their relatively small brains do not 
impede its formation. (This argument 
does not apply to the great oddball 
Homo sapiens, with an enormous 
brain despite its large body.) The larg- 
est australopithecine, Australopithe- 
cus boisei, has a pronounced sagittal 
crest, while smaller members of the 
same genus do not. And gorillas have 
a sagittal crest, while most smaller 
primates do not. We would make a 
great error if, using the sagittal crest 
as a shared derived character, we 
united an australopithecine with a go- 
rilla in a sister group and linked other, 
smaller-bodied australopithecines with 
marmosets, gibbons, and rhesus mon- 
keys. The sagittal crest is a “simple” 
character, probably part of the po- 
tential developmental repertoire for 
any primate. It comes and goes in 
evolution, and its mutual presence 
does not indicate common descent. 
Bennett bases her cladistic analysis 
of the genus Equus on skeletal char- 
acters, primarily of the skull. All 
horses are pretty much alike under 
the skin, and Bennett has not found 
any shared derived characters as con- 
vincing as the chromosomal similar- 
ities of chimps and gorillas. Most of 
her characters are, by her own ad- 
mission, more like the sagittal crest — 
hence the provisional nature of her 
conclusions. 
Bennett argues that the genus 
Equus contains two major cladistic 
groups — donkeys and asses on one side 
and true horses and zebras on the 
other. Thus, zebras pass the first test 
for consideration as a genealogical 
unit. Unfortunately (or not, according 
to your point of view), Bennett claims 
that they fail the second test. She 
does identify the Burchell and Grevy 
zebras {Equus burchelli and E. 
grevyi) as a sister group. But in Ben- 
nett’s scheme, the third species, the 
mountain, or Hartmann, zebra {E. ze- 
bra) does not join its cousins to form 
a larger sister group. Instead, the sister 
species of the mountain zebra is none 
other than Old Dobbin himself, the 
true horse (E. caballus)\ Thus, moun- 
tain zebras join with true horses before 
they connect with other zebras. Old 
Dobbin is inextricably intercalated 
into the cladogram of zebras— and 
since he is not a zebra by any defi- 
nition, then what, if anything, is a 
zebra? 
But Bennett’s analysis is based upon 
only three characters, none very se- 
cure. All are potentially simple modi- 
fications of shape or proportion, not 
presences or absences of complex 
structures. All, like the sagittal crest, 
could come and go. Only one potential 
shared derived character unites true 
horses with E. zebra : the “orientation 
of postorbital bars relative to hori- 
zontal plane” (a relatively less slanted 
position for a bar of bone located on 
the skull behind the eyes — not exactly 
the stuff of which confident conclu- 
sions are made). Only two potential 
shared derived characters unite Bur- 
chell and Grevy zebras: the presence 
of frontal doming (inflation of the top 
part of the skull) and relative skull 
breadth (these two zebras have long 
and narrow snouts). Unfortunately, we 
know that at least one of these char- 
acters doesn’t work very well for Ben- 
nett’s cladistic scheme because she 
admits that a member of her other 
lineage — a horse with the peculiar 
moniker of the Asiatic half-ass (E. 
hemionus)—\t&s independently 
evolved a long, narrow snout. If twice, 
why not three times? 
When we look for corroboration to 
an obvious source — numbers of chro- 
mosomes — we are again disappointed. 
As I discussed in my column of No- 
vember 1980, the various species of 
horses, despite their marked similar- 
ities of form, differ greatly in numbers 
of chromosomes. Fusion or fission of 
chromosomes may be a major mech- 
anism of speciation in mammals, and 
these differences may therefore have 
great evolutionary significance. True 
horses have 64 chromosomes, and one 
of the zebras shares this number ex- 
actly. Unfortunately, it is not E. zebra, 
as Bennett’s cladistic scheme would 
suggest. It is the Grevy zebra, E. 
grevyi. Only two horses have fewer 
than 50 chromosomes — the mountain 
zebra with the minimum of 32 and 
the Burchell zebra with 44 — but these 
species do not form a sister group 
in Bennett’s arrangement. These facts 
do not necessarily count against 
Bennett’s conclusions, however. Sixty- 
four could be the original number for 
the genus Equus. In that case, it would 
be a shared primitive character in true 
horses and Grevy zebras, and would 
not specify a sister group. Similarly, 
the shared low chromosome number 
of mountain and Burchell zebras could 
represent two independent evolution- 
ary sequences of reduction. Still, there 
is nothing in the chromosome data 
that should encourage Bennett. 
I conclude that Bennett’s proposal 
is interesting, but very much un- 
proven. Suppose, however, that she 
is right. What then would a zebra 
be? Or more specifically, how did cla- 
Equus burchelli (Burchell’s zebra) 
■£. \ Equus zebra (mountain zebra) 
1 
Equus grevyi (Grevy’s zebra) 
Joe Le Monnier 
10 
