Dinosaur eggs are common worldwide, but embryos are rare [1]. Among the diverse duck-billed dinosaurs and their nearest relatives (Hadrosauroidea), just three identifiable species are known from perinatal material: Hypacrosaurus stebingeri [2], Maiasaura peeblesorum [3, 4], and Saurolophus angustirostris [5]. Accordingly, the early ontogeny of hadrosauroids is poorly understood, which hinders determination of skeletal development and allometric trends across the clade.
Recently, a construction project in the Upper Cretaceous red beds of the Ganzhou basin, Jiangxi Province, China revealed a fossilized clutch of spheroolithid eggs. In this contribution, we briefly describe two of these eggs and their embryonic contents, accessioned at the Yingliang Stone Natural History Museum (YLSNHM) in Fujian Province, China. The embryos (YLSNHM 01328 and 01373) share several features in common with hadrosauroids, but otherwise lack the more derived features present in embryonic hadrosaurids. The postorbital is distinctive and recalls that of Nanningosaurus dashiensis from Upper Cretaceous red beds elsewhere in southern China [6], suggesting possible affinities. We end this contribution with a consideration of the evolutionary and taphonomic implications of these eggs and their contents.
A comment on taxonomy
Hadrosauridae is traditionally split into two subfamilies, the solid-crested Hadrosaurinae and hollow-crested Lambeosaurinae [7], the former named after the eponymous Hadrosaurus foulkii. However, in a relatively recent phylogenetic analysis, Prieto-Marquez [8] recovered Hadrosaurus as the sister taxon to all other hadrosaurids, and so, abiding by the regulations of the International Code of Zoological Nomenclature, renamed the clade of solid-crested hadrosaurids Saurolophinae. Although this new name has gained some acceptance in the literature (e.g., [9-11], not all current phylogenies recover Hadrosaurus apart from the solid-crested hadrosaurids (e.g., [12, 13]). Therefore, in keeping with tradition and some recent phylogenies, we use the clade name Hadrosaurinae, which is practically synonymous with Saurolophinae, except for the inclusion of Hadrosaurus [14].
Geological provenance
The red beds of Ganzhou area of Jiangxi Province are divided into: (1) the Upper Cretaceous Ganzhou Group consisting of the Maodian and Zhoutian formations, and (2) the Cretaceous-Paleogene Guifeng Group, consisting of the Upper Cretaceous Hekou and Tangbian formations and the Cretaceous-Paleogene Lianhe Formation [15]. Paleomagnetic studies have dated the Guifeng Group to 71.4–65.0 Ma [16, 17]. The eggs and embryos reported here come from the Hekou Formation of the Guifeng Group in Ganzhou area (Figure 1).
The red beds of the Ganzhou area are possibly correlated to the Nanxiong Group of the Nanxiong Basin, Guangdong Province [18]. Within Guangdong Province, the Nanxiong Group is divided from bottom to top into the Cretaceous Changba, Yuanfu, Zhutian and Zhenshui formations [18]. There is little consistency between studies concerning the correlation of the Hekou Formation with the Cretaceous formations of the Nanxiong Group [18, 19]. Most recently, the Hekou Formation has been correlated to the Dafeng and Yuanpu formations of the Lower Nanxiong Group in the study of Zhang et al. [20].
The Hekou Formation varies in thickness between 200 and 1600 m, depending on where it is exposed. It consists of channelized, fining-upward conglomerates, sieve deposits, and alternating sandstone and mudstone beds. The depositional setting is interpreted as primarily fluvially-derived within a proximal alluvial fan system. Mudcracks and caliche deposits are common, and the prevailing palaeoclimate is interpreted as subhumid [21].
The age of the Hekou Formation is debated. Radiometric dating suggests an age range from the Coniacian to Santonian [21]. In contrast, the arguably penecontemporaneous red beds of the nearby Nanxiong Group have been assigned a Maastrichtian age [22]. Indeed, the preserved theropod assemblage, including alioramine tyrannosaurids and oviraptorosaurs [23, 24], agrees well with the dinosaur assemblage of the Nemegt Formation in Mongolia, which is often considered Maastrichtian in age [25]. For this reason, and in keeping with other recent interpretations (e.g., [26, 27]), we accept a correlation with the Nemegt Formation for the Hekou Formation. The formation has thus far yielded fossil algae, plants, dinosaur bones, and trackways, few of which have been formally described [15, 26-30]. Dinosaur eggs assigned to Oolithes sp., Oolithes spheroides [31], “Spheroolithus minor”, Ovaloolithus sp., Paraspheroolithus sp., Macroolithus rugustus, Coelurosauria fam. et gen.indet. [28], and Elongatoolithidae [27] are also known from the Hekou Formation.