What Do Those Pterosaur Embryos Really Look Like?

In this blog you’re going to see the benefits of using DGS (Digital Graphic Segregation) a technique of tracing high resolution digital images on a computer monitor without the specimen at hand. This is widely considered to be inferior to first-hand observations using a microscope, pencil and camera lucida. However one method has not been tested against another, until now. The results speak for themselves.

I know of five pterosaur eggs with embryos inside. Four have been published. Three were reported to contain embryos. Let’s look at them one at a time. Image links will take you to individual taxon pages on reptileevolution.com. Embryo pterosaurs trapped inside their eggshells are exciting subjects because: 1) we can expect all of their bones to be present and 2) we know that each specimen was exactly zero years old.

1. The IVPP specimenIVPP V13758 (Wang and Zhou 2004, Figs. 1, 2) Early Cretaceous, ~125 mya, was the first pterosaur embryo to be published and it was originally considered to be a baby ornithocheirid, like the JZMP embryo. Using DGS to trace and reconfigure the bones into a standing reconstruction, the IVPP embryo turns out to be an anurognathid, but every bit as large as virtually all other adult anurognathids! Except one.

The IVPP embryo pterosaur

Figure 1. Click to enlarge DGS tracing. The IVPP embryo pterosaur (far left) as originally traced, (near left) as originally reconstructed as a baby ornithocheirid, (near right) traced using the DGS method, (far right) adult reconstructed at 8x the embryo size.

The only other anurognathid in the same size category as the IVPP hypothetical adult is a Mexican specimen mistakenly assigned to the genus Dimorphodon, but under the species D. weintraubi. Only its limbs are known and in cladistic analysis it is a sister to the IVPP embryo.

The IVPP embryo

Figure 2. The IVPP embryo scaled to an adult size (based on matching the egg to the pelvic opening diameter, along with various views of the skeleton and an egg and hatchling.

The IVPP embryo has a longer neck than most anurognathids and a longer metacarpus. The humerus is quite a bit shorter. Perhaps this will turn out to be a juvenile trait in this clade. The IVPP embryo is the most primitive clade member with a short tail, but it is also among the latest members to appear chronologically. Apparently it is a late relic. Learn more here.

The JZMP ornithocheirid embryo

Figure 3. Click to enlarge DGS tracings. The JZMP ornithocheirid embryo, in situ and reconstructed.

2.The JZMP embryoJZMP-03-03-2 (Ji et al. 2004, Fig. 3) was the second pterosaur embryo described from China. It was considered close to Beipiaopterus, but it is a basal ornithocheirid close to Haopterus. Here again the humerus was atypically small when compared to those of adult sister taxa, but otherwise the embryo had full size wings and adult proportions, including an elongated, tooth-filled rostrum and small eyes. Here the dorsal vertebrae were separated from the sacral vertebrae and the legs were disarticulated, so this egg was shaken or rolled before it was buried. Learn more here.

Pterodaustro embryo

Figure 3. Pterodaustro embryo. There certainly is no short snout/large eye here!

3. The Pterodaustro embryo MHIN-UNSL-GEO-V246 (Chiappe et al. 2004) was the first embryo found in association with adults and other juveniles of various sizes. The Pterodaustro egg was longer and narrower than the others and no wonder — it had to contain that elongated upturned rostrum! Distinct from the Chinese embryos, the Pterodaustro embryo had a relatively larger humerus and antebrachium (forearm), but it had a relatively smaller metacarpus. The sternal complex was also slightly larger. The details of the embryo in situ will be shown when it is published by its discoverers. Rather than estimating adult size from making comparisons to egg and pelvic opening, with Pterodaustro we have direct evidence for an 8x larger adult associated with an embryo. Learn more here.

Darwinopterus mother and premature embryo

Figure 4. Darwinopterus mother and premature embryo. Click to see in situ tracing.

4. The Darwinopterus egg/embryo – After several specimens of the germanodactylid, Darwinopterus, were published, one  (AMNH M8802) was reported (Lü et al. 2011) with an egg between its legs, evidently just expelled from the pelvis before or during burial. Originally no embryo was reported present in the egg, but the DGS method enabled the tracing of virtually all the articulated, but poorly ossified bones of the less than full term embryo. A reconstruction closely resembled the mother. Learn more here.

