Pterosauria

The pterosaurs (literally 'winged reptiles'), known from just over 100 species, existed for the same span of time as the dinosaurs. They were important small fish-eaters in the Jurassic, and adopted a variety of ecological roles in the Cretaceous when some truly gigantic forms arose.

8.6.1 Pterosaur anatomy and ecology

The first pterosaurs from the Late Triassic, such as Eudimorphodon from northern Italy (Wild, 1978), show all the unique characters of the group (Figure 8.23(a)): the short body, the reduced and fused hip bones, the five long toes (including a divergent toe 5), the long neck, the large head with pointed jaws and the arm. The hand (Figure 8.23(b)) has three short grasping fingers with deep claws and an elongate fourth finger that supports the wing membrane. In front of the wrist is a new element, the pteroid,a small pointed bone that supported a small anterior flight membrane, which joined on to the short robust humerus (Figure 8.23(a)). The pelvis (Figure 8.23(c)) is a solid small structure with short blunt pubes and ischia. An additional element, the pre-pubis, is attached in front and it may have had a func tion in supporting the guts. The tail is stiffened with ossified tendons, as in some dinosaurs, and it may have been used as a rudder during flight.

The pterosaurs diversified in the Jurassic and Cretaceous (Wellnhofer, 1978, 1991; Buffetaut and Mazin, 2004). Basal lines are often grouped in the paraphyletic 'Rhamphorhynchoidea', but most diverse was the clade Pterodactyloidea, which arose in the Late Jurassic and radiated during the Cretaceous.

Much of the diversity of pterosaurs may be appreciated by an examination of a selection of skulls (Figure 8.24). First, skull lengths vary considerably from 90 mm in Eudimorphodon, little larger than a seagull, to 1.79 m in Pteranodon. These skulls also show some broad evolutionary changes: forward shift of the jaw joint to lie below the orbit, elongation of the skull and fusion of the nostril and antorbital fenestra with reduction of the nasal bone.

Pterosaur skulls suggest a range of feeding styles. The long spaced teeth of Rhamphorhynchus, Ptero-dactylus and Ornithocheirus (Figure 8.24(b, c, f)) were probably used for piercing and holding fish, whereas the shorter teeth of Dimorphodon (Figure 8.24(a)) may have been used for insect-eating. Ctenochasma and Pterodaustro (Figure 8.24(d, e)) have huge numbers of slender teeth in each jaw, i.e. 400-500 flexible teeth in nasal bone nasal bone

Pterosaur Skull
Fig. 8.24 Diversity of pterosaurs, shown by their skulls: (a) Dimorphodon; (b) Rhamphorhynchus; (c) Pterodactylus; (d) Ctenochasma; (e) Pterodaustro; (f) Ornithocheirus; (g) Dsungaripterus; (h) Pteranodon.(After Wellnhofer, 1978)

Pterodaustro,which were probably used to filter microscopic plankton from the water. The teeth would have acted as a fine filter mesh in trapping thousands of small organisms that could be licked off and swallowed. The jaws of Dsungaripterus and Pteranodon (Figure 8.24 (g, h)) are deep and hatchet-shaped and bear very few, or no, teeth. These forms also probably fished by beak trawling and swallowed their catch so rapidly that no teeth were needed.

Pteranodon, one of the best-known and largest pterosaurs from the Late Cretaceous of North America (Bennett, 2001), has a wingspan of 5-8 m. The skull is longer than the trunk (Figure 8.25(a)) and its length is doubled by the pointed crest at the back that may have functioned like a weathercock to keep the head facing forwards during flight. However, the crest is sexually dimorphic, so the aerodynamic function might have been less important than display. Each massive cervical vertebra (Figure 8.25(b)) has a pneumatic foramen in the side that led into open spaces inside, a weight-reducing feature. The dorsal vertebrae are nearly all involved in one or two heavily fused girder-like structures, the no-tarium and the synsacrum (Figure 8.25(c, d)), which stabilize and support the shoulder girdle and pelvis. The shoulder girdle is attached to the side of the notarium above and to a large bony sternum (Figure 8.25(e)) below,which holds the ribcage firm. The sternum bears a slight keel for the attachment of flight muscles. This massive stabilization of the shoulder girdle and pelvis is typical of pterodactyloids and it was probably related to flight stresses.

