A new southern Laramidian ankylosaurid, Akainacephalus johnsoni gen. et sp. nov., from the upper Campanian Kaiparowits Formation of southern Utah, USA.

Jelle P Wiersma,Randall B Irmis

doi:10.7717/peerj.5016

Jelle P Wiersma, Randall B Irmis

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https://doi.org/10.7717/peerj.5016

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Abstract

A partial ankylosaurid skeleton from the upper Campanian Kaiparowits Formation of southern Utah is recognized as a new taxon, Akainacephalus johnsoni, gen. et sp. nov. The new taxon documents the first record of an associated ankylosaurid skull and postcranial skeleton from the Kaiparowits Formation. Preserved material includes a complete skull, much of the vertebral column, including a complete tail club, a nearly complete synsacrum, several fore- and hind limb elements, and a suite of postcranial osteoderms, making Akainacephalus johnsoni the most complete ankylosaurid from the Late Cretaceous of southern Laramidia. Arrangement and morphology of cranial ornamentation in Akainacephalus johnsoni is strikingly similar to Nodocephalosaurus kirtlandensis and some Asian ankylosaurids (e.g., Saichania chulsanensis, Pinacosaurus grangeri, and Minotaurasaurus ramachandrani); the cranium is densely ornamented with symmetrically arranged and distinctly raised ossified caputegulae which are predominantly distributed across the dorsal and dorsolateral regions of the nasals, frontals, and orbitals. Cranial caputegulae display smooth surface textures with minor pitting and possess a distinct conical to pyramidal morphology which terminates in a sharp apex. Character analysis suggests a close phylogenetic relationship with N. kirtlandensis, M. ramachandrani, Tarchia teresae, and S. chulsanensis, rather than with Late Cretaceous northern Laramidian ankylosaurids (e.g., Euoplocephalus tutus, Anodontosaurus lambei, and Ankylosaurus magniventris). These new data are consistent with evidence for distinct northern and southern biogeographic provinces in Laramidia during the late Campanian. The addition of this new ankylosaurid taxon from southern Utah enhances our understanding of ankylosaurid diversity and evolutionary relationships. Potential implications for the geographical distribution of Late Cretaceous ankylosaurid dinosaurs throughout the Western Interior suggest multiple time-transgressive biogeographic dispersal events from Asia into Laramidia.

Highlights

A suite of unique anatomical features including the characteristic sculpturing of cranial ornamentation and the unusually wide supraorbital horns, indicate that Akainacephalus johnsoni is distinct from other ankylosaurid specimen reported from the Kaiparowits Basin, including UMNH VP 21000, UMNH VP 19472, and UMNH VP 19473, as well as Nodocephalosaurus kirtlandensis (Sullivan, 1999), Ahshislepelta minor (Burns & Sullivan, 2011a), and Ziapelta sanjuanensis (Arbour et al, 2014) from the stratigraphically younger Kirtland Formation of the San Juan Basin in northwestern New Mexico
Akainacephalus johnsoni and Nodocephalosaurus kirtlandensis (SMP VP-900) do share several similar anatomical features, such as the morphology of the cranial osteoderms and lateral orientation of the external nares (Fig. 8), suggesting a close taxonomic relationship (Fig. 26B). Both taxa are temporally separated by nearly three million years; Akainacephalus johnsoni is recorded from the lower portion of the middle unit of the Kaiparowits Formation, which is late Campanian in age (76.26 ± 0.10 Ma (Roberts et al, 2013)), whereas Nodocephalosaurus kirtlandensis is from the upper-most Campanianlower-most Maastrichtian (73.04 ± 0.25 Ma (Fassett & Steiner, 1997); 73.49 ± 0.25 Ma (Fowler, 2017)) De-na-zin Member of the Kirtland Formation
Intraspecific variation in cranial ornamentation is present in ankylosaurid taxa, for example, Euoplocephalus tutus, Anodontosaurus lambei, Pinacosaurus grangeri (Penkalski, 2001; Arbour & Currie, 2013a), the size proportions and morphology of the quadratojugal horns between Akainacephalus johnsoni and Nodocephalosaurus kirtlandensis (SMP VP-900) (Figs. 8 and 9A–9D) differ significantly and are regarded as taxonomically distinct

Summary

Introduction

The Ankylosauridae is a monophyletic clade of herbivorous, armored ornithischian dinosaurs that are predominantly recorded from the Late Cretaceous (Turonian—late Maastrichtian) of Asia and latest Cretaceous (early Campanian—late Maastrichtian) of western North America (Laramidia) (Lambe, 1902; Brown, 1908; Parks, 1924; Nopcsa, 1928; Sternberg, 1929; Gilmore, 1933; Maryanska, 1977; Sullivan, 1999; Carpenter, 2004; Miles & Miles, 2009; Arbour, Burns & Sissons, 2009; Arbour & Currie, 2013a, 2016; Arbour, Currie & Badamgarav, 2014; Arbour et al, 2014; Penkalski, 2014; Arbour, Zanno & Gates, 2016; Penkalski & Tumanova, 2017; Arbour & Evans, 2017). Several ankylosaurid specimens (UMNH VP 19472, UMNH VP 19473, UMNH VP 21000, UMNH VP 20202) have been recorded from the upper Campanian Kaiparowits Formation of southern Utah (Loewen et al, 2013a; Wiersma & Irmis, 2013) but UMNH VP 20202 is the first newly-described ankylosaurid taxon from the Late Cretaceous of Utah Despite these recent discoveries from New Mexico and Utah, Late Cretaceous southern Laramidian ankylosaurid specimens remain rare from upper Campanian terrestrial deposits of the Kaiparowits, Kirtland, and Fruitland formations, and the majority of taxa are represented by a single specimen

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