miércoles, 22 de diciembre de 2010
The Dinosaur Fossils That Changed Everything
One of the many track-bearing sandstone slabs that enthralled Edward Hitchcock.
Footprints and Feathers on the Sands of Time
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.
Science writer and research associate at the New Jersey State Museum, Brian Switek has done fieldwork on fossils in Utah, Montana and Wyoming. He is a frequent guest on the BBC and has written about paleontology for Smithsonian magazine, London Times, Wired Science and elsewhere. He is the author of the science blog Laelaps here on our Wired Science Blogs network and Smithsonian magazine’s Dinosaur Tracking blog. Written in Stone is his first book.
Have a look at some of the strangest fossils in the author’s gallery of strange, historically significant fossils.
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. 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.
One of the many track-bearing sandstone slabs that enthralled Edward Hitchcock.
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. 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.
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. 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. 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.
Huxley began his scientific career in 1846 by studying marine invertebrates while serving as an assistant surgeon aboard the HMS Rattlesnake. His work was well received by other naturalists, and when he returned to England in 1850 he was set to establish himself among the scientific elite. Like the man who would become his rival, Richard Owen, Huxley was most concerned with the underpinnings of anatomical form, but where Owen cloaked his work in pious rhetoric, Huxley’s distaste for religious interference in science may have attracted him to Darwin’s theory of evolution in the first place. While Huxley disagreed with Darwin on some key points, natural selection was the best mechanism yet proposed for evolutionary change. For natural selection to make sense, however, the absence of graded transitions in the fossil record had to be accounted for, which Huxley explained through the concept of “persistent types.”
Throughout the fossil record there seemed to be little evolutionary change; crocodiles looked like crocodiles no matter what strata they came from. Instead of being evidence against evolution, however, Huxley proposed that the persistent forms were echoes of evolutionary changes that had occurred in a time so distant that it was not recorded in the rock. If most of evolution happened during “non-geologic time,” then the inability of naturalists to explain the origin of major groups of animals with fossil evidence became a moot point. The caprices of geology kept them out of science’s reach.
Thomas Henry Huxley, photographed around 1870.
This concept was a double-edged sword. It removed the problem of missing transitional forms but it made it nearly impossible to determine evolutionary relationships through fossil evidence. But Huxley was not concerned with drawing out ancestors. Instead, he was after the common denominators of animal form, and birds and reptiles provided a key example of how the same plan could be modified to different ends. During his 1863 lectures on vertebrate anatomy at the Royal College of Surgeons, Huxley asserted that birds were “so essentially similar to Reptiles in all the most essential features of their organization, that these animals may be said to be merely an extremely modified and aberrant Reptilian type.” Reptiles, too, shared similarities with birds, and to reinforce these connections Huxley placed both birds and reptiles into an encompassing group called the “Sauropsida” (thus labeling birds “reptile-faced”).
Huxley reiterated this point in his 1867 survey of birds. Reptiles and birds were modifications of the same “groundplan,” with living reptiles being closer to the hypothetical framework from which each had been adapted. If one were to compare a turtle with a dove, this association might seem laughable, but it was not among the lowly lizards and snakes that the best evidence for the connection between reptiles and birds was to be found. The solution of a fossil puzzle provided a better set of candidates.
While traveling through England that same year Huxley met geologist John Phillips, who invited Huxley to visit the museum at Oxford with him. As the naturalists strolled through the geology collection Huxley noticed something strange about the bones of the dinosaur Megalosaurus on display. A portion of its shoulder blade was actually part of the hip. Once this scrap was put in its proper place other fragments caught Huxley’s eye. When the two scientists finished reorganizing the bits of bone, they found they had restored a predator with small forelimbs and a birdlike pelvis. This new shape for Megalosaurus pointed to a deeper relationship between reptiles and birds that Huxley had inadvertently been amassing evidence for since his Royal College lectures. Dinosaurs were much more birdlike than any living reptiles, and inspired by the branching evolutionary trees in the 1866 book Generelle Morphologie by the German embryologist Ernst Haeckel, Huxley started to think of how birds actually could have evolved from reptiles. In January 1868 Huxley outlined a preliminary line of descent in a letter to Haeckel.
