Earlier we looked at the myth of Darwinopterus as the transitional taxon between long-tailed early pterosaurs and short-tailed later pterosaurs. Actually, several series of tiny pterosaurs (Fig. 5 as an example) fill that role and they do it four times, two out of two distinct Dorygnathus and two out of the smallest Scaphognathus (which is why some tiny pterosaurs have a large eye and short snout).
Remember a good transition consists of a beginning, several middles and an end. The Darwinopterus scenario provides a middle, but no specific beginning or end.
Supporting the traditional view, Mark Witton, author of “Pterosaurs“, reports, “Darwinopterus incontrovertibly fills a long-standing gap in pterosaur evolution, bridging the morphological distance between early pterosaurs and Pterodactyloidea.”
Incontrovertibly? Not so. Darwinopterus fails when put to several tests (see below).
Witton (2013) also falls off the Darwinian train when he reports, “Rather than demonstrating a bauplan with a smattering of pterodactyloid and non-pterodactyloid features across the entire skeleton, it [Darwinopterus] possess the characteristic skull and neck of pterodactyloid while retaining a body very similar to those of rhamphorhynchid pterosaurs.” This has been called, “modular evolution” and this is the only animal that this bizarre mode of evolution has _ever_ been applied to. Modular evolution creates chimaeras, but that’s _not_ how evolution works!
Long time readers of the Pterosaur Heresies already know the solution to this problem.
According to the results of the large pterosaur tree (now 204 taxa), Darwinopterus nests at the acme of a small clade of darwinopterids including Wukongopterus, Kunpengopterus and Pterorhynchus at its base, all derived from a sister to Jianchangnathus, which also gave rise to Scaphognathus and a long list of tiny and large descendants.
A clade has been erected (Lü, Unwin, et al. 2009) for Darwinopterus + Pterodactyloidea, the “Monofenestrata.” Unfortunately, Darwinopterus does not have a monofenestra. The naris is small, but still visible (Fig. 2), just like Pterorhynchus.
More unfortunately, Darwinopterus does not nest near the base of any pterodactyloid-grade pterosaurs in the completely resolved large pterosaur tree. Those taxa that do actually nest at the base of pterodactyloid-grade pterosaurs (in the large pterosaur tree) fulfill Witton’s wish for a smattering of pterodactyloid and non-pterodactyloid features. Those features can be found in tiny pterosaurs (Fig. 5).
Let’s put aside all the other problems with Witton’s pterosaur family tree and focus on the Darwinopterus situation.
Here Darwinopterus nests within the Wukongopteridae, a suprageneric taxon. Both the ancestor and descendant taxa are also suprageneric, leaving the transition to and from Darwinopterus rather cloudy. By that I mean, Witton doesn’t tell us which taxa are the direct ancestors and descendants of Darwinopterus. That avoids having to deal with details and data. Actually the original Lü, Unwin et al. (2010) tree generated some 500,000 most parsimonious trees, so from the start there are red flags everywhere with this study and this tree.
there’s complete resolution (one tree results) in the large pterosaur tree. It also provides specific taxa (specimens) that nest with Darwinopterus and others that act as transitions to the four pterodactyloid grades. The former clade “Pterodactyloidea” is not monophyletic when tiny pterosaurs are included in analysis, something Witton and his cohorts refuse to do.
For a reminder, here (Fig. 4) are the closest sisters to Darwinopterus and three Darwinopterus specimens. The upper clade of wukongopterids are monophyletic, not transitional. The real story takes place after Scaphognathus with those half-sized descendants (no, they’re not juveniles).
And here (Fig. 5) are some of the transitional taxa arising out of the small Scaphognathus specimens. These tiny pterosaurs are the real transitional taxa. And there’s not just one. There are four series with gradual decreases and gradual increases in size. The hope that there is just one transitional taxon is a myth. The transition is a spectrum of gradual change. The apparent disappearance of the naris likewise had four paths with some reducing the naris and others merging the naris and antorbital fenestra.
Here, using specimens, you can see that every specimen between the large Scaphognathus and the large Germanodactylus are transitional taxa, creating a spectrum, some closer to Scaphognathus and others closer to Germanodactylus. That’s the beauty of using specimens, rather than suprageneric taxa. You get the real picture without any fudging or imagination.
By convergence, Darwinopterus, like the other four gradual transitions, did reduce the naris and elongate the skull and neck. These traits were derived from Pterorhynchus, which already had a reduced naris, then Kunpengopterus, which had a longer skull and longer neck (Fig. 4).
But then Darwinopterus went nowhere. It became extinct. End of story?
Lü J, Unwin DM, Jin X, Liu Y and Ji Q 2009. Evidence for modular evolution in a long-tailed pterosaur with a pterodactyloid skull. Proceedings of the Royal Society London B (DOI 10.1098/rspb.2009.1603.)
Lü J, Unwin DM, Deeming DC, Jin X, Liu Y and Ji Q 2011a. An egg-adult association, gender, and reproduction in pterosaurs. Science, 331(6015): 321-324. doi:10.1126/science.1197323
Lü J, Xu L, Chang H and Zhang X 2011b. A new darwinopterid pterosaur from the Middle Jurassic of Western Liaoning, northeastern China and its ecological implicaitions. Acta Geologica Sinica 85: 507-514.
Witton M. 2013. Pterosaurs. Princeton University Press. 291 pages.