You're Doing It Wrong: Dinosaur Tails

A new post by W. Scott Persons over at the Art Evolved blog is an excellent overview of why most artists, even the pros, have been getting dinosaur tails completely wrong since the Dinosaur Renaissance. It makes me feel a little better to know I was in the company of Mark Hallett and other titans of paleoart when I'd simply make up cool-looking tail musculature in my older drawings with no regard for anatomy...

You can read the post here.

Heat, Feathers, and Half-Arsed Velociraptor

Above: Comparison of common Velociraptor reconstructions by artist Tomozaurus, used with permission.

A month or so ago, this diagram (also shown above) produced by an artist known as Tomozaurus stirred up a good deal of debate over at DinoForum. Tom was trying to illustrate common misconceptions about the most likely life appearance of dromaeosaurids. By now, everyone (including the birds-are-not-dinosaurs crowd) agree that dromaeosaurids were fully feathered and pretty bird-like. Whether or not to consider them actual birds is a matter of semantics at this point. But, among dino-fans weaned on their depictions in pop-culture, there seem to be a lot of heated reactions to depicting them as too bird-like. Many will admit that they had feathers, but stop short at reconstructing them in a really bird-like manner, preferring a short, cat-like pelt that allows the graceful and well-known contours of the skeleton to show through in life. But compare any bird skeleton to a live specimen, even those with "simple" feathers like chicks or kiwi, and you'll immediately see that this is the wrong way to go. Even most feathered theropod fossils show a feather covering that does not hug the body contours, but like modern birds, consists of a lot of poofy, long feathers (especially at the breast and neck) that would make them look very "bulky."

The debate about the above image concerned how well these principles should apply to larger dino-birds like Velociraptor. Velociraptor is perhaps the worst offender in this area due to its enormous popularity: it has a very well-ingrained image in the popular consciousness that, in all likelihood, doesn't match how it would have appeared in life. But just how likely is any reconstruction?

The fact is, we don't know very much at all about the feathering in Velociraptor. At least one specimen preserves quill knobs on the ulnae where large secondary feathers must have attached, so we know for a fact that it had wings. But what about the body feathers? Tom's picture supposes that we would do best to reconstruct the remaining feathers based on related (but much smaller) species in which the full compliment of feathers has been preserved; things like Microraptor, Anchironis, Archaeopteryx, and the unnamed species nicknamed "Dave". Based on these, Tom gave his "correct" Velociraptor a full set of long primary feathers (present in Micro and Archie but much shorter in Dave and Anchi), a feather crown (present in Micro and Anchi but not Dave and Archie), flight feathers on the hind legs (present in Micro, Anchi and Dave but not Archie, though the last does have "bell bottoms" of long, non-planar pennaceous feathers down to the ankles), and feathers covering the face and most of the snout (present in all but possibly Archie). Last, he restored the body contour feathers as pennaceous rather than downy. This is the most problematic, present (apparently) in Anchiornis but not in the others (Archaeopteryx has pennaceous feathers over the hips but this appears to be a single or paired feather tracts (pterylae), and the rest of the body is covered in plumulaceous feathers or simple protofeathers).

In contrast, Tom presents two "incorrect" versions: the Greyhound/Lizard (which is so obviously wrong there's not much else to say about it) and the "Half-Arse." The later is interesting because while it deviates considerably from the inferences drawn based on its relatives, there's nothing obviously inaccurate about it, or at least implausible. The drawing seems to be identical to the Velociraptor computer models used in the TV show Dinosaur Planet: feathered, but barely so, with a short, mammal-like pelt hugging the contours of the body, a large crown for display, and many featherless areas of the body.

Defenders (or at least devils advocates) for this reconstruction pointed out that it doesn't deviate too wildly from larger, flightless modern birds. Ostriches (Struthio camelus) are famously stripped-down of feathers, more so than many people think (see image above, showing the naked underside of the wings and featherless torso). However, ostirches are also quite a bit larger than Velociraptor, and smaller ground birds from the same environment like the Secretary Bird (Sagittarius sepentarius) lack extensive naked patches. On the other hand, Rhea (Rhea spp.), which are similar to Velociraptor in weight, do have naked patches on their underwings, though not as extensive as ostirches (see image below).

