Polydolops was a scansorial, omnivorous marsupial from the Palaeocene, Eocene and Oligocene. Fossils have been found in Chile, Argentina and Antarctica.
Polydolops was a member of the Polydolopidae and belonged to the Polydolopoidea, which was part of the Polydolopimorphia.
Flynn and Wyss (2004) described Polydolops mckennai from a skull, dated from the early Oligocene or earlier. Polydolopids were among the earliest-described South American fossil marsupials. They have a long temporal range and are moderately common elements of most early Cenozoic faunas. Polydolopids, like most marsupials, are
rare in comparison to placentals within most
Cenozoic South American assemblages and
had been reported only from Paleocene and
Casamayoran assemblages until the 1970s
(see Marshall, 1982: 9). They do, however,
occur with some frequency in many pre-Deseadan
assemblages, with well-documented
occurrences now ranging from the Paleocene
(Itaboraian and Riochican SALMAs; Paula
Couto, 1952; Marshall, 1982; Goin and Candela,
1995; and possibly the older Tiupampan
SALMA, ‘‘Epidolops sp.’’, Gayet et al.,
1991) to the earliest Oligocene Tinguiririca
Fauna (representing a new pre-Deseadan,
post-Mustersan SALMA, the Tinguirirican,
see Flynn and Swisher, 1995, Flynn and
Wyss, 1999, Flynn et al., 2003), a span of
some 30 million years. Polydolops itself is
quite long-ranging, with species known from
that entire span, excepting the earliest (Tiupampan
and Itaboraian SALMAs) part during
which only Epidolops, the polydolopine
nearest outgroup, and Amphidolops (5 Seumadia)
yapa, an advanced polydolopine, are
known from Patagonian Argentina and Brazil
(Paula Couto, 1952; Marshall, 1982; Goin
and Candela, 1995; Bond et al., 1995; see
Gayet et al., 1991, for possible older, Tiupampan
SALMA occurrence of Epidolops in
Bolivia). In addition, the polydolopids reported
from Antarctica (Antarctodolops, Eurydolops)
have been considered to be closely
related to particular Polydolops species
(Woodburne and Zinsmeister, 1984; Case et
al., 1988), either rendering Polydolops paraphyletic
or the Antarctic taxa assignable to
Polydolops (see discussion in Flynn and
MAMMALIA LINNAEUS, 1758 (SENSU ROWE,
METATHERIA HUXLEY, 1880 (SENSU FLYNN
AND WYSS, 1999)
MARSUPIALIA ILLIGER, 1811 (SENSU FLYNN
AND WYSS, 1999)
POLYDOLOPIMORPHIA MARSHALL ET AL., 1990
(SENSU FLYNN AND WYSS, 1999)
POLYDOLOPOIDEA AMEGHINO, 1897 (SENSU
FLYNN AND WYSS, 1999)
82 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 285
Fig. 6.1. Palatal view of skull and upper dentition
of Polydolops mckennai.
POLYDOLOPIDAE AMEGHINO, 1897 (SENSU
FLYNN AND WYSS, 1999)
POLYDOLOPINAE AMEGHINO, 1897 (SENSU
FLYNN AND WYSS, 1999)
Polydolops mckennai new species
Figures 6.1–6.2; tables 6.1–6.2
HOLOTYPE (AND SOLE ELEMENT OF HYPODIGM):
SGOPV 3476 (field number, 3-3-96-
676; Loc. C-96–5). SGOPV designates specimens
housed in the vertebrate paleontology
collections of the Museo Nacional de Historia
Natural in Santiago, Chile.
