Tuesday, December 14, 2010

Tearing Up Pteranodon

Above: Male (foreground) and female Geosternbergia sternbergi, mounted casts at the Royal Ontario Museum. Photo by Kenn Chaplin, licensed.

On the PteroGoss front, I just came across a new paper that doesn't seem to have received much press online yet. A new paper by A.W. Kellner has appeared online concerning the taxonomy of the small but well-known pterosaur family Pteranodontidae. The abstract can be found here.

This paper is interesting in that it attempts to reverse some of the trends of the past few decades concerning pterosaur diversity, in some cases (in others it's merely a re-calibration of the old genericometer). Kellner erects two new species and ressurects a genus, Geosternbergia.

I first encountered Pteranodon sternbergi in Dave Peters' awesome picture book A Gallery of Dinosaurs and other Ancient Reptiles (which also probably instilled my OCD towards drawing scale diagrams). It took my young mind a while to register that this huge, tall/broad crested pterosaur belonged to the same genus as the familiar, backward-pointing-crested Pteranodon lonciceps that flew beside it. I have to admit that I though P. sternbergi just looked... cooler. In this kind of side-by-side comparison, they really don't looks like they belong to the same genus. But as always, genera are subjective things, and nobody doubted that P. longiceps and P. sternbergi were each others closest relatives (they even overlapped in time and geologic range).

P. sternbergi was first described by Harksen in 1966 as a species of Pteranodon, based on a skull which differed from other species by its tall, vertical crest. In 1972, Miller placed it rather arbitrarily in its own subgenus, as Pteranodon (Sternbergia) sternbergi. The name Sternbergia turned out to be pre-occupied (as far as I know, subgenenus names compete with genera for priority). Miller amended the name to Geosternbergia in 1978.

However, this designation fell out of favor by the 1990s, when Chris Bennett published a couple of hefty reviews of Pteranodon from the Niobrara and related formations in Kansas. Bennett stramlined Pteranodon taxonomy, taking the several species that had been considered valid up to that time and showing that much of the variation was likely due to age and/or sexual dimorphism. For example, Bennett re-affirmed the idea that some small-crested Pteranodon specimens represent females (and some juveniles) of the same species as the longer-crested adult males. With this variation in mind, he suggested that all Pteranodon specimens could fit into two species: P. lonciceps and P. sternbergi. Because differences between species are limited almost entirely to the skull and crest (though Kellner 2010 suggests that some consistent differences may be found in the skeleton with further study), Bennett had to rely mainly on stratigraphic position to decide which species a specimen belonged to. While the two did overlap in time, it was only for a very brief period, so even though skulls are very rare compared to skeletons, a specimen from the lower Niobrara could be confidently referred P. sternbergi, while one from higher in the formation probably came from P. longiceps. (Image at left: illustration of various Pteranodon skulls by Matt Martyniuk, licensed).

One problem with this method, which Kellner points out in his new paper, is that many specimens (especially those recovered back during the days of Cope and Marsh) lack information about their provenence detailed enough to allow such an assignment. Bennett himself simply referred these to Pteranodon sp., but if Kellner's new work holds up, they'd need to be assigned even more broadly, only to indeterminate Pteranodontidae.

That's because Kellner has re-assessed the variation within the traditional grouping known as Pteranodon, and found some specimens that seem to represent new species among them. For me, the most interesting is Dawndraco kanzai, or "Kanza Dawn dragon" named for Dawn, apparently an Iroquois sky goddess, not the English word. The type and so far only specimen is UALVP 24238, a really interesting nearly complete skull and skeleton usually attributed to P. sternbergi (as in Bennett, 1994). Aside from being one of the most complete (former) Pteranodon specimens, it has always struck me as very, very odd. One of the primary reasons for making this a new species it its upper jaw. In most Pteranodon skulls (though none are as complete as you may assume based on its ubiquity in paleoart), the jaws can be seen to curve upward toward the tip and taper off into a needle-sharp projection at the tip. In the Dawndraco holotype, the preserved portion of the jaws are extremely long relative to the rest of the skull, but show no signs of tapering. In fact, the top and bottom margins of the upper jaw form essentially completely parallel lines up until the break. Letting your imagination fill in the rest, this must represent either a phenomenally long-billed animal, or one with a very unusual fat, somewhat flattened tip. The bone within this tall, long bill looks like a loose, honeycomb mesh of very thin struts. Taken together, this does seem like it probably comes from something fairly different than Pteranodon proper. (Image at right: Skull of Dawndraco, from Kellner 2010. Scale bar = 500mm).

