Pterosaur, Inter-Modulated

When Helmut Tischlinger and Eberhard “Dino” Frey team up for a paper, you know it’s gonna be good. Almost certainly, there will be UV involved. The pterosaur fossils of the Solnhofen are especially UV reflective, which brings out obscure or often hidden aspects, including soft-tissue that is simply not visible under normal light.

Their latest is a beautiful specimen of a juvenile pterosaur, a maturing “flapling.” Caught in the act of growing, the fossil even preserves several replacement teeth growing in. Some details are hard to see (the sternum is obscured and small, the palate shattered inside the skull) but others are amazing.

The Painten “pro-pterodactyloid” of Tischlinger and Frey (2014). Images modified from the paper as provided by Dave Hone.

It is also apparent that this fossil represents an important transition in pterosaur evolution: The transformation from the “rhamphorhynchoid” grade (short necks, long tails, short hands) to the “pterodactyloid” grade (long necks, short tails, long hands).

Caught in between these two were the Wukongopteridae, represented by a few pterosaurs from the Middle Jurassic of China: Darwinopterus, Wukongopterus, Kunpengopterus. Rather than being of the one grade or the other, they show evidence of both (long necks, long tails, short hands). Other details are apparent:

Many rhamphorhynchoid tails show the presence of long rods formed from the articulations between zygapophyses and haemal bones, processes lying above and below the vertebrae of the tail, whereas pterodactyloids do not, and wukongopterids had these rods. At the same time, the wings of pterodactloids have longer bones in the wing finger, with the first and second bones longer than the forearm, but in rhamphorhynchoids, as in wukongopterids, they are closer to equal in size.

Pterodactyloids also have a long, shallow skull, much longer than the neck even though that neck is so long, but in rhamphorhynchoids the head is closer to the neck in length. And lastly, the fifth toe in rhamphorhynchoids, which supported the edge of the uropatagium (a membrane stretched between ankle and tail) is long, with a pair of slender bones forming a sort of bat-like wing toe; but in pterodactyloids, this toe is absent, reduced to a tiny little nubbin.

Wukongopterids retain the long toe, along with the tail and its complex rods, showing that while its head, neck, and parts of its wings were adapting to whatever pterodactyloids were doing at the time, the hindparts were lagging behind. They were just more useful that way. But this new pterosaur specimen shows something different.

Evolution is modular. It’s not about just random experiments for the most part, but about adaptive selection for what helps an animal, and eventually a group survive. A feature that improves survival tends to pass through a group faster than one that doesn’t improve survival. With pterosaurs from the Middle to Late Jurassic caught in three groups, with “rhamphorhynchoids,” wukongopterids, and pterodactyloids representing three steps along to the “perfection” of azhdarchoids and ornithocheiroids and such, the new baby finds itself slotted in between the wukongopterids and pterodactyloids. It’s filling in what wukongopterids were missing out on, changes in the hindlimb lagging behind head and wing.

Below is the UV photo of Tischlinger & Frey (2014)’s new specimen, termed the “Painten pro-pterodactyloid” and nicknamed by some “Rhamphodactylus” (not a formal name!). I’ve labeled the interesting features. Gold circles represent pterodactyloid features, while purple circles represent “rhamphorhynchoid” features. Transitions that are unique to this new stage (not present in wukongopterids but present in pterodactyloids proper) are marked with an asterisk (*). The remaining features must then have been the last to be modified, but it is unclear if this, too, was transitional. Did the tail rods, pteroid size, or wing-finger bone length come next? It is also likely that some pterosaurs will be found to have changed only one or two of these at a time. Potentially, some may have other novelities, with long fingers and a long tail. Probably, however, this isn’t the case; it’s just not “neat.”

