Node Calibrated. Common ancestor of ruminants and "Suiformes", including the now well-established hippo-whale clade (Gatesy et al., 2002; Geisler and Theodor, 2009).

Fossil Taxon and Specimen. Himalayacetus subathuensis, Roorkee University Vertebrate Paleontology Laboratory (RUSB) specimen 2003, from early Eocene deposits of the Subathu Formation type section in Kuthar Nala, India (Bajpai and Gingerich, 1998).

Phylogenetic Justification. Himalayacetus is known only from a partial dentary and two molars, but it was claimed that these show characterisics of Pakicetidae (Bajpaj and Gingerich, 1998). The phylogenetic placement within Cetacea is controversial, and a conservative phylogenetic analysis (O’Leary and Uhen, 1999) confirmed that Himalayacetus might be more closely related to ambulocetids than to pakicetids. Either way, Himalayacetus is a member of total-group Cetacea, and nested well within crown Artiodactyla, as currently understood.

Minimum Age. 52.4 Ma

Soft Maximum Age. 66 Ma

Age Justification. The cow-pig division is dated minimally by the record of Himalayacetus from the base of the Subathu Formation in Pakistan (Bajpai and Gingerich, 1998) where it co-occurs with Nummulites atacicus whose range correlates with nannoplankton zones 11-12, providing a minimum age of 52.4 Ma (Martín-Martín et al., 2001). The availability of this marine correlation for this fossil enables us to forego use of the corresponding marine stage (Ypresian).

The absence of any crown cetartiodactyls during the Paleocene may point to a soft maximum constraint of 66.04 Ma ± 0.4Myr = 66 Ma.

Discussion. Cetaceans comprise closer relatives to ruminant artiodactyls than do either suids or camels (Gatesy et al., 2002; Spaulding et al., 2009; Geisler and Theodor, 2009). While stem artiodactyls (e.g., Diacodexis) date to the Wasatchian NALMA (Rose, 2006), Himalaycetus is only slightly younger, and as such exceeds other crown artiodactyl lineages in age.