Written in Stone: Evolution, the Fossil Record, and Our Place in Nature

by Brian Switek

  The first step in fossil hunting is pinpointing the most likely place to find your quarry, so Shubin and Daeschler began flipping through textbooks hoping to find Devonian deposits of the right age and type to hold early tetrapods. During this process a dispute broke out about some bit of geological trivia that only another textbook could resolve, but when the scientists opened one to continue their argument they were struck by something they had somehow missed. There was a swath of Devonian rocks of just the kind they were looking for in the upper latitudes of Canada that had not yet been picked over by other paleontologists.

  FIGURE 23 - A map of the Euramerican landmass during the late Devonian. The dashed line indicates the equator. Sites of elpistostegalian fish discoveries: T, Tiktaalik; E, Elpistostege; P, Panderichthys.

  The 1999 expedition of the Nunavut region of Canada led by Shubin and Daeschler did not yield the fossils they had been hoping for. The next year they decided to move to a site near the southwestern tip of Ellesmere Island, but their search only turned up well-known varieties of Devonian fish. They kept searching, each expedition a gamble, but they failed to hit the payoff they had been expecting. It was decided that the 2004 expedition would be their last try.

  They were in luck. Much to the delight of the paleontologists, encased in the approximately 375-million-year-old rock were the remains of a flat-headed, long-snouted fish unlike any the scientists had seen before. When they started to remove it from the encasing rock back in the United States they found that they had recovered the fairly complete remains of a creature that further documented the evolution of limbs. Dubbed Tiktaalik roseae in 2006, this creature was a tetrapodomorph that was even more closely related to Acanthostega than Panderichthys was. Tiktaalik had a similar low profile well suited to life in the shallows, but unlike Panderichthys it also had a neck. The head and shoulder girdles were separated, allowing Tiktaalik to move its head without having to move its body (a useful trait for snapping up fish, which its teeth suggested it did), but the parts of Tiktaalik that garnered the most attention were its arms.

  FIGURE 24 - A simplified restoration of the skeleton of Tiktaalik.

  The well-preserved forelimbs of Tiktaalik were built on the familiar humerus-radius and ulna-fin radials-lepidotrichia pattern, but it possessed rudimentary joints between its fin radials and other arm bones that would have allowed it to flex its forelimbs. It was still very much an aquatic creature, but these intricate traits suggested that it could better support its body weight in the shallow-water environment in which it lived. As the authors of the first description of Tiktaalik concluded, “New discoveries of transitional fossils such as Tiktaalik make the distinction between fish and the earliest tetrapods increasingly difficult to draw.”

  Tiktaalik also provided another reference point with which to compare other early tetrapods. When more complete skull and shoulder material of Ventastega was described by Per Ahlberg and colleagues in 2008 the scientists were able to determine that this creature was part of a diversification of tetrapod types. Ventastega was probably not a direct descendant of Tiktaalik or an ancestor of Acanthostega, but part of a radiation of forms that lived in near-shore environments near the close of the Devonian. The fin-to-limb transition was better documented than ever before, but the fossils could not be slotted into a straight lineup of ancestors and descendants. This not only confirmed that evolution has a branching pattern, but it also suggested that there are many more species of early tetrapod waiting to be discovered.24

  FIGURE 25 - A simplified phylogeny of early tetrapods and their close relatives. The diagram does not indicate a direct line of descent, but successive branching points that illustrate the changes early tetrapods went through during the evolutionary transition.

  FIGURE 26 - The restored head of Ventastega as seen from the side (left) and above (right).

  Ultimately many of these early tetrapod forms succumbed to extinction, but they also formed the basis of a radiation of the first amphibians. One lineage among these varied forms, in turn, gave rise to the first amniotes—lizardlike vertebrates that could reproduce by laying shelled eggs further away from the water’s edge. As disparate as these creatures were, though, they all were modifications of the ancestral tetrapod body plan, themselves modified from fleshy-finned fish. The traits that allowed these radiations of terrestrial vertebrates, from lungs to fins supported by a bony architecture, were in place for millions of years before the first tetrapod crawled on land. This co-option would become a recurring theme in the evolution of vertebrates, including one particular lineage of tetrapod descendants, birds.

