Node Calibrated. Divergence of Dugong from Trichechus.

Fossil Taxon and Specimen. Eotheroides aegypticum cranial endocast BMNH 46722 from the Mokattam Formation of the Mokattam Hills, Egypt (Sickenberg 1934; Domning et al., 2010).

Phylogenetic Justification. Eotherioides is identified by Domning et al. (2010) and Gheerbrant et al. (2005) as more closely related to Dugong than to Trichechus, and the associated specimens of skulls and postcranial remains show apomorphies of Dugongidae, including an upper dental formula of 3.1.5.3, and the deciduous P5 is not replaced (Domning et al., 2010). It is therefore nested within crown Sirenia.

Minimum Age. 47.6 Ma

Soft Maximum Age. 66 Ma

Age Justification. Sirenian remains, referrable to "halitheriine" dugonids from the Mokattam Hills in Egypt, have been interpreted as Lutetian, or middle Eocene (Domning et al., 2010; Sickenberg, 1934) with a minimum bound of 47.8 - 0.2 Ma. Paleogene strata have yielded abundant sirenian remains, including stem taxa, for much of the Eocene (Domning et al., 2010). Tethythere (i.e., Sirenia- Proboscidea) remains are known from the Paleocene and early Eocene (Gheerbrant, 2009), but lack crown Sirenia. Hence, we define the paleontological soft maximum constraint as the base of the Paleocene, equivalent to the base of the Danian marine stage at 66.04 Ma ± 0.4 Myr = 66 Ma.

Discussion. Eosiren is another taxon from the Lutetian of Egypt that is identified by Domning et al. (2010) and Gheerbrant et al. (2005) as more closely related to Dugong than to Trichechus and as such is also a member of crown Sirenia. Both Eotheroides and Eosiren are now known by cranial and postcranial elements, and traditionally placed in the (probably) paraphyletic subfamily Halitheriinae.