The inner ear of a 400 million-year-old 'platypus fish' has yielded new insights into early vertebrate evolution, suggesting this ancient creature may be more closely related to modern-day sharks and bony fish than previously thought.
A team of scientists from the University of Birmingham in the UK, and institutions in China, Australia and Sweden, used 'virtual anatomy' techniques, including MicroCT scanning (using x-rays to look inside the fossil) and digital reconstruction to examine previously unseen areas within the braincase of these mysterious fossils.
They discovered the fish, called Brindabellaspis stensioi and nicknamed 'platypus fish' because of its long beak, has an inner ear which is surprisingly compact in construction. Its closely connected components resemble the inner ears of modern jawed vertebrates such as sharks and bony fishes. Some features of it also appear very similar to a human's inner ear. The team's analysis is published in Current Biology.
The fish belongs to a group called placoderms, a class of armoured fish which thrived for 60 milllion years of the early Palaeozoic era, between 420 and 360 million years ago. Most placoderms have less complex inner ear structures, with a large sac, called a vestibule, placed in the centre and separating all the other components.
The Brindabellaspis stensioi fossils were discovered in Australia in the later part of the 20th century. Several examples were dug out of limestone rocks in an ancient reef near the Brindabella Moutains on the border of New South Wales. The specimens were unusually well preserved, giving palaeontologists a rare opportunity to examine inside the brain cavity. Their early analysis led them to conclude these fish were closely related to primitive, jawless fishes that first appeared some 500 million years ago.
The more recent analysis, however, challenges assumptions that placoderms were a distinct group and supports the possibility that they, in fact, contained the origins of modern jawed vertebrates.
Joint first author on the paper, Dr Sam Giles, of the University of Birmingham's School of Geography, Earth and Environmental Sciences, explains: "The inner ear structure is so delicate and fragile that it is rarely preserved in fossils, so being able to use these new techniques to re-examine specimens and discover this wealth of new information is very exciting."
She adds: "This fossil has revealed a really intriguing mosaic of primitive features and a surprisingly modern inner ear. We don't yet know for certain what this means in terms of our understanding of how modern jawed vertebrates evolved, but it's likely that virtual anatomy techniques are going to be a critical tool for piecing together this fascinating jigsaw puzzle."
The research was funded by the Chinese Academy of Sciences, the National Natural Science Foundation of China, the Australian Research Council, the Swedish Research Council, the Knut and Alice Wallenberg Foundation and the Royal Society.