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The discovery of several exceptionally preserved reproduction-related dinosaur specimens over the past thirty years has advanced our knowledge of dinosaur reproductive biology. Nevertheless, much about dinosaur reproduction remained unclear, due to limited fossil evidence and a lack of quantitative analysis at broad phylogenetic scales, especially pre-Cretaceous evolutionary history.
Now, however, a recent fossil discovery by researchers at the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) of the Chinese Academy of Sciences (CAS), as well as associated analyses, suggests that the first dinosaur egg was leathery and that the great transition in egg morphology occurred early in the evolution of theropod dinosaurs rather than near the origin of birds.
The study, published Oct. 5 in the National Science Review, reports the discovery of specimens of a new Early Jurassic sauropodomorph dinosaur species from the Early Jurassic in Guizhou, China, the Qianlong shouhu, consisting of three skeletons of adult individuals and five egg clutches. This discovery may represent the earliest fossil record of the association between adult dinosaurs and nests, and the species name reflects this association: Qianlong means ‘Guizhou dragon’, while shouhu means ‘guard’ – a reference to the preservation of adult skeletal fossils in context . with embryo-containing egg fossils.
Qianlong was a medium-sized sauropodomorph dinosaur that weighed one ton and was about six meters long. The embryos show some differences from the adults, for example a proportionally longer skull, a more vertical anterior edge of the snout and fewer teeth.
Allometric analyzes of limb proportions between adult and embryonic specimens indicate that adult Qianlong could walk on its hind legs, but the infants were probably quadrupedal. The general taphonomic and sedimentary features indicate that Qianlong may have practiced colonial nesting as a reproductive behavior, similar to other basal sauropodomorphs, including Massospondylus and Mussaurus.
The researchers also examined Qianlong’s eggshell microstructure using multiple techniques, including histological thin sectioning, electron backscatter diffraction, energy-dispersive spectroscopy and scanning electron microscopy. The results showed that Qianlong possessed eggshell microstructures similar to other Cretaceous dinosaur egg fossils, which likely consisted of two layers – the mammillary layer and the continuous layer – and had fully developed eggshell units. The calcareous layer of Qianlong eggs was much thicker than that of most soft-shell eggs, but thinner than that of hard-shell eggs. The comparison of eggshell fragmentation among different types of eggshells also suggests that the eggshell surface of Qianlong contained small fragments, similar to a leathery eggshell, in contrast to the folded surface of soft-shelled eggs or the large fragmented surface of hard-shelled eggs. These observations indicate that Qianlong laid leathery eggs.
To test the macroevolutionary patterns of selected reproductive traits during the transition between dinosaurs and birds, the researchers collected data from 210 fossil and extant species representing all major reptilian clades and tested evolutionary trends using multiple time-scaled phylogenies.
They found that relative egg size decreased from the base of the Diapsida to that of the Saurischia, but showed an increasing trend from early theropods to the crown bird node. The most significant increase in egg size occurred early in theropod evolution. In terms of eggshell thickness, they found 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 the theropod evolution. An increasing trend in eggshell thickness also occurred in the evolution of sauropodomorphs.
The egg shape was generally conserved throughout the evolution of diapsids into living birds. For example, although theropod egg elongation peaked 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 crownbird clades.
Overall, the reconstruction of the ancestral state of several 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.
Fig. 1 Reconstruction of egg nests using fossilized embryos (Image by NICE Vistudio)
Fig. 2 Skeletal reconstructions of adult and juvenile Qianlong (Image by IVPP)
Fig. 3 Comparison of fossil eggs (c) with extant soft-shelled, leathery and hard-shelled eggs (Image by IVPP)
Fig. 4 Summary of ancestral eggshell type for major nodes with the maximum posterior probabilities in all ASR analyzes (Image by IVPP)
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