The systematic position of the enigmatic thyreophoran dinosaurParanthodon africanus, and the use of basal exemplifiers in phylogenetic analysis

Abstract

The first African dinosaur to be discovered, Paranthodon africanus was found in 1845 in the Lower Cretaceous of South Africa. Taxonomically assigned to numerous groups since discovery, in 1981 it was described as a stegosaur, a group of armoured ornithischian dinosaurs characterised by bizarre plates and spines extending from the neck to the tail. This assignment has been subsequently accepted. The type material consists of a premaxilla, maxilla, a nasal, and a vertebra, and contains no synapomorphies of Stegosauria. Several features of the maxilla and dentition are reminiscent of Ankylosauria, the sister-taxon to Stegosauria, and the premaxilla appears superficially similar to that of some ornithopods. The vertebral material has never been described, and since the last description of the specimen, there have been numerous discoveries of thyreophoran material potentially pertinent to establishing the taxonomic assignment of the specimen. An investigation of the taxonomic and systematic position of Paranthodon is therefore warranted. This study provides a detailed re-description, including the first description of the vertebra. Numerous phylogenetic analyses demonstrate that the systematic position of Paranthodon is highly labile and subject to change depending on which exemplifier for the clade Stegosauria is used. The results indicate that the use of a basal exemplifier may not result in the correct phylogenetic position of a taxon being recovered if the taxon displays character states more derived than those of the basal exemplifier, and we recommend the use, minimally, of one basal and one derived exemplifier per clade. Paranthodon is most robustly recovered as a stegosaur in our analyses, meaning it is one of the youngest and southernmost stegosaurs.

Phylogeny of Paleozoic limbed vertebrates reassessed through revision and expansion of the largest published relevant data matrix

“…The addition of taxa in Analyses R3–R6, B2 and EB has had unexpected effects that may have improved the reliability of the tree. As previously demonstrated (Mortimer, 2006; Butler & Upchurch, 2007; Raven & Maidment, 2018), every OTU in a data matrix can influence the position of every other OTU in the resulting trees.…”

The anatomy and palaeobiology of the early armoured dinosaur Scutellosaurus lawleri (Ornithischia: Thyreophora) from the Kayenta Formation (Lower Jurassic) of Arizona

“…Scutellosaurus lawleri was originally described as a member of Fabrosauridae by Colbert [18], although it was suggested it could be ancestral to ankylosaurs and stegosaurs. Fabrosauridae is now considered polyphyletic [1,3,5,26,31,34], and Scutellosaurus lawleri is generally considered an early diverging member of Thyreophora [1][2][3][4][5]20,[25][26][27][28][29][30][31][32][33]. This phylogenetic position is supported by numerous aspects of morphology and subsequently numerous phylogenetic characters.…”

“…Thyreophoran dinosaurs (the armoured dinosaurs-stegosaurs, ankylosaurs and related forms) were important and diverse terrestrial herbivores from the Late Jurassic to the Late Cretaceous [1][2][3]. They split from their sister-clade, Neornithischia (including ornithopods, pachycephalosaurs and ceratopsians), by the Early Jurassic and diverged into two major clades (Stegosauria and Ankylosauria, together forming the clade Eurypoda) by the Middle Jurassic [1][2][3][4][5][6]. However, the paucity of taxa known from these early phases of thyreophoran evolution (Late Triassic-Middle Jurassic) hampers attempts to understand thyreophoran systematics, palaeobiology and evolution.…”

Novel track morphotypes from new tracksites indicate increased Middle Jurassic dinosaur diversity on the Isle of Skye, Scotland

“…[74,112,[114][115][116][117]. Additionally, the Bajocian-aged ornithischian Issaberrysaura mollensis has been found as a stegosaur in recent phylogenies [118][119][120]. The oldest definitive ankylosaur, Sarcolestes leedsi is also from the Oxford Clay [121].…”