In scientific work the main thing just now about which I am engaged is a revision of the Dinosauria — with an eye to the Descendenz Theorie! The road from Reptiles to Birds is by way of Dinosauria to the Ratitae — the Bird “Phylum” was Struthious, and wings grew out of rudimentary fore limbs.
Huxley would unveil this evolutionary trajectory to his peers later that same year. Even though there was no direct evidence for the transition he was proposing, the forms that had already been found suggested that the connection between birds and reptiles was real. Archaeopteryx, for instance, was clearly a bird with reptilian characteristics. He conceded that it was “more remote from the boundaryline between birds and reptiles than some living Ratitae [flightless birds such as ostriches and emus] are,” and therefore not a direct ancestor of modern birds, but it still illustrated the point that birds could have evolved from reptiles. While the complete evolutionary series had yet to be found, the anatomical resemblances between flightless birds and fossil creatures like Megalosaurus suggested that the first birds had been derived from something resembling a dinosaur. This was made possible by a major change in the way paleontologists understood dinosaurs.
Two visions of Megalosaurus. While originally envisioned as an immense crocodilelike beast, as shown by the restoration on the left, by the latter part of the nineteenth century naturalists had greatly revised the appearance of the dinosaur, as shown by the restoration to the right. Unfortunately, since so little is known of Megalosaurus, we can only base our ideas of what it looked like on related theropod dinosaurs.
The first dinosaurs known to science, Megalosaurus and Iguanodon, were initially thought to have looked like enormous crocodiles and lizards. So little was known of them that they were easily cast as larger versions of known reptiles, but when Richard Owen grouped them within the Dinosauria in 1842 he gave them an anatomical overhaul. Dinosaurs, as he envisioned them, were warm-blooded creatures that carried their limbs directly beneath their bodies. They were the “highest” of the reptiles, much more impressive than their degenerate reptilian kin that inhabited the modern world, but the fragmentary nature of their remains left most of their anatomy uncertain. The discovery of a more complete dinosaur revealed that dinosaurs looked strikingly different from what Owen envisioned.
Found in the sandy marl of New Jersey in 1858, and later described by William Parker Foulke and Joseph Leidy, Hadrosaurus was a Cretaceous herbivore related to Iguanodon. Unlike Owen’s reconstruction of Iguanodon, however, its skeleton suggested that it walked upright at least some of the time. A predatory dinosaur from nearby deposits called Laelaps by its discoverer E. D. Cope (later renamed Dryptosaurus by his rival O. C. Marsh) also shattered Owen’s dinosaurian archetype. This New World relative of Megalosaurus walked on two legs, and the fact that the forelimbs of the animal were much shorter than the hind limbs caused Cope to envision an active, hot-blooded dinosaur that relied on its powerful hind limbs to kill:
This relation [between the hind limbs and forelimbs], conjoined with the massive tail, points to a semi-erect position like that of the Kangaroos, while the lightness and strength of the great femur are altogether appropriate to great powers of leaping. . . . If he were warm-blooded, as Prof. Owen supposes the Dinosauria to have been, he undoubtedly had more expression than his modern reptilian prototypes possess. He no doubt had the usual activity and vivacity that distinguishes the warm-blooded from the cold-blooded vertebrates. We can, then, with some basis of probability imagine our monster carrying his eighteen feet of length on a leap, at least thirty feet through the air, with hind feet ready to strike his prey with fatal grasp, and his enormous weight to press it to the earth. Crocodiles and Gavials must have found their bony plates and ivory no safe defence, while the Hadrosaurus himself, if not too thick skinned, as in the Rhinoceros and its allies, furnished him with food, till some Dinosaurian jackalls dragged the refuse off to their swampy dens.
