MONTES-DOMÍNGUEZ, H. M.; AYALA-PÉREZ, L. A.; CASTILLO-RIVERA, M. A.; GONZÁLEZ-ISÁIS, M. & REYNOSO, V. H. Neuroanatomy of two species of genus Myliobatis (Chondrichthyes: Myliobatoidea). Int. J. Morphol., 38(2):499-504, 2020. SUMMARY:Several studies on the elasmobranchs neuroanatomy have shown that their brain is more complex than previously thought, and had significant intra and interspecific variations. The objective of this work was conducting a comparative encephalic neuroanatomy study of two species of genus Myliobatis. In total, 16 organisms of genera Myliobatis californica and Myliobatis longirostris, collected in the coasts of Kino Bay, Sonora, Mexico, were used. In Myliobatis, the brain has a long telencephalon and the posterior central nucleus is poorly developed. Their cerebellum is asymmetric, has several sulci, most of which are transversally oriented, with four lobes (anterior, medium and two posterior), a condition which has not been reported for any other species. It was observed that, despite the morphology of M. californica and M. longirostris is similar, there are some significant differences. Both species have moderate foliation, but M. californica has more sulci. In the diencephalon of M. californica, it was observed that the lobes of the infundibulum are oval-shaped and separated, while in M. longirostris, such lobes are rounded and near the medium line. It has to be highlighted that Myliobatis belongs to the most derived batoid group; nevertheless, its brain is considerably less complex, as compared to what has been reported for the most derived milyobatoids species.

Although skeletal and muscle anatomy has supported Gymnuridae as the sister group of the most derived myliobatoids, recent studies based on molecular characters suggest that the family branches into a more basal position than previously thought. This study aims to understand the brain anatomy of the genus Gymnura and its importance in the evolution of the batoid brain. The brain anatomy of Gymnura lessae and Gymnura marmorata is relatively simple. They exhibit a small brain and telencephalon (T), where the latter is wider than it is longer, and the division of the posterior central nucleus is poorly developed. The cerebellum (C) is symmetrical and is not highly foliated. Unlike other species, the brain auricles are smooth, and the posterior auricles exhibit a diagonal arrangement, not always forming a bridge over the fourth ventricle. These auricles are larger in G. marmorata. A principal component analysis based on 20 morphological variables, revealed a separation between species, and multivariate analysis of variance identified significant differences. The most important variables in species segregation were a deeper olfactory bulb in G. lessae and a greater distance between the bulbs in G. marmorata. Contrary to the body anatomy, the brain anatomy reveals that Gymnura has a simpler and more primitive brain than most derived myliobatoids. Our results are consistent with the evidence from phylogenies developed with molecular data, where gymnurids are a basal group within myliobatoids.