And what’s a placoderm, you ask? It’s an ancient armoured fish. The placoderms were a group of fish that were common during the Devonian (410 – 360 million years ago), but then became extinct. The reason for the title of this post? A group of Australian researchers (Long et al., 2008) have just reported on a placoderm fossil that contained embryos – far and away the oldest mother known.
Materpiscis attenboroughi was found in rocks around 380my old (& named after Sir David Attenborough – isn't that a nice touch?). Most of the time fossils represent just the hard parts of the animal: the bones & teeth, although sometimes you'll also see a smear of carbon that outlines the body or, in exceptional cases, things like feathers (Archaeopteryx, & some of the other early birds & feathered dinosaurs). Just occasionally, the dead organisms were quickly buried in very fine-grained sediments that allowed for 3-D preservation of both hard parts and soft tissues – the wonderful Cambrian animals from the Burgess Shale are an example. But Materpiscis ('mother-fish') trumps them all.
Long and his fellow researchers comment that the new specimen shows additional soft-tissue preservation never before recorded in any fossil …[and] is exceptional in revealing a small partial skeleton located within the upper body cavity of a pregnant, adult female… placoderm.
(From Pharyngula, after Long et al., 2008)
They are sure that the tiny bones belong to the same species as the adult because of characters of the jaws and skull. And they know that it's not an example of cannibalism because if the adult had eaten the smaller fish, at least some of the bones should have been broken, and they'd all show signs of being etched by digestive juices in the stomach.
And there's more. The embyro was found close to the adult's spine, which the researchers interpret as being within a uterus – and the 3-D preservation of soft tissue shows what looks like an umbilical connection between adult and young. Following this discovery, Long et al. re-examined another placoderm fish from the same rocks (the Gogo formation in Western Australia) and found that this specimen also contained 3 small embryos.
So, Materpiscis is the oldest known example of viviparity (live-birth) in vertebrates, more than 200my earlier than the previous record-holder – and therefore the earliest example of internal fertilisation in a vertebrate (you can't have viviparity without it!).
Reference:
J.A. Long, K. Trinajstic, G.C. Young & T. Senden (2008) Live birth in the Devonian period. Nature 453: 650-653