Hints at jaw evolution found in marsupials and monotremes

Date:
June 30, 2020
Source:
eLife
Summary:
Infant marsupials and monotremes use a connection between their
ear and jaw bones shortly after birth to enable them to drink
their mothers' milk, new findings reveal.



FULL STORY ========================================================================== Infant marsupials and monotremes use a connection between their ear and
jaw bones shortly after birth to enable them to drink their mothers'
milk, new findings in eLife reveal.


==========================================================================
This discovery by researchers at King's College London, UK, provides new insights about early development in mammals, and may help scientists
better understand how the bones of the middle ear and jaw evolved in
mammals and their predecessors.

Marsupials such as opossums, and monotremes such as echidnas, are unusual
types of mammals. Both types of animal are born at a very early stage in development, before many bones in the body have started to form. Opossums
latch on to their mother's nipple and stay there while they finish
developing. Monotremes, which hatch from eggs, lap milk collected near
their mother's milk glands as they grow. But how they are able to drink
the milk before their jaw joint is fully developed was previously unclear.

"Given the lack of a jaw joint in marsupials and monotremes at birth, scientists have previously suggested that the animals may use a connection between the middle ear bones and jaw bones to allow them to feed,"
explains lead author Neal Anthwal, Research Associate at the Centre for Craniofacial & Regenerative Biology, at King's College London's Faculty
of Dentistry, Oral & Craniofacial Sciences in the UK.

To find out if this is true, Anthwal and his colleagues compared the
jaw bones in platypus, short-beaked echidnas, opossums and mice shortly
after birth.

Their work revealed that, soon after echidnas hatch, their middle ear
bones and upper jaw fuse, eventually forming a joint that is similar
to the jaws of mammal-like reptile fossils. The team found a similar
connection in mouse embryos, but this disappears and the animals are
born with functioning jaw joints.

Opossums, by contrast, use connective tissue between their middle ear
bones and the base of their skull to create a temporary jaw joint that
enables them to nurse shortly after birth. "This all shows that marsupials
and monotremes have different strategies for coping with early birth,"
Anthwal says.

The findings suggest that the connection between the ear and jaw dates
back to an early mammal ancestor and persisted when mammals split into subgroups.

Marsupials and monotremes continue to use these connections temporarily
in early life. In other mammals, such as mice, these connections occur
briefly as they develop in the womb but are replaced by a working jaw
joint before birth.

"Our work provides novel insight into the evolution of mammals," concludes senior author Abigail Tucker, Principal Investigator and Professor of Development & Evolution at the Centre for Craniofacial & Regenerative
Biology, King's College London. "In particular we highlight how structures
can change function over evolutionary time but also during development,
with the ear bones moving from feeding to hearing. The recent availability
of monotreme tissue for molecular analysis, as showcased here, provides
an amazing future opportunity to understand the biology of these weird
and wonderful mammals, which we are keen to explore."

========================================================================== Story Source: Materials provided by eLife. Note: Content may be edited
for style and length.


========================================================================== Journal Reference:
1. Neal Anthwal, Jane Catherine Fenelon, Stephen D Johnston, Marilyn B
Renfree, Abigail S Tucker. Transient role of the middle ear
as a lower jaw support across mammals. eLife, 2020; 9 DOI:
10.7554/eLife.57860 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/06/200630193202.htm

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