TYPE LOCALITY: ‘‘Cachapoal Locality’’
(new locality, discovered in 1996 by the authors,
Reynaldo Charrier and team; see Charrier
et al., 1997), Rı´o Cachapoal, Chile. This
locality is ;5 km NW of the Rı´o Las Len˜as
fossiliferous sites (Flynn et al., 1995) and
;100 km N of Termas del Flaco, site of the
original discovery of fossil mammals in the
Abanico Formation. More detailed locality
information is on file at the Field Museum,
STRATIGRAPHIC OCCURRENCE: Abanico (5
Coya-Machalı´) Formation, Chile. The precise
stratigraphic provenance of the holotype
is uncertain, as it was recovered from an automobile-
sized block on a talus slope below
steeply dipping cliff exposures. Part or all of
this stratigraphic interval, however, appears
to correlate with or underlie a sequence in
the next drainage south (Rı´o Las Len˜as), preliminarily
dated at 29.3 6 0.1 Ma (unpublished
40Ar/39Ar, C.C. Swisher III, personal
commun.; Charrier et al., 1997). In addition,
the source strata certainly predate stratigraphically
higher levels in the Rı´o Las Len˜-
as drainage that have been dated at 16.1 and
20.09 Ma (Flynn et al., 1995; Charrier et al.,
AGE: Indefinite but possibly correlative
with, or older than, levels dated elsewhere at
ca. 29.3 6 0.1 Ma (see ‘‘Stratigraphic Occurrence’’).
May be equivalent to the Tinguirirican
SALMA in being about earliest
Oligocene in age (;31–32 Ma), but the data
are also permissive of an even older age.
ETYMOLOGY: In honor of Malcolm C. Mc-
Kenna, for his remarkable contributions to
mammalian systematics, his imaginative integration
of geological and biological studies
in paleontology, and his collaboration in our
early work in Chile.
DIAGNOSIS (BASED, OF NECESSITY, SOLELY
ON UPPER DENTAL FEATURES): The taxon is a
polydolopine polydolopid; it is conservatively
assigned to Polydolops as it shares several
synapomorphies with other Polydolops species,
although it differs from all of them in
various dental features. Measurements and
tooth proportions are presented in tables 6.1
2004 FLYNN AND WYSS: POLYDOLOPINE SKULL FROM CHILE 83
Fig. 6.2. Left lateral view of upper dentition and cranial morphology (partial) of Polydolops mckennai.
Dimensions (in mm) for Polydolops mckennai
and 6.2. Upper dental formula tentatively
Differs from all other species of Polydolops
in having proportionally much more
elongate premolars and molars, a longer and
more heavily ribbed, arcing, sectorial blade
spanning P2 and P3, and apparently fewer
and/or less well-developed buccal and stylar
cusps on molars. Upper teeth are unknown
for all Polydolops species except P. thomasi
and P. serra, but proportions of the lower
teeth (relative to upper/lower teeth proportions
in taxa for which both are known) suggest
that those more poorly known taxa also
have proportionally much shorter and wider
teeth than P. mckennai. In absolute size, P.
mckennai is much larger than P. serra (and
presumably also other small Riochican–Casamayoran
Polydolops species not known
from upper teeth [P. winecage, P. clavulus,
P. rothi, P. kamektsen]), and is 10–20% larger
and has much longer cheek teeth than P.
thomasi (and narrower P2–3, but wider M3).
Also differs from P. thomasi in having more
pronounced labial ribs on the anterior half of
P2 and a second very large and strong arced
rib extending from the anterodorsal edge of
P3 to the tip of the crown both lingually and
Differs from Epidolops in having P2 and
P3 sectorial (rather than only P3), P2 large
(P2 tiny or absent in Epidolops), axis of P3
in line with molar series (rather than angled
about 308 anterobuccally to posterolingually),
proportionally and absolutely larger and
more cuspate molars, narrower and more
elongate palate, more gracile skull, and absence
84 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 285
Dental Proportions for Polydolops mckennai and Other Polydolopidae
(Polydolops serra, Polydolops thomasi, Amphidolops serrula, Eudolops tetragonus, Eudolops hernandezi,
Antarctodolops dailyi, Epidolops ameghinoi)a
Differs from Eudolops in having smaller
molars, absolutely and proportionally (relative
to molar size) larger premolars (than E.
tetragonus, the only species for which these
are known), P2 much larger than P3, much
less distinct cusps on M3, M2 larger than M3
(rather than subequal in size), M3 much
smaller, and less wrinkled enamel in molar
Differs from Amphidolops in its much
larger overall size, having P3 roots more
equal in size, presence of large P2 (apparently
absent in Amphidolops) and thus P2
and P3 both sectorial (rather than just P3),
having M3 longer than wide, and possessing
less wrinkled enamel in molar basins.
