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Skeleton Archives
Methuselah is said to have lived for 969 years between about 6000 and 5000 years ago, and he was, according to ancient biblical texts, the longest-lived person in history. In these texts, he was said to have come from a family of very long-lived individuals, being the grandson of Adam and Eve, and the grandfather of Noah. Or perhaps scholarly translation, or mistranslation, might have been involved, as totting up his years simply as months gives an age of just over 78, a more typical human lifespan.
Humans now typically live to be into their ninth decade, and those who reach 100 years are no longer so unusual. Indeed, as we write, the Italian Emma Morano has just passed away, at the age of 117. She was the last person alive who was born in the 19th century, her birthday being November 29th 1899. Emma Morano lived through extraordinary changes. She was a small girl celebrating her fourth birthday when Orville Wright took to the air for the first powered flight on December 17th 1903. She was a teenager of 14 years old when the assassination of Archduke Franz Ferdinand on 28th June 1914 precipitated the First World War, and she was approaching middle age—by normal human standards—at 39 years old when the German invasion of Poland on September 1st 1939 began the Second World War. When Neil Armstrong took the first step on to the surface of the Moon on July 21st 1969 she was approaching old age, but that is already nearly half a century ago. As Emma Morano’s mind took in all of these changes, her bones and teeth became an archive too, reflecting such things as the changing diet of Europeans over more than 100 years, the burning of fossil fuels by her fellow humans, and the explosion of atomic bombs by a few of them. One hundred and seventeen years is a long time from our personal perspectives, but to understand the profound changes to planet Earth over its more than 4-billion-year history, we have to dig much deeper and find archives of information—many in the skeletons of ancient organisms—that have preserved records over such a span of history.
Methuselah and Emma Morano notwithstanding, the skeletons of primates may not be the best archives of past environmental change. For a start, our nearest great ape cousins have similar lifespans to us: the bonobo, chimpanzee, gorilla, and orangutan all clock in at between 40 and 60 years. Some animal and plant species live for much longer. And, as they live, these organisms add to their skeletons, layer by layer, through life. In a good year, with a balmy summer and a rich harvest of food from the land or sea, their skeletons grow rapidly. In a bad year, when the food supply fails, growth may stall. For an animal like a coral or a bivalve mollusc, the changes to the environment around them might show as growth spurts or breaks in the skeleton. And if these growth breaks can be reliably matched between shells, there is the possibility to put together chains of years stretching back over centuries and millennia.
In taking such a journey backwards in time, beginning near the present, and finishing more than 550 million years ago, the nature of the evidence changes. At the beginning of the journey, some of the fossil organisms that one encounters still have living representatives. Later on, these will disappear. Further back still, the fossil archives have been buried, deformed by mountain-building processes, and their chemistry and mineralogy altered. And, in the Precambrian, there are few skeleton archives that can be used; it is difficult, though not entirely impossible, to glean such a picture of the past from such ancient rocks. One might begin, though, with a molluscan Methuselah that—had it not been for a lethal encounter with a shipboard freezer—would probably today have still been extending a quite remarkable lifespan.
A Mollusc for all Seasons
Arctica islandica is a mollusc that lives in the cold waters of the North Atlantic. It is colloquially known by many names, including ocean quahog, Icelandic cyprine, and ‘black clam’ (Figure 36). Ocean quahogs are edible, though they are said to have a very strong taste. They are harvested as shellfish along the east coast of North America. The animal grows quickly to sexual maturity, adding many increments to the shell after just a few years, the shell typically reaching about 5 centimetres in diameter. Thereafter, they grow much more slowly and some, over a very long time, reach a maximum size of about 12 centimetres. An ocean quahog collected in 2006, just to the west of the small Icelandic island of Grimsey, holds the record for the most long-lived animal ever recorded. It has been christened ‘Ming’,72 after the Chinese dynasty founded in 1368. Ming, along with many of its kin on the seabed, was hauled aboard the Icelandic fishery research vessel Bjarni Sæmundsson to meet an unfortunate end, especially after its distinguished 507-year lifespan. Ming was frozen on board the ship, and thawed out for scientific investigation back onshore. When the shells of these ocean quahogs were examined it was quickly realized that several of the others had also lived for more than 300 years. Ming was by far the oldest, though, born in 1499, and hence about midway through the 276-year long Chinese dynasty. For what seems like an eternity to humans, Ming quietly lived on the seafloor of the North Atlantic. Ming was already 121 years old when the Pilgrim Fathers passed to the south on their way to the Americas. When the colonies of the United States of America declared their independence from Britain in 1776, Ming was approaching its 277th birthday. The age of Ming the mollusc can be measured by the number of growth increments on its shell, and by comparing those patterns with other ocean quahog specimens and looking for overlaps in the patterns of growth, it has proved possible to extend this skeleton archive back yet earlier, to the mid-7th century, to Dark Ages Britain under Anglo-Saxon rulers.
Figure 36. Arctica islandica, a bivalve Methuselah from Iceland. Scale bar is in cm.
What stories do these venerable ocean quahog shells tell? One animal, collected from the waters of Iceland in 1868, had lain in the collections of the Zoological Museum at the University of Kiel, Germany for over 100 years, when Bernd Schöne, a palaeontologist at the Johannes Gutenberg University in Mainz, and his colleagues rediscovered it. This particular ocean quahog does not have a name, so one might here christen it Carl, after the founder of the museum in Kiel, Carl Möbius. Like Ming, Carl was also very old, and reckoned to be 374 at death. That takes Carl back to the later part of the 15th century, to about 1494, the year of birth of Suleiman the Magnificent, greatest of the Ottoman emperors. Schöne and his team set about analysing the shell of Carl in minute detail.
