The Origin of Birds
ISBN 0-486-22784-7 | | |
Followed by | Univers og traditionen (1940)[1] |
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The Origin of Birds is an early synopsis of
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
Background
When Heilmann began his research in the early 1900s, the early bird Archaeopteryx was only known from three fossils found in the limestone quarries of Solnhofen near Eichstätt, Germany. The three fossils consisted of two nearly complete skeletons found in 1861 and 1877 and a single feather from 1860. They had been discovered just a few decades after the discovery of the dinosaurs, and as some dinosaurs appeared somewhat birdlike, Archaeopteryx was regarded as a possible "missing link" between reptiles and birds by many paleontologists at the time.[5]
The similarities between Archaeopteryx, known dinosaurs and extant birds were examined and emphasized, with
While the dinosaur-bird connection (or lack thereof) was being pursued in paleontology, the problem of the
Publication
Between 1913 and 1916, Gerhard Heilmann published a series of articles in the journal of the
Heilmann's original articles were publicly ignored by Danish zoologists, but caused considerable unrest behind the scenes. Danish zoologist R.H. Stamm, for example, mocked Heilmann in private letters to Helms shortly after the publication of his first article. Other Danish zoologists openly expressed their disdain for Heilmann's work as well, including the professors of zoology in Copenhagen University J.E.V. Boas and Hector Jungersen. Despite this, Helms continued to steadfastly support Heilmann and his ideas, which eventually began to gain attention from abroad. In April 1913, the American expert in fossil birds R. W. Shufeldt came across Heilmann's first article by chance. Shufeldt, who was married to a Norwegian, could understand some Danish and was able to interpret Heilmann's work and initiated contact with him shortly thereafter. This opened up the opportunity for international correspondence with distinguished paleontologists for Heilmann, which was instrumental to the recognition of his work outside of Denmark.[5]
Heilmann's articles were later collected and published as a book in Danish in 1916 with the title as in his series, Vor Nuvaerende Viden om Fuglenes Afstamming ("our present knowledge about the origin of birds").[9] This met the same lack of interest and is thought to have been a result of publishing in Danish, as this made it inaccessible to many scientists working in the U.S. and much of Europe.[1] The field of paleontology at the time was dominated by American and English scientists, and the discipline in continental Europe was dominated by Germany and to a lesser degree France.[2]
Consequently, and at the urging of Shufeldt, Heilmann devoted much of the next few years attempting to find an English–language publisher willing to undertake a translation of his work. None of the larger publishers he approached were willing to do so unless Heilmann himself was prepared to finance it, which he was unable to do.
Editions
While most of the original material was published in the journal of the Danish Ornithological Society between the years of 1913 and 1916, the first English version of the book was published in London in 1926 by H. F. & G. Witherby. It was published in the United States the following year by D. Appleton & Company. The book was reprinted in 1972 by Dover Publications, Inc.,[10] with the only change being the grayscale reproduction of several illustrations originally published in color.[11]
The English and Danish editions differed in several significant ways. The English edition was somewhat shorter and more concise, and included newer information that Heilmann had acquired by studying the actual fossils of Archaeopteryx in Berlin, as well as from foreign scientists who sent him photographs and plaster casts. It also contained considerably less harsh language towards Boas and others with whom he disagreed. Curiously, the English edition did not contain the transformational sequences inspired by D'Arcy Thompson, though Heilmann still briefly acknowledged the use of his methods in reconstructing his Proavis.[8]
Book outline
Heilmann's book was divided into four main parts. The first three draw evidence for bird evolution from the fossil record, from the embryos of birds and other animals, and from living birds, respectively. The fourth and most groundbreaking section examines several groups of extinct animals in order to determine the probable root of modern birds.[1]
Part I: Some Fossil Birds
In the first section, Heilmann examines in exquisite detail the fossil remains of several extinct birds, including Hesperornis, Ichthyornis, Archaeopteryx, and "Archaeornis", the name used commonly at the time for the Berlin specimen, which was then thought to represent a separate genus. Throughout this section are many finely detailed and labeled renderings of different parts of the skeletal anatomy of these birds, as well as other groups of extinct reptiles and some modern birds. Heilmann's attention to detail in his artwork is inspired at least in part by his dissatisfaction with anatomical renderings of these animals in scholarly works of the time, which he deemed as "unsatisfactory" and "containing misleading errors."[12]
