Paleontologists in China have unearthed and examined three adult skeletons and five clutches of embryo-containing eggs of Qianlong shouhu, a previously unknown species of Early Jurassic sauropodomorph dinosaur. Most significantly, these fossils provide strong evidence for the earliest known leathery eggs.
Reconstruction of Qianlong shouhu’s eggs. Image credit: NICE Vistudio.
Qianlong shouhu lived in what is now China during the Early Jurassic epoch, between 200 and 193 million years ago.
The dinosaur belongs to Sauropodomorpha, a group of plant-eating long-necked dinosaurs that includes the largest animals ever to walk the Earth.
The ancient creature was approximately 6 m (20 feet) long and weighed around 1 ton.
The fossilized remains of Qianlong shouhu — three adult skeletons and five clutches of eggs — were found in the Ziliujing Formation in Guizhou Province, southwestern China.
“The embryos display some differences from the adults, e.g., a proportionally longer skull, a more vertical anterior margin of the snout, and fewer teeth,” said China University of Geosciences paleontologist Fenglu Han and colleagues.
“Allometric analyses of limb ratios between the adult and embryonic specimens indicate that adult Qianlong shouhu was able to walk on its hindlimbs, but the babies were likely quadrupedal.”
“The general taphonomical and sedimentary features indicate that Qianlong shouhu might have practiced colonial nesting as a reproductive behavior, similar to other basal sauropodomorphs including Massospondylus and Mussaurus.”
According to the team, Qianlong shouhu had relatively large eggs with a relatively thick calcareous shell formed by prominent mammillary cones compared with other early-diverging sauropodomorph dinosaurs.
“Our results show that Qianlong shouhu possessed eggshell microstructures similar to other Cretaceous dinosaur egg fossils, which likely consisted of two layers — the mammillary layer and continuous layer — and had fully developed eggshell units,” the researchers said.
“The calcareous layer of Qianlong shouhu eggs was much thicker than that of most soft-shelled eggs but thinner than that of hard-shelled eggs.”
“The comparison of eggshell fragmentation among different eggshell types also suggests that the eggshell surface of Qianlong shouhu featured small fragments, similar to a leathery eggshell, in contrast with the folded surface of soft-shelled eggs or the large-fragmented surface of hard-shelled eggs.”
“These observations indicate that Qianlong shouhu laid leathery eggs.”
To test the macroevolutionary patterns of selected reproductive traits across the dinosaur-bird transition, the scientists assembled data from 210 fossil and living species representing all major reptilian clades and tested evolutionary trends using multiple time-scaled phylogenies.
“We found that relative egg size decreased from the base of the Diapsida to that of the Saurischia but displayed an increasing trend from early theropods to the crown bird node.”
“The most significant egg-size increase occurred early in theropod evolution.”
“In terms of eggshell thickness, we discovered that thickness tended to decrease from the base of the archosaur to the base of the Saurischia, followed by a significant increase in eggshell thickness early in theropod evolution. An increasing trend in eggshell thickness also occurred in sauropodomorph evolution.”
“Egg shape was generally conserved in the evolution of diapsids to living birds. For example, although theropod egg elongation reached its peak in oviraptorosaurs — with the greatest egg elongation among diapsids — it would later return to its ancestral state.”
“As a result, only slightly elongated eggs were inherited by all crown bird clades.”
“All in all, reconstruction of the ancestral state of various eggshell types supports the conclusion that the first dinosaur egg was probably leathery, relatively small, and elliptical.”
“Furthermore, a leathery eggshell was probably the ancestral state of Avemetatarsalia, Archosauria and Testudines.”
This research is described in a paper in the journal National Science Review.
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Fenglu Han et al. Exceptional Early Jurassic fossils with leathery eggs shed light on dinosaur reproductive biology. National Science Review, published October 9, 2023; doi: 10.1093/nsr/nwad258