So, while I still think the rather short body feathers would be a little far-fetched (longer ones would be better for thermoregulation--if overheating is the issue, the feathers would probably simply be lost entirely, which is not the case in modern birds that lose feathers only on extremities), the Half-Arse version doesn't seem all that bad.

But there's more that needs to be taken into account. A great post at Tetrapod Zoology today touches on some aspects of featherless patches on birds and its effects on thermoregulation. A bit counter-intuitively, a heavy feather coat can actually help keep birds cool--they're general insulators, not simply heat-trappers, like a thermos. In the post, Darren Naish discusses male wild turkeys, which famously have bare heads with brightly colored, outlandish soft tissue structures for display. This actually puts the male birds at a disadvantage, because all that bare skin causes the males to be more prone to overheating, requiring them to spend more time cooling in the shade than females. Like the tail of a peafowl, the naked heads of turkeys are a sexual display structure that puts the birds at a disadvantage when it comes to survival. Granted, this is only the example of one bird. I don't know how this might apply to, say, ostriches. Ostriches, like Velociraptor, inhabit a hot, arid environment. Indications suggest that Velociraptor lived in an even more desert-like setting, dominated by barren dune fields, making it a solidly desert bird. Would the extensive bare patches of the Half-Arse be beneficial in this setting, or would they tend to drastically overheat the animal under an unrelenting desert sun with few sources of shade?

Above: Photo of a Greater Rhea (Rhea americana) by Ramon Moller Jansen. This species is about the same size as an adult Velociraptor mongoliensis, but live in more lush environments with some tall shade plants.

According to Willmer, Stone & Johnson (2000), many ratites use similar strategies to regulate their body temperature and prevent overheating. Ratites such as the Rhea store their heat while active, and only actively attempt to shed it while at rest. This is achieved through strategies such as panting, drooping the wings (allowing air to conduct heat from the sparsely feathered underwings while at the same time shading them), and raising or lowering the sparse feathers of the back (known as ptilo-erection). It's interesting to note that in Archaeopteryx, the back is the only region of the body that has pennaceous feathers, better for trapping or shedding heat. With all of these adaptations, ostriches rarely have to seek shade, even when it's available (Levy et al., 1990). So while feather-loss in modern birds is an important strategy for those living in hot, arid climates, note that the pattern of loss is not random or extensive, but rather strategic, allowing for maximum thermal regulation.

If I were to speculate, I'd make an educated guess that bare patches in Velociraptor must have been limited to the legs, flanks and underwings for these reasons. Those areas could be shaded simply by the animals own body and cooled easily as needed. The head and neck would have been most prone to overheating. The animal could only shade these by adapting a Mei long style sheltered posture with the head tucked under the wing and body feathers (but only if a hefty body covering was present, not a form-fitting pelt). One question I can't seem to find an answer to is how ostriches cope with direct sun to their nearly bare heads and necks--the problem faced by male turkeys. My wild guess would be that the heads of turkeys are highly decorated with thickened skin, presumably a better insulator than the plain skin of an ostrich, which would allow more loss of heat to the air. Or maybe they bury their heads in the sand to keep cool... (I kid, I kid!). If anybody knows of any studies or physiological issues that would cause this to be a problem for one and not the other, please leave a comment!

References:
* Levy, A., Perelman, B., Grevenbroek, M.V., Creveld, C.V., Agbaria, R. and Yagil, R. (1990). "Effect of water restriction on renal function in ostriches (Struthio camelus)." Avian Pathology, 19: 385-393.
* Willmer, P., Stone, G. and Johnston, I.A. (2000). Environmental Physiology of Animals. Wiley-Blackwell, Science. 644 pp.

Scientific Anachronism and why Biostratigraphy Matters

Image: Stegosaurus confronts Tyrannosaurus, from Walt Disney's Fantasia. Such anachronistic views of paleontology could never form the basis of peer-reviewed literature, could they?

A new study (Carbone, Turvey &Bielby, 2011) suggests T. rex could not have been a pure scavenger.

Yeah, I know. This is already the universally accepted position of modern paleontologists. Of course T. rex scavenged if it could, but there is ample reason to think (and ample fossil support backing this up) that it hunted as well. Even the originator of the scavenging thoery, Jack Horner, has basically admitted that he only came up with it as a way to get young people to think critically about their own preconceptions (obviously, he has never met any young paleontology fans. I'd have given up that strategy after I witnessed my first T. rex vs. Spinosaurus debate).

