<oolo@calJournal ofthe Linnean Socieb (1994), 112: 29-63. With 26 figures
Vertebrate Palaeobioloa. Edited ly M. 3.Benton and D. B. Noman
The beginning of the equoid radiation
J. J. HOOKER
Palaeontology Department, Jvatural History Museum, Gromwell Road, London S W7 5BD
With the benefit of newly collected material, primitive equoids are analysed cladistically to
determine the detailed relationships of the families Equidae and Palaeotheriidae. The most primitive
equid is shown to be Pliolophw. Cymbalophus cuniculus and ‘Hyracothm’um’ sandrae are closely related
stem equoids. The Pachynolophidae are not equoids, but most closely related to the primitive
tapiromorph family Isectolophidae. Hallaria is sister to these two and therefore also not an equoid.
Hyracofhm’um is restricted to the type species H. leporinum Owen, part of whose lower dentition is
made known for the first time. It is closest to a restricted genus Propachynolophlcs within the family
Palaeotheriidae. The original concept of Propachynolophlcs is polyphyletic. Using the cladogram, newly
extended stratigraphic ranges and palaeogeography, an attempt is made to reconstruct the very
early speciation and biogeographical history of the group. ‘Gmbalophus’ hooka’ Godinot is
recombined in the genus Pachynolophus and ‘H.’ pemk in Pliolophu. Hallensia louisi sp. nov. and
Propachynolophus levei sp. nov. are described.
ADDITIONAL KEY WORDS:-phylogeny
palaeobiogeography.
Hyracofhm’um
~
Equidae
-
-
Palaeotheriidae
CONTENTS
Introduction . . . . . . . . . .
Dispersal scenarios . . . . . . . .
Family definitions . . . . . . . .
Newfinds.
. . . . . . . . .
Cladistic analysis . . . . . . . . .
Choice of taxa . . . . . . . . .
Choice of outgroup. . . . . . . .
Character definitions . . . . . . .
Character explanations . . . . . . .
Results of the analysis . . . . . . .
Stratigraphy . . . . . . . . .
Palaeobiogeography. . . . . . . . .
Palaeoecology . . . . . . . . . .
Systematics. . . . . . . . . . .
Acknowledgements.
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References . . . . . . . . . . . .
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INTRODUCTION
Perissodactyls appear abruptly in the fossil record of northern hemisphere
continents in the latest Palaeocene (Dashzeveg, 1988; Hooker, 1991; Koch,
Zachos & Gingerich, 1992). They are accompanied by the first known members
of the Artiodactyla and true Primates. Although the pattern of appearance is
widely recognized as a dispersal event rather than a macroevolutionary burst,
the area of origin remains elusive (Gingerich, 1989). This is because the nearest
relatives in each case in the earlier Palaeocene appear only distantly related
0024-4082/94/090029+35 $08.00/0
29
0 1994 The Linnean Society of London
30
J. J. HOOKER
and because Palaeocene faunas are poorly known outside the northern
hemisphere.
Among these earliest perissodactyls are some that are recognizable as horse
relatives. Almost all these are currently placed in the genus Hyracotherium and
are restricted to Europe and North America (Kitts, 1956; Savage, Russell &
Louis, 1965). The only Asian representative, ‘Hyracotherium’ gubuniai (Dashzeveg,
1979a), appears not to be an equoid (see below). After the earliest Eocene,
Europe was isolated from North America by the North Atlantic, which initially
divided the two continents east of Greenland (McKenna, 1983). Europe was
also isolated in the Eocene from Asia by the Turgai Straits and fi-om Africa
by the Tethys Ocean. Horse relatives in Europe underwent evolution in isolation
and have long been accepted as remote from the ‘mainstream” evolution to
modern horses, which took place in North America (MacFadden, 1976). The
horse family, Equidae, is usually restricted to the North American representatives,
wherease those that evolved in Europe and became extinct in the Oligocene
are usually placed in the family Palaeotheriidae (but see variations in treatment
below), the two combined in the superfamily Equoidea (e.g. Prothero & Schoch,
1989).
Three questions immediately arise from this scenario. Firstly, if the appearance
of equoids represents a dispersal event, was it really synchronous across the
Euramerican area? Secondly, are there any European equids or North American
palaeotheres? Thirdly are there any equoids that cannot be accommodated in
either Equidae or Palaeotheriidae?
Dispersal scenerios
Different ideas of area of origin for equoids have resulted in the concept of
diachronous appearances and different scenarios for dispersal directions. A
central American origin implies dispersal from North America to Europe (e.g.
Gingerich, 1976). An African (e.g. Gingerich, 1989) or Indian (Krause & Maas,
1990) origin implies dispersal either from Europe to North America or, if Asia
were also involved, from North America to Europe or to both independently
without crossing the North Atlantic. Relative primitiveness for the earliest
representatives in respective continents has also been invoked for determining
dispersal direction (e.g. Hooker, 1980; Godinot, 1981, 1982; Gingerich, 1989),
although this has not been based on a critical evaluation of character states.
Fumib dejinitions
How and where to draw the dividing line between equids and palaeotheriids
has long been and continues to be a problem, because of conceptual differences
as well as a patchy fossil record. Figure 1 shows a distinct difference between
those authors who accept a broad concept of the Palaeotheriidae (only omitting
Hyrucotherium because it is considered paraphyletic or ancestral to both Equidae
and Palaeotheriidae) (e.g. Remy, 1976; Hooker, 1989) and those with a narrow
concept including only the highly derived hypsodont genera (e.g. Savage et al.,
1965; Franzen, 1990). That of Butler (1952a, b) is intermediate. Whether the
authors’ concepts mean that the trees are phylograms (with ancestors at nodes
or as grades along branches), or cladograms does not radically affect their
L - - - - - -
1
EQUIDAE
1
PALAEOTHERI:IDAE
EQUIDAE
] PALAEOTHERIIDAE
EPIHIPPUS
LOPHIOTHERIUM
PACHYNOLOPHUS
ANCHILOPHUS
PROPALAEOTHERIUM
PSEUDOPALAEOTHERIUM
PALAEOTHERIUM
PARAPLAGIOLOPHUS
PLAGIOLOPHUS
LEPTOLOPHUS
ANCHILOPHUS
PACHYNOLOPHUS
PROPALAEOTHERIUM
LOPHIOTHERIUM
PALAEOTHERIUM
PLAGIOLOPHUS
1
I;;:
,
EPIHIPPUS
GE
1
EQUIDAE
E
D
PALAEOTHERIIDAE
- EQUIDAE
PACHYNOLOPHIDAE
1
EUROPEAN
1
HYRACOTHERIINAE
EQUIDAE
N. AMERICAN
HYRACOTHERIINAE
HALLENSIA
PALAEOTHERIIDAE
ANCHILOPHUS
PACHYNOLOPHUS
' H ' . SP. (RIANS)
CYMBALOPHUS
H . TAPIRINUM
H. AFF. VULPICEPS
H. VULPICEPS
H. VASACCIENSE
H , LEPORINUM
P. MALDANI
PROPAL. MESSELENSE
LOPHIOTHERIUM
P. GAUDRYI
PROPAL. HASSIACUM
PALAEOTHERIUM
PLAG. sp.1
PLAG. ANNECTENS
Figure 1. Phylogenetic models of the Equoidea according to: A, Butler (1952a, b); B, Savage et al. (1965), C, Remy (1976); D, Hooker (1989); E, Franzen (1990). N.B., A
is based on text as Butler did not provide a branching diagram. Abbreviations: H = Hyracothm'um, 0 = Orohippu; P = Propachynolophus; Propal = Propalaeothrium; Plag = Plagiolophus.
C
B
A
OROHIPPUS
PROPACHYNOLOPHUS
PACHYNOLOPHUS
PLAGIOLOPHUS
PALAEOTHERIUM
ANCHILOPHUS
PROPALAEOTHERIUM
LOPHIOTHERIUM
w
P
2
s
EM
0
z
f
+
32
J. J. HOOKER
different ideas of group composition, although only A, D and E are implicitly
or explicitly based on shared derived characters.
The broad concept of Palaeotheriidae within the Equoidea was recently based
on two dental synapomorphies (Hooker, 1989, fig. 6.5). These were: (1) presence
of an upper molar metaloph and its occlusal partner a lower molar hypolophid;
and (2) presence in at least some individuals of an upper molar centrocristal
mesostyle. Franzen (1989, 1990) took the view that the palaeothere femur and
pelvis are more primitive than those of ~yracotherium in the low trochanter
major and short simple rounded ilium, which were rather similar to those of
Phenacodus. Those of Equus and various fossil equids back to Hyracotherium have
a high trochanter major and expanded ilium with concave crista ilica and
complex tuber coxae for attachment of muscles associated with cursoriality. He
also considered that the presence of a P2 metacone in Hyracotherium was
advanced and that its absence in Palaeotherium (as also in Paraplagiolophus,
Plagiolophus and Leptolophus) was primitive. Butler (1952a, b) also considered that
Hyracotherium was more advanced dentally than palaeotheres in its premolar and
deciduous premolar molarization pattern, but he included in the Palaeotheriidae
not only Plagiolophus and Palaeotherium but also Propalaeotherium, Anchilophus and
Lophiotherium. He found the order of appearance of cusps to be variable in his
palaeothere grouping, but usually different from that of true equids, more
closely resembling (especially in Plagiolophus) that of various condylarths.
The presence of a metacone on P2 is variable within the Messel assemblage
of Propalaeotherium hassiacum Haupt (see Savage et al., 1965: 62, fig. 28), suggesting
that for this character the species is intermediate between palaeotheres and
equids. In overall dental morphology, P. hassiacum is almost indistinguishable
from Propachynolophus gaudy' (see Savage et al., 1965: 26, fig. 7), where the P'
metacone is also variable.
Franzen (1990) listed a number of autapomorphies for his restricted sense
Palaeotheriidae (comprising Palaeotherium, Paraplagzolophus, Plagiolophus, Leptolophus,
Cantabrotherium and Pseudopalaeotherium). Notable are: metacarpals longer than
metatarsals (rather than the opposite proportions in the Equidae); and length
of cervical vertebral series approaching that of the thoracic series. In addition,
these derived palaeotheres have moderate to deep narial incisions and relatively
elongate braincases, causing anterior shifting of the orbit to at least a central
position (Remy, 1992). Autapomorphies cannot be used to relate palaeotheres
with other groups, although they are important in assessing the content of the
family. Propalaeothrium hassiacum, in addition to being intermediate between
Equidae and Palaeotheriidae (sensu Franzen) in presence of P' metacone, has:
equal length metacarpals and metatarsals (Haupt, 1925: pl. 20, figs 1, 2); narial
incision as far back as P'; and elongate braincase with central orbit (Franzen,
1988: fig. 345).
