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Description
The Mosasaurs, the premier lineage of marine reptiles since the Late Cretaceous, were highly diverse by the end of the Eocene. The largest of all marine animals at the time were members of this group, particularly the clade Hydrachini, which were the apex predators of Alter Earth’s Paleogene oceans. The type genus Hydrachos was a native of the Late Eocene and inhabited a vast swathe of ocean stretching from the eastern United States to the coasts of ancient India. Four species are known, with the largest and last (H. magnificens) reaching lengths of up to 22m and weighing 35 tonnes. All other members of Hydrachini were smaller but were still significantly large predators no less than 8m long. Their skulls and teeth were powerful and robust, suggesting a macropredatorial lifestyle hunting sizeable fish, sharks, Plesiosaurs and other Mosasaurs. Their range was global, with fossils recovered from marine sedimentary deposits on all continents including Antarctica. The clade was closely related to the Late Cretaceous genus Mosasaurus and probably evolved from it during the Paleocene.

Description
The Mosasaurs, the premier lineage of marine reptiles since the Late Cretaceous, were highly diverse by the end of the Eocene. The largest of all marine animals at the time were members of this group, particularly the clade Hydrachini, which were the apex predators of Alter Earth’s Paleogene oceans. The type genus Hydrachos was a native of the Late Eocene and inhabited a vast swathe of ocean stretching from the eastern United States to the coasts of ancient India. Four species are known, with the largest and last (H. magnificens) reaching lengths of up to 22m and weighing 35 tonnes. All other members of Hydrachini were smaller but were still significantly large predators no less than 8m long. Their skulls and teeth were powerful and robust, suggesting a macropredatorial lifestyle hunting sizeable fish, sharks, Plesiosaurs and other Mosasaurs. Their range was global, with fossils recovered from marine sedimentary deposits on all continents including Antarctica. The clade was closely related to the Late Cretaceous genus Mosasaurus and probably evolved from it during the Paleocene.