Ornithocheiroids Built for a Life on the Wing The ornitho

cheiroids, several of which are shown in Figure 4-10, were a diverse, important and highly distinctive group of Cretaceous pterosaurs. Among them were several of the most spectacular animals ever to take to the skies, not least Pteranodon, which, with its long, pointed jaws and bizarre crest, is an icon for flying reptiles the world over. Unlike rhamphorhynchoids, but practically without exception among the main pterodactyloid clans, all known orni-thocheiroids were well over 1 meter (3 feet) in wingspan when fully grown and most reached 3 or 4 meters (10 to 12 feet). Several, including Pteranodon, grew to remarkably large sizes, with wings that reached 6 or even 7 meters (20 to 22 feet) across.

Ornithocheiroids appear to have been even more highly specialized fliers than other pterosaurs and, like modern-day albatrosses and frigate birds, seem to have been particularly well-adapted for soaring. Modifications for this lifestyle are found throughout the body and, in addition to revealing how ornithocheiroids functioned, make it relatively easy to identify fossil remains of these pterosaurs, even, in some cases, just single bones. One obvious adaptation is seen in the hind limbs, which, compared with the forelimbs, are very slender and weak. Presumably, ornithocheiroids spent little time on the ground. Other adaptations are less obvious. Study the shoulder girdle closely, however, and it becomes apparent that the shoulder blade (scapula) is surprisingly short and stout compared with that of other pterosaurs. This means that the wings did not sprout from the body at about half-height, as is usual, but were rooted rather higher, which, according to one recent study,60 increased ornithocheiroids' stability during flight.

Istiodactylus, represented by several incomplete skeletons and fragmentary skulls from Lower Cretaceous beds that crop out along the southern shores of the Isle of Wight, England,61 and by an almost complete individual from the Lower Cretaceous Jehol Biota of northeast China, seems to be the least-evolved ornithocheiroid known at present. Uniquely among pterosaurs, the snout was rather wide and shaped a bit like the beak of a duck, except that its edges were rimmed with a set of sharp-edged blade-like teeth. Thus armed,

FIGURE 4.10 Meet the ornithocheiroids. The skulls of Istiodactylus (22 inches [0.56 meters] long) above; Omiihocheirus (2 feet [0.67 meters] long), middle above; and Pteranodon (3 feet [l meter] long), middle below; and the skeleton of Coloborhynchus, about 15 feet (4.5 meters) in wingspan, below. (Redrawn from Wellnhofer, 1978, 1987, 1988, and Bennett, 2001.)

FIGURE 4.10 Meet the ornithocheiroids. The skulls of Istiodactylus (22 inches [0.56 meters] long) above; Omiihocheirus (2 feet [0.67 meters] long), middle above; and Pteranodon (3 feet [l meter] long), middle below; and the skeleton of Coloborhynchus, about 15 feet (4.5 meters) in wingspan, below. (Redrawn from Wellnhofer, 1978, 1987, 1988, and Bennett, 2001.)

Istiodactylus could deliver a powerful "cookie-cutter"-style bite— quite sufficient to snip gobbets of flesh from the carcass of its prey.

The most diverse and important group of ornithocheiroids, found almost everywhere during the Lower Cretaceous, were the ornithocheirids. Initially known only from fragments of jaws from the Lower Cretaceous Cambridge Greensand of England (dismissed by one colleague as "the ugliest pterosaur material I ever saw"), more complete fossils from the Santana Formation of Brazil show that Ornithocheirus was a large pterosaur with thick, rounded bony crests on the tips of its jaws.62 The most distinctive feature of this pterosaur was its well-developed teeth. The first three pairs were very large— all the better to grab their prey with— and serve today as a useful hallmark of the ornithocheirid clan.

This and several other characteristics reveal that Omithocheirus has several close relatives in the Santana Formation, in the Cambridge Greensand, and, most recently, in the Jehol Biota of China, but they have acquired such a plethora of names that it is not clear how they should be referred to correctly.63 One that we can be sure about is Coloborbynchus, first named by Harry Govier Seeley's contemporary and rival, Sir Richard Owen, and represented by several beautifully preserved skulls and skeletons from Brazil and some lumpy-looking bits of jaws from Lower Cretaceous rocks of North America, Europe and Mongolia.64 One or two individuals of Coloborbynchus reached more than 6 meters (20 feet) in wingspan, but not all ornithocheirids were so large. The single complete skeleton of Haopterus from the Lower Cretaceous Jehol Group of China measured little more than a meter (3 feet) from one wing tip to the other,65 and another astounding find from the same rocks, a pterosaur egg, discussed in depth in Chapter 7, contains an embryonic orni-thocheirid with wings less than a quarter of a meter (10 inches) across.66

The ancestors of ornithocheirids also gave rise to another group, distinguished above all else by the absence of something that most other pterosaurs found indispensable: teeth. This hallmark is reflected in the name of the clan, pteranodontians (meaning the winged toothless ones), and its most important member, Pteranodon. First found in the late 1800s in the Upper Cretaceous chalk bluffs of Kansas, well over 1,000 individuals have now been collected from these rocks, making Pteranodon one of the best represented and best known of all pterosaurs.67 Immediately recognizable from its long, scimitar-like jaws and equally spectacular "look at me" crest, this pterosaur sometimes reached wingspans of almost 7 meters (22 feet), although typical adults seem to have been only about half that size.

