The Hapalodectinae and a phylogeny of the Mesonychidae (Mammalia, Condylarthra)

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BY
THE
AMERICAN
MUSEUM
OF
NATURAL
HISTORY
CENTRAL
PARK
WEST
AT
79TH
STREET,
NEW
YORK,
N.
Y.
I0024
NUMBER
236I
MARCH
IO,
I969
The
Hapalodectinae
and
a
Phylogeny
of
the
Mesonychidae
(Mammalia,
Condylarthra)
BY
FREDERICK
S.
SZALAY1
Hapalodectine
mesonychids
are
relatively
rare
fossils
of
continental
Eocene
sediments
of
Asia
and
North
America.
Among
the
archaic
pla-
centals,
these
predominantly
small
condylarths
represent
one
of
the
most
unusual
specializations
toward
an
inferred
carnivorous
and
piscivorous
mode
of
life.
Specimens
of
the
subfamily
were
recorded,
described,
and
discussed
by
Matthew
(1909,
1915),
Gazin
(1962),
Chow
(1965),
Szalay
and
Gould
(1966),
and
Guthrie
(1967).
In
addition
to
the
description
of
previously
unreported
material
both
from
the
early
Eocene
of
North
America
and
the
late
Eocene
of
Central
Asia,
the
systematics
of
this
subfamily
is
reviewed
here.
It
is
suggested
that,
in
spite
of
the
first
occurrence
of
hapalodectines
in
the
early
Eocene,
the
subfamily
was
probably
distinct
from
meso-
nychines
prior
to
the
middle
Paleocene.
A
few
subtle
but
probably
im-
portant
characters
of
hapalodectines
may
be
more
primitive
than
those
of
middle
Paleocene
mesonychines,
such
as
Dissacus.
One
of
these
is
the
unreduced
M3
(longer
than
the
preceding
molars)
in
Hapalodectes.
There
appear
to
be
three
possibilities
for
the
origin
of
the
Hapalodectinae:
(1)
differentiation
from
early
or
middle
Paleocene
mesonychids,
more
primitive
than
known
mesonychines;
(2)
derivation
from
Asiatic
or
1
Department
of
Vertebrate
Paleontology,
the
American
Museum
of
Natural
History;
Assistant
Professor,
Department
of
Anthropology,
Hunter
College,
City
University
of
New
York.

AMERICAN
MUSEUM
NOVITATES
North
American
(less
likely
than
Asiatic)
mesonychines
during
the
Paleocene;
or
(3)
Hapalodectes
and
their
allies
represent
a
distinct
family
derived
from
unknown
sources.
The
last
possibility
does
not
appear
to
be
probable
at
present.
As
a
result
of
a
re-evaluation
of
the
interrelationships
of
mesonychid
genera,
a
tentative
phylogeny,
together
with
continental
occurrences,
is
presented.
No
functional
analysis
is
detailed
in
this
paper.
Another
work
by
me
on
the
origin
and
evolution
of
the
mesonychid
feeding
mechanism
will
be
submitted
elsewhere,
in
which
pertinent
functional
considerations
of
the
Hapalodectinae
are
treated.
I
thank
Dr.
Glenn
L.
Jepsen,
of
Princeton
University,
for
lending
me
several
specimens.
I
am
grateful
to
Drs.
Bobb
Schaeffer,
Chairman
of
the
Department
of
Vertebrate
Paleontology,
Malcolm
C.
McKenna,
Frick
Associate
Curator,
and
Richard
H.
Tedford,
Associate
Curator,
of
the
Department
of
Vertebrate
Paleontology,
the
American
Museum
of
Natural
History,
for
the
use
of
facilities
to
conduct
this
research.
The
work
was
supported
by
the
National
Science
Foundation
(Grant
GB-7418),
and
by
a
City
University
of
New
York
Doctoral
Faculty
Research
Grant.
Figures
10-12
and
18
were
drawn
by
Miss
Biruta
Akerbergs,
and
figure
19
was
prepared
by
Mr.
Raymond
J.
Gooris.
The
following
abbreviations
are
used:
A.M.N.H.,
the
American
Museum
of
Natural
History
P.U.,
Princeton
University
U.S.N.M.,
United
States
National
Museum
of
the
Smithsonian
Institution
SYSTEMATICS
ORDER
CONDYLARTHRA
COPE,
1881
SUPERFAMILY
MESONYCHOIDEA
OSBORN,
1910
FAMILY
MESONYCHIDAE
COPE,
1875
SUBFAMILY
HAPALODECTINAE
SZALAY
AND
GOULD,
1966
Hapalodectinae
SZALAY
AND
GOULD,
1966,
p.
152.
INCLUDED
GENERA:
Hapalodectes
Matthew
(1909),
and
an
unnamed
new
genus
("Hapalodectes
lushiensis")
in
Chow
(1965).
DISTRIBUTION:
Early
Eocene
of
North
America
and
late
Eocene
of
Asia.
EMENDED
DIAGNOSIS:
Hapalodectines
differ
from
mesonychines
and
andrewsarchines
in
their
distinctly
smaller
size,
in
the
greater
transverse
2
NO.
2361