First, the team cleaned the shell so that they could examine and count its growth increments from 1494 to 1878. Next, they sampled the early part of the shell—when Carl was growing rapidly—drilling out carbonate from the skeleton to extract isotopes of oxygen and carbon that give information about sea temperature and food supply. Back in 1512, for example, when Carl was 18 years old, they could discern that this mollusc had lived in summer waters with a temperature of about 9 °C, while normally Carl lived in waters a little cooler, at about 6°C. A little over 300 years after Carl was born, telltale patterns in the shell showed slower growth during the years 1816 to 1818 that seem to reflect a reduced food supply, and perhaps also cooler surface waters then, in the North Atlantic.
In the Northern Hemisphere, 1816 is recorded as the ‘year without a summer’. The year before, in April, on the tropical east Indonesian island of Sumbawa, the volcano Tambora erupted, blowing away its uppermost kilometre and ejecting tens of cubic kilometres of lava and ash into the air. The explosive eruption was so loud that it could be heard in Jakarta nearly 1300 kilometres to the west. The eruption column of ash reached up to the stratosphere along with large amounts of the gas sulphur dioxide. These materials were then blown around the globe by high-level winds, the sulphur dioxide forming sulphur aerosols that eventually fell back to Earth to be recorded in Greenland ice as a sulphate-rich ice layer. There were yet broader climatic effects. The global dimming caused by the pollution in the atmosphere blocked out the sun’s light and made 1816 one of the coldest years in the past 500 years. In North America and Europe, crops failed and people starved. Joseph Mallord William Turner’s paintings of that year show a red glow in the sky that may literally reflect particles from the eruption in the upper atmosphere over England, while Lord Byron, his spirits lowered by the cold and gloom, then wrote his bleakest poems—and became a muse, too, for monsters, as we shall see later. In the North Atlantic, Carl silently recorded this chill weather in the slower growth of its shell.
Why does this environmental archive deep in the history of Carl’s skeleton matter to our story? It matters because when looked at collectively, the shells—exoskeletons—of these ocean quahogs from the North Atlantic provide a record stretching back perhaps 1400 years, long before humans made historical accounts of the weather or climate from this region. For their latitude, the waters of the North East Atlantic are warm, steeped in the heat of the Gulf of Mexico, from where these surface waters come, to be carried across the Atlantic by the great ocean conveyor, a system of currents that connects the ocean surface to its depths, and the deep Pacific with the Atlantic and Southern Oceans. Ocean quahogs may help to monitor the strength of this ocean conveyor system as it has changed over centuries in the North Atlantic, and hence help determine the controls on North Atlantic climate. Shells of this mollusc from Iceland show cooling waters in the 14th century that presage the interval of time in Europe known as the Little Ice Age, and this was perhaps caused by a weakening of the Gulf Stream. Again in the 17th century, their growth slowed, this time possibly associated with cooler seas and a reduction in sunlight called the Maunder sunspot minimum. This is the time when the Thames River in London regularly froze over in winter, so that frost fairs were held, uniting Londoners south and north of the river. The first of these fairs was in 1608, but the most famous one was during the severe winter of 1683–4, when the Thames upstream from (old) London Bridge was frozen over from late December to early February.
The ocean quahogs show yet another dip in growth during the early 19th century—bracketing the time of the last frost fair in London held in 1814—associated with the Dalton sunspot minimum, and later compounded by the eruption of Tambora.
These shells are eloquent about the sensitivity of the North Atlantic to climate change. The twin continental masses of North America to the west and Europe to the east help deflect and focus the warm waters of the Gulf Stream north-eastwards, feeding the air above with moisture that seeds the ice sheet of Greenland with snow. As these long-lived molluscs show, the climate here is subject to substantial and periodic changes between warmer and much colder spells. And as the fossil record is explored into deeper levels of time, this skeleton archive can be used to observe the Greenland ice sheet growing to cover much of the landmass of northern North America and Europe.
The Saw-tooth Foraminifera
The near-surface waters of the oceans often abound in a planktonic foraminifer called Globigerina bulloides, a species that was first described by Frenchman Alcide d’Orbigny in the early part of the 19th century. Although individuals of Globigerina bulloides have short lifespans, their antecedents have occupied the seas for several million years since the Pliocene Epoch. Globigerina bulloides builds its skeleton from calcium carbonate, growing a series of globular chambers, the last one being the largest. Long-dead specimens look like four interlocking balloons arranged spirally around a large central aperture, but in life its glass-like skeleton is covered in a protective mesh of spines. Though it is a small foraminifer, with a diameter of less than 1 millimetre, Globigerina bulloides is widely dispersed across the oceans in the zone that is penetrated by sunlight, and it is particularly abundant in regions rich in the phytoplankton it feeds on.
Like the ocean quahog, skeletons of Globigerina bulloides record the properties of the ocean around them. These properties include saltiness and temperature, but the foraminifer skeleton can also sense, from a distance of thousands of kilometres, the amount of ice at the poles. This may seem absurdly far-fetched: for how can a planktonic foraminifer living in the near surface waters of the tropical Atlantic Ocean off the coast of East Africa sense what is happening in the high polar regions? Yet the foraminifera possess exactly such sensitivity. Fossils of Globigerina bulloides tell stories of ocean waters from millions of years ago.
To grasp how the skeleton of a long-dead marine creature can do this, the passage of oxygen through the water cycle on Earth must be followed—that is, we must track what happens to the O in H2O, as that molecule moves across the planet. Oxygen is an abundant element, the third most abundant in the Universe. On Earth it makes up 21% of the atmosphere, 90% of the mass of the oceans, and just over half of the mass of the Earth’s crust. Oxygen occurs in three different stable forms—isotopes—all born within different layers of a star late in its life. Oxygen-18, written 18O, has ten neutrons and eight protons in its nucleus and is the heaviest of the isotopes, representing a little more than 0.2% of the oxygen in Earth’s atmosphere. 17O has nine neutrons and eight protons and accounts for a little less than 0.04% of that oxygen. 16O is the commonest form, accounting for 99.76% of the oxygen in the atmosphere, and has the same number of neutrons and protons, there being eight of each.
