21 Birds: Reptiles by Another Name 1bese waved albatrosses (Phoebastria irrorata) are in courtship. 11Jesebehav­ iors m·e common in modern birds.Behavioral links between birdsand thei,· ancestors are difficult to assess;howeve1; many structural characteristics, once thought to be avian, are now providing clues to the reptilianancestry of birds. Chapter Outline 21.1 EVOLUTIONARY PERSPECTIVE 21.1 Evolutionary Perspective Phylogenetic Relationships LEARNING OUTCOMES Archaeopteryx, Eoalulavls, and the Evolution of Flight 1. Describe the characteristics of the members of the class Aves. Diversity of Modern Birds 21.2 Evolutionary Pressures 2. Critique the statement that "vertebrate flight evolved first in the dinosaurs." ExternalStructure and Locomotion Nutrition and the Digestive System Drawings of birds on the walls of caves in southern France and Spain, bird images Circulation, Gas Exchange, and Temperature Regulation of ancient Egyptian and American cultures, and the bird images in biblical writings Gas Exchange are evidence that humans have marveled at birds and bird flight for thousands of Nervous and SensorySystems years. From Leonardo da Vinci's early drawings of flying machines (1490) to Orville Excretion and Osmoregulation Reproduction and Development Wright's first successful powered flight on 17 December 1903, humans have tried to Migration and Navigation take to the sky and experience soaring like a bird. Birds' ability to navigate long distances between breeding and wintering grounds is just as impressive as flight. For example, Arctic terns have a migrato1y route that takes them from the Arctic to the Antarctic and back again each year, a distance of approximately 35,000 km (figure 21.1). Their rather circuitous route takes them across the notthern Atlantic Ocean, to the coasts of Europe and Africa, and then across vast stretches of the southern Atlantic Ocean before they reach their wintering grounds. Phylogenetic Relationships Avian reptiles are traditionally classified as members of the class Aves (a'ves) (L. avis, bird). The major characteristics of this group are adaptations for flight, includ­ ing appendages modified as wings, feathers, endothermy, a high metabolic rate, a ve1tebral column modified for flight, and bones lightened by numerous air spaces. In addition, modern birds possess a horny bill and lack teeth. The similarities between birds and nonavian reptiles are so striking that, in the 1860s, T. H. Huxley described birds as "glorified reptiles" and included them in a single class Sauropsida. As zoologists and paleontologists learn more about the rela­ tionships between birds and other reptiles, many scientists advocate Huxley's origi­ nal idea. Anatomical similarities include features such as a single occipital condyle on the skull (the point of articulation between the skull and the first cervical verte­ bra), a single ear ossicle, lower jaw structure, and dozens of other technical skeletal characteristics. Physiological characteristics, such as the presence of nucleated red blood cells and aspects of liver and kidney function, are shared by nonavian reptiles and birds. Some birds and other reptiles share behavioral characteristics, for exam­ ple, those related to nesting and care of young. Even characteristics that were once 390 CHAPTER TWENTY-ONE thought to be characteristic of birds but not nonavian reptiles, such as endothermy (mesothermy), air spaces in bones, and the presence of feathers, have been demonstrated in some dinosaurs. Birds descended from ancient ar hosaurs-a lineage shared by the dinosaurs and crocodilians sec,figu.r 20.3} The birds are closely related to a group of dinosaurs in the sauris­ chian lineage called the theropods. (This lineage also includes bipedal dinosaurs like Tyrannosaurus and Velociraptor.) Spectacular discoveries from 160-million-year-old fossil beds in no1thern China support theropod ancestry. These fos­ sils may not represent animals directly ancestral to birds, but more importantly they repeatedly show that ancestral features of birds were present in diverse species within one impor­ tant lineage. Fossils of at least a dozen theropod dinosaurs bearing feathers have been discovered (figure 21.2). The first (a) discovered was a chicken-sized dinosaur called Sinosaurop­ teryx. It had small tubular structures, similar to feathers in their early stages of development in modern birds. Another fossil was named Caudipte1yx. It was a turkey-sized theropod with symmetrical feathers on fore appendages and tail. It is assumed that neither of these theropods were fliers because (b) FIGURE 21.2 FIGURE 21.1 An Artist's Representation of Feathered Theropods and Class Aves. (a) The birds were derived from the archosaur Ancient Birds. In the right foreground is Sinosauropteryx. It was luieag of an icnr reptil •s. d;iptatfons for night includ about the size of a chicken. Note the presence of tubular feathers. append:1 >e. modif'ied a wing� fead1crs, end()lh-:rmy, a hi�h In the foreground center is Caudipteryx. It had symmetrical memb lie nit�, a v rtcbrai column mouified fc r flight. and I cml:S feathers and was about the size of a turkey. Microraptor is shown lightened I y n 1mer >tt:- air spaces. 