SZALAY:
HAPALODECTINAE
constriction
of
the
lower
teeth,
in
the
presence
of
a
hypocone
on
the
upper
molars,
and
in
having
deep,
vascularized
embrasure
pits
between
the
lingual
parts
of
the
upper
cheek
teeth.
They
differ
from
all
known
mesonychines
in
having
M3
the
longest
tooth
of
the
lower
molars.
HAPALODECTES
MATTHEW,
1909
Hapalodectes
MATTHEW,
1909,
p.
498.
TYPE
SPECIES:
Hapalodectes
leptognathus
(Osborn
and
Wortman,
1892).
INCLUDED
SPECIES:
Hapalodectes
leptognathus
and
Hapalodectes
serus.
DISTRIBUTION:
Same
as
for
subfamily.'
DIAGNOSIS:
Same
as
for
subfamily.
Hapalodectes
leptognathus
(Osborn
and
Wortman,
1892)
Matthew,
1909
Figures
1-12
?Dissacus
(Pachyaena)
leptognathus
OSBORN
AND
WORTMAN,
1892,
p.
112.
Hapalodectes
leptognathus
(OSBORN
AND
WORTMAN):
MATTHEW,
1909,
p.
498.
Hapalodectes
compressus
MATTHEW,
1909,
p.
499.
HOLOTYPE:
A.M.N.H.
No.
78,
fragment
of
right
horizontal
ramus
with
broken
P4-Ml
and
almost
complete
M2,
from
Gray
Bull
beds
in
1891,
Big
Horn
Basin,
Wyoming.
HYPODIGM:
A.M.N.H.
Nos.
78,
79,
Gray
Bull
beds.
A.M.N.H.
Nos.
12781,
12782,
12783,
P.U.
Nos.
16181,
17698,
18161,
Lysite
beds.
A.M.N.H.
No.
14748,
Lost
Cabin
beds.
U.S.N.M.
No.
22447,
New
Fork
beds.
DISTRIBUTION:
Wasatchian
sediments
of
North
America.
SPECIFIc
DIAGNOSIS:
Differs
from
the
Asiatic
Hapalodectes
serus
in
retain-
ing
a
vestigial
metaconid
and
in
having
slightly
larger
teeth.
DESCRIPTION,
INTERDEME
AND
INTRADEME
VARIATION:
As
discussed
be-
low,
the
Gray
Bull,
Lysite,
and
Lost
Cabin
samples
are
not
large
enough
for
one
to
determine
whether
or
not
there
is
a
meaningful
difference
in
the
relative
depth
of
the
dentaries
of
these
samples.
The
horizontal
ramus
is
deepest
under
M2
and
M3.
On
the
medial
side
of
the
dentary
there
is
a
long
horizontal
groove,
probably
for
the
origin
of
the
mylo-
hyoid
muscle.
The
ascending
ramus
is
unknown.
The
long
symphysis
was
loose.
In
all
the
specimens
known
there
are
two
mental
foramina,
1
I
wish
to
correct
the
mistake
in
Szalay
and
Gould
(1966,
p.
168,
and
table
10)
that
Hapalodectes
occurs
in
Bridgerian
rocks
in
North
America.
The
known
distribution
is
correctly
stated
in
that
paper
on
page
152.
3
1969