  Footprints and Feathers on the Sands of Time

  “Tempora mutantur, et mutandum est avibus (The times have changed, and the birds must change with them)”

  —EDWARD HITCHCOCK, Ichnology of New England, 1858

  “There is no evidence that Compsognathus possessed feathers; but, if it did, it would be hard indeed to say whether it should be called a reptilian bird or an avian reptile.”

  —T. H. HUXLEY, American Addresses, 1877

  Humans have been finding the traces of extinct creatures for thousands of years. Unaware of their true identity, a variety of cultures have interpreted fossil footprints, shells, and bones as the remnants of gods, heroes, saints, and monsters. The cyclops, griffins, and numerous other beings of myth and legend were not just figments of human imagination but monsters restored from the remains of creatures dead for millions of years. It was no different among the Native Americans of North America. The Tuscaroras, Iroquois, Onondagas, and many other tribes had legends inspired by fossils, including the Lenape of the Delaware Valley.25

  At the time Europeans arrived in North America the Lenape occupied the land from northern Delaware to the Hudson Valley of New York, and in the blood-red sandstone of this range they saw three-toed, clawed footprints.26 According to one tale, passed down by Richard Calmet Adams, some of these were said to be the footprints of the primeval progenitor of all the great monsters of the land and sea. It was a living horror, the destroyer of all it could dig its claws into, but perished when it was trapped in a mountain pass and obliterated by lightning.

  Europeans that settled in the Connecticut Valley noticed the tracks, too. While plowing his father’s field in South Hadley, Massachusetts, around 1802 a young man named Pliny Moody turned up slabs of rock indented with weird footprints. At least one of these curiosities was appropriately put to use as a doorstep, and visitors to the Moody farm sarcastically remarked that the Pliny’s family must have raised some hearty chickens if they left footprints in solid stone. The physician Elihu Dwight, who later bought the slab, had a different interpretation. To him the tracks were made by Noah’s raven when the biblical Deluge subsided.

  Such tracks were hardly unique. The abundant three-toed footprints were often called “turkey tracks” (although many indicated turkeys bigger than a full-grown human), and a cache of the impressions were discovered by laborers quarrying flagging stones near Greenfield, Massachusetts, in 1835. These were brought to the attention of the local physician James Deane, who knew they were not made by antediluvian poultry or biblical birds. Just what had created them, though, Deane could not say, and so he contacted Amherst geology professor Edward Hitchcock and Yale academic Benjamin Silliman for their opinions.

  Hitchcock was initially skeptical of Deane’s claims. Some mundane geological phenomenon could have produced tracklike marks, the professor cautioned, but Deane was adamant that the footprints were genuine. Deane sent Hitchcock a cast of one of the footprints to support his case, and despite his doubts Hitchcock was intrigued. Hitchcock soon set out to have a look at the Greenfield tracks for himself and found that Deane was right. The impressions were the footsteps of ancient creatures that had trod the Connecticut Valley long before humans had settled there.

  Hitchcock became enthralled by the tracks. He collected and purchased as many as he could. He fancied himself a scientific pioneer. Although Deane was also researching
the tracks, Hitchcock was the first to publish on them in an 1836 issue of the American Journal of Science .27 There was a variety of footprint types, each given a unique binomial name to indicate a different species, but the three-toed ones were some of the most remarkable. They ranged from giant tracks over seventeen inches long to tiny impressions less than an inch from front to back. A few large slabs even showed the strides of the animals, and the only reasonable conclusion was that they had been made by birds that flocked along the ancient shoreline. “Four out of five, I presume, would draw this conclusion at once,” Hitchcock noted, and he thought that the tridactyl footprints were made by extinct equivalents of storks and herons that strode along the banks of an ancient lake or river.

