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Maintained by pioweb@ucsc.edu. The common ancestor of placental mammals probably looked like Eomaia scansoria, the earliest known placental mammal, shown here in an artist's reconstruction based on a 125-million-year-old fossil skeleton found in China in 2002.aintained by pioweb@ucsc.edu. The common ancestor of placental mammals probably looked like Eomaia scansoria, the earliest known placental mammal, shown here in an artist's reconstruction based on a 125-million-year-old fossil skeleton found in China in 2002.
The common ancestor of birds and crocodiles is much more recent than their common ancestor with mammals. And crocodiles are not warm-blooded. And, AFAIK, there are no known species which have lost the ability to control their body temperature. So, the answer is that we can assume with somewhat hight confidence that the last common ancestor of birds and mammals was not warm-blooded.