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| Coahuilasaurus lipani Longrich, Velasco, Kirkland, Torres & Serrano-Brañas, 2024 Life reconstruction by C. Díaz Frías |
Abstract
The Late Cretaceous of Western North America (Laramidia) supported a diverse dinosaur fauna, with duckbilled dinosaurs (Hadrosauridae) being among the most speciose and abundant members of this assemblage. Historically, collecting and preservational biases have meant that dinosaurs from Mexico and the American Southwest are poorly known compared to those of the northern Great Plains. However, evidence increasingly suggests that distinct species and clades inhabited southern Laramidia. Here, a new kritosaurin hadrosaurid, represented by the anterior part of a skull, is reported from the late Campanian of the Cerro del Pueblo Formation, ~72.5 Ma, in Coahuila, Mexico. The Cerro del Pueblo Formation kritosaur was originally considered to represent the same species as a saurolophine from the Olmos Formation of Sabinas, but the Sabinas hadrosaur is now considered a distinct taxon. More recently, the Cerro del Pueblo Formation kritosaur has been referred to Kritosaurus navajovius. We show it represents a new species related to Gryposaurus. The new species is distinguished by its large size, the shape of the premaxillary nasal process, the strongly downturned dentary, and massive denticles on the premaxilla’s palatal surface, supporting recognition of a new taxon, Coahuilasaurus lipani. The dinosaur assemblage of the Cerro del Pueblo Formation shows higher diversity than the contemporaneous fauna of the Horseshoe Canyon Formation in Alberta. Furthermore, Kritosaurini, Lambeosaurini, and Parasaurolophini all persist into the latest Campanian in southern Laramidia after disappearing from northern Laramidia. These patterns suggest declining herbivore diversity seen at high latitudes may be a local, rather than global phenomenon, perhaps driven by cooling at high latitudes in the Late Campanian and Maastrichtian.
Keywords: Dinosauria; Hadrosauridae; Saurolophinae; Kritosaurini; Cretaceous; Campanian; Laramidia; biogeography
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| IGM 6685, holotype Coahuilasaurus lipani. (A), rostrum with posterior skull reconstructed in left lateral view; (B), scaled silhouette. The animal is estimated to have been around 8 m in length. |
Dinosauria Owen 1842
Ornithopoda Marsh 1881
Hadrosauridae Cope 1869
Saurolophinae Brown 1914 (sensu Prieto-Márquez, 2010)
Kritosaurini Brett-Surman 1989 (sensu Prieto-Márquez, 2014)
Coahuilasaurus lipani gen. et sp. nov.
Etymology. Coahuila in reference to Coahuila State + Greek sauros, ‘lizard’; lipani references the Lipani Apache tribe of Coahuila.
Diagnosis. Large kritosaurin with an estimated length of ~8 m (Figure 5). Distinguished by the following unique character combination (* = autapomorphies). The anterior margin of premaxillae is strongly convex in lateral view; the anterodorsal margin of the narial process is subtly sinusoidal in lateral view, being concave anteriorly then weakly convex posteriorly *. The narial aperture is narrow and slot-like and strongly extended anteroventrally *. The premaxillary labrum is deep anteriorly, becoming shallower posteriorly. The posteroventral corner of the premaxillary beak is strongly angular, with the anteroventral margin and posteroventral margin forming approximately a right angle in lateral view *; the posteroventral margin of the premaxilla is strongly concave in lateral view. The premaxilla is short and broad in dorsal view. The oral margin of the beak bears a series of 5–6 small denticles. The palatal surface of the premaxilla forms a horseshoe-shaped ridge projecting far below the oral margin *; the ridge bears three pairs of massive, diamond-shaped denticles*. The nasal–premaxillary contact is posteriorly positioned, probably near the end of the nasal aperture. Very large narial flange of premaxilla *. The predentary oral margin is strongly concave in the lateral view and has a steeply inclined rostroventral margin, and the tip is hooked up strongly in lateral view (approached in Gryposaurus monumentensis). There is a narrow U-shaped notch between predentary lateral processes and ventral processes (shared with Gryposaurus monumentensis); the ventral processes are long and slender. The edentulous end of the dentary is strongly downturned, with a strongly convex anterodorsal margin, and a long ventral contact between the dentaries.
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| Map showing localities of kritosaurins from the Late Cretaceous of western North America (Laramidia). 1,2, Gryposaurus notabilis (incl. G. incurvimanus) Dinosaur Park Formation, Alberta; 3, Gryposaurus latidens, Two Medicine Formation, Montana; 4, Gryposaurus sp., Bearpaw Shale, Montana; 5, Kritosaurini indet., North Horn Formation, Utah; 6, Rhinorex condrupus, Neslen Formation, Utah; 7, Gryposaurus monumentensis, Kaiparowits Formation, Utah; 8, Anazisaurus horneri, Farmington Formation of the Kirtland Formation of New Mexico; 9, Naashoibitosaurus ostromi, De-Na-Zin Member of the Kirtland Formation of New Mexico; 10, Kritosaurus navajovius, De-Na-Zin Member of the Kirtland Formation of New Mexico; 11, Naashoibito kritosaurin, Naashobito Member of the Kirtland Formation of New Mexico; 12, Gryposaurus (?) alsatei, upper Javelina Formation, Texas; 13, cf. Kritosaurus, UTEP P.37.7, basal Javelina Formation of Texas; Aguja Formation, Texas; 14, Aguja Fm. Kritosaur, AMNH 3079; 15, Coahuilasaurus lipani, Cerro del Pueblo Formation, Coahuila. Map by Ron Blakey. |
Conclusions:
A partial skull from the latest Campanian of the Cerro del Pueblo Formation of Coahuila, Mexico, represents a new species of kritosaurin, Coahuilasaurus lipani (Figure 20). Along with specimens from the Naashoibito Member of the Kirtland Formation in New Mexico, C. lipani documents the persistence of kritosaurins in the south after local extinction in northern Laramidia, a pattern also seen in Parasaurolophini and Lambeosaurini.
Southern faunas not only have distinct species, but different patterns of diversity change, compared to northern Laramidian faunas. Differences in species composition and community structure in different parts of Laramidia mean that further work on the dinosaurs of Mexico and the American Southwest is needed to understand the evolution of dinosaur diversity in western North America. Southern dinosaur faunas may have been characterized by both higher origination rates and lower extinction rates than northern faunas.
Nicholas R. Longrich, Angel Alejandro Ramirez Velasco, Jim Kirkland, Andrés Eduardo Bermúdez Torres and Claudia Inés Serrano-Brañas. 2024. Coahuilasaurus lipani, a New Kritosaurin Hadrosaurid from the Upper Campanian Cerro Del Pueblo Formation, Northern Mexico. Diversity. 16(9), 531. DOI: doi.org/10.3390/d16090531
