Evolution of polacanthid dinosaurs and description of a new skeleton from the Late Jurassic of Portugal

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EVOLUTION OF POLACANTHID DINOSAURS AND DESCRIPTION OF A NEW SKELETON FROM THE UPPER JURASSIC OF PORTUGAL

Publication . Russo, João Paulo Vasconcelos Mendes; Mateus, Octávio

Ankylosaurs are one of the most iconic groups of dinosaurs. Their most conspicuous characters are the widespread dermal ossification, heavily ornamented, hyperossified skull, coossification of posterior dorsal and anterior caudal vertebrae with the sacrum into a synsacrum, and in some, fusion of the posterior half of the tail, forming the recognizable tail club. Specimens have been identified worldwide, dating at least from the Middle Jurassic to the latest Cretaceous. The phylogenetic relationships within the group have always been a matter of debate, particularly of non-ankylosaurid ankylosaurs, and remain poorly understood, especially due to a poor pre-Cretaceous record. The best-preserved specimens come from the Late Jurassic Morrison Formation, USA, as well as the Lourinhã formation, Portugal. The Portuguese record has until recently been restricted to the poorly known Dracopelta zbyszewskii from the upper Tithonian, known from a partial, articulated ribcage and an autopodium. This study reports and describes a second ankylosaur specimen, mostly complete and articulated, from the uppermost Tithonian of the Lourinhã formation, in the coastal cliffs one kilometre North from the beach of Porto da Calada, Mafra, about 40 km North of Lisbon, Portugal. It consists of axial, appendicular, and dermal armour skeletal elements: nearly complete skull, left mandible, complete articulated cervical, dorsal, and sacral vertebral series, as well as 13 anterior caudal vertebrae, ribs, pectoral and partial pelvic girdles, right humerus, both femora, and dermal armour, thus making it the most complete ankylosaur from the Jurassic. Furthermore, the holotype of D. zbyszewskii was redescribed, including hitherto unknown elements of the appendicular skeleton, such as a partial right hindlimb, composed of the distal end of the femur, tibia, fibula, and articulated autopodium, which is herein reidentified as a right pes. Also, its type locality and discovery history were established. Comparisons of both specimens allowed to conclude that D. zbyszewskii is now known from two specimens, and is herein rediagnosed, based on a unique combination of characters, ten of which are autapomorphic. A Maximum Parsimony analysis was performed to assess the phylogenetic position of D. zbyszewskii, using a new dataset (330 characters, 95 taxa). The analysis recovered four major clades within Ankylosauria, Ankylosauridae, Nodosauridae, Struthiosauridae, and Polacanthidae, occurring together with a large polytomy formed of both traditionally considered earlier and later diverging taxa. Also, Scelidosaurus harrisonii is the earliest diverging ankylosaur, placing the origin of Ankylosauria in the Early Jurassic. Moreover, polacanthids are the earliest diverging group of ankylosaurs, appearing as early as the Late Jurassic. D. zbyszewskii is recovered as an early branching polacanthid, and is the sister taxon of Gargoyleosaurus parkpinorum, forming a sister group to Mymoorapelta maysi, both from the Morrison Formation, USA. The three are grouped together in an early diverging polacanthid clade, herein proposed as Jurapelta clade. nov. Jurapeltans suggest a Late Jurassic (Kimmeridgian) North American origin for polacanthids, immediately followed by an Iberian dispersion in the uppermost Tithonian and achieving a Laurasian distribution by the Early Cretaceous. These results not only highlight the paleobiogeographical connections and paleoecological relationships between North America and Iberia during the Late Jurassic, but also underline the need for improved specimen and character sampling to increase the resolution of problematic taxa.

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SFRH/BD/128717/2017