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.

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.”

Tischlinger&Frey pro-pterodactyloid features

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-pterodactyloidshort 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.

About these ads
This entry was posted in Biology, Biomechanics, Paleontology and tagged , , , . Bookmark the permalink.

6 Responses to Pterosaur, Inter-Modulated

  1. Hi Jaime,
    “Rhamphodactylus” is a largely disarticulated specimen different from your featured “Painten” pterosaur. Considering the importance to phylogeny in this case, I was disappointed to see no analysis in the Tischlinger/Frey paper. I ran the traits and found this specimen nested basal to Pterodactylus + Ningchengopterus, but curiously larger than more primitive tiny taxa AND larger than more derived taxa. Notably, when set to scale, the Painten ptero together with its smaller relatives had a very similar metacarpal length in real measurement. So relatively, based on overall size, the Painten pterosaur demonstrates something of a reversal in metacarpal length. These results suggest it is a potential mistake to assume that bones like metacarpals, even in transitional taxa, will necessarily elongate incrementally from point A to point Z. Rather, every bone in every generation is free to lengthen or shorten according to natural selection and despite trends seen in other taxa. So picking out even a dozen putative transitional traits runs the risk of pulling a “Larry Martin.” Phylogenetic analysis with 200+ tested traits is key to nesting this and other pterosaurs, and, of course, don’t discard any taxa just because they fall below a certain height. Images at pterosaurheresies.
    Dave

    • Saying “every bone is free to lengthen or shorten according to natural selection” regardless of inference from trends is to discard a tenet of science: that observations of potential trends are a component of our predictions. We observe the specimens, observe trends by placing taxa in series, and determine if the series is internally consistent. This test revealed how Marsh’s perfect horse lineage, and also the Ascent of Man, are false models of evolution. But the trends are generally true amongst those lineages.

      The specimen safely and neatly places amongst certain taxa, and regardless of how far you move it, it’s doing something novel with some part of its anatomy. You claim secondary shortening of the wing, Tishlinger&Frey, Hone, and apparently befuddled me see otherwise. We can claim these have equal weight, but that means neither supercedes the other. Claiming yours does is a problem, given how egregious the problems with your matrix are (Mickey’s posts on your matrix especially relevant). But I won’t belabor this response with that.

      We observe these trends because they seem apparent. We make predictions on their basis, and assume that there is a basal mosaic of pterosaurs in the transition from “rhampho” to “ptero,” and for the most part this holds true despite disparate phylo matrices by the particulars of phylo analyses (Kellner’s, Unwin’s, and Andres’ groups). Our predictions of these allow us to test new specimens, like this one. And yes, the model rpedicts that mosaic pterosaurs will seem to also reverse a trend, an experimentation in flight. We see the same imperfect series when it comes to birds (an imperfection that delights the Feduccia’s, Olson’s, and anti-evolutionists alike).

      The specimen fits neatly into the model we’ve seen so far, so our predictions for it are consistent. Should features show up that overwhelming counter this, and not just seem to; that it introduces homoplasy; or outweigh the model’s prediction with another one, then we will take that in stride. Tishlinger & Frey did not need to provide a phylo analysis when discussing the specimen to affirm their argument, though one can do this (and get it through review, Dave) and provide that missing analysis.

      • As both you and I recall, Mickey’s theropods descend from beaked dinosaurs. So, that’s a problem. And the pterosaur tree is not the dinosaur tree. Phylogenetic analysis is the bedrock of paleontology. Other attempts are ‘just so’ stories. Harder to defend, as everyone agrees.

        • My discussion on your trees and comments re: Mickey’s assessment are about your horribly flawed cladistic methodologies. They stand regardless of which clade you’re in. Your methods don’t sharply improve when you move into your selectively coded and outright poorly handled pterosaur tree. I firmly, and soundly, denounce whomever you got that “phylogenetic analysis is the bedrock of paleontology” bit from, comedic though it is. I hope it isn’t you, if it’s serious. Just so. Ha!

          • A little vitriolic for a healthy conversation here…

          • I get tired of having the same conversation over and over again. I use and refer to the best available data, include more and more of it, only to be claimed to be inferior to another that uses less and poorer means of analysis, analysis having been pointed out as such on numerous occasions. I wonder why this conversation keeps happening.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s