Bizarre tail weaponry in a transitional ankylosaur from subantarctic Chile

Abstract

Armoured dinosaurs are well known for forms that evolved specialized tail weapons: paired tail spikes in stegosaurs, and heavy tail clubs in advanced ankylosaurs 1 . Armoured dinosaurs from southern Gondwana are rare and enigmatic, but likely include the earliest branches of Ankylosauria 2-4 . Here, we describe a mostly complete, semiarticulated skeleton of a small (~2m) armoured dinosaur from the late Cretaceous of Magallanes in southernmost Chile, a region biogeographically related to West Antarctica 5 . Stegouros elengassen gen. et sp. nov. evolved a large tail weapon unlike any dinosaur: A flat, frond-like structure formed by 7 pairs of laterally projecting osteoderms encasing the distal half of the tail. Stegouros shows ankylosaurian cranial characters, but a largely primitive postcranial skeleton, with some stegosaur-like characters. Phylogenetic analyses placed Stegouros in

New Ankylosaurian Cranial Remains From the Lower Cretaceous (Upper Albian) Toolebuc Formation of Queensland, Australia

“…The only current exception is the Argentinian ankylosaur, a nodosaurid (Figure 8) that appeared in Gondwana following dispersal events from Laramidia during the Campanian-Maastrichtian, a pattern also observed among hadrosaurids and titanosaurs (Brett-Surman, 1979;Coria and Salgado, 2001;Prieto-Márquez, 2010;Arbour and Currie, 2016;Ibiricu et al, 2021). Given the mid-upper Albian ages of Australian ankylosaurs, the Gondwanan dispersal of Parankylosauria considerably predated those of the latest Cretaceous (Arbour and Currie, 2016;Kubo, 2020;Soto-Acuña et al, 2021). Interestingly, the recent identification of the putative ankylsoaur Spicomellus afer from the mid-Jurassic of Morocco (Maidment et al, 2021) hints at an initial, more ancient global diversification of Ankylosauria (Gibbons et al, 2013;Arbour and Currie, 2016;Maidment et al, 2021;Soto-Acuña et al, 2021).…”

“…Previous studies hypothesized that Australian ankylosaurians evolved independently from the Late Cretaceous ankylosaurs found elsewhere in Gondwana (i.e., A. oliveroi and the Argentinian ankylosaur), which were previously considered to be more closely related to Laurasian nodosaurids (Figure 8A; Coria and Salgado, 2001;Salgado and Gasparini, 2006;Arbour and Currie, 2016;. However, the proposed existence of Parankylosauria suggests that most Gondwanan ankylosaurs are more closely related to each other than to those found elsewhere (Soto-Acuña et al, 2021). The only current exception is the Argentinian ankylosaur, a nodosaurid (Figure 8) that appeared in Gondwana following dispersal events from Laramidia during the Campanian-Maastrichtian, a pattern also observed among hadrosaurids and titanosaurs (Brett-Surman, 1979;Coria and Salgado, 2001;Prieto-Márquez, 2010;Arbour and Currie, 2016;Ibiricu et al, 2021).…”

“…By contrast, SAMA P40536 and K. ieversi exhibit a derived condition whereby the choanae are relatively posterior within the palate. The other parankylosaurian Stegouros ellengassen may also exhibit this derived condition, as the secondary palate that marks the anterior extent of the choanae extends posteriorly to approximately the mid-point of the preserved tooth row (Soto-Acuña et al, 2021). Unfortunately, the maxillary (Molnar, 1980;Bell et al, 2018a), Eromanga Basin (Molnar, 1996;Leahey and Salisbury, 2013;Leahey et al, 2015), Gippsland and Otway Basins (Barrett et al, 2010), and Canning Basin (Salisbury et al, 2016) tooth row is incomplete posteriorly and so we cannot confirm its condition at this time, and it was coded as "?".…”

“…However, K. ieversi is now considered an earlybranching ankylosaurian outside of both Ankylosauridae and Nodosauridae (Arbour and Currie, 2016). Recently, the discovery of Stegouros ellengassen introduced a new phylogenetic hypothesis, whereby K. ieversi, S. ellengassen, and Antarctopelta oliveroi form a Gondwanan ankylosaur clade, Parankylosauria, which would have diverged before the Ankylosauridae + Nodosauridae split (Soto-Acuña et al, 2021). In testing the phylogenetic position of SAMA P40536, we were unable to reproduce the same tree from Soto-Acuña et al ( 2021), even when the aforementioned specimen was removed.…”

“…Ankylosaurians are a rare component of Gondwanan Cretaceous terrestrial ecosystems with five described species: Antarctopelta oliveroi, Minmi paravertebra, Kunbarrasaurus ieversi, Spicomellus afer, and Stegouros elengassen (Molnar, 1980;Salgado and Gasparini, 2006;Leahey et al, 2015;Maidment et al, 2021;Soto-Acuña et al, 2021). A. oliveroi and M. paravertebra have been considered nomen dubia due to a perceived lack of autapomorphies (Leahey et al, 2015;Arbour and Currie, 2016).…”