Pteranodon was not the largest pterosaur. That honour goes to Quetzalcoatlus from the upper Cretaceous of Texas (where else?), which is represented by parts of a single wing, giving an estimated wingspan of 12m (Figure 8.25(f)). Quetzalcoatlus was the largest known flying animal, three times the size of the largest bird, and more like a small aeroplane in size than any familiar living animal. Quetzalcoatlus and its relatives, the azhdarchids, are known from fragmentary remains from the uppermost Cretaceous of many parts of the world.

Skeleton Reconstruction Pterosaur

Fig. 8.25 Anatomy of the giant Late Cretaceous pterosaurs: (a-e) Pteranodon: (a) flying skeleton in lateral view, (b) cervical vertebra in lateral view, (c) notarium in dorsal view, (d) synsacrum in dorsal view, (e) sternum in ventral view, (f) Quetzalcoatlus shown in proportion to Pterodactylus and Pteranodon. [Figures (a-e) after Eaton, 1910; (f) modified from Langston, 1981.]

Fig. 8.25 Anatomy of the giant Late Cretaceous pterosaurs: (a-e) Pteranodon: (a) flying skeleton in lateral view, (b) cervical vertebra in lateral view, (c) notarium in dorsal view, (d) synsacrum in dorsal view, (e) sternum in ventral view, (f) Quetzalcoatlus shown in proportion to Pterodactylus and Pteranodon. [Figures (a-e) after Eaton, 1910; (f) modified from Langston, 1981.]

8.6.2 Pterosaur flight

Pterosaurs were sometimes portrayed in the past as rather inefficient gliding animals that were incapable of flight. On the ground, their locomotion was supposed to be an awkward bat-like form of progression, consisting of staggering and tumbling on all fours like a broken umbrella blowing along the street. Current work (e.g. Wellnhofer, 1978; Padian, 1984; Padian and Rayner, 1993; Unwin and Bakhurina, 1994) counters these views and presents a picture of the pterosaurs as efficient flapping flyers like modern birds. The first line of evidence is the possession of wings and other aerody namic and flight adaptations (hollow bones, streamlined head). The second key aspect is that the pterosaurs were probably endothermic, as they appear to have had hair (see Box 8.7). Only endotherms have external insulation and endothermy gave the pterosaurs the high sustained metabolic rates necessary for flight.

The wing is composed of skin that attached to the side of the body and along the entire length of the arm and of the elongated flight finger 4 (Figures 8.25(a) and 8.26(a)). It has been argued that the pterosaur wing was a slender structure rather like that of a gull (Wellnhofer, 1978; Padian and Rayner, 1993),but it was broader as the flight membrane also attaches to the

Pterosaur Skeleton

Fig. 8.26 Pterosaur wings and flight: (a) skeleton ofPterodactyluswith the wing membranes preserved and showing partial attachment to the legs; (b, c) anterior and lateral views of the shoulder girdle and humerus of a pterosaur showing the humerus in the upstroke and downstroke positions and the main flight muscles (pectoralis,downstroke; supracoracoideus, upstroke). [Figure (a) after Wellnhofer, 1987; (b,c) after Padian, 1984.]

Fig. 8.26 Pterosaur wings and flight: (a) skeleton ofPterodactyluswith the wing membranes preserved and showing partial attachment to the legs; (b, c) anterior and lateral views of the shoulder girdle and humerus of a pterosaur showing the humerus in the upstroke and downstroke positions and the main flight muscles (pectoralis,downstroke; supracoracoideus, upstroke). [Figure (a) after Wellnhofer, 1987; (b,c) after Padian, 1984.]

BOX 8.7 HAIRS AND FIBRES

BOX 8.7 HAIRS AND FIBRES

Pterosaurs have been credited with a hair covering for more than a century (Wellnhofer, 1978), and hair has been reported in well-preserved specimens of Dorygnathus, Anurognathus, Rhamphorhynchus and Pterodactylus from the Upper Jurassic of Germany and in Sordes from the Upper Jurassic of Kazakhstan. The details of pterosaur 'hair' are currently disputed.

Studies of exceptionally preserved fossils (Martill and Unwin, 1989; Unwin and Bakhurina, 1994) show unexpected details of the pterosaur wing membrane. The membrane was about 1 mm thick and it was made from several layers of tissue (illus-traion (a)), a thin outer epidermis over a dermis that probably contained capillaries. Beneath this lay a tissue containing (?) collagenous fibres, and on the lower surface a relatively thick layer of striated muscles.