However, as Denver Fowler pointed out on the DML today, drawing an obvious conclusion is the least of the paper's problems.

I recently participated in at least two paleoart discussion threads in which really awesome artists showed off some mind-blowingly fantastic paintings depicting the Jehol biota. I know first hand that both artists are completely on the ball and know their stuff. But both made common errors in the often neglected field of biostratigraphy.

In one, a Sinornithosaurus watches as two Microraptor glide down from the trees. These are two similar animals from around the same time and place. However, there is no evidence that they ever met. All known fossils of Microraptor come from the Jiufotang formation, dated to 120 million years ago, plus or minus 700k years. The youngest Sinornithosaurus fossils are from the upper Yixian formation, dated to around 122 million years ago. The timespan and environment are grossly similar, but 2 million years is still a long time in a world where most dinosaur species, if not genera, don't span more than a couple million years (and the ones that do are probably egregiously over-lumped, like Iguanodon).

Another painting portrayed a Yixian formation scene with Yixian ornithopods and Yixian insects being fed on by Jeholopterus, a pterosaur which lived in the Daohugou biota, in beds dating to at least 150 million years ago, a full 25 million years before the Yixian faunas existed. The error here was probably based on a confused history of dating the formations (old sources placed the Yixian in the late Jurassic), and many sources, both professional and popular, which tended to conflate the various Chinese feather-preserving formations into one amorphous pseudo-fauna.

Artistic depictions throwing together prehistoric animals from disparate times are obviously nothing new. Walt Disney Pictures has done this at least twice, first and most famously in Fantasia (Stegosaurus meets T. rex meets Pteranodon), and later and more flagrantly in Dinosaur (I can't think of any two animals in that movie that were actually contemporaries, and many didn't even live together on the same continent).

This is somewhat excusable when it's done for the sake of art (as long as that art isn't passed off as being scientifically rigorous). But this kind of disregard for, or generalization of, biostratigraphy can creep into science and completely foul up your results.

In Carbone et al. 2011, the authors attempt to calculate the amount of potential carcasses that would have been available to scavenging Tyrannosaurus rex in its environment to make their case. Their lists of T. rex contemporaries are reproduced in part below:

Species and body masses of carnivorous non-avian theropod dinosaurs of Late Cretaceous North America
Dromaeosaurus albertensis
Richardoestesia gilmorei
Richardoestesia isosceles
Saurornitholestes
Velociraptor sp.
Troodon formosus
Chirostenotes elegans
Chirostenotes pergracilis
Nanotyrannus lancensis
Albertosaurus sarcophagus
Tyrannosaurus rex

Species and body masses of herbivorous dinosaurs of Late Cretaceous North America
Parksosaurus warreni
Prenocephale edmontonensis
Ornithomimus velox
Struthiomimus sp.
Thescelosaurus garbanii
Thescelosaurus neglectus
Leptoceratops gracilis
Montanoceratops sp.
Pachycephalosaurus wyomingensis
Edmontosaurus annectens
Edmontosaurus regalis
Edmontosaurus saskatchewanensis
Lambeosaurus sp.
Parasaurolophus walkeri
Edmontonia rugosidens
Ankylosaurus magniventris
Triceratops horridus
Alamosaurus sanjuanensi

The authors state, "our species list is treated as representing a consistent sympatric faunal unit across this region for the purposes of analysis." But they absolutely don't represent that.

If you have even a little bit of a handle on Late Cretaceous biostratigraphy, or the paleoecology of T. rex, you may notice a few problems with these lists. Namely, the fact that they are complete messes, incorporating erroneous or outdated taxonomic assignments or over-generalizations of the geologic column.

This kind of data crunching would require taxa to be broken down on an environment-by-environment basis. That is, in order to be meaningful, all included taxa have to be demonstrated to be contemporaries. Most of the taxa in those lists were not, or can't be said to have been with any confidence.

To be fair, some of the mistakes are due to very new research, some of which has only appeared in abstracts or mentioned briefly in papers. For example, while Edmontosaurus regalis is widely reported from the late Maastrichtian Hell Creek and Lance formations, this is mainly by default, skeletons that are not identifiable to the species level. Ongoing stratigraphic and taxonomic work by Nicolas Campione has shown that E. regalis was actually not a contemporary of E. annectens, and specimens assignable to that species are only known from lower strata. The validity of E. saskatchewanensis, which is from the same stratographic level as T. rex, is pretty dubious. E. annectens is its likely synonym.

