Oligocene cetaceans from Baja California Sur, Mexico

A new group of late Oligocene mysticetes from México

“…Its estimated total length is between 6.4 and 7.7 m, whereas Tlaxcallicetus sp. (MU EcSj5/18/95) is fragmented but comparable in size to Tlaxcallicetus guaycurae under incomplete standard dimensions (Hernández- Cisneros et al, 2017). Both specimens are considered as adults because they present well-developed cranial sutures, although they are not completely fused.…”

“…A layer of grey laminated mudstone is located over the Humboldt bed, and above it rests the grey phosphatic sandstone bed (Schwennicke, 1994;Fischer et al, 1995). These three horizons have cetacean fossils that include archaic odontocetes, toothed and toothless mysticetes, as well as possibly Kekenodon-like animals (Hernández-Cisneros and Hernández-Cisneros et al, 2017). In addition, the Humboldt bed and the laminated mudstone represent shelf deposits under low-oxygen conditions, which are in contact with the grey sandstone that was probably deposited in a transitional zone between offshore and shore face (Schwennicke, 1994).…”

“…Tlaxcallicetus mainly differentiates from other described Oligocene stem Mysticeti (Aetiocetidae, Mammalodontidae, Eomysticetidae, and Sitsqwayk, Mauicetus, Horopeta, and Whakakai) in a periotic bone with a compound posterior process (posterior processes of bulla and periotic fused) and the ovoid head of the periotic. Tlaxcallicetus is interpreted as a chaeomysticete based on the next derived features: a compound posterior process of the periotic; transversally widened intertemporal constriction; and a postglenoid process of the squamosal posteriorly directed in lateral view (Hernández-Cisneros et al, 2017). Thus, Tlaxcallicetus is readily distinct from toothed forms (Llanocetidae, Mammalodontidae, Aetiocetidae) but remnant teeth as in eomysticetids is unknown (see Boessenecker and Fordyce, 2015a).…”

“…Nevertheless, the Oligocene Mysticeti taxonomic composition in the Pacific Ocean has increased recently by descriptions of new species (e.g., Peredo andUhen, 2016, Geisler et al, 2017). Currently there are about 28 recognized species of Oligocene mysticetes, mainly aetiocetids and eomysticetes (Hernández-Cisneros et al, 2017), in a range of ~33 to 24 Ma (see , which represent a portion of the known materials and sites (Fordyce, 2009c(Fordyce, , 2009d.…”

“…Indeed, a place like Mexico offers an important Oligocene cetacean fossil record that is poorly known (Barnes, 1998), but recent advances (Hernández-Cisneros, et al, 2017) suggest a more complete idea on its diversity. The main Oligocene fossiliferous area with cetaceans in Mexico is located in the state of Baja California Sur, which is recognized as the most important southern record of North America, and its fossils include tooth-bearing mysticetes (aetiocetids) and toothless mysticetes, as well as archaic odontocetes, and possible Kekenodon-like animals from the late Oligocene of El Cien Formation and San Gregorio Formation (Hernández- Cisneros and Tsai, 2016;Hernández-Cisneros et al, 2017).…”