The ratio of two of these isotopes, 18O and 16O, can be traced from seawater, through water vapour, rainfall, rivers, lakes, and ice. Because water with the lighter isotope 16O more readily evaporates from the surface of the oceans, water vapour in clouds is relatively enriched in 16O. And conversely, following that evaporation, the water left behind in seawater or a lake is enriched in 18O. Water with the heavier 18O also tends to fall out of clouds more easily as rainfall, which is logical given that it is heavier. This means that as clouds move inland and sequentially lose more rain, the water remaining within the clouds becomes progressively enriched in 16O. As a result, snowfall at the centre of a large continent like Antarctica is enriched in 16O. When the ice sheets of Greenland or the Antarctic grow, they become long-lived reservoirs of 16O, whilst the oceans become relatively enriched in 18O. And that enrichment is then locked into the skeletons of foraminifera, and bivalves, and many other carbonate-making skeletal organisms.
The realization that seawater variations in 16O and 18O are preserved in the skeletons of ancient organisms, and can track changes in climate over great lengths of time, was a discovery made in the late 1940s by Harold Urey at the University of Chicago. Urey is perhaps best known in science as one half of the Miller–Urey team that demonstrated how complex amino acids could be synthesized in the Earth’s early atmosphere during electrical storms. But it was his work on the fractionation of oxygen isotopes (specifically 16O and 18O) between water and carbonate that would make his greatest impact on science, founding, in effect, the entire discipline of palaeoclimatology. It began with a paper in the journal Science in 1948 that reconstructed Cretaceous sea temperatures from the carbonate of fossilized skeletons of belemnites, torpedo-shaped cephalopods that lived in the Mesozoic seas.
Urey noted from the very beginning that in order to extract reliable ancient sea temperatures from fossil materials, a large and reliable dataset was needed. These fossils, too, must be perfectly preserved, as any change from their original composition would alter the primary chemical signal. In reality, much of the material from the fossil record has been changed, as skeletons are buried and subject to heating, pressure, and the migration of fluids through rock. The older the material is, the more problematic it becomes to interpret. Even so, it is possible to find pristine, and largely unchanged, fossil material even in very ancient rocks. Even where some change has occurred, a remnant environmental signal can often still be discerned.
Bivalves provide a very detailed record of climate—sometimes on a seasonal basis—over hundreds of years, and documenting changes over geological timescales requires a near-continuous record of the same or similar creatures making their skeletons for millions of years. And these skeletons need to lie undisturbed on the seabed on which they have fallen—and then to be buried by the ‘eternal snowfall’73 of countless subsequent generations of skeletons. Foraminifera are perfect for this, as they proliferate in huge numbers in the water column, and are preserved on a deep-sea floor that is undisturbed by the action of waves, tides, or currents. And there they accumulate, over millions of years, preserving a continuous record that can then be recovered by drilling through the ocean bed and recovering a core of sediment layers.
Although the oceanographic importance of foraminifera had been suspected at the time of the Challenger expedition in the 1870s, it was really through the work of three pioneer palaeoclimatologists, Jim Hays, John Imbrie, and Nick Shackleton,74 that their importance for palaeoclimatology was realized. Their work followed over 100 years of calculations on how changes in the Earth’s orbit might impact on climate, beginning in the mid-19th century with Frenchman Joseph Alphonse Adhémar. Later, Scotsman James Croll and Serbian Milutin Milanković would build on these ideas, recognizing three important orbital cycles: the eccentricity of the Earth’s orbit around the sun, operating on a 100 000-year cycle; obliquity, the changing angle of the Earth’s rotational axis, operating on a 40 000-year cycle; and precession, the wobble on its axis as the Earth spins, operating at about a 20 000-year cyclicity. Their calculations had shown that as a result of these orbital changes, the amount of sunlight reaching the two hemispheres of the planet would vary, and as a result this could be an important pacemaker of the growth and decline of ice sheets.
Hays, Imbrie, and Shackleton coupled their knowledge of these astronomical cycles with new geological data emerging from sediment cores produced as part of the Deep Sea Drilling Programme. They focused on fossils from two drilling sites in the southern part of the Indian Ocean between 40° and 50° latitude south. Their analyses were published in 1976 and showed, for the first time, physical evidence that the variations in the Earth’s orbit had indeed exerted a control on major changes in the extent of northern hemisphere ice during the past half million years. This was clearly displayed by the oxygen isotope data they extracted from Globigerina bulloides skeletons which, for very many millennia, had been sinking from near-surface waters to accumulate in sediments of the southern Indian Ocean. When these data were plotted, they resembled an earthquake seismogram that seemed to increase its magnitude periodically. The jumps, towards more enriched 16O signatures in the skeletons, are quite sudden on the graph, and represent rapid changes to warmer seas and less polar ice that seem to be paced by the 100 000-year eccentricity cycle, with smaller, but still significant, changes on 40 000-year and 20 000-year patterns. These major jumps were followed by long-term declines in sea temperature and steadily increasing ice volume as more 16O was removed from seawater. Short, warm interglacial periods, then, were separated by long, complex, cold glacial periods—the ‘ice ages’ of vernacular speech. The overall pattern mapped out a ‘saw-tooth’ profile that has been replicated in many subsequent studies, analysing the skeletons of many different foraminifera.
The pattern of change over the past half million years in the northern polar region, recorded in the Indian Ocean foraminifera, is part of a much longer story of global change. Oxygen data from foraminifera at many ocean drilling sites around the world were steadily amassed to show a detailed record of climate, with gradual cooling from 55 million years ago, interspersed with intervals of more pronounced warming and cooling. There is a particularly big jump in the isotope values about 33.6 million years ago that marks a phase of rapid growth of the Antarctic ice sheet, whilst the gradient of the curve towards more 18O-enriched oceans steepens through the past 3 million years, recording the development of ice sheets in the northern hemisphere.
