zalambdalestes - American Museum of Natural History

130 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 281
sess the broader relationships of these three
zalambdalestid taxa.
Our studies have revealed a number of
primitive features of the skull of Zalambdalestes,
which viewed in the context of the
recently reported epipubic bones in this taxon
(Novacek et al., 1997), support a position
for zalambdalestids within Eutheria outside
of extant placentals (Novacek, 1997). Included
among the primitive cranial features of
Zalambdalestes are: the last upper incisor in
the maxilla; nasals expanded posteriorly to
contact the lacrimals; double lacrimal foramina;
pterygoids meeting on the midline; jugal
extending posteriorly to the glenoid process;
and a relatively posterior position of the
glenoid fossa, opposite the anterior half of
the promontorium of the petrosal, almost entirely
on the zygoma and not the braincase
proper. This evidence contraverts the few
craniodental apomorphies that have been cited
as allying Zalambdalestes with certain
placentals, such as lagomorphs (McKenna,
1994) and Glires (Archibald et al., 2001).
Along with these primitive features, Zalambdalestes
exhibits numerous cranial specializations.
As noted by Kielan-Jaworowska
(1975a, 1984a), Kielan-Jaworowska and Trofimov
(1980), Archibald (1996), and Archibald
et al. (2001), the taxon with which Zalambdalestes
shares the most craniodental
apomorphies is the other well-known Mongolian
zalambdalestid Barunlestes. These
taxa share fewer specializations with the older
zalambdalestid Kulbeckia (Archibald et
al., 2001; Archibald and Averianov, 2003).
Beyond Zalambdalestidae, the clade with
which Zalambdalestes shares the most cranial
specializations is the Mongolian Late
Cretaceous Asioryctitheria. Included among
these features are: the postglenoid foramen
in front of rather than behind the postglenoid
process; the postglenoid and entoglenoid
processes continuous; a fusiform ectotympanic
expanded dorsolaterally beneath the
external acoustic meatus and contacting the
entoglenoid process; a fingerlike tympanic
process posterior to the fenestra cochleae
connected via a distinct crista interfenestralis
to the promontorium; a large piriform fenestra
in the anterior roof of the middle ear,
which transmitted the ramus inferior of the
stapedial artery; foramen ovale situated between
the alisphenoid and squamosal; a suprameatal
foramen; a medially positioned internal
carotid artery; and mastoid foramina
entirely within the petrosal bone. In light of
these unusual similarities, we think that a zalambdalestid-asioryctithere
clade is the most
reasonable working hypothesis for these
forms (see Ji et al., 2002). Endemic Asian
clades from the Late Cretaceous have also
been reported for multituberculates (Rougier
et al., 1997; Kielan-Jaworowska and Hurum,
1997), squamates (Gao and Norell, 2000),
and dinosaurs (Currie, 2000; Sereno, 2000).
Testing of the zalambdalestid-asioryctithere
hypothesis awaits broadscale phylogenetic
analysis, a goal of our future research. Interestingly,
some of the unusual characters of
the ear region shared by zalambdalestids and
asioryctitheres are reminiscent of features
occurring in various lipotyphlans (e.g., tenrecids,
solenodontids, and erinaceids), the
most notable exception being the medially
positioned internal carotid artery. Extensive
testing is needed in order to assess the possibility
of a close phylogenetic relationship
between the Mongolian Cretaceous eutherians
and lipotyphlans. If such a relationship
existed, epipubic bones might have been lost
independently and other postcranial specializations
(Kielan-Jaworowska, 1977, 1978;
Horovitz, 2000) convergently acquired in
(some?) lipotyphlans and other placentals.
As reported by us previously (Wible et al.,
1998), the current sample of partial and complete
crania referred to Zalambdalestes lechei
exhibits a striking degree of individual variation.
Included among the variable features
are: the presence/absence of a third incisor in
the maxilla, P1, and p2; the degree of development
of the postpalatine torus; and the position
and number of the major palatine foramina.
Despite this variability, we have chosen
for now to retain all individuals in one
species. However, we add that the entire
sample of Z. lechei (CAE, ZPAL, PIN, and
MAE) should be restudied to assess the possibility
that more than one taxon is represented.
ACKNOWLEDGMENTS
We are grateful to Zofia Kielan-Jaworowska
for access to Cretaceous therian mam-