It thus appears that the palaeothere (s.s.) autapomorphies of deepening narial
incision and of shortening of the hind limbs are associated with reversals
respectively in the premolar and deciduous premolar molarization gradient and
in the cursoriality specializations of the ilium and femur. In fact, earlier species
of Plagzolophus, with relatively shallow (where known) narial incisions, have more
molariform anterior premolars (Hooker, 1986: 37 1) than the advanced P.
annectens and P. minor, which were the species mainly used by Butler in his
studies. Almost complete premolar molarization in advanced Palaeotheriurn species
EARLY HORSE EVOINTION
33
was probably accomplished by subsequent strengthening of the anterior part of
the maxilla where it borders the narial incision. Even Propalaeotherium messelerne
and Lophiotherium cervulum have slight narial incisions, extending nearly as far as
P’ (Franzen, 1988: fig. 344; Deptret, 1917: 78). Moreover, the equid-type ilium
appears to be quite widespread among primitive non-equoid perissodactyls: e.g.
Hyrachyus (Wood, 1934), Lophialetes (Reshetov, 1979: 83, figs 26, 27), Heptodon
(Radinsky, 1965: 92, fig. 14) and Moropus (Holland & Peterson, 1914). It may
therefore be primitive for perissodactyls as a whole.
Therefore, molar dilambdodonty on a primitive equoid plan, perhaps
accompanied by some degree of narial incision (few skulls of primitive members
are known), seems the best way of characterizing the Palaeotheriidae. The
alternative, to restrict the family to only the derived genera (sensu Franzen),
would leave the Equidae paraphyletic (Hooker, 1989). The species Propachynolophus
gaud$ and ‘P’. maldani would thus be the most primitive currently recognized
members of the Palaeotheriidae.
The Equidae are best defined by their mode of molarization of P3. This
involves enlargement and lingual shift of the P3 paraconule to become the
fourth main cusp (pushing the protocone distally) and distal migration of the
P3 metaconid to occupy the distolingual corner of the tooth (being replaced by
a neoformed cusp in the position of the metaconid) (character 31 herein).
Butler (1952b) has argued that this paraconule and neoformed cusp are really
the protocone and metaconid respectively, which have been subjected to delayed
development. If this is so, then the distolingual cusp that forms on molariform
P3 in palaeotheres cannot be a hypocone, as it forms behind the single lingual
cusp of the nonmolariform P3 that Butler has homologized with the hypocone.
Similarly, the palaeothere P, entoconid would instead have to be a neoformed
cusp behind a mesially positioned entoconid, which is indistinguishable from a
metaconid. Whatever the terminology employed, the pattern of molarization of
P3 in equids appears to be unique within equoids and thus a valid synapomorphy
(see also Van Valen, 1982).
Hooker ( 1989) separated Puchynolo~hus,Anchilophus and ‘Hyracot~erium’ sp. from
Rians in the family Pachynolophidae, within a nominate superfamily distinct
from Equoideea but linked with the latter in the Hippomorpha. These
relationships will also be examined below.
Hallensia (based on H. matthesi, from the Middle Eocene of Geiseltal, Germany)
was originally described as a phenacodontid (Franzen & Haubold, 1986), and
later as an equoid (Franzen, 1990). The genus has many characters in common
with species attributed to Hyracotherium and is present amongst the Early Eocene
assemblages so identified by Savage et al. (1965). It is therefore included in the
present study.
New jinds
For the past three decades, intensive collecting from latest Palaeocene and
Early Eocene deposits in the Paris Basin by Messieurs Pierre Louis, Alain
Phtlizon and others has augmented the meagre sample of primitive equoids
originally available to Savage et al. (1965). These provide bigger samples, greater
diversity and stratigraphic range extensions, although assemblages are still
dominated by isolated teeth. Nevertheless, the consistent repetition of distinctive
34
J. J. HOOKER
morphologies, which can be linked to more complete material of the same or
related species, supports the species distinctions recognized here. In some cases,
however, small sample size still makes assessment of intraspecific variation
difficult (e.g. Propachynolophus levei). Future collecting should provide the necessary
test.
Godinot (1981) described but did not name a new species of Hyracotheium
from Provence as the most primitive yet discovered. This was countered by
Gingerich’s (1989) discovery of a tiny equoid in the Clark‘s Fork Basin,
Wyoming, earlier than any formerly known from North America, which he
considered slightly more primitive than any other in that continent or in
Europe. In 1990, Mr John Bruce discovered a lower jaw fragment of Hyracotherium
leporinum in the London Clay of the Isle of Sheppey, Kent, England. It is the
first to be recognized as belonging to the type species of Hyracotheiium, described
by Owen (1841) from a cranium collected over 150 years ago at Herne Bay,
Kent.
These new finds and controversies encourage a new analysis of the relationships
of the most primitive equoids that have usually been included in the genus
Hyracotherium, together with the primitive palaeotheres Propachynolophus gaudvi and
‘P’. maldani. As North American species of Hyracotherium are currently under
revision by David Froehlich, analysis of species from that continent will be
restricted here to two particularly relevant ones recently treated by Gingerich
(1 989, 1991).
CLJDISTIC ANALYSIS
Choice o f taxa
This followed the principle of using only the most primitive representatives
of well-established groups and their nearest sister taxa (Hooker, 1989). Many
of the characters used are dental. This is because so few of the species are
known from cranial and postcranial remains and their inclusion would have
meant a data matrix with much missing data. Dental characters are combined
wherever possible into functional complexes, provided that their co-occurrence
either on the same tooth or on occluding partners is consistent. Some characters
differ slightly from those used in an earlier study (Hooker, 1989). Their states
are tabulated in a data matrix (Table 1).
Choice o f outgroup
In choosing an appropriate outgroup to polarize the characters, it was
regarded as important to avoid using sister orders of the Perissodactyla (e.g.
Proboscidea, Sirenia), which are equally derived. The family Phenacodontidae
is often regarded as a primitive close relative of the Perissodactyla as well as
of several other ungulate orders, recently combined as the Pantornesaxonia (see
Thewissen & Domning, 1992). The Equoidea were considered by MacFadden
(1976) to be nested within the Perissodactyla because they share a derived
posterior position of the optic foramen, unlike other perissodactyls and most
other mammals. In this study it was therefore initially felt that primitive nonequoid perissodactyls were an appropriate outgroup for an analysis of the
EARLY HORSE EVOLUTION
35
TABLE
1. Data matrix of primitive equoids used in the cladistic analysis. ‘0’ indicates the primitive
state. The derived state for binary characters is 1, those for multistate characters are A, B, C,
D, (?) indicates missing data. N.B., the postcranial characters attributed to Cardiolophus are derived
from the closely related Homogalax. The cranial, postcranial and premolar characters attributed
to Halhnsia louisi are derived from H. matthesi.
1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 3 3 3 3
1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
Characters:
~
~
Propachynolophus gaudryi
Propachynolophw leva’
Hyracotherium leponnum
‘Bopachynolophus’ mala’ani
Pliolophus pmix
Pliolophus vulpiceps
‘Hyracothm’um’ sandrae
Cymbalophus cuniculus
Pachynolophus hookm
Hallensia louisi
Cardiolophus
Ectocion
Phenacodus
lABlOBlBClAOBOOBAlOOlllAlBOlllOl???
1AB???1AB1BO???????OlllA1???110???110????
1 A B 1 OB 1 A 0 1 AOBO 0 B A 1 1 1 1 1 1 A1 BO 0 1 1 0 1 l ? ?
lABlOBlABlAAAAlBAlOOlOlAlBOlllO????
1 B B 1 OB 1 A 0 0 B A A A l B A O 0 0 0 0 1 A1 A0 0 1 1 1 I ? 1 1
1 BB I O B I A0 OBAAAl B A O O O 1 0 1 A1 A00 1 1 1 1 1 1 1
lBAOlBlAAOBABBlBBlllOOlAlAOOllOl?ll
lBAOlBlABOBBBBlBCllllOlAlOOOllOl???
1BO 1 OAl A C O B A A A I AD1 1 1 10 1 A1 0 10 100 l ? ? ?
1BO1OA1AAOA??B1AC111lOlOlOOOlOOOOll
lBAllAlBCOABBBlADlllllOBlOlllOOO?ll
1 AAO 0 0 000 1 OAAAO 00 0 L O 1 0 0 . 4 0 0 0 0 0 0 0 0 I 0 1
000000000O0000000000000000000000000
Equoidea. It was nevertheless noticed that the polarities obtained for many
characters were the opposites of those which would be obtained by using
phenacodontids (see Thewissen, 1990) as the outgroup. This has been highlighted
by varying views on the evolution of bilophodonty and selenolophodonty in
perissodactyls (Hooker, 1984, 1989). To resolve this conflict, Phenacodus, considered
to be the most distant, reliable and sufficiently well known perissodactyl relative,
was used as the outgroup taxon. Ectocion, another phenacodontid, and the
tapiromorph Cardiolophus (possibly the most primitive perissodactyl according to
Gingerich, 1991: 209), were added to the data matrix. Other potential outgroup
taxa, such as Meniscotherium (Meniscotheriidae) and Radinsba (Phenacolophidae),
were not used as they were considered to be too autapomorphically derived
(McKenna et al., 1989; Thewissen & Domning, 1992). Lumbdotherium, probably
the most primitive brontothere (Hooker, 1989), was initially included and formed
the basal perissodactyl branch. It was excluded for the final analysis as its
highly derived dilambdodonty, combined with many primitive characters, seemed
to bias the results.
Character dejinitions
See Fig. 2 for dental terminology.
Character I : Cheek teeth generally bunodont with cresting weak (0); more
strongly crested (1).
Character 2: Upper molar preparaconule crista directed towards parastyle (0),
towards preparacrista, sometimes joining it (A); towards preparacrista, constantly
joining it (B).
Character 3: Upper molar paraconule mesiodistally distanced from protocone,
with oblique preprotocrista joining the paraconule lingually (0); paraconule and
protocone in close mesiodistd proximity with less oblique linking preprotocrista
(A); paraconule mesiodistally distanced from protocone with near transverse
J. J. HOOKER
cent
&
v
poproc
poproc
A
C
protd
I
Vd
co
I
lmetd
met&
\
entd
entkl
lmetd
b
entd
D
B
Pmetd
5
1
5
I
E
6
j
EARLY HORSE EVOLUTION
37
preprotocrista joining paraconule distally, producing a step or re-entrant angle
in the protoloph between the paraconule and protocone (B).
