Fossilised pregnant fish was one of the first animals to have sex
A pregnant fossil fish at the Natural History Museum in London has shed light on the possible origin of sex, according to a study published in Nature today by an international team including Museum scientists. Dating from the Upper Devonian period 365 million years ago, the adult placoderm fish Incisoscutum ritchiei is one of the earliest examples of a pregnant vertebrate and contains a five-centimetre-long embryo. Scientists used to think external fertilisation was the earliest form of reproduction, but this fish from the Gogo formation of Western Australia demonstrates that internal fertilisation – sex – started far sooner than previously thought.
Dr Zerina Johanson, palaeontologist at the Natural History Museum explains, ‘We expected that these early fishes would show a more primitive type of reproduction, where sperm and eggs combine in the water and embryos develop outside the fish. Embryos had already been found in the extinct group of armoured fish, the placoderms, but I. ritchiei shows that the type of advanced fertilisation, taking place inside the mother, was more common among early fishes than previously thought. This discovery is incredibly important because evidence of reproductive biology is extremely rare in the fossil record.’
The fish, which has been in the collections of the Museum since the 1980s, was originally thought to have died soon after its final meal, hence the bones of a smaller fish seen within it. But research on similar related fish caused Johanson and her colleagues to reinterpret the fossil, coming to the conclusion that the ‘meal’ was in fact a young fish developing in the womb of an adult.
‘The position of the embryo in this fish is the same as that of the Austroptyctodus specimen which was discovered to contain an embryo last year which is why we decided to reassess the description of the specimen. The embryo in I. ritchiei was in its final stages of development, inside its mother, waiting to be born’ continues Dr Johanson.
A modification of the pelvic fin on the belly of the adult fish is also significant. This structure, or clasper, would have been used by the male to grip the female during mating. Johanson and her colleagues believe the process of internal fertilisation and giving birth to live young, which differentiates some fish and mammals from other animals such as reptiles and amphibians, was the main reproductive method for early fish groups such as the placoderms and could have evolved in other fish groups.
‘Sex’ was far more common in these primitive prehistoric animals. We used to think that external fertilisation was the earliest form of reproduction but copulation appears to be the main way they reproduced, demonstrating that ‘sex’ started a lot sooner than we thought’ concluded Dr Johanson.
For further information, images or to arrange an interview please contact:
Claire Gilby, Senior Press Officer, Natural History Museum
Tel: 020 7942 5106
Mobile: 07799 690 151
(not for publication)