The overwhelming evidence that birds evolved from maniraptoran theropod dinosaurs has rekindled an interest in the work of the Victorian ana...
The overwhelming evidence that birds evolved from maniraptoran theropod dinosaurs has rekindled an interest in the work of the Victorian anatomist Thomas Henry Huxley. Many popular and technical accounts credit Huxley with being the first to propose that birds evolved from dinosaurs, but this is a misinterpretation of Huxley's work. During the 1860s Huxley was preoccupied with identifying the basic ‘groundplans’ that united vertebrate forms. Birds and reptiles were two groups united by a shared body plan, with dinosaurs representing an intermediate form. Huxley did not begin to cast dinosaurs as transitional forms between birds and earlier reptiles until he read Ernst Haeckel's Generelle Morphologie, at which time Huxley amassed ample anatomical evidence to illustrate how birds could have evolved from something dinosaur-like. Even then, however, Huxley did not say that birds had evolved from dinosaurs. As he explicitly stated in public addresses during the 1870s, small bird-like dinosaurs like Compsognathus only represented the form of what the true ancestors of birds might have looked like. Bird-like dinosaurs chiefly served to show that such a transition was possible. Thus, Huxley's views on the evolution of birds were much more complex than many modern authors appreciate.
When Hadrosaurus was first described (Foulke & Leidy 1858), the disparity in fore- and hindlimb length led the authors to suggest that it may have adopted a ‘kangaroo-like’ posture, and Cope came to similar conclusions about the theropod dinosaur ‘Laelaps’ (=Dryptosaurus (Marsh 1877)) (Cope 1867a, 1868). From this Huxley inferred similar bipedal postures for Iguanodon and Megalosaurus, but the description of Compsognathus (Wagner 1861c) and Hypsilophidon (Huxley 1870a) were more important to Huxley's hypothesis that birds had evolved from reptiles. While it was difficult to imagine birds arising from something as monstrous as a Megalosaurus, the smaller dinosaurs more closely resembled the hypothetical reptilian ancestor of birds.
Strangely, Archaeopteryx had little significance to Huxley even though he had published on it in 1868 (Huxley 1868b). Huxley's minimal interest in Archaeopteryx probably stemmed from his view that most evolution had occurred during ancient ‘non-geologic time’, and the consensus that the three-toed tracks from the Triassic of New England (Hitchcock 1836, 1858) were those of birds made the Jurassic Archaeopteryx far too young to be a bird ancestor. Even when Huxley later modified his views on persistence and transitional forms, as reflected in his 1876 lecture tour of America, Archaeopteryx was placed on an evolutionary side branch and he doubted that it resembled a stage in the reptile–bird transition (Huxley 1877). The direct ancestors of birds were also unlikely to be found among the most bird-like of the dinosaurs, and Huxley considered them the ‘modified descendants of Palaeozoic forms through which the transition was actually affected’ (Huxley 1877, p. 67). Marsh's recently discovered Cretaceous toothed birds Hesperornis and Ichthyornis, however, were marshalled as evidence of the relationship between birds and reptiles, and, although Huxley could not identify a direct line of descent, there were enough intermediates to defend the evolution of birds from reptiles.
Huxley's work on this problem was never so simple as to assert that birds evolved from dinosaurs, and the evolution of his arguments about the relationship of birds and reptiles marks a transition in his own thinking (Di Gregorio 1982; Lyons 1993) as well as a period of change in the discipline of vertebrate palaeontology.
When Charles Darwin published On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life (Darwin 1859) palaeontology presented major problems for his still-nascent evolutionary hypothesis. Although palaeontology was still a relatively young science, it was generally believed that the geological strata had been sampled adequately enough by 1859 to reveal the diversity of ancient life in each age (Rudwick 1976). If transitional forms had not yet been discovered there was little chance that they existed. What was present in one locality seemed to be present in all, and it appeared that well-studied fossil sites in Europe were representative of the entire record of life on Earth (Rudwick 1976, pp. 228–229).
Darwin's hypothesis was primarily derived from observations of living organisms (population growth, artificial selection, etc.), but his evolutionary mechanism did make predictions about ancient life. If all of life on Earth shared a common ancestor in the distant past, with evolution branching gradually instead of making ‘jumps’, then the fossil record should provide graded intermediate forms. Unfortunately, such forms were rare and failed to bridge major gaps between groups of animals. Darwin attempted to explain the negative evidence through the imperfection of the fossil record. That any ancient creature, particularly a soft-bodied animal, should be preserved as a fossil seemed unlikely, and many animals that became fossilized were only known from fragmentary remains.
In order for an evolutionary series to be preserved a group of organisms would have to live in a place with regular sedimentation events over huge expanses of time – a doubtful scenario. The problem was further compounded by the fact that the span of geological time contained gaps, blank spots in the history of life on Earth, and there were too many unpredictable factors required to preserve an evolutionary series (Darwin 1859, pp. 310–311):
For my part, following out Lyell's metaphor, I look at the natural geological record, as a history of the world imperfectly kept, and written in a changing dialect; of this history we possess the last volume alone, relating only to two or three countries. Of this volume, only here and there a short chapter has been preserved; and of each page, only here and there a few lines. Each word of the slowly-changing language, in which the history is supposed to be written, being more or less different in the interrupted succession of chapters, may represent the apparently abruptly changed forms of life, entombed in our consecutive, but widely separated formations. On this view, the difficulties above discussed are greatly diminished, or even disappear.
One of the first major responses by the palaeontological community to Darwin's work was Life on Earth by geologist John Phillips (1860). Phillips found little evidence of the gradual evolutionary series predicted by Darwin's hypothesis. Some of the oldest known fossils from the Cambrian and Silurian, for instance, already represented complex forms of life that provided no clues as to their ancestors. Phillips regarded them as new creations consistent across multiple localities generated by some unknown law of nature. As Phillips (1860, p. 214) incredulously asked, ‘How is it conceivable that the second stage should be everywhere preserved, but the first nowhere?’.
Much like the overblown claim that Huxley trounced Bishop Samuel Wilberforce in a debate at Oxford in 1860 (Gould 1991), however, the idea that Huxley perfectly anticipated the modern confirmation that birds are living dinosaurs is an example of ‘textbook cardboard’ (sensuGould 1987). This can be defined as a past notion that appears to have predicted recent discoveries but is, in reality, abstracted and ripped from their proper context, a technique often used to lend weight to a particular idea or deconstruct unfavourable notions. In this particular case, authors and researchers have cited Huxley's work to support the idea that birds were thought to have evolved from dinosaurs as soon as Archaeopteryx was discovered, and that recently discovered evidence confirms what Huxley had hypothesized nearly 150 years ago. A survey of Huxley's work, however, does not bear out such gross summation.
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