The periodic switches between more ice and less ice identified by the foraminifer record reflect another impact of climate change, in that they document swings in sea level that often exceed 100 metres. For the high peaks of the Himalayas such a sea level rise makes little difference, but for low-lying areas, like the coastal plain of the Brahmaputra, it is profound. For an atoll in the Pacific Ocean just a few metres above the sea surface, the change is irrevocable in the short term, shifting the ecosystem from land to sea in just a few years. How life responds to such sea level fluctuations may be gleaned from another archive of skeletons—in this case through a story of Indian Ocean atoll tortoises—a story, here, that does not rely upon subtle chemical signals. What matters with these tortoises is simply the presence or absence of their quite unmistakable gigantic skeletons.
The Return of the Giant Tortoises
Aldabra is an isolated place—a small group of four islands that form an atoll in the Indian Ocean, lying some 600 kilometres to the west of the East African coastline, and to the north of Madagascar. The atoll sits atop a giant submarine mountain and has a complex history that extends back to the Pleistocene Epoch, a history that shows the strong influence of changing sea level influenced by the 100 000-year-long cycle of the Earth’s orbit around the Sun, as it stretched out from a circle to an ellipse and back again.
The youngest of three main limestone layers that make up the atoll is called the Aldabra Limestone. It formed on the floor of a very shallow sea about 125 000 years ago,75 at the time of the last interglacial warm period when global sea level, at its peak, was probably a few metres higher than it is today. After that peak of warmth, climate cooled and sea level dropped until about 17 000 years ago, when polar ice had grown to its maximum level, drawing water out of the oceans so that sea level was about 120 metres lower than it is today. This was followed by a similar amount of sea level rise to its present level about 5000 years ago, as the ice in the polar regions melted in the present interglacial phase. The present rocky surface of the atoll is currently emergent just above sea level in the four small islands. On this land surface, one can see rocks that show alternations from limestone, formed under marine conditions when sea level was high, and fossil soils and erosion surfaces that formed when sea level was low. When global climate was warm, the land area of the atoll was periodically covered by the rising sea, and on at least two (possibly three) occasions the atoll was completely submerged, drowning any land animals then present. The animals that live on the islands today must have arrived by sea as colonizers, possibly by clinging to floating logs—a perilous journey, for sure, but one that must, from time to time, have successfully transplanted animals from the mainland or from neighbouring islands.
Despite its remoteness from land, and until recently its isolation from humans too, Aldabra is not a barren landscape. It is populated with a range of creatures, including most notably about 100 000 giant tortoises, Aldabrachelys gigantea (Figure 37). In captivity the tortoises can weigh in at 250 kilograms, though in the wild they are normally about 150 kilograms.
Figure 37. A female Aldabra tortoise (Aldabrachelys gigantea). This particular animal lives on Curieuse Island in the Seychelles.
Fossil bones of the giant tortoises of Aldabra show that their ancestors colonized the atoll perhaps 100 000 years ago. Yet older bones of giant tortoises are preserved in the fossil soils sitting atop the earlier limestone layers, together with the bones of crocodiles, lizards, and birds. Aldabra, then, has been reinvaded by the tortoises, and other animals, several times during the Pleistocene as sea level changed, in between the episodes of island drowning. The fossil record of the tortoises is, in its own way, an archive of climate and sea level change as effective as—and arguably even more evocative than—that provided by the tiny foraminifera in the deep sea oozes. Each time the Earth’s spin through space changed, ice melted in the polar regions, sea level rose, and the tortoises of Aldabra were cast adrift in the rising eastern Indian Ocean. However, they lived on elsewhere, and so could recolonize the island thousands of years later, as the Earth drifted further away from the sun’s heat again, and ice formed at the poles.
The Aldabra tortoise population is currently recovering after the 19th-century inhabitants of the islands nearly hunted them to extinction. But the populations of giant tortoises on adjacent islands through this region are now mostly extinct. When the sea once again submerges Aldabra—this time through our burning of fossil fuels, and not by the action of astronomical cycles—there may be few sources of tortoises to repopulate the Aldabra of the far future, when sea level falls again. New populations of Aldabrachelys introduced to the neighbouring Mascarene Islands (which have high volcanic peaks, and so cannot be completely drowned by sea level rise), though, offer some hope for the giant tortoise.76
Aldabra is a good example of how Earth’s past environments can be confidently reconstructed using the fossils of species that are alive today. These fossilized skeletons speak directly to us of such things as their ecology, the types of food they ate, and the temperature of the seawater they lived in. In going back more than 10 million years, though, few of the species living today are the same as those in the fossil record. Even if some modern species were present then, they may not have had the same environmental tolerances as their living relatives possess today. In yet older skeletal archives of Earth’s past environments, living species are absent from the record.
It is still possible, though, to discern relatives of living species in the older geological record. Coelacanth fossils are found in Cretaceous (and yet older) strata, while ginkgo trees thrived in the Jurassic. Further back, in the Ordovician seas of more than 440 million years ago, there are distant relatives of the living Nautilus, and yet further back, worms are amongst the common animals of the Chengjiang biota of the Cambrian Period that are related—though not identical—to animals still found today. Before that, and not long after the beginning of life in Earth’s oceans, there are the stromatolites, biogenic structures made by microbes that still form in a few places today. The similarities between organisms from many millions of years ago and living species suggest that there is a record of environmental information stretching far back into the deep time of Earth. Can these ancient skeletons be used as an archive of past Earth environment? One answer lies in the delicate growth lines of Devonian corals.
