Sciencespeak: Pycnofiber

Pterosaurs were into fuzz before dinosaurs. That’s true in a historical sense, at least.

In 1831, over a decade before the word “dinosaur” had even been coined, the German paleontologist August Goldfuss mentioned that a small pterosaur he had studied seemed to have a coat of mammal like “fur”. This was strange. Even though some naturalists had proposed that pterosaurs were weird proto-bats, the consensus was that they were reptiles. Why should reptiles have a fluffy coat? Perhaps it was another indication, as some paleontologists suspected, that pterosaurs were highly-active animals with mammal-like metabolisms and a need to stay warm.

The fossil Goldfuss described wasn’t an anomaly. As the decades flew by, other researchers found weird fibers on the bodies of pterosaurs. One species, Sordes pilosus, was nicknamed the “hairy devil” because it was so damn fluffy, and now it seems that all pterosaurs wore such structures on their bodes. Even though the prospect of enfluffled Velociraptor or Tyrannosaurus can still send comment threads into chaos, everyone seems cool with the idea that pterosaurs had fine coats of simple fuzz.

In 2009, Alexander Kellner and colleagues finally gave a formal name to the fossilized wisps. What had been referred to as “hair”, “fur”, and “integument” would henceforth be known as pycnofibers. But what was this fluff?

Pycnofibers were short, simple structures. The only internal landmark in these filaments was a canal running up the middle, and, unlike mammalian fur, pycnofibers were not deep-set in the skin. These differences set pterosaur pelts apart from those of other prehistoric creatures, including protofeather-covered dinosaurs. And yet, the relationship between pterosaurs and their dinosaurian cousins may have major implications for when prehistory started to get fuzzy.

Pterosaurs were not dinosaurs. They were a separate lineage of flying creatures. Exactly where they fit in the tree of life is a contentious issue, but pterosaurs are often placed as the closest lineage to the dinosaurs. In this arrangement, both shared a common ancestry and form a group called the Ornithodira. And if this truly was the case, the relationship between pterosaurs and dinosaurs raises the possibility that fluff – or the propensity to evolve it – is a very ancient trait.

At the moment, the pycnofibers of pterosaurs and the simple protofeathers of dinosaurs are treated as structures that evolved independently of each other. Yet paleontologists are still learning more about the occurrence and early evolution of these body coverings. Pycnofibers were a universal pterosaur trait, likely present in the earliest members of the lineage, and there’s a mounting body of evidence that protofeathers may have been present in the earliest dinosaurs. If the establishing members of both closely-related lineages were fuzzy, might it be possible that they inherited this trait from an even earlier ancestor? We need more fossils – particularly from the days of Triassic pterosaurs and dinosaurs – to find out.

There’s another possibility, of course. Pterosaurs and dinosaurs may have evolved their pelage independently of each other. (In dinosaurs, it’s possible that feather- and bristle-like structures evolved multiple times.) Even so, this may be a hint that pterosaurs and dinosaurs inherited some sort of developmental pathway to produce simple, fluffy body coverings – perhaps having to do with the mechanisms that link structures as disparate as bird feathers and alligator scales. Whether pycnofibers turn out to be the same as dinosaur protofeathers or not, they’re a sign that prehistory was fluffier than previously imagined.

References:

Kellner, A., Wang, X., Tischlinger, H., de Almeida Campos, D., Hone, D., Meng, X. 2009. The soft tissue of Jeholopterus (Pterosauria, Anurognathidae, Batrachognathinae) and the structure of the pterosaur wing membrane. Proceedings of the Royal Society B. doi: 10.1098/rspb.2009.0846

Witton, M. 2013. Pterosaurs. Princeton University Press: Princeton. p. 51