Friday, August 31, 2018

The Many Crests of Pterodactylus

Little Pterodactylus,  from the late Jurassic period of Bavaria, was one of the first pterosaurs ever discovered (a story you can read all about in my book Beasts of Antiquity). Represented by numerous juvenile and subadult specimens, it's among the better understood pterosaurs as well, especially if you include a few controversial specimens that have recently been argued to represent distinct genera such as Aerodactylus (a conclusion many pterosaur specialists remain skeptical of, but that's a topic for another post).

Although many Pterodactylus specimens preserve soft tissue, one pretty important aspect of their biology is NOT so well understood - their crests. In the past few decades, it has become apparent that crests of one kind or another are a hallmark of most pterodactyloid pterosaurs (and even a good number of non-pterodactyloids). Crests were first reported for Pterodactylus itself by Doderlein in 1929, but it was almost never depicted with a crest in art afterwards. The first-ever crested Pterodactylus was probably a toy. In 1988, Tyco released a crested Pterodactylus toy as part of their "Dino-Riders" line. Though produced under the supervision of Bob Bakker, it's unclear whether or not the crest was based on Bakker's inside knowledge of pterosaurs or was just a lucky guess added to give a fairly plain pterosaur toy more flair. Bakker himself had illustrated Pterodactylus without any crests in The Dinosaur Heresies several years earlier. Despite the fact that Peter Wellnhofer described a lappet "crest" (see below) for Pterodactylus in 1970, and even figured this specimen in his popular 1996 book The Illustrated Encyclopedia of Prehistoric Flying Reptiles, illustrations in that same book depicted Pterodactylus as crestless.

Photo of Pterodactylus specimen BSP 1929 I 18, from Wellnhofer 1996. You can see a thin occipital lappet extending diagonally up from the back of the skull.

The dubious Tyco example aside, the concept of a crested Pterodactylus didn't really reach the popular consciousness (and had apparently been forgotten by science, much like several other "modern" ideas about pterosaurs that were really discovered by 19th and early 20th century German paleontologists) until the first ultraviolet florescence studies done by Eberhard Frey and Helmut Tischlinger in the late 1990s and early 2000s. They produced what, at the time, seemed like a very bizarre reconstruction of a pterosaur, especially one like Pterodactylus which was somewhat famous for being the 'crestless one' (as opposed to its more famous, crested cousin, the giant Pteranodon). The illustration that was sent out in press materials about the early UV studies showed a shaggy mane of filaments on the neck, big, floppy webbed feet, a throat pouch, and a big, teardrop-shaped crest that extended above and behind the eyes. Clearly, the UV analysis had totally overhauled our image of Pterodactylus.

Frey and Tischlinger's reconstruction of Pterodactylus based on UV studies.

Or did it? The actual UV papers are still difficult to come by online, so early on it was difficult if not impossible for many paleoartists to examine the source material themselves. In the mean time, Frey's Pterodactylus became the gold standard for accuracy, with savvy artists beginning to incorporate the mane, webbed feet, and distinctive crest into their own work, all based not on any photos or diagrams of fossils, but simply on Frey's pencil drawing. Here's my own early take on the "new" Pterodactylus. Note that, in order to try and be a little different, I applied the UV soft tissue findings to a different specimen, the holotype of Pterodactylus brevirostris (which may actually be a juvenile Ctenochasma!).

Note that this was done in June 2002, shortly after the publication of Frey's English-language work summarizing the UV findings of the past few years. I later sketched out a version based more directly on Frey's original drawing:

A couple of things turned out to be... maybe not wrong, per se, but definitely speculative and not directly evidence-based, about the Frey-style Pterodactylus.

For one, that shaggy mane. Pterodactylus did indeed have a coat of unusually long pycnofibres on its neck. And by "unusually long", I mean that they are nearly half a centimeter long (compared to a ~10 cm long neck), unlike most of the incredibly tiny fibers coating the rest of the body. The "mane", therefore, would probably have appeared as a particularly fuzzy, bristly section of a short, dense coat.

As for the crest, none of the specimens show an oval shaped crest extending above and behind the eyes. What the few specimens we have of the crest show is actually two discrete crests or crest-like structures. The main crest, as preserved, is roughly triangular, with its peak just in front of the eyes. A second structure protrudes behind the skull. This has been called the "occipital lappet", and was first noticed by Wellnhofer in 1970. Superficially, the lappet resembles a small version of the crest of Pteranodon. Or, maybe more appropriately, the rear spar of the crest of Tupandactylus. In that tapejarid, the crest is comprised of two bony supports. One, roughly triangular in shape, above the snout. The other, a long horizontal spike, extends behind the skull. In between was an enormous, rounded crest composed of keratin or some other rigid soft tissue. The two "obvious" crests are merely the support struts for these larger structures. Frey imagined that in life, the triangular crest above the eyes and the occipital lappet may have been joined together into this kind of single structure, the apparent shape of the crests as preserved being an artifact of decomposition or post-mortem breakage. This interpretation has been followed by a majority of artists since, as a Google Image search will show.

