page_title: The Origin of Birds
section_title: Part III: Some Anatomical and Biological Data
hierarchical_section_title: The Origin of Birds / Book outline / Part III: Some Anatomical and Biological Data
caption_attribution_description: English: Gerhard Heilmann's comparative illustrations of the feet and scale shields of various extant birds and reptiles: green lizard (1), viviparous lizard (2), alligator (3), hooded crow (4), pheasant (5), black grouse (6), and ptarmigan (7).
context_page_description: The Origin of Birds is an early synopsis of bird evolution written in 1926 by Gerhard Heilmann, a Danish artist and amateur zoologist. The book was born from a series of articles published between 1913 and 1916 in Danish, and although republished as a book it received mainly criticism from established scientists and got little attention within Denmark. The English edition of 1926, however, became highly influential at the time due to the breadth of evidence synthesized as well as the artwork used to support its arguments. It was considered the last word on the subject of bird evolution for several decades after its publication.
  Through the course of the research represented in the book, Heilmann considers and eventually rejects the possibility of all living and several extinct groups of reptiles as potential ancestors for modern birds, including crocodilians, pterosaurs and several groups of dinosaurs.
context_section_description: Part III deals with anatomical comparisons between extant birds and reptiles, wherein Heilmann finds traces of the relationship between them in examples of fenestrae, claws, the brain, sense organs, sexual organs, and other features. He concludes that many of these features are "nearly identical" between reptiles and birds. He cites other features as being clearly derived from one another, such as the avian feather essentially being a cylindrical, fringed scale.
  He begins the section with an analysis of the temporal opening found in the skull of many extant birds. After a thorough comparison, he rejects the notion, which was common at the time, that this temporal opening was homologous with the supratemporal fenestra in reptiles. Instead, he concludes that it is a recent feature. Next he makes some observations on the wing structure of modern nestling birds. He finds that some species of extant birds have claws on their first and second fingers when very young, and some, like the hoatzin, will even use these temporary claws to climb about, including in the branches of trees. He also discovers that far more nestlings have a nonfunctional claw on the first digit, and some adult birds do as well.
  Organs are next examined in detail, with various comparisons drawn between reptiles and birds. He starts with the brain, analyzing in detail the cerebral and cerebellar structure of several animals including birds, crocodilians, and mammals. Though noticeably more developed, Heilmann finds that the general structure of the brain is very similar in birds and reptiles, and he describes the brain of modern birds as "further evolution of the peculiar characters already found in the reptile." He also finds the eyes of birds and reptiles to be remarkably similar, especially the development of the lens, as well as the ear, which reveals a much wider gap between the Sauropsida and mammals than with birds. He describes the sexual organs of birds and reptiles to likewise be structurally similar, and finds that while most species of male bird have lost the penis to reduce weight, those that retain it bear remarkable similarities to extant reptiles. He writes that secondary sexual characteristics are also similar between birds and reptiles, with both groups frequently utilizing bright colors and structures for display. He concludes the section by offering a few more comparisons of structures and organs, including the lungs, ambiens muscle, and beak and scale sheaths. Taken together, Heilmann interprets these many similarities as further evidence for the close kinship of birds and reptiles.
caption: Heilmann's comparative illustrations of the feet and scale shields of various extant birds and reptiles
exif: {}