Abstract Background The Late Cretaceous Nemegt Formation, Gobi Desert, Mongolia has already yielded abundant and complete skeletons of the hadrosaur Saurolophus angustirostris, from half-grown to adult individuals. Methodology/Principal Findings Herein we describe perinatal specimens of Saurolophus angustirostris, associated with fragmentary eggshell fragments. The skull length of these babies is around 5% that of the largest known S. angustirostris specimens, so these specimens document the earliest development stages of this giant hadrosaur and bridge a large hiatus in our knowledge of the ontogeny of S. angustirostris. Conclusions/Significance The studied specimens are likely part of a nest originally located on a riverbank point bar. The perinatal specimens were buried by sediment carried by the river current presumably during the wet summer season. Perinatal bones already displayed diagnostic characters for Saurolophus angustirostris, including premaxillae with a strongly reflected oral margin and upturned premaxillary body in lateral aspect. The absence of a supracranial crest and unfused halves of the cervical neural arches characterize the earliest stages in the ontogeny of S. angustirostris. The eggshell fragments associated with the perinatal individuals can be referred to the Spheroolithus oogenus and closely resemble those found in older formations (e.g. Barun Goyot Fm in Mongolia) or associated with more basal hadrosauroids (Bactrosaurus-Gilmoreosaurus in the Iren Dabasu Fm, Inner Mongolia, China). This observation suggests that the egg microstructure was similar in basal hadrosauroids and more advanced saurolophines. Competing Interests One of the authors (FE) is employed by the commercial organization Eldonia. Eldonia provided support in the form of a salary for FE, but did not have any additional role or influence in the study design, data collection and analysis, decision to publish, or preparation of the manuscript and it does not alter the authors’ adherence to all the PLoS ONE policies on sharing data and materials.

Citation: Dewaele L, Tsogtbaatar K, Barsbold R, Garcia G, Stein K, Escuillié F, et al. (2015) Perinatal Specimens of Saurolophus angustirostris (Dinosauria: Hadrosauridae), from the Upper Cretaceous of Mongolia. PLoS ONE 10(10): e0138806. https://doi.org/10.1371/journal.pone.0138806 Editor: Andrew A. Farke, Raymond M. Alf Museum of Paleontology, UNITED STATES Received: April 22, 2015; Accepted: September 3, 2015; Published: October 14, 2015 Copyright: © 2015 Dewaele et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited Data Availability: All relevant data are within the paper and its Supporting Information files. Funding: The author(s) received no specific funding for this work. Eldonia provided support in the form of a salary for FE, but did not have any additional role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript. The specific role of this author is articulated in the ‘author contributions’ section. Competing interests: François Escuillié is employed by Eldonia a company specializing in the preparation and casting of fossils. There are no patents, products in development or marketed products to declare. This does not alter the authors' adherence to all the PLOS ONE policies on sharing data and materials, as detailed online in the guide for authors. Abbreviations: MPC, Institute of Paleontology and Geology, Mongolian Academy of Sciences, Ulaan Baatar, Mongolia; PIN, Paleontologiceski Institut, Academii Nauk, Moscow, Russia; RBINS, Royal Belgian Institute of Natural Sciences, Brussels, Belgium; ZPAL, Institute of Paleobiology of the Polish Academy of Sciences, Warsaw, Poland

Introduction The 'Dragon’s Tomb' dinosaur locality was discovered in 1947, in the Nemegt Formation (late Campanian / early Maastrichtian, Late Cretaceous) of the Gobi Desert, by the Russian Palaeontological Expedition to Mongolia’s Gobi Desert, led by I. A. Efremov. The bone bed at this site has yielded numerous articulated skeletons of the giant saurolophine hadrosaurid Saurolophus angustirostris Rozhdestvensky, 1952 [1]. This dinosaur is particularly abundant in the whole Nemegt Formation, comprising approximately 20% of all vertebrate fossils [2] found. The skull lengths of the known S. angustirostris specimens extend from about 437 mm (MgD-1/159) up to 1220 mm (PIN 551/357), thus already covering a wide array of ontogenetic stages, from juveniles to adult individuals. However, embryonic and neonatal remains have not been described to date. Because of the quantity and quality of dinosaur skeletons from Mongolia, the whole of the Nemegt Formation has become a favorite target for poachers, and untold numbers of Saurolophus specimens are now in private hands around the world or have been destroyed in the process of poaching. Here, we describe and discuss specimen MPC-D100/764: an exceptional block of perinatal specimens of Saurolophus angustirostris, with associated eggshell fragments, from the Nemegt Formation. The skull length of these babies is around 5% of that of the largest known S. angustirostris specimens, so they document the earliest development stages of this giant hadrosaur and bridge a large gap in our knowledge of the ontogeny of Saurolophus angustirostris.

