First mammal debate

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

That’s the headline
of a DiscoverMagazine.com article by science writer Sara Novak.

Novak writes,
“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.“

Novak’s source is Dr Martha Richter, Principal Curator, Vertebrates, Natural History Musuem, London. Richter writes, “My research focuses on fossil fishes and other vertebrates, especially those from the Palaeozoic of Gondwana,”

Note the absence of a last common ancestor
recovered in a phylogenetic analysis in Novak’s debate. In the large repitle tree (LRT, 2338 taxa) that last common ancestor is Late Triassic Megazostrodon (Fig 1).

Listing taxon traits is called “Pulling a Larry Martin.”

Avoid listing traits until AFTER determining a last common ancestor. That way there’s no debate – unless someone else comes up with another last common ancestor.

Is there such a cladogram out there with a different last common ancestor for mammals? If so, please let us know the name of the paper and first author.

Novak wrote:
“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.

“Rodents” are derived placental2 mammals in the LRT. This is probably a typo or a misunderstanding. Novak probably meant ‘rodent-like’. Try to forgive freshman mistakes.

Novak wrote:
“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.”

That Journal of Anatomy paper (cited below) examined the argument for diphyodonty in Brasilodon, a Late Triassic cynodont and a pre-mammal close to Sinoconodon (which had multiple tooth replacements) in the LRT.

“Grew two sets of teeth” is “Pulling a Larry Martin” (= relying on a single trait) no matter the accuracy of the claim. Don’t make this mistake and fall prey to possible convergence. Run your own analysis. Find that last common ancestor.

Clades are defined
by their last common ancestor.

Clades are described by their traits.

References
discovermagazine.com/the-sciences/was-the-first-mammal-to-live-on-earth-the-morganucodon-or-brasilodon-experts

Abdala F et al (7 co-authors) 2023. 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. Brasilodon diphodonty defense in Journal of Anatomy

Source: https://pterosaurheresies.wordpress.com/2025/06/19/first-mammal-debate/