Character 4: Lower molar metaconid single (0); twinned (1).
Character 5: Lower molar protocristid with occlusal edge concave or notched
(0); straight (protolophid) (1).
Character 6 Lower molar metastylid a distinct cusp (0); a distal projection
near tip of metaconid (A); missing (B).
Character 7 M'-' metaconule midway between metacone and hypocone,
MI-* hypoconulid high and forming part of the arc of cusps demarcating the
talonid (0); M1-' metaconule (or its position), mesial of a line drawn between
metacone and hypocone, MI-, hypoconulid low and distal to talonid (1).
Character 8 MI-' metaconule circular and uncrested (0); with short lophoid
mesiobuccally directed premetaconule crista (A); with elongate mesiobuccally
directed premetaconule crista, forming part of well developed metaloph (B).
Character 9: MI-' metaconule strong, MI-, lacking postentocristid (0); M'
metaconule weak, forming slight step in mesiolongual margin of metaloph,
MI-? with weak postentocristid which, together with posthypocristid, forms
complete hypolophid with concave or notched occlusal edge, in a proportion
of individuals (A); state 1 constant (B); M'-' metaconule weak, with metaloph
scarcely or not stepped, Ml-2 with complete hypolophid with straight occlusal
edge (C).
Character 10: Upper molar centrocrista straight or flexed buccally slightly (0);
flexed buccally distinctly (1).
Character I I : Upper molar mesostyle large, conical, sharply demarcated from
centrocrista (0); variably developed by forming a ridge on the buccal wall
between paracone and metacone, often with joining low cinplar cusp, in a
proportion of individuals (A); missing (B).
Character 12: Upper molar buccal and lingual cusp outer walls converge at
100" (0); between 90" and 100" (A); at less than 90" (B).
Character 13: Lower molar buccal and lingual cusp outer walls converge at
40" or more (0); at less than 20" (1).
Character I 4 Lower molar cristid obliqua attaches to trigonid nearer to
metaconid than to protoconid (0); midway between metaconid and protoconid
(A); nearer to protonid than to metaconid (B).
c
Figure 2. Dental terminology. A,B, Phenacodus intenedizu Granger (BMNH.M44732 (A) and M982 1
(B)) from Wyoming; C,D, Cymbulophu cuniculus (Owen) (MNHN-TRY-l7L, from Try (C);
IRSNB.EFM107, from Erquelinnes); E, Ectocion osbomianus (Cope) (BMNH.M16471); F, Puchynolophus
hooken (Godinot) comb. nov. (University of Montpellier, PAT14). A,C are left MI'*, B,D
right M,,,, all in occlusal view. E,F are disto-bucco-occlusal views of left
showing wear
facets numbered according to Butler (1 95Za) and Hooker (1 984). Abbreviations: b = metaconid
butress; cent = centrocrista; co = cristid obliqua; ect = ectocingulum; entd = entoconid;
entld = entoconulid; hyd = hypoconid; hyld = hypoconulid; hyp = hypocone; mes = mesostyle;
met = metacone; lmetd = simple metaconid; Zmetd = twinned metaconid; met1 = metaconule;
metsd = metastylid; mf = mesiolingual metacone fold; pacd = paracristid; pad = paraconid;
par = paracone; pal = paraconule; pas = parastyle; phyc = prehypocrista; pmec = premetacrista;
pmelc = premetaconule crista; poecd = postentocristid; pohycd = posthypocristid; pomec =
postmetacrista; popac = postparacrista; popalc = postparaconule crista; poproc = postprotocrista;
ppac = preparacrista; ppalc = preparaconule crista; pproc = preprotocrista; procd = protocristid;
prot = protocone; protd = protoconid. B-D are shown reversed. Each scale bar measures 5 mm,
the short one for A & B, the long one for C-F.
38
J J HOOKER
Character 15: Lower molar cristid obliqua attaches high (0); low (1) on back
wall of trigonid.
Character 1 6 Lower molar entoconulid a distinct cusp (0); a mesial projection
near tip of entoconid (A); missing (B).
Character 1 7 Lower molar paracristid, when lightly worn, makes angle to
tooth long axis of 50" (0); 40" (A); 30" (B); 20" (C); 10" (D).
Chizracter 18: Lower molar paracristid with mesiobuccal angle rounded (0);
sharp or bulgmg (1).
Character 19: Lower molar trigonid back wall shallow (0); steep (1).
Character 20: Upper molar ectocingulum unbroken (0); may be broken at
paracone (1).
Character 21: M'-* as broad as long (0); broader than long (1).
Character 22: Upper molar protocone and hypocone separated no more widely
than paracone and metacone (0); more widely (1).
Character 23:
postmetacrista and lower molar paracristid short (0); long
11).
Character 24: Upper molar parastyle small (0); medium (A); large (B).
Character 25: M3 smaller than M2, M, not larger than M2 (0); M3 not smaller
than M2, M, larger than M, (1).
Character 2 6 M, hypoconulid as close to hypoconid as this is to entoconid
(0); more distant but closer to hypoconid than this is to protoconid (A); as far
or further from hypoconid than this is from protoconid (B).
Character 2 7 M, hypoconulid forming distal margin of post-talonid lobe (0);
bearing more distal lobe (1). Dashzeveg (1979b: 11) has described a cusp and
crest nomenclature for this region.
Character 28: M3 hypolophid incomplete (0); complete (1).
Character 29: P3 metacone smaller than paracone (0); equal to paracone (1).
Character 30: P3 with trigon relatively narrow, protoloph weak and paraconule
and P, metaconid very small and poorly defined (0) (Godinot, 1981, fig. 22;
1987, pl. 1, fig. f); trigon broader, protoloph stronger, paraconule (usually) and
metaconid much larger and better defined, but smaller than protocone and
protoconid respectively (1) (Hooker, 1980, fig. 2a; Savage et al., 1965, fig. 3e).
Character 31: P3 paraconule midway between parastyle and protocone, P,
metaconid very close to or transversely adjacent to protoconid (0); paraconule
mesiolingual of an imaginary line joining parastyle and protocone, metaconid
well separated distolingually from protoconid (1) (Cooper, 1932, pl. 50, fig. 3;
herein Fig. 23).
Character 32: Lower canine to PI diastema short (0); long (1).
Character 33: Optic foramen of orbit significantly in front of (0) or close to
(1) the posterior group of foramina, viz. anterior lacerate (=orbital fissure or
sphenoidal foramen), foramen rotundum and anterior opening of the alisphenoid
= alar) canal.
Character 34: Navicular facet of astragalus convex (0); saddle-shaped (1).
Character 3.5: Astragalar canal present (0); absent (1).
z
Character explanations
'The protoloph-protolophid complex (characters 2-6)
Thc relative mesiodistal position of upper molar paraconule and protocone
13) is normally closed linked to whether the metaconid is single or twinned (4).
EARLY HORSE EVOLUTION
39
If the paraconule is distant from the protocone, it occludes between the twinned
cusps of the metaconid, producing an extensive buccal phase wear facet-‘a’
(see Fig. 2F and Hooker, 1984: 239). If the paraconule is mesiodistally close
to the protocone and more or less aligned with it on the protoloph, the
metaconid is simple and facet 2a very restricted (see Fig. 2E). There are
exceptions. Cardiolophus and Homogalax have the paraconule aligned with the
protocone but the metaconid twinned; here, however, the twinning is oblique
not mesiodistal, producing a slightly different occlusal pattern (Hooker, 1984:
239). Phenacodus has well separated paraconule and protocone, but single
metaconid; here the difference may be due to the different orientation of the
preparaconule crista. The orientation of the preprotocrista is closely linked to
the relative positions of paraconule and protocone. The simplest way of making
its orientation more transverse is to move the paraconule distally relative to
the protocone. This suggests that the separated state of the paraconule and
protocone, where the preprotocrista is transverse, is not equivalent to that
where the preprotocrista is oblique; hence the sequence of states employed in
character 3 and its separation from character 4.
The twinned state of the metaconid is closely linked to the primitive notched
state of the protocristid, again except in Phenacodus and Cardiolophus. It is
therefore treated as a separate character (5). The more distal of the two
metaconid cuspules has usually been called a metastylid. This is anomalous, as
the paraconule occludes between the cuspules. In other mammals the metastylid
occludes with the mesiolongual side of the protocone (e.g. Hooker, 1986: 277278, text-fig. 28). Court & Hartenberger (1992) have demonstrated the same
pattern of occlusion in the primitive hyracoid Titanohyax and called into question
the homology of paraconule and metastylid in this group. It seems unnecessary
to question the homology of the paraconule, but the perissodactyl ‘metastylid’
appears non-homologous with that of hyracoids, primates, meniscotheriids or
phenacodontids. ‘Qmbalophus’ hookeri has both metastylid (6) and twinned
metaconid cuspules (4) and in comparison with Ectocion demonstrates well the
cusp homologies (Fig. 2E & F). The metaconid, twinned or otherwise, can be
distinguished by its distolingual butress.
T h e metalop~-~po~ophid
complex (7, 8, 9)
The occlusal relationships are similar to but not identical with those of the
protoloph and protolophid (Hooker, 1984: 239). Mesiodistal separation of the
metaconule and hypocone and thus stepping of the metaloph is not matched
by twinning of the entoconid, but by absence of a postentocristid. The occlusal
relationships here, however, are consistent among the taxa, and uppers and
lowers are combined into one character (9).
Polarization of the different perissodactyl states is, however, difficult. The
absence of a postentocristid and mesiodistal separation of metaconule and
hypocone are regarded as more primitive than the more bilophodont states for
two reasons: (1) although phenacodontid hypoconulid and entoconid cusps are
close together, a postentocristid appears to be missing; (2) other mammals
progress in the same way (e.g. nyctalodonty to myotodonty in bat lower molars,
see Menu & Sigt (1971), even though here the occluding crest in the upper
molars is a premetacrista not a metaloph).
The crest previously identified in perissodactyls as a premetaconule crista
J. J. HOOKER
Ficqres 3 5. Hallensia louisi sp. nov.; 3 , mesial \riews of upper preultimate molars; 4, holotype,
MNHN.Mu-2 18-L (left) from Mutigny; B, MNHNAV-4770 from Avenay (right tooth rrversed); 4.
h i c c a l view of incomplete unworn right M ,
(reversed) (h1NHN.Mu- 12391) h m hlutigny.
showing cntoconulid; 5, dorsal view of frapmentaq mandible with symphysis (AV-841 -Ph, cast
BhINH.M.5 I 708) from Avenay. Figure 6, I[vrucvracotherium leporinum Owm, disto-occlusal \iew of lcft
h l , IRI\lNH.%f51682) from Sheppey. Scale bar for Figs 3, 4, 6 = 5 m m , for Fig. 5 = 20mm.
