Professor Jenny Clack
Palaeontologist who found a fishlike creature with limbs that explained the move from water to land
PROFESSOR JENNY CLACK, who has died aged 72, was a palaeontologist 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.
Once dubbed “the diva of the Devonian”, she devoted her career to studying the early development of tetrapods, the four-legged animals said to have evolved from Devonian lobe-finned fishes who colonised the freshwater swamps of the Carboniferous period.
Her breakthrough came about 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 dated from the Devonian era and was seen as the “missing link” between water and land-based animals.
The fossil had been found by the Swedish palaeontologist Erik Jarvik in Greenland in the 1930s. Jarvik believed that lobe-finned fishes walked on to shore as the planet became warmer and drier, and only developed limbs once they got there. Jenny Clack wanted to test that theory, but since his discovery, Jarvik had refused to give any other scientist access to the fossil while the Danish government refused to let any palaeontologist visit the site where Ichthyostega had been found.
Jenny 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, housed in the basement of the Sedgwick Museum of Earth Sciences in Cambridge, had been identified as Ichthyostega, but when Jenny Clack examined them she noticed that there were hornlike projections on the back of each skull, indicating that they were Acanthostega, another “transitional” fossil, which at that time was known only from two partial skull roofs.
In the summer of 1987 Jenny Clack and her husband travelled to the sites in Greenland where the samples had been found. There, they came across some fossil bones sticking through a block of silty sandstone. They dug out the bones encased in the surrounding rock and sent the specimen back to Cambridge, little anticipating how important it would be.
As the fossil emerged from its casing, it turned out to be an almost complete 360 million year-old specimen of Acanthostega. Jenny Clack gave it the nickname “Boris”.
Examination of the fossil, a fishlike animal with limbs, found that, instead of the five digits thought to be standard for early tetrapods, Boris had eight. 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, Jenny Clack was able to examine Jarvik’s Ichthyostega, showing that it, too, had limbs that could not support its weight.)
Jenny Clack’s discovery stimulated a renaissance in collecting and research related to the fish-tetrapod transition. Museum curators around the world began re-examining fossils of “undetermined fish” collecting dust in backroom drawers, some of which turned out to be tetrapods.
In 2009 Jenny Clack became the first woman in her specialism to be appointed a fellow of the Royal Society.
She became known to the wider public through her book Gaining Ground: the Origin and Early Evolution of Tetrapods (2002, second edition 2012).
She was born Jennifer Alice Agnew on November 3 1947 to Ernest and Alice Agnew. Brought up in Manchester, she was educated at Bolton School (Girls’ Division).
She had started collecting fossils at the age of 10, and became the first in her family to attend university, studying Zoology at Newcastle. Missing out on the first-class degree that would have allowed her to progress to a doctorate, she moved to Leicester to take a postgraduate course in museum studies.
In her early 30s she called her old tutor at Newcastle, Alec Panchen, and asked if there was anything she could help him with. There was. A museum in Bradford had a rare tetrapod called Pholiderpeton that he wanted to study and he asked her to see if she could negotiate its release.
She could and she turned Pholiderpeton into a PHD project, in which she described the fossil’s brain case. Newcastle awarded her a PHD in 1984.
In the meantime, in 1981 she had moved to Cambridge as an assistant curator at the University Museum of Zoology. She became a senior assistant curator in 1995, a Fellow of Darwin College in 1997 and a reader at the University in 2000. In 2006 she was awarded a personal chair by the University and took the title Professor of Vertebrate Palaeontology. She retired in 2015 and became Emeritus Professor of Vertebrate
Palaeontology at the museum. She continued to work on tetrapods, her focus shifting to the so-called Romer’s Gap, a period just after the Devonian era during which something happened to cause a mass extinction, resulting in a dearth of fossils from many major groups of organisms.
At the end of the period, the fossil record contains fully terrestrial tetrapods, whereas the ones in the Devonian were all semiaquatic. Until the end of the last century scientists knew of no fossils that represented any sort of intermediate stage between the two types of tetrapod.
In 2002 Jenny Clack formally announced in Nature that a fossil discovered in 1971 in central Scotland and classified as a lobefinned fish, was a primitive tetrapod with at least five digits, dating from within Romer’s Gap – the only fossil of its kind to be found intact from a 20 million-year period.
She gave it the name Pederpes, after its finder Peder Aspen, formerly curator of the Museum of Geology at Edinburgh University. The salamander-like creature was perhaps a yard long, and, she said, “probably split its time between the water and land, where it walked on four feet”.
“It trudged through the swamp catching anything that moved,” she observed. “Not terribly exciting, I suppose.” However, what pushed Pederpes to move to land was anyone’s guess, she said.
In recent years Jenny Clack had been trying to shed more light on this issue by leading the Tw:eed project (Tetrapod World: early evolution and diversification) investigating newly discovered fossils from Northumberland and the Borders Region of Scotland which date from Romer’s Gap.
One of the theories to explain the apparent lack of fossils at this time was that atmospheric oxygen levels were low, but the project has found, through analysis of bore samples, that oxygen levels must have been at least 16 per cent throughout the period, suggesting that the low oxygen theory is implausible.
Jenny Clack was the most modest and unlikely of scientific stars; her idea of a good day out was to put on the leathers and take a high-powered motorbike for a spin. But she was also one of the most quietly determined, her love of knowledge for its own sake helping her to overcome numerous obstacles.
Popular with colleagues, she had a delightful sense of humour. She once wrote an article on her work for Scientific American entitled “Getting a Leg Up on Land” and when she identified another early tetrapod, a four-legged, froglike creature found in West Lothian, she called it Eucritta melanolimnetes (“the creature from the black lagoon”). In 1980 she married Robert Clack, who survives her.