[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

DOI: 10.7717/peerj.17224

Abstract
 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

[Paleontology • 2022] Bisticeratops froeseorum • A New Chasmosaurine Ceratopsid from the Upper Cretaceous (Campanian) Farmington Member of the Kirtland Formation, New Mexico

Bisticeratops froeseorum

 Dalman, Jasinski & Lucas, 2022

Researchgate.net/publication/362680358

 deviantart.com/atrox1

A nearly complete skull of a new ceratopsid dinosaur, Bisticeratops froeseorum, is described from the Farmington Member of the Kirtland Formation (late Campanian, Upper Cretaceous) of New Mexico. Bisticeratops is distinguished by several diagnostic cranial characters, including those of the premaxilla (stepped dorsal margin), maxilla (short jugal process lacking the ventral pocket-like fossa), jugal (short maxillary process of the jugal), and palpebral ornamentation (short with moderate ornamentation). It differs from other known chasmosaurines, especially from stratigraphically older species in the same geographic region, Pentaceratops sternbergi and Titanoceratops ouranos, by a strongly reduced jugal process of the maxilla and an unusual maxilla/jugal contact, which forms a shallow, triangular-shaped lateroposteriorly concave sulcus. A phylogenetic analysis recovers Bisticeratops froeseorum as sister species to the unnamed Almond Formation chasmosaurine and as a member of a potentially new southern clade of chasmosaurines, outside the Triceratopsini, and distinct from other southern Laramidian chasmosaurines such as Pentaceratops. The dinosaur fauna of the Farmington Member is comparatively poorly understood, especially compared to stratigraphically older faunas in the San Juan Basin. Therefore, the new, presumably rare species Bisticeratops froeseorum, together with several recently named and described chasmosaurines such as Navajoceratops sullivani, Sierraceratops turneri, and Terminocavus sealeyi, add to the diversity and disparity of chasmosaurines and provides further support for latitudinal variation in the ceratopsid fauna during the Late Cretaceous interval in the Western Interior Basin of North America.

Sebastian Dalman, Steven E. Jasinski and Spencer G. Lucas. 2022. A New Chasmosaurine Ceratopsid from the Upper Cretaceous (Campanian) Farmington Member of the Kirtland Formation, New Mexico. New Mexico Museum of Natural History and Science Bulletin. 90: 127–153. 

Researchgate.net/publication/362680358_A_NEW_CERATOPSID_FROM_THE_UPPER_CRETACEOUS_OF_THE_KIRTLAND_FORMATION_NEW_MEXICO

 deviantart.com/atrox1/art/Bisticeratops-froeseorum-926150418

twitter.com/TheDarknix/status/1559635224165695492

  

[Paleontology • 2021] Sierraceratops turneri • A New Chasmosaurine Ceratopsid (Dinosauria: Ceratopsidae) from the Hall Lake Formation (Upper Cretaceous) of south-central New Mexico

Sierraceratops turneri 

Dalman, Lucas, Jasinski & Longrich, 2021

DOI: 10.1016/j.cretres.2021.105034

Abstract

The horned dinosaurs (Ceratopsidae) were a diverse family of herbivorous dinosaurs originating in the Late Cretaceous in western North America (Laramidia). As one of the most species-rich dinosaur groups, their diversity and distribution are important to understanding Cretaceous dinosaur evolution. Ceratopsids have previously been hypothesized to have high levels of endemism despite inhabiting a relatively small land mass with few barriers to dispersal. Here, we document a new chasmosaurine ceratopsid, Sierraceratops turneri gen. et sp. nov., from the Upper Cretaceous (latest Campanian–Maastrichtian) Hall Lake Formation of south-central New Mexico, consistent with the hypothesis that southern Laramidia supported an endemic dinosaur fauna. Sierraceratops is distinguished by its relatively short, robust, and mediolaterally compressed postorbital horns; a flattened medial ridge on the posterior end of the pterygoid; a jugal with pronounced anterior flanges; a long pyramid-shaped epijugal horncore; a D-shaped cross section of the median parietal bar; and a squamosal with a pointed tip and low episquamosal ossifications. Phylogenetic analysis recovers Sierraceratops as sister to Bravoceratops and Coahuilaceratops, part of a clade endemic to the southwestern United States and Mexico. Sierraceratops adds to the diversity and disparity of the Chasmosaurinae in the Late Cretaceous and provides additional evidence for Laramidian endemism. Together with Sierraceratops, the Hall Lake Formation dinosaur fauna suggests that the latest Cretaceous of southern Laramidia was characterized by endemic clades and distinct community structures.

