A Small Derived Theropod from Öösh, Early Cretaceous, Baykhangor Mongolia

2007·5.8 MB·English

by  Alan H. Turner

PUBLISHED BY THE AMERICAN MUSEUM OF NATURAL HISTORY
CENTRAL PARK WEST AT 79TH STREET, NEW YORK, NY 10024
Number 3557, 27 pp., 17 figures March 8, 2007
¨
A Small Derived Theropod from Oo¨sh, Early
Cretaceous, Baykhangor Mongolia
ALAN H. TURNER,1,2 SUNNY H. HWANG,3 AND MARK A. NORELL4
ABSTRACT
Anewtheropoddinosaur,Shanagashile,fromtheEarlyCretaceousO¨o¨shdepositsofMongoliais
describedhere.Thenewspecimen(IGM100/1119)comprisesawell-preservedrightmaxilla,dentary,
and partial splenial. This specimen exhibits a number of derived theropod features, including
atriangularanteriorlytaperingmaxilla,alargeantorbitalfossa,andmaxillaryparticipationinthe
caudally elongate external nares. These features resemble the Early Cretaceous dromaeosaurids
Sinornithosaurus millenii and Microraptor zhaoianus, as well as the basal avialan Archaeopteryx
lithographica. A comprehensive phylogenetic analysis including 58 theropod taxa unambiguously
depictsthenewO¨o¨shtheropodasamemberofDromaeosauridae.Relativetootherdromaeosaurids,
Shanag ashile is autapomorphic in its lack of a promaxillary fenestra and in the presence of
interalveolarpneumaticcavities.ThediscoveryofIGM100/1119expandsourknowledgeofEarly
CretaceousdromaeosauridsandthefaunalsimilaritybetweentheO¨o¨shandtheJeholbiotas.
more rare than the plentiful fossils of
INTRODUCTION
Psittacosaurus mongoliensis and sauropods at
Theropods were some of the first dinosaur the locality (Osborn, 1923, 1924b). In some
fossils discovered from the Early Cretaceous places,however,isolatedtheropodmaterialof
locality of O¨o¨sh (5 Ohshih or Oshih), but a range of sizes is encountered frequently—
their described remains are limited to undiag- especially unguals, phalanges, and teeth.
nostic teeth of the large theropod ‘‘Pro- The deposits at O¨o¨sh are considered to be
deinodon’’ (Osborn, 1924a; but see Currie, Early Cretaceous (Berriasian–Barremian).
2000). Theropod remains are generally much This is roughly coeval with beds forming the
1DivisionofPaleontology,AmericanMuseumofNaturalHistory([email protected]).
2Correspondingauthor.
3Division of Paleontology, American Museum of Natural History ([email protected]). Current address: New York
CollegeofOsteopathicMedicine,OldWestbury,NY11568.
4DivisionofPaleontology,AmericanMuseumofNaturalHistory([email protected]).
CopyrightEAmericanMuseumofNaturalHistory2007 ISSN0003-0082
2 AMERICAN MUSEUMNOVITATES NO. 3557
Fig. 1. Map of Mongolia showing geographical relationship of the O¨o¨sh locality to other Gobi
fossillocalities.
Jehol group of northeastern China. The Jehol andtheAmericanMuseumofNaturalHistory
beds of the Yixian and Juifotang formations include a wide array of fossil materials,
have produced abundant vertebrate fossil including gobiconodontids (Rougier et al.,
remains (reviewed in Zhou et al., 2003). 2001), a pterosaur (Andres and Norell,
These discoveries include the ubiquitous 2005), lizards, large and small sauropods,
Psittacosaurus (also present in the O¨o¨sh beds) and psittacosaurus ranging in size from
as well as avialan and nonavialan dinosaurs, hatchlings to senescent adults.
