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

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Written in Stone: Evolution, the Fossil Record, and Our Place in Nature Page 15

by Brian Switek


  FIGURE 44 - The lower jaw of a hawk compared with the lower jaw of a Tyrannosaurus rex, both showing lesions in the bone caused by the microorganism Trichomonas gallinae.

  Traits we think of as clearly identifying birds—feathers, air sacs, behavior, and even peculiar parasites—were present in a wide variety of dinosaurs first. Distinguishing the first true birds from their feathered dinosaur relations has become increasingly difficult. If we define birds as warm-blooded, feathered, bipedal animals that lay eggs, then many coelurosaurs are birds, so we have to take another approach.

  Living birds, from kiwis to chickadees, fall within the group Aves, which also includes extinct birds like Confuciusornis, Jeholornis, Zhongornis , Lonipteryx, Hesperornis, and Archaeopteryx. Overall, Aves is the taxonomic equivalent of what are often informally referred to as “birds,” but the earliest birds share many features with their closest relatives among the non-avian dinosaurs. What the closest dinosaurian relatives of birds may be, though, is presently under debate. Deinonychosaurs, the group containing both the dromaeosaurs (i.e. Deinonychus , Microraptor) and troodontids (Mei, Anchiornis), has often had pride of place as the dinosaurs nearest to birds and the group from which birds evolved. The identification of Archaeopteryx as a feathered dromaeosaur certainly reinforces this view, but the research describing the dinosaurs Scansoriopteryx and Epidexipteryx has placed them even closer to birds than the dromaeosaurs.

  If the new analyses are supported by further evidence, Scansoriopteryx and Epidexipteryx together would make up a group called the Scansoriopterygidae and be the closest relatives to Aves. Thus, Aves plus the Scansoriopterygidae would form a group called the Avialae, with the deinonychosaurs being the next closest relatives to both groups. This placement does not reveal direct ancestors and descendants, but rather represents the group of dinosaurs from which birds arose and what they might have looked like. It is extremely unlikely that a direct line of descent from birdlike dinosaur to the first dinosaurlike bird will be found.

  In his 1871 critique of evolution by natural selection, On the Genesis of Species, George Jackson Mivart considered the wings of birds a damning example of how Darwin’s theory failed. To him a bird’s wing was an atrophied organ, degenerate in the number of digits and bones in each finger. “Now, if the wing arose from a terrestrial or subaerial organ, this abortion of the bones could hardly have been serviceable—hardly have preserved individuals in the struggle for life.” In other words, how could organisms have survived with half-formed wings?

  What we know now about evolution has undermined Mivart’s contention. The limbs of birds are only the modified limbs of dinosaurs; all the bones in the wing of a bird were present in the terrible, grasping hands of Deinonyonus and the delicate manus of Epidexipteryx. There is scarcely anything about a pigeon perched on a statue or a chicken you eat for dinner that did not first appear in dinosaurs, long before Confuciusornis flew in great flocks over what is now China. The majority of their relatives sunk into extinction sixty-five million years ago, but they are perhaps the most successful dinosaurs ever to have evolved. If you want to see living dinosaurs, you don’t have to trek to a steaming jungle or isolated plateau. All you have to do is put up a bird feeder and look out the window.

  FIGURE 45 - A simplified evolutionary tree of theropod dinosaurs highlighting the relationships of coelurosaurs and birds.

  But dinosaurs and birds were not the only terrestrial vertebrates evolving during the Mesozoic. The first mammals evolved alongside early dinosaurs, but they remained small creatures that lived in the corners of the world’s ecosystems. The worst mass extinction ever to strike the planet had almost entirely wiped out their ancestors, making them only the remnants of a family that once flourished, but, 150 million years later, a stroke of bad luck for the dinosaurs would prove to be an unexpected boon.

  The Meek Inherit the Earth

  “The relatively late time at which mammals took over the world’s supremacy from the reptilian dynasties would lead one to think that the stock from which they sprang must have been developed at a comparatively late date in reptilian history. This, however, is exactly the reverse of the true situation.”