Keywords: Dinosauria, Ceratopsidae, Chasmosaurinae, Hall Lake Formation, New Mexico

    

Sierraceratops turneri gen. et sp. nov.

 Sebastian G. Dalman, Spencer G. Lucas, Steven E. Jasinski and Nicholas R. Longrich. 2021. Sierraceratops turneri, A New Chasmosaurine Ceratopsid from the Hall Lake Formation (Upper Cretaceous) of south-central New Mexico. Cretaceous Research. In Press, 105034. DOI: 10.1016/j.cretres.2021.105034

 media.newmexicoculture.org/press_releases.php?action=detail&releaseID=1357

 krqe.com/news/new-mexico/name-given-to-horned-dinosaur-discovered-in-southern-new-mexico

[Paleontology • 2021] Menefeeceratops sealeyi • The Oldest Centrosaurine: A New Ceratopsid Dinosaur (Dinosauria: Ceratopsidae) from the Allison Member of the Menefee Formation (Upper Cretaceous, early Campanian), northwestern New Mexico, USA

Menefeeceratops sealeyi 

Dalman, Lucas, Jasinski, Lichtig & Dodson, 2021

DOI: 10.1007/s12542-021-00555-w

Artwork by S. Krasovskiy.

  twitter.com/StevenJasinski

 

Abstract

An associated incomplete skeleton of a ceratopsid dinosaur from the Campanian deposits of the Allison Member of the Menefee Formation in New Mexico, USA is described. Although it was originally described over two decades ago, newly prepared portions of the Menefee Formation skeleton and reinterpretations of previously known morphology, in addition to newly described specimens have provided new information on ceratopsids, and centrosaurines in particular. These new data allow for a thorough reassessment of the specimen and the erection of a new taxon: Menefeeceratops sealeyi gen. et sp. nov., potentially the oldest recognized member of Centrosaurinae. Menefeeceratops sealeyi is represented by diagnostic cranial and postcranial skeletal elements. The cranial elements include a portion of the left premaxilla, a nearly complete left postorbital horncore, a parietal fragment, the right and left squamosals, the left jugal, the predentary, and the left dentary. Postcranial material consists of two cervical vertebrae, eight dorsal vertebrae, a partial sacrum with six sacral vertebrae, 11 dorsal ribs, the distal left radius, proximal and distal portions of the left ulna, the left femur, and a left metatarsal II. The taxonomic validity of Menefeeceratops sealeyi is supported by a combination of several morphological characters. These include a lack of epiossifications on the lateroposterior edge of the parietal (shared with Machairoceratops), three epiossifications on the squamosal, and three smaller, secondary undulations as part of episquamosal locus S1. There are also two subequal embayments on the posterior free margin of the squamosal with the more dorsal embayment (between episquamosal loci 1 and 2) distinctly larger than the ventral (= lateroventral) one (between episquamosal loci 2 and 3), three ridges on the lateral (dorsolateral) surface of the squamosal, an elongate posterior portion of the squamosal, the presence of a shallow but distinct groove on the medial surface of the squamosal nearly paralleling the ventrolateral and ventroposterior edges, elongate postorbital (= supraorbital) horns that are anteriorly curved distally, and two elongate ridges on the lateral surface of the dentary that diverge anteriorly, creating a distinct anterior triangular fossa. Phylogenetic analysis of Menefeeceratops sealeyi places this new species as a basal centrosaurine, most closely related to Crittendenceratops krzyzanowskii, thus adding to the growing record of centrosaurines discovered in western North America. It thus provides new information about the diversity of morphologies throughout different species and the temporal and paleobiogeographic distribution of these animals throughout Laramidia during the Late Cretaceous. Its presence as one of the, if not the, oldest members of the Centrosaurinae also suggests centrosaurines originated in the southern portions of western North America and the southern Rocky Mountain region, and subsequently radiated north during the upper middle to late Campanian.

 

Keywords: Dinosauria, Ceratopsidae, Centrosaurinae, Late Cretaceous, New Mexico, North America, Taxonomy, Evolution

Systematic paleontology

Ornithischia Seeley, 1887

Ceratopsia Marsh, 1890

Neoceratopsia Sereno, 1986

Ceratopsidae Marsh, 1888

Centrosaurinae Lambe, 1915

Menefeeceratops gen. nov.