Early in this section Heilmann embarks on a thorough description of the Berlin Archaeopteryx specimen, which includes detailed comparisons to specific aspects of modern birds. Following a comparison of its skull to that of
He goes on to state that other features of Archaeopteryx, however, are remarkably birdlike and bear little resemblance to their reptilian analogues. The hand of Archaeopteryx is observed as being one of its most remarkable features, having what Heilmann calls a "reptilian basis" which has come to support primary feathers. He compares this to a primitive five-digit reptile hand, noting the obvious differences, before outlining the striking similarity of the hand to that of the theropod Ornitholestes.[17] Here Heilmann goes into considerable detail about the wing arrangement of Archaeopteryx, drawing from his observation of the Berlin specimen.[18] The latter part of this section deals with analyzing the skeletal anatomy of the fossil birds Hesperornis and Ichthyornis, but Heilmann ultimately decides that they are of no importance to his investigations.[19] He concludes the section by stating that Archaeopteryx "may be characterized as a reptile in the disguise of a bird", and states that his studies must turn from the skeleton to the soft tissue in order to reach a final conclusion.[20]
Part II: Embryonic Stages of Reptiles and Birds
In this section, Heilmann draws evidence from his observations of
He examines in detail the expression of evolutionary stages in the development of embryos, tracing from the process of cell division to the development of specific anatomical features. He finds a striking resemblance between the embryonic development of reptiles and birds, including details of the skeletal anatomy (with special attention to the hands and feet) and various organs. He notes that bird and reptile embryos develop visceral arches, hinting at their aquatic ancestry. Of more interest to his goal, Heilmann writes in a similar vein that the embryos of certain birds clearly show a three-clawed finger structure, at least one of which (the hoatzin) retains actual claws after hatching. He mentions other anatomical features of bird embryos that hint at their reptilian ancestry as well, such as the embryonic splitting of the pygostyle into distinct separate vertebrae.[23]
Part III: Some Anatomical and Biological Data
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.[24]
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.[25] 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.[26]
Organs are next examined in detail, with various comparisons drawn between reptiles and birds. He starts with the brain, analyzing in detail the
Part IV: The Proavian
In the final section Heilmann seeks to synthesize the information in the previous three sections to uncover the probable origins in a particular group of ancestors. In doing so he discusses the specific morphology of a hypothetical creature, which he refers to as "the proavian", that must have existed between modern birds and their reptilian ancestors. After making a bold assertion that birds are descended from reptiles, Heilmann cites Dollo's law of irreversibility as the primary reason why he believes birds cannot be descended from theropod dinosaurs, despite their many morphological similarities. Dollo's Law states that a feature or organ once lost by evolution cannot be regained. One consistently confounding issue Heilmann had encountered in his research into the bird-reptile link was that modern birds possess a wishbone and theropod dinosaurs, by his observations, did not. Since ancient reptilian fossils that predated dinosaurs clearly possessed a different sort of wishbone, Heilmann concluded that this feature could not have been lost and regained again over the course of evolution. Based on this law, he therefore rejected the possibility of a direct theropod ancestor of birds, though he acknowledged that theropods and birds must have shared a close relationship.[31]
Throughout this section, Heilmann examines several groups of possible ancestors in addition to
He concludes this final section by fleshing out his Proavis and summarizing his view of bird origins, in which birds would have departed from reptiles at the pseudosuchians. From this branching point, birds and dinosaurs would have evolved along parallel evolutionary tracks for millions of years, as cousins rather than ancestors.[33] He imagines that these reptiles would have gradually assumed a bipedal gait, and transformed eventually from terrestrial runners to arboreal climbers, developing leaping capabilities ever increasing in length. Along the way, the ancestral reptilian scales would have become "frayed" and gradually developed into feathers, beginning along the forearm and tail and gradually spreading to the entire body. The need for this animal to be an adept climber would have catalyzed the lengthening of its phalanges, which would eventually become long and strong enough to support a wing. Powerful muscles would have developed to anchor these limbs, which would have reacted upon the breastbone. All of this together would have facilitated the origin of an accelerated metabolic rate, resulting in the warm-blooded state known of modern birds. This development would have occurred alongside of the enlargement of the brain, needed to coordinate and supervise these refined features. It is in this way, Heilmann concludes, the reptile has been changed into a bird.[34]