  —ALFRED SHERWOOD ROMER, Vertebrate Paleontology, 1933

  Scotland did not have much to offer nineteen-year-old Andrew Geddes Bain. Both his parents had died when he was a child, and even though he was educated his job prospects were few. So when his uncle, Lieutenant Colonel William Geddes, left for South Africa in 1816, young Andrew tagged along.

  Once he arrived in the Cape colony Bain worked as a saddler, an explorer, an ivory trader, a soldier, and a road builder, but in 1837 he read a book that inspired him to look a little bit closer at the rugged landscape around him. It was Charles Lyell’s influential Principles of Geology , and just like young Charles Darwin, Bain was smitten with Lyell’s work. It allowed him to see the traces of lost worlds right beneath his feet, and his job as a road builder gave him the chance to see in the field what Lyell had described in print.

  As he searched the dusty, shrub-flecked landscape of South Africa’s Karoo desert for fossils one day in 1838, Bain discovered a skull unlike any he had seen before. The petrified creature had a short, turtlelike head complete with a beak, but instead of being toothless it possessed two large tusks that jutted down from the upper jaw. It was a fantastic find, and its relatively good state of preservation made it all the more spectacular. Most fossils from the Karoo were crushed, distorted, extremely fragile, and encased in nearly impenetrable sandstone.

  Bain determined that the skull had come from a hitherto unknown type of “bidental” animal, but he did not have the background in anatomy to fully comprehend what he had found. To find out more, Bain sent the skull to the Geological Society in England in 1844. Astonished by the unusual find, the Society sent Bain a £20 reward for his “judicious course” of action. Encouraged by the warm reception of the distant scholars, Bain started shipping more fossils and acted as the man in the field for London’s cadre of professional naturalists.

  FIGURE 46 - The skull of Bain’s “Bidental animal,” named Dicynodon by Richard

  While the rest of Bain’s fossils were en route from the colony, the task of describing the enigmatic cranium fell to England’s most eminent comparative anatomist, Richard Owen. The skull had belonged to some kind of reptile, Owen surmised, but it did not correspond to any known type of lizard, crocodile, or turtle. This extinct animal, which he named Dicynodon for the two enlarged fangs sticking out from the upper jaw, was a chimerical creature that consisted of both mammalian and reptilian parts. The ever-pious Owen could not help but think that the enigmatic fossil spoke to “a power transcending the trammels of the scientific system.”

  Dicynodon was only the first such creature to be described. South Africa proved to be unexpectedly fossil-rich, and Owen became the unofficial interpreter of the colony’s prehistory. By the late 1850s Owen had seemingly everyone who ventured into the field, from laborers to visiting politicians, funneling fossils back to England for him to scrutinize. Even Prince Alfred, who visited South Africa in 1860, returned home with two more Dicynodon skulls for Owen to add to his expanding collection. His annoyance at the hubbub over Darwin’s evolutionary theory aside, Owen was at the height of his scientific power.

  The fossil flood from South Africa inundated Owen with a collection of “reptilian” skulls that also possessed mammalian traits, like differentiated kinds of teeth in different parts of the jaw. Owen celebrated these strange amalgamations of features by giving the new forms names like Galesaurus (“weasel reptile”), Lycosaurus (“wolf reptile”), and Tigrisuchus (“tiger crocodile”). Owen placed these creatures into a new group united by their mammal-like dentitions, the Theriodontia, and by his estimate the fossils showed that some time in the distant past reptiles had begun to change into something more mammal-like.

  Owen’s interpretation was difficult for other naturalists to challenge. The senior scientist had a corner on the
Karoo fossil market. If a significant specimen was found it was often sent right to him. So pervasive was his reach that paleontologists who later visited the Karoo lamented that almost all the best specimens had already been extracted for Owen. This was made all the more intolerable by Owen’s jealous love for “his” fossils. Though kind to friends, Owen could be ruthless with colleagues; he was a powerful and ornery figure aware of his own brilliance.

