Longisquama

Skeletal diagram (with smaller soft tissue silhouette)

Researchers Haubold and Buffetaut believed that the structures were long, modified scales attached in pairs to the lateral walls of the body, like paired gliding membranes.[1] They published a reconstruction of Longisquama with plumes in a pattern akin to gliding lizards like Draco species and Kuehneosaurus latus, allowing it to glide, or at least parachute. Though the reconstruction is now thought to have been inaccurate, versions of it are still often portrayed in modern paleoart.

Other researchers place the scales differently. Unwin and Benton interpreted them as a single, unpaired row of modified scales that run along the dorsal midline.[2] Jones et al. interpreted them as two paired rows of structures that are anatomically very much like feathers, and which are in positions like those of birds' spinal feather tracts.[3] Feather-development expert Richard Prum (and also Reisz and Sues) see the structures as anatomically very different from feathers, and thinks they are elongate, ribbonlike scales.[4][5]

Still other observers (e.g. Fraser in 2006) believe that the structures are not part of Longisquama at all, that they are simply plant fronds that were preserved along with the reptile and were misinterpreted.[6] Buchwitz & Voigt (2012) argue that the structures of Longisquama are not plant remains, because all of the structures except for the last in the holotype PIN 2584/4 are arranged regularly, and that they are not preserved as carbon films, the usual mode of preservation for plants in the Madygen Formation.[7] The only plant from Madygen with similarities to the Longisquama structures is Mesenteriophyllum kotschnevii, but its leaves do not have the distinct hockey-stick shape of the structures attributed to Longisquama.[7]

Restoration

Longisquama is characterized by distinctive integumentary structures along its back. The holotype (specimen PIN 2584/4) is the only known fossil preserving these appendages projecting from the back of an associated skeleton. It has seven appendages radiating in a fan-like pattern, but the tips are not preserved. PIN 2584/9 preserves five complete appendages spaced close together. PIN 2584/6 preserves two long, curved appendage running side by side. Other specimens, such as PIN 2585/7 and FG 596/V/1, preserve only one appendage. These structures are long and narrow throughout most of their lengths, and angle backward near the tip to give the appearance of a hockey stick. The proximal straight section is divided into three longitudinal lobes: a smooth lobe on either side and a transversely ridged lobe running between them. The middle ridged lobe is made up of raised "rugae" and deep "interstices", which Sharov compared to rosary beads. The distal section is thought to be an extension of the middle and anterior lobes of the proximal section. While the anterior lobe widens in the distal section, the posterior lobe of the proximal section narrows until it ends at the base of the distal section. In addition, an "anterior flange" appears about two-thirds the way up the proximal section and continues to the tip of the distal section. Both lobes in the distal section are ridged and separated by a grooved axis. In some specimens, the rugae of either lobe in the distal section line up with each other, while in other specimens they do not. Some specimens have straight rugae projecting perpendicular to the axis, while others have rugae that curve in an S-shape. One specimen of Longisquama, PIN 2584/5, has small spines projecting from the axis of the distal section.[7]

The holotype skeleton shows each structure attaching to a vertebral spine. These anchorage points are visible as raised knobs. The base of each appendage is slightly convex, unlike the flattened shape of the rest of the structure. The convex shape may be evidence that the base of each structure was tubular in life, anchoring like other integumentary structures such as mammalian hair or avian feathers into a follicle. Moreover, the proximity of each structure to its corresponding vertebra suggests that a thick layer of soft tissue, possibly including a follicle, surrounded each base.[7]

A consensus of paleontologists agrees that birds evolved from theropod dinosaurs. The scenario for this hypothesis is that early theropod dinosaurs were endothermic, and evolved simple filamentous feathers for insulation. These feathers later increased in size and complexity and then adapted to aerodynamic uses. Ample evidence for this hypothesis has been found in the fossil record, specifically for such dinosaurs as Kulindadromeus, Sinosauropteryx, Caudipteryx, Microraptor and many others. Longisquama is thus regarded as a diapsid with strange scales, ambiguous skeletal features and no real significance to bird evolution.

A minority of scientists prefer the hypothesis that birds evolved from small, arboreal archosaurs like Longisquama. They see these as ectothermic animals that adapted to gliding by developing elongated scales and then pennaceous feathers. This hypothesis, however, is not supported by cladistic analysis.[11]