Legacy
In 1868, Thomas Huxley published On the animals which are most nearly intermediate between birds and reptiles, making a strong case for the bird-dinosaur ancestral link.[35][failed verification] Huxley's proposal that birds arose from dinosaurs – based primarily on his observation of the similarities between Archaeopteryx and Compsognathus – remained respectable and widespread in the paleontological community until the publication of The Origin of Birds.[36] Heilmann more than anyone else was responsible for a widespread rejection of the dinosaur-bird link.[37] His conclusions involved more than the mere rejection of a dinosaurian ancestry for birds, however: for instance, he favored the arboreal origin of avian flight, he allied birds firmly with reptiles instead of mammals, and he was responsible for finally putting an end to the idea that birds descended from pterosaurs.[38] Several aspects of his research have continued to be influential long after its publication.[39]
Wishbones and the dinosaur-bird link
The proposed thecodontian ancestry eventually fell out of favor, in part because the clade "thecodontia" is not monophyletic, meaning that it has no unique, diagnostic characteristics (and is largely considered an obsolete grouping today).[40] But the principal reason why Heilmann's hypothesis was proven incorrect lies in the issue of the clavicle. Heilmann came extremely close to linking theropods and birds to one another, even going so far as to write that "it would seem a rather obvious conclusion that it is amongst the Coelurosaurs that we are to look for the bird ancestor." However, he was prevented from doing so by the apparent lack of the clavicle in the predatory dinosaurs, which reflected his strict adherence to Dollo's Law: reptilian ancestors had possessed a clavicle, but had lost it at some point during their evolution to the dinosaurs. Therefore, in order for Heilmann to believe that bird ancestry lie in the dinosaurs, there would need to be proof of clavicles in dinosaurian clades.[38]
Most coincidentally, a small theropod dinosaur named Oviraptor philoceratops had been discovered while Heilmann was working on his book, and was described by Henry Fairfield Osborn in 1924. Figure 8 of Osborn's paper, redrawn from the fossil, shows what was later proven to be an oviraptorid wishbone between the animal's arms, interpreted by Osborn as the interclavicle ("Ic." in Figure 8).[41] Unfortunately, this important structure was misidentified.[42] If Heilmann had examined this paper as closely as he had much of his source material – or had travelled to New York to see the specimens in person – he may have reversed his conclusions entirely.[38]
As it was, Heilmann's conclusion was so persuasive that it forestalled further debate on the subject even in the face of additional conflicting evidence. In 1936, paleontologist Charles Lewis Camp described a new theropod from the Jurassic of North America, Segisaurus. Like Oviraptor, Segisaurus had an unmistakable clavicle,[43] but unlike Oviraptor, it was also plainly identified as being such in the paper that described it. Despite this, the implication of a wishbone-bearing dinosaur was blatantly ignored until much later, and for many years the state of dinosaur research stagnated,[38] possibly due to the effects of the Depression and World War II.[44]
The reinvigoration of interest in the dinosaur–bird link was largely due to the discoveries and research of paleontologist
Models of flight evolution
Heilmann envisioned that birds evolved from ground–dwelling animals that became arboreal and capable of jumping between branches over time. Their descendants would eventually be able to glide as the length of leaps increased, leading to greater specialization and eventual flapping capabilities. This "from the trees down" hypothesis was originally proposed by Othniel C. Marsh in 1880. This general theory about the mode of the evolution of flight in birds has persisted to modern times, especially (but not exclusively) among opponents of a theropod origin of birds.[47] This includes paleontologists such as Alan Feduccia, who essentially agree with Heilmann's assertion that the ancestors of birds must have been arboreal.[48]
The origin of bird flight itself is still hotly debated. It is clear that feathers must have been a prerequisite for flight in birds (though flight may have not been a prerequisite for feathers). There has yet to be a consensus on whether flight involved from the ground up or the trees down, and Heilmann was largely responsible for popularizing the trees-down idea early on. The modern idea of the arboreal (or trees-down) hypothesis has changed little since Heilmann's time, and it states that bird flight would have originated by climbing birds gliding down from the tops of trees, making gliding a precursor to flapping or powered flight. As these early birds became more efficient gliders, they would have begun to extend their range and capability by developing more powerful flight. In this proposed mode of bird evolution, Heilmann's "Proavis" is strongly implicated, which would have probably been a climbing, perching, early-stage gliding quadruped. This hypothesis for the origin of bird flight has had many adherents, including Walter J. Bock and Alan Feduccia.[40]