(Hooker, 1989) appears to be partly homologous in Phenucodus with an enamel
fold on the mesiolingual metacone wall unassociated with the metaconule. It
links up with a neoformed premetaconule crista, which develops mesiobuccally
from the metaconule in primitive perissodactyls and gradually disappears as it
becomes bypassed by the lengthening crista (8) (all stages in development of
the metaloph). Elongation of the premetaconule crista leading to advanced
devdopment of the metaloph is not coupled with development of the hypolophid
(9).
Bucial crest complex ( 10- 18)
There is a complex relationship between flexing of the centrocrista (lo),
convergence angles of buccal and lingual cusps (12, 13) and orientations of the
cristid obliqua (14) and paracristid (17). The state of each structure is clearly
influenced to some degree by the others, but the relationships are not consistent
among taxa. Several characters have therefore been employed to describe the
different elements. The great variation in upper and lower molar cusp
convergence angles (from 0 to B) within Hullensiu louisi (Fig. 3A,B) was impossible
to code for any one of the states described. The variation is considered to be
rather an autapomorphy of the taxon, and both characters 12 and 13 were
coded as ‘?’ to avoid biasing the result.
The mesostyle (1 1) is variable in both presence/absence and detailed structure
in the perissodactyls studied here, although it appears to be constant in
phenacodontids. As has been argued elsewhere (Hooker, 1989: 82-85), this cusp
probably rcpresents reduction and loss in perissodactyls. It does not seem
possible, however, in practice to distinguish between ‘centrocristal’ and ‘cingular’
types. Its ‘reduced’ state (A) usually consists of a narrow blunt ridge running
up the buccal wall midway between the paracone and metacone, and this may
merge with (Fig. 14) or be distinct from (Hooker, 1989, fig. 6.3C) a small cusp
arising from the ectocingulum. Alternatively the ridge may be stronger but lack
the cingular cusp (Fig. 18), or the mesostyle may simply be a small conical
cusp in the middle of the centrocrista (Fig. 13). In all the species studied here,
EARLY HORSE EVOLUTION
41
where such mesostyles occur, there are some individuals which lack a mesostyle
altogether. Therefore the only practical solution seemed to be to lump all these
in a state intermediate between constant presence and absence.
The angle of the paracristid (17) varies slightly with wear and the character
states are scored for the lightly worn state. When unworn, the occlusal edge
of the paracristid tends to flare buccally towards its mesial end, reducing overall
obliquity by about 10”. In contrast, advanced wear increases the obliquity. The
difference between Cymbalophus cuniculus and ‘Hyracotheriunz’ sandrae for this
character is subtle, with a small mount of overlap. Six specimens of C. cuniculus
and 4 of ‘H:sandrae were examined.
P3 paraconule enlargement and Py metaconid distal sh$ (31)
The normal development of the metaconid leads it to shift from a position
very close to but slightly distolingual to the protoconid in its incipient state, to
one transversely opposite the protoconid as it enlarges and occludes with the
stronger, more transverse protoloph usually associated with a broader trigon
(Butler, 195210). The distolingual position of the large metaconid in the 1 state
of character 10 is clearly related occlusally to the position and saliency of the
paraconule.
Optic foramen (33)
This has been discussed by MacFadden (1976: 8, fig. 6), who recognized
that the derived state was restricted to horses and palaeotheres and the primitive
state was widespread in other perissodactyls (e.g. Radinsky, 1965: 72, fig. 2)
and non-perissodactyl mammals. His figure shows the position of the optic
foramen changing in relation to both the posterior group of foramina and the
more anterior ethmoid foramen, whose positions remain constant. Whereas in
the derived state this is true of more primitive horses (e.g. Mesohippus,
Hyracotherium-Savage
et at., 1965, fig. 23), in modern Equus, the ethmoid
foramen has migrated posteriorly too (Sisson & Grossman, 1953: 71, fig. 48).
The significance of the character is also less clear in view of some seemingly
anomalous occurrences. Perhaps it is related to the position of the orbit. A
specimen of the brontothere Palaeoyops has been shown to have a posterior
optic foramen, although the ethmoid foramen was apparently not visible
(Osborn, 1929: 326, fig. 275). Hallensia matthesi and Pachynolophus lavocati appear
to have the primitive condition (Franzen, 1990: 190, fig. 7; Remy, 1972, fig.
8). In Pachynolophus liuinierensis (Savage et al., 1965: 44, fig. 20) the state seems
intermediate, although this might be influenced by crushing. Pa~aeotherium(Remy,
1992, fig. 24) and Plagiolophus (pers. obs.) also have the primitive condition. A
strange occurrence of the derived state is in Ectocion (Thewissen, 1990, fig.
55A), whilst the primitive state occurs in Phenacodus (Thewissen, 1990, figs 53A,
54A).
Results
of the anabsis
The computer program used was PAUP 3.0 (Swofford, 1990). The search
for the shortest ‘tree’ was conducted using the branch and bound method.
Multistate characters were ordered. Rooting was by outgroup (Phenacodus) with
all character states at zero. The analysis produced three topologically different
42
J. J. HOOKER
maximum parsimony cladograms of 92 steps, with a consistency index excluding
uninformative characters of 0.544 and a retention index of 0.653. The three
cladograms differ only in the interrelationships of three terminal taxa. Hyracotherium
Lepoiinum, Propachynolophus leuei and P. gaudyi. The relationship which fitted best
with stratigraphic occurrence was preferred (i.e, P. leuei and P. gaudyi more
closely related to each other than to H. Leporinum). Both ACCTRAN and
DELTRAN optimizations were employed, and the characters which showed
different distributions according to each (3, 4, 9, 11, 14, 17, 20, 26, 28, 33)
are listed in the explanation to Figure 7. Choices between the equally
parsimonious but different patterns of character state changes between the two
optimizations are discussed below.
The choice for characters 3 and 4 is to ensure that equivalent changes
appropriate to their occurrence and occlusal relationships took place at the
same nodes. In character 9 it is to avoid a 2-step reversal to the primitive
statc in H. leporinum. That for character 11 favours reversal in line with
dilambdodont trends. That for character 26 avoids unnecessary reversal. The
primitive state for character 28 in ‘Propachynolophus’ aff. maldani (see systematic
descriptions), which occurs earlier than in typical members of the species,
indicates independent acquisition of the derived state in this species and P,
gaudyi. If a stem perissodactyl position for brontotheres is correct, acquisition
of character 33 in Ectocion plus perissodactyls and subsequent reversal in
tapiromorphs is the likely alternative.
Major conclusions from the analysis are:
( 1) Hyracotherium leporinum, the type species, nests within the Palaeotheriidae.
(2) ‘Hyracotherium’ penzix and ‘H.’ uulpiceps are sister taxa and equids, for which
the genus Pliolophus is available.
(3) Equidae and Palaeotheriidae are sister groups.
(4) CymbaLophus cuniculus and ‘H.’ sandrae are closely related stem group equoids.
N.B. ‘H.’ sandrae Gingerich, 1989, is not a Hyracotherium. It may belong in
the genus Systemodon Cope, 1881, because of a similar degree of bilophodonty
to that of the type species-S. tapirinus (Cope, 1875) (see Radinsky, 1963:
10; Gingerich, 1991: 183, 185, for discussion of the genus, including early
incorrect referral of species now included in Homogalax).
(5) Pachynolophus hookeri (recombined from Cymbalophus) and thus the family
Pachynolophidae are sister groups to Cardiolophus in the Isectolophidae and
therefore not equoids.
(6) Hallensia louisi (and thus the genus) is stem group to the Pachynolophidae
plus Isectolophidae and therefore not an equoid.
Pigurt. 7. Maximum parsimony cladogram of 92 steps, best fit with stratigraphy and using
combination of ACC’IRAN and DELTRAN options (see text), generated using PAUP 3.0 from
ddtd matrix in Table 1. Broad bar = synapomorphy; narrow bar = normal polanty Iiomoplas):
X = revwsal. Reversal to 0 state of multistate character indicated by character number in brackets.
Optimization differences per numbered node: ACCTRAN: I 3A, 14A, 33; 2 4, $)A, I1A: 14B,
17A-B, 20; 3 l I B , 26A; 4 3B, 14AR, 17AR, 20R; 5 9B, IIAR, 26B, 28; 6 j14)R; 8 9C; 10
9A- (9)R, 20, 28R; I 2 (9)R; 15 4R; 17 SB, 17C, (26)R; 18 (3)R, 17C, 33R; I 9 9B-C, 17D; 20
I I B , 14AR; 21 3A, 28; DELTRAN: I 14A, 2 9A, 11A, 17A, 3 3A, 4 3B, 4, 26A, 33; 5 26B:
6 (14)R; 7 9B; 8 9C, 28; 10 (9)R, 20; 11 9B, 28; 12 (9)R, 11B; 15 l l B , 14B, 1 7 K , 20; 16 26A;
17 9B, 17C; 18 4, 17B-C, 20; 19 9B-C, 17D; 20 11B; 21 3A, 14B, 28; 22 14B; 23 3A, 33.
W W W
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SANDRAE
ECTOCIOIY
HALLENSIA L O U I S I
CARDIOLOPHUS
PACHYNOLOPHUS HOOXERI
'HYRACOTHERIUM'
PLIOLOPHUS VULPICEPS
PLIOLOPHUS PERNIX
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J J HOOKER
The Equoidea are supported as a monophyletic group so long as Hallensza
and the Pachynolophidae are excluded. If C. cuniculus and ‘H.’ sandrae are
included, then it is shown to be defincd by three synapomorphies. Of these,
the incipient molarization of P3 (30) is the most important. The other two
(loss of metastylid and entoconulid) are paralleled by non-equoid perissodactyls
more advanced than those analysed here. Equoids may not be defined by the
posterior position of the optic foramen, if the derived state is really present in
brontotheres. This region is not yet known for C. cuniculus or ‘H.’ sandrae and
the state is reversed in more advanced palaeotheres. The Palaeotheriidae are
defined principally by greater elongation of the M3 hypoconulid (26B) and by
upper molar characters that are either reversals or paralleled elsewhere: incipient
secondary breakage of the link between preparaconule crista and preparacrista,
flexing of the centrocrista and probably incipient reacquisition of a mesostyle
[although this may never have been completely lost); all three are minor trends in
dilanibdodonty which become accentuated in more derived members of the family.
