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An important key assemblage to understand the evolutionary traits of Mysticeti clade and their relationship between stemward and crownward groups are Oligocene toothless mysticetes. Few specimens have been described for the genus Mauicetus, and for both families Eomysticetidae and Cetotheropsidae. It is known that there are many others undescribed specimens that support the great cetaceans diversity that started in the late Eocene to Oligocene, which originated the Neoceti group. Baja California Sur (BCS) is an important locality in the Oligocene cetacean paleontological framework and represents the southernmost record in North America. In the late Oligocene deposits of San Gregorio and El Cien formations in BCS, occur baleen whale fossils that show a significant diversity, including eomysticetids forms and various undescribed groups. The relationship with other Oligocene records from Oregon, and Washington, USA, as well as Vancouver, Canada is unknown. Actually, four craniums from the Museo de Historia Natural de La Universidad Autónoma de Baja California Sur (MNH-UABCS), are in preparation and description. Preliminary observations of the MHN-UABCS_EcSj1/29/141 show a juvenile cranium that retains a supraoccipital shield with a semicircular outline like some aetiocetids or archaeocetes forms, short and slightly thicker intertemporal region with a thin sagittal crest, two lobed prominence like fingertip on the posterolateral part of squamosal (perhaps homologous to squamosal prominence), triangular nasal bones elevated in its anterior end like Aetiocetidae, wide anteroposterior supraorbital process of frontal like Yamatocetus. The cranium, EcSj1/29/142, represents an adult form similar to Yamatocetus, with a long and wide mesorostral groove contrary to both Eomysticetus withmorei and Yamatocetus canalicualtus, thin nasal bones with a maximum breadth of 43.2 mm at its anterior end, the posterior end of the nasal bones are behind of posteriormost end of the ascending process of premaxilla, which is in line with the posterior half of supraorbital process and close to the temporal crest. The other two specimens ECSj5/06/31 and ECSj2/18/95 are considered as one group, perhaps a new family, based on a characteristic periotic bone that differs from Eomysticetus, Micromysticetus and Mauicetus. The periotic has a longer posterior process, a pars cochlearis with ovoid form in oblique position, the anterior process expands dorsoventrally and grows to cerebral direction, finishing in apical and fused form with the cranial wall by a very thin lamina. Now many anatomical details and phylogenetic relationships are uncertain, but these materials will give more knowledge to a better understanding of evolutionary patterns between mysticetes.

Palabras clave: San Gregorio; El Cien; fossil cetaceans

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