No limit can be set on the features used to distinguish the Mammalia Group.

http://en.wikipedia.org/wiki/Mammal

For the most part, these characteristics were not present in the Triassic ancestors of the mammals.

For palaeontologists who define Mammalia phylogenetically, no limit can be set on the features used to distinguish the group. Any feature may be relevant to a fossil's phylogenetic position. Palaeontologists defining Mammalia in terms of traits, on the other hand, need only consider those features that appear in the definition. The dentary-squamosal jaw joint is generally included.