The genetic machinery controlling diphyodont teeth is intertwined with other mammalian traits
Across 225 million years of silence, a small shrew-like creature from southern Brazil is asking humanity to reconsider where it came from. A new study proposes that Brasilodon quadrangularis, no longer than a human hand, lived alongside the first dinosaurs and possessed the dental hallmark of true mammals — teeth replaced only once in a lifetime. The claim is bold, the fossil evidence is real, and the argument over what to call this ancient animal reveals how much of our own origin story remains unresolved.
- A team of international researchers has declared a 225-million-year-old Brazilian fossil the earliest known mammal, potentially rewriting the timeline of our own lineage by twenty million years.
- The evidence rests on a single but significant trait: diphyodont dentition, the two-set tooth pattern that distinguishes mammals from reptiles and fish, observed through destructive analysis of three fossilized jawbones.
- Critics are pushing back hard, arguing the study blurs the line between true mammals and the broader category of mammaliaforms, warning that one shared trait cannot carry the weight of a reclassification.
- The feathered-dinosaur analogy looms over the debate — just as feathers don't make a T. rex a bird, two sets of teeth alone may not make Brasilodon a mammal.
- What both sides agree on is the fossil itself: the creature existed, it had those teeth, and it walked beside early dinosaurs — the fight is over the name, and names in science carry enormous consequence.
A team of international researchers has proposed that the earliest known mammal was a shrew-like creature no longer than twenty centimeters, living alongside the first dinosaurs some 225 million years ago in what is now southern Brazil. Published in the Journal of Anatomy, the finding would push back mammalian origins by roughly twenty million years — a significant revision — though it has already drawn sharp criticism from specialists in the field.
The creature, Brasilodon quadrangularis, was known from fossils unearthed at the Linha São Luiz site decades ago. What drew the researchers' attention was its teeth. Led by Martha Richter of the Natural History Museum in London, the team examined three fossilized lower jawbones using a destructive sectioning technique, revealing that Brasilodon replaced its teeth only once during its lifetime — a pattern called diphyodonty, and a defining mammalian trait. Richter argued that the genetic machinery behind diphyodont teeth is developmentally linked to other mammalian characteristics: body heat regulation, milk production, and hair growth. If Brasilodon possessed this trait, the team contends, it deserves the title of earliest known mammal.
Not everyone agrees. Simone Hoffmann, an evolutionary biologist at the New York Institute of Technology, argues the study conflates true mammals with mammaliaforms — the broader group that includes mammals and their closest fossil relatives. In her view, Brasilodon sits even further back on the evolutionary tree, closer to a sister group of mammaliaforms, and the Morganucodon remains the oldest confirmed true mammal.
The deeper dispute is about what a single trait can prove. Hoffmann offered a sharp analogy: feathers appear in many dinosaurs, but no one classifies Tyrannosaurus rex as a bird. Shared characteristics can be inherited across branches of the evolutionary tree without conferring group membership. Richter acknowledged the challenge ahead, framing the study as a contribution to an ongoing and genuinely unresolved conversation. The fossil record itself is not in dispute — Brasilodon lived 225 million years ago, had two sets of teeth, and coexisted with early dinosaurs. What remains open is the question of what, precisely, that makes it.
A team of international researchers has made a bold claim: the earliest known mammal was a shrew-like creature no longer than twenty centimeters, living alongside the first dinosaurs roughly 225 million years ago. The finding, published in the Journal of Anatomy, would push back the origins of mammals by about twenty million years—a significant revision to our understanding of early mammalian history. But the claim is already drawing fire from skeptics who say the researchers have overreached their evidence.
The animal in question is Brasilodon quadrangularis, known from fossilized remains discovered decades ago at the Linha São Luiz site in southern Brazil. What caught the researchers' attention was the creature's teeth. The team, led by scientists including Martha Richter from the Natural History Museum in London, examined three lower jawbones—one from a juvenile and two from adults—using a destructive sectioning technique to observe how teeth developed inside the bone. What they found was that Brasilodon replaced its teeth only once during its lifetime, a pattern known as diphyodonty: two complete sets of teeth, one for juveniles and one for adults.