The next new species is Geosternbergia maysei. Kellner considers Geosternbergia a distinct genus based on its unique skull characteristics, but again, there is currently no analysis to suggest that it is any more or less closely related to Pteranodon than anything else, so it remains a subjective decision (though it would be interesting to see someone perform an analysis using all of Kellner's species and Nyctosaurus). Anyway, G. maysei is named for a partial skull (KUVP 27821) from the South Dakota Sharon Springs formation. It was a large individual that Bennett previously referred to P. longiceps. However, Kellner notes that the crest is inclined further upward than it should be for that species, and that the premaxilla is arranged differently in forming part of the crest. It also appears to have a larger nasoantorbital fenestra, and a lower and larger temporal fenestra, than in G. sternbergi.

How well either of these identifications remains to be seen. I'm more inclined to be convinced by Dawndraco than G. maysei, simply because the very strange bill of the former seems harder to explain by age or gender variation. Kellner has also tended to be the 'leader' of one 'camp' when it comes to pterosaur taxonomy, usually opposed to Dave Unwin -- see their drastically different recent taxonomies of the ornithocheirids, for example. It will be interesting to see not only if these new species are accepted, but by whom.

While on the subject of new pterosaurs, two new species have also just been reported from the mid-Jurassic Tiojishan formation, some with soft tissue: Kunpengopterus sinensis and Darwinopterus linglongtaensis seem to add support for a monophyletic group of Tiaojishan pterosaurs somewhat intermediate between pterodactyloids and "rhamphorhynchoids", the Wukongopteridae. If I have time to read the paper more closely I'll try to follow up with a full post on these guys.

Kellner, A.W.A. (2010. "Comments on the Pteranodontidae (Pterosauria, Pterodactyloidea)
with the description of two new species." Anais da Academia Brasileira de Ciências, 82(4): 1063-1084. doi: 10.1590/S0001-37652010000400025.

Sunday, December 12, 2010

Borsti Lives Up to Its Name

[Above: Photo of the type specimen of Juravenator under UV light. From Chiappe & Göhlich, 2010.]

This month, Luis Chiappe and Ursula Göhlich published the first real English-language follow-up paper on the small German compsognathid Juravenator starki (named for its discovery at Stark Quarry near the Jura mountains, which also gave their name to the Jurassic period). You may remember this critter causing a stir when it was first described in 2006. Juravenator was the first non-avialan theropod found from the limestone deposits of Germany and France (which have also yielded Compsognathus and Archaeopteryx) to preserve clear, fairly extensive impressions of its skin and other soft tissue.

The specimen was discovered in 1998, but as far as I know news of this discovery first hit the Internet back in 2001, by way of a German-language news story reported to the DML. The new fossil was nicknamed "Borsti", from the German borstig, meaning "bristly." As some early-release photos show, at this point much of the fossil was not yet prepped, and the bulk of the skeleton was still encased in rock. I remember getting my hands on some photos from the early 2000s showing only the skull exposed, with no trace of soft tissue. Nevertheless, being a compsognathid, scientists fully expected that in life, this animal would have been covered in short, bristly stage 1 or 2 (in Richard Prum's model of feather evolution) protofeathers like its close relative Sinosauropteryx prima.

By 2006, the whole skeleton had been exposed, along with unexpected soft-tissue traces. While limestones from this area are world famous for their preservation of feathers, this is usually limited to the large, vaned feathers present on the wings and tails of aviremigian birds (those with feathered wings like Archaeopteryx). Small theropods like Compsognathus usually preserve very little, if any, soft tissue traces, and even the down-like or proto-feathery body covering of Archaeopteryx is only very rarely preserved, and then only as the faintest wisps in the rock. For this reason, even though the two known specimens of Compsognathus itself didn't preserve any feathers, it wasn't necessarily scaly all over (only a few hints of possible scales have been noted from the tail of one specimen, and even interpretation of those has been ambiguous).

