Marine mammal mass strandings have occurred for millions of years, but their origins defy singular explanations. Beyond human causes, mass strandings have been attributed to herding behaviour, large-scale oceanographic fronts and harmful algal blooms (HABs). Because algal toxins cause organ failure in marine mammals, HABs are the most common mass stranding agent with broad geographical and widespread taxonomic impact. Toxin-mediated mortalities in marine food webs have the potential to occur over geological timescales, but direct evidence for their antiquity has been lacking. Here, we describe an unusually dense accumulation of fossil marine vertebrates from Cerro Ballena, a Late Miocene locality in Atacama Region of Chile, preserving over 40 skeletons of rorqual whales, sperm whales, seals, aquatic sloths, walrus-whales and predatory bony fish. Marine mammal skeletons are distributed in four discrete horizons at the site, representing a recurring accumulation mechanism. Taphonomic analysis points to strong spatial focusing with a rapid death mechanism at sea, before being buried on a barrier-protected supratidal flat. In modern settings, HABs are the only known natural cause for such repeated, multispecies accumulations. This proposed agent suggests that upwelling zones elsewhere in the world should preserve fossil marine vertebrate accumulations in similar modes and densities.

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Along the south‐western coast of South America, three genera of fossil phocids (true seals) have been formally described from the late Neogene: Acrophoca and Piscophoca from Chile and Peru, and, more recently, Hadrokirus from Peru, which all represent medium‐ to large‐sized phocids. Here, we report the discovery of Australophoca changorum gen. et sp. nov., a diminutive phocid from the late Miocene of the Bahía Inglesa Formation (northern Chile) and Pisco Formation (southern Peru), comparable in size with the smallest species of modern phocids. This taxon is based on diagnostic postcranial material, including a humerus that has an elongated deltopectoral crest but lacks an entepicondylar foramen; a femur with a subtrochanteric fossa, among other characters; in combination with a relatively small body size. All these features together distinguish A. changorum from all other reported pinnipeds. This new taxon not only increases the taxonomic and morphological diversity of phocids of the late Neogene of the eastern South Pacific Ocean, but it also provides new insights about the evolutionary history of fossil pinniped assemblages in South America and, broadly, in the southern hemisphere.