Discussion Ontogenetic stage of the perinatal specimens Multiple features indicate that the Saurolophus angustirostris specimens studied in this paper are perinatal individuals, but do not help in determining whether the individuals are still embryonic or neopionic (postembryonic) when they died. The skull length is estimated to be in the order of six centimeters (Fig 3), about five percent of the skull length of the largest known individual of Saurolophus angustirostris (1220 mm in PIN 551/357). It is about the same size as the skull of embryonic specimens in the lambeosaurine Hypacrosaurus stebingeri [10]. Some bones are obviously spongy at larger magnification (Fig 6), indicating that bone growth is still dominated by growth of trabeculae and woven bone. This corresponds with histological observations in the section of the distal femur and dorsal vertebra described above. The predominance of primary trabecular woven bone is indicative of rapid bone growth, a characteristic for embryos, hatchlings and juveniles (e.g., [27, 28]). The complete closure of the notochordal canal may indicate a postembryonic stage (for saurischians, see [44, 45]), however it remains unclear if vertebral development in ornithischian and saurischian dinosaurs followed the same timing and ossification patterns. Therefore, these observations do not aid in the unequivocal differentiation between a late embryonic or hatchling stage for MPC-D100/764, but give a preliminary indication of a postembryonic stage, in the absence of further osteohistological studies of perinatal hadrosaurid vertebrae. The poor preservation of the extremities of the long bones testifies for their poor ossification. Poor ossification of the joints is also observed in many other dinosaurian embryos and (possibly altricial) hatchlings (e.g., [27, 28]). Few vertebral centra are associated with their neural arches and neural spines. This is especially true for the caudal vertebrae of the partially articulated skeleton, where no neural arches are present at all (Fig 9). Thus, contact of the vertebral centra with their associated neural arches might have been very weak or absent at the time death of the individuals of this specimen. However, fusion between the neural centra and their associated neural arches usually occurs relatively late in the ontogeny of archosaurs (see [46]). Unfused halves of neural arches also reveal a very early developmental stage, as it has only been observed in an embryonic Camptosaurus specimen, in dinosaurs [22]. Hence, Saurolophus angustirostris is only the second dinosaur species with perinatal specimens actually showing unfused neural arches. Consequently, the observation of unfused neural arches is strongly supportive of an embryonic stage for MPC-D100/764, because many other hadrosaurids such as Hypacrosaurus [10] and Maiasaura [47] already show fused neural arches while still in the embryonic stage. The presence of a relatively narrow medullary cavity in cross-section with resorption of the cortex indicates that the studied individuals experienced maximal rates of bone growth and remodeling and, hence, that they were in the embryonic or hatchling stage (e.g., [9, 27]). The association of two eggshell fragments with small Saurolophus angustirostris individuals suggests that they belong to the eggs that contained the perinatal individuals. Together with the cratering of many of the cones on the internal surface of the eggshell, this strongly suggests that the eggshell fragments stem from a near-term egg or an egg from a nestling that already hatched [23, 33]. Early ontogeny in Saurolophus angustirostris The specimens in the MPC-D100/764 display several features, widely distributed amongst terrestrial tetrapods and reflecting their earliest ontogenetic stages: a proportionally large skull, large orbits, a proportionally robust postcranium, and unfused neural arches and centra. Ontogenetic changes within Saurolophus angustirostris have already been described by Rozhdestvensky [48], Maryańska and Osmólska [11], and then by Bell [8], on the basis of an ontogenetic series including juveniles, subadults and adults. For the first time, we are now able to reconstruct the morphological changes that took place during the early ontogeny of a saurolophine dinosaur. During development from the perinatal stage to the juvenile stage, the snout became proportionally longer, the orbit became more elongated dorsoventrally and inclined caudodorsally, the doming of the frontal became less prominent, and the coronoid process stood up straighter, perpendicular to the mandibular axis. Those changes continued throughout more advanced ontogenetic stages, from the juvenile to the adult stages ([11]; Table 2). Ontogenetic changes described by Bell [8] on the lateral wall of the braincase could not be adequately observed in the perinatal specimens, due to poor preservation of the braincase. PPT PowerPoint slide

