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Devonian embryos and the origin of internal fertilisation in vertebrates
A paper, published in the current issue of Nature (available online 25 February 2009) provides new evidence of reproduction by internal fertilisation in placoderms - one of the most primitive jawed vertebrates, now completely extinct. Although evidence has been found in a smaller group of placoderms, this new discovery confirms that live birth was much more widespread than has previously been thought.
Evidence of reproductive biology is extremely rare in the fossil record. Recently the first known embryos were discovered within the placoderms, an extinct class of armoured fish, indicating a viviparous mode of reproduction in a vertebrate group outside the crown-group Gnathostomata (Chondrichthyes and Osteichthyes). These embryos were found in ptyctodontids, a small group of placoderms phylogenetically basal to the largest group, the Arthrodira.
Now, team of scientists, led by John A. Long of the Museum Victoria, Melbourne reports a discovery of embryos in the Arthrodira inside specimens of Incisoscutum ritchiei from the Upper Devonian Gogo Formation of Western Australia (approximately 380 million years ago), providing the first evidence for reproduction using internal fertilisation in this diverse group.
The authors show that Incisoscutum and some phyllolepid arthrodires possessed pelvic girdles with long basipterygia that articulated distally with an additional cartilaginous element or series, as in chondrichthyans, indicating that the pelvic fin was used in copulation. As homology between similar pelvic girdle skeletal structures in ptyctodontids, arthrodires and chondrichthyans is difficult to reconcile in the light of current phylogenies of lower gnathostomes, they explain these similarities as being most likely due to convergence (homoplasy).
These new finds confirm that reproduction by internal fertilisation and viviparity was much more widespread in the earliest gnathostomes than had been previously appreciated.
The paper is titled 'Devonian arthrodire embryos and the origin of internal fertilisation in vertebrates.'
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