Looking for the sternal complex in a tiny pterosaur

All pterosaurs have a sternal complex
(sternum + interclavicle + wrap-around clavicles), even the flightless ones. This tiny specimen (Fig. 1) probably had a sternal complex, but where is it? As everyone knows, it should be between the elbows, but it’s not there.

Figure 1. Tiny pterosaur mistakenly named Pterodactylus? pulchellus. I cannot find the sternal complex here. It should be between the elbows. That tiny red triangle under the mid-humerus is the ventral coracoid.

Figure 1. Tiny pterosaur mistakenly named Pterodactylus? pulchellus. I cannot find the sternal complex here. It should be between the elbows. That tiny red triangle under the mid-humerus is the ventral coracoid.

Pterodactylus? pulchellus BM NHM 42735 is the same size as the closely related Gmu-10157 specimen, but has a longer rostrum. The BM NHM specimen is one node closer to the common ancestor of cycnorhamphids + ornithocheirids in the large pterosaur tree (LPT, 242 taxa). The sternal complex appears to be missing or displaced in this otherwise undisturbed tiny specimen. Soft tissue confirms the narrow chord wing membrane and dual uropatagia. Pedal digit 5 remained long.

Figure 2. The GMU 10157 specimen and the P? pulchellus BM NHM 42735 specimens to scale and full size.

Figure 2. The GMU 10157 specimen and the P? pulchellus BM NHM 42735 specimens to scale and full size.

These tiny adults,
(Fig. 2) derived from slightly larger scaphognathids (Fig. 3) are transitional taxa undergoing phylogenetic miniaturization at the genesis of Cycnorhamphidae + Ornithocheiridae. They have not been given novel generic names by established workers because the traditionalists among them consider these to be babies/juveniles of larger, undiscovered taxa. Thus they have remained relatively ignored, despite their pristine preservation and sometimes gravid condition.

Figure 3. Click to enlarge. Taxa in the lineage of Cycnorhamphidae + Ornithocheiridae in the LPT.

Figure 3. Click to enlarge. Taxa in the lineage of Cycnorhamphidae + Ornithocheiridae in the LPT.

The key to finding the missing sternal complex
on this relatively undisturbed specimen is to look to the only area of the skeleton that is slightly disturbed (Fig. 4). The gastralia basket is expanded beyond its natural contours in the BM NHM specimen and that’s where I find (thanks to DGS) a displaced sternal complex, separated from the coracoids and jammed back into the stomach, surrounded by gastralia, almost hidden from view.

Figure 4. Here the sternal complex of the BM NHM 42735 specimen is colored indigo.

Figure 4. Here the sternal complex of the BM NHM 42735 specimen is colored indigo.

Not sure how that happened during taphonomy,
but there you go: mystery solved!

Shenzhoupterus skull in situ with sternum in blue.

Figure 5. Shenzhoupterus skull in situ with sternum in blue.

Earlier a sternal complex was found beneath the skull
of Shenzhoupterus (Figs. 5, 6) using the same techniques, contra traditional reconstructions (Lü, Unwin, Xu and Zhang 2008; see skull diagram insert matching no other pterosaur skull morphology in Fig. 6). Despite its derived state, the newly reconstructed Shenzhoupterus skull (Fig. 6 standing skeleton) bears all the hallmarks of sister taxa.

Shenzhoupterus reconstructed alongside original interpretation of skull.

Figure 6. Shenzhoupterus reconstructed alongside original interpretation of skull.

While we’re on this subject,
Shenzhoupterus does not nest with azhdarchoids, as originally hypothesized, but with tiny Nemicolopterus, between dsungaripterids and tapejarids in the LPT—and neither of these clades are related to azhdarchids in the LPT, contra traditional thinking that excludes tiny taxa and large swathes of congeneric taxa.


References
both of the tiny taxa listed above await description and publication other than in:
Wellnhofer P 1970. Die Pterodactyloidea (Pterosauria) der Oberjura-Plattenkalke Süddeutschlands. Abhandlungen der Bayerischen Akademie der Wissenschaften, N.F., Munich 141: 1-133.

Shenzhoupterus was first described in:
Lü J, Unwin DM, Xu L and Zhang X 2008. A new azhdarchoid pterosaur from the Lower Cretaceous of China and its implications for pterosaur phylogeny and evolution. Naturwissenschaften 95 (9): online (preprint). doi:10.1007/s00114-008-0397-5. PMID 18509616.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.