including small feathered theropods (reviewed The Jehol biota and O¨o¨sh locality contain
in Norell and Xu, 2005). Additionally, the some elements in common such as the
Jeholbedscontainadiversemammalianfauna primitive ceratopsian Psittacosaurus. Un-
including one of the largest Mesozoic mam- fortunately, dates of the Jehol and O¨o¨sh
mals, Repenomamus (Li et al., 2001). The faunas are poorly constrained. Despite an
Jehol flora is abundant, containing the puta- abundance of andesitic layers that allow for
tive earliest angiosperm Archaeofructus (Sun radiometricdatingofrocksthatproduceJehol
et al., 1998). fossils, several different ages have been pub-
The O¨o¨sh locality lies in the Altai region of lished. Recent dating efforts agree with pre-
Central Mongolia (see Watabe and Suzuki, viousresults(Swisheretal.,1999;Wangetal.,
2000; this paper, fig. 1). Fossils were first 2001) in proposing a Barremian date for the
collected here by American Museum field feathered dinosaur beds in Sihetun and
parties in the 1922 (Andrews, 1932). New bracket the lowermost Yixian beds (one of
collections from the O¨o¨sh locality by field the most fossiliferous formations in the Jehol
partiesoftheMongolianAcademyofSciences group) as being between 128 m.y. and
2007 TURNER ETAL: O¨O¨SHTHEROPOD 3
139 m.y. (Swisher et al., 2002). Older dates process projecting into the orbit, like that in
have been published that would put much of derived tyrannosaurids. In overall appearance
theYixianFormationintheLateJurassic(Lo it is similar to the postorbital of Sinraptor
etal., 1999).Theseolder dates, however,have dongi (Currie and Zhao, 1993) and tentatively
beencriticizedonanumberofgrounds(Zhou considered a noncoelurosaur (allosauroid?) or
et al., 2003). basal tyrannosauroid postorbital.
Similarly, the stratigraphic and temporal A partial pes along with other identified
divisions for the deposits at O¨o¨sh are poorly material pertaining to a small theropod was
constrained (Jerzykiewicz and Russell, 1991). collected and is currently being prepared.
According to Shuvalov (2000), rocks that Slightly weathered postcranial remains col-
overlie the strata of the O¨o¨sh locality exam- lectedasfloatfurtherindicatedthepresenceof
ined here have yielded dates of 141 6 8 m.y. at least one small, derived theropod taxon.
and 126 6 9 m.y. This indicates a Berriasian– This material includes a proximal femur and
Hauterivian stage for these strata. Correlated tibia. The femoral head is weathered but
exposureshavebeendatedataround130 m.y. preservesaprominent,moundlikelateralridge
(Samilov et al., 1988; Rougier et al., 2001), and a well-developed posterior trochanter.
andareconsistentwiththeresultsofShuvalov The proximal tibia lacks a medial cnemial
crest,unlikethatseeninMononykusolecranus
(2000). A broader overview of the geology of
O¨o¨sh is given by Andres and Norell (2005). (Perle et al., 1994) and other alvarezsaurids,
and bears a well-developed fibular crest.
Unfortunately,thedorsalsurfaceofthefemur
COMPONENTS OF THE
ispoorlypreserved,soitisunclearifthelesser
O¨O¨SH LOCALITY
and greater trochanters were confluent as in
derived avialans. Nonetheless, the moundlike
Consistent with previous collecting,
lateral ridge and posterior trochanter indicate
Mongolian Academy of Sciences–American
this material likely is referable to a small
Museum of Natural History Mongolian ex-
paravian theropod.
peditions in 1997, 1999, 2001, and 2005
In 1999, the Mongolian Academy of
collected ubiquitous Psittacosaurus material
Sciences–American Museum of Natural
and numerous undiagnostic large theropod
History Mongolian expedition recovered
teeth. In addition to this material, partial
a partial skull of a small theropod dinosaur
vertebrate material was recovered that is
from O¨o¨sh in a coarse, well-sorted sand
referable to varying levels of clade specificity.
channel. Although incomplete, this skull
Isolated squamate vertebrae lacking zygo-
(IGM 100/1119; fig. 2) is notable for its
phenes/zygantra, exhibiting an unconstricted
possession of several extremely derived coe-
precondylar region, and nonoblique condyles
lurosaurcharacters,clearlydistinguishingitas
are insufficient for referral to a specific clade,
a small dromaeosaurid.
but these characters do rule out varanoid,
cordyliform, lacertiform, or snake affinities.