Parasaurolophus is known exclusively from the Campanian-age Dinosaur Park Formation, over five million years before the earliest known T. rex fossils. Same goes for Lambeosaurus. While the former was tentatively identified in the Hell Creek by Sullivan & Williamson (1999), this was based on very fragmentary remains that almost certainly belong to Edmontosaurus instead. Montanaceratops is from the St. Mary River Formation, dated to the early Maastrictian and also pre-dating T. rex.

Some cases are even more nuanced. Alamosaurus did coexist with T. rex, but not with many of the other listed species. Current indications are that the southern part of North America during the late Maastrichtian supported a different fauna from the north, comprised many of species which are related to, but distinct from, their northern counterparts. Alamosaurs, for example, did not coexist with Triceratops, but Ojoceratops (assuming they're distinct). It didn't coexist with Torosaurus latus (which the authors apparently lump with Triceratops), but with "Torosaurus" utahensis. Indications are that these beds are a bit earlier than the late Maastrichtian as well, so while Alamosaurus lived alongside Edmontosaurus, it was E. regalis rather than E. annectens.

The carnivores don't fare much better. Most are tooth taxa, like Troodon (another Dinosaur Park critter from the Campanian). While "Troodon" teeth are known from the same beds as T. rex, they're almost certainly not Troodon formosus. The same goes for Dromaeosaurus. However, these are taxonomic issues, not biostratigraphic ones, and don't really affect species count--whatever we name them, there were at least one troodontid and at least two dromaeosaurids present (though the authors erroneously list both Velociraptor and Saurornitholestes, based on the same specimens, first referred to the former and then the later, both incorreclty). Not so for the inexplicable inclusion of Albertosaurus. I can figure out where these other misplaced species came from, but I don't know of any albertosaur remains having been reported from Lancian-age deposits. Anybody? Either way, it's almost certainly an error (as is making Nanotyrannus a distinct taxon, but that's another story).

So you can see why failing to understand which dinosaurs lived together, specifically, can have major implications for actual science. This kind of paper also illustrates why it's a bad idea to keep non-diagnostic genera around as nomina dubia and not sink them into their better known, probably-synonymous counterparts or simply designate neotypes from the good material. These authors avoided pitfalls like including Thescelosaurus infernalis (=T. sp.), Manospondylus gigas (=Tyrannosaurus rex), Aublysodon molnari (=Tyrannosaurus rex), Thespesius occidentalis (=Edmontosaurus annectens), Trachodon mirabilis (=Edmontosaurus annectens), or Agathaumas sylvestris (=Triceratops horridus), but those taxa aren't doing science any favors by cluttering the playing field.

Here's my preliminary attempt to clean up their faunal lists, based on a Lancian-age, northern ecosystem: (updated thanks to additional information provided by Mickey Mortimer in the comments. note that I'm following the authors in not including avialans)
Dromaeosaurinae sp.
Zapsalis abradens
Richardoestesia gilmorei
Richardoestesia isosceles
Troodontidae indet. spp. (multiple species)

Pectinodon bakkeri

Paronychodon sp.

Avimimidae sp.
Chirostenotes elegans

Chirostenotes? sp.
Tyrannosaurus rex (=Manospondylus gigas)
Struthiomimus sedens

Ornithomimus velox

Ornithomimidae sp. (="Orcomimus")

Dromeiceiomimus sp.

Alvarezsauridae sp.

Therizinosauridae sp.
Thescelosaurus garbanii
Thescelosaurus neglectus
Leptoceratops gracilis
Pachycephalosaurus wyomingensis
Edmontosaurus annectens (=Thespesius occidentalis)
Edmontonia schlessmani (=Denversaurus schlessmani)
Ankylosaurus magniventris
Torosaurus latus?
Triceratops horridus (=Agathaumas sylvestrus?)


References:
* Campione, N.E. (2009). "Cranial variation in Edmontosaurus (Hadrosauridae) from the Late Cretaceous of North America." North American Paleontological Convention (NAPC 2009): Abstracts, p. 95a.

Brilliant New Anchiornis, the Bone Wars, and More

A few quick news items while I finish up a post exploring the extent and structure of beaks in theropods...

