Seventee N the Early Tetrapods and Modern Amphibians
Total Page：16
File Type：pdf, Size：1020Kb
Hickman−Roberts−Larson: 17. The Early Tetrapods and Text © The McGraw−Hill Animal Diversity, Third Modern Amphibians Companies, 2002 Edition 17 chapter •••••• seventeen The Early Tetrapods and Modern Amphibians Vertebrate Landfall The chorus of frogs beside a pond on a spring evening heralds one of nature’s dramatic events. Masses of frog eggs soon hatch into limbless, gill-breathing, fishlike tadpole larvae. Warmed by the late spring sun, they feed and grow.Then, almost imperceptibly,a remarkable transfor- mation takes place. Hindlegs appear and gradually lengthen. The tail shortens. The larval teeth are lost, and the gills are replaced by lungs. Eyelids develop. The forelegs emerge. In a matter of weeks the aquatic tadpole has completed its metamorphosis to an adult frog. The evolutionary transition from water to land occurred not in weeks but over millions of years. A lengthy series of alterations cumulatively fitted the vertebrate body plan for life on land. The ori- gin of land vertebrates is no less a remarkable feat for this fact—a feat that incidentally would have a poor chance of succeeding today because well-established competitors make it impossible for a poorly adapted transitional form to gain a foothold. Amphibians are the only living vertebrates that have a transi- tion from water to land in both their ontogeny and phylogeny.Even after some 350 million years of evolution, few amphibians are com- pletely land adapted; most are quasiterrestrial, hovering between aquatic and land environments. This double life is expressed in their name. Even the amphibians that are best adapted for a terrestrial existence cannot stray far from moist conditions. Many,however, have developed ways to keep their eggs out of open water where the larvae would be exposed to enemies. A frog tadpole undergoing metamorphosis. 325 Hickman−Roberts−Larson: 17. The Early Tetrapods and Text © The McGraw−Hill Animal Diversity, Third Modern Amphibians Companies, 2002 Edition 326 chapter seventeen daptation for life on land is a major theme of the Early Evolution remaining vertebrate groups treated in this and the fol- A lowing chapters. These animals form a monophyletic of Terrestrial Vertebrates unit known as tetrapods. The amphibians and the amniotes (including reptiles, birds, and mammals) represent the two Devonian Origin of Tetrapods major extant branches of tetrapod phylogeny. In this chapter, we review what is known about the origins of terrestrial verte- The Devonian period, beginning some 400 million years ago, brates and discuss the amphibian branch in detail. We discuss was a time of mild temperatures and alternating droughts and the major amniote groups in Chapters 18 through 20. floods. During this period some primarily aquatic vertebrates evolved two features that would be important for permitting the subsequent evolution for life on land: lungs and limbs. Movement onto Land The Devonian freshwater environment was unstable. During dry periods,many pools and streams evaporated,water The movement from water to land is perhaps the most dra- became foul, and dissolved oxygen disappeared. Only those matic event in animal evolution, because it involves the inva- fishes able to acquire atmospheric oxygen survived such con- sion of a habitat that in many respects is more hazardous for ditions. Gills were unsuitable because in air the filaments col- life. Life originated in water. Animals are mostly water in com- lapsed, dried, and quickly lost their function. Virtually all position,and all cellular activities occur in water.Nevertheless, freshwater fishes surviving this period, including lobe-finned organisms eventually invaded land, carrying their watery com- fishes and lungfishes (p. 314), had a kind of lung that devel- position with them. Vascular plants, pulmonate snails, and tra- oped as an outgrowth of the pharynx. It was relatively simple cheate arthropods made the transition much earlier than to enhance the efficiency of the air-filled cavity by improving vertebrates, and winged insects were diversifying at approxi- its vascularity with a rich capillary network, and by supplying mately the same time that the earliest terrestrial vertebrates it with arterial blood from the last (sixth) pair of aortic arches. evolved. Although the invasion of land required modification Oxygenated blood returned directly to the heart by a pul- of almost every system in the vertebrate body,aquatic and ter- monary vein to form a complete pulmonary circuit. Thus the restrial vertebrates retain many basic structural and functional double circulation characteristic of all tetrapods originated: similarities. We see a transition between aquatic and terrestrial a systemic circulation serving the body and a pulmonary