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We describe a new complex burrow system produced by geomyids in southern Mexico. Yaviichnus inyooensis igen. isp. nov. is composed of main large chambers near the top of the paleosol, from which shafts showing different morphologies and orientations radiate, some of them ending in or connected to small deeper chambers. Gregorymys spp. is proposed as the producer based on its fossorial habits, abundance in the outcrops, presence of remains inside the burrows, and paired grooves in the walls, which are compatible with the traces of geomyid incisors. The complexity of these burrows attests to an extended underground life that would have been triggered by semiarid to arid conditions. Morphological complexity also suggests that the burrows were excavated and inhabited by more than one individual, indicating that Oligocene Gregorymys of southern Mexico would be a unique gregarious geomyid.

An integrated study of Foraminifera, Ostracoda and lithostratigraphy was conducted on samples of the Tepetate Formation from the Arroyo Colorado locality, Baja California Sur (Mexico). Based on the planktonic Foraminifera, Acarinina pentacamerata (latest lower Eocene) and Hantkenina nuttalli (earliest middle Eocene) zones were recognized that indicate an age range from 51.2 to 48.4 Ma. Benthic Foraminifera suggest correlation with the Penutian to Narizian California stages, which agrees with the planktonic foraminiferal biozones. The sedimentary structures and microfossils throughout the sequence indicate deposition within a transgressive-regressive-transgressive cycle, including environments from the inner to outer marine shelf in depths shallower than 150 m. Gravity flows dominate the sequence, and storm-deposited beds are present in the middle of the section. Considering the nature of this formational unit, we recommend the designation of a composite stratotype, according to the North American Stratigraphic Code.

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