Compsognathus, as restored in Huxley
If Hadrosaurus and Dryptosaurus walked on two legs it was reasonable that Iguanodon and Megalosaurus could have done the same. Three-toed tracks from the same deposits that yielded Iguanodon were in accord with the idea that it was bipedal at least some of the time, and Huxley’s own revision of Megalosaurus at Oxford suggested that it also stalked about on two legs. Yet these animals presented a substantial problem for Huxley’s evolutionary program. They were enormous animals, far too large to be good models for the forerunners of birds.
The chicken-sized dinosaur Compsognathus was a far better candidate for the sort of creatures from which birds evolved. Discovered in 1861 from the same quarries that yielded Archaeopteryx, it was more birdlike than any of its gargantuan relatives, especially in details of its hind limbs and ankles. This similarity had been recognized by the German anatomist Carl Gegenbaur in 1864, and even the anti-evolutionist Wagner drew attention to it in his description of the animal; but where Wagner disavowed that the similarities were evidence for evolution, Compsognathus was Huxley’s prime evidence that birds had sprung from reptiles. Speculating upon what it might have looked like in life, Huxley wrote:
It is impossible to look at the conformation of this strange reptile and to doubt that it hopped or walked, in an erect or semi-erect position, after the manner of a bird, to which its long neck, slight head, and small anterior limbs must have given it an extraordinary resemblance.
With this new vision of dinosaurs in place, Huxley continued to accumulate evidence that birds had been derived from the dinosaur body plan. The small dinosaur Hypsilophodon, while less birdlike than Compsognathus, was significant as it provided Huxley with the first good look at a complete dinosaurian pelvis. The process that normally extended forward in reptiles, the pubis, was rotated backward to meet the ischium, as in birds. Huxley thought it reasonable that all dinosaurs had this arrangement, and he also appealed to embryology to imply that at certain states developing chicks exhibited dinosaurlike traits.
If the whole hind quarters, from the ilium to the toes, of a half-hatched chicken could be suddenly enlarged, ossified, and fossilized as they are, they would furnish us with the last step of the transition between Birds and Reptiles; for there would be nothing in their characters to prevent us from referring them to the Dinosauria.
The English paleontologist Harry Seeley criticized this interpretation. The congruence between the hind limbs of birds and dinosaurs could be attributed to a shared mode of life, Seeley argued, and not a family relationship. In Seeley’s view, walking bipedally on land had caused the legs of both dinosaurs and birds to take similar form, and thus the resemblance was only skin-deep. This was particularly significant as Seeley had specialized in studying another group of reptiles that he thought were closer to birds.
The first pterosaur known to science was discovered in 1784 in a German limestone quarry. With a tooth-studded snout, lizardlike hind limbs, and a ludicrously long fourth finger on each hand, the creature was unlike any that had been seen before. The man who described it, Italian naturalist Cosmo Alessandro Collini, thought it was a swimmer, since it had come from marine deposits. Others disagreed and proposed that it was closely related to bats, but in 1809 Georges Cuvier recognized it as a unique kind of extinct flying reptile. He dubbed it Pterodactylus, or “wing finger.”
Not everyone was in agreement with Cuvier. In 1830, the German researcher Johannes Wagler reconstructed the animal as something of a cross between a swan and a penguin, which sculled about the surface of the water with a paddle supported by the elongated finger. Another specimen discovered in 1828 by fossil hunter Mary Anning was investigated by William Buckland. The creature was clearly a reptile, but Buckland was perplexed by its features, and he thought that, like Milton’s “Fiend” in Paradise Lost, the pterosaur could have swum, sunk, waded, crept, or flown through a strange ancient world. By the 1840s, however, there was little doubt that Cuvier had been correct, and some naturalists were very impressed by resemblances between the skeletons of the flying fiends and birds. As Richard Owen stated in an 1874 monograph of Mesozoic fossil reptiles:
Every bone in the Bird was antecedently present in the framework of the Pterodactyle; the resemblance of that portion directly subservient to flight is closer in the naked one to that in the feathered flier than it is to the forelimb of the terrestrial or aquatic reptile.
