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2 Posts tagged with the evolution tag

The evolutionary rates of sea urchins are more complex than previously thought, a finding that could apply across the evolutionary tree.


Evolution within groups of organisms was first thought to occur continuously, at a constant rate. Fossil-based analyses soon led to the belief that many groups quickly reach maximum diversity early on in their history, followed by a decline in evolutionary rates as habitat types fill up.


Now, in a detailed analysis of a group of marine invertebrates called echinoids, Dr Melanie Hopkins of the American Museum of Natural History and Museum palaeobiologist Dr Andrew Smith have found a branch of the evolutionary tree that has increased its evolutionary rate over time.

Slow starters

Modern echinoids originated 265 million years ago, just before the Permian-Triassic mass extinction, an event that wiped out around 96% of all marine species. They still exist today as sea urchins and sand dollars.


Despite the abundance of ecological space left behind after the mass extinction, Dr Hopkins and Dr Smith found that echinoids experienced the lowest rates of evolutionary diversification during this early phase. Said Dr Smith of the result:

This slow start is very different from the standard model of high initial rates of diversification followed by a slowing down as ecological space gets filled that we have come to expect.

Bursts of diversity

When they looked in more detail at sub-groups of echinoids through time, they discovered that some that underwent episodes of 'early bursts' in evolution, primarily associated with the adoption of new feeding strategies.


For example, one particular group of echinoids - the sand dollars - evolved a novel method of 'deposit-feeding' that allowed them to filter nutrients from the sand, and this innovation coincided with a marked increase in morphological innovation.



Regular echinoids like the sea urchin (left) have five-fold symmetry and can head in any direction, whereas irregular echinoids, like the sand dollar (right) have two-fold symmetry, with defined 'front' and 'back' ends.


A question of scale

The overall pattern of slowing evolutionary rates punctuated by smaller 'early burst' events within certain subgroups points to the importance of considering scale when assessing the evolutionary history of any group. Said Dr Smith:

Rates of evolution turn out to be quite different when viewed at different scales, and both 'continuous' and 'early burst' patterns of evolution may apply to the same group depending upon how you view them.


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Some of the earliest mammals had more specialised diets than previously thought, leading to key evolutionary traits we carry today.


Shrew-sized mammals living 200 million years ago in the Jurassic period were thought to be opportunistic insect-eaters with a generalised diet. But a new study by a team of researchers including the Museum's Nature Live science communicator Dr Nick Crumpton shows that two core taxa of early mammals had teeth and jaws adapted to specific kinds of insects.


At this time, small early mammals were known to be evolving the precise chewing and better hearing that are traits of mammals worldwide today. However, it was thought that, because of their general diets, these traits did not evolve in response to different hunting and feeding behaviours.


The new research shows teeth and jaws of early mammals were in fact becoming specialised as a response to different diets.



The Early Jurassic basal mammals, Morganucodon and Kuehneotherium, hunting their prey on the small island they shared in what is now Glamorgan, southern Wales. © John Sibbick.


Dr Crumpton said this gives us new ideas on how the earliest mammals lived:


The idea of the first mammals eking out a meagre living, hiding in the shadows whilst dinosaurs ruled the land is a pervasive one, but we have revealed that even the earliest mammals were already showing specialisations for certain lifestyles.


Tale of the teeth


The team, led by the Universities of Bristol and Leicester, analysed 2cm long jaws and tiny teeth from the mammals Morganucodon and Kuehneotherium found in Glamorgan, South Wales. When the creatures were alive 200 million years ago, the area was made up of small islands in a shallow sea.


Bits and pieces of jaw were scanned and the images stitched together to allow the researchers to determine the bite and strength of the creatures' jaws. This was combined with evidence of 'microwear' on the teeth, patterns of pits and scratches that indicate what the animal was eating.


The patterns on the ancient mammal teeth were compared to those of insect-eating bats alive today that have specialised diets. The combined evidence shows that Morganucodon favoured harder, crunchier food such as beetles, while Kuehneotherium prefered softer prey such as moths and scorpion flies.


Old specimens, new techniques


Dr Crumpton said this research also highlights the importance of specimens that may have been in the collections for decades, but still have stories to tell:


Although our methods were very modern, the fossils themselves had been stored in collections including the Natural History Museum for decades. It's work like this that shows how important museum collections are, and that even though those techniques didn't exist in the 1950s, we were able to study them in fresh new ways in order to discover the secrets they held.