Differs from Antarctodolops in being
smaller (based on Antarctodolops P3 alveoli
size estimates, M1 length and width, and M2
length [based on P. mckennai alveolus minimum
measurements]), having a proportionally
larger P3 (compared to M1 dimensions),
having P2 broader (posteriorly) than P3 (the
reverse was inferred by Woodburne and
Zinsmeister, 1984, from alveoli in Antarctodolops),
having higher L/W ratio for M1 and
a slightly lower ratio for M2, and possessing
many fewer labial and stylar molar cusps.
Differs from Eurydolops (a taxon known
only from a single P3) in having a P3 that is
much larger, much more elongate, doublerooted,
and possessing more and better-developed
serrations, a labial cingulum, and a
more expanded posterolingual shelf.
DESCRIPTION: The specimen is well preserved,
but is crushed in various parts, especially
through dorsoventral flattening. The
animal was a mature adult, as all the teeth
appear to have been erupted and moderately
worn, and sutures are not readily observable
on the skull.
Although anterior teeth are not preserved,
alveoli are present on both sides (better on
the left). These alveoli provide the first evidence
of the anterior upper dentition for a
polydolopine, allowing comparison to the
conditions in their nearest outgroup, Epidolops.
There likely was one large, laterally
compressed tooth near the anterior end of the
snout, with two smaller alveoli flanking it.
The smaller alveoli are closely appressed to
the alveolus for the enlarged central tooth,
being separated from the latter by a bony
transverse partition (the posterior alveolus is
more pronounced on the left side, appearing
as a shallower and more laterally constricted
pit on the right side). In general form and
position, these alveoli in P. mckennai are
very similar to those in Epidolops ameghinoi
(see Paula Couto, 1952: fig. 2, and Marshall,
1982: fig. 62), suggesting comparable morphology
of the anterior tooth battery.
The posteriormost of these three anterior
alveoli almost certainly held P1, given its position
in the tooth row, the ancestral marsupial
premolar complement, and the presence
of two premolars posterior to it. Judging
from alveolar size and position, this tooth
was quite small and sat tightly against its enlarged
anterior neighbor. This is the first
demonstrated occurrence of P1 in a polydol2004
FLYNN AND WYSS: POLYDOLOPINE SKULL FROM CHILE 85
opine (as P. abanicoi from Chile had earlier
shown for p1; Flynn and Wyss, 1999).
The middle of the alveolus for the large
tooth appears laterally constricted, although
this is much more pronounced on the left
than the right side probably due to post-depositional
distortion. We interpret this as a
single alveolus for an enlarged tooth, given
the left-right asymmetry in the apparent constriction,
lack of a transverse bony partition
within the alveolus on either side, and the
continuous smooth rim of the alveolus. This
interpretation seems much more likely than
the alternative of two subequally-sized teeth,
given the above observations, the complementary
enlarged procumbent lower tooth
(canine) in polydolopids, and similarity to
the morphology of the anterior upper dentition
in Epidolops (in which there is an almost
identical elongate and compressed alveolus
for a single enlarged tooth, also flanked by
closely appressed and much smaller alveoli
for incisors and P1). The enlarged tooth in
Polydolops mckennai is interpreted as the canine;
as the premaxillary-maxillary suture is
not visible, this is based on its occurrence
well posterior to the anterior end of the
snout, its position immediately anterior to the
tooth identified as P1, the morphology of the
anterior upper dentition (especially the canine)
in Epidolops, and the additional homology
arguments presented by Marshall
(1982). The orientation of the alveoli suggests
that the canine would have been vertical
or only slightly procumbent.
At least one and possibly two teeth (incisors)
were present anterior to the enlarged
tooth. These are on the buccal border of the
palate and well posterior to the anterior rim
of the snout (within which no alveoli are obvious).
A very long diastema (nearly 15 mm) separates
the canine/P1 from P2.
The entire cheek-tooth series is elongate
and aligned anteroposteriorly, parallel to the
midline. The second and third upper premolars
are both enlarged, sectorial teeth,
whereas the molars are flattened crushing
teeth, consisting of large enamel-rimmed basins.
The cheek-tooth-bearing portion of the
palate is somewhat longer and significantly
broader than the rostral part. The palate extends
posteriorly to M3 and does not vary
appreciably in breadth posterior to P2; a distinct
foramen or vacuity occurs near the posterobuccal
corner of the palate.