  FIGURE 27 - One of the many track-bearing sandstone slabs that enthralled Edward Hitchcock.

  Hitchcock was deeply inspired by the varied assemblage of birds that had once lived in the Connecticut Valley, and he attempted to do justice to his fascination in the anonymously published poem “The Sandstone Bird.” In the geologist’s verse, science is placed in the guise of a sorceress who conjures up the most majestic of the primeval birds:Bird of sandstone era, wake!

  From thy deep dark prison break.

  Spread thy wings upon our air,

  Show thy huge strong talons here:

  Let them print the muddy shore

  As they did in days of yore.

  Pre-adamic bird, whose sway

  Ruled creation in thy day,

  Come obedient to my word,

  Stand before Creation’s Lord.

  So restored, Hitchcock’s fictional bird could only lament the dismal state of the modern world. The earth was cold and the impressive giants it knew so well were all gone. Even the trees were so Lilliputian that the dinosaur “Iguanodon could scarce here find a meal!” The haughty bird could not stand the sight of what had become of its home.

  . . . all proclaims the world well nigh worn out,

  Her vital warmth departing and her tribes,

  Organic, all degenerate, puny soon,

  In nature’s icy grave to sink forever.

  Sure ’tis a place for punishment designed,

  And not the beauteous happy spot I loved.

  These creatures here seem discontented, sad:

  They hate each other and they hate the world,

  I can not, will not, live in such a spot.

  I freeze, I starve, I die: with joy I sink,

  To my sweet slumbers with the noble dead.

  The sullen bird was then swallowed up by the earth, leaving the geologist with no evidence to prove what he had seen.28 Hitchcock was in a similar bind. No skeleton had been found to reveal the true form of his birds. Storks and herons provided fair analogs, but even the largest of the living wading birds was puny compared to the birds that made the largest fossil tracks. Without skeletons, Hitchcock could only guess what they looked like.

  At the same time that Hitchcock was researching the Connecticut Valley tracks, Richard Owen was examining a strange chunk of bone from New Zealand. It was said to have belonged to an enormous eagle, but Owen took it to be part of the femur of a gargantuan, ostrichlike bird he called Dinornis (commonly known as the moa). From the osteological scrap he reconstructed an entire skeleton, and it was later proven to be correct when more remains of the flightless birds were found. Owen had raised a giant bird from the dead, and it provided the perfect proxy for the sandstone birds.

  For Hitchcock, though, there were more than just scientific lessons to be learned from the tracks. What he saw in the fossil record spoke of God’s benevolence, and he expounded upon this belief as a Congregationalist pastor and professor of natural theology at Amherst. (Part of his inspiration for collecting so many tracks was to build a testament to God’s glorious works in nature.) He was astonished by the vast array of stupendous creatures that crawled, swam, flew, and dashed over the surface of the earth in time immemorial. Though facts from the geological strata were shaking the foundations of a literal interpretation of Genesis, Hitchcock attempted to bridge the gap between geology and theology as the Bridgewater Treatises had in England. In his Ichnology of New England Hitchcock concluded:And how marvellous the changes which this Valley has undergone in its inhabitants! Nor was it a change without reason. We are apt to speak of these ancient races as monstrous, so unlike existing organisms as to belong to another and quite different system of life. But they were only wise and benevolent adaptations to the changing condition of our globe. One common type runs through all the present and the past systems of life, modified only to meet exigencies, and identifying the same infinitely wise and benevolent Being as the Author of all. And what an interesting evidence of his providential care of the creatures he has made, do these modifications of structure and function present! Did the same unvarying forms of organization meet us in every variety of climate and condition, we might well doubt whether the Author of Nature was also a Providential Father. But his parental care shines forth illustriously in these anomalous forms of sandstone days, and awakens the delightful confidence that in like manner he will consult and provide for the wants of individuals.