Just like Owen, Seeley saw no way to “evolve an ostrich out of an Iguanodon,” but Huxley turned the argument from convergence against his opponents. The traits supposedly shared between birds and pterosaurs had to do with flight, and given that both lineages had become adapted to flying, common traits in their skeletons were to be expected. The diagnostic traits in the hips, legs, and feet of dinosaurs, on the other hand,were found in all birds, not just ground-dwelling ones. This meant that these characters marked a true family relationship and not just a shared way of life.
To formalize this new image of dinosaurs Huxley placed them in new taxonomic groups to underline their avian characteristics. The dinosaurs and Compsognathus (which Huxley considered to be the closest relative to dinosaurs but not one itself) were put together under the name Ornithoscelida, making them the “bird-legged” members of the “reptile-faced” Sauropsida. Yet, despite all the work he had done on the topic, Huxley could not rule out any of the dinosaurs then known to be bird ancestors. Some represented the form the real ancestors may have taken, but that was all.
Huxley explained this argument in an 1870 presidential address before the Royal Society. In searching for evolutionary lineages, Huxley warned, “it is always probable that one may not hit upon the exact line of filiation, and, in dealing with fossils, may mistake uncles and nephews for fathers and sons.” To prevent this sort of confusion he drew a distinction between intercalary types, or representations of the form of ancestors and descendants, and linear types, which were the actual ancestors and descendants.
At the present moment we have, in the Ornithoscelida the intercalary type, which proves that transition ["from the type of the lizard to that of the ostrich"] to be something more than a possibility; but it is very doubtful whether any of the genera of Ornithoscelida with which we are at present acquainted are the actual linear types by which the transition from the lizard to the bird was effected. These, very probably, are still hidden from us in the older formations.
After 1870 Huxley’s paleontological work slowed.He was in over his head delivering lectures, writing papers, and engaging in the politics of science — so much so that he burned himself out. His wife, Nettie, sent him on a vacation to Egypt in 1872 with the hope that he would recover from the stress, and when Huxley returned he started on a new tack. He turned his attention to the minutiae of anatomy under the microscope, largely setting aside the old bones that had previously transfixed him.
The reconstructed skeleton of Hesperornis. As a bird with teeth, it further confirmed the connection between birds and reptiles that Huxley highlighted.
But Huxley did not abandon the evolution of birds entirely. In 1876 he set out on a lecture tour of the United States, and one of his first stops was Yale’s Peabody Museum run by American paleontologist O. C. Marsh. Though little new information about the origin of birds had been found since the time of Huxley’s 1870 address, Marsh had recently found the remains of toothed birds in the Cretaceous-age chalk of Kansas. One of the birds, Hesperornis, had tiny nubs for wings and looked like a loon with a tooth-studded beak; the other, Ichthyornis, would have looked more like a toothed gull in life.
Marsh’s odontornithes (“toothed birds”) strengthened the link between reptiles and birds, but they were geologically too young to indicate from what group birds had evolved. Along with Archaeopteryx and Compsognathus, and the early Jurassic dinosaurs which made the Connecticut Valley tracks, they could not be placed on a straight evolutionary line but instead signaled what Huxley believed was an earlier transition:
It is, in fact, quite possible that all these more or less avi-form reptiles of the Mesozoic epoch are not terms in the series of progression from birds to reptiles at all, but simply the more or less modified descendants of Palaeozoic forms through which that transition was actually effected.
We are not in a position to say that the known Ornithoscelida are intermediate in the order of their appearance on the earth between reptiles and birds. All that can be said is that if independent evidence of the actual occurrence of evolution is producible, then these intercalary forms remove every difficulty in the way of understanding what the actual steps of the process, in the case of birds, may have been.