AMERICAN
MUSEUM
NOVITATES
^:~~~~~~~
~
'
&t/t
FIG.
1.
Hapalodectes
leptognathus,
medial
view
of
broken
right
dentary
with
P4-M2,
holotype,
A.M.N.H.
No.
78,
Gray
Bull
beds,
Big
Horn
Basin,
Wyoming.
under
P1
and
under
P2
(or
slightly
posterior
to
P2),
respectively.
The
canine
is
relatively
large
and
slender;
it
is
known
only
in
A.M.N.H.
No.
14748
(fig.
6).
The
crown
of
P1
is
not
known.
It
appears
that
this
tooth
was
in
many
cases
double-rooted.
However,
it
cannot
be
established
at
present
whether
the
two-rooted
or
the
one-rooted
con-
dition
of
P1
was
more
prevalent.
On
the
right
dentary
of
P.U.
No.
17698
(from
Lysite
beds)
and
on
A.M.N.H.
No.
14748
(from
Lost
Cabin
beds) there
is
a
shallow
alveolus,
anterior
to
the
large
one,
for
the
anterior
root
of
P1.
In
contrast
to
P1,
which,
judged
from
the
alveoli,
was
procumbent,
the
double-rooted
P2
(known
only
in
A.M.N.H.
No.
14748)
stood
erect
NO.
2361
4

SZALAY:
HAPALODECTINAE
in
the
dentary.
The
protoconid
of
this
tooth
points
slightly
posteriorly,
whereas
that
of
P1
was
probably
inclined
anteriorly.
The
crown
of
P3
is
unknown,
but,
judged
from
the
size
of
the
alveoli,
it
was
the
same
size
as
P2,
or
slightly
larger.
FIG.
2.
Hapalodectes
leptognathus,
lateral
view
of
broken
right
dentary
with
P4-M2,
holotype,
A.M.N.H.
No.
78,
Gray
Bull
beds,
Big
Horn
Basin,
Wyoming.
Complete
specimens
of
P4
are
not
known.
It
is
clear
from
A.M.N.H.
No.
14748
(Lost
Cabin)
and
A.M.N.H.
No.
78
(Gray
Bull),
however,
that
there
was
no
well-developed
paraconid,
but
only
an
incipient
nubbin
representing
this
cusp.
P4
is
posteriorly
inclined.
There
are
small
diastemata
between
P1
and
P2,
and
between
P2
and
P3.
M1
is
the
smallest,
and
M3
the
largest,
of
the
lower
molar
series.
This
size
relationship
is
best
displayed
on
P.U.
No.
16181,
but
it
can
also
be
recognized
from
the
size
relationship
of
the
alveoli
of
other
speci-
5
1969
IV
r
..
I'-
&
'
I-
.11,
Ni
-a
1111

AMERICAN
MUSEUM
NOVITATES
mens.
Although
M2
and
M3
bear
a
vestigial
metaconid
on
all
known
specimens,
the
chipped
protoconid
of
M1
of
P.U.
No.
16181
may
not
ss,
,N'
d.!
27It
B
..
...
.
Z
I.I
.,WI'
f
7
-4
01
.,r
.
_*
_
'-
-
r*
I
'.
.
V
49
FIG.
3.
Hapalodectes
leptognathus,
left
dentary
fragment
with
M3
(type
of
"Hapalodectes
compressus"),
A.M.N.H.
No.
12781,
Wind
River Formation.
A.
Lingual
view.
B.
Buccal
view.
have
a
metaconid.
Because
this
specimen
is
broken
on
the
anterior
medial
surface
of
the
protoconid,
it
cannot
be
determined
with
cer-
tainty
whether
the
vestigial
metaconid
was
present
or
not.
In
general,
the
molars
are
transversely
narrow,
with
a
dominant
proto-
conid,
small
paraconid,
vestigial
metaconid,
and
the
trenchant
remnant
6
NO.
2361

