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but those that were allowed us to make a fairly accurate reconstruction
of what the skull had looked like.
As in any other baby, the skull of our sauropod embryo was large in
proportion to the size of the body, even though the whole head was
only about two inches long. Likewise, the eye socket in our embryos
was probably slightly larger in relation to the rest of the skull than the
eye socket in adult sauropods, another characteristic of most infant
animals. In addition, a couple of embryos had tiny, pencil-shaped
teeth. The crown, or upper surface, was formed by enamel, the same
extremely durable material that forms the crowns of human teeth, as
well as those of many other animals. Dinosaur teeth come in a variety
of shapes: some are designed like steak knives to cut flesh; others form
tightly packed assemblages that serve as a grinding surface for mac-
erating tough leaves and other kinds of vegetation; others are less spe-
cialized and shaped like tiny leaves. Despite all this dental diversity,
however, only two groups of dinosaurs have pencil-shaped teeth—like
the teeth from our embryos—and both are sauropods. Pencil-shaped
teeth evolved once in the common ancestor of dicraeosaurids and
diplodocids, but similar pencil-shaped teeth also evolved within
titanosaurs. Which kind of sauropod did our teeth represent?
Titanosaurs are especially difficult to place on the evolutionary tree
of sauropods. Some paleontologists argue that they are most closely
related to diplodocids and dicraeosaurids, believing that the pencil-
shaped teeth that are typical of all of these dinosaurs evolved only
once. However, most students of sauropods argue that titanosaurs are
most closely related to brachiosaurids. These researchers suggest
that titanosaurs and brachiosaurs inherited a small claw on the first
finger from their common ancestor, and that this claw was com-
pletely lost in some later members of the group. Additional charac-
teristics in the hip and hind limb of these dinosaurs support the
idea that brachiosaurids and titanosaurs are closely related. According
to this argument, pencil-shaped teeth evolved twice—once in the
common ancestor of dicraeosaurids and diplodocids and again in
the common ancestor of titanosaurs. The fact that certain sauropods
thought to be primitive titanosaurs—animals whose skeletons are
very much like those of typical titanosaurs—lack pencil-like teeth
supports this latter interpretation of a double evolutionary origin
for this peculiar type of sauropod dentition.
