Lactation , along with other characteristically mammalian features , is also thought to characterize the Mammaliaformes , but these traits are difficult to study in the fossil record .
Lactation ( milk ) and fur , along with other features , also characterize the Mammaliaformes , though these traits are difficult to study in the fossil record .
Lazarus taxa occur either because of ( local ) extinction , later resupplied , or as a sampling artifact .