The name eomysticetids refers to the “dawn” of the baleen whale, suggesting the critical role in understanding the origin and early evolution of baleen whales. Eomysticetids represent an early diverging lineage showing the baleen-assisted feeding mode—an essential feature for baleen whales, including giant animals such as the blue or fin whales. This paper describes a new eomysticetid species—Echericetus novellus—from the Oligocene of Mexico, about 28 million years old. Our discovery of a new species of Oligocene eomysticetids from Mexico shows a much broader global distribution and higher diversity than previously recognized. The existence of Echericetus novellus from Mexico indicates that eomysticetids also inhabited warmer waters in the Northern Hemisphere instead of primarily higher latitudinal regions. Last, our discovery of a new but extinct eomysticetid from the Oligocene of Mexico provides new insights into the distribution pattern and habitat use of Eomysticetidae, which helps to decipher the ultimate demise of this once-successful baleen whale lineage.

Among the several evolutionary lineages of the baleen whales (Mysticeti), the eomysticetids are an ancient successful family that retain possibly nonfunctional teeth and functional baleen, a transitional stage between toothed and baleen-assisted filter-feeding mysticetes. The patchy fossil record leaves gaps in eomysticetid paleobiology 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, the 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 eomysticetid inhabited subtropical regions in the Northern Hemisphere. Last, 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.