Additionalsquamatematerialincludesaprob- SYSTEMATIC PALEONTOLOGY
able nonvaranoid, nonsnake autarchoglossan
(based on the presence of small dorsolateral THEROPODAMARSH,1884
zygosphenes) and a nonautarchoglossan cau-
COELUROSAURIAVONHUENE,1914
dal vertebra with a fracture plane that is on
the transverse process. A nearly complete MANIRAPTORAGAUTHIER,1986
gekkonomorph skull is also known from
O¨o¨sh (Conrad and Norell, 2006). DROMAEOSAURIDAEMATTHEWAND
Alarge (,6 cm), heavily rugose postorbital BROWN,1922
was collected. Its straight frontal process
Shanag ashile, new taxon
precludes its referral to most coelurosaur
clades except ornithomimosaurs or tyranno- HOLOTYPE: IGM 100/1119, nearly com-
sauroids.Itssizeandrugosetextureexcludesit plete right maxilla, dentary, and partial
from ornithomimosaurs. It lacks an anterior splenial.
4 AMERICAN MUSEUMNOVITATES NO. 3557
Fig. 2. IGM 100/1119in right lateralview.Anatomical labels in appendix3.
ETYMOLOGY: Shanag, Black-hatted dan- a medial lamina, or the portion of the maxilla
cers in the Buddhist Tsam festival, and ashile, that resides within the antorbital fossa, and
in reference to the old O¨o¨sh locality and a lateral lamina that forms the lateral surface
formation name used by Dr. Henry F. of the maxilla outside the antorbital fossa.
Osborn. These laminae correspond to the lamina
DIAGNOSIS: Small dromaeosaurid thero- lateralis and lamina medialis of Witmer
pod diagnosed by the following combination (1997). Nomenclature for the structures of
of characters: triangular, anteriorly tapering the antorbital cavity follows that used by
maxilla; lateral lamina of nasal process of Witmer (1997).
maxilla reduced to small triangular exposure; The maxilla is triangular, tapering anter-
absenceofapromaxillaryfenestra;presenceof iorly as in Archaeopteryx lithographica
interalveolar pneumatic cavities; incipient (Wellnhofer, 1974: fig. 5; Mayr et al., 2005:
dentary groove on posterolateral surface of fig. 2) and Sinornithosaurus millenii (Xu and
dentary. Wu, 2001). The maxilla contributes to the
narial border, as suggested by the slight
DESCRIPTION depression on its dorsal margin near its
anterior tip. This is unlike the condition in
IGM 100/1119 consists of an almost com- derived dromaeosaurids, but similar to the
plete right maxilla and dentary in articulation basal dromaeosaurids Microraptor zhaoianus
(fig. 2). The maxilla is eroded laterally, re- (Xu et al., 2000) and Sinornithosaurus millenii
vealing details of its alveoli, pneumatic cham- (Xuetal.,1999)andderivedtroodontidssuch
bers, and fenestrae. Like most theropods, the as Troodon formosus (Makovicky and Norell,
maxilla of Shanag ashile can be divided into 2004),Saurornithoidesjunior(IGM100/1)and
three parts: a posteriorly projecting subantor- Byronosaurus jaffei (IGM 100/983). The artic-
bitalprocessthatcontactsthejugal,adorsally ulation with the premaxilla is complex.
projecting ascending ramus of the nasal pro- Anteromedially, a thin projection of bone,
cess, and an anteriorly projecting ventral the surface of which is slightly depressed,
ramus of the nasal process (Witmer, 1990, extendsrostrallypastthelimitofthedentiger-
1997).The maxillacanbefurtherdivided into ous portion of the maxilla. A complementary
2007 TURNER ETAL: O¨O¨SHTHEROPOD 5
groove separates this process from the lateral the internal antorbital fenestra. Furthermore,
lamina of the maxilla. In articulation, a por- there is no indication that the maxilla was
tion of the posterior ramus of the premaxilla recessedmedially fromthenasaland thusitis
would have fit into this groove. A similar unlikely that the nasal formed the dorsal
construction is present in a disarticulated boundary of the antorbital fossa.
maxilla of Velociraptor mongoliensis (IGM The antorbital cavity contains two fenes-
100/976). Neurovascular foramina are present trae, the antorbital and maxillary fenestrae.
on the labial surface with each foramen There is no sign of a promaxillary (tertiary
generally corresponds to a single tooth antorbital) fenestra. Archaeopteryx lithogra-
position. phica,whichpossessesthemostplesiomorphic
The antorbital fossa is large and occupies antorbital cavity of any avialan (Witmer,
much of the lateral surface of the maxilla. 1997), has both a maxillary and promaxillary
Anteriortothefossa,asmalltriangularlateral fenestra in its recessed antorbital cavity.
