The Origin of Birds

Birds are usually regarded as being archosaurs and, therefore, a sister group of the crocodiles (Benton 2004). This view originated with E. Haekel who, in 1866, claimed the mammals as the sister group of the remaining amniotes. Haekel was supported by T.H. Huxley and Ray Lankester in 1870. A contrary opinion, however, has been expressed by Brian Gardiner (1982,2002) who argued that crocodiles share their ancestry with both birds and mammals - having six synapomorphies with birds (including an eustachian tube that passes through the base of the cranium, a gizzard, and reduction of the fifth toes to the metatarsal) and two synapomorphies with mammals (calcaneal tubers and secondary palate). The fossil record of dinosaurs shows all the evolutionary changes that took place between crocodiles and birds (Gardiner 1993,2002; Sect. 7.8).

11.5.1 Feathers

Among the more remarkable fossil discoveries of recent years have been those of small Upper Jurassic theropod dinosaurs which, although unable to fly, yet possessed feathers. One of these, Sinosauropteryx (Fig. 126a), has already been mentioned as being closely related to Compsognathus (Sect. 11.3.1). Another was Protarchaeopteryx (Fig. 126b), a typical coelurid but very similar to the earliest bird Archaeopteryx (Fig. 126c). Protarchaeopteryx had wrists that could twist to seize prey, elongated arms and hands with sharp claws. There was a clump of feathers at the end of the tail, which might perhaps have been used for display, and feathers on the forelimbs (Milner 2002). It seems highly

Reptiles Mesozoic Era
■ Fig. 126a-c. Dino-birds and Archaeopteryx the first bird (Upper Jurassic). a Sinosauropteryx (length ca. 1 m), b Protarchaeopteryx (length ca. 1 m), c Archaeopteryx (length ca. 37 cm)

probable, even if unprovable, that both feathers and hairs evolved from reptilian scales. The integumentary structure covering the bodies of Sinosauropte-ryx and the other 'dino-birds' were simpler than true feathers. They were composed of a central rachis or spine from which the barbs branched, but they lacked the aerodynamic quality of avian feathers. The longest ones covering the body were only about 3 cm in length and were probably more suitable for insulation than for display. The discovery of feathered dinosaurs provides support for the hypothesis not only that the theropods were direct ancestors to the birds but also that some, if not all of them, were tachymetabolic (Sect. 7.6). At the same time, the caudal feathers of Protarchaeopteryx, like those of Caudip-teryx, might well have been used for display, possibly in conjunction with the longer feathers attached to the forearms.

11.5.2 Flight

The origin of flight in birds has aroused considerable speculation over the years. As Norman (1985) pointed out, there have been essentially two main schools of thought: flight evolved either in fast-running dinosaurs such as coe-lurosaurs or from gliding tree-dwellers. Leading advocates of the first of these hypotheses were Samuel Williston in 1879, Francis von Nopsca in 1907 and J.H. Ostrom in 1974. Williston suggested that gradual lengthening of the outer fingers and greater development of the scales might have aided a bipedal cursorial dinosaur in running. The "change of scales to feathers would have been easy. The wings must first have been used in running, next in leaping and descending from heights, and finally in soaring." Williston's hypothesis enjoyed little success among his contemporaries, and the following year O.C. Marsh proposed that the power of flight probably originated among small arboreal forms of reptilian birds. Nopsca's idea was similar to that of Williston, but more detailed and aerodynamically absurd, as Feduccia (1980) has clearly explained.

Reasoning from the anatomy of Archaeopteryx and from the features of dinosaurs contemporary with it, Ostrom (1974a) argued that the first feathers of tachymetabolic dinosaurs served as a thermoregulatory pelt. Enlargement of the primordial primaries and secondaries on the prey-catching forelimbs would have transformed these into "large, continuous trapping surfaces - or natural insect nets - activated by powerful ventral adductor muscles (the pec-toralis group). These adaptations were admirably preadaptive for active, flapping flight." The desert islands in which Archaeopteryx lived did not contain any tall trees, and some scientists have suggested that perhaps it ran after flying insects, leapt to catch them in mid-air and then stayed aloft, flapping its wings. Its breast bone was too small to anchor strong muscles and it could not have flown at all well. During the Cretaceous period, however, many lines of birds with well-developed powers of flight evolved. Nevertheless, they died out some 5 my before the end of the Mesozoic Era.

Several important morphological, functional and physiological systems that are characteristic of modern birds were acquired only late in avian history. Chief among these was the modern pattern of hind limb kinematics - flexion of the knee and tibio-tarsal displacement, rather than the ancestral theropod pattern of hip extension. This modern pattern involves extensive retraction of the femur during each stride. It is probable, too, that throughout the evolution of the theropods, reduction in caudal musculature was the principal agent implicated in this mechanism. The tail was decoupled functionally from the primitive pattern of kinematics and, instead, coupled with the flight apparatus. At the same time, the centre of gravity moved forward and the modern avian stance developed (Chiappe 1995).

The arboreal origin of flight, first proposed by Marsh in 1926 is now generally favoured by palaeontologists (Feduccia 1980; Norman 1985; Milner 2002; Benton 2004). Kenneth Dial, for instance, has very recently produced evidence to support the view that the first feathered wings were used, not to keep jumping dinosaurs in the air for longer, but to increase the traction between their feet and the substrate when they were clambering up slopes steeper than about 45°. The idea was tested by filming partridges (Perdix perdix) as they scrambled up steep slopes and measuring the g-forces exerted by the birds' feet on the ground. Even newly hatched partridge chicks were able to use their stubby, undeveloped wings to climb a 59° incline (cited in The Independent, 17 January 2003).

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