The Thomas Farm deposit in northern Florida has yielded the largest
number of bat fossils found on the North American continent. About
2000 bat fossils have been unearthed in this paleontological treasure
trove. The Chiropteran fossil fauna of this location include at least
9 species from the following families of bats:

Vespertilionidae (5)
Molossidae (2)
Emballonuridae (1)
Natalidae (1)

Bat fossils discovered in this area date to a time span that was 31 -
16.5 million years ago. This was during the Oligocene and Miocene
periods.
The peninsular part of Florida emerged from the sea about 31 million
years ago. The type of fossil bats found in Thomas Farm in Florida
were of families, genera, and species whose fossils had previously
been discovered in the West Indies, ones that predate those of Thomas
Farm. It can therefore be concluded that a large number of bats
migrated north to Florida as it became dry land.

Neal Robbins

P.S. The Thomas Farm deposit was first discovered in 1931. It is 12
km. north of Bell in Gilchrist County in northern Florida.
Paleontologists have been excavating it for decades. The sediments
consist of alternating layers of sand and clay that fill a 30 m.
sinkhole/cave which developed during the Eocene (54-38 million years
ago). The majority of bat fossils of the site were extracted from
nearly 2 million metric tons of seiment. It was screenwashed between
1981 and 1985 by Ann Pratt and Arthur Foyer. Ann Pratt used data
collected during this project for her doctoral dissertation. The
fossils were especially heavily concentrated in a 1 m. thick layer of
lime sand.