Fossil

• 1. Introduction

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  A fossil can be as small as a single bacterium trapped in stone or as long as 88 meters, the length of a Sequoia preserved at Coaldale, Nevada. Fossils are the preserved remains, impressions, or traces of anything that once lived, from bones and shells to petrified wood, hardened resin, and fragments of ancient DNA. Together they form what scientists call the fossil record, a patchy but revealing archive of life that stretches back somewhere between 3.48 billion and 4.1 billion years. For most of human history, the people who dug up these strange shapes in rock had no idea what they were looking at. Some saw dragon bones. Others saw the claws of mythical creatures, or the tongues of poisoned snakes. It took centuries of careful observation, and a handful of thinkers willing to contradict scripture and superstition, before fossils were recognized as evidence of a much older and stranger Earth than anyone had imagined. How did civilizations an ocean apart arrive at such similar, and similarly wrong, explanations for the same kinds of objects? And how did fossils eventually become the tool that lets scientists date rock layers, track the course of evolution, and search for life on Mars?

• 2. Ancient Myths and Beliefs

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  Long before recorded history, Paleolithic toolmakers set fossil echinoderms into the hand grips of stone knives found across Europe, a practice traced back to Homo heidelbergensis and to Neanderthals. Those same ancient peoples drilled holes through the center of round fossil shells, apparently to string them as beads on necklaces.

  Egyptian priests kept fossilized bones resembling hippopotamuses inside the temples of the god Set, while five-rayed fossil sea urchin shells were associated with Sopdu, a deity tied to the Morning Star. The scholar Adrienne Mayor has argued that skulls of the dinosaur relative Protoceratops, exposed in the deserts of Central Asia, inspired Greek legends of the griffin, though paleontologists Mark Witton and Richard Hing have challenged that idea. Skulls of Deinotherium giganteum, found in Crete and Greece, have a single hole in front for a trunk, much like a modern elephant, and scholars since the 13th century have suggested they gave rise to the Greek myth of the one-eyed Cyclops.

  Roman naturalist Pliny the Elder called fossil shark teeth glossopetra, or tongue stones, because some cultures thought they resembled the tongues of people or snakes. He also described fossil ammonites as the horns of Ammon, the origin of the group's modern name, and wrote of toadstones, fossil teeth from the Cretaceous ray-finned fish Lepidotes that were believed until the 18th century to cure poison. In Japan, fossil shark teeth were linked to the mythical tengu and treated as the creature's razor-sharp claws by the 8th century AD.

  In medieval China, the fossilized bones of ancient mammals, including Homo erectus, were sold as dragon bones and used in medicine and aphrodisiacs. The Song dynasty scholar Huang Tingjian engraved his own poem onto a fossil seashell he collected in the 11th century, and his contemporary Shen Kuo used marine fossils found hundreds of miles from the Pacific Ocean, described in his 1088 Dream Pool Essays, to argue that a prehistoric coastline had once stood there. In Norse mythology, round echinoderm shells were tied to the god Thor and kept in homes for protection, a belief that survived into English folklore as the shepherd's crown, and among Suffolk bakers as the fairy loaf.

  In prehistoric Africa, entire tribes moved fossils to ceremonial sites and treated the bones with a reverence that left behind no written myth to explain why.

• 3. Scholars Rewrite the Story

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  The Greek philosopher Aristotle noticed that fossil seashells pulled from rock closely resembled shells found on beaches, and concluded that the fossils had once been living animals. The Persian polymath Avicenna took a different view, describing in his 1027 Book of Healing a "powerful mineralizing and petrifying virtue" that turned buried plants and animals into stone. Albert of Saxony built on the sea-origin theory in the 14th century, and by the 16th century most naturalists had accepted it.

  Leonardo da Vinci pointed out a problem with the popular theory that Noah's flood had scattered seashells across mountainsides: shells of the same species were found grouped together, exactly as they cluster on modern beaches, rather than jumbled together as a single flood would leave them. In 1666, Nicholas Steno dissected a shark and matched its teeth to the old "tongue stones" of Greco-Roman legend, proving they were shark teeth rather than snake tongues. Robert Hooke, writing in his Micrographia and in observations published after his death in 1705, described fossils as the petrified remains of creatures, some of which no longer existed.

