Phylogeny is the biological development and history of an organism or its class. Paleontologists have been researching the class Mammalia’s phylogeny for some time, particularly its trajectory from tiny rodent-like creatures in the late Triassic 200 million years ago, to modern-day humans. There are currently around 5,000 mammals on the planet, and it’s a very diverse group at that.
Mammals are split into three subgroups. The first are the monotremes (Prototheria) who still lay eggs; among them only the duck-billed platypus and the spiny anteater. The second are the marsupials (Metatheria) among them kangaroos, wallabies, and the lowly possum. The last, latest, and most common type are placental mammals (Eutheria, Placentalia). These are mammals kept in a placenta in a mother’s womb before being born live and fully formed. This group comprises over 4,000 species with fantastic diversity, from mice and horses to whales and humans, and so many others.
Unfortunately, we don’t know much about the origins of placental mammals. But now, fossils of the earliest placental mammal—and as such our first ancestor—have been found. They are two teeth belonging to two rat-like creatures who lived 145 million years ago, during the Mesozoic era. Researchers at the University of Portsmouth in the UK found the teeth, belonging to creatures who probably scurried around the feet of dinosaurs. Their findings were published in the journal, Acta Palaeontologica Polonica.
There’s been conflicting evidence until now of when placental mammals first emerged. Other findings have pointed to the Cretaceous or Jurassic periods. This discovery puts their emergence squarely in the Cretaceous. This is the oldest fossil ever found belonging to the line that leads to us.
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