follows at a distance.  He watches the tribe for a moment before he too enters the water.

---

Elephants are the largest land mammals, and are at home in hot, dry savannah environments. They migrate hundreds of miles in search of food, and when required will cross rivers, lakes, and even undertake marine excursions.  They are surprisingly strong swimmers, and are the only mammals able to snorkel.

This provides a vital evolutionary clue to their past.  Although we associate elephants with dry land, their bodies reveal clear evidence of an aquatic ancestry.

Fossils and DNA put elephants amongst the swimmers

DNA evidence:

Comparing DNA sequencesenables usto build molecular phylogenies, or ‘family trees’, defining the genetic relatedness between species.  These phylogenies show that elephants’ closest living relatives are the fully aquatic sea cows; the dugongs and manatees.

Fossil evidence:

Water molecules (H₂O) contain three isotopes of oxygen; ¹⁶O, ¹⁷O and ¹⁸O.  The lighter forms evaporate more easily, slightly enriching lakes and oceans with ‘heavy’ water.  This then can become incorporated into aquatic plants and the bodies of animals that graze on them.  Analysing the ratio of ‘light’ ¹⁶O to ‘heavy’ ¹⁸O isotopes (i.e. d¹⁸O) in teeth from fossils of the extinct elephant ancestor Moeritherium, show that they contain a higher proportion of ¹⁸O than teeth from land grazing animals.  This tells us that it ate mostly freshwater plants, suggesting its lifestyle was at least semi-aquatic.

Elephant embryos reveal adaptations to breathing under water

Elephants can snorkel because of their elongated trunks, and elastic connective tissues that fill the space between their lungs and their body wall.  Both of these features appear together early in the foetus, suggesting that they were important for survival of the elephant’s ancestors.

As in other large mammals, elephants’ blood pressure is high; this is needed to keep the brain well supplied with oxygenated blood.  Under water, the differences between the pressure of inhaled air (atmospheric; 0mmHg) and the blood pressure in the capillaries (150mmHg) would be enough to rupture blood vessels and the delicate linings of the lung.  Their elastic connective tissues act as shock absorbers, protecting the lungs against such damage.

Elephants’ testes are inside the body like other aquatic mammals

The location of a mammal’s testes is related to temperature.  Excessive heat or cold will kill mammal sperm (including humans), causing sterility.  Hence in puberty, boys’ testes descend into sacs that are slightly cooler than the interior of their bodies.  However aquatic mammals continue to carry their testes inside their abdomen to protect them from cold.  We would expect land-dwelling elephants’ testes to be carried in a scrotum.  Instead, their testes remain inside the body like those of aquatic mammals.

Early in the foetal development of both male elephants and dugongs, an artery develops which directly connects their kidneys and testes.  In contrast, the testes of most land mammals have a less direct connection, allowing them to descend into a scrotum during puberty.  This suggests that like modern dugongs, the elephants’ ancestors lack a scrotum.

Aquatic mammals including dugongs have a plexus of blood vessels that cool their internal testes and uterus through a counter-current heat exchange  mechanism.  Testicular cooling by a blood plexus has not yet been investigated in elephants; however in the absence of heat stress, they maintain a body temperature of around 34-36⁰C, which is similar to the temperature of the scrotum in most other land mammals.

Elephants ears close for swimming

After swimming we often have to shake our head to clear water from our ears.  However elephant ears have a unique sphincter-like muscle that closes the ear canal and prevents water entry when swimming.  This also creates a sealed ‘acoustic tube’, which works with the aerated bones and ‘acoustic fat’ lenses of the elephant skull to amplify low frequency ‘infrasound’.  Dugongs have similar aerated skull bones and fatty deposits; again implying that they share an aquatic ancestor.

Text copyright © 2015 Mags Leighton. All rights reserved.

References

Fowler, M.E. and Mikota, S.K. (eds) (2006) Biology, Medicine, and Surgery of Elephants.  Blackwell.

Gaeth, A.P. et al. (1999)  The developing renal, reproductive and respiratory systems of the African elephant suggest an aquatic ancestry.  Proceedings of the National Academy of Sciences, USA  96, 5555-5558.

Hildebrandt, T. et al. (2007)  Foetal age determination and development in elephants.  Proceedings of the Royal Society of London, B 274, 323-331.

Johnson, D.E. (1978)  The origin of island mammoths and the Quaternary land bridge history of the Northern Channel Islands, California.  Quaternary Research 10, 204-225.

Johnson, D.E. (1980)  Problems in the land vertebrate zoogeography of certain islands and the swimming powers of elephants.  Journal of Biogeography 7, 383-398.

Liu, A.G.S.C. et al. (2008)  Stable isotope evidence for an amphibious phase in early proboscidean evolution.  Proceedings of the National Academy of Sciences,USA 105, 5786-5791.

McKenchie, A.E. and Mzilikazi, N. (2011)  Heterothermy in Afrotropical mammals and birds: a review.  Integrative and Comparative Biology 51, 349-363.

O’Connell-Rodwell, C.E. (2007)  Keeping an ear to the ground: seisemic communication in elephants.  Physiology  22, 287-294.

Poulakakis, N. et al. (2006)  Ancient DNA forces reconsideration of evolutionary history of Mediterranean pygmy elephantids.  Biology Letters 2, 451-454.

Poulakakis, N. and Stamatakis, A. (2010)  Recapitulating the evolution of Afrotheria: 57 genes and rare genomic changes consolidate their history.  Systematics and Biodiversity 8, 395-408.

Seiffert, E.R. (2007)  A new estimate of Afrotherian phylogeny based on simultaneous analysis of genomic, morphological and fossil evidence.  BMC Evolutionary Biology 7, 224.

Springer, M.S. et al. (2004)  Molecules consolidate the placental mammal tree.  Trends in Ecology and Evolution 19, 430-438.

Weissenbrock, N.M. et al. (2012)  Taking the heat: thermoregulation in Asian elephants under different climatic conditions.  Journal of Comparative Physiology, B 182, 311-319.

West, J.B.  Why doesn’t the elephant have a pleural space?  Physiology 17, 47-50.

West, J.B. et al. (2003)  Fetal lung development in the elephant reflects the adaptations required for snorkelling in adult life.  Respiratory Physiology and Neurobiology 138, 325-333.

West, J.B. (2001)  Snorkel breathing in the elephant explains the unique anatomy of its pleura.  Respiration Physiology 126, 1-8.