Abstract

Baleen whales (Mysticeti) are the largest animals on Earth, thanks to their ability to filter feed huge volumes of small prey from seawater. Mysticetes appeared during the Late Eocene, but evidence of their early evolution remains both sparse and controversial, with several models competing to explain the origin of baleen-based bulk feeding. Here, we describe a virtually complete skull of Llanocetus denticrenatus, the second-oldest (ca 34 Ma) mysticete known. The new material represents the same individual as the type and only specimen, a fragmentary mandible. Phylogenetic analysis groups Llanocetus with the oldest mysticete, Mystacodon selenensis, into the basal family Llanocetidae. Llanocetus is gigantic (body length ca 8 m) compared to other early mysticetes. The broad rostrum has sharp, widely-spaced teeth with marked dental abrasion and attrition suggesting biting and occlusal shearing. As in extant mysticetes, the palate bears many sulci, commonly interpreted as osteological correlates of baleen. Unexpectedly, these sulci converge on the upper alveoli, suggesting a peri-dental blood supply to well-developed gums, rather than to inter-alveolar racks of baleen. We interpret Llanocetus as a raptorial or suction feeder, revealing that whales evolved gigantism well before the emergence of filter feeding. Rather than driving the origin of mysticetes, baleen and filtering likely only arose following an initial phase of suction-assisted raptorial feeding. This scenario strikingly differs from that proposed for odontocetes, whose defining adaptation ? echolocation ? was present even in their earliest representatives.