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The osteology of the holotype and referred specimens of the saurolophine hadrosaurid, Gryposaurus latidens, from the lower Two Medicine Formation of Montana (USA), is described in detail. With an estimated early Campanian (Late Cretaceous) age, this dinosaur is one of the oldest known hadrosaurids. Gryposaurus latidens shows several autapomorphies, including dentary tooth crowns, with an apicobasal length/mesiodistal width ratio of 1.7-2.2, a single median carina, and large marginal denticles in sections of the dental battery; deep and oval depression on the posteroventral premaxillary surface; and, in the context of Hadrosauridae, a shallow lacrimal process of the rostral process of the jugal. Furthermore, G. latidens differs from G. notabilis and G. monumentensis in having a broadly arcuate anterolateral oral margin of the premaxilla, a posterior margin of the narial fenestra that is wider than the dorsoventral depth of the proximal region of the narial bar, with a nasal crest that rises above the level of the frontals (in adults) and is located above the posterior margin of the narial fenestra, and wide and shallow posteroventral margin of the rostral process of the jugal, among other characters. This study confirmed a previous hypothesis supporting G. latidens as the sister taxon to a clade composed of the other species of Gryposaurus. L'osteologie de l'holotype et des specimens affectes de l'hadrausoride saurolophine Gryposaurus latidens, de la partie inferieure de la Formation de Two Medicine au Montana (Etats-Unis), est decrite en detail. Ce dinosaure, dont l'age estime est du Campanien precoce (Cretace tardif), est l'un des plus anciens hadrosaurides connus. G. latidens presente plusieurs autapomorphies, dont des couronnes des dents dentaires ayant un rapport longueur apicobasale/largeur mesiodistale allant de 1,7 a 2,2, une seule carene mediane et de grands denticules marginaux dans certains segments de la batterie dentaire, une profonde depression ovale sur la surface premaxillaire posteroventrale et, dans le contexte des Hadrosaurides, un processus lacrymal peu profond du processus rostral de l'os jugal. G. latidens se distingue en outre de G. notabilis et G. monumentensis par une bordure posterieure de la fenetre nariale plus large que la profondeur dorsoventrale de la region proximale de la barre nariale, avec une crete nasale s' elevant au-dessus du niveau des os frontaux (chez les adultes) et situee au-dessus de la bordure posterieure de la fenetre nariale, et une bordure posteroventrale large et peu profonde du processus rostral du jugal, entre autres caracteristiques. L'etude confirme l'hypothese etablie anterieurement voulant que G. latidens soit le taxon frere d' un clade compose des autres especes de Gryposaurus. [Traduit par la Redaction], Introduction Hadrosaurid dinosaurs were among the most diverse and abundant large terrestrial herbivores of the Late Cretaceous (Lull and Wright 1942; Horner et al. 2004). This is evidenced by a [...]

Huehuecanauhtlus tiquichensis gen. et sp. nov. is the southernmost diagnostic basal hadrosauroid in the Americas. The holotype and referred material of this taxon came from Santonian strata in the Michoacan State, southwestern Mexico. Huehuecanauhtlus tiquichensis is diagnosed on the basis of a combination of dental, axial, and appendicular characters, including the following: at least two teeth exposed on the occlusal plane of the dentary and maxilla; seven sacral vertebrae; tall neural spines of caudal vertebrae; supraacetabular process long; and short and trapezoidal (in lateral view) postacetabular process. It differs from other hadrosauroids in having an ilium with extreme ventral deflection of the preacetabular process. Maximum parsimony cladistic analysis placed H. tiquichensis as a closely related outgroup to Hadrosauridae. The occurrence of H. tiquichensis in the Santonian of North America may be explained as a dispersal event from Asia to North America that occurred no later than the Albian or, alternatively, as a vicariant event of a most recent common ancestor widespread in both landmasses. Huehuecanauhtlus tiquichensis gen. et sp. nov. est l'hadrosauroide basal diagnostique le plus meridional des Ameriques. L'holotype et le materiel de reference de ce taxon proviennent de strates santoniennes de l'Etat du Michoacan, dans le sud-ouest du Mexique. Le diagnostic de H. tiquichensis repose sur une combinaison de caracteres dentaires, axiaux et appendiculaires dont les suivants : au moins deux dents exposees sur le plan occlusal du dentaire et du maxillaire, sept vertebres sacrales, les longues neurepines des vertebres caudales, un long processus supraacetabulaire et un processus postacetabulaire court et trapezoidal (en coupe laterale). Le taxon se distingue des autres hadrosauroides par la presence d'un ilium presentant une deflection ventrale extreme du processus preacetabulaire. L'analyse cladistique par la methode du maximum de parcimonie situe H. tiquichensis en tant qu'exogroupe etroitement relie aux Hadrosaurides. La presence de H. tiquichensis dans le Santonien de l'Amerique du Nord pourrait s'expliquer par un evenement d'expansion de l'Asie a l'Amerique du Nord s'etant produit au plus tard a l'Albien ou par la vicariance d'un des plus recents ancetres communs repandus dans les deux masses continentales. [Traduit par la Redaction], Introduction Hadrosauroids constitute a Late Cretaceous group of derived ornithopod dinosaurs with an extensive fossil record in Europe, Asia, both Americas, and the Antarctica (Lull and Wright 1942; Weishampel et [...]

Perinatal specimens of hadrosaurids discovered in the late 1970’s by field crews from Princeton University were significant in providing evidence of the early ontogenetic stages in North American dinosaurs. These specimens from the Campanian (Upper Cretaceous) Two Medicine Formation of Montana consist of over a dozen skeletons referable to the saurolophine hadrosaurid Maiasaura peeblesorum, but never fully figured or described. Here, we provide a more complete documentation of the morphology of these specimens, along with an examination of variation during a large span of the development of saurolophine hadrosaurids. Many ontogenetic changes in the available facial and mandibular elements are associated with the progressive elongation of the preorbital region of the skull and mandible. In the postcranium, limb bones change nearly isometrically, with exception of certain elements of the forelimb. Some cranial and postcranial characters commonly used for inferring hadrosaurid phylogenetic relationships remain invariable during the ontogeny of M. peeblesorum. This indicates that early ontogenetic stages may still provide a limited amount of character information useful for systematics and phylogenetic inference. [ABSTRACT FROM AUTHOR]