A new analysis of extinct sea creatures suggests that the
transition from egg-laying to live-born young opened up
evolutionary pathways that allowed these ancient species to
adapt to and thrive in open oceans. The evolutionary sleuthing
is described this week (Sept. 16) in the journal Nature by
scientists at Harvard University and the University of Reading
who also report that the evolution of live-born young depended
crucially on the advent of genes — rather than incubation
temperature — as the primary determinant of offspring sex.
Having drawn this link in three lineages of extinct marine
reptiles — mosasaurs, sauropterygians, and ichthyosaurs — the
scientists say that genetic, or chromosomal, sex determination
may have played a surprisingly strong role in adaptive
radiations and the colonization of the world’s oceans by a
diverse array of species. “Determining sex with genetic
mechanisms allowed marine reptiles to give live birth, in the
water, as opposed to laying eggs on a nesting beach,” says Chris
Orga