1980 MACFADDEN & WALDROP: FLORIDA NANNIPPUS PHLEGON 29

However, the concurrent range zone of N. phlegon and Equus is an in-
dicator of Blancan time at all other published localities of this time in-
terval. Also, the turtle Chrysemys platymarginata appears to be
restricted to the Blancan (Webb 1974). More work is needed to increase
the faunas from southwestern Florida. The complex interfingering of
marine and non-marine facies in that region (see e.g. DuBar 1958)
would provide an excellent opportunity to analyze late Cenozoic faunal
correlation and geological history of the Gulf Coast.
 ORIGIN OF Nannippus phlegon.-Dental characters have tradi-
tionally been used to distinguish fossil horse taxa. In many cases the
phylogenetic significance of minor dental variations so used is poorly
understood. This practice has been especially true for hipparions,
which consist of an unnatural or horizontal group of several genera of
predominantly Mio-Pliocene horses with isolated protocones in the up-
per molars and tridactyl feet. Attempts have been made to resolve this
undesirable taxonomic assemblage by recognizing discrete groups of
hipparions based on other characters, as well as those of the dentition.
The configuration of facial fossae has recently been shown to be of
great significance in determining phylogenetic interrelationships of
fossil horses (MacFadden and Skinner 1977, Skinner and MacFadden
1977, MacFadden 1980).
 As stated above, Nannippus phlegon is one of the rarest and least-
studied members of the late Cenozoic Equidae. It is instructive to ex-
amine the cranial morphology of this horse in order to discuss its
phylogenetic affinities later in this section. Until now, skulls of this
horse have not been described in the literature. Unfortunately no
skulls of this horse are known from Florida. There is one skull in the
AMNH collected during the 1924 expedition to Crawfish Draw, Mt.
Blanco, Texas'.
 AMNH 104708 is a fairly well-preserved skull of a subadult N.
phlegon (Fig. 14). The cheek teeth include right and left dP2-M'.
Despite some crushing, the preorbital region of this specimen
preserves some important characters. The nasal notch is not retracted
and it lies above the buccinator fossa anterodorsal to dP2. The premax-
illary bone extends posteriorly to a position that lies over dP2. There is
a moderately inflated malar crest. The lacrimal bone extends anterior-
ly to a position that lies over M'. The preorbital region is smooth with
no trace of a facial fossa. The infraorbital foramen lies over dP3-dP4.
 Numerous workers have speculated on the origin of N. phlegon.
Matthew and Stirton (1930), Webb (1969), and Dalquest and Donovan
(1973) stated that "Nannippus" lenticularis appears to be closely
'Two other partial skulls of Nannippus hlegon. JWT (Johnston West Texas) 771 and JWT 1028, from Cita Can-
yon. Texas, are housed in the Panhande Plains Museum, Canyon, Texas.