Pterosaurs - The Flying Reptiles - Evolution

Pterosaurs - The Flying Reptiles - Evolution Pterosaurs (winged lizards) hold a special place in the history of life on earth: they were the first creatures, other than insects, to successfully populate the skies. The evolution of pterosaurs roughly paralleled that of their terrestrial cousins, the dinosaurs, as the small, basal species of the late Triassic period gradually gave way to bigger, more advanced forms in the Jurassic and Cretaceous. (See a complete, A to Z list of pterosaurs.) Before we proceed, though, its important to address one important misconception. Paleontologists have found indisputable proof that modern birds are descended not from pterosaurs, but from small, feathered, land-bound dinosaurs (in fact, if you could somehow compare the DNA of a pigeon, a Tyrannosaurus Rex and a Pteranodon, the first two would be more closely related to each other than either would be to the third). This is an example of what biologists call convergent evolution: nature has a way of finding the same solutions (wings, hollow bones, etc.) to the same problem (how to fly). The First Pterosaurs As is the case with dinosaurs, paleontologists dont yet have enough evidence to identify the single ancient, non-dinosaur reptile from which all pterosaurs evolved (the lack of a missing linksay, a terrestrial archosaur with half-developed flaps of skinmay be heartening to creationists, but you have to remember that fossilization is a matter of chance. Most prehistoric species arent represented in the fossil record, simply because they died in conditions that didnt allow for their preservation.) The first pterosaurs for which we have fossil evidence flourished during the middle to late Triassic period, about 230 to 200 million years ago. These flying reptiles were characterized by their small size and long tails, as well as obscure anatomical features (like the bone structures in their wings) that distinguished them from the more advanced pterosaurs that followed. These rhamphorhynchoid pterosaurs, as theyre called, include Eudimorphodon (one of the earliest pterosaurs known), Dorygnathus and Rhamphorhynchus, and they persisted into the early to middle Jurassic period. One problem with identifying the rhamphorhynchoid pterosaurs of the late Triassic and early Jurassic periods is that most specimens have been unearthed in modern-day England and Germany. This isnt because early pterosaurs liked to summer in western Europe; rather, as explained above, we can only find fossils in those areas that lent themselves to fossil formation. There may well have been vast populations of Asian or North American pterosaurs, which may (or may not) have been anatomically distinct from the ones with which were familiar. Later Pterosaurs By the late Jurassic period, rhamphorhynchoid pterosaurs had been pretty much replaced by pterodactyloid pterosaurslarger-winged, shorter-tailed flying reptiles exemplified by the well-known Pterodactylus and Pteranodon. (The earliest identified member of this group, Kryptodrakon, lived about 163 million years ago.) With their larger, more maneuverable wings of skin, these pterosaurs were able to glide farther, faster, and higher up in the sky, swooping down like eagles to pluck fish off the surface of oceans, lakes and rivers. During the Cretaceous period, pterodactyloids took after dinosaurs in one important respect: an increasing trend toward gigantism. In the middle Cretaceous, the skies of South America were ruled by huge, colorful pterosaurs like Tapejara and Tupuxuara, which had wingspans of 16 or 17 feet; still, these big fliers looked like sparrows next to the true giants of the late Cretaceous, Quetzalcoatlus and Zhejiangopterus, the wingspans of which exceeded 30 feet (far larger than the largest eagles alive today). Heres where we come to another all-important but. The enormous size of these azhdarchids (as giant pterosaurs are known) has led some paleontologists to speculate that they never actually flew. For example, a recent analysis of the giraffe-sized Quetzalcoatlus shows that it had some anatomical features (such as small feet and a stiff neck) ideal for stalking small dinosaurs on land. Since evolution tends to repeat the same patterns, this would answer the embarrassing question of why modern birds have never evolved to azhdarchid-like sizes. In any event, by the end of the Cretaceous period, the pterosaursboth large and smallwent extinct along with their cousins, the terrestrial dinosaurs and marine reptiles. Its possible that the ascendancy of true feathered birds spelled doom for slower, less versatile pterosaurs, or that in the aftermath of the K/T Extinction the prehistoric fish that these flying reptiles fed on were drastically reduced in number. Pterosaur Behavior Aside from their relative sizes, the pterosaurs of the Jurassic and Cretaceous periods differed from one another in two important ways: feeding habits and ornamentation. Generally, paleontologists can infer a pterosaurs diet from the size and shape of its jaws, and by looking at analogous behavior in