It seems there are several orders of reptiles similar to and closely
related to the dinosaurs. Remains of these reptiles are found in the
sedimentary rocks which contain the earliest known dinosaurs. A number
of them resembled the dinosaurs but do not quite meet the requirements
as far as details of the skeleton are concerned. In the scheme of
classification these orders of reptiles are grouped together into the
subclass _Archosauria_. This subclass includes the dinosaurs,
crocodiles, and the flying reptiles. The lizards, snakes, turtles, and
the tuatera of New Zealand belong to other subclasses of reptiles which
have been distinct from that of the dinosaurs as far back in geologic
time as we can trace them. The kinship between the dinosaurs and the
small lizards living in the monument today lies only in that both are
reptiles. The only living relatives of the dinosaurs are the alligator
and the crocodile.
The dinosaurs were so numerous, and so dominated the whole of the
Mesozoic Era, that this period of earth history is frequently referred
to as the Age of Reptiles.