Type species. Menefeeceratops sealeyi gen. et sp. nov.

Menefeeceratops sealeyi gen. et sp. nov.

 

Material. Holotype. NMMNH P-25052, a fragmentary skeleton consisting of the following elements: a left partial premaxilla, the nearly complete left postorbital horncore, a partial right squamosal, the left squamosal, an incomplete left parietal, the left jugal, the predentary, the left dentary, one cervical vertebra, eight dorsal vertebrae, a partial sacrum consisting of six sacral vertebrae, 11 dorsal ribs, the left ilium, the distal left radius, the proximal and distal portions of left ulna, the left metatarsal II, the left femur, and the distal end left fibula.

The holotype of Menefeeceratops sealeyi was collected under a permit issued by the U.S. Bureau of Land Management to the New Mexico Museum of Natural History and Science, Albuquerque, New Mexico.

Etymology. The generic epithet includes “Menefee” in reference to the Menefee Formation in which the type specimen was found. The Greek suffix “ceratops” (= horn-face) denotes membership in Ceratopsidae, which includes the new species. The specific epithet honors Paul Sealey, who discovered the type specimen.

Life reconstruction of Menefeeceratops sealeyi gen. et sp. nov.

(artwork by S. Krasovskiy)

Conclusions: 

Menefeeceratops sealeyi is a new genus and species that represents the oldest current record of centrosaurine ceratopsid in North America (Fig. 24). It is basal to Centrosaurini and Pachyrhinosaurini and potentially is a member of the “Nasutoceratopsini”. Recognition of Menefeeceratops sealeyi adds to the growing species diversity of the Centrosaurinae in North America, and its presence in southern Laramidia fills in part of the evolutionary gap between the earlier Zuniceratops christopheri (~ 90 Ma) and the more derived ceratopsids known from the late Campanian in North America. The phylogenetic analysis suggests that derived members of the Centrosaurinae evolved rather quickly and lived at the same time as basal members. The presence of the oldest members of the Centrosaurinae in southern Laramidia suggests the clade evolved in the south and radiated north through the late Campanian. This is further corroborated by the presence of Campanian strata (~ 80 Ma) to the north (Alberta and Montana) that currently lacks evidence of centrosaurines. More material is needed, particularly of the parietal, to determine a more confident phylogenetic position of M. sealeyi and other potential “nasutoceratopsins” at, or near, the base of the Centrosaurinae.

   

Sebastian G. Dalman, Spencer G. Lucas, Steven E. Jasinski, Asher J. Lichtig and Peter Dodson. 2021.  The Oldest Centrosaurine: A New Ceratopsid Dinosaur (Dinosauria: Ceratopsidae) from the Allison Member of the Menefee Formation (Upper Cretaceous, early Campanian), northwestern New Mexico, USA. PalZ. DOI: 10.1007/s12542-021-00555-w

  twitter.com/StevenJasinski/status/1391831457195335680

[Paleontology • 2020] Navajoceratops sullivani & Terminocavus sealeyi • Transitional Evolutionary Forms in Chasmosaurine Ceratopsid Dinosaurs: Evidence from the Campanian of New Mexico

Navajoceratops sullivani & Terminocavus sealeyi

Fowler & Fowler, 2020

 DOI: 10.7717/peerj.9251 

Illustration: Robert Boessenecker
dickinsonmuseumcenter.com

Abstract 

Three new chasmosaurines from the Kirtland Formation (~75.0–73.4 Ma), New Mexico, form morphological and stratigraphic intermediates between Pentaceratops (~74.7–75 Ma, Fruitland Formation, New Mexico) and Anchiceratops (~72–71 Ma, Horseshoe Canyon Formation, Alberta). The new specimens exhibit gradual enclosure of the parietal embayment that characterizes Pentaceratops, providing support for the phylogenetic hypothesis that Pentaceratops and Anchiceratops are closely related. This stepwise change of morphologic characters observed in chasmosaurine taxa that do not overlap stratigraphically is supportive of evolution by anagenesis. Recently published hypotheses that place Pentaceratops and Anchiceratops into separate clades are not supported. This phylogenetic relationship demonstrates unrestricted movement of large-bodied taxa between hitherto purported northern and southern provinces in the late Campanian, weakening support for the hypothesis of extreme faunal provincialism in the Late Cretaceous Western Interior.

