Perinatal Specimens of Saurolophus angustirostris (Dinosauria: Hadrosauridae), from the Upper Cretaceous of Mongolia

Rare juvenile material constrains estimation of skeletal allometry in Gryposaurus notabilis (Dinosauria: Hadrosauridae)

In studying the skeletal allometry of any vertebrate, it is important to sample the ontogenetic extremes to ensure the accuracy of parameter estimation; this is particularly true for fossil taxa, where sampling of ontogenetic series is incomplete and sporadic. Previous studies have examined allometry in the skull of the duck-billed dinosaur Gryposaurus notabilis, but these did not include individuals smaller than ~65% the maximum known size (based on linear dimensions). Here, we report on the two smallest known examples of this species (a mostly complete skeleton and a partial skull), which are ~37% the known maximal size of G. notabilis. Osteohistology indicates that these represent individuals ~2 years of age. Allometric analysis demonstrates that most aspects of the skull of G. notabilis grew isometrically, although the height of the nasal arch grew with positive allometry. Early in the ontogeny of G. notabilis, the dentary teeth possessed secondary ridges, which were lost later in life. This finding has important bearing on hadrosaurid tooth taxonomy. The limb proportions of G. notabilis largely grew isometrically (or with weak negative allometry, at most), like some other hadrosaurids, suggesting that the species did not undergo a gait shift with increasing age (unless it occurred very early in ontogeny). We argue that the lack of significant locomotory performance compensation exhibited by young hadrosaurids helps to explain why they apparently formed small, mutualistic aggregations, presumably for protection from large predators.

Hadrosauroid eggs and embryos from the Upper Cretaceous (Maastrichtian) of Jiangxi Province, China

BACKGROUND

Dinosaur eggs containing embryos are rare, limiting our understanding of dinosaur development. Recently, a clutch of subspherical dinosaur eggs was discovered while blasting for a construction project in the Upper Cretaceous red beds (Hekou Formation) of the Ganzhou Basin, Jiangxi Province, China. At least two of the eggs contain identifiable hadrosauroid embryos, described here for the first time.

RESULTS

The eggs, attributable to Spheroolithidae indet., are thin-walled and small (~ 660 mL) compared to those of Lambeosaurinae. The shape of the embryonic squamosal is reminiscent of that seen in the Late Cretaceous hadrosauroids Levnesovia transoxiana, Tanius sinensis, and Nanningosaurus dashiensis, suggestive of possible affinities.

CONCLUSION

The small size of the eggs and embryos, similar to those of Hadrosaurinae, indicates that the larger eggs and hatchlings typical of Lambeosaurinae are evolutionarily derived.

High-latitude neonate and perinate ornithopods from the mid-Cretaceous of southeastern Australia

Dinosaurs were remarkably climate-tolerant, thriving from equatorial to polar latitudes. High-paleolatitude eggshells and hatchling material from the Northern Hemisphere confirms that hadrosaurid ornithopods reproduced in polar regions. Similar examples are lacking from Gondwanan landmasses. Here we describe two non-iguanodontian ornithopod femora from the Griman Creek Formation (Cenomanian) in New South Wales, Australia. These incomplete proximal femora represent the first perinatal ornithopods described from Australia, supplementing neonatal and slightly older ‘yearling’ specimens from the Aptian–Albian Eumeralla and Wonthaggi formations in Victoria. While pseudomorphic preservation obviates histological examination, anatomical and size comparisons with Victorian specimens, which underwent previous histological work, support perinatal interpretations for the Griman Creek Formation femora. Estimated femoral lengths (37 mm and 45 mm) and body masses (113–191 g and 140–236 g), together with the limited development of features in the smallest femur, suggest a possible embryonic state. Low body masses (<1 kg for ‘yearlings’ and ~20 kg at maturity) would have precluded small ornithopods from long-distance migration, even as adults, in the Griman Creek, Eumeralla, and Wonthaggi formations. Consequently, these specimens support high-latitudinal breeding in a non-iguanodontian ornithopod in eastern Gondwana during the early Late Cretaceous.

A new hadrosauroid (Dinosauria: Ornithopoda) from the Late Cretaceous Baynshire Formation of the Gobi Desert (Mongolia)

A new genus and species of non-hadrosaurid hadrosauroid, Gobihadros mongoliensis, is described from a virtually complete and undeformed skull and postcranial skeleton, as well as extensive referred material, collected from the Baynshire Formation (Cenomanian-Santonian) of the central and eastern Gobi Desert, Mongolia. Gobihadros mongoliensis is the first non-hadrosaurid hadrosauroid from the Late Cretaceous of central Asia known from a complete, articulated skull and skeleton. The material reveals the skeletal anatomy of a proximate sister taxon to Hadrosauridae in remarkable detail. Gobihadros is similar to Bactrosaurus johnsoni and Gilmoreosaurus mongoliensis, but can be distinguished from them in several autapomorphic traits, including the maximum number (three) of functional dentary teeth per tooth position, a premaxillary oral margin with a ‘double-layer morphology’, and a sigmoidal dorsal outline of the ilium with a well-developed, fan-shaped posterior process. All of these characters in Gobihadros are inferred to be convergent in Hadrosauridae. Phylogenetic analysis positions Gobihadros mongoliensis as a Bactrosaurus-grade hadrosauromorph hadrosauroid. Its relationship with Maastrichtian hadrosaurids from Asia (e.g., Saurolophus angustirostris, Kerberosaurus manakini, Wulagasaurus dongi, Kundurosaurus nagornyi) are sufficiently distant to indicate that these latter taxa owe their distribution to migration from North America across Beringia, rather than having a common Asian origin with Go. mongoliensis.