New fossil discovery of a pregnant fish provides rare evidence on reproductive biology
The importance of fossils in understanding evolution has been proved beyond doubt and most of the new discoveries by paleontologists helps us gain valuable insights into different life forms that existed in the past. Sometimes scientists unearth such a fossil that will put our biological understanding of some extinct group of organisms on a much firmer footing. Recently John A. Long and colleagues discovered such a fossil of a placoderm fish with preserved five cm long embryo in body cavity .
Placoderms are extinct jawed fishes that lived from approximately 430 million years ago to the end of the Devonian 360 million years ago.Embryos in the body cavity imply internal fertilization.This fossil of a pregnant fish has shed light on the possible origin of sex and also helped our understanding of early vertebrate evolution.
Generally evidence of reproductive biology is extremely rare in the fossil record (for obvious reasons as soft structures rarely gets preserved) and this valuable discovery of embryo in the Arthrodira inside specimens of Incisoscutum ritchiei from the Upper Devonian Gogo Formation of Western Australia provides first evidence of reproduction using internal fertilization in this diverse group.
Researchers were amazed by this find as they didn’t expect such advanced way of reproduction in this kind of primitive fish species. External fertilisation (where sperm and eggs combine in the water and embryos develop outside the fish) is believed to be the earliest form of reproduction and obviously scientists thought that placoderms and other early fish groups used this method. The process of internal fertilisation and giving birth to live young, or viviparity, is more advanced and differentiates some fish and mammals from other animals such as reptiles and amphibians.
“Copulation appears to be the main way they reproduced,demonstrating that sex started a lot sooner than we thought.” –Zerina Johanson
With this discovery the concept of origin of jawed vertebrates or gnathostomes, are also into state of flux. Read an interesting piece by Per E. Ahlberg in the PALAEONTOLOGY secetion of in recent nature issue.
Journal Reference :
Devonian arthrodire embryos and the origin of internal fertilization in vertebrates.
Long JA, Trinajstic K, Johanson Z.
Nature. 2009 Feb 26;457(7233):1124-7.
Birth of the jawed vertebrates
Per E. Ahlberg
Nature. 2009 Feb 26;457(7233):1094-5.
Image Credit :
T.Hart49 @ FlickR
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