  English canal engineer William Smith, working between 1769 and 1839, found that rock layers of different ages held different, predictable groups of fossils, a pattern he called the principle of faunal succession. Georges Cuvier concluded that most of the animal fossils he studied belonged to extinct species, and in a 1796 paper on living and fossil elephants he wrote that the evidence pointed to "the existence of a world previous to ours, destroyed by some kind of catastrophe." In Britain, Mary Anning's discovery of the first complete ichthyosaur skeleton and a complete plesiosaurus skeleton drew both public fascination and scientific attention to fossils in the early 19th century.

  Smith's principle of faunal succession would go on to become one of the pieces of evidence Charles Darwin leaned on hardest.

• 4. Darwin's Missing Fossils

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  When Charles Darwin wrote On the Origin of Species, the oldest known animal fossils came from the Cambrian period, now dated to about 540 million years ago. Darwin worried that the absence of any older fossils threatened his theory, though he held out hope they would eventually turn up, noting that "only a small portion of the world is known with accuracy." He was also troubled by the sudden appearance of many animal groups in the oldest Cambrian rock layers.

  Since Darwin's death, the fossil record has been pushed back to between 2.3 and 3.5 billion years, mostly in the form of microscopic bacteria. The Ediacara biota, dating to 575 million years ago, revealed a richly diverse community of early multicellular organisms living long before the Cambrian period Darwin knew. Paleontologist Niles Eldredge's study of the Phacops trilobite genus showed that changes to the arrangement of its eye lenses happened in sudden bursts rather than smoothly over millions of years during the Devonian period, an observation that helped Eldredge and Stephen Jay Gould publish their theory of punctuated equilibrium in 1971.

  Synchrotron X-ray scans of 543-million-year-old Cambrian embryos, including a worm-like fossil called Markuelia found in China and Siberia, have revealed three-dimensional cell structures preserved through a process called phosphatization. The scans suggest Markuelia's closest living relatives are priapulid worms, placing it near the evolutionary branch point of priapulids, nematodes, and arthropods. In a more recent case, the 2004 discovery of Tiktaalik in the Canadian Arctic filled a specific gap in the fossil record, documenting a transitional step in the evolution from fish to four-limbed land animals.

  Even with these discoveries, researchers estimate that the number of species known through fossils amounts to less than 5 percent of all species alive today, let alone the far larger number that have ever lived.

• 5. How Rock Preserves Life

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  Permineralization begins when an organism is buried quickly enough to escape scavengers, allowing mineral-rich groundwater to fill the empty spaces once occupied by soft tissue, sometimes down to the level of a single plant cell wall. Replacement, by contrast, swaps the original shell or bone entirely for another mineral, occasionally so gradually that microscopic structural detail survives despite the loss of every original atom. Silicification fills an organism's pores with a silica gel that later hardens into quartz, chalcedony, agate, or opal, each still bearing the shape of what it once encased.

  Pyritization occurs when organic matter decaying in iron-rich marine sediment releases sulfide that reacts with dissolved iron to form the mineral pyrite, replacing carbonate shell material outright; phosphatization instead replaces tissue with dense calcium-phosphate minerals, producing fossils that range in color from dark orange to black. Compression fossils, formed by chemical reduction of an organism's original tissue, often leave behind a thin carbonaceous film called a phytoleim; when that film survives, the fossil is called a compression, and when only its outline remains in the rock, it is called an impression. Carbonized fossils are reduced almost entirely to a thin film of carbon, while coalified fossils, typically from woody plant tissue, are made mostly of coal.

  Bioimmuration is stranger still: a skeletal organism such as a bryozoan or oyster grows directly over another organism, preserving it, or an impression of it, inside its own skeleton, a process documented in the fossil record from the Ordovician period to the present.

  Some of the most striking fossils form as casts and molds, where the original remains dissolve away entirely and leave a hollow, organism-shaped void that later fills with sediment. Fossil resin, known popularly as amber, can trap insects, spiders, and occasionally small lizards so completely that fragments of their DNA survive; the oldest fossil resin dates to the Triassic period, though most comes from the more recent Cenozoic era. In 2014, paleontologist Mary Schweitzer and her colleagues reported iron particles associated with soft tissue recovered from dinosaur fossils, including blood vessels, proposing that iron from hemoglobin helped stabilize and preserve that tissue for tens of millions of years.

