Geoscience Reference
In-Depth Information
about 2 m (6.7 ft) thick and across a horizontal distance of 23 m (75 ft) were ten
egg clutches; eight of these were apparently unmoved, implying these groupings of
eggs marked the original sites of the nests. (Keep in mind, though, that this is only
a two-dimensional sample, and many more nests might be inside the roadcut await-
inggeologicalunveiling.)Thenestswereonfourhorizonsandclosetooneanother,
which the paleontologists interpreted as good reasons to infer site fidelity and nest-
ing colonies. If so, these were the oldest known examples from the fossil record for
dinosaurs, or any other vertebrates for that matter.
One of the clutches had as many as 34 eggs, with perhaps a few more lost to
erosion, a significant number of eggs compared to other dinosaurs. From an ichno-
logical perspective, the paleontologists could not find any other evidence of actual
nest structures that fulfill all ofthe previously mentioned criteria fordinosaur nests,
but they inferred their presence from depressions that held the tightly packed eggs.
The eggs, which formed discernible rows within each clutch, also may have been
organized thusly by one of the parents after laying. Invertebrate trace fossils (bur-
rows) in the sediments surrounding the eggs show that the clutches may have been
partially buried in soil by one or more of the parents before being buried in a more
lethal way by river floods. This assumption is strengthened by the close-fitting ar-
rangement of the egg clutches. If these eggs had been left in an open nest when a
nearby river flooded, they surely would have been scattered about like balls in the
opening break of a billiards game.
Nonetheless, it was not only the nests and eggs that made this suite of fossils
trulyextraordinary,butalsosomestartlingdinosaurtracefossilsintherocksaround
the nests: teeny footprints. These four-toed tracks, preserved as natural casts on
blocks of rock that fell from the roadcut, just happened to match the front and rear
feet of a sauropodomorph like
Massospondylus
. Yet these tracks were only about
15 mm (0.6 in) long, just smaller than a U.S. dime. When compared to embryonic
Massospondylus
footbones,thetracksweremadebysauropodomorphsabouttwice
