The “strange” anatomy of a family of giant marsupials that roamed eastern Australia and Tasmania for much of the past 25m years has been revealed in a new study.

Scientists had already figured out that palorchestids had tapir-like skulls and large “scimitar-like claws”, but little was known about the limbs of one of the “strangest marsupial lineages to have existed”, according to the paper published by a group of Australian researchers.

By examining 60 fossil specimens of palorchestids of varying geologic ages, the scientists were able to get an idea of how their legs and arms would have looked, functioned and evolved over time.

They found that the palorchestid had “muscular bent forelimbs” with fixed elbows that were “unlike any of their marsupial kin, living or extinct”, the study published in journal PLOS ONE this week said.

“We are beginning to build a picture of the palorchestids as they were in life,” it said.

The image gives an indication of how the palorchestids compare with humans in terms of height and weight. Photograph: H Richards

Palorchestids were a sister family to the better-known Diprotodontidae and became extinct some time during the Late Pleistocene, probably as a result of human activity and climatic change.

But mystery has surrounded the exact make-up of palorchestids because very few fossils have been found.

What is known is that they died out before other mega-fauna, said the Monash university researcher Hazel Richards.

“The fact that there just aren’t that many of these guys in the fossil record probably tells us something about the way they were distributed,” Richards told Guardian Australia.

“We find them all over Australia but the population size was probably quite small. And they were quite sparsely arranged. It’s not like these guys were travelling in herds or in large numbers as has been kind of hypothesised for some of the other megafauna.”

The study found that as they evolved, palorchestids grew larger and stranger. While they varied in size, the researchers said they were “bigger than previously thought, with the largest species likely weighing over 1000 kg”.

“We used a method where you take the circumference of the bone and that tells you something about how much weight is being borne through each of the four legs,” Richards said.

“We came up with a number that was substantially larger than what has been estimated before and puts them some way up there among some of the the largest marsupials that we ever had in Australia.”

The fossils were obtained from museums in Australia and London’s Natural History Museum.

“What makes them so weird is the kind of combination of features that they have,” Richards said.

“They’ve got an extremely long pointed face, really tiny little eyes, protrusible tongue, something maybe something a bit like a giraffe, and extremely muscular fore quarters ... and huge claws.

“Some of the other large megafauna ... like diprotodon they’re not as muscular proportionately as ... and also their claws are much smaller. So these guys were clearly specialised for doing something with their forelimbs.”

Their claws would have been made digging difficult, “like trying to dig with a chef’s knife”, while their elbows, which could not “bend or straighten, are “totally unheard of in any other mammal, living or extinct”.

Richards said future research would aim to determine why palorchestids came to be how they were.

“The plan now is to zero in and conduct some functional, biomechanical analyses to try and understand exactly what they were adapted for,” she said.

“What foods are they targeting, why would they have become so different from everything else?”