  Lagerstätten, a German term meaning storage places, are sites of exceptional preservation where even soft tissue can survive, including the Cambrian Burgess Shale, the Devonian Hunsrück Slates, the Jurassic Solnhofen Limestone, and the Carboniferous Mazon Creek deposits in Illinois. At Mazon Creek, organisms sometimes became nuclei for mineral nodules that grew rapidly enough around them to capture fine three-dimensional detail before decay set in.

  None of these processes, however, can turn rock into a calendar. For that, scientists had to look elsewhere.

• 6. Reading Time in Stone

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  Radiometric dating can date a fossil-bearing rock to within 0.5 percent accuracy or better, but only volcanic ash layers carry the radioactive elements the method requires, leaving most sedimentary rock beds impossible to date directly. Paleontologists instead lean on stratigraphy, the principle that rock forms in horizontal layers with each layer younger than the one beneath it, so a fossil found between two dated layers must fall somewhere between their two ages.

  Because rock sequences are often broken by faults or erosion, scientists use short-lived species as index fossils to match up rocks that are not directly adjacent. The conodont Eoplacognathus pseudoplanus, for example, existed only briefly during the Middle Ordovician period, so any rock containing its remains can be dated to that narrow window. A useful index fossil must be common, easy to identify, and spread across a wide geographic area, or the odds of ever finding it twice become too low to be of use.

  Family trees can also narrow down a fossil's age: if species B and C are dated to a given time and their shared ancestor A appears earlier in the same lineage, A must have evolved earlier still. So-called molecular clocks estimate how long ago two living groups diverged by assuming DNA mutations build up at a roughly constant rate, but different techniques applied to the same question, such as when animal groups first appeared before the Cambrian period, can produce estimates that differ by a factor of two.

  Even the best-dated fossils, though, remain rare survivors of organisms that mostly vanished from the record without a trace.

• 7. Fossils in All Shapes

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  Not every fossil is a body. Trace fossils record biological activity rather than the organism itself, and include tracks, burrows, and coprolites, or fossilized feces, first described by William Buckland in 1829, before which they were mistaken for "fossil fir cones" and "bezoar stones." Coprolites can range from a few millimeters to more than 60 centimeters and give paleontologists direct evidence of what an extinct animal actually ate.

  Microfossils sit at or below the edge of what the naked eye can resolve, with the cutoff between "micro" and "macro" fossils usually set at 1 millimeter; they include whole organisms such as foraminifera and coccolithophores as well as fragments like small teeth or spores, and serve as a key source of paleoclimate data. Subfossils are remains, such as bones or nests, whose fossilization is incomplete, often because too little time has passed; unlike true fossils they still retain organic material, which makes them useful for radiocarbon dating and DNA extraction.

  Stromatolites, layered structures built by cyanobacteria that trap and bind sediment grains together, provide some of the oldest confirmed fossil evidence of life on Earth, with the earliest confirmed example dating to 2.724 billion years ago and a 2009 discovery pushing possible evidence back to 3.45 billion years. Stromatolites peaked in abundance around 1.25 billion years ago before declining to just 20 percent of that peak by the start of the Cambrian period, a drop researchers attribute mainly to the rise of grazing animals.

  Not everything that looks like a fossil is one. Pseudofossils, such as branching dendrite crystals or the round shapes called kidney ore, form through purely geological processes, and rounded sedimentary nodules called concretions were once mistaken for dinosaur eggs.

• 8. Fossils in the Modern World

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  Illegal fossil trading remains a persistent problem, particularly in China, where researchers say important scientific specimens are stolen from dig sites every year before they can be studied. Traditional Chinese medicine has continued to prescribe dinosaur bones as "dragon bones" into the 21st century, with mid-Cretaceous dinosaur remains reportedly still consumed in Ruyang County.

  On the 24th of January 2014, NASA announced that its Curiosity and Opportunity rovers on Mars would begin searching for signs of an ancient biosphere, including evidence of past habitable, fluvio-lacustrine environments shaped by rivers or lakes. The search for organic carbon and signs of past habitability on Mars has since become a primary NASA objective, applying the same taphonomic thinking paleontologists use to interpret fossils on Earth.

  A different piece of evidence points to how far back the human fascination with fossils really goes. Some researchers argue that a Corinthian vase from the 6th century BCE, held in Boston, depicts a giraffe skull from the Miocene epoch blended with features from other species. Julián Monge-Nájera has challenged that reading, arguing the painted creature's skull anatomy fits a living monitor lizard the ancient Greeks would have recognized, not a fossil at all.