Keeping Time in the Devonian
The passage of Earth through the heavens has a strong influence on climate, and this is recorded in the skeletons of both foraminifera and tortoises. On a shorter timescale, a sense of the Earth’s motion may be obtained simply by watching the sky at night, as the stars and planets drift across the heavens. This motion, the Earth’s spin momentum, began long ago, at the time that our solar system formed from a spiralling disc of condensing gas and dust. But the energy of that spin is gradually dissipating through the tidal effects generated by the Earth–Moon system, with part of that continuous, ongoing energy expenditure being felt as the ebb and flow of the tides. As time passes, the average day becomes slightly longer, so that over each passing century, the day becomes about 2 milliseconds longer. Eventually, the Earth’s spin will slow, and the planet will begin to wobble—quite dramatically—on its axis, like a spinning top losing its energy. As this rate of dissipation of energy can be calculated, it is possible to estimate the length of a day and how many days there used to be in a year, going backwards in time. The further back, the faster the Earth should have been spinning relative to today, and the shorter each day would have been.
The skeletons of many marine creatures are known to record seasonal changes in weather and tides, as evident from the ocean quahogs of the North Atlantic. But if the Earth was spinning faster in its younger life, then there should have been more days in a month, and more months in a year. Could these changes be captured in the growth patterns of very ancient skeletons? Two geologists working in the 1960s, Colin Scrutton, then at Oxford University, and John West Wells at Cornell University in New York State, set out to provide a test of the changing motion of the Earth using fossils. Wells was accustomed to studying the growth patterns of modern corals, having worked in the Pacific Ocean during the 1940s, including on Bikini Atoll, the site used for testing nuclear bombs by the US military. The two men examined well-preserved fossil corals from the Devonian Period, a time stretching back from 359 to 419 million years ago.
Corals grow their skeletons from calcium carbonate, materials that are readily available in seawater, adding new layers that appear as a series of growth lines in the wall of the coral skeleton. Wells and Scrutton totted up the number of growth lines on the skeletons of an extinct group of corals called the rugose corals (Figure 38). These corals mostly lived a solitary existence on the seabed, never forming the giant reef systems of modern corals. But many individual rugose corals nevertheless grew large, to look like inverted horns of a Viking’s helmet, and were clearly long-lived organisms. Though rugose corals were common in the seas of the Palaeozoic Era, they had become extinct at the end of the Permian Period, about 252 million years ago and so the question remained, could a skeleton belonging to an extinct group of corals from more than 360 million years ago really record the length of a Devonian year? Wells’ careful examination of the growth patterns in the coral skeletons was published in the scientific journal Nature in 1963,77 and it suggested a Devonian year of about 400 days. A year later,78 after corresponding with Wells, Scrutton confirmed these observations showed, based on fossils from both North America and Europe, that rugose corals from the Devonian Period added increments to their skeletons in groups that reflected lunar months, with an average of 30.6 days, and therefore with 13 lunar cycles in a year.
Figure 38. The rugose coral Caninia, from the Carboniferous of County Sligo, Ireland. From tip to tip the specimen is 10 cm long.
Could these techniques for discerning ancient months and years be taken back yet further in time, into the Precambrian when the range of skeletons available for analysis dwindles? Some scientists suggested that the calcium carbonate layers of stromatolites might reflect daily growth layers, or responses to tides, or both. Suggestions of a Precambrian year with about 425 to 450 days have been made, but there is no consensus on whether stromatolites really do record these trends.
The jury is out on the viability of stromatolites as timekeepers, but this does nothing to devalue the work of Wells and Scrutton. Their papers spurred the geological community to examine a whole range of different fossils for their potential environmental signature. Most importantly, the two men’s coral work showed that even when organisms were long extinct, the fossil evidence from skeletons retained a powerful narrative of astronomical influence. The field was now open to interrogate the climate signature of some of Earth’s most ancient animal skeletons.
The Warm Seas of the Ordovician
It soon proved obvious that fossil corals were not the only useful archive of environmental change in the Palaeozoic seas, and that skeletons made from a diversity of different materials, including carbonate, silica, apatite, and complex organic materials, might yield important stories too. Mostly, analysis showed that the original chemistry of these ancient fossils had been changed by heat and pressure, as they lay deeply buried in the Earth’s crust, the original minerals being recrystallized and the original chemistry being scrambled. However, amongst these ancient fossils, the teeth-like conodonts were seen as more resilient to such change, and scientists sought to use them to measure the temperature of the seas in which these extinct early vertebrates lived.
Modern tropical oceans typically have sea temperatures of around 30°C, though in a few more land-locked seas, temperatures may rise a little higher. Warmed by the sun’s rays, the heat of the tropical seas is carried to higher latitudes by ocean currents, and by the air circulating above. This redistribution of heat in the open oceans helps prevent the tropics from getting too hot, and the polar regions from getting too cold. This engine of ocean heat transfer seems to have functioned even during the global greenhouse conditions of Cretaceous and Paleogene times, with tropical sea temperatures only sporadically rising much above 30°C.
Hays, Imbrie, and Shackleton (and their many colleagues) had plotted a clear path in establishing the climate history of the time since the dinosaurs. However, that path became more overgrown as scientists looked deeper into the geological record—and then, seemingly, vanished altogether. For all of the marvellous fossils recovered from the early Palaeozoic strata of the Burgess Shale and Chengjiang Biota, the picture of early life that these provided was accompanied by scant information on the degree of warmth of the seas in which these animals lived. Then, about a decade ago, a team of scientists led by Julie Trotter, a geologist at the Australian National University, set out to clear this path. They examined the oxygen isotope signatures from Ordovician conodonts living in the tropical regions between 443 and 489 million years ago.79 To make their calculations they had to make some bold assumptions about the chemistry of Ordovician seawater. And they also had to be sure that the conodonts had not been recrystallized in the more than 440 million years that had elapsed since they formed. Trotter and her team worked to overcome these problems, and found a dramatic trend, with the chemistry of the oldest conodonts suggesting surface ocean temperatures of about 40°C. These very warm tropical seas seem to have persisted for several million years, and they may have characterized much of the Cambrian Period before. Only some 25 million years after the beginning of the Ordovician Period did the conodonts begin to show tropical sea temperatures more like typical modern values of around 30°C.
