On Ancient Lizardy Things

For a fossil so controversial, Longisquama insignis is a nondescript little beast. I've actually seen the one collected specimen, or at least a high-grade cast of it, at the ROM a couple of years ago, touring with the Russian dinosaurs exhibit. The entire animal could probably have stood in the palm of my hand.

But Longisquama has assumed a significance all out of proportion to its size due to the role it has played in the debate over bird origins. As you can see, the creature possessed a series of long scale structures growing on its back. Some (Feduccia, among others) have interpreted these structures as being feathers, or at least "proto-feathers", which the animal employed to glide among the Triassic tree-tops, or as a kind of parachute. They have suggested that Longisquama therefore counts as one of the earliest bird ancestors in the fossil record. Thus its existence tells against the dominant theory in paleontological circles: that birds have descended from dinosaurs (are dinosaurs, as a matter of fact).

Now there's a new paper out--"Feather-like development of Triassic diapsid skin appendages"--which may settle the matter once and for all.

And, nope, they're not feathers:

Based on new finds and their comparison with the type material, we show that Longisquama’s appendages consist of a single-branched internal frame enclosed by a flexible outer membrane. Not supporting a categorization either as feathers or as scales, our analysis demonstrates that the Longisquama appendages formed in a two-stage, feather-like developmental process, representing an unusual early example for the evolutionary plasticity of sauropsid integument.

And nope, they're not any kind of flight structure. They are not even paired as in the illustration above:

The theory of an airborne Longisquama using its elongate appendages as gliding devices was introduced for the first time by Sharov (1970) and later adopted and adapted by others Haubold and Buffetaut 1987; Jones et al. 2000; Martin 2004). It is rejected here for two reasons: (1) According to our and previous observations (Sharov 1970; Unwin et al. 2000; Unwin and Benton 2001), the holotype comprises only a single fanned-out row of appendages with no indication for the postmortem bending and loss of a second row, as assumed by Jones et al. (2000). (2) Unlike reptilian gliders, such as Coelurosauravus, Sharovipteryx, kuehneosaurids, and draco lizards, whose wing membranes are spanned close to the trunk and supported by limbs, ribs, or bony spines (Fig. 4a, see Schaller 1985), a gliding Longisquama with two rows of dorsal appendages would possess a continuous airfoil only distally where the expanded portions may partially overlap (Fig. 4b). With the bulk lift created far from the center of mass, the strain on the anchoring would be high, maximizing the risk of structural failure, especially if a flexible joint existed as in the reconstruction of Haubold and Buffetaut (1987). The orderly arrangement of appendages in PIN 2584/9 (Fig. 4c) has been interpreted as a “thoracic wing” by Martin (2004), but preservation effects or an alternative function can explain it as well: If the animal could fan out a single row of appendages by rotation in the sagittal plane (Fig. 4d), it might have used this mechanism only occasionally—e.g., for protection mimicry or for sexual display—most of the time, the appendages might have rested in a horizontal position. As in PIN 2584/9, they would overlap and form a series of successively smaller elements. Erection would have been realized by a system of longitudinal muscles attached to the deeply countersunk follicles.

Oh well. Here's a reconstruction showing the scales in their correct position. They might have been some kind of sexual display structure, which is not nearly as exciting.

The full .pdf is here.