http://en.wikipedia.org/wiki/Procellariidae
"According to the famous DNA hybridization study into avian phylogenetic relationships by Sibley and Ahlquist, the split of the Procellariiformes into the four families occurred around 30 million years ago; a fossil bone often attributed to the order, described as the genus Tytthostonyx, has been found in rocks dating around the Cretaceous-Tertiary boundary (70-60 mya), but the remains are too incomplete for placement within the Procellariiformes to be certain.[2] The molecular evidence suggests that the [swimming] storm-petrels were the first to diverge [develop] from the ancestral stock [ichthyornithes], and the albatrosses next, with the procellariids and diving petrels splitting [developing] most recently. Many taxonomists used to retain the diving petrels in this family also, but today their distinctiveness is considered well supported."
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