An Adams Consensus of a total of thirty 91, 92 and 93 step cladograms
iafter removal of P. leuei) shows consistency of relationship between H. ~ ~ p o r i ~ u ~
and P. g a u d y and between P. pernix and P. uulpiceps; ‘P.’maldani, however, is
equally related to these two clades and to a fourth, an unresolved polychotomy
of ‘H.’ sandrae, C. cuniculus, P. hookeri, H. louisi and Cardiolophus.
Stratigraphy
The order of branching can be readily compared with that of stratigraphic
occurrence only in northern Europe, where time correlation by other organisms
is fairly reliable. If one adds the ‘aff taxa (see systematics section) to the ranges
of the definitely identified species, the correspondence between the order of
stratigraphic appearance and that of the cladogram branches is quitc close,
although ‘P.’maldani appears too late (Fig. 8). ‘Propachynolophus’ sp. 1 may be
closely related to the latter, and when better known may resolve the problem.
In the Clarks Fork Basin, ‘H.’ sandrae certainly occurs below P. pernix (Gingerich,
199l), but lack of detailed correlation with Europe prevents the stratigraphic
testing of these branches relative to the rest of the cladogram.
l i p r e 8. Chart showing Stratigraphic ranges of ~irimitiveequoids in the Clarks Fork 8~7\’yomingJ,
L.ondon and Paris plus Belgian Basins. Rangcs ending in arrows indicate continuation of the
or clade; those with T-bars indicate extinction. Transverse arrows indicate suggcsted
dispersal. Lorahtics in capitals, lilhostratigraphy in l o w r case (lithostratigraphy of‘ Frcrich and
Belgian localities can be found in Systematics st=ction). Lt’a0-4 arc mammal z o n e (Gingerich,
199 I ) . Probable position of Palaeocenr-Eo~.ene boundary shown with heavy line. Peckzchara
charophy~r~,,
Tt’etzeliella (dinocyst) and NP (,calcareous nannoplankton) are biostratigraphic indicator5
(Hooker. 1991). Anomalies 24A arid B help date the opening of the Denmark Strait and the
Fhglish Channrl. Levels brlow 24B in the Clarks Fork and London Basins all Ilelong to the
i-~.\-ersedi n t e n d of Cliron 21 (Aubry, 1985). itlthough the Pans and Belgium basins are 1-oml,int.d,
rhr perissodactyl record in the latter Is restricted to Dormaal and Erquelinnes. Brackcts indicate
rcu orkrd fixsils; the Suffolk Pebble Reds fauna is reworked almost certainly from the Reading
Krds, which contain P. dz.wvza;. NR: no time or thickness dimension is signified by widths of
atratigmphic units; morrovcr, mammal rangcs s h v n for Europe represent only one or a few spot
(recurrences per unit, not a continuous record. ‘l’he North American record is considrrilhly more
rstensivc than shown, with a succession of species in !Val-3 that appear closely related to ‘Ff.’
m i d m e (see Gingerich, 1991).
EARLY HORSE EVOLUTION
I
I
I
I
m
I3A37 S
'I3NNVH3
I
dOTONXh'3VdO8d
HSI?
-J
45
16
J. J HOOKER
The dating of the southern French locality of Palette containing P. hookeri is
problematic. It was thought by Godinot et al. (1987) to be coeval with the
Dormaal reference level (MP7), although no restricted range taxon was in
common with any from Dormaal or a well correlated locality (Brunet et al.,
1987). An MP7 age is essentially based on the rodent Pseudoparamys cezannei
Hartenberger (in Godinot et al., 1987), which is judged to be of similar
evolutionary grade to an undescribed species of Pseudopnramys from Dorrnaal.
However, P. hookeri may also occur at Fournes and Monze, which fit within
calcareous nannoplankton zones NP10 (high) or NP11 (low) (Marandat, 1991)
which almost certainly postdate North Sea Basin MP7 localities, which correlate
with NP9 (Hooker, 1991).
There are several taxa which are closely related to those analysed here but
are too poorly known to be analysed themselves. Some are described herein.
Pliolophus aff. uu@ceps from Soissons and Sinceny appears transitional between
typical P. uulpiceps and Cymbalophus cuniculus, as single M”‘ and MI,, have the
B state of character 9 (the metaloph-hypolophid). Unfortunately, variation cannot
be established and the two teeth could simply be members of a variable
assemblage displaying state 9A. If not, then C. cuniculus and ‘H.’ sandrae might
branch successively below the palaeothere-equid split, rather than form a clade
of their own. O n present information, however, this would mean a less
parsimonious cladogram.
Mexican ‘Hyracothm‘um’ seekinsi Morris, with its large upper molar parastyle
and well developed lower molar hypolophid, yet short premetaconule crista and
centrally attached cristid obliqua (Novacek et al., 1991: 48-50), is also poorly
known but would fit well as a sister taxon to Cardiolophus and thus not be an
equoid (cf. Hooker, 1989: 98).
It seems clear that it is in deposits of MP7 age or older that we must
search to improve the record of primitive equoids in order better to resolve
their relationships.
PALAEOBIOGEOGRAPHY
The cladogram is plotted on a latest Palaeocene-earliest
Eocene
palaeogeographic map of North America and Europe (Fig. 9). The earliest
equoid in each continent is shown by the analysis to be also the most primitive.
As the most primitive members of the nearest sister group occur in Europe,
this continent is the current best candidate €or the equoid centre of origm.
From there, dispersal to North America would have been either via Greenland
or via Asia in the latest Palaeocene prior to the palaeothere-equid split. The
choice depends on the authenticity of the Asian Eocene equoid record. This is
based on isolated teeth and a few jaw fragments of Hyracotherium gabuniai
Dashzeveg, 1979a, and Gobih$pus menneri Dashzeveg, 1979b, from Mongolia,
and Propalaeotherium sinense Zdansky, 1930, and P. henaangense Young, 1944, from
China. The Hyracotherzum has molars with strong bilophodonty , the uppers
having a large parastyle; these characters remove it from the Equoidea and
ally it with tapiromorphs. The other three species show dilambdodonty typical
of either brontotheres or fairly advanced palaeotheres. The close proximity of
the M, hypoconulid to the hypoconid (primitive state of character 26) occurs
in brontotheres, whereas the derived distant (26B) state occurs in palaeotheres.
EARLY HORSE EVOLUTION
47
Figure 9. Maximum parsimony cladogram from Fig. 7, minus Cadiotophus, Ectocioii and Phenacodus,
superimposed on palaeogeographic map of North America and Europe around the PalaeoceneEocene boundary (mainly after Tiffney, 1985, map 3). Continuous thin lines indicate ancient
shorelines. Thicker lines encircle geographic ranges. Opposed arrows indicate position of opening
of Denmark Strait (D) and English Channel (C). 1: Propachynolophus gaud& 2: Propachynolophus ha’;
3: Hyracothmum leporinum et aff.; 4: ‘Propachynolophus’ maldani; 5: Pliolofihus uulpiceps; 6: Pliolophus perniq
7 : Qmbalophus cunuulus; 8: ‘Hyracothenum’ sandrae; 9: Pachynolophus hookni, 10: Hallensia louisi.
Moreover, they have an oblique M3 hypolophid, like brontotheres and unlike
palaeotheres, where this crest is nearly always transverse. Although meagre, the
weight of evidence favours the identification of these three taxa as brontotheres,
and no characters make them unequivocal equoids. In view of the abundance
of Asian Eocene mammal localities recording a diversity of tapiromorph
perissodactyls (Russell & Zhai, 1987), the absence of equoids appears real and
supports the westward Greenland route of equoid dispersal to North America.
Speciation of palaeotheres in the north European area seems to have been
endemic, there being no authenticated records of the family in North America.
Land bridges existed between North America and western Europe via
Greenland at this time (McKenna, 1983). However, it seems that by magnetic
Anomaly 24 time, rifting east of Greenland had all but changed from subaerial
to submarine (Anderson, 1988) and cut off the intercontinental land route. At
the same time the London Clay sea (division B, calibrated to zone NP11 and
Anomaly 24B) had reached its maximum extent (King, 1981; Aubry, 1985;
Townsend & Hailwood, 1985). This was when the first influx of nummulites
and planktonic foraminifera into the North Sea Basin took place, an event
usually taken to represent the opening of the English Channel and consequent
separation of the British and continental west European landmasses (Curry et
a k , 1978: 10; Murray & Wright, 1974).
P. pernix appeared abruptly in the Clarks Fork Basin, Wyoming, just above
the 2000 m level but still within Chron 24R (Gingerich, 1991; Butler, Gingerich
48
J. J. HOOKER
& Lindsay, 1981). As P. vulpiceps is slightly more primitive, it is likely that
dispersal took place a second time from Europe. It could not have been long
after this that interchange of terrestrial faunas between North America and
Europe ceased.
The opening of the English Channel may have caused the isolation of
Hyrarotherium leporinum in Britain and Propachynolophus in France. P. uulpiceps
apparently became extinct in France earlier than in England. Because of a
poor mammal record in England in the very late Palaeocene (Soissons-Meudon
time), it is not certain whether P. aff. uulpiceps or H. aff. leporinum existed there
then as well as in northern France. H. aff. leporinum therefore either dispersed
there or differentiated into typical H. leporinum in England and into the
Proparhynolophus levei-gaud$ lineage in France. It is interesting that, overall, more
primitive morphotypes survived longer in southern England than in northern
France. In contrast, none of the isolated English species appears to have given
rise to latcr equoids, whereas differentiation in northern France led to more
derived palaeotheres, which radiated through the remaining Eocene.
PA1AEOECOLOGY
‘The dental variation in bilophodonty, selenodonty and bunodonty in these
primitive perissodactyls reflect adaptations to diets of leaves, fruits or complex
mixtures of the two. The pattern of frequent parallelisms and reiersals in these
adaptations suggests fluctuating conditions and may reflect the climatic changes
documented around the Palaeocene-Eocene boundary (Koch et al, 1992), pcrhaps
influencing length of fruiting seasons. Trends in dilambdodonty, producing a
seleriolophodont or bunoselenodont dentition, may have allowed more efficient
mastication of both fruit and leaves by specialization in different areas of the
tooth. Low crowned selenolophodont ‘Propalaeotherium’ messelense (Haupt) had a
diet of leaves and occasionally fruits, according to gut contents preserved at
Messel (Franzen, 1988). The different early perissodactyls may also have occupied
different microhabitats. A given European site tends to be dominated by one
species: e.g. P. uulpiceps at Abbey Wood and Pourcy; Hallensia louisi at Mutigny.
