Scientist traced line from sea to land

Jenny Clack, who has died aged 72, was a paleontologist who solved one of the greatest mysteries in the history of life on Earth: how vertebrates made the transition from sea to land, from animals with fins to animals with legs; in 2012 her career was the subject of a BBC documentary, Beautiful Minds.

She devoted her career to studying the early development of tetrapods, the fourlegged animals said to have evolved from Devonian lobefinned fishes who colonized the freshwater swamps of the Carboniferous period.

Her breakthrough came in a roundabout way in the mid-1980s when, working as an assistant curator at the University Museum of Zoology, Cambridge, she wanted to get her hands on a tetrapod fossil called Ichthyostega that was seen as the “missing link” between water and land-based animals.

The fossil had been found by the Swedish paleontologist Erik Jarvik in Greenland in the 1930s. Jarvik believed that lobe-finned fishes only developed limbs once they got to shore. Clack wanted to test that theory, but Jarvik had refused to give access to the fossil, while the Danish government refused to let any paleontologist visit the site where it had been found.

Clack turned to a Cambridge colleague, the geologist Peter Friend, who had done fieldwork in Greenland in the 1960s and 1970s and had unearthed many tetrapod fossils while studying sediments. The specimens had been identified as Ichthyostega, but when Clack examined them she thought they were another “transitional” fossil.

In the summer of 1987 Clack and her husband travelled to the sites in Greenland, and came across some fossil bones sticking through a block of silty sandstone. They sent the specimen to Cambridge, little anticipating how important it would be.

The fossil turned out to be an almost complete 360-million-year-old Acanthostega.

The fossil, a fishlike animal with limbs, had eight digits instead of the five thought to be standard for early tetrapods. It also had gills and a fishlike tail, and the structure of its wrist showed that it could not have supported weight.

The discovery overturned assumptions about how vertebrates moved from water to land, showing that limbs had evolved in the oceans, tetrapods such as Acanthostega using their paddle-like hands to clamber through weed-choked swamps. (Later, Clack was able to examine Jarvik’s Ichthyostega, showing that it, too, had limbs that could not support its weight.)

Clack’s discovery stimulated museum curators around the world to re-examine fossils of “undetermined fish” collecting dust in backroom drawers, some of which turned out to be tetrapods.

In 2009 Clack became the first woman in her specialty to be appointed a fellow of the Royal Society.