The cooling trend that Trotter and her colleagues had identified coincides with a great flowering of marine life that is called the ‘Great Ordovician Biodiversification Event’, or, as it has come to be known now by palaeontologists, ‘GOBE’. This event did not introduce new types of animals, but rather was a diversification of species of the major animal groups that had originated in the late Precambrian and Cambrian seas. GOBE is also associated with an increase in creatures that used calcium carbonate to make their skeletons. This trend can be clearly seen by, say, examining a kilogram of sedimentary rock from the early Ordovician and noting that it might contain the skeletons of a few ostracods belonging to a couple of species. In contrast, a grab sample of fossilized seabed from some 30 million years later will typically contain a thousand or so ostracod skeletons, belonging to 30 or more species. GOBE also involved a race to colonize the plankton, with skeletonized groups such as the nautiloid cephalopods, arthropods, and graptolites all becoming well established and diverse. The changes to the ocean ecosystem that ensued increased the food supply for animals living on the seabed, as larger zooplanktonic animals, such as arthropods, delivered a continual stream of organic-rich poo to the seabed. This rain of nutrients, which has been termed the ‘biological pump’ (oceanographers also refer to it as the ‘faecal express’), may have had a fundamental effect on the ocean’s carbon cycle too: one that may have helped to cool the Ordovician seas. The pump forms a major part of the Earth’s life support system, exporting carbon from the surface to be stored in the deep ocean, and so it therefore prevents carbon, as carbon dioxide, from building up to excessive levels in the atmosphere.
The increasing range of planktonic creatures that occupied the Ordovician seas might therefore be linked with the cooling ocean temperatures. These creatures would have helped the biological pump to function by trapping carbon in their bodies and then delivering this to the seabed, in turn reducing the level of carbon dioxide in the atmosphere. The negative greenhouse effect so produced would have cooled the oceans too—and so put in place the conditions for the huge diversification of life in the sea that was GOBE.
Sea temperatures more or less then stabilized for some 20 million years, but at the end of that period the Earth was plunged into a severe icehouse climate state. As ice grew enormously on South America and southern Africa, which were then on the South Pole, sea level dropped precipitously, exposing the continental shelves and triggering the first of the big five mass extinctions of the past half-billion years. The cause of this event is still mysterious, but current suspicions do not place animal bodies, and animal skeletons, as a principal cause of cooling. Rather, large-scale collision of continents, the uplift of giant mountain chains, and changing patterns of ocean circulation are considered possible causes.
Such climate riddles are bound up with ancient geography—and the changing patterns of the geography of our planet were, famously, to provide a cornerstone of plate tectonic theory. As oceans open and close, and as continents are separated and coalesce once more, the animals and plants caught up in these planetary changes respond by changing their evolutionary pathways. Their skeletons are an archive of the movement of the continental and oceanic plates too.
Skeleton Coasts
In the 16th century, the great Flemish cartographer Abraham Ortelius noted the good fit of the continents of South America and Africa (Figure 39). He was able to do this because his maps were the best of their kind in the world, being founded on more than a century of trans-Atlantic shipping. Ortelius thought that the two sides of the Atlantic had been ripped asunder by some catastrophic event, but he did not have the geological evidence to support this notion, not least because the science of geology had not yet been born. The idea that the continents might somehow move apart at the surface of the planet found support slowly, as geological data grew gradually. And only in the early part of the 20th century did it resurface again as a serious idea, through the work of German scientist Alfred Wegener. It was then in the mid-20th century, when technology at last caught up with Ortelius’ insights, that physical evidence was found that oceans could widen.
Figure 39. Ortelius’ world map of 1570.
The 1960s were a heady time for geologists. The decade saw the publication of Marie Tharp and Bruce Heezen’s map that clearly showed for the first time the structure of the deep ocean seafloor, revealing a gigantic line of submerged mountains, of the mid-ocean ridge, extending down the middle of the Atlantic. In tandem, Canadian John Tuzo Wilson and colleagues recognized that ocean crust showed symmetrical magnetic stripes, in parallel on either side of these mid-ocean mountains. These stripes, like a gigantic barcode, had been detected from ship-borne magnetic surveys, and they showed conclusively that new ocean crust, formed at the mid-ocean ridges, as it cooled, fossilized a remnant of the orientation of the Earth’s magnetic field within it. As newly formed crust moved away from the ridge, the crust forming behind it recorded the periodic switches in the polarity of the Earth’s magnetic field, as the north and south magnetic poles changed places.
It took until the mid-20th century to understand how the topographic and magnetic properties of the deep ocean reflected the movement of oceans and continents. However, the fossil archive that was to provide its own stories of evolving oceans and continents was already well established, and had been quietly and systematically accumulating since the mid-19th century.
Amid the majestic mountains and glens of Scotland, fossils from hundreds of millions of years ago showed that the rocks were completely different from those of similar age in England and Wales. Therefore, long ago, these two regions of the today’s British Isles must have been separated by a formidable geographical barrier. Little sense of this geographical dislocation is apparent on the short drive from the English Lake District in Cumbria to the hills of the Southern Uplands of Scotland. Nevertheless, the fossils clearly show that 500 million years ago, these landscapes were an ocean apart.
The great ocean that separated these land masses existed over more than 100 million years, from its birth in the late Precambrian, to its demise in the early Silurian Period. Its story is recorded in the skeletons of animals in rocks from either side of this great seaway, in North America and continental Europe, and its demise, when those land masses finally collided, can be seen in the mountain chains of the eastern United States and Canada, the highlands of Scotland, and those of Norway too.