modern birds (such as pelicans and seagulls). Pterosaurs with sharp, narrow beaks most likely subsisted on fish, while anomalous genera like Pterodaustro fed on plankton (this pterosaurs thousand or so tiny teeth formed a filter, like that of a blue whale) and the fanged Jeholopterus may have sucked dinosaur blood like a vampire bat (though most paleontologists dismiss this notion). Like modern birds, some pterosaurs also had rich ornamentationnot brightly colored feathers, which pterosaurs never managed to evolve, but prominent head crests. For example, Tupuxuaras rounded crest was rich in blood vessels, a clue that it may have changed color in mating displays, while Ornithocheirus had matching crests on its upper and lower jaws (though its unclear if these were used for display or feeding purposes). Most controversial, though, are the long, bony crests atop the noggins of pterosaurs like Pteranodon and Nyctosaurus. Some paleontologists believe that Pteranodons crest served as a rudder to help stabilize it in flight, while others speculate that Nyctosaurus may have sported a colorful sail of skin. Its an entertaining idea, but some aerodynamics experts doubt that these adaptations could have been truly functional. Pterosaur Physiology The key trait that distinguished pterosaurs from land-bound feathered dinosaurs that evolved into birds was the nature of their wings which consisted of wide flaps of skin connected to an extended finger on each hand. Although these flat, broad structures provided plenty of lift, they may have been better suited to passive gliding than powered, flapping flight, as evidenced by the dominance of true prehistoric birds by the end of the Cretaceous period (which may be attributed to their increased maneuverability). Although theyre only distantly related, ancient pterosaurs and modern birds may have shared one important feature in common: a warm-blooded metabolism. Theres evidence that some pterosaurs (like Sordes) sported coats of primitive hair, a feature usually associated with warm-blooded mammals, and its unclear if a cold-blooded reptile could have generated enough internal energy to sustain itself in flight. Like modern birds, pterosaurs were also distinguished by their sharp vision (a necessity for hunting from hundreds of feet in the air!), which entailed a bigger-than-average brain than that possessed by terrestrial or aquatic reptiles. Using advanced techniques, scientists have even been able to reconstruct the size and shape of the brains of some pterosaur genera, proving that they contained more advanced coordination centers than comparable reptiles. Pterosaurs (winged lizards) hold a special place in the history of life on earth: they were the first creatures, other than insects, to successfully populate the skies. The evolution of pterosaurs roughly paralleled that of their terrestrial cousins, the dinosaurs, as the small, basal species of the late Triassic period gradually gave way to bigger, more advanced forms in the Jurassic and Cretaceous. Before we proceed, though, its important to address one important misconception. Paleontologists have found indisputable proof that modern birds are descended not from pterosaurs, but from small, feathered, land-bound dinosaurs (in fact, if you could somehow compare the DNA of a pigeon, a Tyrannosaurus Rex and a Pteranodon, the first two would be more closely related to each other than either would be to the third). This is an example of what biologists call convergent evolution: nature has a way of finding the same solutions (wings, hollow bones, etc.) to the same problem (how to fly). The First Pterosaurs As is the case with dinosaurs, paleontologists dont yet have enough evidence to identify the single ancient, non-dinosaur reptile from which all pterosaurs evolved (the lack of a missing linksay, a terrestrial archosaur with half-developed flaps of skinmay be heartening to creationists, but you have to remember that fossilization is a matter of chance. Most prehistoric species arent represented in the fossil record, simply because they died in conditions that didnt allow for their preservation.) The first pterosaurs for which we have fossil evidence flourished during the middle to late Triassic period, about 230 to 200 million years ago. These flying reptiles were characterized by their small size and long tails, as well as obscure anatomical features (like the bone structures in their wings) that distinguished them from the more advanced pterosaurs that followed. These rhamphorhynchoid pterosaurs, as theyre called, include Eudimorphodon (one of the earliest pterosaurs known), Dorygnathus and Rhamphorhynchus, and they persisted into the early to middle Jurassic period. One problem with identifying the rhamphorhynchoid pterosaurs of the late Triassic and early Jurassic periods is that most specimens have been unearthed in modern-day England and Germany. This isnt because early pterosaurs liked to summer in western Europe; rather, as explained above, we can only find fossils in those areas that lent themselves to fossil formation. There may well have been