Dimorphodon and the Short Winged Big Heads Starting at the

base of the tree, the first branch encountered belongs to Dimorphodon and its relatives (Figure 4.6). As might be expected from their basement location, dimorphodontids,39 with their large, reptilian-looking skulls and relatively broad wings, are the least evolved of all pterosaurs known at present. Often reaching well over a meter in wingspan, the most distinctive feature of members of this clan is summarized in their name— dimorphodontids. This refers to the two strikingly different types of teeth found in their jaws: a few large fangs at the front (for grabbing the prey); and a row of tiny spikes at the back (for holding onto it afterward). Another instantly recognizable feature that distinguishes dimorphodontids from all other pterosaurs is the big, deep skull with its large openings framed by long, slender bars of bone. This, as one pterosaurologist has pointed out,40 follows the design principles of a bicycle, combining the minimum amount of material with the maximum amount of effect.

Dimorphodon, from Lower Jurassic rocks of southern England, is easily the best-known dimorphodontid and is represented by several skeletons, the first of which was collected in the 1820s by one ofpaleontology's greatest legends, Mary Anning. Based in Lyme Regis, Anning, the inspiration for the nursery

FIGURE 4.6 Short-winged big heads. The dimorphodontids, represented by the restored skull and skeleton of Dimorphodon, with a wingspan of about 3 feet (1 meter).

FIGURE 4.6 Short-winged big heads. The dimorphodontids, represented by the restored skull and skeleton of Dimorphodon, with a wingspan of about 3 feet (1 meter).

rhyme "she sells sea shells on the seashore," was perhaps the most famous and important British fossil collector of the early 19th century. Apart from Di-morphodon, she made many other astounding finds, including some of the first fossil remains of the so-called sea dragons, ichthyosaurs and plesiosaurs.

The only other dimorphodontid known at present, Peteinosaurus, is represented by just two incomplete skeletons that number among some of the oldest pterosaur fossils in the world, having been found in 220-million-year-old Triassic limestones that now form part of the beautiful and dramatic landscape of the Dolomites in northern Italy.41 These same rocks have produced other fossil reptiles, among which are poorly preserved remains of another Triassic pterosaur called Preondactylus.42 This small, rather enigmatic form might belong on a twig that branched off even earlier than the dimorph-odontids, because in some respects, such as the construction of the lower jaw and the tail, Preondactylus is more similar to other diapsids than any other pterosaur found so far.

Anurognathus and the Fabulous Flying Frog-Heads Returning to the main trunk of pterosaur evolution, after dimorphodontids had branched off along their own path the principal development seems to have been linked to flight ability. The forelimbs became much longer, mainly through the lengthening of bones in the forearm and wing-finger, and the hind limbs were lightened and streamlined, mainly by the reduction of the outer bone of the lower leg (fibula) to a thin, slender, splint-like element. The first clan in which these features are to be seen also happens to be one of the most extraordinary and enigmatic of all pterosaur groups— the anurogna-thids, named for their remarkably bulbous and frog-like heads.

The first evidence of anurognathids, a "road kill with exploded head" was recovered from the Solnhofen Limestones in the 1920s and for many years was practically the only evidence of this particular clan. Another superb example of this pterosaur, Anurognathus, which came to light quite recently and is illustrated in Figure 1.1, consists of an entire skeleton laid out in a remarkably lifelike pose, with the added bonus of fossilized wing membranes. Apart from Anurognathus, several incomplete, rather crushed remains of anurog-nathids have been found in ancient lake sediments deposited in Kazakhstan in the Upper Jurassic and northeast China in the Lower Cretaceous.43 One of these, a Chinese pterosaur calledJeholopterus, shown in Figure 8.4, has astonishingly complete and well-preserved wing membranes and even some patches of skin, replete with a covering of "hair."44

Anurognathids were small pterosaurs, usually only half a meter (2 0 inches) or less in wingspan, with a short, compact body and relatively broad wings. The most distinctive feature was the skull: Short, broad and deep, and perforated by large openings (Figure 4-7)? it looks so frog-like that one anurognathid, Batrachognathus, literally "amphibian jaw," was named for this similarity. The teeth are also quite peculiar: rare, widely spaced, and resembling sharp little spikes.

