Geoscience Reference
In-Depth Information
crowned night herons (
Nyctanassa violacea
) and black-necked stilts (
Himantopus
mexicanus
),both long-legged birds with thin toes, had been hunting forcrabs in the
muddy areas around the pond, and both species made hundreds of tracks. Wherever
these birds' feet had punctured the muddy surfaces, sunlight dried the mud exposed
along the edges of the footprints, which started cracks that grew and radiated from
these edges. The mudcracks grew enough that they sometimes joined, especially
where the birds' tracks were closer together (a result of slowing down while stalk-
ing crabs). From then on, I've looked for avian-caused mudcracks elsewhere and
was gratified to find them in Arctic environments. In the summer of 2007, while
doing field work at a dinosaur dig site on the North Slope of Alaska, I noticed bird
tracks—this time from seagulls, geese, swans, and plovers—in the muddy patches
of a river floodplain. Sure enough, wherever the muddy surfaces had dried, mud-
cracks developed, and nearly all connected perfectly to the bird footprints. The ex-
tendedsunlightofapolarsummer—withthesunsettingforonlyafewhoursduring
each 24-hour cycle—accelerated the drying after birds had pierced the muddy sur-
faces with their feet.
Didnon-avian theropods ever cause mudcracks fromtheir tracks? Yes,indeed.
Among the extensive collections of dinosaur tracks at the Amherst Museum of
Natural History in Massachusetts are slabs of Early Jurassic sandstones with gor-
geously defined natural casts of mudcracks. Some of these mudcracks have thero-
pod tracks in the middle of them, with cracks joining theropod clawmarks. The
same apparent causal relationship between theropod tracks and mudcracks shows
up in Early Jurassic rocks of southwestern Utah, in the same strata holding thou-
sands of dinosaur swim-tracks, and is one of the better surviving examples of such
intersections between dinosaur trace fossils and their correspondingly altered land-
scapes.
Nests, Empty and Otherwise
Bird nests are extremely variable as traces, ranging from simple scrapes in the
ground to some of the most elaborate structures made by any living vertebrates.