The second upper premolar is the longest
cheek tooth, being slightly longer than M1
(M2 represented only by alveoli). It is anteroposteriorly
elongate, laterally compressed,
and generally ovoid in outline. The
second upper premolar is significantly larger
than P3, especially in length and crown
height. Like P3, however, P2 is dorsoventrally
elaborated into slicing ‘‘blades’’, with
the axial ridges on both forming a continuous,
gently S-curved, arcing ridge. On P2 the
axial ridge is strong, serrated, and slightly
concave buccally. The ridge is marked by six
distinct cusps, from which climb a series of
dorsoventral ribs forming the serrations on
the labial face of the tooth. The posterior four
serrations extend dorsally only about onefourth
the height of the tooth crown. The two
anterior cusps and rib serrations are the most
prominent, both extending from the dorsal
base of the crown to its ventral tip, with the
second forming the highest cusp on the tooth
and its rib creating a marked rim separating
the anterior third of the tooth from the posterior
two-thirds. A shallow depression exists
on the posterobuccal side of this rim. A series
of shallow grooves is present on the anterior
face of the tooth, as is a slight anterodorsal
The third upper premolar is irregularly triangular
in outline, with a distinct and slightly
basined posterolingual lobe. As on P2, an axial
ridge runs the entire length of the tooth,
but it is less well developed, slightly convex
buccally, and located more lingually on the
crown. On the anterior half of the tooth, four
weak cusps and associated dorsoventral serrations
mark the crest of the axial ridge. The
posteriormost serration is the strongest, extending
almost to the base of the tooth, while
the other serrations become progressively
The molars decrease progressively in size
posteriorly (M1 . M2 . M3). The crowns
are fairly heavily worn, making it difficult to
determine the number and positions of all the
distinct cusps and cuspules on an unworn
The first upper molar is quadrate in outline.
Both the lingual and buccal edges are
86 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 285
pinched by shallow grooves near the middle
of the tooth, dividing it into distinct anterior
and posterior lobes. On the crown surface the
basins of the two lobes are continuous, with
a bordering enamel rim. Buccal cusps appear
to be obliterated early in wear, although at
least four large cusps were present (two on
each lobe). The two anterior cusps are widely
separated by a deep valley, whereas the two
posterior cusps are more proximate, with a
small ‘‘pocketed’’ enamel basin forming between
them on the posterobuccal corner of
The second upper molar is not preserved
on either side of the specimen, but the general
shape and size of the alveoli indicate that
the tooth would have been smaller than, but
similar in shape, proportions, and morphology
The third upper molar is preserved on the
left side of the skull; it is a gently rounded,
isosceles triangle in outline. The triangle’s
most acute angle forms the posteromedial
end of the tooth, and the anterior edge of the
tooth represents the triangle’s base, making
the tooth much wider anteriorly than posteriorly.
A continuous rim of enamel surrounds
a central basin. No distinct cusps are present,
but there are indications of two closely appressed
cuspules at the anterobuccal corner
and at least one at the posterobuccal end of
the buccal rim.
The palate posterior to M3 is complete
enough to indicate that no M4 was present
in this animal.
The skull clearly is crushed, but most of
the distortion appears to be dorsoventral flattening
rather than lateral or oblique distortion,
allowing approximate estimates of
lengths and widths of some features. Overall,
the skull appears quite gracile, with a very
long but generally narrow form. The snout is
elongate and narrow, but the palate broadens
significantly across the cheek-tooth region,
being broadest near the anterior base of the
zygomatic arches. There are slight lateral
constrictions just anterior to P2, at the anterior
base of the zygoma (P3-M1 junction),
and posterior to the cheek-tooth row (separating
an expanded braincase from the rostral
part of the skull). The anterior root of the
zygomatic arch is moderately robust, forming
a strong bridge extending buccally from
the area lateral to M1–M2.