Subtle differences in the frill shape distinguish between different species of chasmosaurine ceratopsids.

Illustration: Robert Boessenecker

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

CHASMOSAURINAE Lambe, 1915, sensu Dodson, Forster & Sampson, 2004

Pentaceratops sternbergii (Osborn, 1923)

Type specimen - AMNH 6325 (Osborn, 1923), nearly complete skull, missing the mandible and the posterior half of the parietal and squamosals.

Referred specimens - AMNH 1624, nearly complete skull, missing mandible and the medial part of the parietal; AMNH 1625, nearly complete frill, missing anterior end of the parietal and right squamosal, and most of the left squamosal. Referred to as cf. Pentaceratops sternbergii.

Locality and Stratigraphy - AMNH 6325, 1624, and 1625 were all collected by C.H. Sternberg in 1922 and 1923 from the Fruitland Formation, San Juan Basin, New Mexico (Figs. 1 and 2; see Supporting Information 1 for discussion).

Figure 4: Navajoceratops sullivani holotype SMP VP-1500 parietal.
Dorsal (A) and ventral (B) views. Cross section of median bar (mb) illustrated on dorsal view. Ep1 mostly removed during extraction or preparation (see Fig. S4 for original extent). em, median embayment of the posterior bar; ep, epiparietal loci numbered by hypothesized position (no epiossifications are fused to this specimen); f, parietal fenestra; L-lr/R-lr, Left/Right lateral rami of the posterior bar; te, tapering lateral edges of the median bar.

Scalebar = 10 cm. Reconstruction adapted from Lehman (1998).

Navajoceratops sullivani gen. et sp. nov.

Etymology - Navajoceratops, “Navajo horned face”, after the Navajo people indigenous to the San Juan Basin; sullivani, after Dr. Robert M. Sullivan, leader of the SMP expeditions to the San Juan Basin that recovered the holotype.

Holotype - SMP VP-1500; parietal, squamosal fragments, fused jugal-epijugal, other unidentified cranial fragments. Collected in 2002 by Robert M. Sullivan, Denver W. Fowler, Justin A. Spielmann, and Arjan Boere.

Diagnosis - Can be distinguished from aff. Pentaceratops n. sp. by the following characters: Lateral rami of the parietal posterior bar meet medially at a more acute angle (~60°, rather than 87 or 88°; KUVP 16100, MNA Pl.1747, respectively). Median embayment of the parietal posterior bar especially deep, extending anterior to the posteriormost extent of the parietal fenestrae (which consequently overlap anteroposteriorly slightly with ep2).

Figure 5: Terminocavus sealeyi holotype NMMNH P-27468 parietal.
Dorsal (A) and ventral (B) views. Paired ep1 are deflected dorsally. em, median embayment of the posterior bar; ep, epiparietal loci numbered by hypothesized position (no epiossifications are fused to this specimen); f, parietal fenestra; lb, lateral bar; L-lr/R-lr, Left/Right lateral rami of the posterior bar; mb, median bar; te, tapering lateral edges of the median bar.

Scalebar = 10 cm. Reconstruction adapted from Lehman (1998).

Terminocavus sealeyi gen. et sp. nov.

Etymology - Terminocavus, “coming to the end of (or “last stop for”, as in a train terminus) the cavity” after the nearly-closed parietal embayment; sealeyi after Paul Sealey who discovered the holotype specimen.

Holotype - NMMNH P-27468; parietal, partial squamosal, jugal, epijugal, partial quadratojugal, partial sacrum, vertebral fragments. Collected in 1997 by Paul Sealey.

Diagnosis - Differs from Navajoceratops holotype SMP VP-1500 by the following characters: Posterior bar flattened and plate-like (i.e., not bar-like). Lateral rami of the parietal posterior bar strongly expanded anteroposteriorly both medially and laterally. Maximum anteroposterior thickness of the posterior bar ~35% of the parietal maximum width (compared with <30% in Navajoceratops and ~19–30% in aff. Pentaceratops n. sp.). Median embayment of the posterior bar narrower and more notch-like. Parietal fenestrae subrounded rather than subangular.

.....

