BULLETIN FLORIDA STATE MUSEUM


 INTRODUCTION
 Nannippus phlegon is one of the most poorly known species of late
Cenozoic horses. It is traditionally recognized as a hipparion, which is
a horizontal or polyphyletic group of late Cenozoic (Neogene) horse
genera with isolated protocones in their upper cheek teeth and tridac-
tyl limbs. At present the phylogenetic interrelationships of N. phlegon
to other hipparions remain obscure. Matthew and Stirton (1930) ex-
anded the concept of the genus Nannippus to include the earlier
species "N." lenticularis and suggested a close interrelationship with
N. phlegon, but this hypothesis has not been adequately justified.
 N. phlegon is somewhat of an evolutionary curiosity as it was ap-
parently adapted to a highly cursorial mode of life, even more so than
most of the presumably more advanced one-toed horses. Its relatively
small skelton is graceful and antelope-like. The limbs are long and
slender, and the lateral digits are extremely reduced in function. The
cheek teeth are the most hypsodont (index of crown area to height) of
the Equidae. In short, Nannippus phlegon was an extremely pro-
gressive grazer and open-country runner.
 It is extraordinary to find N. phlegon in the Blancan of Florida. The
richest accumulations of this open-country antelope-like little horse
are found at the Santa Fe River sites in some 10 m of water bordered
by lush hydric hammock vegetation. The following evidence shows N.
phlegon flourished in Florida only a few million years ago. Clearly, as
indicated by studies of paleoclimate, there were significant fluctua-
tions in Florida habitat types during the latest Cenozoic.
 Cope (1893) originally described this hipparion as Equus minutus
based on a fragmentary lower molar from Mt. Blanco in the Texas
Panhandle. Hay (1899) provided the replacement name Equus phlegon
because the name minutus had previously been used for a Eurasian
species of Equus. Although referring phlegon to Protohippus, Gidley
(1907) suggested that if upper cheek teeth had been available to him,
he might have placed this species in either Neohipparion or Hipparion.
Matthew (1926) proposed Nannipus as a subgenus of Hipparion for
very hypsodont, small, and extremely graceful hipparions with three
toes but without remains of the fifth digit or trapezium. His concept of
this subgenus was based on the N. phlegon material collected from
Crawfish Draw during the American Museum of Natural History ex-
pedition to Mt. Blanco in 1924 (for an anecdote about collecting at this
locality, see Simpson 1951:93-95). This important 1924 collection has
never been adequately described in the literature. Subsequently, Nan-
nippus was elevated to generic rank (e.g. Stirton 1940), and numerous
species of small North American hipparions have been included in it


Vol. 25, No. 1