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Microdocodon gracilis, a small Jurassic animal about 165 million years old, is the earliest known fossil evidence of mammal-like milk suckling—not proof that it was the first animal ever to nurse. The clue is a set of preserved throat bones that could have helped it suck and swallow.
What was Microdocodon?
Microdocodon gracilis was a small docodont from the Jurassic, dated to about 165 million years ago. Docodonts are commonly discussed among early mammaliaforms and close relatives of true mammals, though where “true mammals” begin on the fossil family tree remains debated. Nature’s 2019 feature described the animal as the earliest mammal then known to have suckled milk.
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What evidence suggests it could suckle?
The fossil preserves the hyoid apparatus, a group of bones in the throat involved in swallowing. In living mammals, the hyoid and its supporting muscles help coordinate sucking and swallowing. Microdocodon’s hyoid has mobile joints resembling the modern mammalian arrangement, making a similar suckling mechanism plausible.
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1Fix the driver behind crashes, sound loss and screen glitches2Clear out junk files and repair common Windows errors3Scan for outdated or missing drivers - takes under a minuteThat is anatomical evidence, not a snapshot of nursing. The fossil does not preserve milk, mammary glands, or a suckling event. The bones support an inference about what the animal could do; they do not directly record the behavior.
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Was it really the first animal to drink milk?
“Earliest known” is the careful wording. It means this is the oldest evidence currently identified for the relevant ability, not that no earlier animal suckled. Suckling itself does not leave a hard structure in the way bones or teeth do, so its evolutionary origin is difficult to establish directly from fossils. A 2023 review of maternal-infant feeding evolution discusses this challenge.
The label “mammal” also needs care. Scientists differ over which early fossil groups count as true mammals and where that boundary falls. The strongest claim is therefore about Microdocodon’s suckling-related anatomy and its place in the known fossil record, rather than a settled taxonomic definition of the first mammal.
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What other fossils tell us—and what they do not
Other fossils help show that reproduction and development were changing among mammal relatives, but they are not competing evidence for suckling.
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- A 2018 study of Kayentatherium wellesi reported a clutch of perinates alongside a presumed maternal skeleton. Its authors called it the first fossil record of pre- or near-hatching young of any non-mammalian synapsid. It is evidence about reproduction and parental investment, not milk nursing.
- A 2024 study of juvenile and adult Middle Jurassic Krusatodon used growth increments in dental cementum to investigate early mammaliaform life history. That approach can inform questions about growth and development, but it does not replace the hyoid-based evidence for Microdocodon.
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