Conclusions: 

Description of the new taxa Navajoceratops sullivani and Terminocavus sealeyi, and the fragmentary Taxon C, provides critical stratigraphic and morphologic links between the Campanian Pentaceratops, and the Maastrichtian Anchiceratops, reinstating the phylogenetic hypothesis originally postulated by Lehman (1993, 1998). Combined with significant revision of other chasmosaurine taxa, this reveals a deep split of the Chasmosaurinae into Chasmosaurus and Pentaceratops clades. Morphological divergence from similar basal forms suggests the clades diverged from a common ancestor probably in the early Campanian.

Analysis of paleogeographic maps suggest that high sea level in the Santonian through to middle Campanian may have acted as an agent of vicariance, separating an ancestral chasmosaurine population into northern and southern subpopulations which over time led to divergence and speciation. This lends support to recent hypotheses of latitudinally arrayed differences in terrestrial faunal composition (e.g., Lehman, 1987, 1997, 2001), but stops short of supporting basinal-level endemism in the middle to late Campanian (e.g., Sampson et al., 2010).

Description of the new material places San Juan Basin chasmosaurines as among the best documented of their clade, second only to Triceratops in number of specimens and quality of accompanying data.

Although this work presents significant revision of many chasmosaurine taxa, much reanalysis and redescription remains. Inclusion of more recently described taxa and separation of problematic taxa and specimens (see Supporting Information 1) will be attempted in forthcoming manuscripts based on Fry (2015) and Fowler & Freedman Fowler (2017).

Denver W. Fowler and Elizabeth A. Freedman Fowler. 2020. Transitional Evolutionary Forms in Chasmosaurine Ceratopsid Dinosaurs: Evidence from the Campanian of New Mexico. PeerJ. 8:e9251. DOI: 10.7717/peerj.9251

TWO NEW TRANSITIONAL SPECIES FILL IN MISSING LINKS IN THE EVOLUTIONARY HISTORY OF HORNED DINOSAURS

http://dickinsonmuseumcenter.com/badlands_research-2020newceratopsids/

[Paleontology • 2020] Stellasaurus ancellae • A New, Transitional Centrosaurine Ceratopsid from the Upper Cretaceous Two Medicine Formation of Montana and the Evolution of the ‘Styracosaurus-line' Dinosaurs

 Stellasaurus ancellae

Wilson, Ryan & Evans, 2020

DOI: 10.1098/rsos.200284 

twitter.com/DrMichaelJRyan

Illustration: Andrey Atuchin

Abstract

Ceratopsids are among the most ubiquitous dinosaur taxa from the Late Cretaceous terrestrial formations of the Western Interior of North America, comprising two subfamilies, Chasmosaurinae and Centrosaurinae. The Two Medicine Formation of northwestern Montana has produced numerous remains of centrosaurine dinosaurs, which represent three taxa previously considered valid: Rubeosaurus ovatus, Einiosaurus procurvicornis and Achelousaurus horneri. Here, we reassess the previous referral of specimens to Rubeousaurus ovatus and demonstrate that this taxon is represented solely by its holotype specimen, which was first diagnosed as Styracosaurus ovatus. One of the specimens previously referred to ‘Rubeosaurus’ ovatus instead represents a new eucentrosauran centrosaurine taxon diagnosed here, Stellasaurus ancellae gen. et sp. nov. Stellasaurus expresses a unique combination of eucentrosauran centrosaurine characters, including an elongate nasal horncore, diminutive supraorbital horncores, and a parietal bearing straight, elongate P3 processes, semi-elongate P4 processes and non-elongate P5, P6 and P7 processes. Within the stratigraphic succession of Eucentrosaura, Stellasaurus occurs intermediate to Styracosaurus albertensis and Einiosaurus, and likewise reflects intermediate morphology. Assessed within the stratigraphic, geographical, taphonomic, ontogenetic and phylogenetic framework of Unified Frames of Reference, we fail to reject the hypothesis that Stellasaurus ancellae represents a transitional taxon within an anagenetic lineage of eucentrosauran centrosaurines.

Keywords: centrosaurine, evolution, ceratopsid, dinosaur, anagenesis, Cretaceous

Figure 4. Left lateral parietal bar of Stellasaurus ancellae holotype MOR 492 in dorsal and ventral views.

EPS, epiparietosquamosal; PSC, parietosquamosal contact. Scale bar 10 cm. 

Parietal line drawing modified from Evans & Ryan [2015], Public Library of Science (PLoS), used under Creative Commons Attribution 4.0.

