Wednesday, 1 July 2020

Evolution News - How The Mammalian Jaw Evolved

When mammalian middle ear bones develop, they begin as part of the arch of cartilage that makes up the embryonic jaw. In reptiles, these structures remain connected to the jaw as developmental processes gradually convert the cartilage to bone.

Hints at jaw evolution found in marsupials and monotremes | For the press | eLife

It's not a good day for creationists.

Back in 2017, researchers from the University of Illinois at Urbana-Champaign, King’s College London, and the University of Chicago showed how the mammalian ear evolved from the reptilian jaw, so, incidentally, showing how the mammalian jaw evolved.

Now researchers at King’s College London, UK, have found more evidence for this in the developing young of marsupials and monotremes.

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.

Images of the join between the ear bones and upper jaw in the echidna at hatching. The left image shows the cartilages in blue, while the right shows the expression of type II collagen (green) and Sox9 (red), both necessary for cartilage development.

Image credit: Anthwal et al. CC BY 4.0
“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.”

Their findings were published open access in elife yesterday.

Mammals articulate their jaws using a novel joint between the dentary and squamosal bones. In eutherian mammals, this joint forms in the embryo, supporting feeding and vocalisation from birth. In contrast, marsupials and monotremes exhibit extreme altriciality and are born before the bones of the novel mammalian jaw joint form. These mammals need to rely on other mechanisms to allow them to feed. Here we show that this vital function is carried out by the earlier developing, cartilaginous incus of the middle ear, abutting the cranial base to form a cranio-mandibular articulation. The nature of this articulation varies between monotremes and marsupials, with juvenile monotremes retaining a double articulation, similar to that of the fossil mammaliaform Morganucodon, while marsupials use a versican-rich matrix to stabilise the jaw against the cranial base. These findings provide novel insight into the evolution of mammals and the changing relationship between the jaw and ear.

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.
, 2020; 9 DOI: 10.7554/eLife.57860

Copyright: © 2020 The authors
Published by elife, open access
Reprinted under a Creative Commons Attribution 4.0 International License (CC BY 4.0)

This is just a little more evidence of the evolution of mammals from a mammal-like reptile which marsupials and monotremes, because they are born with such poorly-developed skeletons, have retained to assist with feeding.

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