Prolonged suckling has vastly limited marsupial evolution

By looking at the evolution of mammal jaw bones, a team of Museum researchers have found that marsupials have evolved at a slower pace and with much less diversity than placentals. 

Marsupials are mammals that nourish their young in a pouch, such as kangaroos and possums, while placentals are mammals that nourish their young in a uterus, such as humans and whales. 

The study, published in Proceedings of the Royal Society B, has found that mammal jaw shape is significantly influenced by both diet and reproduction.

They found that marsupial feeding behaviour when young has stopped the group from evolving the diversity seen in placental mammals, which includes flying bats, swimming whales and giant elephants.  

Why are there no marsupial whales?  

Marsupial newborns are born very early, at most a few weeks after conception at a point when their body is not well-developed. They have big arms to crawl from the birth canal to their mother's pouch, and face and jaw bones for the non-stop suckling when they get there. 

Prof Anjali Goswami, a Research Leader at the Museum and senior author of the study says, 'At this stage, marsupial newborns look like little jellybeans with hands. They just have some face bones and forelimbs - everything else is basically cartilage and goo.

'They don't even have much of a brain.'

During this time, they suckle more intensively than placentals, which means their jaws are under a lot of pressure to function properly within just a few weeks of conception. 

Because their jaws need to function so early in their development, they can't later remodel their jaws into different shapes for other kinds of feeding behaviours in the way placentals can. This is why, for example, placentals could evolve the unusual jaws of baleen whales, but marsupials could not. 

In contrast, placental young are born more developed and mostly don't suckle for as much or as long as marsupials. The only placental mammals that do suckle as long as marsupials are primates, including humans. 

Placentals and marsupials have evolved similar jaw shapes

The study found some great examples of marsupials and placentals evolving similar jaw shapes to adapt to similar diets and lifestyles. 

Some of the most extreme convergences in shape between these groups include the marsupial mole and the star-nosed mole, the recently extinct Tasmanian wolf and the (placental) wolf, and the black rhinoceros and the fossil giant wombat.

Using 3D data sets to look at jaw shape 

To study mammal evolution, Museum researchers looked at the adult jaw bone of 151 living and extinct species of mammals.

Anne-Claire Fabre, postdoctoral researcher and lead author of the study describes how they conducted the study, 'We use micro-CT scanning and laser scanners to build gigantic 3D data sets of living and fossil species, using the collections here at the Museum and at other museums around the world.

'With these scans, we use methods to describe the shape of each bone with hundreds of individual points which are then used to reconstruct how the shapes differ across species and how fast they evolved. 

'This dataset is particularly exciting because we included some really unusual extinct marsupials, including a sabre-toothed marsupial that lived in South America, marsupial lions, and marsupial rhinos.'

The researchers chose to use the jaw bone because it's the perfect bone to study feeding and it is also functional immediately after birth in all marsupials, unlike the forelimbs. 

Their work is part of a bigger study of skull evolution across all vertebrates (fish, amphibians, reptiles, birds and mammals) led by Anjali.

The research group within the Museum are studying life on the planet to understand how factors like life cycles, ecology, habitat and extinction drive the diversity of species in the past, present and future. 

All of the 3D scans used in the study are available free online on the Museum's Phenome10K.org database.