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gills of their earlier marine incarnation. Turtles went back to the sea a very long time ago and, like all vertebrate

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gills
of their earlier
marine incarnation. Turtles went back to the sea a very long time ago
and, like all
vertebrate
returnees to the water, they breathe air. Howev-
er, they are, in one respect, less fully given back to the water than whales
or dugongs, for turtles still lay their eggs on beaches.
There is evidence that all modem turtles are
descended
from a terrestrial
ancestor which lived before most of the dinosaurs. There are two key
fossils called
Progaochelys quenstedti
and
Palaeochersis talampayensis
dating from early dinosaur times, which appear to be close to the ances-
try of all modem turtles and tortoises. You might wonder how we can
tell whether fossil animals lived on land or in water, especially if only
fragments are found. Sometimes it`s obvious.
Ichthyosarus
were reptili-
an contemporaries of the dinosaurs, with
fins
and streamlined bodies.
The fossils look like dolphins and they surely lived like dolphins, in the
water. With turtles it is a little less obvious. One way to tell is by meas-
uring the bones of their
forelimbs
.
Walter Joyce and Jacques Gauthier, at Yale University, obtained
three measurements in these particular bones of 71 species of living tur-
tles and tortoises. They used a kind of triangular graph paper to plot the
three measurements against one another. All the land tortoise species
formed a tight cluster of points in the upper part of the triangle; all the
water turtles cluster in the lower part of the triangular graph. There was
no overlap, except when they added some species that spend time both in
water and on land. Sure enough, these amphibious species show up on
the triangular graph approximately half way between the ‘wet cluster' of
sea turtles and the ‘dry cluster' of land tortoises. 'The next step was to
determine where the fossil fell. The bones of
P quenstedti
and
P. talam-
payensis
leave us in no doubt. Their points on the graph are right in the
thick of the dry cluster. Both these fossils were dry-land tortoises. They
come from the era before our turtles returned to the water.
You might think, therefore, that modem land tortoises have probably
stayed on land ever since those early terrestrial times, as most mammals
did after a few of them went back to the sea. But apparently not. If you
draw out the family tree of all modern turtles and tortoises, nearly all the
branches are aquatic. Today’s land tortoises constitute a single branch,
deeply nested among branches consisting of aquatic turtles. This sug-
gests that modern land tortoises have not stayed on land continuously
since the time of
P. quenstedti
and
P. talampayensis
. Rather, their ances-
tors were among those who went back to the water, and they then re-
emerged back onto the land in (relatively) more recent times.
Tortoises therefore represent a remarkable double return. In common
with all mammals, reptiles and binds, their remote ancestors were marine

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fish and before that, various more or less worm-like creatures stretching
back, still in the sea, to the primeval bacteria. Later ancestors lived on
land and stayed there for a very large number of generations. Later an-
cestors still evolved back into the water and became sea turtles. And fi-
nally, they returned yet again to the land as tortoises, some of which now
live in the driest of deserts.

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