The braincase is missing its posterior portion,
and is ventrally flattened; nonetheless,
it is clear that the braincase was fairly expanded
and globular. Although it is uncertain
whether the breakage of the braincase resulted
from postmortem transport or predation/
scavenging, it is reminiscent of the damage
done to mammal skulls preyed on by
modern birds (e.g., owls) and the breakage
of the posterior ends of the only other polydolopoid
skulls known (Polydolopidae:
Epidolops ameghinoi, Itaborai, Brazil, Itaboraian
SALMA, Paula Couto, 1952, Marshall,
1982; Bonapartheriidae: Bonapartherium
hinakusijum, Lumbrera Formation, Salta,
Argentina, Casamayoran SALMA, Pascual,
The skull of Polydolops mckennai is the
first recovered for a polydolopine and only
the second for any polydolopid, providing
evidence bearing on questions of polydolopid
monophyly, basal diversification, and the
features typifying the group and subclades
ancestrally. A full revision of polydolopoid
phylogenetics would be worthwhile, especially
in light of advances in systematic
methods, description of many new taxa since
Marshall’s (1982) review of polydolopids,
and proposals that some taxa currently recognized
may not be monophyletic or that the
important Antarctic taxa (Antarctodolops and
Eurydolops) may be nested within various
species assigned to Polydolops. Although
outside the scope of this brief initial report,
such an analysis will yield better understanding
of patterns of morphological evolution
within the clade and of early Cenozoic biogeographic
relationships among ‘‘Gondwanan’’
continents. For example, Eurydolops is
a taxon known only from a single tooth (considered
P3 by Case et al., 1988). Although
this taxon has reasonably been considered to
be a polydolopine, the shape and morphology
of the available tooth has many resemblances
to the P2 of Epidolops; if Eurydolops
is in fact an epidolopine rather than polydolopine,
this would have important biogeographic
and temporal ramifications.
2004 FLYNN AND WYSS: POLYDOLOPINE SKULL FROM CHILE 87
Polydolops mckennai is readily distinguishable
from the other taxon known from
a skull (Epidolops ameghinoi), as well as
other non-Polydolops polydolopines, in a variety
of features (see diagnosis). In overall
size, P. mckennai is smaller than Epidolops
and Eudolops, but larger than most other polydolopines.
It is more difficult to distinguish P. mckennai
from other species of Polydolops, as
many of them are known from less complete
or noncomplementary material (e.g., P2–3
known only for P. thomasi; upper molars
known only for P. thomasi and P. serra), but
it is larger than almost all of them (see diagnosis)
and differs in cheek-tooth proportions.
The largest species of Polydolops, P.
mckennai and P. mayoi, are represented by
material that is not directly comparable (i.e.,
skull and upper dentition versus lower dentition
only), so the approximate sizes can
only be inferred through relative sizes of upper
and lower teeth in closely related taxa for
which both are known. Upper cheek teeth of
P. mckennai are ;10–20% larger than those
of P. thomasi, and the lower cheek teeth in
the holotype of P. mayoi are 2–26% longer
and 20–35% wider than the average for P.
thomasi. Thus, among Polydolops species, P.
mckennai may most closely approximate P.
mayoi in size (and possibly in having elongate
cheek teeth), although they likely differ
in shape and proportions, particularly in P.
mckennai having proportionally larger and
more elongate P2–3. The most distinctive
autapomorphy of P. mckennai, distinguishing
it from all other species of Polydolops, is the
extreme elongation of all the cheek teeth (table
6.2). In P. mckennai all cheek teeth are
at least 15% longer than they are wide, and
P2 is almost twice as long as it is wide. In
other species of Polydolops the upper cheek
teeth are squared (approximately equal in
length and width) or wider than long, except
P2, which is much less elongate in those other
species. The extreme elongation of both
P2 and P3 appears to be a diagnostic autapomorphy
for P. mckennai, distinguishing
the species from all other polydolopids. Although
Eudolops tetragonus also has an
elongate P2 and slightly elongate P3, they
are not nearly so elongate as in P. mckennai
(e.g., P2: 60% versus 90% longer than broad)
and although both teeth are sectorial in both
taxa, their morphologies are quite distinct.
The primitive condition for molar shape in
polydolopids appears to be either squared (as
in most polydolopines) or broader than long
(as most marked in the polydolopine proximal
outgroup Epidolops). Although the elongate
molars in Eudolops hernandezi make inferences
about the ancestral molar shape in
polydolopids and polydolopines a bit problematic,
clear distinction of this taxon from
Polydolops and the roughly squared or relatively
broad molars in all other polydolopids
(including the other species of Eudolops)
support the proposed reconstruction. Thus,
the very elongate molars in P. mckennai appear
to be diagnostic autapomorphies. Peculiarly,
P. mckennai appears to have many
fewer buccal and stylar cusps on the upper
molars than do other species of Polydolops.
