The Most Primitive Stem-Bat

Man, lots of news pertaining to the evolution of flight lately. I saw a presentation on this bit of news at SVP last year—nice to see that it's been published. (Or, uh, will be tomorrow—today it's online early.)

Bats are mysterious creatures—not just in the poetic sense, but in the phylogenetic sense. What are they related to? Where did they come from? For a long time they were considered primate relatives—in fact, Linnaeus included them (as Genus Vespertilio) in his original version of Ordo Primates. In particular, they were often thought to be related to colugos (Cynocephalus), also known as "flying lemurs". Colugos aren't really lemurs, which are true primates, but they are relatives of primates (as are tree shrews, Tupaiidae). Like bats (and several other groups of therian mammal), colugos are arboreal and have large skin membranes used for aerial locomotion. Several characters seemed to unite bats and colugos, or at least put them in the same neighborhood, and this was the status quo for a while.

Then the whole placental tree got shaken up by molecular studies. Soon it became clear that bats were not at all close to colugos or primates, but were instead part of a large radiation with the imaginative name of Pegasoferae (previously mentioned here). Pegasoferae also includes Perissodactyla (horses, rhinos, tapirs), Carnivora (dogs, bears, weasels, seals, cats, hyenas, mongooses, civets, etc.), and Manis (pangolins, a.k.a. "scaly anteaters"—very odd creatures which look like pine cones).

The upshot is that we lost our model (colugos) for what a flightless stem-bat might look like. Now we have nothing (unless some fossil out there has not been recognized as such), so it's hard to tell much about how flight evolved in bats.

Enter a new species from Wyoming. Onychonycteris finneyi is now the most primitive known stem-bat, edging out the previous titleholder, Icaronycteris index. It still has wings, but it also has more primitive characters than any other known stem-bat. As an example, look at bat claws. Modern bats may or may not have a claw on their first manual digit (thumb). I. index has this, as well as a claw on its second manual digit (hence the name—"index finger"). But O. finneyi has claws on all five of its manual digits (hence its name—"onychus" means "claw").

O. finneyi is not a bat ancestor, since it coexisted with more derived stem-bats like I. index. But it does provide some clues as to the ancestral state of flying bats ("apo-chiropterans"). Scans of its ear canals reveal that, unlike modern bats and other known stem-bats, it did not have sonar capabilities. So, in stem-bats, flight evolved before echolocation.

Analysis of its proportions suggests that, like some modern bats, it probably had a flight style where it did not flap continuously, but had a pattern of fluttering up and then gliding down, fluttering up and then gliding down, and so on. (Sort of like a big, leathery butterfly.)

Unlike most modern bats, but like I. index, O. finneyi has a long tail. This evolutionary pattern is also seen in pterosaurs and birds (avialans), where the ancestral trait is a long tail, but the most successful later lineages evolve shortened tails (with occasional reversals).

The work of integrating fossils into the new paradigm of placental evolution is still in its early stages. Hopefully the characters in O. finneyi can help elucidate this problem, just as they have helped to elucidate the origins of bat flight. This fossil is an important clue in The Mystery of the Bat.

References:

• Nishihara, H., M. Hasegawa and N. Okada (2006 June 27). Pegasoferae, an unexpected mammalian clade revealed by tracking ancient retroposon insertions. Proceedings of the National Academy of Sciences 103: 9929–9934. doi:10.1073/pnas.0603797103
  
• Simmons, N. B., K. L. Seymour, J. Habersetzer and G. F. Gunnell (2008 February 14). Primitive Early Eocene bat from Wyoming and the evolution of flight and echolocation. Nature 451:818–821. doi:10.1038/nature06549

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