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On this page you will learn about a past project related to the stapes, the smallest bone of the body.
Summary
Summary
In this project we look at the smallest bones of the body situated in the ear cavity, the stapes.
We describe the stapes of various extinct metatherians, such as Sparassodonta.
We reconstruct the shape of the stapes in the last common ancestor of marsupials.
Main result:
The stapes of the last common ancestor of marsupials is triangular, with an intracrural foramen and a rounded footplate
Introductory terms
Introductory terms
Metatheria
Metatherian are mammals like us, cats, horses, and marsupials.
Metatherians are extinct mammals closely related to marsupials.
Examples of metatherians are Sparassodonta.
Sparassodonta
Sparassodonts lived in South America between 56 to 3 million years ago.
They are diverse with more than 60 recognized species.
A species is a concept in biology used to group animals that are alike and can have fertile young together.
When biologist refer to a species, they write it in italics.
An example of one species of sparassodont is Sipalocyon gracilis.
The stapes
The stapes is the third bone of the middle ear chain.
The middle ear chain is part of the ear.
The stapes is also called the stirrup because of its shape in human.
The stapes is the smallest bone of the body in mammals.
One stapes consists of three parts:
the head
the body
the footplate
The body of some stapes is perforated by a hole, called the intracrural foramen.
The presence or absence of the intracrural foramen is of interest for paleontologists.
Why do we study the stapes?
Why do we study the stapes?
One of the main interest of the stapes is their shape.
The shape of the stapes can give pieces of information on two elements:
the blood system of an animal.
the identification if the stapes belonged to a marsupial or a placental.
The intracrural foramen is the hole in the body of some stapes.
The presence or absence of the intracrural foramen has implication for the blood system of an animal.
This hole is linked to the stapedial artery.
The stapedial artery may pass through the intracrural foramen.
This artery is important during the development of young mammals, but not as much in the adults.
In fact many adult mammals do not have the artery anymore.
This is the case for most marsupials.
If the artery is lost early in the development of the young, the foramen can be lost as well.
This is the case of most stapes that do not have the intracrural foramen.
However, if the artery is lost later in the development of the young, the foramen can remain present.
This is the reason why the intracrural foramen can be present or absent in the stapes of adult marsupials.
Apart from the arterial system, the stapes is interesting for its footplate.
The footplate of the stapes is useful to differentiate marsupials from placentals.
The footplate of marsupials is usually round.
On the contrary, the footplate of placental is oval.
Paleontologists are interested in the shape of the stapes of the common ancestor of marsupials.
They want to know if the stapes had an intracrural foramen.
They also want to see if the stapedial footplate was round or oval.
To do so, paleontologists study the stapes of fossil marsupials and metatherians such as sparassodonts.
How do we study the stapes?
How do we study the stapes?
First we x-ray the cranium of the fossil in a scanner.
Then we use the images of the x-rays to search for the presence of preserved stapes.
Once we find a stapes, we virtually model it using specific computer softwares.
To learn more on how to create virtual models of fossils, click on this link.
📽️Let's see how the 3D model of one stapes is created in the video below:
With the 3D models of the stapes, we take virtual measurements of its parts on the computer.
In comparing three measurements, we characterize the stapes into one of the following four shapes:
triangular
columelliform
stirrup-shaped
rectangular
We document the presence or absence of the foramen intracrural in the stapes.
We measure the length and width of each stapedial footplate.
By dividing the width by the length, we obtain the stapedial ratio.
This ratio was designed by W. Segall in 1970.
When the stapedial ratio is less than 1.8, the footplate is considered round.
When the stapedial ratio is more than 1.8, the footplate is considered oval.
Ancestral state reconstruction
Paleontologists are interested in the shape of the stapes of the common ancestor of marsupials.
To reconstruct it, we map the shape of the stapes of all the marsupials and their extinct relatives on a phylogenic tree.
A phylogenetic tree is a representation of the relationships between species.
The illustration below shows the phylogenetic tree used to reconstruct the ancestral shape of the marsupial stapes.
At the bottom of the figure are the species with the drawings of their stapes.
The lines grouping the species are branches.
Branches show the relationship between the species.
💭Look for example at Sparassodonta:
Two branches group Borhyaena with Arctodictis (indicated by number 4).
Another branch then groups Sipalocyon with these two species (indicated by number 3).
This means that Borhyaena is more closely related to Arctodictis than to Sipalocyon.
Fossils of marsupials and their extinct relatives are important for that purpose because they fill gaps in the phylogenetic tree.