So, when Göhlich and Chiappe described Juravenator, they may have been surprised to find extensive and well-preserved soft tissue surrounding the tail and part of the legs, showing very clear impressions of small, bumpy scales like those known of more primitive theropods and most sauropod and ornithischian dinosaurs. [See photo at right, from Chiappe & Göhlich 2010]. This caused a bit of a scandal, and rendered the name Borsti ironic: here was a specimen which phylogenetic bracketing methods predicted feathers, but the prediction failed. A number of explanations were offered for this. It could be that our phylogenetic analyses were off: that is, Juravenator was not a compsognathid at all, but something more primitive, having arisen before the origin of feathers. Alternately, since impressions were known only from the tail, it could have been feathered elsewhere on its body.

The first suggestion was complicated by the fact that the relationships of primitive coelurosaurs are notoriously poorly understood (some later analyses even found Juravenator to be more advanced than compsognathids), not to mention that the only known specimen came from a juvenile, so testing its relationships are a tricky proposition to begin with. The second suggestion, that Juravenator was only partly feathered, sounded a bit like special pleading given that there was only absence of evidence to go by.

The first hint that the second explanation may have been correct came in a little-known German-language follow-up paper published by the original authors later in 2006 in the journal Archaeopteryx. Apparently, this paper reported that, on closer examination, very faint, thin impressions of some kind of filament were present on the top edge of the tail. But that was all we had to go on until this month, when the complete osteology of the specimen was published.

Examination of the specimen under UV light (performed by H. Tischlinger, one of the co-authors of the paper in Archaeopteryx) has revealed more soft tissue than reported in the description. Additional impressions of scales can be seen under UV on the snout and lower legs, as well as the visible-light impressions on the tail. This new paper confirms the 2006 reports of proto-feather-like filaments on parts of the tail. As expected for these deposits, the feather remains are very poorly preserved, and only the tips are evident. But their size and arrangement seems to closely match those of Sinosauropteryx [see diagram at left, from Chiappe & Göhlich 2010]. Additionally, these impressions lie above the level of the clear, in-tact scale impressions. Impressions of internal tissues, including what may be collagen, can also be seen under UV below the scales and between the vertebrae. This makes the standard interpretation of the filaments as frayed collagen fibers by the birds-are-not-dinosaurs crowd pretty much impossible.

So what does all this mean? Clearly, the second explanation for the surprisingly "featherless" Juravenator seems to have been correct. As the authors note, it's possible that at this early stage of evolution, feathers and scales co-existed across the body of dinosaurs like Juravenator, and possibly even Sinosauropteryx and Dilong, where no scales are preserved but feathers are still found only in certain parts of the body. So far, this kind of co-existence of widespread scaly skin with fringes of feathers has only been known in the ornithischian Psittacosaurus but, they point out, it's not inconsistent with theoretical models of feather development and evolution. [Accompanying image: my life restoration of Juravenator starki from 2006. I'll have to update this to reflect the position of filaments on the tail.]

Of course, it is also possible that explanation #1 is also correct. The authors noted that compsognathids have sometimes been found to be an evolutionary grade, not a natural grouping. In studies which have found a natural, monophyletic Compsognathidae, only a few species (usually of Compsognathus, Sinosauropteryx and Huaxiagnathus) have been included in the analysis. More testing, with more included taxa, are needed to suss out where on the dinosaur family tree other supposed compsognathids belong.

Goehlich, U.B., Tischlinger, H., and Chiappe, L.M. (2006). "Juravenator starki (Reptilia, Theropoda) ein nuer Raubdinosaurier aus dem Oberjura der Suedlichen Frankenalb (Sueddeutschland): Skelettanatomie und Wiechteilbefunde." Archaeopteryx, 24: 1-26.

Chiappe, L.M. and Göhlich, U.B. (2010). "Anatomy of Juravenator starki (Theropoda: Coelurosauria) from the Late Jurassic of Germany.Neues Jahrbuch für Geologie und Paläontologie - Abhandlungen, 258(3): 257-296.