Focus on Angustinaripterus – transitional between Dorygnathus and Ctenochasma

Angustinaripterus (Fig. 1) has been difficult to classify for other pterosaur paleontologists. Here in the large pterosaur tree the reason becomes quite evident. Add a few taxa and Angustinaripterus become a transitional taxon between the more primitive Dorygnathus (R154) and the more derived Gnathosaurus and Ctenochasma.

Figure 1. Angustinaripterus as a transitional taxon between Dorygnathus and Gnathosaurus.

Figure 1. Angustinaripterus as a transitional taxon between Dorygnathus and Gnathosaurus. Seems pretty obvious. Phylogenetic analysis confirms.

This possibility flips out traditional pterosaurologists who have pinned their hopes on Darwinopterus, which doesn’t look much like either Ctenochasma or Dorygnathus.

But that’s not all
Here (Fig. 2) are the many other specimens that smooth the evolutionary transition, as we noted before. Here they are. And many of them are tiny.

Figure 2. The same sequence with the addition of Dorygnathus purdonti and four tiny pterosaurs variously misassigned to Pterodactylus and Ctneochasma.

Figure 2. The same sequence with the addition of Dorygnathus purdonti and four tiny pterosaurs variously misassigned to Pterodactylus and Ctneochasma. Black illustrations are to scale. Gray figures are enlarged to show detail.

Angustinaripterus is the reason why I delved deeper into pterosaur phylogeny than anyone has done before. At first glance it’s clearly something not quite Dorygnathus and not quite Gnathosaurus or Ctenochasma.

The next step after Angustinaripterus was to add in some…

Tiny pterosaurs
This (Fig. 2) is only one sequence of many in which tiny adult pterosaurs are transitional between larger forms, both more primitive and more derived. At first glance, and according to tradition, these tiny pterosaurs are only juveniles or hatchlings, which makes perfect sense—until you add them to your matrix—and you realize juvenile pterosaurs of other types are virtual copies of adults. So there was no allometric growth after hatching, but chiefly isometric growth.

But there’s an important clue here staring you right in the face: Note how the tiny pterosaurs are ALL the same size. As it turns out, this is the minimum size at which pterosaurs could fly, evidently, as we find no smaller pterosaurs in the fossil record. Smaller pterosaurs, all juveniles, must have been living in damper environments and probably not flying. Some hatchlings, like the IVPP embryo, were as large as these adult pterosaurs (Fig. 2) and other adult tiny pterosaurs, so they could fly immediately. 

References
He X-L, Yang D-H and Su C-K 1983. A New Pterosaur from the Middle Jurassic of Dashanpu, Zigong, Sichuan. Journal of the Chengdu College of Geology supplement 1: 27-33.

wiki/Angustinaripterus

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