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Published online 28 May 2008 | Nature 453, 575 (2008) | doi:10.1038/453575a
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The oldest pregnant mum
Devonian fossilized fish contains an embryo.
Researchers in Australia have uncovered the oldest record of live birth — viviparity — in any vertebrate (see page 650.
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The report fortunately straightens a possibly inadvertent confusion created by the "Breaking News" item, which mentions that the newly discovered case involves an umbilical chord, but neglects to mention that it involves a fish. This may indeed be the most ancient case of vivipary so far known in vertebrates. But, it may not be the most ancient case of vivipary in the whole animal kingdom. Vivipary is well known today in scorpions, (arachnids), as well as in aphids, (insects). As far as I know, vivipary in these two groups does not involve any umbilical chords. And I am not aware of any fossils ever found of a pregbnant scorpion, or a pregnant aphid. Both arachnids and insects are, however, very ancient invertebrates, and vivipary in these groups mmay very well have preceded vivipary in vertebrates.
What's really significant is not simply how early live birth appeared, but what it means in relation to the history of vertebrates. This new fossil shows full viviparity, not just ovoviviparity. As noted here, live birth appears independently in placoderms and chondrichthyans. Teleosts also include species that have viviparity (ovoviviparity), again not inherited from an ancestral form. According to the supplementary information (where, for an odd change, the "further discussion" of evolutionary implications is hidden), it is noted that "...the complex behavioural, morphological, and physiological mechanisms required for successful copulation and internal fertilisation in chondrichthyans (Hamlett & Koob 1999) must have evolved independently and non-reversibly 12 times in teleosts..." It also claims that "Live bearing evolved from egg laying independently in all classes of vertebrates except birds, and also in many invertebrate clades." That's a lot of independent, sudden appearances of a complex reproductive strategy. It must take a lot of faith to believe in all that being the result of purely natural evolutionary processes, which haven't yet been shown to be capable of producing such changes.