Updated April 5, 2016 with a new image of Nemicolopterus and Germanodactylus.
Happy Fourth of July.
If you’re looking for fireworks, you came to the right post.
Earlier we looked at various problems with Mark Witton’s new book, “Pterosaurs” here, here and elsewhere. Today we’ll look at Witton’s misconceptions regarding ontogeny, as we study pterosaur maturity from hatchling to old age. Witton’s hypothesis follows traditional thinking, which we looked at earlier here, here and here.
Witton, like other pterosaur workers before him, is a lumper.
As he did with Rhamphorhynchus, Witton proposes that several discrete taxa (Fig. 1) represent a possible ontogenetic growth series of one genus, rather than a phylogenetic evolutionary series of many genera, as recovered by the large pterosaur tree.
Witton Supports Bennett’s View on Germanodactylus
Witton illustrated a tiny pterosaur (Figs. 2, 3, (SoS 4593 (formerly PTHE No. 29 III 1950, No. 9 of Wellnhofer 1970)), which Bennett (2008) considered a juvenile Germanodactylus cristatus (Fig. 3). If pterosaurs changed their appearance from head to toe from hatchling to adult, Bennett and Witton might have had a case. But embryo pterosaurs and phylogenetic analysis demonstrate they don’t.
In the large pterosaur tree, the only analysis that has ever considered a large number of tiny pterosaurs, SoS 4593 (Fig. 2), nests with certain tiny pterosaurs as a tiny adult following a series of decreasing size pterosaurs (fig. 3) and preceding a series of increasing size pterosaurs. Sos 4593 nests with these tiny pterosaurs because it shares more traits with them. The changes are gradual and completely resolved, independent of tiny pterosaurs in other clades.
At first glance, I can see Bennett’s and Witton’s point.
Sos 4593 does have a juvenile look to it. And the size is right. That’s why it’s important to test it with phylogenetic analysis. To their fault, Bennett and Witton omitted this step. They just eyeballed it — and perhaps not closely enough (compare Fig. 2 to Fig. 4).
Here (Fig. 3) No. 4072 (lower left), with its longer rostrum and soft tissue precursors to the hard crests, is a much closer match to G. cristatus. No. 6 (lower right) is actually the right size to be a hatchling of G. cristatus and is phylogenetically closer to it than Sos 4593, but it, too, was ignored.
Sos 4593 is phylogenetically closer to Ornithocephalus (#2) and one of the smaller Scaphognathus specimens (upper right), because it shares more traits with them than with G. cristatus.
Why wasn’t Sos 4593 considered a juvenile Scaphognathus? Probably due to the naris/antorbital situation. The tiny pterosaurs, #3 (middle right) and Ornithocephalus (#2, middle) demonstrate how the ascending process of the maxilla became more and more slender. We can’t just accept broken parts ‘as is.’ We have to put them back together, if possible. That’s why precise tracings and reconstructions are necessary. A closer look at Sos 4593 shows the naris and antorbital fenestra may have remained separated by a very slender maxillary ascending process prior to crushing during burial. In this clade the naris and antorbital fenestra did become confluent, but a secondary naris developed anteriorly, beginning with Scaphognathus.
Nemicolopterus, the tiny pterosaur at the base of the Tapejaridae and Shenzhoupterus, is a tiny descendant of Germanodactylus cristatus and is phylogenetically closer (fewer nodes away) to it than is Sos 4593.
Pterosaur paleontologists can’t just make assertions that one specimen is the juvenile or hatchling of another larger specimen unless they first do the phylogenetic analysis (or find the juvenile and parent together in a mass death assemblage). The large pterosaur family tree has strongly demonstrated several times that tiny pterosaurs like Sos 4593 and Nemicolopterus are transitional taxa that link larger clades together, like scaphognathids and germanodactylids and tapejarids. Pterosaur embryos, as we’ve seen before, nest with their parents, not with other tiny pterosaurs that may be no larger than those embryos. Since pterosaur clades tend to disappear (become extinct) over time, these tiny pterosaurs are the means by which pterosaur lineages evolve and survive. This has gone unrecognized by Witton, Bennett and other pterosaur workers.
And now, let’s tie it in all together with the “baby face” hypothesis.
It’s widely recognized that pterosaur hatchlings were ready to fly right after hatching. Not so widely recognized (due to the antiquated archosaur hypothesis) pterosaurs hatched out of thin-shelled, singleton, unburied eggs laid just before hatching (common enough among lizards). Embryo pterosaurs benefitted in several regards by being carried by the mother until just before hatching (warmth, protection, fluids).
Careful examination of long-rostrum pterosaur embryos demonstrates that they did not have the short rostrum common to those tetrapods (birds, crocs, mammals) in which parents care for their hatchlings. Such a short rostrum biochemically inspires other parents to care for their hatchlings. Since hatchling pterosaurs did not have this “cuteness” trait, it appears unlikely that pterosaur parents would have had any interest in caring for their hatchlings. Rather, the hatchlings were on their own from day one, flying and feeding on their own, probably far from typical areas that support fossilization. As discussed earlier, hatchlings of tiny pterosaurs, in danger of desiccation due to their large surface/volume ratio, likely matured in damp, moist, shady, leafy environs.
Bennett SC 2006. Juvenile specimens of the pterosaur Germanodactylus cristatus, with a review of the genus. Journal of Vertebrate Paleontology 26:872–878.
Wang X, Kellner AWA, Zhou Z and Campos DA 2008. Discovery of a rare arboreal forest-dwelling flying reptile (Pterosauria, Pterodactyloidea) from China. Proceedings of the National Academy of Sciences, 106(6): 1983–1987. doi:10.1073/pnas.0707728105
Wellnhofer P 1970. Die Pterodactyloidea (Pterosauria) der Oberjura-Plattenkalke Süddeutschlands. Abhandlungen der Bayerischen Akademie der Wissenschaften, N.F., Munich 141: 1-133.
Witton M. 2013. Pterosaurs. Princeton University Press. 291 pages.