SZALAY:
HAPALODECTINAE
of
a
talonid
that
probably
represents
the
cristid
obliqua
of
less-advanced
ancestry.
There
are
well-defined
re-entrant
grooves1
on
the
anterior
face
of
the
paraconids.
The
only
remains
of
the
upper
dentition
or
part
of
the
skull
of
this
species
is
P.U.
No.
17698
from
Lysite
beds,
a
left
maxilla
fragment
with
the
alveoli
of
P4
and
M',
and
the
somewhat
broken
last
two
molars,
M2
and
M3.
Guthrie
(1967),
who
identified
the
specimen,
pub-
lished
a
rough
drawing
of
it.
I
have
thoroughly
cleaned
the
specimen
with
acetic
acid
solution
and
an
Airdent
machine,
and
present
a
brief
description
below.
M2
is
the
largest
of
the
molars,
with
a
large
paracone,
a
distinctly
smaller
metacone,
and
a
small
but
distinct
metastyle,
or
at
least
a
cus-
pule
corresponding
in
position
to
the
metastyle.
The
anterior
part
of
the
molar
is
broken
off,
but
there
is
little
doubt
that
there
was
a
large
parastyle
(or
a
cusp
corresponding
to
it
in
position)
on
this
tooth.
The
buccal
half
of
M3,
although
smaller
than
M2,
is
essentially
like
that
of
the
preceding
tooth,
with
the
large
parastyle
intact.
The
lingual
half
of
M2
is
square,
with
the
protocone
broken
off,
and
with
a
large
hypo-
cone.
Although
the
apex
of
the
protocone
is
broken,
the
cusp
probably
pointed
in
an
anterobuccal
direction.
There
is
no
trace
of
a
paraconule
or
metaconule;
the
tooth
is
badly
worn
in
these
areas
because
of
the
propalinal
action
of
the
lower
molars
across
the
upper
ones.
This
wear
surface
anterolingual
to
the
paracone
on
M2
is
roughly
triangular
and
is
deeply
etched
into
the
tooth.
A
similar
wear
surface
was
present
on
M3,
although
much
of
the
lingual
part
of
the
molar
is
missing.
The
lingual
root
of
Ml
is
larger
than
either
of
the
buccal
roots,
whereas
the
lingual
and
posterobuccal
roots
of
P4
are
approximately
the
same
size.
One
of
the
most
striking
peculiarities
of
the
maxilla
fragment
is
the
deep
embrasure
pits
thoroughly
traversed
with
foramina
(presumably
for
vessels
of
the
circulatory
system).
The
embrasure
pit
between
P4
and
M'
is
smaller
than
that
between
P4
and
P3,
possibly
because
either
P3
lacked
a
protocone
or
this
cusp
on
that
tooth
was
very
small.
These
1
The
re-entrant
grooves
on
the
lower
molars
of
Hapalodectes
clearly
indicate
the
im-
portance
of
having
the
teeth
form
a
straight,
and
not
a
crooked,
tooth
row.
These
grooves,
apparently
for
the
purpose
of
guiding
teeth
into
their
correct
occlusal
position,
as
well
as
adding
to
the
resistance
of
the
tooth
against
lateral
and
medial
forces
during
mastication,
occur
also
in
triconodont
mammals
(Bob
H.
Slaughter,
personal
communi-
cation)
as
well
as
in
hyaenodontids,
such
as
Hyaenodon
James
S.
Mellett,
personal
com-
munication).
It
is
impressive
that
the
selective
forces responsible
for
the
evolution
of
mesonychid
"shear"
were
stringent
enough
to
produce
de
novo
mechanical
means
of
insur-
ing
occlusion
in
hapalodectines.
7
1969