cir- vertebrates most clearly today in the many living amphibians culation supplying the lungs. that make this transition during their own life histories. Vertebrate limbs also arose during the Devonian period. Beyond the obvious difference in water content, there Although fish fins at first appear very different from the jointed are several important physical differences that animals must limbs of tetrapods, an examination of the bony elements of the accommodate when moving from water to land.These include paired fins of the lobe-finned fishes shows that they broadly (1) oxygen content, (2) density, (3) temperature regulation, resemble the equivalent limbs of amphibians. In Eusthenop- and (4) habitat diversity. Oxygen is at least 20 times more teron, a Devonian lobe-fin, we can recognize an upper arm abundant in air and it diffuses much more rapidly through air bone (humerus) and two forearm bones (radius and ulna) as than through water. Consequently, terrestrial animals can well as other elements that we can homologize with the wrist obtain oxygen far more easily than aquatic ones once they pos- bones of tetrapods (figure 17.1).Eusthenopteron could walk— sess the appropriate adaptations, such as lungs. Air, however, more accurately flop—along the bottom mud of pools with its has approximately 1000 times less buoyant density than water fins,since backward and forward movement of the fins was lim- and is approximately 50 times less viscous. It therefore pro- ited to about 20 to 25 degrees. Acanthostega, one of the earli- vides relatively little support against gravity, requiring terres- est known Devonian tetrapods, had well-formed tetrapod legs trial animals to develop strong limbs and to remodel their with clearly formed digits on both fore- and hindlimbs, but the skeleton to achieve adequate structural support. Air fluctuates limbs were too weakly constructed to enable the animal to in temperature more readily than water does, and terrestrial hoist its body off the surface for proper walking on land. environments therefore experience harsh and unpredictable Ichthyostega, however,with its fully developed shoulder girdle, cycles of freezing,thawing,drying,and flooding.Terrestrial ani- bulky limb bones, well-developed muscles, and other adapta- mals require behavioral and physiological strategies to protect tions for terrestrial life, must have been able to pull itself onto themselves from thermal extremes; one such important strat- land, although it probably did not walk very well. Thus, the egy is homeothermy (regulated constant body temperature) of tetrapods evolved their legs underwater and only then, for rea- birds and mammals. sons unknown, began to pull themselves onto land. Despite its hazards, the terrestrial environment offers a Evidence points to lobe-finned fishes as the closest rela- great variety of habitats including coniferous, temperate, and tives of tetrapods; in cladistic terms they contain the sister tropical forests, grasslands, deserts, mountains, oceanic islands, group of tetrapods (figures 17.2 and 17.3). Both the lobe- and polar regions.Provision of safe shelter for protection of vul- finned fishes and early tetrapods such as Acanthostega and nerable eggs and young may be accomplished much more read- Ichthyostega shared several characteristics of skull, teeth, and ily in many of these terrestrial habitats than in aquatic ones. pectoral girdle. Ichthyostega (Gr. ichthys, fish, + stege¯, roof, or Hickman−Roberts−Larson: 17. The Early Tetrapods and Text © The McGraw−Hill Animal Diversity, Third Modern Amphibians Companies, 2002 Edition Eusthenopteron Acanthostega Cleithrum Humerus Skull Ulna Ulnare Clavicle Radius Intermedium Humerus Dermal fin rays Radius Ulna Phalanges Carpals Ichthyostega Limnoscelis Pelvis Femur Tibia Fibula Fibulare Tarsals Phalanges figure 17.1 Humerus Evolution of the tetrapod leg. The legs of tetrapods evolved from fins of Paleozoic fishes. Eusthenopteron, a late Devonian lobe-finned fish had paired muscular fins supported by bony elements that foreshadowed the bones of tetrapod limbs. The anterior fin contained an upper arm bone (humerus), two forearm bones (radius Radius Ulna and ulna), and smaller elements homologous to the wrist bones of tetrapods. As Carpals typical of fishes, the pectoral girdle, consisting of the cleithrum, clavicle, and other Phalanges bones, was firmly attached to the skull. In Acanthostega, one of the earliest known Devonian tetrapods (appearing about 360 million years BP), dermal fin rays of the anterior appendage were replaced by eight fully evolved fingers. Acanthostega was probably exclusively aquatic because its limbs were too weak for travel on land. Ichthyostega, a contemporary of Acanthostega, had fully formed tetrapod limbs and must have been able to walk on land. The hindlimb bore seven toes (the