‘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.

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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.

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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.

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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.

Adam Sedgwick (1785-1873) was Professor of Geology at the University of Cambridge, holding the Woodwardian Chair, and was one of the most respected geologists in England in the nineteenth century. It was an era when the major geological time periods were being defined, a task in which Sedgwick played a key role with his identification of both the Devonian and Cambrian periods. It was through the definition and clarification of the major geological epochs that the immense antiquity of Earth was recognised. This realisation was critical in the development of our understanding of Earth history because prior to this period, calculations based predominantly on the Bible had led to the prevailing wisdom that Earth was only around 6000 years old. It is due to the meticulous work of scientists such as Sedgwick, that we now know it is actually around 4.6 billion years of age.

Sedgwick’s crowning glory was the identification of a period of geological time that he named the ‘Cambrian’ (from the Latin ‘Cambria’, an ancient name for Wales). This interval – which spans the period from around 543 million years ago to around 505 million years ago – is significant because it is a period where we see a remarkable ‘explosion’ in the number and diversity of fossils in the geological record.

In the early 1830s, Sedgwick teamed up with fellow geologist Roderick Murchison to map the strata of Wales. Sedgwick identified the Cambrian by relying on the physical characteristics of the rocks in northern Wales, where Murchison identified another geological period, the Silurian (named after a Celtic tribe, the Silures), by concentrating on the fossil-bearing strata in southern Wales. In 1835 they presented a paper to the British Association for the Advancement of Science on their findings entitled On the Silurian and Cambrian Systems, exhibiting the order in which the older sedimentary strata succeed each other in England and Wales. The paper founded the Palaeozoic timescale that we know today. Click here to view chronostratigraphic charts from the International Commission on Stratigraphy (ICS).

However, controversy arose because Sedgwick and Murchison could not agree on where the boundaries between the Cambrian and Silurian lay. The crux of the problem was that Murchison had misinterpreted the age of certain rocks in several areas in the Welsh borders. This led him to claim that the rocks forming the baseline of his system belonged to the Lower Silurian, whereas Sedgwick argued that they should be placed in the upper part of the Cambrian system.

The dispute was eventually resolved by another geologist, Charles Lapworth (1872), who introduced an intermediate period (the Ordovician, named after another ancient tribe, the Ordovices) separating the Cambrian and the Silurian, and equivalent to the disputed “upper Cambrian-lower Silurian” beds. Each of these periods is now known to be characterized by distinct fossil assemblages and remain standard in geology. Sadly the acrimonious nature of this disagreement culminated in the Geological Society of London, of which Murchison was President, and Sedgwick was a past-president, refusing to publish any of Sedgwick’s papers on the subject.

Sedgwick also mentored a young Charles Darwin, who at the time was a fledgling naturalist reading theology at Christ’s College, Cambridge. Darwin accompanied Sedgwick on fieldwork to Wales and attended his lectures. Darwin’s endorsement by his mentors, primarily John Henslow but also Sedgwick, was instrumental in gaining Darwin a position on the famous voyage of HMS Beagle, which in turn was key to developing Darwin’s thoughts behind the groundbreaking On The Origin of Species. However, Darwin’s book had enormous theological implications and Sedgwick was notably anti-evolutionary in his thinking. He was a devout Christian and believed that acceptance of the idea of evolution would cause a breakdown in the moral fabric of society. After all, he would have no doubt reasoned, if we are simply descendants of monkeys what warrant do we have for our ethics? In a famous letter written to Darwin shortly after he received a copy of the first edition of On The Origin of Species, Sedgwick set out those fears as well as his appreciation for Darwin’s great work. Despite Sedgwick’s outspoken opposition to Darwin’s theory of evolution by natural selection, it is testimony to the open-mindedness of both men that they remained on good terms throughout Darwin’s lifetime.

Darwin clearly enjoyed Sedgwick’s mentorship and it is easy to understand why. Sedgwick was a charismatic speaker and his lectures were always well attended despite not being compulsory for students at that time. He gave a course in Cambridge which ran nearly every year from 1819 to 1870, by which time he was 85 years of age. As Colin Speakman (1982, p.88) notes, Sedgwick’s inspirational persona is summed up by a statement made in one of his lectures:

“I cannot promise to teach you all geology, I can only fire your imaginations.”

Most scientists are inspired by their own research, but the ability to capture that excitement and inspire scientific curiosity in others is a much rarer gift.

Sedgwick also stressed the importance of fossils as a means of determining relative age and was the first Cambridge scholar to earnestly begin collecting fossils for the University’s geological collection. Many of these specimens were acquired from other collectors, including a number from the pioneer fossil collector Mary Anning. The fossils that Sedgwick amassed built upon a collection of around 10,000 specimens which were bequeathed to the University of Cambridge in the will of John Woodward, which was recognized in the early eighteenth century as one of the most significant collections in the world. These are still housed in the Sedgwick Museum, a building that arose as the result of a wildly successful subscription after Sedgwick’s death, and forms part of the Department of Earth Sciences in the University of Cambridge.

Text copyright © 2015 Victoria Ling. All rights reserved.

References

Lapworth, C. (1872)  On the tripartite classification of the Lower Palaezoic rocks.  Geological Magazine 6, 1-15.

Levin, H. (2010)  The Earth Through Time.  Wiley.

Rupke, N.A. (1983)  The Great Chain of History. William Buckland and the English School of Geology (1814-1849).  Clarendon.

Sedgwick, A. and Murchison, R.I. (1835)  On the Silurian and Cambrian Systems, exhibiting the order in which the older sedimentary strata succeed each other in England and Wales. British Association for the Advancement of Science Report, 5th Meeting, 59-61.

Speakman, C. (1982)  Adam Sedgwick: Geologist and Dalesman 1785-1873.  Broad Oak Press, The Geological Society of London, and Trinity College, Cambridge.