Ornithocephalus pterosaur egg.

Figure 5. Ornithocephalus pterosaur egg. Click to see in situ specimen.

5. The Ornithocephalus embryo – (Soemmerring 1812-1817) Pterodactylus micronyx von Meyer 1856, No. 29 in the Wellnhofer (1970) catalog). Ornithocephalus was the second pterosaur ever described. The tiny size of the specimen and its short snout immediately earned it juvenile status in the eyes of every paleontologist who saw it. Unfortunately, for that hypothesis, Ornithocephalus nests with other tiny pterosaurs in a transitional series from Scaphognathus to the bases of Pterodactylus and Germanodactylus (more on this in future blogs). Between the femora of this pterosaur is an elongated egg-like structure with an apparent embryo inside. This observation needs to be tested because this egg/embryo has not been published and confirmed. I happened upon it when I was tracing the bones of the specimen. Like Darwinopterus, the Ornithocephalus egg was apparently discharged while the mother lay motionless. Learn more here.

Immediately able to fly?
The size of pterosaur eggs is key to understanding the abilities of the hatchlings to fly–or not. The IVPP egg was ~50 mm in length. The JZMP egg was ~60 mm in length. The Pterodaustro egg was ~60 mm in length. The snout/vent length of each emerging hatchling would have been greater than these measurements because the pterosaur was tucked in with its snout down prior to hatching. These examples all fit in the category of: “hatchlings ready to fly” because the embryos equalled or exceeded the sizes of many tiny adult pterosaurs.

Please consider that living lizards with a snout/vent length under 18mm (Hedges and Thomas 2001) dry up and die when removed from their moist leaf litter environment. Now increase that surface area with wing membranes, crests and uropatagia and you have a real problem as a hatchling of a tiny pterosaur with a relatively greater surface/volume ratio will spend every flying moment evaporating precious moisture in a steady airstream.

At ~27 mm in length, the Darwinopterus egg produced a much smaller hatchling only about 35 mm tall. This was slightly shorter than the smallest known pterosaur not associated with an eggshell, B St 1967 I 276 (No. 6 in the Wellnhofer 1970 catalog), which stood at ~40mm tall with a similar snout/vent length. Thus the Darwinopterus hatchling was likely able to fly, but it was nearing what appears to be some sort of theoretical limit.

Now the real problem: Hatchlings of no. 6 would have been ~5 mm in snout/vent length, the size of house flies. Their high surface/volume ratios would have grounded such tiny hatchlings until they were able to grow up to the theoretical limit (unless they were somehow able to keep themselves hydrated some other way). The various problems tiny grounded pterosaurs likely encountered likely provided certain natural selection pressures for whatever traits followed, such as the reduction of the tail and the elongation of the metacarpus. These changes occurred at least four times, by convergence, according to the present fully resolved tree.

Archosaur Eggs or Lizard Eggs?
Archosaurs, like birds and crocs, lay their eggs shortly after fertilization and the embryos develop chiefly outside of the mother. Grellet et al. (2007) suggested that, as archosaurs,
pterosaurs would have buried their eggs for months at a time, forcing the tiny hatchlings to crawl through the sand, mud and rotten leaves to get to the surface to fly. Seems unlikely since any tear to the wing fabric would have been a death sentence, but that’s the traditional hypothesis.

Lizards, generally lay their eggs long after fertilization, sometimes at the moment of hatching or shortly before. Since pterosaurs are lizards, you should always look for embryos, even poorly ossified embryos, inside of pterosaur eggs. The extremely thin pterosaur eggshell is most comparable to the eggshells of living lizards that retain the egg until just before hatching. Finally, just think of the benefits for the embryo. Inside of its warm-blooded mother for most of its development, a pterosaur embryo could develop quickly and safely.

