Institutional abbreviations – AMNH , American Museum of Natural History, New York, USA; ANSP , Academy of Natural Sciences of Drexel University, Philadelphia, Pennsylvania, USA; CPE2 , Coleção Municipal, São Pedro do Sul, Brazil; DMNH , Perot Museum of Natural History, Dallas, Texas, USA; DMNH , Denver Museum of Nature and Science, Denver, Colorado, USA; FMNH , Field Museum, Chicago, IL, USA; FR , Frick Collection, American Museum of Natural History, New York, USA; MCCDP , Mesalands Community College Dinosaur Museum, Tucumcari, New Mexico, USA; MCSNB, Museo Civico di Scienze Naturali Bergamo, Bergamo, Italy; MCP , Museo de Ciencias e Tecnología, Porto Alegre, Brazil; MCZ , Museum of Comparative Zoology, Harvard University, Cambridge, Massachusetts, USA; MCZD , Marischal College Zoology Department, University of Aberdeen, Aberdeen, Scotland, UK; NCSM , North Carolina State Museum, Raleigh, North Carolina, USA; NHMUK , The Natural History Museum, London, United Kingdom; NMMNH , New Mexico Museum of Natural History and Science, Albuquerque, New Mexico, USA; MNA , Museum of Northern Arizona, Flagstaff, Arizona, USA; PEFO , Petrified Forest National Park, Petrified Forest, Arizona, USA; PFV , Petrified Forest National Park Vertebrate Locality, Petrified Forest, Arizona, USA; PVL , Paleontología de Vertebrados, Instituto ‘Miguel Lillo’, San Miguel de Tucumán, Argentina; División de Paleontología de Vertebrados del Museo de Ciencias Naturales y Universidad Nacional de San Juan, San Juan, Argentina, SMNS , Staatliches Museum für Naturkunde, Stuttgart, Germany; TMM , Texas Memorial Museum, Austin, Texas, USA; TTUP , Museum of Texas Tech, Lubbock, Texas, USA; UCMP , University of California, Berkeley, California, USA; ULBRA PVT , Universidade Luterana do Brasil, Coleção de Paleovertebrados, Canoas, Rio Grande do Sul, Brazil; UMMP , University of Michigan, Ann Arbor, Michigan, USA; USNM , National Museum of Natural History, Smithsonian Institution, Washington, D.C., USA; VPL , Vertebrate Paleontology Lab, University of Texas at Austin, Austin, Texas, USA; YPM , Yale University, Peabody Museum of Natural History, New Haven, Connecticut, USA; VRPH , Sierra College, Rocklin, California, USA; ZPAL, Institute of Paleobiology of the Polish Academy of Sciences in Warsaw, Warsaw; Poland.

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Revueltosaurus callenderi is included in the analysis as an outgroup because it is currently recovered as the sister taxon of Aetosauria ( Nesbitt, 2011 ). Furthermore, it is known from several specimens, which preserve nearly the entire skeleton ( Parker et al., 2007 ). Postosuchus kirkpatricki is utilized as an outgroup because it is relatively complete, well-described and illustrated ( Weinbaum, 2011 ; Weinbaum, 2013 ). Furthermore, it represents a more crownward clade (Paracrocodylomorpha) within Pseudosuchia providing a deeper optimization of character states than can be provided by Revueltosaurus . Both of these taxa have been utilized as outgroups in previous phylogenetic studies of the Aetosauria (e.g., Heckert & Lucas, 1999 ; Parker, 2007 ; Desojo, Ezcurra & Kischlat, 2012 ; Heckert et al., 2015 ). Unfortunately neither Postosuchus nor Revueltosaurus can presently be scored for lateral osteoderm characters and therefore these characters have been scored as inapplicable for these taxa. Furthermore, most of the paramedian osteoderm characters were scored as inapplicable for Postosuchus because even though Postosuchus possesses trunk osteoderms, the homology of characters such as ornamentation pattern and presence of certain processes cannot be determined.

