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A comparative embryological study of two ornithischian dinosaurs

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Abstract

Embryonic dinosaur remains are extremely rare1,2 and with the exception of the specimens described here, are so fragmentary that they have yielded little or no information useful for palaeobiological interpretation. We report here the first well-documented association of dinosaur eggs containing embryos, juveniles and adults from the fossil record. The material, attributable to a new hypsilophodontid, Orodromeus makelai, n. gen., n. sp., and the hadrosaurid Maiasaura peeblesorum, comes from the Upper Cretaceous Two Medicine Formation of western Montana3. Osteo-logical and histological studies of these two animals indicate structural and morphological adaptations for their respective growth patterns and neonate behaviour.
... 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. ...
... Lambeosaurine embryos (Hypacrosaurus stebingeri and an indeterminate form), by contrast, have femora that vary between 60 and 80 mm long, and are derived from eggs approaching 4000 mL [2,4]. Horner [4] hypothesized that these differences were typical of their respective subfamilies, and further suggested that the smaller hadrosaurine hatchlings were altricial, based on their poorly ossified epiphyses [3,52]. ...
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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.
... Paleoecologic contributions followed this work, principally based on nests, embryos, and hatchlings from the Cretaceous Two Medicine Formation of Montana that provided insights on dinosaurian paleoecology including hypothesized parental strategies and nesting-ground revisitation (e.g., Horner and Makela, 1979;Horner, 1982;Horner and Weishampel, 1988;Hirsch and Quinn, 1990;Yang et al., 2018). To a certain extent, such approaches may have catalyzed paleohistological investigation of diagenetically well-preserved dinosaur remains from the Two Medicine and Hell Creek formations, which in ensuing years yielded insights into the growth rates, metabolism, diet, sexual dimorphism, and feeding and nesting strategies of dinosaurs and other archosaurs (e.g., Schweitzer and Horner, 1999;Woodward et al., 2015; also see review in Bailleul et al., 2019). ...
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Science is a verb. Its approach, the scientific method, brings us closer to understanding nature through discovery and hypothesis testing. In most deep-time and deep-Earth science, complete understanding is unachievable. Fortunately, we move closer to it with each new generation of scientists, creating frameworks of knowledge that evolve with ever-more-refined observations, interpretations, and hypotheses. The field of paleontology epitomizes this phenomenon, and its evolution is intimately tied to fossils and strata of the Rocky Mountains. Using examples from the American West, the ensuing article highlights how a field anchored in the archaic has a very bright future because paleontology has grown to help our community answer Earth-science questions spanning tectonics, climate, the evolutionary history of life, and beyond.
... He listed several shared features in support of this node, only one of which can be coded for the holotype of P. warreni (flattened lateral surface of the greater trochanter). Boyd (2015: table 1) also defined a stem-based subclade Thescelosaurinae Sternberg, 1937 as comprising all neornithischians more closely related to Thescelosaurus neglectus Gilmore, 1913 than to Orodromeus makelai Horner and Weishampel, 1988 or Parasaurolophus walkeri Parks, 1922. Unfortunately, neither of the two diagnostic features listed by him in support of this grouping can be coded for the holotype of Parksosaurus warreni. ...
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... Two traits that collectively characterize dinosaurs, birds, and other archosaurs are oviparous mode of reproduction and cledoic egg, the latter of which was instrumental in the rise of amniotes 1 . These cledoic eggs are notably observed in the oological records of theropods, sauropodomorphs, and ornithischians more or less from all over the globe [2][3][4][5] , along with the rare occurrences of embryos [6][7][8] . These oological records have helped in analyzing dinosaur reproduction and nesting behavior which fall between the other archosaurs, such as crocodiles and birds. ...
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The Campanian Two Medicine Formation of northwestern Montana, USA, is richly fossiliferous, and discoveries made within the unit over the past century have greatly advanced our appreciation of dinosaur paleobiology and evolution. Previously undifferentiated from a lithostratigraphic perspective, the formation is now subdivided into four new members that include (from base to top) (1) the Rock City Member, (2) the Shields Crossing Member, (3) the Hagans Crossing Member, and (4) the Flag Butte Member. These new formal units and their associated fossil occurrences are also now included in an age model founded on eight high-resolution chemical abrasion−isotope dilution−thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb ages. New age data confirm that the Two Medicine Formation accumulated during much of the Campanian, with deposition spanning ca. 82.4 Ma to 74.4 Ma. New age data further indicate that a major reorganization of depositional systems, marked by a shift from predominantly lacustrine to alluvial facies and accompanied by a dramatic increase in accommodation, transpired near the base of the new Flag Butte Member at ca. 76.3 Ma. This change in depositional regime correlates in age with the Judith River−Belly River discontinuity, which marks the contact between the McClelland Ferry and Coal Ridge Members in the Judith River Formation and coincides with the onset of the Bearpaw transgression in north-central Montana. The new lithostratigraphic and chronostratigraphic framework for the Two Medicine Formation serves to contextualize and calibrate the formation’s rich dinosaur fossil record, which can now be interrogated with increased clarity and precision. These results also provide ground truth for numerical models that explore the structure of the fossil record in relation to alluvial architecture and terrestrial sequence stratigraphy.
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Dinosaur embryos cause a lot of excitement in the scientific literature and are often widely reported because of the general public's interest in dinosaur biology. Well-preserved, articulated oviraptorosaur embryos in eggs are usually interpreted as representing a stage of development close to hatching because of their large size and good level of skeletal ossification. Based on this evidence, a recent report suggested that the position of the one embryo's head was reminiscent of an avian-like hatching position. Here we explore how the developmental stage of well-preserved oviraptorosaur embryos can be estimated, rather than assumed. This will help in our understanding of their developmental biology and its evolutionary consequences. Using quantitative methods and comparison with modern crocodilian embryos, we show that all articulated oviraptorosaur embryos are small relative to the egg and most likely at a stage of development equivalent to around 50%–60% of the developmental period, that is, not even close to hatching. This conclusion is supported by the fact that many elements of the crocodilian skeleton are well ossified many weeks before hatching and the position of oviraptorosaur embryos’ heads was also comparable to a crocodilian embryo many days before hatching. Misunderstandings about the stage of the developmental biology of these well-preserved oviraptorosaur embryos hampers our understanding of the true nature of their reproductive biology. We urge a more conservative approach to their interpretation. This is important, because misunderstandings in the minds of the public about dinosaur biology are hard to counter once poorly evidenced ideas have been reported around the world.
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The avian palaeognath phylogeny has been recently revised significantly due to the advancement of genome-wide comparative analyses and provides the opportunity to trace the evolution of the microstructure and crystallography of modern dinosaur eggshells. Here, eggshells of all major clades of Palaeognathae (including extinct taxa) and selected eggshells of Neognathae and non-avian dinosaurs are analysed with electron backscatter diffraction. Our results show the detailed microstructures and crystallographies of (previously) loosely categorized ostrich-, rhea-, and tinamou-style morphotypes of palaeognath eggshells. All rhea-style eggshell appears homologous, while respective ostrich-style and tinamou-style morphotypes are best interpreted as homoplastic morphologies (independently acquired). Ancestral state reconstruction and parsimony analysis additionally show that rhea-style eggshell represents the ancestral state of palaeognath eggshells both in microstructure and crystallography. The ornithological and palaeontological implications of the current study are not only helpful for the understanding of evolution of modern and extinct dinosaur eggshells, but also aid other disciplines where palaeognath eggshells provide useful archive for comparative contrasts (e.g. palaeoenvironmental reconstructions, geochronology, and zooarchaeology).
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