AMERICAN
MUSEUM
NOVITATES
FIG.
4.
Hapalodectes
leptognathus,
left
dentary
with
M2-M3,
A.M.N.H.
No.
12781,
and
part
of
lower
jaw
with
roots
of
teeth,
P.U.
No.
17698,
Lysite
Member
of
the
Wind
River
Formation.
embrasure
pits
are
apparently
limited
both
anteriorly
and
posteriorly
by
the
lingual
halves
of
the
cheek
teeth.
The
zygomatic
arch
originates
lateral
to
M2
and
begins
to
descend
posteroventrally
at
the
level
of
the
metacone
of
M2.
The
preserved
part
of
the
zygoma
is
partly
split
off
as
a
result
of
either
preburial
or
postburial
damage.
Any
attempt
to
clean
out
this
crack
and
refit
the
two
parts
might
result
in
serious
damage
to
this
unique
specimen.
As
a
point
of
interest
it
must
be
added
that
the
broken
tip
of
the
protoconid of
M3
was
removed
from
the
embrasure
pit
posterior
to
M2
while
the
fossil
was
being
cleaned.
8
NO.
2361

SZALAY:
HAPALODECTINAE
The
palatal
view
of
Hapalodectes
leptognathus
(fig.
12)
was
based
on
P.U.
No.
17698.
In
addition
to
the
skull
fragment,
the
tooth-bearing
part
of
the
right
dentary
from
the
canine
to
M3
and
the
anterior
part
of
the
left
dentary
are
also
present.
I
superimposed
the
lower
tooth
row
(fig.
11,
reconstructed
from
various
dentitions)
on
the
drawing
of
the
known
upper
teeth
and
added
the
missing
anterior
part
in
correct
TABLE
1
MEASUREMENTS
(IN
MILLIMETERS)
OF
LOWER
TEETH
AND
MANDIBLES
OF
Hapalodectes
leptognathus
Depth
Depth
P4
M1
M2
M3
Below Below
L
W
L
w
L
W
L
W
M2
M3
A.M.N.H.
No.
78
5.6
1.9
5.2
-
5.8
2.3
-
-
11.75
13.00
A.M.N.H.
No.
79a
--
-
-
-
11.4
12.2
A.M.N.H.
No.
1278..
-
---
-
6.15
2.3
10.35
11.65
A.M.N.H.
No.
1278
-
--
5.5
1.9
-
-
10.4
11.6
A.M.N.H.
No.
1278..
-
- -
-
-
6.0
2.45
-
-
P.U.
No.
16181b
-
-
-
-
-
-
7.00
2.25
12.55
13.10
P.U.
No.
17698.
-
-
-
-
-
-
-
-
11.00
-
A.M.N.H.
No.
14748c
4.7
1.7
5.8
2.0
5.9
2.1
-
-
8.5
-
a
bFrom
Gray
Bull
beds.
From
Lysite
beds.
From
Lost
Cabin
beds.
proportions.
Because
the
symphyseal
parts
of
both
dentaries
are
pre-
served
in
P.U.
No.
17698,
it
is
possible
to
give
the
accurate
distances
between
the
lower
teeth
on
opposite
sides
at
any
one
point
on
the
lower
tooth
row.
Consequently,
from
the
occlusal
relationship
of
these
lower
cheek
teeth
with
the
upper
ones,
the
width
of
the
palate
at
any
point
can
be
determined
and
reconstructed.
DISCUSSION:
There
are
no
meaningful
morphological,
size,
or
propor-
tional
differences
among
the
meager
samples
of
Hapalodectes
from
the
Gray
Bull,
Lysite,
and
Lost
Cabin
beds.
I
believe
that
the
distinctions
made
by
Matthew
(1909)
based
on
jaw
depth
differences
and
the
rela-
tive
compression
of
molars,
which
were
subsequently
used
by
Gazin
(1962),
break
down
when
applied
to
samples
rather
than
to
individual
specimens.
P.U.
No.
16181
is
a
dentary
fragment
with
teeth,
collected
from
Lysite
beds
as
was
A.M.N.H.
No.
12781,
the
type
of
"Hapalodectes
compressus."
The
mandible
of
P.U.
No.
16181
is
as
deep
as
that
of
the
type
of
Hapalodectes
leptognathus,
from
Gray
Bull
beds.
The
dentary
of
1969
9