lamina is present below the external nares. Confuciusornis sanctus, a basal avialan phylo-
This is similar to the condition in Si- genetically intermediate between Archae-
nornithosaurus millenii (Xu and Wu, 2001), opteryx lithographica and Ornithothoraces
Microraptor zhaoianus (Xu et al., 1999), and (sensuChiappe,1995),hasonlyasmall,round
Archaeopteryx lithographica (Wellnhofer, accessory antorbital fenestra (Chiappe et al.,
1974: fig. 5; Mayr et al., 2005: fig. 2). The 1999). Chiappe et al. (1999) tentatively ho-
rostral boundary of the antorbital fossa is mologized this fenestra with the maxillary
situated approximately 7 mm caudal to the fenestra. All other known avialans have
premaxillary contact, above the 3rd maxillary complex diverticula extending medially, ros-
toothposition.Thecaudalmarginofthenaris trally, and caudally from one large antorbital
overlaps the rostral border of the antorbital cavity (Witmer, 1990), but laterally, the
fossa as in Archaeopteryx lithographica antorbital opening is not subdivided into
(Wellnhofer, 1974: fig. 5; Mayr et al., 2005: separate accessory fenestrae. Witmer (1997)
fig. 2) but unlike dromaeosaurids (Norell suggested that the promaxillary fenestra char-
and Makovicky, 2004) and troodontids acterizes Neotheropoda. The lack of a pro-
(MakovickyandNorell,2004).Theantorbital maxillary fenestra is not, however, entirely
fossa is defined shallowly anteriorly, but the unusual among nonavialan theropods. The
dorsal and posteroventral posterior margins troodontids Saurornithoides junior (Barsbold,
aresharplyboundedbythin,laterally project- 1974;Norelletal.,inprep.)andByronosaurus
ingshelvesofbone.Althoughtheproportions jaffei (IGM 100/983) lack a promaxillary
of the antorbital fossa and general shape of fenestra, while the more basal troodontid
the maxilla are similar to that of Sinovenator changii (Xu et al., 2002) retains
Archaeopteryx, the raised dorsal rim of the a promaxillary fenestra. Additionally, the
antorbital fossa differs from the recessed oviraptorosaur Incisivosaurus gauthieri (Xu et
antorbital fossa seen in Archaeopteryx al., 2002) and the therizinosauroid Erliko-
(Witmer, 1990). In almost all avialans, the saurus andrewsi (Clark et al., 1994) lack
ascending ramus of the maxilla is lost, and as a promaxillary fenestra.
a result, the nasal forms the majority of the The internal antorbital fenestra is large and
dorsal border of the internal antorbital fenes- is defined anteriorly (and probably dorsally)
tra and antorbital fossa (Cracraft, 1986). by the ascending process of the maxilla. The
Archaeopteryx lithographica is the only avia- antorbital fossa does not extend ventrally
lan that retains an extensive ascending ramus beyond the border of the internal antorbital
of the maxilla, but the ramus is recessed fenestra, as in troodontids (Saurornithoides
medially so that the nasal forms the dorsal junior,IGM100/1;Saurornithoidesmongolien-
borderoftheantorbitalfossaasinotherbasal sis, AMNH FR 6515; Troodon formosus
avialans (Witmer, 1990), but not the internal [Witmer, 1997]). The surface bone is missing
antorbitalfenestraunlikeotheravialans.IGM fromtheanteriorrimoftheinternalantorbital
100/1119 retains the typical theropod condi- fenestraexposing asubstantial rostralexcava-
tion in which maxilla excludes the nasal from tion.Anteriorly,theexcavation extendstothe
6 AMERICAN MUSEUMNOVITATES NO. 3557
root of the 6th maxillary tooth. The thin caudodorsally open fossa, both of which are
lamina of bone forming the anterior wall of located within the antorbital fossa. The fossa
the excavation wraps around the entire length containing the internal aperture of the maxil-
of the root. This excavation may correspond lary fenestra of Shanag ashile, however, is
tothedeepfossainvadingthepilapostantralis much deeper and more sharply defined than
(the strut of bone reinforcing the poster- those in the previously mentioned taxa. It is,
omedial wall of the maxillary antrum) noted therefore, more similar to Bambiraptor fein-
byWitmer(1997:43)inOrnitholestes herman- bergorum (AMNH FR 30554), which has
ni, Allosaurus fragilis, Marshosaurus bicentesi- a maxillary fenestra that is nearly continuous
mus, and Deinonychus antirrhopus. The la- with the excavatio pneumatica. In Bam-
teral surface of the medial wall of the biraptor feinborgorum (AMNH FR 30554)
excavation is not smooth, but crossed by both structures are contained within a caudo-
ridges and sculptured with rounded depres- dorsallyopeningfossa.Thus,itispossiblethat
sions. Ventrally, the dorsal surfaces of the the oval structure in IGM 100/1119, consid-
posterior alveoli protrude slighting into the ered here to be the maxillary fenestra, may in
fossa. fact represent a maxillary fenestra confluent
The maxillary fenestra in Shanag ashile is with an excavatio pneumatica.
