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By David Peters
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Rethinking the “fused ribs” of Triassic gliders

Tuesday, November 13, 2012 2:50

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Living from the Permian through the Cretaceous, the so-called “rib” gliders are an interesting lot. And I’m still trying to figure out what’s going on with the ribs and transverse processes. All their closest kin have ribs and none have transverse processes. Here’s the latest (a very minor change of thinking):

Figure 1. Coelurosauravus. It had ribs, but no transverse processes. The extradermal rods were more numerous than the ribs. Click to learn more.

Coelurosauravus is the earliest one (Late Permian). It had no transverse processes. It had ribs. It had extradermal rods likely supporting membranes, and many more rods than ribs anteriorly, but that became a one-to-one relationship posteriorly.

The other rib gliders were distinctly different. Icarosaurus (Fig. 2), Kuehneosaurus (Fig. 3), Mecistotrachelos and Xianglong all had long transverse processes, few to no ribs and the extradermal rods matched one to one with the transverse processes.

Traditional thinking (everyone else) considers the gliding membrane rods to be the ribs, following the pattern of Draco (Fig. 4), the living and genuine rib glider, which likewise has no transverse processes.

Since no other close taxa to the extinct “rib” gliders had transverse processes this led to the heretical idea that the long transverse processes WERE the ribs now fused to the centra and that the gliding spars continued to be extradermal in origin, as in Coelurosauravus (Fig. 1).

Figure 2. Icarosaurus. Note the tiny ribs near the shoulders. Or are those unfused ribs?

Icarosaurus and Kuehneosaurus display mid-change solutions. They have short transverse processes anteriorly and long ribs. They also have long transverse processes starting at the shoulder and tiny to no ribs. Look closely. You’ll see them.

Figure 3. Kuehneosaurus. Note the elongation of the transverse processes replacing the gradually shortening ribs posteriorly.

This is different from the situation in the convergent and largely unrelated living glider, Draco (Fig. 4) in which there are no transverse processes and those membrane spars definitely are the dorsal ribs. The spars on Icarosaurus (Fig. 2) are waaaay too long to be even considered as ribs.

Figure 4. Draco volans in dorsal view based on an X-ray. Click for more info.

Ultimately, I now see the evolution of increasingly longer transverse processes (restricted only to members of this clade) and the reduction of the ribs, not their fusion as I thought before. So, dermal extensions attached directly to transverse processes and the ribs are missing posteriorly.

Not sure how the rest of the gut was supported, or how the lungs expanded without traditional ribs. These oddballs figured out some other way to respire.

The trend was for a shorter body and fewer membrane spars taken to extremes in the most derived of these gliders, Mecistotrachelos and Xianglong, which was originally considered a lizard related to Draco. Here’s the family to scale.