The "species recognition hypothesis' does not explain the presence and evolution of exaggerated structures in non-avialan dinosaurs

The hypothesis that the exaggerated structures in various non-avialan dinosaurs (e.g. horns, crests, plates) primarily functioned in species recognition, allowing individuals of a species to recognize one another, is critically examined. While multifunctionality for many such structures is probable given extant analogues, invoking species recognition as the primary selective mechanism driving the evolution of such structures is problematic given the lack of evidence for this in extant species. Furthermore, some of the evidence presented does not support the hypothesis as claimed or is equivocal or erroneous. Suggestions that certain evolutionary patterns of diversification in these exaggerated structures are indicative of a role in species recognition are unreliable, as both a degree of phylogenetic directionality and of randomness are seen in extant species where similar structures function in sexual selection. Claims that an absence of sexual dimorphism in the exaggerated structures of non-avialan dinosaurs rule against a role in sexual selection ignores the possible existence of mutual sexual selection and is also sometimes limited in view of sample sizes. The suggestion that the existence of species recognition is supported by the presence of exaggerated structures in sympatric, closely related relatives is also erroneous because adorned dinosaur species sometimes exist in the absence of unadorned relatives. We conclude that species recognition was not the evolutionary mechanism most likely to be driving the appearance and persistence of exaggerated structures in non-avialan dinosaurs.