A Ceratopsian Dinosaur from the Lower Cretaceous of Western North America, and the Biogeography of Neoceratopsia

The fossil record for neoceratopsian (horned) dinosaurs in the Lower Cretaceous of North America primarily comprises isolated teeth and postcrania of limited taxonomic resolution, hampering previous efforts to reconstruct the early evolution of this group in North America. An associated cranium and lower jaw from the Cloverly Formation (?middle–late Albian, between 104 and 109 million years old) of southern Montana is designated as the holotype for Aquilops americanus gen. et sp. nov. Aquilops americanus is distinguished by several autapomorphies, including a strongly hooked rostral bone with a midline boss and an elongate and sharply pointed antorbital fossa. The skull in the only known specimen is comparatively small, measuring 84 mm between the tips of the rostral and jugal. The taxon is interpreted as a basal neoceratopsian closely related to Early Cretaceous Asian taxa, such as Liaoceratops and Auroraceratops. Biogeographically, A. americanus probably originated via a dispersal from Asia into North America; the exact route of this dispersal is ambiguous, although a Beringian rather than European route seems more likely in light of the absence of ceratopsians in the Early Cretaceous of Europe. Other amniote clades show similar biogeographic patterns, supporting an intercontinental migratory event between Asia and North America during the late Early Cretaceous. The temporal and geographic distribution of Upper Cretaceous neoceratopsians (leptoceratopsids and ceratopsoids) suggests at least intermittent connections between North America and Asia through the early Late Cretaceous, likely followed by an interval of isolation and finally reconnection during the latest Cretaceous.


Contents
Text S1. Character list used for phylogenetic analysis. Text S2. Codings for phylogenetic analysis, in TNT format. See also File S2. Text S3. Codings for phylogenetic analysis, in TNT format, with ontogeny-dependent characters of Aquilops scored as unknown. See also File S3. Figure S1. Most parsimonious tree, recovering Ajkaceratops as most basal neoceratopsian, with synapomorphy list. Figure S2. Most parsimonious tree, recovering Ajkaceratops as sister to Bagaceratops, with synapomorphy list. Figure S3. Bootstrap and Bremer support values for phylogenetic hypothesis. Figure S4. Strict consensus of 18 equally parsimonious trees, with selected ontogenetic-variant characters for Aquilops americanus scored as unknown, assuming that the holotype is a juvenile. Figure S5. Strict consensus of 18 equally parsimonious trees, with selected ontogenetic-variant characters for Aquilops americanus scored as unknown, assuming that the holotype is a juvenile. In this version of the tree, the wildcard taxon Ajkaceratops has been pruned post hoc. Figure S6. Phylogenetic tree recovering Ajkaceratops as sister to Bagaceratops, used for DEC modeling, with nodal numbers referenced in results (Table S1). Table S1. Temporal calibrations used for DEC analysis. Figure S7. Summary figure of DEC modeling results. Table S2. Node-by-node results for DEC analysis.
Ectopterygoid exposed in palatal view (0) or reduced and concealed in palatal view (1).
Dorsal edge of squamosal temporal bar curves medially at the posterior end, arcing confluently into posterior frill margin (0) or dorsal edge of squamosal meets posterior margin of frill at acute angle (1). 54.
Basioccipital participates in foramen magnum (0) or basioccipital is excluded from foramen magnum and exoccipitals form less than onethird of condyle (1) or exoccipitals form about half or more of occipital condyle (2). 58.

61.
Notch between posteroventral edge of basisphenoid and base of basipterygoid process deep (0) or notch shallow and base of basipterygoid process close to basioccipital tubera (1).
Predentary less than two-thirds of dentary length (0) or equal to or more than two-thirds of dentary length (1).
Tip of dentary smooth (0) or grooved dorsally for reception of the lateral process of the predentary (1) or bears large pit for reception of the lateral process of the predentary (2). 73.

77.
Prominent medial expansion of the central mandible in the middle of the tooth row formed by wide Meckelian groove separating tooth-bearing part of the jaw from external surface absent (0) or present (1).
Posterior end of splenial simple or with shallow dent (0) or with bifid overlap of angular (1). 81.
Coronoid process positioned close to main axis of dentary and posterior to tooth row (0) or set lateral to tooth row, and end of tooth row covered by anterior part of coronoid process (1) or tooth row level with posterior edge of coronoid process (2). 83.
Coronoid straplike and with subequal depth throughout (0) or with lobate, highly expanded dorsal end much deeper than ventral end that slots between splenial and dentary (1).
Lateral surface of surangular flat or only weakly convex (0) or with pronounced laterally convex curvature (in the transverse plane) between the coronoid process and glenoid region (1). 86.
Angular without one or more small, lateral tubercles along ventral rim below glenoid articulation (0) or tubercles present (1).

88.
Lateral surface of angular flat or slightly convex (0) or angular bears a raised emargination along posteroventral margin of mandible, lateral surface distinctly concave (1).
Mandibular glenoid narrow and flush with medial margin of surangular flange in dorsal view (0) or glenoid region medially expanded and forming lingual process in dorsal view (1). 91.
Teeth occlude at an oblique angle (0) or at a vertical angle (1) or at a vertical angle, but dentary teeth have a horizontal shelf on the labial face (2). 98.
Teeth without distinct median primary ridge (0) or with very weak and wide median ridge on at least some maxillary teeth (1) or all maxillary and dentary teeth with distinct primary ridge (2). 99.