Competing with the arboreal hypothesis is the idea that birds evolved from running bird ancestors, known as the cursorial (or ground up) hypothesis. This scenario may have involved ancient birds jumping or running along the ground and briefly becoming airborne, perhaps to avoid obstacles or catch insects. As these animals strove to overcome the force of gravity, powered flight may have appeared early on. They may also have used their ground speed to run up trees or other steep slopes, developing increasingly sophisticated flapping mechanism to assist with this. This model requires a highly cursorial and feathered ancestor. Proponents of the cursorial hypothesis cite the legs, feet and hands of Archaeopteryx as inheritance from a cursorial maniraptoran ancestor. This model has also had many adherents over the years, including John Ostrom and Jacques Gauthier.[40]
The arboreal hypothesis was popular in Heilmann's day, even prior to his research, as it had been advanced by Marsh. It fell out of favor following Ostrom's research in the 1960s and 1970s, which suggested that the ancestors of birds were fast–running bipedal animals, lending credence to the cursorial model. The focus shifted back to the arboreal model when several Chinese non–avian theropods from the Early Cretaceous were found in the early 2000s. These new finds, represented chiefly by
Beebe's leg-wings
In the fourth section of The Origin of Birds, Heilmann examines the Tetrapteryx hypothesis proposed by William Beebe in 1915. This hypothesis was based on observations of bird embryos and hatchlings, which Beebe found to possess a presumably atavistic fringe of flight feathers on their hindlimbs. His main evidence came from examination of incipient quill feathers on the thigh of a four-day-old white-winged dove. He theorized based on this embryological fringe and the recapitulation theory that birds had once passed through a "Tetrapteryx" stage in their distant evolution, which he represented as a hypothetical four-winged gliding animal.[50]
Heilmann, though excited about Beebe's idea, found little evidence for these leg-wings when studying the nestlings in the Zoological Museum collection in Copenhagen. He also examined the nestlings of more basal bird species, such as the ostrich and the emu, searching for a trace of leg-wings there, again without success. Even after studying the nestlings of birds closely related to Beebe's doves, including pigeons, he still found no trace of leg-wings. Instead he found on the nestlings' thighs "a series of permanent feathers, and no atavism. If it were a genuine relic from such a very remote past, it would make its appearance, like a glimpse, in the embryo or squab, quickly to vanish again." Heilmann went on to outline the morphological difficulties involved in such a leg-wing, indicating that it could hinder survival. In effect, Beebe's Tetrapteryx theory was completely disregarded by Heilmann, and this remained the consensus in ornithological literature until much later.[51] Despite this, Heilmann's Proavis illustrations sport a short border of feathers behind the thigh, which some authors have suggested was inspired by Beebe's Tetrapteryx idea.[52]
Beebe was relatively undeterred by the scientific community's acceptance of Heilmann's rejection of his theory, as is evidenced by the fact that he was still writing about his Tetrapteryx hypothesis well into the 1940s.[53] His adherence to his theory was well-placed, as in 2003 a revolutionary discovery was made in the early Cretaceous Jiufotang Formation of Liaoning, China: Microraptor gui, the small, four-winged dromaeosaur that had led to renewed credence of the arboreal model for the origin of bird flight. This animal's most remarkable feature was the existence of long, pennaceous feathers on both its arms and legs, forming a set of four aerodynamic wings that its discoverers theorized were used for gliding.[54] This discovery had the immediate effect of resurrecting the idea that leg feathers may have had some bearing on the origin of flight in birds, building on the idea originally proposed by Beebe and rejected by Heilmann.[55]
Today, Beebe's 1915 description of his hypothetical four-winged bird ancestor is regarded as prescient, and there is no doubt that Microraptor looks startlingly similar to the almost-century old Tetrapteryx illustrations.[56] After its discovery, Microraptor had the effect of both reinvigorating the arboreal hypothesis as well as finally putting an end to the widespread acceptance of Heilmann's disregard for the Tetrapteryx theory.[57]
Footnotes
- ^ a b c d e f Chambers (2002) pp. 154–185.
- ^ a b Nieuwland (2004) pp. 1–11.
- ^ Shipman (1998) p. 110.