AMERICAN
MUSEUM
NOVITATES
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FIG.
5.
Hapalodectes
leptognathus,
right
dentary
fragment
with
M1-2,
A.M.N.H.
No.
14748,
Lost
Cabin
beds,
Wind
River
Basin,
Wyoming.
Above:
Lateral
view.
Below:
Medial
view.
P.U.
No.
18161,
from
Lysite
beds,
is
also
comparable
in
depth
with
that
of
the
Gray
Bull
holotype.
Samples
of
homologous
teeth
are
scarce.
It
cannot
be
demonstrated
that
differences
either
in
jaw
depth
or
in
the
relative
compression
of
the
molars
are
consistent
with
the
stratigraphic
provenance
of
the
speci-
mens.
The
only
taxonomic
decision
that
can
be
made
from
the
known
samples
of
the
North
American
Hapalodectes
is
to
unite
the
specimens
under
one
species.
The
recognized
differences
can
be
explained
by
sexual
dimorphism,
or
by
normal
variation
resulting
from
age,
or
simply
by
intraspecific
variability.
10
NO.
236
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A.
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AMERICAN
MUSEUM
NOVITATES
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IIII
IIIIIIIII
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FIG.
7.
Hapalodectes
leptognathus,
occlusal
view
of
right
maxilla
fragment
with
M2-3,
P.U.
No.
17698,
Lysite
Member,
Wind
River
Formation.
In
addition
to
the
conspicuous
absence
of
Plagiomene,
Plesiadapis,
Ectoganus,
Dissacus,
Palaeonictis,
Meniscotherium,
Probathyopsis,
and
Homogalax
from
the
Four
Mile
fauna
noted
by
McKenna
(1960),
Hapalodectes
is
also
missing
from
this
well-sampled
fauna.
NO.
2361
12

1969
SZALAY:
HAPALODECTINAE
13
FIG.
8.
Hapalodectes
leptognathus,
buccal
view
of
left
maxilla
fragment
with
M2-3,
P.U.
No.
17698,
Lysite
Member,
Wind
River
Formation.
Hapalodectes
serus
Matthew
and
Granger,
1925
Figures
13-18
Hapalodectes
serus
MATTHEW
AND
GRANGER,
1925,
p.
3.
The
holotype
of
this
Asiatic
species
was
studied
by
Szalay
and
Gould
(1966,
pp.
153-154).
Hapalodectes
serus
is
clearly
distinct
from
the
North
American
Hapalodectes.
The
absence
of
a
metaconid
from
the
lower
molars
of
H.
serus
is
an
advanced
feature
compared
with
those
of
the
North
American
Hapalodectes
which
still
retain
a
vestigial
metaconid.

AMERICAN
MUSEUM
NOVITATES
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FIG.
9.
Hapalodectes
leptognathus,
dorsal
view
of
left
maxilla
fragment,
P.U.
No.
17698,
Lysite
Member,
Wind
River
Formation.
The
lingual
and
buccal
profiles
of
the
only
known
lower
molar,
the
type
(a
right
M2),
and
the
general
morphology
of
the
referred
Asiatic
upper
molar
are
too
similar
to
those
of
the
North
American
form
to
warrant
separation
on
the
generic
level.
If
anything,
the
new
material
of
Hapalodectes
serus
identified
by
me
and
described
below
only
strengthens
the
generic
allocation
of
the
species.
14
NO.
2361

SZALAY:
HAPALODECTINAE
A.M.N.H.
No.
80802,