The JZMP embryo and its adult sister taxa

Currently the only known embryo/adult pterosaur pairing is in the genus Pterodaustro. Unfortunately, not much has been made about the allometry/isometry in the growth patterns of this genus, even though the data is available. The best data, unfortunately, is right here, rather than in an academic journal. Adults are 8x larger than hatchlings and ontogeny is chiefly isometric (as shown most clearly in Zhejiangopterus) and other pterosaurs like Ptweety the Pteranodon.

For the other two embryos, the IVPP embryo and the JZMP embryo we currently do not have congeneric adults and have to look to sister taxa or simple isometry to estimate the adult proportions and traits.

Today we’ll look at the sister taxa of the JZMP embryo (Fig. 2).

Figure 1. The closest sisters to the JZMP embryo, a basal ornithocheirid without an adult skeleton known for it.

Figure 1. The closest sisters to the JZMP embryo, a basal ornithocheirid without an adult skeleton known for it. Click to enlarge. This is not the phylogenetic order. Yixianopterus is the most primitive. Then the JZMP embryo followed by the smaller forms. These are followed by the boreopterids. 

Generally what we find at clade bases is a gradual increase in size from tiny ancestors. While we do have a tiny ancestor in Pterodactylus(?) pulchellus, we don’t have a gradual increase from that point forward. Yixianopterus the basalmost ornithocheirid, is large AND primitive. The JZMP embryo as an adult, was similar in size to Yixianopterus, but twice as tall as Haopterus and two Lebanon basal ornithocheirids. Click here to see a phylogenetic lineup of ornithocheirids and their outgroup.

Given these various sizes in basal ornithocheirids, and no gradual increase in size, one wonders if Haopterus and the Lebanon ornithocheirids are juveniles. Finding lots of larger congeneric taxa would be helpful. Checking out the annular rings in their long bones would also give clues.

The alternative, that the small ornithocheirids are adults, might represent the next phase in derived ornithocheirid evolution, in which the wings get longer and the feet get smaller, among other traits, which appears to be the case in the Lebanon ornithocheirids.

References
Ji Q, Ji S-A, Cheng Y-N, You HL, Lü J-C, Liu Y-Q and Yuan CX 2004. Pterosaur egg with leathery shell. Nature 432:572.

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