positionedhigh ontheascending ramusofthe Some unusual accessory cavities—interal-
maxilla. This location more generally corre- veolar pneumatic cavities—are present be-
sponds to the excavatio pneumatica of other tween the pairs of adjacent alveoli separated
theropods (Witmer, 1997: 43). The plesio- by a diastema. The interalveolar chambers of
morphic position of the maxillary fenestra is IGM 100/1119 would have been closed off
more ventral, such that the center of the fromtherestofthecraniofacialairsacsystem
maxillary fenestrais aligned with thecenterof in life, surrounded on all sides by a thin layer
the internal antorbital fenestra. Our interpre- of bone, and are visible only because of the
tationofthestructureasthemaxillaryfenestra erodedlabialsurfaceofthemaxilla.Theroots
is supported, however, by the presence of of the teeth that bracket them define the
a maxillary antrum medial to the opening, boundaries of the chambers. Thin laminae of
a similarly positioned maxillary fenestra in bone envelop the roots of the flanking teeth,
Deinonychus antirrhopus (Ostrom, 1969; Wit- defining the anterior and posterior limits of
mer, 1997), and a generally dorsally displaced the chambers. In addition, the chambers
maxillaryfenestraindromaeosaurids. extend dorsally no farther than the dorsal
The maxillary fenestra is a relatively large, margin of the shorter bounding tooth, no
anteroventrally inclined oval structure with fartherventrallythanthecrown–rootjunction
bothaninternalandexternalaperture,similar of the teeth, and no farther medially than the
in structure to the well-developed antorbital medial limit of the teeth. The medial walls of
cavities of Erlikosaurus andrewsi (Clark et al., the chambers have the same irregular surface
1994),Bambiraptorfeinbergorum(AMNHFR as that of the internal antorbital fenestra.
30554), and Saurornithoides mongoliensis These are not the ‘‘recessi pneumatici inter-
(AMNH FR 6516). The external aperture is alveolares’’describedbyWitmer(1997),which
confluent with the lateral surface of the are ventrolateral invaginations of the maxil-
maxilla and defines a fairly deep fossa that is lary antrum into the alveolar process.
continuous with the maxillary antrum. The Structures possibly homologous to the pneu-
internal aperture is positioned at the rostro- matic interalveolar recesses are present within
ventral end of this fossa and perforates the theantorbitalfenestrainIGM100/1119where
maxilla. A somewhat similar arrangement is the alveoli protrude into the fenestra.
seen in taxa such as Marshosaurus bicentesi- At least nine widely and unevenly spaced
mus (see Witmer, 1997), Deinonychus anti- alveoli, six of which contain teeth, are present
rrhopus (Ostrom, 1969), and Velociraptor in the maxilla. The number of maxillary teeth
mongoliensis (AMNH FR 6515, IGM 100/ is similar to the eight in Archaeopteryx
982). In these taxa, the maxillary fenestra is lithographica (Wellnhofer, 1988), the nine in
contained within a shallow, caudally or Dromaeosaurus albertensis (AMNH FR 5356;
2007 TURNER ETAL: O¨O¨SHTHEROPOD 7
becoming more inset posteriorly. The ventral
and dorsal margins of the dentary are sub-
parallel, as in dromaeosaurids (Currie, 1995)
and Archaeopteryx lithographica (Wellnhofer,
1992:fig.6).Tworowsofmentalforaminaare
presentonthelabialsurface,althoughaseries
of smaller anteriorly directedforaminacluster
rostrally. The dorsal row contains a greater
numberofforaminathatareanteroposteriorly
elliptical and elongate. The dorsal row of
mental foramina does not lie in a deep
subalveolar groove as in troodontids
(Makovicky et al., 2003) and Buitreraptor
gonzalezorum (Makovicky et al., 2005).