The figure above is an example of the phylogenetic tree without fossils.
❓How many species are absent when fossils are not included in the phylogenetic tree?
💡Click here for the answer:
Seven species are absent from the phylogenetic tree when fossils are not included.
Without fossils, we have little information on the evolution of the stapes.
In conclusion, we would not be able to do ancestral state reconstructions without fossils.
This means that, without fossils, we would not be able to study the evolution of the stapes in mammals.
What did we find?
What did we find?
In this project we found four stapes of sparassodonts.
Not all the stapes are intact.
Two of them were broken, but they are still useful to study.
Two stapes belong to the species Sipalocyon gracilis.
One broken stapes belong to the species Arctodictis sinclairi.
One broken stapes belong to the species Borhyaena tuberata.
By taking observation and measurements on these specimens, we characterized the stapes of Sparassodonta as triangular, with intracrural foramen, and round footplate.
We also found stapes of extinct marsupials.
One stapes belong to the species Sparassocynus bahiai.
One stapes belong to the species Thylophorops cf. T. chapalmalensis.
These two specimens are similar to the stapes of Sparassodonta because they are triangular, with intracrural foramen, and round stapedial footplate.
Lastly, we also found the stapes of a controversial fossil.
It is controversial because we are not sure where it belongs in the phylogenetic tree of Metatheria.
This means that we are unsure about the relationships the fossil has with other marsupials and metatherians.
This fossil is Argyrolagus scagliai.
For the purpose of this study, we followed recent studies that placed Argyrolagus as a marsupial related to Paucituberculata.
This is the broken stapes that belong to the species Argyrolagus scagliai.
The stapes of Argyrolagus differs from the stapes of extinct marsupials and their relatives, the sparassodonts.
Argyrolagus has a columeliform stapes with no intracrural foramen and a round footplate.
⏳Coming soon! Learn more about Argyrolagus.
How are these findings important?
How are these findings important?
The stapes of Sparassodonta give information on the shape of the stapes in extinct relatives to marsupials.
As illustrated below, they fill a gap in the phylogenic tree of metatherians.
The three stapes of extinct marsupials also added knowledge to the phylogenetic tree of Marsupialia.
With this new information, we can reconstruct the ancestral state of the stapes in Metatheria.
For that purpose we use the software Mesquite (Maddison and Maddison, 2019-2025).
In Mesquite, we enter the relationships between all the species for which we have stapes.
In doing so, we create the phylogenetic tree that will serve as a basis for the analysis.
Then we enter the three features, called characters, that we used to describe the stapes:
1. The stapes shape.
2. The presence or absence of the intracrural foramen.
3. The shape of the stapedial footplate.
The figure below shows each group of species with its character information entered in Mesquite.
We then ask the software to ran the reconstruction of the ancestral state for the stapes of the last common ancestor of Marsupialia.
In conclusion, the analysis reconstructed a trangular stapes with intracrural foramen and round stapedial footplate for Marsupialia.
Where does the study take us?
Where does the study take us?
In this study we are interested in the stapes of metatherians for two reasons:
The stapes gives pieces of information on the blood system of the cranium.
The stapedial footplate helps identify if the stapes belonged to a marsupial or a placental.
With the results of this study, we interpret that the stapes of the last common ancestor of marsupials most likely had an intracrural foramen.
In paleontology, the presence of the foramen in the last common ancestor of marsupial is called the basal state.
From this basal state, changes have brought new states, such as the loss of the foramen, which are called derived states.
Knowing if the presence of a foramen is a basal or derived state is part of learning about the evolution of organisms.
With this piece of information on the stapes, we can infer on the evolution of other anatomical features.
For example, the presence of the foramen in the last common ancestor of marsupials indicates that the stapedial artery was likely present during at least part of its development.
This gives details on the evolution of the arterial system in marsupial relatives.
Can we infer functional information from the stapes?
The stapes is part of the middle ear chain that transmits sound waves to the inner ear.
In paleontology there has been interest as to whether the stapes could predict hearing capabilities in fossils.
However, the middle and inner ear are complex systems involving at least five bones, which capabilities cannot be estimated solely with the stapes.
In addition, the stapes represents as little as 6% of the total mass of the ossicular chain.
This means that the other ossicular bones, the anvil and hammer, are more important for understanding the middle ear physics.
To date, we have however not yet found any complete anvil and hammer of an extinct metatherian.
We hope that future scanning and excavations will bring us these missing pieces of information!
What do we do next?
What do we do next?
More and more fossils are scanned and 3D modeled.