 Systematic palaeontology

Dinosauria Owen, 1842

Ornithischia Seeley 1887

Ceratopsia Marsh, 1890

Neoceratopsia Sereno, 1986

Ceratopsidae Marsh, 1888

Centrosaurinae Lambe, 1915

Stellasaurus gen. nov.

Stellasaurus ancellae gen et. sp. nov.

Etymology: The genus name Stellasaurus, ‘star lizard', is derived from Stella, Latin for star, and –saurus, Greek for lizard, in reference to the overall star-like appearance of the cranial ornamentation, and in homage to the song ‘Starman' by David Bowie. The species name ancellae honours Museum of the Rockies field palaeontologist and fossil preparator Carrie Ancell, who discovered and prepared MOR 492, the holotype specimen of Stellasaurus ancellae, as well as the holotype of Achelousaurus horneri, and co-discovered the holotype of Einiosaurus procurvicornis, and whose decades of extraordinary fossil preparation have furthered vertebrate palaeontology beyond measure.

Holotype: MOR 492, an isolated partial skull of a centrosaurine ceratopsid preserving the left lateral parietal bar, proximal portion of the midline parietal bar, near-complete paired and fused nasals, partial left premaxilla, and partial left postorbital and associated supraorbital ornamentation.

Locality, horizon and age: MOR 492 was recovered from Lithofacies 5 of the uppermost Two Medicine Formation, 65 m below the upper contact between the Two Medicine Formation and the overlying Bearpaw Formation. ....

Diagnosis: Centrosaurine ceratopsid exhibiting a unique combination of characters intermediate in distribution between the stratigraphically preceding Styracosaurus albertensis and stratigraphically successive Einiosaurus procurvicornis: elongate, erect and recurved nasal horncore and diminutive supraorbital ornamentation with pointed apex, as in Styracosaurus albertensis; parietal with elongate, straight P3 processes (spikes), partially elongate P4 processes (spikes) less than half as long as P3, and non-elongate P5, P6 and P7 processes, as in Einiosaurus procurvicornis; unique P4 elongation, intermediate between that of Styracosaurus albertensis and the stratigraphically successive lowest bonebed of Einiosaurus. As stated above, elongate, erect nasal horncores are variably present in Centrosaurus apertus, though rarely with the same recurvature and degree of hypertrophy as in both Styracosaurus albertensis and Stellasaurus. Einiosaurus procurvicornis specimens of equivalent maturity (e.g. MOR 456 8-9-6-1; young adult, based on degree of P7 and P6 parietal process imbrication and bone surface texture) express strongly procurved nasal horns and rounded masses of bone as supraorbital ornamentation, differing significantly from the facial ornamentation seen in Stellasaurus.

Figure 11. Stratigraphic and temporal relationship of the centrosaurine taxa hypothesized as representing an anagenetic lineage, with the evolution of their parietal ornamentation, within their respective geological formations. Letters are colour coded by taxon to match transformation of parietal ornamentation listed. (a) Styracosaurus albertensis, (b) Stellasaurus ancellae, (c) and (c') Einiosaurus procurvicornis MOR 373 and MOR 456, respectively, indicating slight stratigraphic separation with associated morphological change of the P4 process, (d) Achelousaurus horneri, (e) Pachyrhinosaurus lakustai. Generalized stratigraphic columns are based on [Fowler, 2017].

Relative taxon placement is based on stratigraphic occurrence of specimens within their respective geological formations and the temporal relationships of host strata based on [Fowler, 2017] (see Material and methods and Discussion). Styracosaurus ovatus is omitted due to its uncertain stratigraphic placement within the Two Medicine Formation. Parietal line drawings used and modified from Evans and Ryan ([2015] fig. 15), Public Library of Science (PLoS), used under Creative Commons Attribution 4.0.

Figure 12. Comparison of the two major evolutionary mode hypotheses for the Two Medicine Formation centrosaurines and the associated lines of evidence supporting or absent. Parietal line drawings modified from Evans & Ryan ([2015] fig. 15), Public Library of Science (PLoS), used under Creative Commons Attribution 4.0.

John P. Wilson, Michael J. Ryan and David C. Evans. 2020. A New, Transitional Centrosaurine Ceratopsid from the Upper Cretaceous Two Medicine Formation of Montana and the Evolution of the ‘Styracosaurus-line' Dinosaurs. Royal Society Open Science. DOI: 10.1098/rsos.200284

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