Origins of pterosaur eggs

[Evan Robinson <evanrobinson2004@hotmail.com>]

(From the Pterosaur embryo fossil tread where David Peters suggests 
viviparity as an explanation for the small young that he sees accompanying 
various prolacertiforms and pterosaurs. The recent finding of a pterosaur 
embryo in a hard-shelled, chicken size egg endangers his theory.)

All known archosaurs, as well as turtles, and even the extant lepidosaur 
Sphenodon have a condition known as embryonic diapause. This means that 
their eggs suspend development if retained in the uterus, and only resume 
development after they have been laid. Thus, prolonged retention of the 
eggs, which could occur for various adaptive reasons, and which is the 
predecessor to viviparity, will never occur, and therefore viviparity will 
never develop. For this reason, viviparity in all archosaurs is unlikely.

In squamates, however, the transition from oviparity to viviparity has 
occurred many times, and incidentally, reversals may never have occurred 
(the theory being that the egg laying structures cannot be regained). Taking 
the Gekkonidae family as an example, the extant Gecko gecko, as well as 
Phelsuma Madigacaraian, lay hard-shelled eggs. Yet many others in the family 
have soft eggs, and others have developed viviparity.

> --- K and T Dykes <ktdykes@arcor.de> wrote:
<<The embryonic skeleton, which is exquisitely preserved in its egg, is
> associated with eggshell fragments, wing membranes and skin imprints.
> > > > That seems to suggest at least one pterosaur laid eggs, the shells of 
> > which
> could fragment; ie. an egg a with hard shell as known from crocs, non-birdy 
> dinos and birdies. That sounds like a very archosaurian kind of thing to 
> do. .

> T. Michael Keesey wrote:

> Or at least archosauromorphan. The major hypotheses of pterosaur origins 
> place
> them either in the immediate outgroup to _Dinosauromorpha_ ("The 
> Ornithodiran Hypothesis") or outside _Archosauria_ sensu stricto (the most 
> exclusive clade containing crocodylians and
> avians), but still within _Archosauromorpha_ (the most inclusive clade 
> containing archosaurs but
> not lepidosaurs), possibly as part of _Prolacertiformes_ ("The 
> Prolacertiform Hypothesis"). Under the Ornithodiran Hypothesis, any feature 
> seen in both crocodylians and dinosaurs would be expected to be present in 
> at least basal pterosauromorphs. If, on the other hand, pterosaurs are 
> non-archosaurian archosauromorphs, any feature seen in both crocodylians 
> and
> dinosaurs still *might* be present in basal pterosauromorphs.

Good point, archosaurmorphans, and even our extant member of the 
lepidosaurs, the Tura Tura, have embryonic diapause, and lay eggs. 
Prolacertiforms are often said to be closely related to the lepidosaurs, 
sharing some synapomorphies. Is it possible that the prolacertiforms 
diverged from an ancestor closer to a common ancestor of Lepidosauromorphs, 
Archosauromorphs, and squamates then presently hypothesized? If this 
ancestor had not yet developed embryonic diapause, then various 
prolacertiforms could have laid soft eggs, become viviparous, or developed 
hard-shelled eggs independently, as did the Gekkonidae. Turtles, 
interestingly, have embryonic diapause. At  least some Ichthyosaurs and 
Mosasauroids are viviparous, demonstrating that they likely diverged 
earlier.

Thus far, hard-shelled crocodilian eggshell dates back to the Late Triassic, 
and represents the oldest known fossil eggshell. Dinosaurian: 
Preprismatoolithus, Spheroolithid, and Megaloolithadae show up in the 
Jurassic, and Ratite-Ornithoid in the Late Jurassic. Mr. Carpenter can 
correct me if this has changed. Although the timing works out right, I don?t 
know that there is of yet any proof that the crocodilian eggshell evolved 
into the dinosaurian types. Incidentally, fossil eggshell of the Geckonoid 
morphotype also dates back to the Jurassic. The pterosaur eggshell, if 
related, would presumably have diverged in the Triassic as well. 
Alternatively, pterosaurs could have developed a hard shell independently of 
crocodilians, as we know that turtles did.

I like the idea of David?s long-armed babes clinging to Longisquama. 
However, there are difficulties with the involvement of viviparity. Perhaps 
it need not be a factor.

> Thus, eggs like those of crocodylians and dinosaurs are compatible with 
> either hypothesis, and do nothing to resolve the relationships further.

I think that a electron microscope scan of a thin section of that pterosaur 
eggshell would be fascinating! It could shed some light on relationships by 
showing that it did or did not develop independently from other oospecies. 
Wouldn?t it be something if it has mammillae for example? Mr. Carpenter: get 
over there (and then call me to come look)!

Thanks,

Evan Robinson

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