  If God provided for birds that could neither sow nor reap their own food surely He would have also cared for the enormous avians of old (and even more so the human “lords of creation”). Hitchcock believed that only God could have so perfectly fitted organisms to their surroundings, but this view of nature crumbled as naturalists increasingly tried to understand nature on its own terms and not as a moral lesson. Charles Darwin’s 1859 treatise slammed the door shut on the concept of natural theology as science, which Hitchcock subscribed to, but this new perspective on life’s history raised new questions.

  Birds were so different from other vertebrates that they appeared to be perched on their own lonely branch in the tree of life. How could they have evolved? Hitchcock’s tracks hinted that true birds had been present nearly as long as reptiles and amphibians, and the discovery of a fossil feather in 1860 from Solnhofen, Germany, did nothing to change this quandry. Found in the Jurassic-aged limestone of a quarry mined for stone to make lithographic plates, the delicate fossil was acquired by the German paleontologist Christian Erich Hermann von Meyer. In 1861 he named it Archaeopteryx lithographica, the “ancient feather from the lithographic limestone.”

  Not long after von Meyer described the feather, another nearby limestone quarry produced an enigmatic skeleton. The jumbled creature had a long bony tail but was surrounded by feather impressions; it was as much a reptile as it was a bird. Rather than going straight to a museum, however, the specimen was given to the local physician Karl Häberlein in exchange for medical services.

  Rumors of the specimen began to circulate among naturalists, but Häberlein would not part with it easily. He stipulated that the fossil would only be sold along with the rest of his fossil collection, raising the cost beyond the reach of many prospective buyers. Richard Owen and George Robert Waterhouse, certain that Archaeopteryx would bring prestige to the British Museum, were able to convince the trustees of the institution to forward £700 for the fossil (or what the museum would normally have spent on new fossil acquisitions over the course of two years). By November 1862 the fossil was in London.

  Some German naturalists were upset that the slab had been expatriated to England, but the august University of Munich professor Johann Andreas Wagner had opposed efforts to acquire Archaeopteryx for his college. He was sure it was not all it seemed. Although Häberlein tried to restrict access to the specimen amid rumors it was a fake, a verbal report and sketch of the fossil reached Wagner, who argued that rather than a bird, it was a kind of reptile he called Griphosaurus, or “riddle reptile.”

  Wagner’s fears over evolution had spurred his impulsive description. Archaeopteryx sounded like just the type of transitional form that would throw support to Darwin and Wallace’s evolutionary theories, and Wagner’s warnings about the fossils were among the last of his publications before his death.


  Owen’s description of the fossil was read before the Royal Society in 1863. He appraised it as the “by-fossil-remains-oldest-known feathered Vertebrate.” More than that, the fossil was most certainly a bird despite its reptilian characteristics, and Owen upheld von Meyer’s original name Archaeopteryx.29 This diagnosis allowed Owen to make a particular prediction. The head of Archaeopteryx was missing, but Owen reasoned that “by the law of correlation we infer that the mouth was devoid of lips, and was a beak-like instrument fitted for preening the plumage of Archaeopteryx.”

  While some naturalists felt that Owen’s description was rather crude, the news of the fossil was welcome among evolutionists. In an 1863 letter to Darwin the fossil mammal expert Hugh Falconer beamed,Had the Solenhofen quarries been commissioned—by august command—to turn out a strange being à la Darwin—it could not have executed the behest more handsomely—than in the Archaeopteryx.

  This news made Darwin eager to hear more about the “wondrous bird,” yet he ultimately did little to present Archaeopteryx as a confirmation of his evolutionary ideas. In the fourth edition of On the Origin of Species published in 1866, Darwin primarily used Archaeopteryx and Hitchcock’s tracks—by now thought to have been made by dinosaurs—to illustrate that the fossil record still had secrets to divulge. “Hardly any recent discovery,” Darwin wrote of Archaeopteryx, “shows more forcibly than this how little we as yet know of the former inhabitants of the world.” Even as it hinted at a connection, Archaeopteryx was too weak to unequivocally bridge the gap between reptiles and birds by itself. The necessary evidence would be supplied by the anatomist Thomas Henry Huxley.