Like most water living animals they are believed to have lost their fur during this period, while they developed a deep layer of fat under their skin as isolation.
(they; are believed; ) (they; to have lost; their fur; T:during this period; T:while they developed a deep layer of fat under their skin as isolation) (they; developed; a deep layer of fat under their skin as isolation)
they they they
are believed to have lost developed
their fur T:during this period T:while they developed a deep layer of fat under their skin as isolation
a deep layer of fat under their skin as isolation