  Interestingly, however, Owen’s rival Thomas Henry Huxley was not especially interested in these transitional forms, and on March 6, 1879, Huxley stood before the Royal Society of London to present his views on where the hairy, milk-producing group of vertebrates known as “mammals” had come from. Though the origin of mammals was still unresolved, Huxley argued, the identification of distinct evolutionary series in the bird, whale, and horse lineages had shown that evolution could be seen in the fossil record, after all. With the principle of evolution thus established clearly there must have once existed forms that would connect mammals of modern aspect to their “protomammal” antecedents.

  At that time the “lowest” mammals known were the monotremes, a group entirely represented by the duck-billed platypus and echidna. The echidna had been described first, but it was the platypus that caused a stir among naturalists when reports of it began to trickle back from the far-flung British outpost of Australia at the turn of the nineteenth century. The platypus was covered in hair and secreted milk for its young, yet it also had a ducklike bill and reproduced by laying eggs. It seemed to defy the divinely imposed order of nature, and the naturalist Thomas Bewick remarked that the platypus “appears to possess a three fold nature, that of a fish, a bird, and a quadruped, and is related to nothing that we have hitherto seen.”

  So odd was the platypus that the first person to describe the remains of one, George Shaw, confessed that it was “impossible not to entertain some doubts as to the genuine nature of the animal, and to surmise that there might have been practiced some arts of deception in its structure.” Such deception had often been practiced to fill the curiosity cabinets of Europe, precursors to true museums, with “authentic” remains of mythical creatures, but the acquisition of additional platypus specimens soon convinced naturalists like Shaw that it was neither freak nor fake.

  FIGURE 47 - A pair of duck-billed platypus, as painted by John Gould in 1863.

  Though their relationship to other vertebrates was debated for a time, the monotremes were ultimately identified as a group of archaic mammals, and they could be distinguished from other mammals by their peculiar mode of reproduction: the young of monotremes hatched from reptilelike eggs. This contrasted with the types of early development seen in the other groups of mammals, the marsupials and placentals. Marsupials are the “pouched” mammals, like opossums and kangaroos, which give birth to tiny, underdeveloped young who crawl into their mother’s pouch where they continue the rest of their early growth. The young of placental mammals, however, gestate for a longer period of time and are born more developed than newborn marsupials. There are other minute characteristics that distinguish these groups, but they are most easily separated from each other by the way in which they reproduce.

  Despite their peculiar reptilelike traits, however, Huxley did not think that the monotremes indicated a direct evolutionary link between mammals and reptiles. Using the anatomy of pelvic bones as his guide, Huxley could find little that connected the form of the platypus to any known reptiles. Rather than mammals evolving from reptiles, then, Huxley proposed that both groups had gone through a few parallel stages of development from a salamanderlike amphibian.

  It seems to me that, in such a pelvis as Salamandra, we have an adequate representation of the type from which all the different modifications which we find in the higher Vertebrata may have taken their origins.

  This wrapped the evolution of birds, reptiles, and mammals up nicely. Mammals and reptiles had evolved from salamanderlike amphibians, with birds being an offshoot of a later, dinosaurlike reptile.

  The American paleontologist O. C. Marsh, who had recently astounded Huxley with his collection of toothed birds and miniature proto-horses during Huxley’s visit to America, was inclined to agree with his British colleague. In an 1898 letter to the journal Science, Marsh expressed similar thoughts about the origin of mammals. Even two decades after Huxley’s Royal Society address it seemed the fossils that would elucidate the origin of mammals were still missing. Marsh lamented:Too often in the past a discussion on the origin of mammals had seemed a little like the long philosophico-theological controversies in the Middle Ages about the exact position of the soul in the human body. No conclusion was reached, because, for one reason, there were no facts in the case that could settle the question, while the methods of investigation were not adapted to insure a satisfactory review.