Posteriorly, the dorsal foramina lengthen to
Fig.3. Detailofposteriortoothmorphologyin form a shallow groove similar to that present
IGM 100/1119. Note serrations on posterior in Archaeopteryx lithographica (Wellnhofer,
carinaeandunconstricted root-crown transition.
1974: plate 22), the holotype of Velociraptor
mongoliensis (AMNH FR 6515), and Tsaagan
Currie, 1995), and the 10 in Velociraptor mangas (IGM 100/1015; Norell et al., 2006).
mongoliensis (AMNH FR 6515). The tooth There are fewer foramina in the lower row,
count in IGM 100/1119 is, however, different although they are of the same general shape.
from the numerous maxillary teeth present in A relatively deep Meckelian groove runs
Jinfengopteryx elegans (Ji et al., 2005) and along the medial surface of the dentary.
troodontids—even basal taxa such as Anteriorly, the groove begins just caudal to
Sinovenator changii (Xu et al., 2002) and Mei the rugose symphyseal tip of the dentary. A
long(XuandNorell,2004).Thelateralsurface small fragment of the splenial covers the
of the maxilla is missing from the central posterior end of the Meckelian groove, so
portionofthemaxillasothattherootsoftwo the posterior extent of the groove cannot be
teeth are exposed. Although there are several determined. The posterior margin of the
emptyalveoliinthemaxillarytoothrow,some dentary is broken, so the posterior articula-
of the gaps are solid stretches of interdental tions are unknown.
bone separating adjacent alveoli. The maxil- There are at least 15 teeth in the dentary.
lary teeth are laterally compressed, with the Due to the overhanging maxilla and its teeth,
carinae positioned lingually. Posterior teeth itisdifficulttodeterminetheexactnumberof
have serrations on their posterior carinae alveoli present. Like the maxillary tooth
(fig. 3). The three central maxillary teeth are count, the dentary tooth count of Shanag
unusual in that they have long roots consti- ashileiscomparabletothatofsomedromaeo-
tuting approximately 70% of the entire tooth saurids, such as the 16 in Deinonychus
length. Tooth root bulges of these teeth are antirrhopus (Ostrom, 1969), 15–16 in
visible on the medial face of the ascending Saurornitholestes langstoni (Sues, 1978), and
ramus of the maxilla. 14–15 in Velociraptor mongoliensis (AMNH
The dentary is long and relatively dorso- FR 6515; Currie, 1995). The dentary teeth,
ventrally shallow, its height comprising only like the maxillary teeth, are interspersed with
about 12% of its length, although it is broken diastemata. The anterior teeth are quite small
posteriorly. The dentary is straight and does and especially closely packed like in troodon-
not curve medially at the symphysis as in tids (Makovicky and Norell, 2004) and
derived troodontids like Troodon formosus Microraptor zhaoianus (Xu et al., 2000; this
(Currie, 1985) or Saurornithoides (Barsbold, paper, fig. 4). Overall, the dentary teeth are
1974; Norell et al., in prep). It is relatively smaller and more closely packed than the
labiolingually thick, with the tooth row maxillary teeth (1.0 versus 1.4 mm basal
slightly inset from the labial margin and crown widths and 0.37 versus 0.70 mm gaps
8 AMERICAN MUSEUMNOVITATES NO. 3557
Fig. 4. Detail of the tooth morphology and size variation seen in the dentary of IGM 100/1119.
Anatomical labels in appendix3.
between adjacent teeth not separated by a di- Makovicky, 2004), Archaeopteryx lithographi-
astema).Theanteriordentaryteetharedevoid ca (Wellnhofer, 1993), and the troodontid
of serrations, while the posterior teeth have Saurornithoides junior (Barsbold, 1974; Norell
smallserrationsonlyontheirposteriorcarinae et al., in prep). A long, narrow notch at the
(12.4 serrations per millimeter). This is the posteriorofthesplenialfragment,correspond-
same pattern seen in Microraptor zhaoianus ing perhaps to the mylohyoid foramen (see
(Hwang et al., 2002). Unlike Microraptor Currie, 1995: fig. 7E), is confluent with the
zhaoianus teeth and the teeth of troodontids Meckelian groove.