'Flight has given birds, like in flight in the background. Note the representation of feathers on this Arctic Lem (Stema c1rcll<;u), Lile ability LO explolc re.sour.•:; all four appendages. No one is certain that Microraptor could fly. It unuvailable Lo other vert I ,·at ·s. (/J) 1\lligrntion route of d1e Arctic may have climbed trees and glided, but it did possess asymmetrical tern. Ar ·tit rems breed 11'1 nori:hern Nnrth /\merk:a, Grecnl:,nd, feathers characteristic of bird flight feathers. Other species shown and the . rcti ·. Migrating I lrds cross rbc AtlanLk Ocean )fl L11eit include two ancient birds found in fossil beds of northeastern trip to Antarctica during the Northern Hemisphere's winter China, Jixiangornis (midground right) and Jeholornis (midground season, In the process, they flyabout 35,000 km (22,000 mi) left). Both had skeletal structures that suggest the ability to fly. each year. Psittacosaurus (below Jeholornis) was a bipedal plant-eating dinosaur. The colors shown here, and the interactions between species that are implied, are the artist's interpretation. Image by Luis Rey fromwww.luisrey-nclriJdarn.uk. Reprinted ·by permission of Luis ·Rey. Birds: Reptiles by Another Name 391 asymmetrical feathers are required for the aerodynamics of 160 million-year-old theropod, is unique among other known flight. These fossils demonstrate that feathers predate flight. theropods in having a feather-covered body and membranous The earliest feathers may have provided insulation in tem­ wings similar in appearance to bat wings. It was probably a perature regulation, water repellency, courtship devices, gliding dinosaur. New fossils like Yi qi are blurring the dis­ camouflage, or balancing devices while running along tinction between birds and their ancestors and pushing the the ground. Flight was apparently a seconda1y function origin of gliding flight earlier into the theropod lineage. of feathers. Another of the dozen feathered theropods is Microraptor. Microraptor lived 125 mya and had asymmetri­ Archaeopteryx, Eoalulavis, cal feathers on both fore and hind appendages as well as a feathered tail. Did Microraptor fly? Probably, it had the and the Evolution of Flight right kind of feathers and two pairs of limbs that might have In 1861, one of the most important vertebrate fossils was formed an airfoil. Other skeletal features suggest Microrap­ found in a slate quarry in Bavaria, Germany (figure 21.3). tor was a climber. Perhaps it climbed into trees and used It was a fossil of a pigeon-sized animal that lived during its wings for gliding flight. The discovery of dinosaur fos­ the Jurassic period, about 150 mya. It had a long, reptilian sils showing a furcula, or wishbone, further supports the tail and clawed fingers. The complete head of this speci­ theropod ancestty hypothesis. (The furcula is derived from men was not preserved, but imprints of feathers on the tail a fusion of the clavicles and, as described later, is an adapta­ and on short, rounded wings were the main evidence that tion for flight in birds.) led to the interpretation that this was a fossil of an ancient Treasure troves of new fossils are being discovered. bird. It was named Archaeopteryx (Gr.archaios, ancient + As this edition goes to press, another Chinese fossil discov­ pteron, wing). Sixteen years later, a more complete fossil ery has been analyzed. Yi qi (Chinese, wing + strange), a was discovered, revealing teeth in beaklike jaws. Four later (a) (b) FIGURE 21.3 Archaeopteryx, an Ancient Bird. (a) Archaeopteryx fossil. (b) Artist's representation. Some zoologists think that Archaeopte1yx was a ground dweller rather than the tree dweller depicted here. 392 CHAPTER TWENTY-ONE discoveries of Archaeopteryx fossils have reinforced the niches. Most of the lineages that these fossils represent ideas of reptilian ancestry for birds. became extinct, along with (other) dinosaurs, at the end of Although Archaeopteryxis often thought of as an ancient the Mesozoic era. An asteroid impact, possibly in combina­ bird, it is probably more accurate to think of it as transitional, tion with volcanism in the Deccan Traps region of India, again recognizing the blurred distinction between bird and created cataclysmic atmospheric and climatic changes nonbird theropods. It seems clear that Archaeopteryx was not 66 mya (see page xviii). These climatic changes resulted the direct ancestor of modern birds. In spite of this uncer­ in dinosaur extinctions, including the extinction of bird tainty, interpretations of the lifestyle of Archaeopteiyx have lineages. been important in the development of hypotheses on the ori­ The toothless birds that su1vived into the Tertiary period gin of flight. The clavicles (wishbone) of Archaeopteryx were were ancestors of modern, toothless birds (Neornithes). These well developed and probably provided points of attachment bird ancestors underwent a ve1y rapid radiation. Modern for wing muscles.