John Bois asked, "Are there any burrows known from the Mesozoic--burrows
from small to middle size mammals?"
I have recovered six burrows from the Early Cretaceous of Maryland,
right in your 'stomping ground', John. They are not the long, beautifully
spiraled type I have seen at the Smithsonian's NMNH, however. Each is
hollow, with walls more consolidated (and by whatever means tougher) than
the matrix. The longest I have found is only about 20 cm. In most cases
there is a wider, oval opening that tapers into the burrow. In a different
type, there is a small opening of about 18 mm diameter, but this opens into
a rather remarkable chamber of about the volume of a medium to large
grapefruit.
Having no well-informed idea of what kind of animals made the burrows,
crayfish
Another interesting burrow-shaped find from the Early Cretaceous of
Maryland in just under 8 cm long, with a wall thickness of roughly 17 mm,
and with its internal cylindrical hollow being roughly 4 cm in diameter. It
was constructed by mud-cementing what seem to have been small sticks. While
the several dinosaur paleontologists who have seen it have been puzzled as
to its maker, a well-informed geologist/naturalist friend assures me that it
is a fossilized "crayfish tower". It is in great shape, and if the friend
is right, I suspect it may be a rather rare fossil find, for the Late
Cretaceous.