Two pioneering 19th-century geologists of the British Geological Survey, Benjamin Peach and John Horne, first developed these ideas. ‘Peach and Horne’ are legendary in the history of British geology, particularly for their masterly work in unravelling the structure and history of the ancient, tectonically deformed rocks of the remote northwest Scottish mountains. The two geologists complemented each other near perfectly. Ben Peach, a genial man, was inspirational in the field, having an extraordinary ability to intuit the complex three-dimensional structures of these rocks, and draw them out as sketches and geological cross-sections—but he was often disorganized, slow with his administrative duties, and was reluctant to put pen to paper. John Horne was logical, organized—and a good and prolific writer. Between them, they produced an account of these rocks that is still the starting point for all subsequent studies.
Peach and Horne noticed how the Scottish bedrock was more akin to that of Greenland and North America, now thousands of kilometres away across the Atlantic Ocean, than it was to that of its English neighbour. This far northern landscape is founded on Lewisian Gneiss—ancient, highly deformed rocks named after the Hebridean island of Lewis that are now known to be nearly 3 billion years old. There is nothing like these rocks in England—but they are akin to the ancient rocks that form the bedrock of Greenland and North America. Upon these ancient rocks, and separated by an irregular and weathered surface called an unconformity, which represents a huge break in the geological record, lie the younger Precambrian rocks of the Torridonian, which make dramatic highland landscapes. But it is the rocks that sit above the Torridonian, separated by another unconformity, that yield telltale skeleton evidence of Scotland’s North American heritage.
Peach and Horne, in making these deductions, were generous in acknowledging Survey palaeontologist John William Salter, who half a century earlier in 1859 had first recognized how fossils also pointed to this relationship. Salter was one of the great palaeontologists of the 19th century. He had worked alongside those two influential British geologists who between them carved out the early Palaeozoic timescale, Englishman Adam Sedgwick and Scotsman Sir Roderick Impey Murchison, who founded the Cambrian and Silurian systems, and then warred over where the boundary should lie between them until that other great geologist (and practical diplomat) Charles Lapworth set up the Ordovician System to separate both the Cambrian and Silurian and their (by now deceased) protagonists.
Salter, in the midst of these discoveries and battles, developed a profound knowledge of a wide range of fossils, including trilobites, making a name for himself as someone who could identify these fossils from as far afield as the Himalayas and Australia. Cambrian cephalopods and gastropods of northern Scotland, he stated, had more affinity with North American forms than with European ones. He then had no inkling of the workings of plate tectonics, but, like Abraham Ortelius, he is a key part of the story that was to play out a century later. Salter, unwisely, resigned his post at the Survey one year short of the term needed to secure a proper pension. Plagued by increasingly poor health and by financial woes, he jumped to his death in the Thames in 1869, leaving a widow and seven children. A year later, no less than Thomas Huxley wrote his epitaph in the presidential address he gave to the Geological Society, noting Salter’s boyish enthusiasm for all things palaeontological.
Salter’s recognition that the fossils of the Northwest Highlands had a North American affinity was just the tip of the iceberg of a torrent of palaeontological data that showed this was so. From the Northwest Highlands to the Midland Valley of Scotland, the Cambrian and Ordovician fossils all showed the same signature. In Girvan, for example, a small fishing town on the western coast of the Midland Valley, the Ordovician mudrocks and limestone have fossil species—including ostracods—that are well known from Virginia in the United States.
Across the Solway Firth to the south, the Cambrian and Ordovician fossils are distinctly different, at least until the end of the Ordovician. Thereafter, the fossils either side of the Solway gradually converge in their biological affinities, as a narrowing ancient ocean, the Iapetus Ocean, was destroyed, subducted beneath the continents on its northern and southern margins. The first contacts are seen in the fossils of animals that lived in offshore deep marine settings of Iapetus that had the capacity to cross the narrowing sea. Later, as the ocean disappeared, and only narrow seaways remained, the animals from very inshore settings also began to increasingly resemble each other.
As the Iapetus Ocean closed, thrusting up its great mountain chains between the ancient terranes of Scotland, Wales, and England, the land was changing colour too. For countless millennia it had been weathered rock, perhaps with a thin veneer of bacteria and algae. Now, gaining a toehold slowly at first, in low-lying damp coastal plains, plants were invading. A once silicate-grey landscape was turning green. And the green shoots of these plants would, in turn, provide an eloquent archive of information about terrestrial environmental change over geological timescales.
The Long Sleep of the Antarctic
Skaar Ridge lies 84° south, on the flanks of Mount Augusta in the Queen Alexandra Range of the Trans-Antarctic Mountains. It is a small ridge of rock, marked by a cairn of stones, which extends for just a few miles towards the mighty Beardmore Glacier. The icy landscape here is blanketed in darkness for 6 months of the year, and even during the height of summer, temperatures stay well below zero. Long ago, in the warm climate of the Permian Period, plants grew here well beyond the southern latitude of modern tree growth. And these trees were of a type that was widespread across many parts of the world, from Africa to India and Australia.
The Skaar Ridge site has yielded many fossil plant specimens of Late Permian age.80 These fossils are part of a long tradition of geological collecting from this part of Antarctica. On 12 February 1912, on route back from their failed attempt to be the first human visitors to the South Pole, Robert Falcon Scott’s ill-fated expedition stopped off at the top of the Beardmore Glacier and, observing some interesting rocks, decided it would be a good place to look at the geology. This rock-hunting sojourn might seem a bizarre act of self-confidence, given that within a few weeks Scott was to write the final entry in his diary. But a day’s collecting added some ‘35 lb’ (about 16 kilograms) of rocks to the expedition’s load, and given how little was known about the Antarctic continent at that time, the samples were akin to returning rock from the surface of the Moon. Scott’s expedition abandoned some of their field gear in an attempt to lighten their load on the return journey, but they clung on to these scientifically important specimens to the end, a measure of true greatness in the face of severe, indeed fatal, adversity. Amongst the rocks that Scott’s expedition collected was a fossil of the Permian plant Glossopteris, which now resides in the Natural History Museum in London. Glossopteris is proof that once upon a time Antarctica had been linked with its African, Indian, and Australasian cousins in one mighty landmass.
