ancestors? What did sabretooths feed on and how?
How and why did sabretooths become extinct?
The relationship ofDinofelisandMetailurusto Ma-
chairodontinae (and to each other) has always been
controversial. Some authors, (e.g. Beaumont 1978;
Werdelin and Lewis 2001), have considered them
to be members of the Machairodontinae with slight
to moderate sabretooth adaptations, while others
(Kretzoi 1929b; Hendey 1974) have considered them
to be conical-toothed cats with a tendency todevelop
sabretooth adaptations. The main feature they share
with Machairodontinae is a reduced lower canine rela-
tive to the upper canine.Dinofelisfurther shares with
Machairodontinae a deep groove or pit supero-medial
to the trochlear notch of theulna(Werdelin and Lewis
2001). This feature seems not to be present inMetai-
lurus(Roussiakiset al. 2006). Thus, it appears likely
thatDinofelisbelongs in the Machairodontinae, but
the position ofMetailurusis equivocal. This also, of
course, makes the relationship between the two
genera uncertain. Thus, the position of this group at
letter J of Fig. 2.2 is problematic, as is the placement,
even existence, of the node at letter M.
Amphimachairodusis clearly paraphyletic, asHomo-
theriumevolved from within this species group. This
is reflected in the intermediate position of letter G
(Fig. 2.2), between letter F whereAmphimachairodus
splits off from a similarly paraphyleticMachairodus,
and letter H, at the base of the monophyletic Homo-
theriini. What is not clear is exactly which species
gave rise to Homotheriini (Fig. 2.2, letter H).L. ema-
geritusfrom Kenya has a more derived dentition than
any species currently assigned toAmphimachairodus,
but is too primitive in other respects and too derived
in a few to be the ancestral taxon. Of the species of
Amphimachairodus, A. kurteniseems the most derived
dentally, butA. kabir(if the material from Sahabi
belongs there; cf. Sardella and Werdelin 2007) has
the most derived mastoid region. Whichever of these
(or some as yet unknown taxon) is ancestral toHomo-
therium, Amphimachairodusas presently conceived
becomes paraphyletic. To resolve this issue, the de-
tailed relationships ofAmphimachairodusspp. need
to be better understood.
The relationship betweenHomotheriumandDino-
bastis(andXenosmilus) is particularly interesting
(Fig. 2.2, letter I). Traditionally, they are synony-
mized in the genusHomotherium(Turner and Anto
´n
1997). However, early North American homother-
iines such as that from the Delmont Local Fauna,
South Dakota (Martin and Harksen 1974) (c.2.9–2.6
Ma) differ considerably from contemporary forms in
Eurasia (see, e.g., Ficcarelli 1979), suggesting a long,
separate evolution. In addition,Homotheriumand
Dinobastisdiffer in a number of aspects of their mor-
phology. As an example, the upper canine ofDino-
bastisis smaller than that ofHomotheriumin
specimens of approximately equal skull size (Werde-
lin and Sardella 2006, plate 1, fig. 1). This is an area
that deserves further in-depth study.
Regardless of which species in theAmphimachair-
odusgroup is closest toHomotherium, it is nearly
universally acknowledged that there is, broadly con-
ceived, an ancestor–descendant relationship be-
tween the two genera. However, the origins of the
other major Plio-Pleistocene sabretooth lineage, the
Smilodontini (Fig. 2.2, letter L), is much less clear.
This group consists of the generaMegantereonand
Smilodon, which share features such as reduced or
absent serrations on the teeth and extremely long
and relatively mediolaterally broad upper canines
compared to Homotheriini (the latter probably a
plesiomorphic feature). It is tempting to associate
them with the other Miocene sabretooth lineage,
Paramachaerodus(Turner and Anto
´n 1997) (Fig. 2.2,
letter K), but the morphological distance between
that genus and Plio-Pleistocene Smilodontini is con-
siderable and the hypothesized relationship is not
based on any clear synapomorphies. Another ques-
tion germane to this issue is the difference between
Smilodontini and Homotheriini: why is it there and
what does it mean for the functional morphology
and ecology of the respective groups? One answer
would be that the former were closed-habitat taxa
and the latter open-habitat taxa, but can such a sim-
plistic view be maintained? Martin (1980) and Mar-
tinet al. (2000) discuss some of these questions, but
more research needs to be done on the functional
differences between Homotheriini and Smilodon-
tini, and in particular on the latest Miocene species
ofParamachaerodus(P. orientalisandP. maximiliani),
to understand their ecology and feeding behaviour,
and whether these can be directly related to those of
Smilodontini.
The extinction of Homotheriini and Smilodontini
occurs at different times on different continents. In
76Biology and Conservation of Wild Felids
OUP CORRECTED PROOF – FINAL, 1/5/2010, SPi
