Was the First Mammal to Live on Earth the Morganucodon or Brasilodon? Experts Still Debate

Key Takeaways on the First Mammal

• According to most experts, the first mammal was Morganucodon, a small, shrew-like rodent that appeared during the Jurassic Period, approximately 200 million years ago. It weighed a few ounces and probably ate mostly insects and small invertebrates.

• However, some experts contend that mammals might go back even further. A January 2023 study published in the Journal of Anatomy considered Brasilodon, the earliest mammal, because it grew two sets of teeth like modern mammals.

• Morganucodon was relatively widespread, living in what is now Wales, U.K., and other parts of Europe, as well as in China and certain parts of North America. Brasilodon would have lived alongside early dinosaurs 225 million years ago. The creature lived in modern Brazil.

Mammals are distinct from reptiles, amphibians, fish, and birds because they have fur and mammary glands used to feed their young. They usually have live births, with a few exceptions. Lesser-known features include an extra set of replacement teeth, a particular ear construction, and one pair of bones in their lower jaw.

While we don’t see all of these features in the earliest mammals because things like mammary glands are soft tissue that rarely preserve into fossils, according to Martha Richter, science associate and paleontologist at the Natural History Museum in London, they did show signs of what was to come. These were the species that had what it took for mammals to move to the next level.

“Many extinct species, potential candidates to be accepted as an ‘early mammal’ show a mosaic of features that indicate they did not fully match the list of characteristics shown by modern-day mammals but were instead at earlier stages in the evolution,” says Richter.

Read More: Did Prehistoric Mammals Live With Dinosaurs, and What Were They Like?

Hot Debate: What Was the First Mammal?

The first mammal is still debated among scientists. According to most experts, the first mammal was Morganucodon, a small, shrew-like rodent that appeared during the Jurassic Period, approximately 200 million years ago. It weighed a few ounces and probably ate mostly insects and small invertebrates.

Covered in fur with a rat-like tail, the species had a brain smaller than modern mammals, but it did have acute hearing.

However, some experts contend that mammals might go back even further. A January 2023 study published in the Journal of Anatomy considered Brasilodon, the earliest mammal, because it grew two sets of teeth like modern mammals.

“Some dento-skeletal traits (mouth bones and their teeth) and clues gathered about the type of dental replacement they had (either polyphyodonty, like in reptiles, or diphyodonty, like modern mammals) are very important but can be the subject of disagreements among scientists,” says Richter.

Key Traits of the First Mammal

Some scientists call them “mammal-like,” and others call them mammals. Still, there’s a lot that we don’t know about the species because no full specimens have been found. Brasilodon was around 12 centimeters, and also had a shrewd-like look to it, like the Morganucodon.

Morganucodon was relatively widespread, living in what is now Wales, U.K., and other parts of Europe, as well as in China and certain parts of North America.

Brasilodon would have lived alongside early dinosaurs 225 million years ago. The creature lived in modern Brazil.

When Did Mammals Begin to Thrive?

These were the first mammals, and they’re what mammals evolved from, but it would take many more years before mammals exploded onto the scene. Mammals began to diversify and expand in numbers around 66 million years ago, following the extinction of non-avian dinosaurs, according to a study published in October 2020 in the journal EMBO Reports.

Early mammals were small and primarily ate insects, but as ecosystems on Earth expanded, so too did the ability of larger, more intelligent mammals to thrive. As the continents broke up and the landmasses became separate, species that lived in different parts of the world became diversified. In Australia, marsupials became a unique group, and placental mammals, those that develop during pregnancy, started to thrive on other continents. The earliest known placental mammal, called Juramaia sinensis, lived in China around 160 million years ago.

By the Cenozoic era, the period right after an asteroid barreled into planet Earth, nearly 4,000 species of mammals developed. These included fully aquatic whales and flying mammals like bats. And most importantly, the primates that would one day become us.

The earliest mammals lived in a world where dinosaurs thrived. They were survivors in a place where an estimated 700-1,000 dinosaurs ruled their ecosystem, many of which would have made for a delicious meal. Still, these early mammals would set the stage for a world where we humans could one day take over at the top of the food chain.

Read More: Established Science Is Wrong About Mammalian Evolution, Study Claims

Article Sources

Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:

• Encyclopedia Britannica. Morganucodon

• Journal of Anatomy. On taxonomic issues, ontogenetic series and tooth replacement. Comments on Diphyodont tooth replacement of Brasilodon—A late Triassic eucynodont that challenges the time of origin of mammals by Cabreira et al.

• EMBO Reports. The rise of the mammals

• The American Museum of Natural History. Dinosaur Facts

Sara Novak is a science journalist based in South Carolina. In addition to writing for Discover, her work appears in Scientific American, Popular Science, New Scientist, Sierra Magazine, Astronomy Magazine, and many more. She graduated with a bachelor’s degree in Journalism from the Grady School of Journalism at the University of Georgia. She’s also a candidate for a master’s degree in science writing from Johns Hopkins University (expected graduation 2023).

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