Thursday, June 20, 2024

[Paleontology • 2024] Lokiceratops rangiformis gen. et sp. nov. (Ceratopsia: Ceratopsidae: Centrosaurinae) from the Campanian Judith River Formation of Montana reveals Rapid Regional Radiations and Extreme Endemism within centrosaurine dinosaurs

Lokiceratops rangiformis
 Loewen​​, Sertich​, Sampson, O’Connor, Carpenter, Sisson, Øhlenschlæger, Farke, Makovicky, Longrich & Evans, 2024

 The Late Cretaceous of western North America supported diverse dinosaur assemblages, though understanding patterns of dinosaur diversity, evolution, and extinction has been historically limited by unequal geographic and temporal sampling. In particular, the existence and extent of faunal endemism along the eastern coastal plain of Laramidia continues to generate debate, and finer scale regional patterns remain elusive. Here, we report a new centrosaurine ceratopsid, Lokiceratops rangiformis, from the lower portion of the McClelland Ferry Member of the Judith River Formation in the Kennedy Coulee region along the Canada-USA border. Dinosaurs from the same small geographic region, and from nearby, stratigraphically equivalent horizons of the lower Oldman Formation in Canada, reveal unprecedented ceratopsid richness, with four sympatric centrosaurine taxa and one chasmosaurine taxon. Phylogenetic results show that Lokiceratops, together with Albertaceratops and Medusaceratops, was part of a clade restricted to a small portion of northern Laramidia approximately 78 million years ago. This group, Albertaceratopsini, was one of multiple centrosaurine clades to undergo geographically restricted radiations, with Nasutuceratopsini restricted to the south and Centrosaurini and Pachyrostra restricted to the north. High regional endemism in centrosaurs is associated with, and may have been driven by, high speciation rates and diversity, with competition between dinosaurs limiting their geographic range. High speciation rates may in turn have been driven in part by sexual selection or latitudinally uneven climatic and floral gradients. The high endemism seen in centrosaurines and other dinosaurs implies that dinosaur diversity is underestimated and contrasts with the large geographic ranges seen in most extant mammalian megafauna.

Skull of Lokiceratops rangiformis n. gen et n. sp. (EMK 0012).
(A) Skull of Lokiceratops rangiformis in dorsal view. (B) Skull reconstruction in anterior view. (C) Skull reconstruction in lateral view interpreted from both sides. Reconstructions are based on 3D surface scans with deformation and parallax removed. The mandible was not found with EMK 0012.
Stippled artwork by Sergey Krasovskiy. Scale bar equals 1 m.

Systematic Paleontology
Dinosauria Owen, 1842; sensu Padian & May, 1993
Ornithischia Seeley, 1887; sensu Sereno, 1998

Ceratopsia Marsh, 1890; sensu Dodson, 1997
Ceratopsidae Marsh, 1888; sensu Sereno, 1998
Centrosaurinae Lambe, 1915; sensu Dodson, Forster & Sampson, 2004

Albertaceratopsini clade nov.
Diagnosis—Albertaceratopsini is defined as a stem-based clade that consists of all taxa more closely related to Albertaceratops nesmoi than to Centrosaurus apertus.

Lokiceratops gen. nov.

Lokiceratops rangiformis gen. et sp. nov.

Diagnosis—Lokiceratops rangiformis is an albertaceratopsin centrosaurine ceratopsid distinguished from other centrosaurines by the following autapomorphies: presence of unadorned nasal; elongate, uncurved ep1 epiossification directed in plane of frill along posterior margin of parietosquamosal frill; hypertrophied, lateral curving epiparietal ep2 directed in plane of frill. The hypertrophied ep2 is relatively larger than any other parietal epiossification within Centrosaurinae. Both ischia are distinctly kinked distally at about two-thirds of the length of the shaft, at the point where the two ischia contact medially. Postorbital horncore bases are deeply excavated by sinuses penetrating a distance equivalent to the orbital diameter of each horncore, to an extent unobserved in other long horned centrosaurs.
Etymology— The generic name refers to the god Loki from Norse mythology, and ceratops, (Greek) meaning “horned face.” The species name refers to the bilateral asymmetry of frill ornamentations, similar to the asymmetry in antlers of the reindeer/caribou genus Rangifer.


Holotype Locality—EMK 0012 was recovered from the Loki Quarry in Kennedy Coulee, south of the Milk River in Hill County, northern Montana (Fig. 1). The quarry is 3.6 km from the Montana-Alberta border and 922 m above sea level. Exact coordinates are available at the Evolutionsmuseet, Knuthenborg, Maribo, Denmark, and the Natural History Museum of Utah, United States of America.

Holotype Horizon—EMK 0012 was recovered from lower Judith River Formation horizons that correlate to the McClelland Ferry Member to the south, and to the lower part of the Oldman Formation of southern Alberta, 3.6 km to the north. EMK 0012 was located 11.4 m above the Marker A Coal equivalent to the Taber Coal Zone (at the top of the Foremost Formation) and laterally equivalent to the Herronton Sandstone Zone near the base of the Oldman Formation 3.6 km to the north in Alberta.

Age—High-precision U–Pb analyses of zircons by the CA-ID-TIMS method in a bentonite within the Marker A Coal (KC061517-1; 11.4 m below the Loki Quarry) date to 78.549 ± 0.024 Ma (Ramezani et al., 2022). Using the median Bayesian age model developed by Ramezani et al. (2022) for the stratigraphic position of the Loki Quarry recovers a date of roughly 78.1 Ma, with a lower modeled bound of 78.38 Ma and an upper modeled bound of 77.18 Ma.


Mark A. Loewen​​, Joseph J. W. Sertich​, Scott Sampson, Jingmai K. O’Connor, Savhannah Carpenter, Brock Sisson, Anna Øhlenschlæger, Andrew A. Farke, Peter J. Makovicky, Nick Longrich and David C. Evans. 2024. Lokiceratops rangiformis gen. et sp. nov. (Ceratopsidae: Centrosaurinae) from the Campanian Judith River Formation of Montana reveals Rapid Regional Radiations and Extreme Endemism within centrosaurine dinosaurs. PeerJ. 12:e17224. DOI: 10.7717/peerj.17224