Darwinopterus – Rio Ptero Symposium

The Rio Ptero Symposium (May 2013) produced several interesting abstracts. We’ll look at some of those in the next few days.

An abstract by Unwin and Lü (2013) once again promoted Darwinopterus as the pterosaur transitional taxon between early long-tails and later short-tails. They also continued to promote the unfounded hypothesis of modular evolution, which has not been shown in any other lineage or transitional taxon EVER.

Unfortunately, as we learned earlier, in the large pterosaur tree, Darwinopterus did not nest as a transitional taxon, but as a dead-end taxon, producing no derived descendants (but we’ll review that subject below!). Rather, along with Pterorhynchus, Darwinopterus formed a clade close to Scaphognathus and derived from Jianchangnathus (Fig. 1), derived from basal Dorygnathus specimens. Darwinopterus does not nest near any “pterodactyloid”-grade pterosaurs when you increase the number taxa employed.

The many species within the genus Darwinopterus (Fig. 1) are potentially confusing because they all have a long low skull, closer in proportion to “pterodactyloid”-grade pterosaurs and a long neck. However, these traits are also found in the related Pterorhynchus, Kunpengopterus and Wukongopterus.

Ironically, Kunpengopterus (Fig. 1) does not have a longer skull, but rather a smaller, lower skull. Compare it to the big-headed Scaphognathus or Jianchangnathus. However, the neck is relatively elongated compared to ancestor taxa. And the torso is smaller than the others. So it looks like the skull is bigger.

Darwinopterus had a longer skull on a slightly shorter neck. There were other changes postcranially, but all in line with this particular clade. So nothing special or out of the ordinary here. Certainly nothing presaging “pterodactyloids.”

Figure 1. Click to enlarge. All taxa are to scale with one another. Unwin and Lü note a resemblance between Darwinopterus and Germanodactylus. And that is certainly so, but only by convergence. Phylogenetic analysis indicates a closer relationship between the smaller descendants of Scaphognathus, like no. 12, and Germanodactylus. Arrows indicate phylogenetic order. Here the long neck evolved first with a notably smaller skull. Then the skull became longer and larger in the genus Darwinopterus. Lack of reconstructions by Unwin and Lü may prevent them from seeing the subtleties.

Figure 1. Click to enlarge. All taxa are to scale with one another. Unwin and Lü note a resemblance between Darwinopterus and Germanodactylus. And that is certainly so, but only by convergence. Phylogenetic analysis indicates a closer relationship between the smaller descendants of Scaphognathus, like no. 12, and Germanodactylus. Arrows indicate phylogenetic order. Here the long neck evolved first with a notably smaller skull. Then the skull became longer and larger in the genus Darwinopterus. Lack of reconstructions by Unwin and Lü may prevent them from seeing the subtleties.

Unwin and Lü noted resemblance to Germanodactylus rhamphastinus, which is certainly reasonable at first glance.

However a large gamut phylogenetic analysis recovers a different tree, principally because Unwin and Lü refused to include tiny pterosaurs. Many of these are the actual transitional taxa linking four convergent lines of long-tailed pterosaurs to short-tailed pterosaurs, as we learned two years ago in the large pterosaur tree (still standing!). Until tiny pterosaurs are included in phylogenetic analysis, pterosaur workers will continue to be frustrated in the trees they recover and will find false leads, like Darwinopterus.

Babies, Juveniles and Teens
Abstracts are notable for their news value and Unwin and Lü do not disappoint. They tell us that several small to tiny Darwinopterus specimens are now known. According to Unwin and Lü, this growth series “shows, for the first time, how the short tail of pterodactyloids originated. Only 15 vertebrae are ossified in the smallest known individual and these are short and simple” and shorter overall than dorsal+sacral series. In adults, they note, 30 vertebrae are ossified.

Contra Unwin and Lü (2013) there’s is not the first time.
You only have to look at several transitional taxa in the large pterosaur tree to see this happening over and over again. Here, here, here and here are previously known examples. But they have cast a blind eye to these taxa.

This brings up another possibility with regard to little darwinopterids
We know from phylogenetic analysis that pterosaur genetic survival was enhanced by size reduction. We’ve seen it over and over throughout the pterosaur tree. Tiny pterosaur adults survived while their larger ancestors and sisters became extinct.

Here’s where it gets interesting…
It is possible that the tiny pterosaurs associated with the Darwinopterus specimens were small derived adults, not juveniles. This would duplicate the evolutionary patterns we see elsewhere several times within the Pterosauria. I’d sure like to get that data.

If the small darwinopterids were indeed juveniles
they would have to be isometric copies of the adults, based on the evidence we find in known embryos and known juveniles, like Zhejiangopterus, Tapejara, Pterodaustro and Pteranodon. But that’s not how the little darwinopterids were described.

If the small darwinopterids had shorter rostra and shorter tails that would involve allometric changes inside the smaller egg that become fixed upon hatching. Then isometric changes would take over to adulthood. A smaller rostrum on a smaller specimen would indicate a smaller adult, probably with distinct pedal proportions, if they follow basic pterosaur growth patterns. This could turn out to be another Solnhofen!

I have requested Lü for access to the unpublished data to help them sort out the juveniles from the small adults, whatever they may be. This is a job for DGS and phylogenetic analysis not “eyeballing it.”

Reference
Unwin DMW and Lü J-C 2013. The basal monofestratan Darwinopterus and its implications for the origin and basal radiation of pterodactyloid pterosaurs. Rio Ptero Symposium 98-101.

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