The Tanis pterosaur egg

BBC.com
published a photo of one of the fossil pterosaur eggs (Fig 1) found at the Tanis site (last day Cretaceous, SW corner of North Dakota, Fig 5). As in other pterosaur eggs, the embryo preserves nearly every bone, some wing membrane and uropatagia. The contents have been jostled, as if dropped, as evidenced by the broken and displaced mandible tip. Distinct from other Tanis fossils, but similar to other pterosaur embryos, this egg is crushed flat. So far, no adult pterosaurs have been reported at the Tanis site.

Figure 1. From BBC.com, a ctenochasmatid pterosaur egg from the Tanis site, last day Cretaceous. Colors added here. See figure 2 for reconstruction. — Figure 1. From BBC.com, a ctenochasmatid pterosaur egg from the Tanis site, last day Cretaceous. Colors added here. See figure 3 for reconstruction.

According to the BBC:
“There’s no doubting the pterosaur egg is special. With modern X-ray technology it’s possible to determine the chemistry and properties of the egg shell. It was likely leathery rather than hard, which may indicate the pterosaur mother buried the egg in sand or sediment like a turtle. It’s also possible with X-ray tomography to extract virtually the bones of the pterosaur chick inside, to print them and reconstruct what the animal would have looked like. Mr DePalma has done this. The baby pterosaur was probably a type of azhdarchid, a group of flying reptiles whose adult wings could reach more than 10m from tip to tip.”

As lepidosaurs pterosaurs could and likely did retain their leathery eggs within the mother’s body until just about ready to hatch. Getting buried in any substance, especially sand, would not have suited a wet, gooey, fragile hatchling ready to fly, but also just taking its first breath.

Pterosaur workers have been and remain loathe to test Peters 2007 who moved pterosaurs and their kin into the Lepidosauria. ‘Adding taxa’ is not on their ‘to do’ list. Analysis showed late surviving Early Cretaceous Huehuecuetzpalli was a basal member of the pterosaur clade, nesting close to Middle Triassic Macrocnemus. That 15-year-old finding has been confirmed and cemented in the large reptile tree (LRT, 2079 taxa, 2010–2022.

Figure 2. From the BBC with the caption, "Pterosaur model Image caption, "A pterosaur embryo inside an egg, found at the Tanis site ...here digitally extracted and constructed into a model." This is an azhdarchid model, not a ctenochasmatid. Perhaps this was based on another egg. — Figure 2. From BBC.com with the caption, “A pterosaur embryo inside an egg, found at the Tanis site here digitally extracted and constructed into a model.” As reported by BBC.com, this is an azhdarchid model, not a ctenochasmatid, and note how perfect and smooth it is, like a freehand drawing. Perhaps this was based on yet another egg. I would hate to think the authors guessed based on coeval azhdarchids.

The published Tanis pterosaur embryo
(Fig 1) and the published digitally extracted model (Fig 2) don’t match. The embryo is a ctenochasmatid (Fig 3) closely related to the B St 1875 XIV 501 specimen of Ctenochasma elegans (Fig 4, Zittel 1882, number 45 in Wellnhofer 1970) from the Latest Jurassic of Solnhofen, Germany. Here it nests in the large pterosaur tree (LPT, 264 taxa).

By contrast
the extracted and constructed model (Fig 2) is an azhdarchid, coeval in the Latest Cretaceous, but a much larger pterosaur with shorter wings, a longer neck and lacking teeth. Perhaps the authors created the model from yet another egg/embryo from the Tanis site and BBC.com got them mixed up. Or perhaps the authors (David Unwin among them) assumed only azhdarchids lived during the last days of the Cretaceous. That would be the easy way to go, but incorrect.

Figure 3. Reconstruction of the Tanis pterosaur embryo. This is a ctenochasmatid, not an azhdarchid. Scale bars represent the embryo and the 8x larger isometrically larger adult size, estimated using phylogenetic bracketing.