This distinction, as well as the very elongate
cheek teeth, might be used to argue for exclusion
of mckennai from a Polydolops
clade. However, currently lacking complementary
material for many of the other polydolopine
taxa (e.g. skulls for all, anterior
dentitions for most, and any upper teeth in
some), and given the apparent synapomorphies
in P2–3, cheek-tooth row orientation,
and other molar morphology, we conservatively
choose to ally the new species with
other species of Polydolops.
The anterior teeth (and consequently dental
formulas) are poorly known in polydolopids,
owing to the dearth of specimens with
a preserved anterior mandibular ramus or
rostrum; indeed, until recently this region of
the dentition was reasonably well known in
just a single taxon, Epidolops ameghinoi.
Preservation of almost the entire rostrum and
mandibular ramus in taxa from the Abanico
Formation, Polydolops mckennai (this paper)
and P. abanicoi (Flynn and Wyss, 1999),
providing the first evidence of the anterior
upper dentition in any polydolopid other than
Epidolops, strengthens homology inferences
for the anterior teeth in polydolopids (particularly
identification of P1/p1 and the enlarged
tooth) and supports the conclusion
that these are synapomorphic in all polydolopids
P. mckennai documents the occurrence of
upper incisors (at least one, and possibly two
88 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 285
or more) in Polydolops; Epidolops had three
upper incisors (Paula Couto, 1952; Marshall,
1982). As in Epidolops, the incisor(s) were
not located on the anterior rim of the snout,
however, but rather were set more posteriorly
on the buccal rim of the palate, closely appressed
to several other teeth. The lateral position
and development of a battery of anterior
upper teeth [incisor(s), canine, P1] seem
congruent with the great enlargement and
procumbency of the lower canine (located
near the midline and symphysis of the mandibular
rami), and associated great reduction
or loss of all lower incisors. In addition, this
specimen establishes the expected, but previously
undocumented, presence of a large,
vertically oriented upper canine. The clear
presence of P1 in P. mckennai contradicts the
earlier inference that P1 and p1 were both
absent in Polydolops (Marshall, 1982), and
is consistent with the evidence from P. abanicoi
of a small, anteriorly positioned p1 in
the clade (Flynn and Wyss, 1999). Thus, the
presence of P1 in P. mckennai and p1 in P.
abanicoi (also likely in P. clavulus, as noted
by Ameghino, 1903; see Flynn and Wyss,
1999), and similar P1/p1 in the polydolopine
outgroup Epidolops, suggests that a small
P1/1, located far anteriorly on the ramus or
maxilla (close to the canine and separated
from P2/2 by a long diastema), was the ancestral
condition for polydolopids and polydolopines.
The homology of this suite of
modifications (very large upper and lower
canines; procumbency of the lower anterior
teeth, particularly the canines; great reduction
or loss of upper and lower incisors; reduction
of P1/p1; and long P1/p1–P2/p2 diastema
separating the anterior dental battery
from the cheek-tooth row) between Epidolops
and polydolopines strengthens the hypothesis
of polydolopid monophyly supported
by these synapomorphies (in addition to
those in the cheek-tooth battery).
Goin and Candela (1995) suggested that
Polydolopidae might not be monophyletic,
given the highly modified lower first molar
of polydolopines and the difficulty in deriving
them from a hypothesized epidolopine
(5 Epidolops) ancestry. Epidolops represents
a lineage that diverged from the remaining
polydolopids (a clade termed the Polydolopinae)
at least as early as the Itaboraian SALMA.
Although Epidolops (E. ameghinoi)
represents the earliest known occurrence of
a polydolopid (polydolopines are first recorded
in the somewhat younger [Flynn and
Swisher, 1995] Riochican SALMA), and
does have some quite different aspects of
molar morphology, it is the proximal outgroup
of the polydolopines (with its own autapomorphies)
rather than an ancestor. Therefore,
one should not necessarily expect to be
able to derive every feature seen in polydolopines
from the condition in Epidolops, but
rather from a more generalized condition that
marked the common ancestor of both lineages.