Isometric Growth is Proven with Pterosaur Embryos
Each of the pterosaur embryos had the proportions of an adult (with humerus length the chief exception). Their eyes were not larger and their beaks were not shorter than in adults. Pterosaur hatchlings did not have “cute” facial proportions like baby mammals, birds and crocodilians. That falsifies decades of earlier traditions supporting allometric development in pterosaurs by Wellnhofer (1970) and others, especially Bennett (1993a, b, 1995, 1996a, 2001a,b, 2006, 2007) who both wrongly considered pterosaurs to be archosaurs and tiny pterosaurs to be juveniles. Precise tracings and reconstructions of the embryos demonstrate that pterosaurs matured isometrically, like their precursor, Huehuecuetzpalli, the basal taxon in the Tritosauria. That strategy for growth supports the hypothesis that all the tiny pterosaurs listed on the pterosaur cladogram (except the juvenile Pterodaustro) represent distinct taxa and unique genera.

Everything about pterosaurs points to a lizard ancestry. The archosaur hypothesis cannot be defended except by excluding all lizard and fenestrasaur candidates, which is how it is so often done nowadays (Hone and Benton 2007, 2008, Nesbitt 2011).

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.

References:
Bennett SC 1993a. The ontogeny of Pteranodon and other pterosaurs. Paleobiology 19, 92–106.
Bennett SC 1993b. Year classes of pterosaurs from the Solnhofen limestone of southern Germany. Journal of Vertebrate Paleontology. 13, 26A.
Bennett SC 1995. A statistical study of Rhamphorhynchus from the Solnhofen limestone of Germany: year classes of a single large species. Journal of Paleontology 69, 569–580.
Bennett SC 1996a. Year-classes of pterosaurs from the Solnhofen limestones of Germany: taxonomic and systematic implications. Journal of Vertebrate Paleontology 16:432–444.
Bennett SC 1996b. The phylogenetic position of the Pterosauria within the Archosauromorpha. Zoological Journal of the Linnean Society 118:261–309.
Bennett SC 2001a, b. The osteology and functional morphology of the Late Cretaceous pterosaur Pteranodon. Part I. General description of osteology. Palaeontographica, Abteilung A, 260:1–112. Part II. Functional morphology. Palaeontographica, Abteilung A, 260:113–153.
Bennett SC 2006. Juvenile specimens of the pterosaur Germanodactylus cristatus, with a review of the genus. Journal of Vertebrate Paleontology 26:872–878.
Bennett SC 2007. A review of the pterosaur Ctenochasma: taxonomy and ontogeny. Neues Jahrbuch fur Geologie und Paläontologie, Abhandlungen 245:23–31.
Chiappe LM, Codorniú L, Grellet-Tinner G and Rivarola D. 2004. Argentinian unhatched pterosaur fossil. Nature, 432: 571.
Grellet-Tinner G, Wroe S, Thompson SB and Ji Q 2007. A note on pterosaur nesting behavior. Historical Biology 19:273–277.
Hedges SB and Thomas R 2001.At the Lower Size Limit in Amniote Vertebrates: A New Diminutive Lizard from the West Indies. Caribbean Journal of Science 37:168–173.
Hone DWE and Benton MJ 2007. An evaluation of the phylogenetic relationships of the pterosaurs to the archosauromorph reptiles. Journal of Systematic Palaeontology 5:465–469.
Hone DWE and Benton MJ 2008. Contrasting supertree and total evidence methods: the origin of the pterosaurs. Zitteliana B28:35–60.
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.
Lü J-C, 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
von Meyer CEH 1856.  Zur Fauna der Vorwelt. Saurier aus dem Kupferschiefer der Zechstein-Formation. Frankfurt-am-Main. vi + 28 pp., 9 pls.
Nesbitt SJ 2011. The early evolution of archosaurs: relationships and the origin of major clades. Bulletin of the American Museum of Natural History 352: 292 pp.
von Soemmering ST 1812. Über einen Ornithocephalus. – Denkschriften der Akademie der Wissenschaften München, Mathematischen-physikalischen Classe 3: 89-158.
von Soemmering ST 1817. Über einer Ornithocephalus brevirostris der Vorwelt. Denkschriften der Akademie der Wissenschaften München, Mathematischen-physikalischen Classe 6: 89-104.
Wang X-L and Zhou Z 2004. Palaeontology: pterosaur embryo from the Early Cretaceous. Nature 429: 623.
Wellnhofer P 1970. Die Pterodactyloidea (Pterosauria) der Oberjura-Plattenkalke Süddeutschlands. Abhandlungen der Bayerischen Akademie der Wissenschaften, N.F., Munich 141: 1-133.

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