vast populations of Asian or North American pterosaurs, which may (or may not) have been anatomically distinct from the ones with which were familiar. Later Pterosaurs By the late Jurassic period, rhamphorhynchoid pterosaurs had been pretty much replaced by pterodactyloid pterosaurslarger-winged, shorter-tailed flying reptiles exemplified by the well-known Pterodactylus and Pteranodon. (The earliest identified member of this group, Kryptodrakon, lived about 163 million years ago.) With their larger, more maneuverable wings of skin, these pterosaurs were able to glide farther, faster, and higher up in the sky, swooping down like eagles to pluck fish off the surface of oceans, lakes and rivers. During the Cretaceous period, pterodactyloids took after dinosaurs in one important respect: an increasing trend toward gigantism. In the middle Cretaceous, the skies of South America were ruled by huge, colorful pterosaurs like Tapejara and Tupuxuara, which had wingspans of 16 or 17 feet; still, these big fliers looked like sparrows next to the true giants of the late Cretaceous, Quetzalcoatlus and Zhejiangopterus, the wingspans of which exceeded 30 feet (far larger than the largest eagles alive today). Heres where we come to another all-important but. The enormous size of these azhdarchids (as giant pterosaurs are known) has led some paleontologists to speculate that they never actually flew. For example, a recent analysis of the giraffe-sized Quetzalcoatlus shows that it had some anatomical features (such as small feet and a stiff neck) ideal for stalking small dinosaurs on land. Since evolution tends to repeat the same patterns, this would answer the embarrassing question of why modern birds have never evolved to azhdarchid-like sizes. In any event, by the end of the Cretaceous period, the pterosaursboth large and smallwent extinct along with their cousins, the terrestrial dinosaurs and marine reptiles. Its possible that the ascendancy of true feathered birds spelled doom for slower, less versatile pterosaurs, or that in the aftermath of the K/T Extinction the prehistoric fish that these flying reptiles fed on were drastically reduced in number. Pterosaur Behavior Aside from their relative sizes, the pterosaurs of the Jurassic and Cretaceous periods differed from one another in two important ways: feeding habits and ornamentation. Generally, paleontologists can infer a pterosaurs diet from the size and shape of its jaws, and by looking at analogous behavior in modern birds (such as pelicans and seagulls). Pterosaurs with sharp, narrow beaks most likely subsisted on fish, while anomalous genera like Pterodaustro fed on plankton (this pterosaurs thousand or so tiny teeth formed a filter, like that of a blue whale) and the fanged Jeholopterus may have sucked dinosaur blood like a vampire bat (though most paleontologists dismiss this notion). Like modern birds, some pterosaurs also had rich ornamentationnot brightly colored feathers, which pterosaurs never managed to evolve, but prominent head crests. For example, Tupuxuaras rounded crest was rich in blood vessels, a clue that it may have changed color in mating displays, while Ornithocheirus had matching crests on its upper and lower jaws (though its unclear if these were used for display or feeding purposes). Most controversial, though, are the long, bony crests atop the noggins of pterosaurs like Pteranodon and Nyctosaurus. Some paleontologists believe that Pteranodons crest served as a rudder to help stabilize it in flight, while others speculate that Nyctosaurus may have sported a colorful sail of skin. Its an entertaining idea, but some aerodynamics experts doubt that these adaptations could have been truly functional. Pterosaur Physiology The key trait that distinguished pterosaurs from land-bound feathered dinosaurs that evolved into birds was the nature of their wings which consisted of wide flaps of skin connected to an extended finger on each hand. Although these flat, broad structures provided plenty of lift, they may have been better suited to passive gliding than powered, flapping flight, as evidenced by the dominance of true prehistoric birds by the end of the Cretaceous period (which may be attributed to their increased maneuverability). Although theyre only distantly related, ancient pterosaurs and modern birds may have shared one important feature in common: a warm-blooded metabolism. Theres evidence that some pterosaurs (like Sordes) sported coats of primitive hair, a feature usually associated with warm-blooded mammals, and its unclear if a cold-blooded reptile could have generated enough internal energy to sustain itself in flight. Like modern birds, pterosaurs were also distinguished by their sharp vision (a necessity for hunting from hundreds of feet in the air!), which entailed a bigger-than-average brain than that possessed by terrestrial or aquatic reptiles. Using advanced techniques, scientists have even been able to reconstruct the size and shape of the brains of some pterosaur genera, proving that they contained more advanced coordination centers than comparable reptiles.