  Marsh insisted that the level of scientific discourse about mammalian origins had risen to a “higher plane” by the time of his writing, but hard evidence was still urgently needed. It was still unclear, for one thing, whether all mammals shared a common ancestry. The known types of mammals were so diverse that it would make the job of evolutionary scientists easier if the monotremes, the marsupials, and the placental mammals had separate origins.

  Even if mammals had evolved multiple times, though, the identity of their ancestral stock remained mysterious. Reptiles were a popular choice, with some authorities pointing to the Karoo fossils Owen had described, but Marsh waved them away as instances of “parallel development.” The possession of mammal-like teeth in the skull of a reptile, Marsh argued, could not be used as a clue to the true evolutionary relationship between groups because mammal-like dental arrangements had evolved multiple times in different societies of vertebrates. Using such a feature to trace evolutionary history would only throw scientists off the trail. Like his friend Huxley, Marsh thought that the earliest known fossil mammals were too unlike any known extinct reptiles to have evolved from them, and it was more probable that both reptiles and mammals had sprung from an unknown amphibian ancestor.

  By the time Marsh wrote his letter to Science, though, many more “mammal-like reptiles” had been recognized. Numerous specimens had been uncovered as a side effect of copper mining in Russia, and the first of these were described at about the same time as Owen turned his attention to the Karoo fossils. In contrast to Owen, S. S. Kutorga, of the University of St. Petersburg, thought some of the forms he described, such as Brithopus and Syodon, were actually mammals rather than reptiles with mammal-like features.

  The great “Bone Rush” of the late nineteenth century also turned up an array of bizarre new forms from North America. Named pelycosaurs by E. D. Cope, many of the North American fossils had been dug out of the rust-colored rock of Texas and seemed to show some resemblances to the Karoo fossils, such as the possession of differentiated canine and incisor teeth. But Cope also noted that the pelycosaurs, like the sail-backed Dimetrodon, closely resembled extinct amphibian relatives like the immense salamanderlike tetrapod Eryops. How they related to mammals, reptiles, amphibians, and even Owen’s theriodonts was open for debate.

  Other naturalists were just as puzzled by the collection of mammal-like creatures that had been accumulated by the twilight of the nineteenth century. Georg Baur, a young assistant to O. C. Marsh who was so committed to his research that he literally worked himself to death in 1898, considered Cope’s fossils to be a collateral branch that only shared a common ancestor with the earliest mammals. Owen’s theriodonts were either ancestral or very close to the earliest mammals, Baur hypothesized, yet they still did not create the graded series paleontologists were hoping to find. The British paleontologist H. G. Seeley was similarly frustrated. The fossils from North America and the Karoo possessed amphibian, reptilian, and mammalian characteristics. These animals were obviously close to the junctures where all three of these vertebrate groups split, but eventually Seeley, too, decided that the ancestors of mammals would be found among the early tetrapods of the
Devonian or even further back in geologic time.

  But the Scottish paleontologist Robert Broom though otherwise. Broom, who is more often remembered for his studies of early humans, spent decades studying the friable fossils of the Karoo desert, and in a 1915 review of mammalian origins Broom concluded that the features shared by both mammals and “mammal-like reptiles” indicated a close relationship between them, not an aberrant evolutionary pathway. Their bearing on the origin of mammals was “beyond question.” According to Broom the pelycosaurs had given rise to the theriodonts from which the ancestors of mammals had evolved.

  Broom initially proposed that a group of squat, weasel-like animals called cynodonts were the theriodont ancestors of mammals. These were relatively small creatures that held their limbs in a more vertical position under their bodies than their earlier relatives, and though early synapsids had only incisorlike and caninelike teeth, the cynodonts had the full complement of incisors, canines, and molars.

  If Broom was correct, the root of the mammal family tree was not to be found among the early tetrapods of the Devonian, but among the anomalous “reptiles” of the Permian (which lasted from about 295 million years ago to 251 million years ago). As the next generation of paleontologists began to study these fossils in more detail they realized that the most powerful evidence of the relationship between them and the first mammals was among the most subtle. The general outline of synapsid evolution has changed quite a bit since the time of Owen and Cope. It all started with an egg.

 

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