| Summary: | Among the several evolutionary lineages of the baleen whales (Mysticeti), the eomysticetids are an ancient successful family that retain possibly non-functional teeth and functional baleen, a transitional stage between toothed and baleen-assisted filter-feeding mysticetes. However, the patchy fossil record leaves gaps in eomysticetids palaeobiology interpretations but their diversity and widespread geographical distribution can be a relevant proxy to understanding the evolution of crown Mysticeti. Here, we describe a new baleen whale, Echericetus novellus n. gen. n. sp., from the Oligocene of Mexico (slightly older than 27.95 million years ago). This new taxon has morphological features that show its affinity to Eomysticetidae, such as the intertemporal region longer than wide, elongate and oval temporal fossa, and a well-developed and lobate coronoid process of the mandible. Similarly, our cladistic analyses confirm the inclusion of Echericetus in the Eomysticetidae. Echericetus reinforces our notion of the eomysticetid diversity and disparity. Geographically, the existence of Echericetus from Mexico also indicates that eomysticetids inhabited subtropical regions in the Northern Hemisphere. Lastly, our discovery of a new eomysticetid from the Oligocene of Mexico provides new insights into the distribution patterns and habitat use of Eomysticetidae, essential to further explain the demise of this transitional lineage between toothed and baleen-bearing whales. Funding provided by: Consejo Nacional de Humanidades, Ciencias y Tecnologías Crossref Funder Registry ID: https://ror.org/059ex5q34 Award Number: 290143 Funding provided by: National Science and Technology Council Crossref Funder Registry ID: https://ror.org/02kv4zf79 Award Number: 108-2621-B-002-006-MY3 Funding provided by: National Science and Technology Council Crossref Funder Registry ID: https://ror.org/02kv4zf79 Award Number: 111-2621-B-002-006 Funding provided by: National Science and Technology Council Crossref Funder Registry ID: ...
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