Anurognathus ammoni is a small pterosaur, about the size of the embryo pterosaur found in China (IVPP 13758). So it was with some surprise that I found tiny impressions in the matrix (Fig. 1) that looked like a miniature (6x smaller) version of Anurognathus, still enrolled, apparently expelled from the pelvis during taphonomy.
We know of very few embryo/adult relations in the fossil record and even fewer among pterosaurs.
The Pterodaustro embryo
We know from the Pterodaustro embryo that it was 8x smaller than an adult Pterodaustro. Like the adult, the jaws were very long, putting to rest any hypotheses that imagined a short rostrum on every embryo pterosaur.
The IVPP embryo
The IVPP embryo is not associated with an adult, but using the 8x formula we can surmise what it looked like here. It was originally imagined to be an ornithocheirid embryo with a very short snout. But pterosaurs grow isometrically, not like archosaurs. Moreover, phylogenetic analysis nests it with anurognathids. Detailed tracings of the embryo using DGS also make the case for an anurognathid affinity, even though it is a VERY big anurognathid.
We’ve also learned that pterosaurs are sexually mature at half their final adult size (Chinsamy et al. 2008), but we’re not sure what that means with regards to egg and embryo relative size.
The Darwinopterus egg
In the Darwinopterus egg, (AMNH M8802, Lü et al. 2011) the eggshell is clearly marked, but the embryo is immature and poorly ossified, invisible to the original workers, but traced here using DGS.
Back to Anurognathus
So Anurognathus is a small, but not a tiny pterosaur. The embryo is 6x smaller than the adult and appears to be complete and enrolled with the wings and feet breaking out of the imaginary ellipse that surrounds the rest of the bones. None of these embryonic bones are ossified or differently colored. They are slight impressions and hard to see until you start colorizing them (Fig. 1). This poor ossification is not unexpected as many of the bones in the adult are likewise poorly ossified or incompletely prepared.
For an illusion,
the embryo Anurognathus matches the adult pretty well. It is curled up like an embryo would be and would have fit into a broad elliptical egg shape. The size, at one-sixth the size of the adult, is larger than the Pterodaustro embryo at one-eighth the size of the adults. The egg-shape could have passed through the pelvis identified here (Fig. 1). The displacement of the adult tail matches the displacement of the embryo from the pelvic opening.
I encourage anyone interested in details to click on figure 1 to see a larger rollover image that shows the in situ fossil on one level and the interpretation on the other. Perhaps this is the only way to see the bones that are, admittedly, barely visible.
I saw the embryo had a longer neck before I realized the adult had a long neck. That observation led to further study chronicled earlier. The small orbit, as in other pterosaurs, is in the back of the skull, behind the large antorbital fenestra. This is in contrast to the Bennett version of Anurognathus discussed here and here.
No egg shell that I can tell
There is no indication of an egg shell here, but then again, pterosaur egg shells of larger specimens are among the thinnest of all reptiles. In three prior larger pterosaurs (IVPP embryo, JZMP embryo, Pterodaustro embryo) the adult and the egg shell are both fairly well-ossified. In the Darwinopterus egg, the egg is clearly marked but the embryo is immature and poorly ossified, invisible to the original workers.
What this mother/embryo relationship tells us
Intent on avoiding any association with lizards, previous interpretations of pterosaur eggs and embryos had them laid under rotting heaps of vegetation (Lü et al. 2011), the way certain superprecocial birds do in the present*, rather than having the eggs retained by the mother until just before hatching, as many lizards do**. Darwinopterus was the first instance of a mother and embryo, even though the original workers did not see the embryo, only the eggshell. So Anurognathus is the second instance of same, but this time, with the embryo, not the eggshell. As lizards, mother pterosaurs kept their embryos with them, until just before hatching from the thinnest of egg shells, contra traditional thinking.
The Anurognathus embryo is further evidence that pterosaurs were able to fly out of their eggs, but they would have remained in their eggshells for only minutes or hours, like certain lizards, not weeks, like birds or crocs.
The Anurognathus embryo argues against pterosaurs producing clutches of eggs, but rather one at a time. No one has ever found two pterosaur eggs in the same area.
It is intriguing to wonder whether the mother pterosaur providing care for the new embryo by allowing it to cling to her for the first few days, or whether the embryo was on its own from the get-go. Evidence for the former would come from an adult/infant relationship preserved as a fossil without the enrolling that characterizes the embryo within the eggshell.
It would be worthwhile to keep looking for enrolled tiny skeletons near the pelves of other pterosaurs. This one was overlooked for 90 years.
As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.
Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.
*NewScientist.com reports, “Unwin suggests that pterosaurs, like modern reptiles, may have buried many small eggs in ground where moisture could seep through their parchment-like eggs during incubation, nearly doubling their mass before hatching.”
** Ovoviviparity: this is oviparity with retention of zygotes in the female’s body or in the male’s body, but without major trophic interactions between zygote and parents (there may be minor effects, such as maintenance of water and oxygen levels). Anguis fragilis is an example of ovo-viviparity. (from Wikipedia)
Chinsamy A, Codorniú L and Chiappe LM 2008. Developmental growth patterns of the filter-feeder pterosaur, Pterodaustro guinazui. Biology Letters, 4: 282-285.
Döderlain L 1923. Anurognathus ammoni, ein neuer Flugsaurier. Sitzungsberichte der Königlich Bayerischen Akademie der Wissenschaten, zu München, Mathematischen-physikalischen Klasse: 117-164.
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