Fossil hunters say they have unearthed the earliest evidence yet of four-limbed vertebrates looking after their young, after discovering the entwined remains of two lizard-like creatures preserved in an ancient plant stump.

The fossil found in Nova Scotia, Canada, is thought to be the remains of an adult and young of a newly identified species of varanopid.

“[The adult] probably would have been about 20cm in length from the tip of the snout to the base of its tail, and it would have had a long tail,” said Hillary Maddin, a co-author of the research and an assistant professor at Carleton University in Canada.

The smaller individual was found beneath the upper leg bone of the larger one, and was encircled by the larger creatures’s tail – an entwinement that the team say suggests the two animals were curled up together in a den.

The den appears to have been made in a hollowed-out stump of a plant known as a lycopsid. “These very fragile fossils, especially the baby, are preserved in a very natural-like position and would have to have been buried very quickly,” said Maddin, adding that the stump was probably inundated with sediment, possibly in a storm.

The animals are thought to have lived just over 300m years ago, pushing back the record for evidence of extended parental care among four-limbed vertebrates by about 40m years.

Parental care is any behaviour that aids the survival of offspring, such as guarding eggs or nest-building. Extended parental care is any support for young after birth or hatching, for example providing food or protection.

Experts say prolonged care is seen across a wide range of animals, from reptiles to birds, and is particularly common to mammals as they produce milk for their young. Such behaviour can help offspring to survive but comes at a cost to parents, who must invest time, energy and resources.

Writing in the journal Nature Ecology and Evolution, the team report that the new species has been named Dendromaia unamakiensis, derived from the ancient Greek words for tree and mother and the Mi’kmaq’ indigenous people’s name for Cape Breton Island, where the fossil was found.

The find was made by Brian Hebert, a fossil enthusiast who runs a gift shop and tours in the fossil-rich Joggins area of Nova Scotia and is part of Maddin’s field team.

There is a debate over whether varanopids are part of a group of animals that eventually gave rise to mammals, or whether they a sit on a different evolutionary branch leading to reptiles.

If the former, traditional view is correct, Maddin said the new fossil suggests parental care of young cropped up early on the path that led to mammals. That such behaviour is also seen in birds and some reptiles, she said, was probably down to it evolving independently in these groups.

Steve Brusatte, a palaeontologist at the University of Edinburgh who was not involved in the study, said the fossil find was exciting.

“The discovery of an adult and a much smaller juvenile side by side is good evidence that they were interacting with each other in life, as it would be quite the coincidence for them to randomly die, get buried and fossilise together,” he said. “And although it’s always tricky to infer behaviour from fossils that are hundreds of millions of years old, to me this seems to be good evidence that there was parental care going on.”

He said this was significant because it suggested such behaviour is an ancient trait. “It’s incredible to think that something we consider so human – parents taking care of children – was developed so long ago, in such distant ancestors, when the world was so much different.”