This matters because diphyodonty is a hallmark of mammals. Reptiles and fish, by contrast, typically regenerate teeth continuously throughout their lives, cycling through multiple sets as they grow. Richter explained to Live Science that the genetic machinery controlling diphyodont teeth is intertwined with other mammalian traits—body heat regulation, milk production, hair growth—all linked together in a developmental chain. If Brasilodon had cracked this code, the argument goes, it deserves to be called a mammal, and the earliest one we know of. The creature's long tail and shrew-like appearance only strengthened the case.
Yet the scientific community is far from convinced. Simone Hoffmann, an evolutionary biologist at New York Institute of Technology who specializes in early mammal history, told Live Science that the study conflates two distinct groups that should remain separate: mammals themselves, and mammaliaforms, the broader category that includes both true mammals and their closest fossil relatives. By Hoffmann's reading of the evidence, Brasilodon is neither a true mammal nor even a mammaliaform. It belongs further back on the evolutionary tree, closer to a sister group of actual mammaliaforms. The Morganucodon, she notes, remains the oldest known true mammal—and Brasilodon typically sits beside it or near it on the family tree, not above it.
The crux of the disagreement hinges on what a single trait can tell us. Hoffmann pointed out that diphyodont teeth, while characteristic of mammaliaforms, do not automatically make something a mammal. She offered a pointed analogy: feathers were once thought to define birds, but we now know feathers appeared much earlier and show up in many dinosaurs. Yet no one calls Tyrannosaurus rex a bird just because it had feathers. Characteristics can be inherited from ancestors and shared across branches of the evolutionary tree without conferring membership in a particular group.
Richter acknowledged that her team's findings will likely face challenge. She noted that classifying these early groups remains genuinely controversial and that the conversation about how to sort them is ongoing. The study, she suggested, will help sharpen that discussion. What neither side disputes is the fossil record itself: Brasilodon lived 225 million years ago, it had two sets of teeth, and it walked the earth alongside the earliest dinosaurs. The question of what name to give it—and what that name means—remains open.
Citas Notables
What we discovered is that Brasilodon only changed its teeth once, making it a diphyodont and suggesting it is more closely related to mammals than to reptiles.— Martha Richter, Natural History Museum in London
Based on all other studies, Brasilodon is not a mammal, certainly not the oldest mammal, and not even a mammaliaform.— Simone Hoffmann, New York Institute of Technology
La Conversación del Hearth Otra perspectiva de la historia
Why does it matter whether we call this creature a mammal or something else? Isn't it the same fossil either way?
The name carries the weight of everything we think we know about mammalian evolution. If Brasilodon is truly the first mammal, then mammals survived two mass extinctions and diversified across the planet. If it's not, then the story of how mammals arose is still incomplete.
But the researchers found two sets of teeth—isn't that the definition of a mammal?
That's where it gets complicated. Two sets of teeth is a mammalian trait, yes, but traits can appear in different branches of the family tree without making something a true member of that family. The researchers may have found a mammal-like ancestor rather than a mammal itself.
So how do we know the difference if we only have bones and teeth?
We don't, not with certainty. Mammals today produce milk and have hair and regulate their body heat—none of which fossilize. We're reading the genetic code written in bone structure, trying to infer what the whole animal was capable of.
And the critics think the researchers are reading too much into it?
Exactly. They say the study blurs the line between "mammal" and "mammal-like ancestor" in ways that earlier research had already clarified. It's a step backward in precision, even if the fossil itself is genuinely interesting.
Will this settle the question?
No. It will sharpen it. Both sides agree the fossil is real and important. They just disagree on what it means. That disagreement is where science actually lives.