- ^ Weishampel, Dodson & Osmólska (2004) p. 220.
- ^ a b c d e f g h Ries (2007) pp. 1–19.
- ^ Alexander & Vogel (2004) p. 197.
- ^ Long (2008) pp. 3–4.
- ^ a b Ries (2010) pp. 69–91.
- ^ Salomonsen (1946) pp. 146–149.
- ^ Worldcat.org
- ^ Heilmann (1926)
- ^ Heilmann (1926) p. 3.
- ^ Heilmann (1926) pp. 5–9.
- ^ Heilmann (1926) pp. 15–21.
- ^ Heilmann (1926) pp. 22–23.
- ^ Heilmann (1926) p. 30.
- ^ Heilmann (1926) pp. 23–25.
- ^ Heilmann (1926) pp. 26–32.
- ^ Heilmann (1926) pp. 38–56.
- ^ Heilmann (1926) p. 57.
- ^ Heilmann (1926) pp. 61–63.
- ^ Heilmann (1926) pp. 64–65.
- ^ Heilmann (1926) pp. 61–94.
- ^ Heilmann (1926) pp. 97–132.
- ^ Heilmann (1926) pp. 97–100.
- ^ Heilmann (1926) pp. 100–105.
- ^ Heilmann (1926) pp. 105–109.
- ^ Heilmann (1926) pp. 110–114.
- ^ Heilmann (1926) pp. 115–125.
- ^ Heilmann (1926) p. 132.
- ^ Heilmann (1926) pp. 139.
- ^ Heilmann (1926) pp. 196–199.
- ^ a b Bakker (1986) pp. 306–318
- ^ Heilmann (1926) p. 202.
- ^ Huxley (1868) pp. 66–75
- ^ Paul (1988) p. 195.
- ^ Wilford (1985) pp. 186–187.
- ^ a b c d Paul (2002) pp. 9–10.
- ^ Feduccia (1999) p. 55.
- ^ a b c Fastovsky & Weishampel (2005) p. 324.
- ^ Osborn (1924) pp. 1–12.
- ^ Paul (2003) p. 191.
- ^ Camp (1936) pp. 39–56.
- ^ Paul (2010) p. 10.
- ^ Ostrom (1969) pp. 1–165.
- ^ Ostrom (1976) pp. 91–182.
- ^ Chiappe (2007) pp. 33–34.
- ^ Tudge (2010) p. 58.
- ^ Martin (2006) p. 470.
- ^ Beebe (1915) pp. 38–52.
- ^ Welker (1975) pp. 168–169.
- ^ Christiansen (2003) pp. 99–118.
- ^ Beebe (1942)
- ^ Xu et al (2003) pp. 335–340.
- ^ Fucheng et al (2006) pp. 395–404.
- ^ Prum (2003) pp. 323–324.
- ^ Chinsamy-Turan (2005) p. 135.
See also
References
- Alexander, D.E. & Vogel, S. (2004). Nature's Flyers: Birds, Insects, and the Biomechanics of Flight. The Johns Hopkins University Press. ISBN 0-8018-6756-8.
- Bakker, Robert T. (1986). The Dinosaur Heresies. William Morrow and Company. ISBN 0-688-04287-2.
- Beebe, C. W. A. (1942). "Evolution of a Lizard into a Bird". Proceedings of the Eighth American Scientific Congress. 3 (296).
- Beebe, C. W. A. (1915). "Tetrapteryx stage in the ancestry of birds". Zoologica. 2: 39–52.
- Camp, C. (1936). "A new type of small bipedal dinosaur from the Navajo sandstone of Arizona" (PDF). Bull. Dept. Geol. Sci. (University of California). 24.
- Chambers, Paul (2002). Bones of Contention: The Archaeopteryx Scandals. John Murray Ltd. ISBN 0-7195-6054-3.
- Chiappe, Luis M. (2007). Glorified Dinosaurs: the origin and early evolution of birds. John Wiley. ISBN 978-0-471-24723-4.
- Chinsamy-Turan, Anusuyu (2005). The Microstructure of Dinosaur Bone: Deciphering Biology with Fine-Scale Techniques. The Johns Hopkins University Press.
- Christiansen, Per & Bonde, Niels (2003). "Body plumage in Archaeopteryx: a review, and new evidence from the Berlin specimen" (PDF). .
