Pterosaurs of the Triassic

Sharovipteryx of the Middle Triassic

Pterosaur origins aren’t fully understood. Pterosaurs emerge in the fossil record fully adapted to a flying lifestyle. Scholars speculate that the earliest known ancestor was Sharovipteryx, a gliding reptile from the Middle Triassic that preceded pterosaur emergence and diversification in the Late Triassic. Sharovipteryx’s hind legs were extremely long and suggest that it jumped like a grasshopper; as it did so, its gliding membrane would catch the air, allowing it to glide. The membrane may have started off as a display device and only later became a function for flight. Its extremely long legs were supported by a panel of skin called a patagium, which stretched from the hind legs to the tail and from the hind legs to the tiny forelimbs. The miniscule forelimbs were possibly used only for adjusting the trim of the patagium when in flight. 

How pterosaurs evolved flight is also unknown; as is the case with the origin of flight for birds, scholars tend towards one of two theories. The ‘Ground Up’ theory holds that they were terrestrial creatures that somehow developed flight whereas the ‘Tree Down’ theory postulates that they were arboreal creatures that weaseled their way into powered flight. This latter theory is the most likely. Perhaps pterosaur ancestors were tree climbers that developed a membrane between limbs that was first used to ‘parachute’ tree-to-tree; over time, the gradual lengthening of the fourth fingers enabled them to glide. How they got from gliding to powered flight is unknown, as the mechanics necessary to go from gliding to flying are entirely different.

Carniadactylus

The first pterosaur, Carniadactylus, appeared during the Carnian stage of the Late Triassic. It was closely related to the better-known Norian-age Eudimorphodon, though it was much smaller. Like Eudimorphodon, it had complex multi-cusped teeth. The significant size difference has been attributed to them occupying different niches and relying on different food surfaces. This is supported by studies of their teeth: Carniadactylus’ teeth show little to no wear, while the fish-eating Eudimorphodon has worn-down teeth from chewing food. This has led scientists to speculate that the smaller Carniadactylus probably fed on smaller, soft-bodied prey like worms and insect larvae.

a pair of Eudimorphodon in flight

Pterosaurs radiated in the Norian skies. The most famous of all Norian pterosaurs was Eudimorphodon – not only because of the wealth of specimens (including numerous juveniles) but also because of an array of teeth that would’ve been an orthodontist’s worst nightmare. It resembled the Carnian’s Carniadactylus but was larger with a wingspan of just over three feet. It weighed around twenty pounds, and its gargantuan fourth finger attached to the membrane making up its wing. Its panoply of heterodont teeth – packed to the gills with 110 in a jaw only six centimeters long! – make it notable among pterosaurs; most pterosaurs either had simple teeth or lacked them altogether. The front of its jaw had fangs, four per side in the upper jaw and two per side in the lower jaw, and these fangs gave way to a line of twenty-five multi-pointed teeth in the upper jaw and twenty-six in the lower jaw. This array of teeth suggests a piscivorous (or fish) diet, and this assumption was verified by the discovery of bony fish remains in the stomach regions of one fossilized Eudimorphodon. It likely also fed on insects and hard-shelled invertebrates. The end of its bony tail may have been diamond-shaped like the later Rhamphorhynchus; if so, it may have helped Eudimorphodon steer while in the air. Eudimorphodon likely caught fish by plunging like an arrow into the water and scooping prey into its mouth in the same manner as modern gannets. Such a fishing method requires excellent eyesight to see under the water (which Eudimorphodon had, for its eye sockets were large) and strong flight muscles so that it could flap hard to pull its body out of the water and rise back into the air (Eudimorphodon had a wide breastbone with a raised ridge called a keel to anchor strong wing-flapping muscles, so it was a more than capable flyer). Eudimorphodon’s jaws could dart forward or swish sideways to grab prey while underwater. Youngsters may have been insectivorous until they grew strong enough to dive into the shallow, fish-teeming Tethys Sea. 

Preondactylus in flight over the Tethys coastline

Another Norian pterosaur was Preondactylus. It was larger than both Eudimorphodon and Peteinosaurus with a wingspan of up to one and a half meters, and like both it was found in Italy. It had a long tail and relatively long legs. It had single cusp teeth (meaning they had one point on each tooth) and likely fed on fish and insects. One specimen had a sad turn of luck when it dived for a fish and was instead eaten by one: this specimen was discovered in the gastric pellet of a predatory fish that had vomited up the pterosaur’s indigestible parts (and which were later fossilized). The artist’s rendition of Preondactylus to the left shows it covered in pycnofeathers (a type of early feather); though we tend to imagine pterosaurs as leathery-winged creatures, the discovery of feathered pterosaurs in China has contested this notion. The fossil beds in China are of a sort that enabled many exquisite features to fossilize, and we know that at least some pterosaurs had feathers. We also know that many dinosaurs, of both ornithischian and saurischian stock, had feathers; because dinosaurs and pterosaurs diverged early on in archosaur history, it’s surmised by many that early feathers originated in basal archosaurs and passed on to pterosaurs, dinosaurs, and (in a highly-specialized form) modern birds. While the recent finds don’t tell us that all pterosaurs had feathers, we can at least surmise that all had the genetic potential for some time of feathered augmentation. These early feathers (unlike those found in modern birds) weren’t designed for flight; pterosaur flight, while working on the same mechanics as birds, was engineered with membrane wings rather than feathered wings. In their case, the feathers would’ve likely been for show – a way to warn rivals or attract mates.

Peteinosaurus

Another Norian pterosaur was Peteinosaurus. This pterosaur hunted along the Tethys coastline in Europe. It had a two-foot wingspan and was the size of a pigeon. Its thumb-sized head was mostly jaws with two large front fangs. It likely swooped down from a perch to grab insects like dragonflies midair. It had shorter wings than most later pterosaurs, and the vertical ‘paddle’ at the end of its long tail would’ve been used to steer mid-flight. Its tail bones were stiffened by bony rods. 

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