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URL: https://pubmed.ncbi.nlm.nih.gov/25494182/

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

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Abstract

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.

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Conflict of interest statement

Andrew A. Farke has read the journal's policy and the authors of this manuscript have the following competing interests: Andrew A. Farke is a volunteer section editor and academic editor for PLOS ONE. This does not alter the authors' adherence to PLOS ONE policies on sharing data and materials.

Figures

👁 Figure 1
Figure 1. Summary phylogeny of Ceratopsia with nomenclatural conventions used in this paper.
Marginocephalia and Coronosauria are node-based clades, indicated by circles. The rest of the clades shown here are stem-based, indicated by half-brackets. See text for definitions and taxonomic authorities. Silhouettes are not to scale (Acrotholus by G. Monger, Psittacosaurus by J. Headden, Triceratops by R. Amos; all others by A. Farke; all images are CC-BY and provided via ).
👁 Figure 2
Figure 2. Location map for the holotype of Aquilops americanus within the contiguous United States of America.
Outcrops of Cloverly Formation within the Bighorn Basin of Montana and Wyoming are shown in orange; the actual width of the outcrop band has been exaggerated for visual clarity. The approximate location of OMNH locality V1057, within Carbon County, Montana, is indicated by the blue arrow.
👁 Figure 3
Figure 3. Skull of Aquilops americanus, OMNH 34557 (holotype).
Partial cranium in A) dorsal and B) right lateral views. Partial lower jaw in C) right lateral and D) dorsal views. This interpretive figure is based on surface scans of the original specimen, with sutures highlighted. The lower jaw is reversed, to facilitate placement with the skull. Abbreviations: aof, antorbital fossa; d, dentary; ecp, ectopterygoid; fr, frontal; itf, infratemporal fenestra; j, jugal; la, lacrimal; m, maxilla; na, nasal; pal, palpebral; pd, predentary; pf, prefrontal; pm, premaxilla; po, postorbital; r, rostral; spl, sutural surface for splenial; sq, squamosal; stf, supratemporal fenestra.
👁 Figure 4
Figure 4. Cranium of Aquilops americanus, OMNH 34557 (holotype).
A) right lateral and B) left lateral views.
👁 Figure 5
Figure 5. Cranium of Aquilops americanus, OMNH 34557 (holotype).
A) dorsal and B) ventral views. The rostral end of the skull is to the right side of the image.
👁 Figure 6
Figure 6. Cranial reconstruction and life restoration of Aquilops americanus.
Cranium in A) right lateral and B) dorsal views; C) life restoration in right lateral view. The rendering is based on OMNH 34557 (holotype), with missing details patterned after Liaoceratops yanzigouensis and Archaeoceratops oshimai. Life restoration by Brian Engh.
👁 Figure 7
Figure 7. Lower jaw of Aquilops americanus, OMNH 34557 (holotype).
Predentary and left dentary in A) medial; B) ventral; C) dorsal; and D) lateral views. The three fragments, although unattached, were placed into articulation for these photos. The rostral direction is to the right in A and to the left in B–D.
👁 Figure 8
Figure 8. Dentition of Aquilops americanus, OMNH 34557 (holotype).
A) Upper right dentition in buccal view. B) Lower left dentition in lingual view. C) Detail of maxillary tooth in buccal view, showing selected features. The photographs shown here were taken of casts coated with ammonium chloride, to even out color variations in the specimen. Abbreviations: ar, accessory ridge; cing, cingulum; pr, primary ridge.
👁 Figure 9
Figure 9. Occlusal (lingual) surface of seventh right maxillary tooth in Aquilops americanus, OMNH 34557 (holotype).
Note that enamel (highlighted by arrows in the inset) only occurs on the labial surface, and does not continue to the buccal extremity of the tooth at the bottom of the image. The discoloration adhering to the tooth at the top of the image (lingual side of the tooth) is sediment impregnated with consolidant resin, not enamel.
👁 Figure 10
Figure 10. Hypothesis of phylogeny and biogeography for Neoceratopsia.
Some terminal taxa have been combined for space considerations, and the range bars for each taxon indicate uncertainty rather than known geological ranges. Continent icons indicate the ancestral areas reconstructed by DEC modeling. Silhouettes depict representative members of major clades and grades (Psittacosaurus by J. Headden, Zuniceratops by N. Tamura and modified by T. M. Keesey; Triceratops by R. Amos; all others by A. Farke; all images are CC-BY and provided via ). Full results are presented in File S1.

References

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