We expect that additional stapes will be revealed in the upcoming years.
We also hope that other bones of the middle ear can be accessed, such as the anvil and hammer.
Every new find gives precious information on the evolution of the middle ear in marsupials.
Meanwhile we try to understand better the sense of hearing in fossil metatherians by studying their inner ear.
⏳Coming soon. 🔗 Learn about the current project on the inner ear of metatherians.
Are there any updates on this project? ✔️Yes!
Are there any updates on this project? ✔️Yes!
✨In 2023 new scans of sparassodonts revealed four additional stapes of three species of sparassodonts.
Two stapes belonging to the species Prothylacynus patagonicus.
One stapes belonging to the species Cladosictis patagonica.
Another stapes belonging to the species Borhyaena tuberata.
Overall these stapes have similar shape than those previously studied.
These finds supports better the results and interpretations of the project.
✨In 2025 while working on the inner ear of metatherians, the stapes of a Cretaceous metatherian was found!
The shape of this stapes is similar to the shape of the stapes of sparassodonts.
It is triangular with intracrural foramen and round footplate.
This newly-found stapes gives additional support to the results of this project.
And we keep looking for other stapes!
Information on the article
Information on the article
Authored by Charlène Gaillard, Ross D.E. MacPhee, and Analía M. Forasiepi
Titled "The stapes of stem and extinct Marsupialia: implications for the ancestral condition"
Published in 2021 in the Journal of Vertebrate Paleontology
List of main references for this project (click here)
List of main references for this project (click here)
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Evolution of the mammalian middle ear.
Journal of Morphology 147:403-437
Archibald JD (1979)
Oldest known eutherian stapes and a marsupial petrosal bone from the Late Cretaceous of North America.
Nature 281:669-670
Carlisle A (1805)
The physiology of the stapes, one of the bones of the organ of hearing; deduced from a comparative view of its structure, and uses, in different animals.
Philosophical Transactions of the Royal Society of London 95:198-210
De Beer G (1937)
The development of the vertebrate skull.
Clarendon Press, Oxford
Diamond MK (1989)
Coarctation of the stapedial artery: an unusual adaptive response to competing functional demands in the middle ear of some eutherians.
Journal of Morphology 200:71-86
Doran AHG (1878)
XVIII. Morphology of the mammalian ossicula auditûs.
Transactions of the Linnean Society of London, Zoology 1(7):371-497
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Studien am Skelett des Gehörorgans der Säugetiere, einschlißlich des Menschen.
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Hemilä S, Nummela S and Reuter T (1995)
What middle ear parameters tell about impedance matching and high frequency hearing.
Hearing Research 85:31-44
Henson OW (1974)
Comparative anatomy of the middle ear.
Chapter in Keidel WD and Neff WD (eds) Auditory System. Handbook of Sensory Physiology, Springer, Berlin
Maddison, WP and Maddison DR (2019-2025)
Mesquite: a modular system for evolutionary analysis. Various versions between 3.61-4.02.
Available at https://www.mesquiteproject.org/
McClain JA (1939)
The development of the auditory ossicles of the opossum (Didelphys virginiana).
Journal of Morphology 64:211-265
Novacek MJ and Wyss (1986)
Origin and transformation of the mammalia stapes.
Chapter in Flanagan KM and Lillegraven (eds) Vertebrates, Phylogeny, and Philosophy. University of Wyoming, Laramie
Nummela S and Sánchez-Villagra (2006)
Scaling of the marsupial middle ear and its functional significance: Marsupial middle ear.
Journal of Morphology 270(2):256-267
Rosowski JJ and Greybeal A (1991)
What did Morganucodon hear?
Zoological Journal of the Linnean Society 101(2):131-168
Schmelzle T, Nummela S and Sánchez-Villagra MR (2005)
Phylogenetic transformations of the ear ossicles in marsupial mammals, with special reference to diprotodontians: A character analysis.
Annals of Carnegie Museum 74(3):189-200
https://doi.org/10.2992/0097-4463(2005)74[189:PTOTEO]2.0.CO;2
Segall W (1970)
Morphological parallelisms of the bulla and auditory ossicles in some insectivores and marsupials.
Fieldiana Zoology 51(16):169-205
Wible JR (1987)
The eutherian stapedial artery: character analysis and implications for superordinal relationships.
Zoological Journal of the Linnean Society 91(2):107-135
Wible JR (1990)
Petrosals of Late Cretaceous marsupials from North America, and a cladistic analysis of the petrosal in therian mammals.
Journal of Vertebrate Paleontology 10(2):183-205
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