Another feature that sets anurognathids off from all other rhamphorhyn-choids is the reduction of the tail to a short little stub just like that found in pterodactyloids. Is this a hint that the two groups were related? Earlier it was thought that anurognathids, with their swollen heads, might have descended directly from dimorphodontids, which have similar, if rather more elongated, swollen heads, but this is now considered unlikely.45 At the same time, anurognathids do not, at least superficially, look very much like ptero-dactyloids. Yet, in addition to the short tail, they share several other special features in common with this group, such as the loss of the neck ribs and a reduction in the number of vertebrae in the main part of the spinal column. It is doubtful that pterodactyloids descended directly from anurognathids, but the two clans might have shared a common ancestor.

Eudimorphodon— First of the Long Snouts At some point soon after anurognathids had branched out on their own, pterosaurs experienced another major evolutionary event. It happened to the skull. Unlike dimorph-odontids and anurognathids, whose skull shape harks back to pterosaurs' reptilian ancestry, all later clans have a longer, lower skull. This evolved mainly by lengthening of the snout region— almost as if someone had pinched a pterosaur by the nose and tugged very hard. Why the snout evolved in this way is not clear, but most likely it was driven by adaptations for feeding that endowed pterosaurs with a longer "prey-grabbing" tool. As the snout extended forward, several things happened to the skull: The bones upon which the lower jaw articulated— long, strong, rod-like elements called the quadrates— began to slope forward, rather than standing vertically, as they do in early pterosaurs. This opened up two possibilities: more room both for muscles that operated the jaw and for the bony capsule that enclosed the brain.

Campylognathoidids,46 illustrated in Figure 4.8, were the first pterosaurs to show these important changes. The flag-bearer for this group, Campylogna-thoides, is best known from the Lower Jurassic Posidonia Shales of Germany,

FIGURE 4«7 Anurognathids, the fabulous flying frog heads, represented by the restored skull of Amtrognathus, with a wingspan of about 16 inches (40 centimeters).

Eudimorphodon

FIGURE 4.8 First of the long snouts. The campylognathoidids, represented by the restored skull and skeleton of Eudimorphodon, with a wingspan of about 3 feet (l meter). Inset: a single tooth of Eudimorphodon illustrating its multiple points. (Redrawn from Rupert Wild, 1978, and Peter Wellnhofer, 2003.)

FIGURE 4.8 First of the long snouts. The campylognathoidids, represented by the restored skull and skeleton of Eudimorphodon, with a wingspan of about 3 feet (l meter). Inset: a single tooth of Eudimorphodon illustrating its multiple points. (Redrawn from Rupert Wild, 1978, and Peter Wellnhofer, 2003.)

Dimorphodon Skull

FIGURE 4«7 Anurognathids, the fabulous flying frog heads, represented by the restored skull of Amtrognathus, with a wingspan of about 16 inches (40 centimeters).

in which the remains of more than 20 individuals, generally a bit less than a meter (3 feet) in wingspan, have come to light over the last 100 years.4' Campylognathoides demonstrates several typical features of the lineage, most noticeably the "drooped" tip of the lower jaw, from which erupt two large "prey-grabbing" fangs.

The other important member of this clan, Eudimorphodon, is mostly known from small individuals, though one or two also reached over a meter (3 feet) in wingspan. The first example of Eudimorphodon, an old individual with a most impressive-looking skull, was found quite by chance in 19'3 by an Italian paleontologist, Rocco Zambelli, in debris from a rockfall at Cene near Bergamo in northern Italy. The first pterosaur to be described from Triassic rocks, one of the most surprising features of Eudimorphodon is its teeth: Unlike those of other pterosaurs, which have a single point, some of them have three or even five points, a shape that presumably helped them to grip their prey (slippery, writhing fish) more effectively. Several species of Eudimorphodon are now known, mainly from northern Italy, but also from elsewhere in Europe and even, in one case, from Greenland.48 Austriadactylus, represented by a single, paper-thin, picture fossil from the Upper Triassic of Austria,49 is probably a close relative of Eudimorphodon, because it too has multi-pointed teeth. But its crowning glory lies above: a magnificent crest that adorned the skull from the tip of the snout to the top of the head, present in a pterosaur right at the beginning of its 150-million-year history.

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  • emma
    How big is a dimorphodon?
    8 years ago

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