SYSTEMATICS
This section contains descriptions of the new material, new species and new
combinations. For the new material, length and width measurements in
millimetres are given in that order, separated by a ‘ x ’ in the ‘Material’
sections; in figures brackets are estimates. Abbreviations are as follows:
BMNH = Natural History Museum, London, MNHN = Muskurn National
d’Histoire Naturelle, Paris; the suffix Ph attached to a specimen number
indicates the private collection of M. A. Phklizon. The synonymy format follows
Matthews (1973).
Suborder Tapiromorpha?
family uncertain
Genus Hallensia Franzen & Haubold, 1986
l j p e sfieries. H. matthesi Franzen & Haubold, 1986, from the Middle Eocene
of Geiseltal, Germany.
EARLY HORSE EVOLUTION
49
Included species. H. parisiensis Franzen, 1990, Sables a Unios et TCrtdines,
Monthelon (Marne), France, H . louisi sp. nov.
Hallensia louisi sp. nov.
(Figs 3-5, 12-15)
v.
1922
v?
vp.
vp.
1922
1965
1965
Propachynolophus nov. sp.; Teilhard de Chardin, p, 69, fig. 33C, pl.
3, fig. 30.
Hyracotherium; Teilhard de Chardin, p. 52, fig. 26B.
Hyracotherium; Savage, Russell & Louis, pp. 6-8, figs 2d, f, i, j.
Hyracotherium larger form; Savage, Russell & Louis, pp. 10 & 11,
13, figs 4f-h.
Egmology. After M. Pierre Louis, the collector of most of the material known
of this species.
Holoppe. Left M”‘ (MNHN.Mu-218-L), from the Argiles a Lignites d’Epernay,
Mutigny (Marne), France (Figs 3A, 13) (8.0 x 10.2).
Parappes. Right M3 (MNHN.Mu-12371) (Fig. 14) (7.9x9.3); 2 left M3s
(MNHN.Mu-201-L, Savage et al., 1965, fig. 2c and Mu-221-L, Savage et al.,
1965, fig. 29; fragment of right dentary with MI-, (MNHN.Mu-12303, Fig. 15)
(6.8 x 5.6; 7.8 x 6.5); 2 right M I p (MNHN.Mu-6283, Savage et al., 1965, fig.
2d; Mu-12391, Fig. 4); all from Mutigny.
Referred material. Right MI’‘ (MNHN.Av-4770, Savage et al., 1965, fig. 4f);
part of left and right dentaries with symphysis, worn P,-M,, right P,-, and left
and right P, alveoli (Av-841-Ph) cast BMNH.M51708); 2 left M3s (MNHN.Av65519, Savage et al., 1965, fig. 4g; Av-(61)14685 (broken mesially)); from the
Argiles a Lignites d’Epernay, Avenay (Marne), France. Right P3-M1 (Louis
Colln, MNHN) (7.9 x 8.4; 7.6 x 9.3; 8.1 x 9.6) from the Sables de Guise, Condten-Brie (Aisne), France. Buccal half of left MI-‘ (MNHN.Py-16-CN) from the
Falun de Pourcy, Pourcy (Marne), France. Left DP3 (Lepage Collection, cast
BMNH.M44757, Fig. 12) from the Sands of Dormaal, Brabant, Belgium.
Doubful material. Right M, minus trigonid (Laboratoire de Gtologie, Universitt
de Marseille; Teilhard de Chardin, 1922, fig. 26B) from the ConglomCrat de
Meudon, Meudon (Hauts-de-Seine), France.
Diagnosis. Small species of Hallensia (MIp 8mm long); upper molars with
distinct but small parastyle on MI-’, variable on M3; upper molar premetaconule
crista, short, joining prominent mesiolingual metacone fold.
D~erential diagnosis. H. parisiensis is slightly larger and H. matthesi larger still.
Both have the upper molar parastyle so reduced that it is scarcely differentiated
from the cingulum that almost encircles the tooth. The extent of the cingulum,
used to differentiate H. parisiensis from H. matthesi, seems to be too variable to
be a useful character. H. matthesi has a longer premetaconule crista.
Description and discussion. The keynote of the dental characters of Hallensia is
variability. This is particularly true of the convergence angle of buccal and
lingual cusps of both upper and lower molars (Fig. 3). This varies between the
primitive and most advanced states of characters 12 and 13 (above). The upper
molar centrocrista may be straight or slightly flexed buccally, with or without
mesostyle at the crest of the flexure (cf. Fig. 14 with Savage et ak, 1965, fig.
2f,g). The enamel surface may be strongly rugose or smooth. The very
variability appears to be an autapomorphy of the genus rather than an incipient
50
J. J. HOOKER
trend towards the states of any of the other taxa treated here, where convergence
angles and other characters are intraspecifically constant.
Some of the more characteristic dental features are: the lingual position of
the metaconule and the notch separating it from the hypocone on upper
molars; and the relatively high lingual cusps of both upper and lower molars.
If the upper molar buccal and lingual cusp convergence is high, as in the
holotype and Mu-12303, the cusp tips approach one another rather closely
(Figs 13, 15). l’he notch between the metaconule and the hypocone is therefore
also distinctly lingual. This can be seen well in the M’s from hlutigny and the
Dorrnaal DP3. Unfortunately, a distinct metaconule with a lingual notch is not
a constant feature, although apparently the small size of the parastyle on
311-2 .
1s.
?‘he broken mandible (Fig. 5) has teeth that are both very worn and corroded
,perhaps by a predator’s stomach juices). It shows a short symphysis broken
immediately in front of the P, alveoli, suggesting a very short postcanine
diastema, as in H. matthesi (see Franzen & Haubold, 1986). The post-P, diastema
is of moderate length. Few diagnostic features can be gleaned fr-om the tooth
crowns, except the relatively broad inflated outline of MI-*, which compares
well with those in Figure 15.
Fami& Pachynolophidae Pavlow, 1888
Discussion. Hooker (1989) placed the genera Pachynolophus and Anchilophus in
this family and in superfamily Pachynolophoidea, as sister group to the
Equoidea comprising Equidae plus Palaeotheriidae. The new analysis shows
Pachynolophidae not to be equoids and makes the Pachynolophoidea unneccssary.
Genus Pachynolophus Pomel, 1847
73’pe species. Pachynolophus duualii Pomel, 1847.
Included species. P. livinierensis Savage, Russell & Louis, 1965; P. cesserasicus
Ger\,ais, 1849; P. garimondi Remy, 1967; P. lauocati Remy, 1972; P. boixedaten.ri,s
Crusafont & Remy, 1970; P. hooken (Godinot, 1987); P. sp. nov. (‘Hyracotheriurn’
aff. cunzculus Owen (Godinot, I98 I)).
Emended diagnosis. Primitive perissodactyl characters: P3 with narrow trigon,
weak protoloph and essentially no paraconule; P, with very small, poorly
differentiated metaconid; upper molars with preparaconule crista confluent with
preparacrista and with unstepped protoloph; lower molars with twinned
metaconid cuspules, concave protocristid, cristid obliqua joining back of trigonid
midway between protonid and metaconid, and steep protoconid backwall; buccal
and linLpal cusps converge at 90” on upper molars; and at 40,‘ on lower
molars. Derived characters: upper molars with unstepped metaloph, without or
with weak metaconule and no mesostyle; lower molars with straight complete
hypolophid and postcingulid extending lingual of hypoconulid on M, :a; postcaninc
diastema long.
Discussion. The advanced bilophodonty of the metaloph-hypolophid complex
relates Pachynolophus and other genera in the family with primitive tapiromorphs.
EARLY HORSE EVOLUTION
51
Figures 10 & 11. Puciynolophus hooken’ (Godinot) comb. nov. from Palette, occlusal views. 10, left
P’-M3 (FSL.2641); 11, left MI-, (UM.PAT14). Scale bar = 5mm. Photographed from casts coated
with ammonium chloride.
Pachynolophus hookeri (Godinot et al., 1987) comb. nov.
(Figs. 2B, 10, 11)
v.
1965
%.
?
1987
1991
Hyracotherium cf. cuniculum Owen or Propachynolophus sp.; Savage,
Russell & Louis, pp. 14-15, fig. 6.
Cymbalophus hookeri Godinot et al.: pp. 281-283, pl. 1, figs f-1.
cf. Cymbalophus hookeri Godinot ou Propachynolophus sp.; Marandat,
pp. 113-115, pl. 7, figs 1-4.
Dzstribution. Early Eocene of Palette (Provence) (type locality) and probably
Fournes (Minervois) and Monze (Corbieres); all southern France.
Emended diagnosis. Very small Pachynolophus (M‘ 5.43-5.72 mm long). Primitive
features are: upper molars with straight centrocrista, width only slightly greater
than length, premetaconule crista short joining prominent mesiolingual metacone
fold, no flattening of metacone buccal wall, ectocingulum broken at paracone,
M3 not longer than M’ or M2; lower molars with entoconulid high on mesial
entoconid wall, metastylid retained, longitudinal paracristid with mesial bulge
and M, with incomplete hypolophid.
Discussion. Godinot (1987) has thoroughly described this species, recognizing
its mosaic of primitive and advanced features. However, the analysis here shows
that it does not belong to Cymbalophus. A closely related species still unnamed
from the Early Eocene of Rians (Provence), but referred by Godinot (1981) to
Hyracotherium aK cuniculus and by Hooker (1989) to ‘Hyracotherium’ sp., is slightly
more advanced than P. hookeri in a number of characters: it is slightly larger;
the upper molars are distinctly broader than long and their premetaconule
crista is longer; the lower molar paracristid is oblique; and M3 has a complete
J. J. HOOKER
32
hypolophid. The lower molars are too worn to detect if they retain metastylid
or entoconulid. The grade of molarization of P, (very small metaconid) fits that
of P3 of P. hookeri (narrow trigon with weak protoloph). P, is not yet known
for P. hookeri and part of the assessment of character 31 above was based on
the species from Rians.
Suborder Hippomo?pha
Superfarnib Equoidea Gray, 1821
Farnib Equidae Gray, 1821
Genus Pliolophus Owen, 1858
q p e species. Pliolophus vulpiceps Owen, 1858.
Included species. ‘Hyracotherium’ pernix Marsh, 1876, and probably also ‘H.’
uasacciense (Cope, 1872) should be placed in Pliolophus.