Microscopic investigation of wing membranes from several species (Padian and Rayner, 1993; Unwin and Bakhurina, 1994) has shown that they are reinforced with parallel stiff fibres, termed actinofibrils, particularly in the distal region. The acti-nofibrils are clear in Sordes (Unwin and Bakhurina, 1994), in which each fibre can be seen to be made from bundles of smaller strands (illustrations (b, c)). Sordespilosus, or 'hairy devil', is renowned for its supposedly thick pelt of hair. Most of the so-called 'hair', however, consists of displaced actinofibrils. Nonetheless, hairs have been detected in non-flight areas and Padian and Rayner (1993) confirm the distinction between actinofribrils and hairs in German specimens. The actinofibrils were located primarily in the outer sector of the wing, radiating backwards from the wing finger. They acted to spread the wing and keep it spread by transferring forces in the wing membrane back to the fourth digit along the leading edge (Bennett, 2000).

continued

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Exceptional preservation of skin and hair in pterosaurs: (a) block reconstruction of a section of pterosaur wing membrane from the Lower Cretaceous of Brazil; (b) the Late Jurassic Sordes, showing wing membranes and a membrane between the hindlegs; (c) detail of the wing-strengthening fibres of Sordes. [Figure (a) modified from Martill and Unwin, 1989; (b, c) courtesy of David Unwin.]

femur (Figure 8.26(a)) in well-preserved specimens (Wellnhofer, 1987; Unwin and Bakhurina, 1994).

The pterosaur power stroke was directed down and back and the recovery stroke up and forward, so that the wing tip, viewed from the side, described a figure-of-eight shape. At slow flight speeds, the downstroke was powered by the massive pectoralis muscle and the upstroke by the supracoracoideus muscle (Figure 8.26(b, c)), which ran from the sternum, over a pulley arrangement at the shoulder joint, to the dorsal face of the humerus. When it contracted, the supracoracoideus muscle, although placed below the wing, actually pulled it up, just as in birds (Padian, 1984).

Pterosaurs flew relatively slowly because of their large wings, but efficiently, and they were highly manoeuvrable. Their wing designs were comparable to those of modern marine soarers such as frigate birds and albatrosses, and aerial predators such as gulls and falcons (Hazlehurst and Rayner, 1992). Pterosaurs probably took off from trees or cliffs, or jumped into the air after a short run to pick up speed. Even in the larger pterosaurs, the take-off speed was low, possibly 4ms-1 in Pteranodon. Landing was awkward for the larger pterosaurs,just as it is for large birds, and the reinforced pelvis and sacrum would have had to withstand large impacts. Pterosaur senses and brains seem to have been adapted for flying—pterosaurs have the large eyes and bulbous heads of birds. Indeed, reconstructed pterosaur brains suggest (Witmer et al., 2003) that pterosaurs had good vision and balance areas in the brain, although overall their brains were relatively smaller than those of birds.

If there is relatively little controversy over the flying abilities of pterosaurs, there certainly is a debate over how well they could walk. Padian (1984) and Padian and Rayner (1993) argue that they could walk well on fully erect hindlimbs. Padian reconstructs the pelvic girdle of various pterosaurs as firmly fused beneath, and the limb motions just like those of a small bipedal dinosaur. The wings are held tucked horizontally beside the body during running. This view has been fairly conclusively disproved on the basis of three independent lines of evidence.

1 New, three-dimensionally-preserved pterosaur specimens show that the pelvis is wide open at the bottom and that the hindlimbs point sideways in an awkward sprawling posture (Wellnhofer, 1988a). The legs could not be pulled into an upright posture, and hence bipedality would be impossible.

2 Calculations of balance show that bipedality would have been hard for the smaller pterosaurs and impossible for the larger Cretaceous forms (Henderson and Unwin, 2004).

3 Fossil tracks show that pterosaurs walked quadrupedally, with the feet wide apart (hindlimbs in the John Wayne posture) and the hands far out on either side (Unwin, 1999).

During walking, the pterosaur used all four limbs, its legs in the middle and its hands a short distance in front and to the side, with the wing tips sticking up on either side of the head. The rolling, awkward locomotion of the Early Cretaceous pterosaur Anhanguera may be viewed at http://palaeo.gly.bris.ac.uk/dinosaur/ animation.html.

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