Nyctosaurus, another toothless pteranodontian, is from the same rock layers as Pteranodon, but much rarer and considerably smaller, with a wingspan of only 2 to 3 meters (6 to 9 feet). Although they were long thought to be completely crestless, two recent finds of this pterosaur show that some individuals, perhaps the males, bore an extraordinary, antler-like structure that erupted from the top of the head, rivaling even the best that Pteranodon had to offer.68

Ctenochasmatoids: Stressing the Straining While ornithocheiroids took to the skies, the ctenochasmatoids, a clan that includes almost all the Upper Jurassic pterodactyloids found so far, set off in another direction— into the water. These pterosaurs, generally only of small or medium size, opted for more teeth, rather than less, as in many other lineages, and became highly adapted to a lifestyle that involved wading in rivers and lakes, using their comb-like dentition to sieve for their supper. The ctenochasmatoids illustrated in Figure 4.11 demonstrate two key features of the group: a highly modified skull design in which the quadrate bone, upon which the lower jaw hinged, lay in an almost horizontal position, and a neck that was extremely long, achieved not by adding more vertebrae, as birds do, but by stretching several of the existing ones into long tube-like structures.

Pterodactylus, one of the geologically earliest known ctenochasmatoids, was relatively unspecialized. Several hundred specimens of this pterosaur are known from the Solnhofen Limestone and seem to belong to two or three different species, distinguished only by small differences in the shape of their teeth and jaws.69 Like Rhamphorhynchus, there are so many specimens of Pterodactylus with evidence of soft tissues that their external appearance at least (shown in Figure 1.5) can be restored with some confidence. Most excitingly, Pterodactylus has one of the most complete growth series for any pterosaur, ranging from hatchlings only a few days or weeks in age, to adults of half a meter (20 inches) or so in wingspan and even big, old individuals that were half as large again.

The general evolutionary trend within ctenochasmatoids seems to have been toward longer and longer jaws and ever more teeth but, as Figure 4-5 shows, this was a development that eventually went two separate ways. On the one hand Gnathosaurus from the Solnhofen Limestone and its relatives, to be found in Lower Cretaceous rocks of Europe, Asia and South America, opted for fewer, larger, often strongly curved teeth, and seem to have concentrated on larger prey items.

FIGURE 4«11 Ctenochasmatoids, long-necked superstrainers, illustrated by the skulls of Ctenochasma (4 inches [10 centimeters] long), above; and Gnathosaurus (11 inches [28 centimeters] long), middle; and the skeleton of Pterodactylus, with a wingspan of about 20 inches (50 centimeters), below. (Redrawn from Wellnhofer, 1970.)

FIGURE 4«11 Ctenochasmatoids, long-necked superstrainers, illustrated by the skulls of Ctenochasma (4 inches [10 centimeters] long), above; and Gnathosaurus (11 inches [28 centimeters] long), middle; and the skeleton of Pterodactylus, with a wingspan of about 20 inches (50 centimeters), below. (Redrawn from Wellnhofer, 1970.)

On the other hand, Ctenochasma and its relatives evolved ever more and finer teeth. This culminated in the so-called flamingo pterosaur, Pterodaus-tro, from the Lower Cretaceous of Argentina. With more than 1,000 long, needle-fine teeth in the lower jaw, this pterosaur was armed with the mother of all filtering apparatuses'0 Thanks to a series of highly successful expeditions in the 1990s to the Lomo del Pterodaustro, the fossil locality in Argentina where this pterosaur was first found, Pterodaustro, like Pterodactylus, is now also known from a large number of individuals ranging from hatchlings to old adults. Most excitingly of all, an egg with remains of an embryo has just been reported (see Chapter ') and extends this growth series into the prenatal realm.'1

Dsungaripterus and the Clam-Cracking Crew Another important pterodactyloid clan, the dsungaripteroids, is principally distinguished by adaptations for cracking open and feeding on shellfish. Naturally, these are most clearly seen in the jaws, illustrated in Figure 4-12, the winkle-picking tips of which are long, pointed and toothless, and in the teeth, those at the back being especially massive. In Dsungaripterus, they were packed up tight against one another so that they formed small, but doubtless highly effective, anvils. This development is paralleled elsewhere in the robust design of the skull and might even be related to another peculiar dsungaripteroid feature: the remarkably thick and heavy construction of their vertebrae and limb bones.

Early dsungaripteroids were only small or medium-size pterosaurs and represented in the main by Germanodactylus, based on just a handful of skeletons from the Solnhofen Limestone, and several similarly sized and shaped species known from fragmentary but distinctive pieces of jaw from Upper

FIGURE 4.12 The quintessential nature of the clam-cracking dsungaripterids is demonstrated by the 16 inch (41 centimeters) long skull of Dsungaripterus. (Redrawn from Wellnhofer, 1978.)

Jurassic rocks of Europe and East Africa.72 More evolved dsungaripteroids, with fully developed shell-cracking teeth, are almost exclusively known from East Asia. Here, extensive remains, including several beautifully preserved uncrushed skulls, have been recovered from Lower Cretaceous lake sediments that crop out in Dsungaria, a remote region of northwest China, and, as detailed in the previous chapter, from similar aged rocks in the region of Tatal, Mongolia.73 Dsungaripterus, which was found and described by the legendary Chinese paleontologist Young Chung-Chien, seems to have been the largest member of the clan, and reached up to 3 or 4 meters (10 to 12 feet) in wingspan.

0 0

Post a comment