Pliolophus vulpiceps Owen, 1858
(Figs 22 & 23)
(For earlier synonymy, see Hooker, 1980, under H. vulpiceps and H. aK.
z‘u@zceps).
vp.
v.
v.
\,.
1965
1980
1980
1989
Hyracotherium; Savage, Russell & Louis, pp. 8-10, figs 3c, e, f.
Hyracotherium vulpiceps (Owen); Hooker, pp. 104, 106, 108, fig. 3.
Hyracotherium aK. vulpiceps (Owen); Hooker, pp. 106-1 08, fig. 2.
Hyracotherium aff. vulpiceps (Owen); Hooker, pp. 83-84, figs 6.3L,
6.41-K.
HoloQpe. Partial skeleton (see Hooker, 1980) (BMNH.44 1 1.J, 44 1 15a &
M10657-61), from the Harwich Stone Band, Harwich Member, London Clay
Formation, Harwich, Essex, England.
Xeu material. Left P2 (MNHN.PY-157 15) (5.2x 4.5); left P4 (MNHN.unnumbered) (6.3 x 7.5); 1 left and 3 right M’% variously damaged (MNHN.PY2431, PY unnumbered (Fig. 22) (7.1 X-), PY-42-L, PY-72-L); 1 right and 2
left M’s (MNHN.PY-41-L (Savage et al., 1965, fig. 3c), PY-39-L ( 7 . 4 9.0),
~
PY-40-1, (ibid. fig. 3d)); left D F (MNHN.PY-46-L, ibid. fig. 3b); 2 left P,s
(MNHN.PY-45-L, ibid. fig. 3e; PY-34-Cn) herein, Fig. 23, 6.4 x 3.6:; 2 left P,s
(MNHN.PY-3-L (Savage et al., 1965, fig. 3f); PY-20-Ph, 6.2 x 4.3); left lower
molar trigonid fra<pent (MNHN.PY-11-L). All from the Falun de Pourcy,
Pourcy (Marne), France.
Description and discussion. This species was previously definitely recorded only
from the London Clay of the London Basin. Two P3s from Pourcy show
variation for position of the inetaconid. One shows this cusp lingual of the
protoconid (Savage et al., 1965, fig. 3e), the other shows it distinctly distolinLgual
of the protoconid (Fig. 23), the advanced state of character 31 as in the
holotype (Cooper, 1932, pl. 51, fig. I). The distolingual position indicates a P3
with a large mesiolingual paraconule, as in the holotype and other specimens
from the London Clay. The only P’ and P, from the Blackheath Beds of
Abbey Wood (referred to Hyracotherium aff. vulpiceps by Hooker, 1980) have the
primitive state for character 3 1, but the Pourcy specimens indicate a variability
Figures 12-15. Hallensia louisi sp. nov.; 12, left DP3 (original Lepage Collection, cast BMNH.M44757) from Dormaal; 13, holotype left M”‘ (MNHN.Mu-218-L) from
Mutigny; 14, paratype right M’ (reversed) (MNHN.Mu-12371) from Mutigny; 15, paratype right M, (reversed) (MNHN.Mu-12303) from Mutigny. Figures 16 & 17.
Pliolophus aff. uulpiceps Owen; 16, right MI,, (reversed) (MNHNSN-34-BN) from Soissons; 1 7, left MI’* (MNHNSY-4-De) from Sinceny. Figure 18. ‘Propachynolophus’ aff.
maldani (Lemoine), left MI/* (MNHN.AV-1012-L) from Avenay. Figure 19. ‘Pfpachynolophur’ sp. 1, left MIr2 (MNHN.PY-133-L) from Pourcy. Figure 20. Hyrucotha’um aK
leporinum Owen, damaged right M’” (reversed) (MNHN.Mu-12329) from Mutigny. Scale bar = 5mm. All occlusal views, photographed from casts coated with ammonium
chloride.
w
Ln
54
J. J. HOOKER
which it is impossible to test in the other assemblages because so few specimens
exist. For this reason, the Abbey Wood assemblage is here referred to this
species.
It is possible that acquisition of the derived state of the P3s spread through
time from only some individuals to the entire population. It is significant that
the largest P3 paraconules (e.g. Cooper, 1932, pl. 50, figs 1, 3) belong to
specimens occurring in phosphatic nodules (reworked into the Pliocene Red
Crag), which typift- levels of the London Clay above division 123 (C. King,
pers. comm.) and are thus younger than the holotype or the Pourcy or Abbey
IYood assemblages.
The other teeth show variation for such features as presence/absence of
upper molar mesiolingual metacone fold, but are constant in their near straight
upper molar centrocrista, strongly stepped metaloph, with distinct metaconule,
and strongly convergent lower molar buccal and lingual cusps. One M3 (PY39-L) and the upper molars of BMNH.38801 (see Cooper, 1932: pl. 50, fig.
1; Hooker, 1980) have a small mesostylar ridge on the buccal wall; all other
specimens examined lack one, and the species is coded state 2 in Table 1.
Pliolophus aff. uulpiceps Owen, 1858
(Figs 16 & 17)
,thzteriul. Left MI/' (MNHN.SY-4-De) (7.3 x 8.9) from the Sables de Sinceny,
Sinceny (Aisne), France; right
(MNHN.SN-34-BN) (7.2 x 5.1) and left M,
lacking hypoconulid (MNHN.Braillon Colln unnumbered) (- x 5.5) from the
Argiles a Lignites du Soissonnais, Seminaire Pit, Soissons (Aisne), France.
Description. The M"' shows a fairly weakly stepped metaloph with short
premetaconule crista, joining mesiolingual metacone fold, and weak metaconule.
It is otherwise typical of P. uulpiceps. The M,/' has a complete notched
hypolophid, but is otherwise typical of P. uulpiceps. Only one of 6 individuals
from Abbey Wood has MI,? with a complete hypolophid (Hooker, 1989, fig.
6.41).
Discussion. These teeth cannot reliably be identified with P. uu@ceps because
of certain primitive traits. They are nevertheless apparently very close and
extend the range of the genus Pliolophus down to the latest Palaeocene in the
Paris Basin.
Fami& Pulueotheriidue Bonupurte, 1850
Genus Hyrucotherium Owen, 1841
7jpe species. Hyracotherium leporinum Owen, 1841. As the cladistic analysis shows
this species to be nested within the Palaeotheriidae, the concept of the genus
is restricted to the type species, other described species being distributed in
other genera (Pliolophus, Cymbalophus, Systemodon, Pachynolophus, Hallensiu).
Hyrucotherium leporinum Owen, 184 1
(Figs 6, 21)
Hologpe. Facial part of cranium with alveoli or roots of both canines, both
P's, left P2 3, left M7 and crowns of right P2-M3 and left F-M2 (BM(NH) no
Figure 21. Hyrucotherium leporinum Owen, left dentary fragment with M2-3 and damaged M , (BMNH.M51682) from Sheppey; A, occlusal view; B, buccal view; C, lingual
view. Figures 22 & 23. Pliolophus uulpiceps Owen, from Pourcy, occlusal views; 22, right MIr2 (reversed) (MNHN.unnumbered); 23, left P, (MNHN.PY-34-CN). Scale
bar = 5mm. Coated with ammonium chloride; figs 22-23 are casts.
J. J. HOOKER
56
M16336) from division B of the London Clay Formation, Studd Hill, near
Herne Bay, Kent.
Topo&pe. Palate with left and right P2-M3 (Sedgwick Museum, Cambridge no
C2 1361).
.New material. Part of left dentary with M2-3, the hypoconid part of M I and
parts of the roots of P, from the London Clay Formation (not in situ, beach
collected) probably division D or E (which form the adjacent cliffs), near
Barrows Brook, west of Warden Point, north coast, Isle of Sheppey, Kent,
BMNH.M51682. M,: 8.9x6.3; M,: 10.8x5.9.
Description. The Sheppey specimen (M51682) has a lightly naturally worn M3
with pro<gressively heavier wear forwards to M I . M3 is complete but M2 has
much of the lingual side of the metaconid broken away. This makes it difficult
to judge whether there were twinned metaconid cuspules, although mesiodistal
extent of this cusp complex suggests that they were present. In contrast, the
mesiodistally shorter metaconid of M, lacks distinct twinning (often a feature
of equoid M,s). The most striking feature of the teeth is the narrowness of the
talonid basins. The cristid obliqua is straight and joins the protolophid just
lingual of the midline of the tooth and extends high up it. This contrasts with
H. uulpiceps and H. aff. uulpiceps, where the cristid obliqua is lower mesially and
joins the protolophid at the midline. There is no postentocristid and the
posthypocristid is deeply concave distobuccally. These features plus narrowness
of the teeth makes them relatively elongate and restricts the area of the talonid
basins. The cingulum is complete buccally. The M, hypoconulid terminates a
long narrow lobe. A weak, mesially bowed crest links the hypoconid and
entoconid on M, and M,.
Di.rcussion. Despite the high convergence angle of the buccal and lingual cusps
of the upper molars, that of the lowers is much lower. There are considerable
similarities with Propachynolophus gaudvi (see Teilhard de Chardin, 1922; Savage
et al., 1965) and P. levei sp. nov. (Fig. 24), the main differences being the
lingually broken lower molar hypolophid and strongly stepped upper molar
metaloph. These similarities, as well as reasonable occlusion obtained by
manipulating the lowers and uppers, strongly support attribution of the new
dentary to Hyracotherium leporinum. The large mesially situated metaconule and
mesially bowed hypoconid-entoconid crest may be autapomorphies of the species.
A notable skull feature of H. leporinum is the shallow narial incision, which
extends almost to the front of Pi (Owen, 1841, pl. 21, fig. 3). Admittedly, P'
has a more anterior position than in more advanced palaeotheres because of
the presence of a post-PI diastema, but the incision is significantly deeper than
in P. uu@ceps, where it scarcely reaches the canine (Owen, 1858, pl. 2). This
is further support for the inclusion of Hyracotherium in the Palaeotheriidae.
Hyracotherium aff. leporinum Owen, 1841
(Fig. 20)
v.
v.
1922
1965
Hyracotherium; Teilhard de Chardin, p. 52, fig. 26C.
Hyracotherium; Savage, Russell & Louis, p. 9, fig. 3g.