You can see that an image search for "Pterodactylus crest" brings up some fossils and diagrams, 2 reconstructions of Pteranodon (of course), 1 of an ornithocheirid (somebody got confused?), 1 old-fashioned reconstruction with no crest, 3 different reconstructions with a triangular crest and separate lappet (one of which is my own), and 9 reconstructions with a Frey-style joined crest (again including one of my own!). 
Mark Witton, in his 2013 book Pterosaurs, was influential in popularizing an even larger tapejarid-like crest, which he included both in his reconstructions and skeletal diagrams. His reasoning for taking the Frey-style crest to the next level was based mainly on the general rule that pterosaur crests tend to be larger than they appear.

There are some important differences, though, that we should consider before speculating too much about a tapejarid-style crest in Pterodactylus. First, the two are not particularly close relatives, and tapejarid-like crests have not yet been found in any other pterodactyloid groups. Given the enormous crest diversity among pterosaurs, I'm not sure it's appropriate to assume they were all basically big ovals and differences are just preservational. Some other pterosaurs unrelated to Tupandactylus did have big, rounded crests, but these were more like semicircles erupting from the skull, not extending behind it or significantly above it (like wukongopterids and even some ctenochasmatoids closer to Pterodactylus itself). One other example of "enormous crest supported by bony struts" has been proposed in the form of Nyctosaurus, but despite some spectacular looking restorations out there, it's unlikely those enormous spars supported any soft tissue.

The huge, oval-shaped crest of Tupandactylus was supported by long bony crests that graded into soft tissue, unlike the totally soft crests of Pterodactylus. Photo from the AMNH pterosaur exhibit by Lisa Brormann.
Second, the supposed spars of the Pterodactylus crest are not made of bone! The reason Tupandactylus and other tapejarids can have those huge oval crests sitting on their heads is because they have bony supports. Even the smaller species like Tapejara wellnhoferi has a significant hard, bone-based component to its (possibly) large oval crest. In Pterodactylus, not only is the main crest comprised entirely of soft tissue with an unusually minimal amount of bone as an underlying base, the occipital lappet is not made of keratin at all. Upon close examination of the internal structure of the lappet, it seems to be supported internally by twisted fibers similar to those that make up the pycnofibre coat. The lappet would not have been flat in life, like the crest of Pteranodon, but conical. The fact that it is composed internally of fibers may imply that it was flexible, a result that would explain why it is preserved in different positions in different specimens (some curving upward, some straight). The lappet seems to have been more an extension of the skin integument than a typical crest, sort of like the wattles and caruncles of a turkey.

Possible crest reconstructions for Pterodactylus (based on specimen BSP 1929 I 18). Clockwise from top: Crest and lappet as preserved; joined crest after Frey 2002; joined crest after Witton 2013; minimally extended unjoined crest.
Diagram by M. Martyniuk 2018, all rights reserved. 
It's entirely possible that future specimens will show that we have Pterodactylus crest shapes wrong, or that the main crest was in some way attached to the lappet. But given the evidence right now, that interpretation is one of the less likely possibilities. A few prominent paleoartists who helped popularize the tapejarid-style crest have since produced lappeted ones, including Mark Witton and John Conway, both of whom, intriguingly, depicted the lappet as just part of the larger pycnofiber assemblage - Conway as an extension of the "mane", Witton as part of a larger set of display fibers). You can read more about Witton's new Pterodactylus reconstruction on his blog.

All of these reconstructions still go a bit beyond the known evidence by depicting large, flamboyant crests. As they probably should - Witton was correct when he pointed out that pterosaur crests were probably larger, in general, than traditionally thought. All of our crested Pterodactylus specimens are also sub-adult, so even though the soft tissue crests we have preserved seem to be pretty small, it's likely the crest would have gotten at least a little bigger with maturity. We just don't know how much bigger.

Reconstruction of a subadult Pterodactylus by M. Martyniuk.

Sunday, June 10, 2018

Review: "Beasts of the Mesozoic" Tsaagan by Creative Beast Studios

Quick Facts
2018 Beasts of the Mesozoic Raptor Series Tsaagan mangas action figure
Size: 20cm long
Scale: 1:6
Sculpted by: David Silva
Produced by: Creative Beast Studios

Back in April 2016, toy industry veteran David Silva launched a Kickstarter campaign to produce scientifically accurate "raptor" (eudromaeosaur) figures. Unlike the vast majority of static PVC dinosaur figures on the market, these would be super articulated, with up to 24 joints allowing significant posability. Now, over two years later, the project has become a reality, and my selection of a Wave 2 Tsaagan mangas figure has finally arrived. So, how does it stack up to the high expectations and lofty claims that these are the most scientifically correct dinosaur action figures on the market?

