The unearthing of numerous remarkably well-preserved dinosaur specimens tied to reproduction within the past thirty years has significantly advanced our understanding of dinosaur reproductive biology.
Despite this progress, a lack of extensive fossil evidence and comprehensive quantitative analyses on a broad evolutionary scale has left much about dinosaur reproduction obscure, particularly regarding the pre-Cretaceous evolutionary history.
However, a recent fossil discovery by researchers from the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) of the Chinese Academy of Sciences (CAS) and subsequent analyses now suggest that the initial dinosaur egg had a leathery structure. Moreover, the pivotal shift in egg morphology occurred early in the evolution of theropod dinosaurs rather than closer to the emergence of birds.
Published in National Science Review on Oct. 5, the study reveals the unearthing of specimens from a new early Jurassic sauropodomorph dinosaur species in Guizhou, China—Qianlong shouhu—consisting of three adult skeletons and five egg clutches. This discovery potentially marks the earliest fossil evidence of the link between adult dinosaurs and nests. Reflecting this association, the species name ‘Qianlong’ means “Guizhou dragon,” while ‘shouhu’ signifies “guarding,” referencing the preservation of adult skeletal fossils alongside embryo-containing egg fossils.
Qianlong, a medium-sized basal sauropodomorph dinosaur, weighed approximately one ton and measured about six meters in length. The embryos exhibited certain discrepancies from the adults, such as a proportionally elongated skull, a more vertically inclined anterior snout margin, and a reduced number of teeth.
Analyzing the limb ratios between adult and embryonic specimens indicated that while adult Qianlong likely ambulated on its hind limbs, the juveniles were probably quadrupedal. Based on general taphonomical and sedimentary features, it’s plausible that Qianlong partook in colonial nesting behavior for reproduction, akin to other basal sauropodomorphs like Massospondylus and Mussaurus.
The researchers scrutinized the eggshell microstructure of Qianlong using various methods, including histological thin-sectioning, electron backscatter diffraction, energy-dispersive spectroscopy, and scanning electron microscopy. Their findings revealed that Qianlong possessed eggshell microstructures akin to Cretaceous dinosaur egg fossils.
These eggshells likely comprised two layers—the mammillary layer and continuous layer—comprising fully developed eggshell units. The calcareous layer of Qianlong eggs appeared thicker than that of most soft-shelled eggs but thinner than hard-shelled eggs. Comparing eggshell fragmentation among various eggshell types indicated that Qianlong’s eggshell surface resembled small fragments, resembling a leathery eggshell, in contrast to the folded surface of soft-shelled eggs or the larger fragmented surface of hard-shelled eggs. These observations strongly suggest that Qianlong laid leathery eggs.
To scrutinize the evolutionary patterns of selected reproductive traits spanning the dinosaur-to-bird transition, the researchers compiled data from 210 fossil and extant species, encompassing major reptilian clades. They tested evolutionary trends using multiple time-scaled phylogenies.
Their analysis revealed that relative egg size decreased from the Diapsida base to the Saurischia base but showcased an increasing trend from early theropods to the crown bird node. The most significant egg-size augmentation occurred early in theropod evolution. Regarding eggshell thickness, they observed a tendency towards decreased thickness from the archosaur base to the Saurischia base, followed by a marked increase in eggshell thickness early in theropod evolution. An escalation in eggshell thickness was also noted in sauropodomorph evolution.
While egg shape remained mostly consistent from diapsid evolution to modern birds, theropod egg elongation reached its pinnacle in oviraptorosaurs, displaying the most extensive egg elongation among diapsids before reverting to its ancestral state. As a result, all crown bird clades inherited slightly elongated eggs.
In conclusion, the reconstruction of ancestral eggshell types supports the inference that the first dinosaur egg was likely leathery, relatively small, and elliptical. Furthermore, it suggests that a leathery eggshell was potentially the ancestral state of Avemetatarsalia, Archosauria, and Testudines.
Header Image Credit : NICE Vistudio