Features of the Painten “pro-pterodactyloid,” from Tischlinger and Frey (2014). Features are colored respective to their “typical” arrangement, with blue features for “rhamphorhynchoids” and gold for pterodactyloids; feature 7 is half and half. Features marked with an asterisk are novel to this “phase” in pterosaur evolution. Features noted are 1, long head; 2, long neck with tall neural spines, being squarish; 3, short and high-placed deltopectoral crest on humerus; 4, short pteroid; slightly elongated metacarpals of hand; 6, first and second wing phalanges longer than radius+carpus+metacarpal IV combined; 7, distinct fifth toe with two elements, but much smaller than normal; 8, tail with long stiffening rods; 9, tail short, not much longer than the femur.

Theoretically, the reason the tail shortens with the shorter fifth toe is that the wing is developing into a more adaptive design, taking over duties earlier pterosaurs had used, such as roll and pitch control, or the ability to fly at slow speeds without stalling, as birds and bats do today. Reversing this, with long tails and long wings and long pteroids merely overloads the control modules for a flying animal: too much, all at once. So this seems more likely:

“Rhamphorhynchoids” – long tail with stiffening rods and long fifth toe, short neck with short neural spines, short and high deltopectoral crest, short hand, pteroid and first bones of the wing finger;
Wukongopterids – long tail with stiffening rods and long fifth toe, long neck with short neural spines, short and high deltopectoral crest, slightly longer hand, short pteroid and first bones of the wing finger;
pro-pterodactyloid – short tail with stiffening rods and moderately short fifth toe, long neck with tall neural spines, short and high deltopectoral crest, slightly longer hand, short pteroid and first bones of the wing finger;
“Pterodactyloids” – short tail without stiffening rods and very short fifth toe, long neck with tall neural spines, long and distal deltopectoral crest, slightly longer hand, long pteroid and longer first bones of the wing finger.

I’d predict from this that we’d likely see the reduction in tail to loss of those rods and the shortening of the toe to a nubbin happen around the same time as the pteroid lengthens, transferring the tail and uropatagium flight control surfaces to the wing itself. We already know that the transition from “rhamphorhynchoids” into wukongopterids itself isn’t smooth, with an earlier, long-ish-necked version called Pterorhynchus wellnhoferi; wukongopterids themselves seem to have built upon that successful lineage. The position and size of the deltopectoral crest influences the size and moment of the muscles that pull the upper arm around, especially the deltoid muscle that in pterosaurs seems to help pull the humerus towards the head (extending the wing), as well as having a slight upstroke component.

One thing that catches me is the longer, low, very Pterodactylus-like head. There are a lot of short, triangular and semiconical, almost ziphodont teeth in the jaw. The tip of the lower jaw has a pair of teeth, but not the tip of the upper. We know from work by Chris Bennett on Pterodactylus and Germanodactylus that juveniles of these pterodactyloids add teeth as they age, so it is quite likely that the tooth count in this animal, which is very low, isn’t the adult tooth count. The head shows no cranial crests, even though it is nested among animals that had them (again, Pterodactylus and Germanodactylus), but that also seems to be ontogenetic.

As a final thought, while one cannot be but amazed at the quality of preservation and discussion, and that this transitional form seems almost deserving of a name, it is also a juvenile. It may not develop the features needed for diagnosis, and one might see some strive to reach that conclusion. Certainly, some of the novelties of the skeleton, representing a unique character complex and itself useful for diagnosis, may be seen to be enough. I disagree in general, though some pterosaurs now known are known either fully or initially through juveniles (e.g., Nemicolopterus, Aurorazhdarcho elegans, Bellebrunnus). Some has been written qualifying that pterosaur proportions do not change substantially with growth, and how much they grow when they do, but the cranial ontogenetic features, and possibly also the vertebral and pelvic features, certainly do change with age. I suggest great caution.

Much thanks to Dave Hone, who blogged this here and supplied tantalizing photographs, for supplying the paper and promptly.

Tischlinger, H. & Frey, E. 2014. Ein neuer Pterosaurier mit Mosaikmerkmalen basaler und pterodactyoider Pterosaurier aus dem Ober-Kimmeridgium von Painen (Oberpfalz, Deutschland) [A new pterosaur with moasic characters of basal and pterodactyloid Pterosauria from the Upper Kimmeridgian of Painten (Upper Palatinate, Germany)]. Archaeopteryx 31: 1-13.