"Dinosaur Wars" on PBS's American Experience
I haven't had a chance to check out this new PBS special on the Bone Wars (darn you, unreliable internet connection!) but the whole thing can be seen here.

"Dinomorphosis" on National Geographic's Naked Science
Airing next week, this special explores current knowledge of feathered dinosaurs. You can see the trailer here (not bad, aside from the rampant bunny hands and baffling statement that all feathered dinosaurs were carnivorous), and a related article from the magazine with photo gallery.

Be sure to check out the image of an undescribed Anchiornis specimen in the photo gallery. The preservation is utterly phenomenal, and even the previously-described color pattern is visible to the naked eye.

Also exciting is a new paper on sexual dimorphism and reproduction in the pterosaur Darwinopterus. Check out the great article summarizing the findings at New Scientist, and this skimpier Discovery article highlighting an awesome new restoration of male and female specimens by Mark Witton.

Theropods That Fit the Bill

[Image: Skull of a gull compared to a living specimen. The apparent differences in the extent of the beak are caused by the presence of different kinds of rhamphotheca.]

One aspect of Mesozoic birds and other theropods that doesn't get discussed much are their bills. Beaks or beak-like structures (generally, rhamphotheca) have a wide but extremely spotty distribution among Mesozoic theropods, and numerous theropod groups seem to have evolved them independently of one another.

Perhaps the most basal known theropod that likely bore a beak (assuming we can safely exclude the skull material attributed to the infamous "first bird" Protoavis) is Limusaurus inextricabilis from the Oxfordian, about 160 million years ago. Though it belonged to the otherwise toothy clade Ceratosauria, Limusaurus was completely toothless. To be precise, most theropods have teeth in the three major sections of their jaws: the premaxilla at the front of the skull, the maxilla behind it, and the dentary, or lower jaw. Ornithischian dinosaurs had an additional bone, the predentary, at the tip of the lower jaw that was always toothless, indicating that almost all ornithischians probably bore a beak, at least on the lower jaw (and ceratopsians added an additional, pre-premaxilla: the rostral). Limusaurus lacked teeth in the premaxilla, maxilla, and dentary, so it was completely toothless and, therefore, it is assumed to have been beaked.

[Image: Skull reconstruction of Limusaurus, posted by Dave Hone at his Archosaur Musings blog.]

As far as I know, Limusaurus is the only non-coelurosaurian theropod that bore a beak. Generally, this fits well with the idea that non-coelurosaurian theropods were hypercarnivorous flesh eaters with little variation in their diets. However, as a number of recent papers by Lindsay Zanno and colleagues has shown, coelurosaurs started playing around with omnivory and herbivory, and as a consequence, many lineages cast off some or all the sharp teeth that characterized their ancestors.

[Image: Skull reconstruction of Pelecanimimus by Ville Sinkkonen]

The first coelurosaur lineage to start shedding their teeth permanently were the ornithomimosaurs. This group is well-known for their toothlessness, and evidence from specimens with preserved keratin sheaths on the jaws (Norell et al, 2001) show definitively that they bore keratinous beaks on both the upper and lower jaws, at least in advanced species. As expected, the most primitive ornithomimosaurs still retained teeth. Pelecanimimus polydon, as its name implies, had teeth in the premaxilla, maxilla and dentary. The next most advanced ornithomimosaur, Harpymimus, is the first example among theropods of the "half beak." While its upper jaw was toothless and likely beaked, it retained teeth in the lower jaw. I'll come back to the implications of these "half beaks" further down. The most basal ornithomimosaur wich lacked teeth completely seems to be Garudimimus, though the status of Beishanlong is unknown.

After the ornithomimosaurs, beaks are found in therizinosaurs. Again, early forms like Falcarius had a full compliment of teeth, but in laker ones like Beipiaosaurus, the teeth became restricted to the middle of the jaws, leaving room for a beak in the front, similar to ornithischians. Next, oviraptorosaurs seem to have developed beaks fairly early in their evolution. While Incisivosaurus had teeth in both jaws (though restricted to the tips), Caudipteryx had only a few teeth in the premaxilla, another example of a "half beak" where a toothless lower jaw is matched with a toothy upper jaw. While the several possible intermediate forms lack skulls, both caenagnathids and oviraptorids proper had fully toothless jaws, so presumably they were fully beaked by the time they diverged.