Epidolops is distinguished from polydolopines
by marked differences in P2/p2 and
some aspects of molar morphology, loss of
M4/m4 (in polydolopines), and significant
rotation of the greatly expanded P3/p3 blades
in Epidolops. With discovery of the first polydolopine
skull (Polydolops mckennai),
comparison to the other available polydolopid
cranium (Epidolops ameghinoi, Paula
Couto, 1952; Marshall, 1982) reveals differences
in several distinctive features between
the two lineages, although caution must be
exercised in interpreting these differences
(due to dorsoventral crushing of both specimens),
and understanding their distribution
in other polydolopids and phylogenetic implications
awaits discovery of crania for other
taxa. The cheek-tooth row is straight in P.
mckennai, rather than V-shaped or arcuate
(P3 is angled sharply outward and the tooth
rows converge posteriorly in E. ameghinoi),
proportionally much longer, and with more
similarly sized cheek teeth (contra the dominance
of a huge P3 in E. ameghinoi). The
skull of Epidolops ameghinoi appears to be
markedly more robust than P. mckennai,
with a very broad rostrum and massive zygomatic
arches. The anterior root of the latter
is much broader (forming a flattened shelf)
and more anteriorly placed relative to the
cheek-tooth row (spanning the anterior edge
of P3 to the posterior end of M2) in E.
ameghinoi. In contrast, the anterior zygomatic
root in P. mckennai spans only the anterior
edge of M1 to the middle of M2. The
snout is narrower and less flattened anteriorly
in P. mckennai, and the canine alveolus is
less robust and more laterally compressed.
2004 FLYNN AND WYSS: POLYDOLOPINE SKULL FROM CHILE 89
The basicranial portion of the skull and the
braincase appear to be much smaller, proportionally,
in E. ameghinoi than in P.
All polydolopids share a large suite of unusual,
and apparently homologous, dental
and gnathic features, including inflexion of
the portion of the mandibular ramus bearing
the anterior dentition relative to that bearing
the cheek teeth, great enlargement of upper
and lower canines, procumbency of the enlarged
lower canine, reduction of P1/p1,
elongate diastema between the canine and
cheek teeth, greatly enlarged and bladelike
sectorial P3/p3, and quadrate and highly cuspate
molars. The two available skulls suggest
polydolopids are characterized by elongate
snouts, broad and relatively flat (dorsoventrally)
skulls, and robust zygomatic arches,
although crushing of both of the known
skulls makes interpretation of these as synapomorphies
of the clade tentative. Although
similar conditions for some of these appear
sporadically among other marsupial taxa,
some of these resemblances can be clearly
shown to be nonhomologous (e.g., see discussions
in Ride, 1962; Marshall, 1982). For
example ‘‘diprotodont’’ procumbent anterior
teeth, or ‘‘plagiaulacoid’’ shearing blades,
occur in other marsupials, but these generally
are in different tooth loci than in polydolopids.
Similarly, quadrate cheek teeth (sometimes
accompanied by a diastema from the
anterior tooth battery) occur in other taxa,
but anterior and cheek teeth typically are
aligned linearly along the ramus. Given their
close correspondence in form and apparent
homology, we consider this suite of features
as synapomorphies indicative of the unique
common ancestry of all polydolopids, and
thus we conservatively regard Epidolops and
polydolopines as constituting a monophyletic
group (see also Flynn and Wyss 1999).
P. mckennai is the first specimen described
from new localities in the Abanico Formation
of the Rı´o Cachapoal drainage. It is not
yet clear whether the material collected from
the Cachapoal locality represents a single
fauna/horizon, or several horizons, as our initial
collection from this area (currently numbering
several dozen specimens) was recovered
both in situ and in blocks rolled from
the steep valley slopes. The holotype of P.
