Morphology of the tympano-periotic complex in stranded odontocetes in northeast Brazil

Morphological descriptions of the tympano-periotic complex (TPC) are fundamental for understanding odontocete auditory adaptations, as well as their relationships with habitat, behavior, and evolutionary processes. This study analyzed the TPC morphology of six Delphinidae species stranded along the northeastern coast of Brazil: Peponocephala electra (n = 4), Pseudorca crassidens (n = 2), Sotalia guianensis (n = 39), Stenella attenuata (n = 4), Stenella longirostris (n = 4), and Tursiops truncatus (n = 4). A total of 57 TPCs were examined, with 24 morphometric measurements taken, including two novel parameters introduced in this study. The results revealed similarities in TPC morphology among species, particularly among S. guianensis, S. attenuata, and S. longirostris, which exhibited more comparable anatomical features in the structures analyzed. Species identification was supported by distinct features: in the tympanic bone, the posterior process, inner and outer prominences, and sigmoid process; and in the periotic bone, the cochlear portion, apertures for the cochlear and vestibular aqueducts, and the transverse crest. A previously undescribed anatomical structure, termed the "mesocochlear opening," was identified in S. attenuata. No remarkable ontogenetic variations were observed in the TPC of S. guianensis, P. electra, or S. longirostris, supporting the hypothesis that auditory structures reach full development early in life. These findings highlight key morphological features of the tympano-periotic complex that contribute to species differentiation while providing new insights into the evolutionary and ecological adaptations of odontocetes. Furthermore, this study underscores the value of detailed morphological analyses for elucidating structural taxonomic variation and supporting future studies on the auditory capabilities of odontocetes.