- Fastovsky, David E. & David B. Weishampel (2005). The Evolution and Extinction of the Dinosaurs, 2nd edition. Cambridge University Press.
- Feduccia, Alan (1999). The Origin and Evolution of Birds. Yale University Press. ISBN 978-0300078619.
- Fucheng, Zhang, Zhonghe, Z. and Dyke, G. (2006). "Feathers and 'feather-like' integumentary structures in Liaoning birds and dinosaurs". Geological Journal. 41 (3–4): 395–404. S2CID 128577038.)
{{cite journal}}
: CS1 maint: multiple names: authors list (link - Heilmann, Gerhard (1926). The Origin of Birds. New York: Dover Publications, Inc. 1972 edition. ISBN 0-486-22784-7.
- Huxley, T.H. (1868). "On the animals which are most nearly intermediate between birds and reptiles". Annals and Magazine of Natural History. 4th. 2.
- Long, John (2008). ISBN 978-0-19-537266-3.
- Martin, Anthony J. (2006). Introduction to the Study of Dinosaurs (2 ed.). Blackwell Publishing. ISBN 0-632-04436-5.
- Nieuwland, I.J.J. (2004). "Gerhard Heilmann and the artist's eye in science, 1912–1927". PalArch's Journal of Vertebrate Paleontology. 3 (2). ISSN 1567-2158.
- OCLC Online Computer Library Center. WorldCat: all editions for 'The Origin of Birds'. OCLC 702765.
- Osborn, H.F. (1924). "Three new Theropoda, Protoceratops zone, central Mongolia". American Museum Novitates (144): 1–12.
- Ostrom, J. H. (1976). "Archaeopteryx and the origin of birds" (PDF). Biological Journal of the Linnean Society. 8 (2): 91–182. .
- Ostrom, J. H. (1969). "Osteology of Deinonychus antirrhopus, an unusual theropod from the Lower Cretaceous of Montana". Peabody Museum of Natural History Bulletin. 30.
- Paul, Gregory S. (2010). The Princeton Field Guide to Dinosaurs. Princeton University Press.
- Paul, Gregory S., ed. (2003). The Scientific American Book of Dinosaurs. Macmillan. ISBN 0-312-26226-4.
- Paul, Gregory S (2002). Dinosaurs of the Air: the evolution and loss of flight in dinosaurs and birds. CJHU Press. ISBN 0-8018-6763-0.
- Paul, Gregory S (1988). Predatory Dinosaurs of the World. Simon & Schuster. ISBN 0-671-61946-2.
- Prum, R. O. (2003). "Dinosaurs Take to the Air". Nature. 421 (6921): 323–324. S2CID 31214197.
- Ries, C.J. (2010). "Angels, Demons, Birds and Dinosaurs: Creativity, Meaning and Truth in the Life, Art and Science of Gerhard Heilmann (1859–1946)". Interdisciplinary Science Reviews. 35 (1): 69–91. S2CID 144697123.
- Ries, C.J. (2007). "Creating the Proavis: bird origins in the art and science of Gerhard Heilmann 1913–1926". Archives of Natural History. 34 (1): 1–19. .
- Salomonsen, F. (1946). "Gerhard Heilman, 26 Juni 1859 - 26 Marts 1946". Dansk Ornithologisk Forenings Tidsskrift. 40: 146–149.
- Shipman, Pat (1998). Taking Wing: Archaeopteryx and the Evolution of Bird Flight. Simon and Schuster. ISBN 0-684-81131-6.
- Tudge, Colin (2010). The Bird: A Natural History of Who Birds Are, Where They Came From, and How They Live. Reprint edition. Three Rivers Press.
- Weishampel, David B.; Peter Dodson; Halszka Osmólska (2004). ISBN 0-520-06726-6.
- Welker, R. H. (1975). Natural Man: The Life of William Beebe. Indiana University Press. ISBN 0-253-33975-8.
- Wilford, John Noble (1985). The Riddle of the Dinosaur. Alfred A. Knopf. ISBN 0-394-74392-X.
- Xu, X., Zhou, Z., Wang, X., Kuang, X., Zhang, F. and Du, X (2003). "Four-winged dinosaurs from China" (PDF). Nature. 421 (6921): 335–340. S2CID 1160118.)
{{cite journal}}
: CS1 maint: multiple names: authors list (link