Mother fish

29 May, 2008

Source: Museum Victoria">

Placoderm gives birth.
Source: Museum Victoria

380 million year old fishes found with unborn embryos.

In 2005 Museum Victoria’s expedition to the Gogo fossil sites in north Western Australia, lead by Dr John Long, made a swag of spectacular fossil discoveries, including that of a complete fish, Gogonasus, showing unexpected features similar to early land animals.

Today the team announced its latest discovery: a remarkable 380 million year old fossil placoderm fish with intact embryo and mineralised umbilical cord.

The discovery, published in Nature and one of the most significant ever made by Australian scientists, makes the fossil the world’s oldest known vertebrate mother. It also provides the earliest evidence of vertebrate sexual reproduction, wherein the males (which possessed clasping organs similar to modern sharks and rays) internally fertilised females.

This fossil has been named Materpiscis attenboroughi, meaning ‘mother fish’, in honour of Sir David Attenborough, who first drew attention to the significance of the Gogo sites in his 1979 series Life on Earth.

Armour-plated shark-like fishes with no modern relatives, a second placoderm specimen containing three embryos was found earlier in 1986 and only recently recognised. These embryos also provided the first data on their developmental biology, indicating the early sequence of bone formation in the placoderm’s growth stages.

Studied using an ultra-fine CT scanner at the Australian National University in Canberra, such extraordinary preservation in such an old fossil is unprecedented. The team had also previously announced the first 3-D preserved muscle, nerve and circulatory tissues in a Devonian age (380 million year old) fish in 2007 paper in Biology Letters.

This research project was funded by Australian Research Council Grant DP0772138 ‘Old Brains, New Data’.

References
Long, J.A., Trinajstic, K., Young, G.C. & Senden, T. 2008. Live Birth in the Devonian. Nature 453, 650-652.

Long, J.A., Young, G.C.,Holland, T., Senden, T.J. & Fitzgerald, E.M.G. 2006. An exceptional Devonian fish from Australia sheds light on tetrapod origins. Nature 444, 199-202.

Trinajstic, K., Marshall, E., Long, J. & Bifield, K. 2007. Exceptional preservation of nerve and muscle. Biology Letters 3, 197-200.

Fossil fish shows oldest live birth

02 June 2008

Illustration of how this ancient fish, a placoderm, gave birth to live young © Museum Victoria

A 380-million-year-old fossil fish that shows an unborn embryo and umbilical cord has been discovered, scientists report in the journal Nature.

The extremely rare specimen shows incredible detail. The umbilical cord is attached to an area of small bones, corresponding to the embryo. This means the fish would have given birth to live young, known as viviparity and is the oldest record of this kind known.

'The find is important because it provides concrete evidence for vivparity,' says Zerina Johanson, fossil fish curator (expert) at the Natural History Museum.

'It is extremely rare to find preservation like this in the fossil record. This new discovery extends the record of viviparity further back almost 200 million years'.

Advanced reproduction

Viviparity, where a mother gives birth to live young, is an advanced form of reproduction and it is most often found in mammals.

This specimen belongs to an ancient species of fish that was previously thought to have had a more primitive form of reproduction where fertilisation happens externally.

However, the specimen shows evidence of viviparity and a reproductive biology that is comparable to that of some modern sharks and rays.

Careful unearthing

The well-preserved specimen is from the Gogo Formation in western Australia where many other fossil fishes have been discovered.

These fossils are unearthed using an acid solution to break down the limestone around the fossil. John Long (Museum Victoria, Melbourne, Australia) and colleagues were able to reveal even finer details because they used a lower concentration of acid.

'Long and his colleagues observed small placoderm bones within the body cavity of an adult placoderm,' says Johanson.

'One could say that this indicates that the adult simply ate the smaller specimen, but the concrete evidence comes in the form of a fine, curled cord in association with the small bones. Long and his colleagues suggest this is an umbilical cord, indicating a placental connection between adult and embryo.'

New species

The fossil fish belonged to an extinct diverse group of fish called placoderms. The fish is a new species, named Materpiscis attenboroughi after David Attenborough, who first drew attention to the Gogo fish sites in the Life on Earth TV series in 1979.

'Placoderms as a group represent the most primitive group of jawed vertebrates, so the work of Long and colleagues shows that viviparity evolved very early during vertebrate history,' Johanson concludes.

Further information

• Read the Fish new first about sex article

External links

• Nature journal
• Read the Museum Victoria article about the fossil fish showing live birth
• BBC article about Fossil fish shows live birth article

Fish knew first about 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 the journal Nature by an international team including Museum scientists.

Close-up of the 350-million-year-old fossil fish. The spine runs along the top left and the embryo is below.

The fossil is an adult placoderm, an extinct group of armoured fish, and it contains a 5cm-long embryo.

It is dated to the Upper Devonian period 350 million years ago and was found in the Gogo formation of western Australia.

The fish species is Incisoscutum ritchiei and this specimen is one of the earliest examples of a pregnant vertebrate and shows that internal fertilisation, or sex, started far sooner than previously thought.

‘This discovery is incredibly important because evidence of reproductive biology is extremely rare in the fossil record,’ says Dr Zerina Johanson, fossil expert (palaeontologist) at the Natural History Museum.

Different types of fertilisation

External fertilisation is believed to be the earliest form of reproduction and scientists used to think the placoderms and other early fish groups used this method. ‘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,’ says Dr Johanson.

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.

Johanson and her colleagues believe it 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,’ adds Dr Johanson. ‘However, copulation appears to be the main way they reproduced, demonstrating that ‘sex’ started a lot sooner than we thought.’

Look again at Museum specimen

The fish fossil has been in the Museum collections since the 1980s. Along with thousands of other fish specimens, they are used in scientific study by Museum scientists and other researchers from around the world.

The Incisoscutum ritchiei fish specimen was originally thought to have died soon after its final meal, because the bones of a smaller can be fish seen within it.

However, recent findings such as embryos found in other placoderms, caused Johanson and her colleagues to take another look at the fossil.

‘The position of the embryo in this fish is the same as that of another found with a placoderm embryo in it in 2008.’ So they decided to reassess the description for the Incisoscutum ritchiei fossil.

The team came to the conclusion that the ‘meal’ was in fact a young fish developing in the womb of an adult.

‘The embryo in Incisoscutum ritchiei was in its final stages of development, inside its mother, waiting to be born,’ continues Dr Johanson.

‘This specimen shows just how important the Museum collections are because knowledge evolves and we have new interpretations that we can apply to fossils in the collections.'

Pelvic fin

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.

Armoured fish

Illustration of a placoderm, an extinct group of armoured fish that lived around 350 million years ago © John Long, Museum Victoria

1. Placoderms were a diverse group of prehistoric fish. Their head and thorax were covered by articulated armoured plates and the rest of the body had scales or was naked, depending on the species.
2. Thought to be the most primitive jawed vertebrates, placoderms were the dominant vertebrate group during the Devonian period.
3. It was thought they went extinct due to competition from the first bony fish and early sharks.
4. But now research has shown that the last placoderms died out one by one as each of their ecological communities suffered the environmental catastrophes of the Devonian/Carboniferous extinction event.