Adorable Science

Scientists recently announced the discovery of one of the smallest pterosaurs of all. Pterosaurs were hairy, skin-winged, flying creatures commonly, and somewhat incorrectly, referred to as "pterodactyls". This adorable little guy has been named Nemicolopterus crypticus, meaning "hidden, forest-dwelling wing".

Painted restoration by Chuang Zhao. Taken from the New Scientist article, which has a larger version.
The wingspan is a mere 25 cm. Although the only known specimen is not a fully grown adult, it would not have gotten too much larger had it lived that long. Smaller pterosaur specimens are known, but they represent less mature individuals (i.e., hatchlings).

As I mentioned, pterosaurs are commonly known as "pterodactyls" outside of the paleontological community. "Ptero-dactyle" is an old term for Pterodactylus, a genus of pterosaur known from the Solnhofen deposits in Germany. That term and other terms like "ornithosaurs" have not been in use in paleontology for a very long time. There is, however, a large group of short-tailed pterosaurs called pterodactyloids; Nemicolopterus is not really a "pterodactyl", but it is a pterodactyloid.

Photo showing just how small this thing was. Taken from the ITN article, which erroneously calls it a "dinosaur".
A far more egregious mistake is being committed by many places reporting this creature, though: calling it a dinosaur. Pterosaurs are not dinosaurs, but are generally considered dinosaur cousins. Most scientists consider them non-dinosaurian stem-avians, although the possibility of a stem-archosaurian position has also been investigated. Unfortunately, primitive pterosauromorphs are not known (with the possible exception of Sharovipteryx mirabilis, a poorly-known Triassic animal seemingly having "leg-wings"), so it is difficult to pin down their exact relationships. But it is quite certain that they were not dinosaurs, that is, there is no way that they were descended from the final common ancestor of Iguanodon bernissartensis and Megalosaurus bucklandii.

pan-Archosauria
|?-Pterosauromorpha (possible position)
`--Archosauria
   |--pan-Crocodylia
   `--pan-Aves
      |--Scleromochlus taylori
      |--Pterosauromorpha (generally favored position)
      |  |?-Sharovipteryx (or in Prolacertiformes)
      |  `--Pterosauria
      `--Dinosauromorpha
         |--Lagerpetonidae
         |  |--Dromomeron romeri
         |  `--Lagerpeton chanarensis
         `--Dinosauriformes
            |--Marasuchus lilloensis
            `--+--Pseudolagosuchus major
               |--Dinosauria (including Aves)
               `--+--Eucoelophysis baldwini
                  `--Silesaurus opolensis

All these nomenclatural goof-ups aside, it's a pretty cool animal, and not just for its size. The toes are curved more than in any other pterosaur, and the authors interpret this as an adaptation to arboreality. I can just picture it hanging like a bat from a Mesozoic tree limb.

Preliminary anatomical reconstruction by John Conway. Click here for the full version.
What's interesting here is that it has arboreal adaptations, but it's not a primitive pterosaur. This same evolutionary pattern is seen in birds (avialans): the earliest members lack arboreal specializations, but some later members developer them. But this pattern is not, so far as we know, seen in bat (chiropteran) evolution. The earliest known stem-bats have arboreal adaptations, while the earliest pterosaurs and avialans do not. Might this reflect a fundamental difference in body plan between pan-mammals (including apo-chiropterans) and sauropsids (including pterosaurs and avialans)?

Two N. crypticus in a tree, by John Conway. Click here for the full version.

Reference:

Wang, X., A. W. A. Kellner, Z. Zhou, and D. A. Campos. (2008 February 12). Discovery of a rare arboreal forest-dwelling flying reptile (Pterosauria, Pterodactyloidea) from China. Proceedings of the National Academy of Sciences 106(6):1983–1987. doi:10.1073/pnas.0707728105

Who Ate What Now?

Most fossils, especially the older ones, are annoyingly incomplete. They'll find a few bones, or some footprints, a smudgy outline, etc. Then there are the more-or-less complete specimens, which are rarer but still not infrequent. And finally there are real "snapshots of life" which come along once in a great while. This latest discovery, for example:

Shark Ate Amphibian Ate Fish: First "Food-Chain Fossil"

Specimens with the remains of ingested animals are sometimes found, but I've never heard of this kind of "Russian doll" fossil before. Truly remarkable.

[image of the fossil, with reconstructions]

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And now that I've finished marveling at it, it's time for me to return to character and bemoan the nomenclatural inaccuracies!

First, what are the actual animals involved? It's an Acanthodes bronni, eaten by a temnospondyl, eaten by a Triodus sessili. Is the headline accurate? Let's establish a few ground rules first.

Most taxa (at least of those that have been around a while) are based on living organisms. When related fossil organisms are found, there is often some dispute as to whether to include them or not. For example, when Archaeopteryx was discovered, people couldn't decide if it was a bird or not. It had feathered wings and bird-like feet, but also had teeth and a long tail. Did it fly? Who knows.

