What makes an animal a mammal? Well, in school we all learned the characteristics that determine what is a mammal: "warm blooded" (endothermy), body hair, three middle ear bones, live birth, and functional mammary glands in mothers with young. Most mammals also have sweat glands and specialised teeth. Of course, we also were taught about that funny exception that was still deemed a mammal, the duck-billed platypus. But, even that was not quite a complete description of the exception, because there are five species of monotremes (which lay eggs) which are still deemed mammals: our friend the platypus and and the four species of spiny anteaters. Okay, a few exceptions, but it still seemed comfortable.
Then, we began to hear about warm-blooded dinasours. A few, okay, perhaps many. Maybe endotherms, or somewhat endotherms. And, wait a minute, birds were really dinosaurs who had merely evolved. Lots of debate about the which species gave rise to birds, but then we found dinosaur fossils with feathers. It was getting messy.
Of course, some scientists had to remind us what we had forgotten, or never knew, which were the mammal-like reptiles, a term used in classical systematics, but are sometimes referred to as "stem mammals" (or sometimes "protomammals") under cladistic terminology [see Cladistics]. Okay, so things were not neat, but kinda messy. What did we expect from evolution, discrete animal types? Perish the thought.
Now, just to keep things interesting, a recent discovery indicates that Plesiosaurs, which lived at the time of dinosaurs, were large carnivorous sea animals with broad bodies and two pairs of flippers [see Plesiosauria] gave birth to live young. A giant carnivorous reptile with a key mammalian characteristic? Apparently so.
Researchers have been puzzled why a predecessor to Plesiosaurs showed evidence of live births, but not Plesiosaurs themselves. A recent fossil find now provides that evidence (well, seems to; keep in mind that a lot about what we know about ancient animals is based on a statistically small sample of fossils, which may or may not represent the characteristics of the species as a whole or over time]. The fossil can be reasonably interpreted to show a fetus rather than another animal that was eaten by the Pleasiosaur.
The report of the finding can be found at http://www.sciencemag.org/content/333/6044/870.
To note again. We should not be surprised at the fuzziness of classifications and definitions. Evolution moves in many ways. Sometimes there is punctuated equilibrium, sometimes slow, incremental change. Such variety should not be a surprise.
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