I wondered the same thing. I looked it up and this was the most commom answer...they are mammals and millions of years ago they walked on the land but they evolved into a sea creature. ?? http://evolution.berkeley.edu/evolibrary/article/evograms_03
Are you all f'ing kidding me. The elephants went through three changes. Elephant umbilical cord is broken before they get out of uteri (sign that they were aquatic mammals). So aquatic - land - aquatic - land! That's why you should always keep your falangi.
Like how horses don't need extra fingers(yes, their hooves are one giant fingernail)
I think"vertebrates" is too broad a term. These animals are all mammals