Friday, May 4, 2018

The Step-wise Bird: Andrea Cau on Bird Evolution

Above: WIP reconstrcution of one potential Connecticut River Valley trackmaker, the bird-like reptile Anchisaurus polyzelus. By M. Martyniuk, all rights reserved.
This morning saw the publication of a new paper by Andrea Cau, titled Assembly of the Avian Body Plan, and what a mammoth (dinosaurian?) work it is! Cau does an amazing job of synthesizing the step-wise nature of bird evolution that is so often hidden behind imprecise or muddy nomenclature. Far from a dichotomy between "non-avian" and "avian" dinosaurs, the important features we associate with modern birds gradually accumulated in a particular lineage of stem-birds ever since the early Triassic period. I should have a lot more to say on the nitty-gritty of this paper this weekend after I've had a chance to fully digest this important work on avian origins. In the mean time, I wanted to share a brief excerpt from (one of) my upcoming book(s), this one dealing with the struggles to interpret some of the earliest known dinosaur remains in an era before the nature of dinosaurs as weird transitional members of the bird lineage was fully understood. The chapter this comes from is discussing Edward Hitchcock's work in the early-mid 1800s on bird-like footprints found in the Connecticut River Valley. The footprints date to the early Jurassic (Cau's "Huxleyan stage" of bird evolution).

Several, more prominent, scientists of the time criticized Hitchcock’s interpretation of the footprints as having been made by birds. He was ridiculed for imagining huge birds that must have been many times the size of the largest living bird, the ostrich. Soon, the rediscovery of giant extinct birds like the moa granted him some level of vindication. But more serious criticisms followed. The sandstone of the Connecticut River Valley was simply too old, other scientists argued. Birds, being “higher” life forms in the ranked scheme of life most believed in at the time, must have also been newer, having developed fully only after the so-called “age of reptiles”. Some scientists went so far as to argue that the three-toed tracks belonged to giant frogs, and that only the large, strong hind limbs left impressions while the lighter forelimbs often did not. And, indeed, one fact which was very inconvenient to Hitchcock’s explanation was that some of the tracks preserved light forelimb impressions, and some were found along with tail drag marks.

What could Hitchcock do to save his bird hypothesis from the facts? By 1861, the discovery of an archaic proto-bird named Archaeopteryx lithographica provided the answer. Here was an example of a “bird” with primitive, reptilian features and a long tail. Perhaps, Hitchcock suggested, his sandstone prints were not made by giant moa-like birds, but giant Archaeopteryx-like birds. And what of the occasional forelimb impressions? Hitchcock actually suggested that, along with its primitive skeletal anatomy, the Archaeopteryx may have been a facultative quadruped! In his view, the Archaeopteryx was halfway between birds and reptiles in both anatomy and gait. Hitchcock had, rather unscientifically, crafted his hypothesis to be immune to all criticism. His peers weren’t buying it.

By the time of Hitchcock’s death in 1864, the bipedal, bird-like nature of many Mesozoic reptiles like Hadrosaurus and Compsognathus had been discovered. For most scientists, these creatures provided a more plausible explanation for Hitchcock’s “sandstone bird” tracks than actual birds. By the late 1800s, the tracks were universally accepted as having been made by prehistoric reptiles, though intriguingly bird-like ones. Today, we know that these ancestrally bipedal reptiles, the dinosaurs and their kin, did indeed have more in common with modern birds than with any of the modern reptile groups, and in fact included the evolutionary ancestors of true birds.

In the end, it turns out that Hitchcock was half-right. His sandstone bird tracks were made by creatures in many ways more like Archaeopteryx than any modern bird or reptile, some of which were partly or fully quadrupedal, with great sweeping tails and enormous body sizes compared to any birds alive today. Many of them even had feathers and feather-like filaments covering parts of their bodies. What Hitchcock had actually discovered were the bird-like reptiles, creatures descended from the same ancestors as crocodiles and turtles, but which had evolved a wide array of uniquely avian features. At a time when most mainstream scientists envisioned dinosaurs as huge, quadrupedal, mammal-like reptiles (in appearance and gait if not lineage), Hitchcock was able to use the traces they made in life to arrive at a conclusion that was actually much closer to the truth in many ways. The Mesozoic was not an “age of reptiles”, at least not on land. It was an age dominated by the bizarre, archaic relatives of birds.

I think the above is a good example of Cau's thesis that a false, dichotomous paradigm, like "bird" vs. "reptile", or "non-avian dinosaur" vs. "bird", and focusing mainly on "key" specimens like Archaeopteryx, can actively mask the reality behind fossil evidence. What do you think?