From National Geographic:
Which Animals Have Barely Evolved?
That said, two mammals that have undergone the fewest evolutionary shifts are the platypus and the opossum, says Samantha Hopkins, associate professor of geology at the University of Oregon.
You could say the platypus is a survivor: It’s one of the few living descendants of an ancestor that diverged from all the other mammals about 150 million years ago, Hopkins says.
The platypus has “a number of primitive features,” Ibrahim says, “both from what we know from fossils and from what we can see in their [modern-day] anatomy.” More.
What does “primitive” mean in the context? Maybe we should call it a “durable species”?
Oh, and the opossum,
A 2009 study published in the journal PLOS ONE traces the opossum lineage back to a sister group of marsupials called the peradectids, which lived at the time of dinosaur extinction in the Cretaceous–Paleogene period. The evolutionary split of opossums from other marsupials occurred about 65 million years ago.
One factor in opossus’ survival, specifically, may be that they will eat just about anything.
Actually, stasis is quite common in evolution. See Stasis: Life goes on but evolution does not happen
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