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larger image TIFF original image Download: Table 2. Cranial ontogenetical changes in Saurolophus angustirostris, as listed by Maryanska and Osmolska [ Cranial ontogenetical changes in Saurolophus angustirostris, as listed by Maryanska and Osmolska [ 1 1 ], with incorporation of observations on the hatchling specimens. https://doi.org/10.1371/journal.pone.0138806.t002 Nasal crest structures are obviously absent in the earliest developmental stages in Saurolophus angustirostris, even thought it cannot be completely excluded that taphonomic processes explain this absence. However the crest in the latter stages of Saurolophus is particularly robust [8], so it is unlikely that the crest would have been broken off and removed taphonomically, while more fragile and slender skull bones are still present and particularly well preserved. The supracranial crest is also absent in the youngest lambeosaurine specimens, including Lambeosaurus [49], Hypacrosaurus [10, 16], Corythosaurus [15], and Parasaurolophus [50]. It must be noted that the crest becomes proportionally larger throughout later ontogenetic stages, being particularly small in the juvenile ZPAL MgD-1/159 then progressively larger in subadults (PIN 551/356) and adults (MPC 100/706 and PIN 551/357). However, the ambiguous incipient bifurcation of the frontal into a caudodorsal and a rostroventral process might indicate the absence of a supracranial crest, or the existence of only an indistinctly small crest in the perinatal specimen, similar to the very small rounded nasal crest observed in the juvenile lambeosaurine Kazaklambia convincens [17]. The separation between the left and right halves of the neural arches, present on some cervical vertebrae of the perinatal specimens, has never been documented in hadrosaurids before. In the embryonic development of mammals [51], two neural processes develop around the spinal cord; these neural processes subsequently fuse dorsally to the spinal cord and form a neural arch that grows dorsally to form the neural spine.

Conclusions MPC-D100/764 represents part of a nest of perinatal hadrosaurids. The eggs were originally laid on a point bar along a riverbank. Whether the individuals are still embryonic or neopionic (postembryonic) when they died cannot be accurately determined. The babies were apparently already dead and partly decomposed when they were buried by sediment entrained by the river current during the wet summer season. Coincidence of hatching and the wet summer season has widely been assumed but rarely been observed among hadrosaurids. The babies already displayed diagnostic characters for Saurolophus angustirostris, including premaxillae with strongly reflected oral margin and an upturned premaxillary body in lateral aspect. They represent the earliest ontogenetic stages known for this species and thus bridge a large gap in our knowledge of the ontogeny of S. angustirostris. The absence of a supracranial crest and unfused halves of the cervical neural arches characterize the earliest stages in the ontogeny of S. angustirostris. The eggshell fragments associated with the perinatal individuals can be referred to as the Spheroolithus oogenus and closely resemble those found in older formation (e.g. Barun Goyot Fm in Mongolia) or associated with more basal hadrosauroids (Bactrosaurus-Gilmoreosaurus in the Iren Dabasu Fm, Inner Mongolia, China). This observation suggests that the egg microstructure did not evolve significantly during the last stages of the hadrosauroid evolution.

Acknowledgments The authors thank J. dos Remedios Esteves for the drawings, Th. Hubin and W. Miseur for the photographs. We would like to thank the editor, Andrew A. Farke, and the reviewers, Kirstin S. Brink, Phil Bell and Terry A. Gates, for their helpful comments on earlier versions of this article.

Author Contributions Conceived and designed the experiments: LD GG KS. Performed the experiments: LD GG KS. Analyzed the data: LD KT RB GG KS FE PG. Contributed reagents/materials/analysis tools: LD GG KS. Wrote the paper: LD KT RB GG KS FE PG.