mckennai, as most of the other fossils obtained
from this drainage, was found in a
block derived from an uncertain stratigraphic
horizon within the Abanico Formation. Preliminary
assessment of the stratigraphic,
structural, and topographic (from analysis of
aerial photos) data, relative to more intensively
studied sequences only a few kilometers
to the south (the Rı´o Las Len˜as drainage;
Flynn et al., 1995; Charrier et al., 1997,
2002; work in progress by R. Charrier and
the authors), suggests that the very steeply
dipping strata in this area likely represent
only a relatively short interval of time, predating
16.1 Ma and 20.09 Ma levels (the latter
containing a fossil platyrrhine, among
other taxa; Flynn et al., 1995; Charrier et al.,
2002) along the Rı´o Las Len˜as. Much or all
of the stratigraphic interval at the Cachapoal
locality appears to correlate with or underlie
strata exposed at Rı´o Las Len˜as, which have
been preliminarily dated at 29.3 6 0.1 Ma
(40Ar/39Ar, unpublished, C.C. Swisher III,
personal commun.). We thus tentatively consider
the material and localities in the immediate
vicinity of the polydolopid skull find
as representing a single faunal interval, herein
termed the Cachapoal locality, although
the presence of a mixed fauna from strata of
several ages within the suite of material collected
from rolled blocks must remain a viable
alternative (Charrier et al., 1997). Continuing
study of the fauna, geology, and geochronology
by R. Charrier and the authors
may provide stronger support for interpretation
of a single fauna or further evidence for
multiple horizons (and their ages).
The youngest previously known polydolopid,
P. abanicoi, is from the pre-Deseadan,
post-Mustersan Tinguiririca Fauna (Flynn
and Wyss, 1999). Thus, the Cachapoal locality
from which P. mckennai derives is no
younger than earliest Oligocene (Tinguirirican
SALMA, Flynn and Swisher, 1995;
Flynn et al., 2003), or this taxon marks an
age extension (Deseadan or younger) for the
last appearance of polydolopids; the available
evidence suggests that the former interpretation
is more likely. The Cachapoal locality
strata seem to lie within the core of a
major anticline, with even more tightly folded
anticlinal strata to the east (Charrier et al.,
2002). Radioisotopic and new biochronol90
BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 285
ogic data suggest that deposition in this area
initiated by the earliest Oligocene (if not earlier,
within the Eocene), and that compression
and uplift postdated the early Miocene.
Alternatively (but unlikely), if the Cachapoal
locality strata (earliest Oligocene or older)
are shown not to lie at the core of an anticline,
but instead to overlie the 16–29 Ma
sequence, it would document eastward
thrusting of older strata after the middle Miocene.
Such thrusting, although of uncertain
timing relative to that in the Las Len˜as/Cachapoal
area, has been documented in the
Abanico Formation along the Rı´o Tinguiririca
valley well to the south (Wyss et al.,
1994, 1996; Charrier et al., 1994, 1996; see
also Godoy and Lara, 1994; Sempere et al.,
1994; Godoy et al., 1999). More robust information
on the age and tectonostratigraphic
relations of the Cachapoal locality strata will
be essential to refining the model for basin
development under an initial extensional tectonic
regime and the timing of subsequent
tectonic deformation, compression, and uplift
(Charrier et al., 2002).
Although many colleagues have provided
assistance with this long-term program in the
Chilean Andes (acknowledged in the resultant
series of publications), we especially
thank Reynaldo Charrier for his continuing
collaboration and co-leadership of many facets
of our work in Chile, including the expedition
to the Rı´o Cachapoal that yielded
this new polydolopid specimen and information
on its tectonostratigraphic occurrence.
We are grateful for the long-term support
of this research by the U.S. National
Science Foundation (DEB 9020213,
9318126, 9317943) and the authors’ home
institutions. Preparation of material from the
Abanico Formation volcaniclastics always is
a great challenge—Andrew Leman ably
completed preparation on this difficult specimen.
Marlene Donnelly prepared the drawings,
continuing her outstanding series of illustrations
of taxa from the Chilean Andes.
For this study, we are extremely grateful for
the assistance provided by the Museo Nacional
(Santiago), Gabriel Carrasco and Andre
´s Charrier (Cachapoal field team), Darin
Croft and Gina Wesley (specimen curation
and measurements), and Francisco Goin,
Marcelo Sa´nchez-Villagra, Michael Woodburne,
and Judd Case (discussions of systematics
of fossil marsupials). Finally, we doff
our hats to Susan Bell and Gina Gould for
organizing these volumes and for their forbearance.
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1) Flynn, John J. and Andre R. Wyss (2004) "A polydopine Marsupial Skull from the Cachapoal Valley, Andean Main Range, Chile" (http://digitallibrary.amnh.org/dspace/bitstream/2246/453/13/B285a06.pdf)