The 26 in-group taxa include the majority of aetosaurian taxa currently considered valid ( Desojo et al., 2013 ; Roberto-Da-Silva et al., 2014 ; Heckert et al., 2015 ). They are listed below, and this study is the first to investigate the phylogenetic positions of Adamanasuchus eisenhardtae , Apachesuchus heckerti , Stagonolepis olenkae , Redondasuchus rineharti as well as a new taxon, Scutarx deltatylus gen. et sp. nov. Other taxa are rescored (e.g., Coahomasuchus kahleorum ; Typothorax coccinarum ) based on new referred material.

In order to test these questions about taxon sampling, character independence, and tree topology, the matrix has been expanded to include more taxa and characters. The new matrix ( Appendix A ) utilizes 83 characters for 26 ingroup taxa. The characters are well-divided between anatomical regions, with endoskeletal characters constituting the majority (34 cranial, 16 axial/appendicular, 33 osteoderm).

The original basis for aetosaurian phylogenetic characters and character transformations is a table of information published by Long & Ballew (1985:58) where comparisons are provided between various North American taxa, establishing a key early character-based taxonomic scheme for aetosaurians (also see Walker, 1961 ). Several of these characters are still utilized in recent phylogenetic analyses. The first computed phylogenetic analysis of aetosaurians ( Parrish, 1994 ) examined 15 characters (six osteoderm, nine non-osteoderm) and eight taxa. However, nine of those characters are parsimony-uninformative for the ingroup, and there are several incorrect scorings and typographical errors that affect the analysis; thus the published tree is neither well-resolved, nor accurate in its character state distributions ( Harris, Gower & Wilkinson, 2003 ). Heckert, Hunt & Lucas (1996) expanded on Parrish’s (1994) work, inflating the matrix to nine taxa and 22 (potentially 23) characters (17 armor, five non-armor). That study was also affected by some scoring errors, as well as the lack of use of a non-aetosaurian outgroup to root the resulting trees ( Harris, Gower & Wilkinson, 2003 ), but did include many new characters that have been used in subsequent aetosaurian phylogenetic studies. Furthermore that study was the first to unambiguously recover the major clades Desmatosuchinae and Typothoracisinae ( sensu Parker, 2007 ).

The goal of phylogenetic systematics is to determine phylogenetic relationships of organisms based on shared homologous character states, and to use this information to interpret the evolutionary histories of clades, or monophyletic lineages of organisms, as well as the histories of various evolutionary character transformations (e.g., Wiley & Lieberman, 2011 ). This presents special challenges for vertebrate groups with extensive carapaces of dermal armor like those of aetosaurian and ankylosaurid archosaurs, which are comprised of hundreds of individual osteoderms (e.g., Desojo et al., 2013 ). Whereas these osteoderms may be common in the fossil record, they are generally dissociated from the rest of the skeleton prior to burial ( Heckert & Lucas, 2000 ). It has been asserted for aetosaurians that osteoderms provide an exhaustive source of phylogenetically informative character data above and beyond that provided by the underlying skeleton (e.g., Long & Ballew, 1985 ; Heckert & Lucas, 1999 ; Parker, 2007 ), but it has also been argued that, while informative, these data may be plagued with phylogenetically confounding homoplasy ( Parker, 2007 ; Parker, 2008a ). The specific goal of this paper is to confront these assertions analytically, first by undertaking an expanded phylogeny of aetosaurian archosaurs based on the largest taxonomic sample yet assembled, using a suite of characters that samples both osteoderms and endoskeletal characters; and second, by applying a new method (Partitioned Bremer Support) to assess character support and conflict within an entirely morphological dataset.

Terminal Taxa

The phylogenetic study by Nesbitt (2011) is currently the basis for most studies of archosauriform relationships (e.g., Nesbitt & Butler, 2013; Butler et al., 2014). This study utilizes the format used in that study for the listing of terminal taxa and characters to make this work compatible.