Next up were the deinonychosaurs, and while some seem to have dabbled in omnivory, they were mainly carnivorous, and none seem to have developed beaks. Greg Paul is infamous for restoring them with cornified tissue or "proto beaks" at the tips of their jaws adjacent to teeth, but there isn't much evidence to support this idea as far as I know. However, many deinonychosaurs which preserve feathers show a (usually small) portion near the tip of the snout that is unfeathered. This featherless snout tip is also seen in some toothed, presumably beakless enantiornithines. It's possible this could be evidence of "rhamphotheca" in its loosest sense--the very lightly cornifies, flexible bill skin found toward the back of the beaks in some modern birds, where the horn-like, kerationous portion thins out into normal skin. More on these flexi-bills down the page.

[Image: My own simplified cladogram of Aviremigia showing some notable occurrences of beaked and half-beaked and beak-free birds. Excuse the quality in close-up, some of these were yoinked from works in progress.]

Around the base of Avialae, the occurrence of half-beaks seems to explode. Almost all known basal, non-pygostylian avialans lack teeth in the upper jaw (e.g. Jeholornis, Yandangornis) or lower jaw (omnivoropterygids) and, possibly, both (Jixiangornis preserves no teeth and may have been full-beaked, but the skull is badly crushed, and crushing has been known to obscure the tiny teeth that should be present in other basal avialan specimens). This trend appears to culminate with the confuciusornithids, which are not only toothless but have sharply pointed jaws that, in some very rare specimens, preserve the actual keratin of the beak. These impressions show that even in these fully beaked birds, the rhamphotheca was thin and delicate and probably not as heavily mineralized as in modern birds.

The next bit is rather odd given the trends seen at the base of avialae. In most basal ornithothoraces (the group that includes enantiornithines and ornithuromorphs), the jaws are fully toothed, with no evidence for beaks. It's tempting to think that this could unite the enantiornithines with the toothy, beakless deinonychosaurs and Archaeopteryx in a "Sauriurae" to the exclusion of the beaked birds. However, given the numerous times beaks have evolved independently in vertebrates, it's not unthinkable that each of the half-beak examples above arose independently of one another (or, that some reversal occurred at the base of ornithothoraces to return birds to a state of fully-toothed maws). Either way, while many enantiornithines preserve jaw material, only a few exhibit the kind of toothlessness at the front of the jaws that could imply a beak. These include the half-beaked Alethoalaornis (with a toothless dentary but toothed premaxilla), the fully-beaked Gobipteryx, and the half-beaked Boluochia. Boluochia had teeth in the dentary but none in the upper jaw and, in fat, had a strongly hooked premaxilla similar to the hooked beaks of modern raptors. Enantiornithine relationships are too unresolved to be able to tell if these beaked birds all form a natural group, or if beaks evolved multiple times among enantiornithes, but given their diversity it's a safe bet that many different lineages came up with their own particular feeding solutions.

[Image: The compound rhamphotheca of an albatross is visible as distinct sutures, and is also found in some non-avian birds. Image from the RSPB.]

As expected based on the enantiornithine record, most primitive ornithuromorphs (the fan-tailed birds, including Aves) either lakced beaks or exhibited a half-beaked configuration. While Hongshanornis was originally reported to have a beak, Jingmai O'Connor and colleagues later showed that it had tooth sockets preserved in the upper and possibly lower jaws (same goes for the related Longicrusavis). The earliest fully-beaked ornithuromorph is also one of the most primitive, however: Archaeorhynchus lacked teeth and had a flattened, Spoonbill-like beak. The songlingornithids (such as Yixianornis), and the later hesperornithines and Ichthyornis all had a therizinosaur-like configuration, with toothless premaxilla (and even predentary-like bones in the hesperornithines) with toothy maxilla and dentaries. Evidence from bone texture shows that likely had keratinous beaks at the tips of their jaws, and either feathery toothed jaws or pliable, skin-like rhamphotheca further back. Since both major lineages of avians lack teeth, it's likely their common ancestor was also fully beaked, so teeth must have been lost for good in the bird lineage shortly after Ichthyornis diverged. Interestingly, studies of ichthyornithines and hesperornithine bone structure shows that they likely had "compound rhamphotheca", and this may have been the ancestral condition for modern birds (Heironymous & Witmer, 2010). While the quintessential bird beak is made up of a single keratin sheet covering the jaw, in species with compound beaks, the keratin is arranged in discrete plates on the jaws. This can best be seen in some modern seabirds like the Albatross.