The rocks on Skaar Ridge preserve a fossilized peat, and within this are the leaves and stumps of glossopterids, the latter so well preserved that individual tree rings can be discerned after more than 250 million years. These fossils tell a story of trees growing at high latitude in a warm world quite different from that of today. Each year that a tree grows, it adds a new circle of growth to the girth of its trunk. This growth occurs in a layer of cells just beneath the bark of the tree, and in modern temperate zone trees, where there is a strong contrast in seasonal growth, these rings are clearly defined. A dendrochronologist—one who studies the signature of these rings—can distinguish wood formed in the spring (early wood) from summer growth (late wood). The width of the ring may vary between a good year and a bad, drought-prone year. Tree ring changes accumulating over time can be compared across trees, and rather like the techniques of measuring growth increments in bivalves such as Ming, a long record of seasonal variation can be built up from the overlapping records of progressively more ancient trees, and this has been reconstructed back in time over many thousands of years. The idea is not a new one. The French polymath and genius the Comte de Buffon had already in the 1730s, with his colleague Henri-Louis Duhamel du Monceau, observed the impact of the severe winter of 1709 on tree ring growth.81
What then of wood found in the very ancient fossil record? Might this reveal something of the environment the trees were growing in so long ago? At Skaar Ridge the fossilized leaves of Glossopteris occur in dense mats showing that, just as deciduous trees do today, these high latitude trees dropped their leaves as winter approached. But the glossopterid trunks show something very different, with almost no late wood, and instead tree rings dominated by early wood. This is a pattern not seen in living trees, and it suggests a transition into winter dormancy that occurred very sharply. The glossopterid trees of Skaar Ridge show that as the sun dipped below the horizon at the start of the long Antarctic night, the trees fell quickly into a deep sleep, only to reawaken when, after 6 months of darkness, the sun once again appeared above the horizon.
A Brief Royal Epilogue
The excavators dug through the car park surface opposite Leicester Cathedral at the point where, spookily, a crudely painted ‘R’ on the tarmac took its place in a series of alphabetically marked parking spaces. Beneath the ground, in a crude and hastily made grave, there was a male human skeleton (Figure 40). The skull showed the marks of the sword blows that had killed him, the back of the cranium hacked away. These bones held many secrets, and one was to identify him. DNA extracted from tiny samples of them showed that these were the long-lost mortal remains of the last Plantagenet King of England, Richard III. The story of his discovery and forensic analysis, by a team of archaeologists and historians from the University of Leicester, the city council, and the Richard III Society, is now a classic. And the bones told their stories in more ways than one.
Figure 40. King Richard III of England, shortly after the discovery of his remains beneath a car park in Leicester.
Richard III has had a very bad press. As vividly portrayed by William Shakespeare, he personified villainy and ambition, his outward appearance of a hunchback with a withered arm matching his moral darkness as the murderer of the young Princes in the Tower, justly meeting his comeuppance on Bosworth Field against the heroic Henry VII, the first of the Tudors. Was it really so? Truth, mused Josephine Tey, is the daughter of time,82 a thought that allowed her to pursue the notion that perhaps it was not Richard, but the first Henry Tudor, who murdered the young princes, and then successfully covered his tracks, with Shakespeare, more than a century later as the Tudor regime still held sway, caught up in creating some of the most atmospheric and convincing propaganda ever written.
What do the bones say? The physical deformity clearly was not a rumour. Richard III suffered from severe scoliosis of the spine, something that may have given him a hunched posture, perhaps with one arm held higher than the other. The bones betray other parts of his history. The nature of the evidence would have mystified Shakespeare—the pattern of isotopes of lead, strontium, nitrogen, oxygen, and carbon—but the stories they told would surely have allowed him to embellish his play with vivid, and true, detail to illuminate the king’s life. In some respects, time’s daughter can faithfully recall fragments of the past.83
The tooth enamel of Richard III formed early, just as it does in all normal mortals. As it formed, it absorbed elements of the landscape amid which the boy grew up. The strontium isotopes reflect the local food, which in turn reflect the local geology upon which wheat grew and sheep and cattle grazed, and the oxygen isotopes reflect rainfall patterns and their interaction with geography. The patterns of these isotopes, extracted from the king’s teeth, clearly show that Richard moved away from Northamptonshire, where he had been born, at about the age of seven, to a place of different geology and heavier rain, probably somewhere in western England (perhaps Ludlow on the Welsh Borders, the authors of the study suggested).
A little later in life, patterns of these isotopes in bones that carry on growing past childhood—dentine in the teeth, the femur—show that the king-to-be moved back to the drier areas and the geology of the east. The lead in his teeth showed that, when he lived, the air, soil, and water were no longer as pristine as they had been in the time of his ancient ancestors: the traces of pollution from metal smelting in the region are evident.
The king’s rib showed another pattern. This bone renews itself quickly through life, and provides clues to his last few years, already as king. The carbon and nitrogen isotopes here betray a rich diet, with wildfowl such as swan, egret, and heron and freshwater fish figuring prominently—and wine, too: the analysis of his bones was the first example where the pattern of nitrogen isotopes has been linked with copious wine drinking. The feasting and banqueting of a royal life in medieval times were no myth, it seems.
As to the moral character—or guilt and innocence—of Richard III, the isotopes remain silent. The daughter of time, here, remains enigmatic. But one can at least raise a glass to the long-dead monarch, knowing that to be an entirely fitting farewell.