Muterial. Right
(Laboratoire de Gkologie, Universiti. de Marseille) from
EARLY HORSE EVOLUTION
57
the Conglomkrat de Meudon, Meudon (Hauts-de-Seine), France (Teilhard de
Chardin, 1922, fig. 26C). Right M"' (MNHN.PY-146-LX) (7.2 x 8.4) and left
M, (MNHN.Py-43-L, Savage et al., 1965, fig. 3g), from the Falun de Pourcy,
Pourcy (Marne), France. Right MI/', broken mesiobuccally (MNHN.Mu-12329;
Fig. 20) from the Argdes Lignites d'Epernay, Mutigny (Marne), France.
Description. These few broken and worn teeth have the diagnostic characters
of H. leporinum but are smaller. They extend the record of the now restricted
genus Hyracotherium (possibly the species H. leporinum) back to the latest
Palaeocene (Conglomkrat de Meudon) in the Paris Basin. The upper molars
show the large mesial metaconule, strongly convergent buccal and lingual cusps
and incipient dilambdodonty. The lowers show the obliquely orientated cristid
obliqua, which extends high up the back of the trigonid, lingual of its midline.
Both upper molars also show a mesostyle, variably present in typical H.
leporinum.
Genus Propachynolophus Lemoine, 1891
l j p e species. P. gaudvi (Lemoine, 1878) from the Sables a Union et Ttrtdines
of the Epernay area, France.
Propachynolophus levei sp. nov.
(Fig. 24)
Epmology. After Dr J. Levt, who collected the holotype.
Holoppe. Left maxilla with P3-M3 (Levt Collection, casts in MNHN and
BMNH.M49399) from the Sables de Cuise, Condt-en-Brie (Aisne), France. P3:
6.6 x 7.5; p: 7.1 ~ 8 . 8 M'.
;
8.1 ~ 9 . 7 M2:
,
9.5 x 11.3; M3: 8.7 x 10.0.
Referred material. Two right M'/'s (MNHN.AV-842-Ph) (8.5 x 10.3) from the
Argiles a Lignites d'Epernay, Avenay (Marne), France; and (MNHN.SZ-7337)
(8.2 x 10.5) from the Sables A Unios, Stzanne-Broyes (Marne), France.
Lhagnosis. Small Propachynolophus, M' 8.1 mm long. Upper molars with metaloph
weakly to strongly stepped, with metaconule of variable saliency; premetaconule
crista may be elongate mesiobuccally or shorter, but the mesiolingual metacone
fold is largely missing; centrocrista slightly flexed buccally but mesostyle absent
in the three available individuals.
Description. Only upper teeth are known but they are mainly very similar in
morphology to the type species, although lacking the constancy of certain
features. P. hei thus shows generally more primitive traits. The P3 shows a
strong protoloph detached from the preparacrista but with a weak but double
paraconule; postprotocrista and metaconule are completely lacking; the protocone
is in a relatively mesial position opposite the paracone and its lingual wall
interrupts an otherwise encircling cingulum. P" differs from P3 in being more
transversely elongate, the cusps forming an isosceles triangle, having a continuous
cingulum, large paraconule and small metaconule. On the preultimate upper
molars of all three specimens, the protoloph and metaloph are detached at
their buccal ends from more buccal cusps or crests. There is, however, variation
in length of the premetaconule crista; it is longer in the Avenay and SkzanneBroyes specimens than in the holotype, its elongation being associated with
weakness of the metaconule. The well marked metaconule of the holotype is
Figure 24. Propachynofophus fevei sp. nov. holotype left P”-M’ (orignal Lcvk Collection. cast RMNH.RT49399) from CondP-cn-Brie. 1Figures 25 & 26. ‘Propachynolophus’
sp. 1 ; 25, right P’ ~ M ”(reversed) (Ipswich Museum, unnumbered\ from ?Fdlkenham: 26, right R I ’ (rwerscdj (MNHN.Collii Berton, 196-1. innurnbered) from Sincrny. Scale
bar = 5 mm. Occlusal views, coated with ammonium chloride, figs 2.1 and 26 arc casts.
EARLY HORSE EVOLUTION
59
nevertheless smaller and less mesially salient than in Hyracotherium leporinum. The
Avenay specimen contrasts with the holotype in having a large parastyle.
M3 in the holotype is only slightly contracted distally but differs from M’-2
mainly in having no metaconule and in the protoloph contacting the preparacrista.
Discussion. The slightly more primitive appearance and smaller size, together
with earlier occurrence, suggests that P. levei evolved into P. gaudryi. The
constant mesostyle absence (1 1B) may reflect the inadequate sample and it may
really have been as variable as P. gaudryi.
‘Propachynolophus’ aff. maldani (Lemoine, 1878)
(Fig. 18)
vp. 1965
Hyracotherium; Savage, Russell & Louis, pp. 6, 8, figs 2a,e.
Material. Right P (MNHN.Mu-5919, Savage et al., 1965, fig. 2a); left M3
(MNHN.Mu-104-Ph) (7.6 x 8.6); and two left M,s broken mesially (MNHN.Mu6567, Savage et al., 1965, fig. 2e; Mu-1210) (-x 5.2) from the Argiles a Lignite
d’Epernay, Mutigny (Marne) France. Right P4 (MNHN.AV-42-Ph) (5.7 x 6.8)
and left M”‘ (MNHN.AV-1012-L) (7.4 x 9.1) from the Argiles a Lignite
d’Epernay, Avenay (Marne), France. Left M,12 (MNHN.SZ-7336) (7.9 x 5.4)
from the Sables a Unios, SCzanne-Broyes (Marne), France.
Description. Size is very similar to that of ‘P.’ maldani (see Savage et al., 1965:
27-34, figs 14 & 15). The M”’ is nearly identical in morphology to their fig.
14d, but lacks a premetaconule crista. The M,s on the other hand show a
lingually broken instead of a complete hypolophid (cf. their fig. 2e with their
fig. 15a-f). The MI/‘ in contrast has a complete hypolophid, as in typical ‘P.’
maldani.
Discussion. These specimens occur earlier than typical assemblages of ‘P.’
maldani from the Sables i Unios near Epernay. The cladistic analysis clearly
shows Propachynolophus to be polyphyletic, if ‘P.’ maldani is included. ‘P.’ maldani
has a unique combination of advanced characters, even though they are
individually shared with other taxa. Its trends in dilambdodonty (characters 10
and 11) as well as high convergence angle of lower molar buccal and lingual
cusps (character 13), while maintaining the distinctness of the upper molar
metaconule, link the species with ‘Propalaeothenum’ parvulum (Laurillard) and
‘Propalaeothenum’ messelense (Haupt) (if distinct) and with Lophiothenum Gervais.
The latter differ mainly in more convergent upper molar buccal and lingual
cusps, constant mesostyles and trends in premolar molarization. They also tend
to have very long postcanine diastemata (unknown for ‘P.’ maldani). Savage et
al. (1965: 67-72) argued against ‘P,’ messelense belonging to Lophiotherium as
originally assigned by Haupt (1925). It seems, nevertheless, together with ‘P.’
parvulum, to be very close to Lophiotherium. As Lophiotherium currently comprises
a lineage of three ‘chronospecies’, the possibility of enlarging its concept to
include ‘P,’ parvulum, ‘P.’ messelense and F.’ maldani would be seriously worth
investigating.
J. J. HOOKER
ti0
‘Propachynolophus’ sp. 1
(Figs 19, 25 & 26)
1‘.
1965
Hyracotherium (cf. cuniculum Owen-the
Russell & Louis, pp. 10-13, figs 4a-e.
small animals); Savage,
New material. Left MI’* (MNHN.PY-133-L, Fig. 19) (6.4 x 7.8) fi-om the Falun
de Pourcy, Pourcy (Marne), France. Left P3 (MNHN.unnumbered, Cahuza
Colln) (5.4 x 5.8), left P4 (MNHNSY-5-De) (5.9 x 6.6) and right M3
jMNHN.unnumbered, Berton Colln 1984, Fig. 26) (6.7 x 7.5) from the Sables
de Sinceny, Sinceny (Aisne), France. Crushed cranium, with abraded left and
right P” M”, largely embedded in phosphatic nodule (Ipsuich Museum,
unnumbered, Fig. 25), Red Crag (reworked from London Clay, probably above
division A3), ?Falkenham, Suffolk, England; right P3: 5.4 x 6.0; P’: 5.8 x 7.0;
h f ’ : (6.6)x 7.3-t; M’: (6.7) x -; M3: 6.6 x 7.2+.
Description. The teeth described by Savage et al. (1965), although small, clearly
do riot belong to Cymbalophus cuniculus. The upper molar (no lower teeth known)
shows trends in dilambdodonty (including mesostyle), convergence of buccal and
lingual cusps and distinct stepping of protoloph and metaloph, with discrete
metaconule, very similar to ‘P.’ maldani, although the teeth are distinctly smaller.
The new teeth are similar except that they show little (Pourcy) or no
(Sinceny) sign of centrocristal flexing and no development of a mesostyle. The
damage that the teeth of the Falkenham cranium suffered during reworking
has removed most useful features of the molars, particularly buccally. However,
the premolars are better preserved and P3 shows a broad trigon with strong
protoloph and paraconule and slightly distally placed protocone. The worn
upper molar metalophs show stepping but no clear metacanule swelling and
the specimen may not be conspecific with those from the Paris Basin.
Discussion. The material indicates at least one distinct species of \7ery small
equoid, probably closely related to P ’ maldani and therefore a palaeothere. It
is neither named nor included in the cladistic analysis here, as at least part of
the lower dentition is required before its characters can be assessed accurately.
.\CKNOLVLEDGEMENTS
I would like to thank the following for access to collections in their care:
Dr D. E. Russell (MNHN, Paris), Dr P. D. Gingerich (University of Michigan,
Ann Arbor), Mr R. A. D. Markham (Ipswich Museum), Drs B. Engesser and
F. Wiedenmeyer (Naturhistorisches Museum, Basel), Dr M. Hugueney (Facultt
des Sciences, Lyon) and Dr M. Godinot (USTL, Montpellier 11). Special thanks
t o illr J. Bruce for making known to me the lower jaw of Hyracotherium leporinum
that he collected and for presenting it to the Natural History Museum, London
(BMNH). I am grateful to Dr P. L. Forey for help with thc computing.
Professor P. M. Butler, Dr D. E. Russell, Dr N. Court, Dr P. Tassy, Dr M.
Godinot and Dr J. A. Remy provided stimulating discussion; and Dr N. Court,
Mr D. Froehlich, Dr D. E. Russell and Dr. J. G. M. Thewissen critically read
the manuscript. Mr P. Hurst of the Photo Unit, NHM, produced the
photographs.
EARLY HORSE EVOLUTION
61
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