Unfortunately, in interesting taxa like Hollanda, Gansus, and Patagopteryx, the condition is unknown. However, we can use parsimony and phylogenetic bracketing to try and come to a reasonable guess. Most studies find these three to be ornithuromorphs more primitive than the hesperornithines and Ichthyornis. Given the condition in Ichthyornis, it's most likely that Archaeorhynchus evolved its toothless beak independently of more advanced birds, most of which have toothed upper and lower jaws with beaked tips in the grade between Archaeorhynchus and Aves. While the three intermediate birds may well have lost some or all of their teeth independently again, it's probably more parsimonious to assume that like hesperornithids, hongshanornithids and songlingornithids, they were either fully toothed or had beaks restricted to the tips of the jaws.

[Image: Cassowary chick by Michael Thirnbeck, from Flickr. Note the horny beak only covers the tip of the bill, leaving the nostril surrounded by more skin-like tissue. Thanks to meidamon over at DinoForum for finding this and other pictures illustrating beak anatomy.]

So, to rephrase an old fallacy, what use is half a beak? Many Mesozoic coelurosaurs had teeth only on the upper or lower jaw, and most researchers have assumed that edentulous = beaked (see for example Zanno et al., 2009). But "beaks" are more complex than simple sheathes of keratin covering toothless parts of the jaw. As anatomist Larry Witmer has explained (in print as well as on the DML), rhamphotheca can consist of everything from solid, hard keratin (which, obviously, is a bit antithetical to tooth growth and replacement) to softer, pliable, barely mineralized tissue. For example, it's a general rule of thumb that the keratin rhamphotheca never completely encloses the external nares, meaning the nostrils of a bird are never encased in keratin. This is most obvious in species that have a cere, a prominent fleshy part of the posterior rhamphotheca found in pigeons, parrots and hawks, among other birds. But in many other birds (especially aquatic forms like ducks and gulls), the nostrils do appear to erupt from mid-bill. What's going on?

[Image: Duck skull by ~Oryx gazella~, from Flickr. Note that only the tip is fully keratinized.]

Basically, the "beak" of a duck (and many other birds) is only really solid towards the tip. The broad bill of a duck is, laterally, fairly soft, and made of rhamphotheca that is only lightly mineralized. Essentially, it's mostly toughened skin (this is why ducks and other anseriforms are particularly prone to bill injuries). In the kiwi, the nostrils don't just erupt from the "beak", they occur at the very tip. Again, the bill of a kiwi is not highly keratinized, but rather made up of soft, pliable integument full of sensory organs (Martin et al., 2007).

So, our mysterious half-beaked birds may have had some cere-like tissue covering the upper portion of the toothed bill, while the lower portion contained more keratinized rhamphotheca. Perhaps the upper jaw had a small keratinized tip wedged between the front teeth, as seen in toothed pterosaurs like Rhamphorhynchus and Pterodactylus. Or maybe the half-beaks of some of these birds contained little to no keratin at all. As the kiwi shows, it's not always necessary for all food-aquisition behaviors to have a beak or teeth.

Hopefully, this rough guide can also serve as a reference for artists, many of whom are a bit over-eager to add a beak to anything that is called a "bird." While a wide range of rhamphotheca like structures may have been present on Mesozoic birds, it wasn't necessarily present on all of them, and many probably looked a bit strange compared to the simple, smooth yellow bills of modern birds.

References
* Heironymous, L. and Witmer, L.M. (2010). "Homology and Evolution of Avian Compound Rhamphothecae." The Auk, 127(3): 590-604. doi: 10.1525/auk.2010.09122
doi: 10.1525/auk.2010.09122
* Martin, G.R., Wilson, K.-J., Wild, J.M., Parsons, S., Kubke, M.F., et al. (2007). "Kiwi Forego Vision in the Guidance of Their Nocturnal Activities." PLoS ONE, 2(2): e198. doi: 10.1371/journal.pone.0000198
* Zanno, L.E., Gillette, D.D., Albright, L.B., and Titus, A.L. (2009). "A new North American therizinosaurid and the role of herbivory in 'predatory' dinosaur evolution." Proceedings of the Royal Society B, 276(1672): 3505–3511. doi: 10.1098/rspb.2009.1029.

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.

References:
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.

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.

References:

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.