In the past, some people have recommended letting taxa extend as far as possible, encompassing what is known as the total group. A total group includes everything sharing closer ancestry with the living members of a group than with any other living organisms. This approach can run into some serious problems, though. For example, it would make all dinosaurs birds. More generally, it lumps the earliest, barely differentiated members of a lineage in with their derived, living descendants. This encourages the use of unjustified inferences. For example, I know that all living birds have tertial feathers (long feathers along the upper arm), so, since Archaeopteryx is a bird, it should have them, too, right? WRONG. Analysis of the available fossils has yet to indicate the presence of any tertials.

This is why many others encourage the use of crown groups for common taxonomic names. A crown group is the final common ancestor of certain living organisms, and all descendants of that ancestor. Using a crown group definition for Aves, for example, would limit it to the clade of modern birds, thereby excluding Archaeopteryx. This practice has the effect of discouraging unjustified inferences.

One other term to note here is stem group. This is simply a total group minus its included crown group. For example, a stem-avian is anything sharing closer ancestry with birds than with other living organisms (e.g., crocodylians), but outside the crown group Aves. Archaeopteryx and all the classic dinosaurs are stem-avians, as are (probably) pterosaurs.

Archosauria
|-Pan-Crocodylia (including Crocodylia)
`-Pan-Aves
  |-Pterosauria *
  `-+-Marasuchus *
    `-+-Silesaurus *
      `-Dinosauria *
        |-Ornithischia *
        `-+-Herrerasauridae *
          `-+-Sauropodomorpha *
            `-+-Eoraptor *
              `-+-Coelophysoidea *
                `-+-Dilophosauridae *
                  `-+-Ceratosauria *
                    `-Tetanurae

Tetanurae
|-Spinosauroidea *
`-+-Carnosauria *
 `-+-Compsognathidae *
   |-Tyrannosauroidea *
   `-+-Ornithomimosauria *
     `-+-Oviraptoriformes *
       `-+-Deinonychosauria *
         `-+-Archaeopterygidae *
           `-+-Confuciusornithidae *
             `-+-Enantiornithes *
               `-+-Hesperornithes *
                 `-+-Ichthyornithes *
                   `-Aves

* extinct stem-avian taxon

Cladograms (somewhat abridged) showing the avian stem group.

Is Acanthodes a fish? Well, yes, they got that right. "Fish" is a really, really broad category, tantamount to "any craniate that isn't a tetrapod" (more or less). Specifically it is an acanthodian, which makes it a stem-osteichthyan (Osteichthyes being the crown group that includes bony vertebrates, e.g., ray-finned fishes, lungfishes, coelacanths, and tetrapods).

Are temnospondyls amphibians? O.K., this is harder. Traditionally, Amphibia was used as a "wastebasket taxon" for any tetrapod that was not an amniote. More recently, it has been limited to one of two groups: the total group including frogs, salamanders, and caecilians; or the crown group including frogs, salamanders, and caecilians. There is actually quite a bit of debate on this matter and I don't expect it to be resolved any time soon. According to the crown group usage, temnospondyls are definitely not amphibians. But according to the total group usage, they might be. (But they might also be stem-tetrapods or stem-amniotes.)

Is Triodus a shark? "Shark" is a really abused term in paleontology. (In fact, acanthodians are sometimes called "spiny sharks" even though they're not even chondrichthyans.) Among modern animals, the term is generally limited to Selachii, the flatter members of which are called skates or rays. Triodus is a stem-selachian, closer to sharks, rays, and skates than to ratfishes (Holocephali). So is it a shark? If we want to limit unjustified inferences, then we should say no.

Gnathostomata (jawed vertebrates)
|--Chondrichthyes (cartilaginous fishes)
|  |--Holocephali (ratfishes)
|  `--+--Triodus *
|     `--Selachii (sharks, rays, skates)
`--+--Acanthodes *
   `--Osteichthyes (bony vertebrates)
      |--Actinopterygii (ray-finned fishes)
      `--Sarcopterygii (flesh-finned vertebrates)
         |--Dipnoi (lungfishes)
         |--Latimeria (Recent coelacanths)
         `--Apo-Tetrapoda (limbed vertebrates)
            |?-Temnospondyli *
            `--Tetrapoda
               |--+?-Temnospondyli *
               |  `--Amniota
               `--Amphibia sensu lato
                  |?-Temnospondyli *
                  `--Amphibia sensu stricto

* extinct

Cladogram of the taxa in question, with related extant taxa.

Okay, smarty, how would YOU dumb this down? Argh, that's pretty hard. Personally I think the lay public might be ready for the term "stem-", but I seem to have a bad habit of overestimating the lay public. Anyway, the best I can come up with is "Stem-Shark Ate Limbed Creature Ate Fish." Doesn't roll off the tongue, you say? Well, I guess this is why I'm not a journalist....

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(References to be added later, if I have time.)