Professor David Unwin is a co-author
on a Goddard Scientific Colloquium abstract about the Tanis site (Palmer et al. 2022). A portion is quoted here. “The pterosaur, consisting of a well-developed semi-articulated prenatal skeleton in ovum, and the dinosaur, a subadult ornithischian (Thescelosauridae), were examined via synchrotron rapid scanning X-ray fluorescence (SRS-XRF), scanning electron microscopy (SEM), and traditional light microscopy, which revealed extensive preservation of soft tissue consisting of distinct signatures of organic residues as well as three-dimensional microstructure. Because the pterosaur embryo comprises the first known specimen from the Late Cretaceous and the only example thus far from North America, it contributes vital information about the prenatal development and early ontogeny of these animals from a time interval for which no such data exists.”

The authors say it contributes ‘vital information’, but concluded this embryo was an azhdarchid (Fig 2). So something is amiss here. The authors missed the most vital information about the embryo: what it is. Otherwise there is no ‘vital information’ here. Everything about this embryo has been seen before in other pterosaur embryos.

Too often pterosaur paleontologists like to grab headlines, sometimes for no reason or under false pretenses. Click here for Dr. Unwin’s history of observational and phylogenetic errors. Dr. Unwin is not a tracer nor a builder of reconstructions. His work on pre-natal pterosaurs was reviewed several years ago here.

Then there’s the Late Cretaceous ‘The Thing‘
the large (29cm), leathery, empty egg shell from Antarctica. That specimen prompted a reverse engineering on the size and shape of azhdarchid eggs, large enough to carry a 1/8 size hatchling yet narrow enough to pass through the mother’s cloaca. More on ‘The Thing’ here.

Figure 4. Ctenochasma elegans from the Late Jurassic of Solnhofen is nearly identical to the Tanis embryo, but twice its size as an adult. With typical isometric growth patterns in pterosaurs, the Tanis ctenochasmatid would grow 8x larger, or 4x larger than its Late Jurassic relative.

The Tanis embryo is the last known pterosaur to exist.
Other than size, it is nearly identical to a Late Jurassic basal ctenochasmatid from Germany (Fig 4). Distinctively the embryo has a longer than wide deltopectoral crest, but retains the big feet and longish, spindly tail common to ctenochasmatids. Membranes trailing the legs and wing finger are preserved in the embryo (Fig 1).

We looked at the reason why pterosaurs died out
at the K-Pg boundary earlier here. This latest discovery does not change that hypothesis.

Figure 1. Latest Cretaceous North America with the Tanis site identified. DePalma presents evidence that a tsunami-like waves traveled north on the inland sea to the Tanis site. — Figure 5. Latest Cretaceous North America with the Tanis site identified. DePalma presents evidence that a seismic (earthquake) wave traveled north over the inland sea to the Tanis site setting up seiche waves over the area.

Some workers and others
keep wondering why the media keeps getting the first news and photos from the Tanis site. That’s an interesting question, but ultimately that is DePalma’s choice.

References:
DePalma RA et al. (10 co-authors) 2019. A seismically induced onshore surge deposit at the KPg boundary, North Dakota. Proceedings of the National Academy of Sciences. 116(17): 8190–8199.
DePalma RA et al. (15 co-authors) 2022. A Blast from the Past: Geochemical Identity of the Chicxulub Bolide and Immediate Effects of the Impact, recorded at Tanis, North Dakota. Goddard Scientific Coloquium Spring 2022.
Peters D 2007. The origin and radiation of the Pterosauria. In D. Hone ed. Flugsaurier. The Wellnhofer pterosaur meeting, 2007, Munich, Germany. p. 27.
Wellnhofer P 1970. Die Pterodactyloidea (Pterosauria) der Oberjura-Plattenkalke Süddeutschlands. Abhandlungen der Bayerischen Akademie der Wissenschaften, N.F., Munich 141: 1-133.
Zittel KA 1882. Über Flugsaurier aus dem lithographischen Schiefer Bayerns. – Palaeontographica 29: 7-80.

wiki/Ctenochasma
wiki/Tanis_(fossil_site)

pnas.org/doi/full/10.1073/pnas.1817407116
bbc.com/news/science-environment-61013740
scicolloq.gsfc.nasa.gov/DePalma_2022.html

pterosaurheresies.wordpress.com/2019/04/04/pbs-attempts-to-throw-cold-water-on-the-tanis-site/

pterosaurheresies.wordpress.com/2022/04/16/james-2022-has-a-deep-sense-of-skepticism-about-the-tanis-site-latest-cretaceous-on-the-day-the-bbc-presents-its-marvels/