The veritable horns of a dilemma: Assessing homology of the parietal and squamosal asperities of Phrynosoma (Squamata: Iguania: Phrynosomatidae)

Members of the genus Phrynosoma all exhibit asperities upon the parietal and squamosal. We examined mu-CT scans of the dermatocrania of ontogenetic series of all but two of the species of Phrynosoma, and defined parietal and squamosal "horns", distinguishing them from tubercles. We counted parietal and squamosal horns and used phylogenetic analysis to reconstruct the plesiomorphic horn numbers for these bones. Based upon this, we propose and test hypotheses of evolutionary homology for these asperities and conclude the following: (1) a plesiomorphic horn array is retained among the species of Phrynosoma; the horns are paramorphs, each identifiable as an evolutionary homologue; (2) an autapomorphic additional pair of parietal horns in P. solare has been recruited from a pair of tubercles associated with the plesiomorphic parietal horns; (3) an additional anterior squamosal horn in P. solare and the Brevicauda species is derived from a tubercle recruited homoplasiously in these taxa; (4) the anteriormost squamosal horn of the plesiomorphic series in the species of the coronatum species group is highly reduced and is subsumed within the posteriormost jugal asperity and (5) an apparent additional squamosal horn exhibited by P. ditmarsi and P. modestum is instead an enlarged submarginal tubercle, a homoplasy for these species.