andearlyavialans,theteethofIGM100/1119
do not have constrictions between their roots DISCUSSION
and crowns. This unconstricted root/crown
transition is present in Buitreraptor gonzale- Thetaxonpreservesaninterestingmosaicof
zorum (Makovicky et al., 2005), Sin- avialan and dromaeosaurid characteristics.
ornithosaurus millenii (Xu and Wu, 2001), Like most avialans, Shanag ashile possesses
and other dromaeosaurids. a tapering, triangular shaped maxilla, which
Only a small sliver ofthesplenialprotrudes participatesinthenarialborderandhasalarge
past the posterioredge ofthe dentary,butthe antorbital fossa. Additionally, the caudal
anterior portion is nearly intact. It cannot be margin of the external nares overlaps the
determined if the splenial was laterally ex- rostral border of the antorbital fossa, as in
posed at the contact of the dentary and avialans. A number of dromaeosaurid syna-
postdentary bones as in deinonychosaurs, as pomorphies, however, are preserved in IGM
thisregionofthemandibleisnotpreserved.It 100/1119. These include a straight, parallel-
overlaps the posterior third of the dentary, sided dentary and a dorsally displaced maxil-
reachingtoaboutthelevelofthe13thdentary lary fenestra that is itself recessed in a caudo-
alveolus.Thepreservedportionofthesplenial dorsally directed depression.
isthin,long,andsharplytriangular,muchlike To test the phylogenetic placement of
thesplenialofthedromaeosaurids(Norelland Shanag ashile we have expanded the Norell
2007 TURNER ETAL: O¨O¨SHTHEROPOD 9
et al. (2006) study by including the new taxon Saurornitholestes langstoni. The Adams con-
and 15 additional characters, several of which sensus demonstrates that consistent structure
are novel and pertain to the maxilla morphol- exists within the large non-unenlagiine dro-
ogy of dromaeosaurids and avialans. A total maeosaurid clade. This phylogenetic structure
of 56 coelurosaurian taxa and 251 characters is nearly identical to that recovered in the
(19 ordered) were used in the analysis, with analysis of Norell et al. (2006). In the present
Allosaurus fragilisandSinraptordongiusedto analysis, an Adams consensus shows the
rootthe most parsimonious trees.The dataset consistent presence of a Deinonychus +
was treated with equally weighted parsimony Velociraptor sister group relationship when
analysisimplementedinTNTv.1.0(Goloboff the position of Saurornitholestes is not consid-
et al., 2003). A heuristic tree search strategy ered as well as the consistent presence of an
was conducted performing 1000 replicates of Achillobator + Utahraptor clade. However, in
Wagner trees (using random addition se- the latter case, both Adasaurus and Dro-
quences) followed by TBR branch swapping maeosaurus are depicted closer to Achillo-
(holding 10 trees per replicate). The best trees bator than Utahraptor or closer to Utah-
obtained at the end of the replicates were raptor than Achillobator insomeof thefunda-
subjected to a final round of TBR branch mental trees. The largely incomplete nature of
swapping. Zero length branches were col- Dromaeosarus albertensis and Adasaurus mon-
lapsed if they lack support under any of the goliensis reduced the resolution within this
mostparsimoniousreconstructions(i.e.,rule1 subcladeoflarge-bodieddromaeosaurids.
of Coddington and Scharff, 1994). This Although the exact sister taxon of Shanag
analysis resulted in 552 most parsimonious ashile is currently ambiguous, it is most simi-
trees of 764 steps (CI 5 0.381, RI 5 0.709) lar in general shape and morphology to
found in 625 out of the 1000 replicates. Sinornithosaurus millenii (Xu et al., 1999).
Shanag ashile was found to be a member of Bothtaxapossessatriangle-shaped,anteriorly
a dromaeosaurid clade less inclusive than tapering maxilla as well as maxillary partici-
Unenlagiinae (i.e., Buitreraptor gonzalezorum, pation in the external nares. In the most
Unenlagiacomahuensis,andRahonavisostromi) parsimonious trees that recover Shanag ashile
inallofthemostparsimoniousreconstructions closelyrelatedtoSinornithosaurusmillenii,the
(fig. 5). This placement is supported by the topology is supported by a single synapomor-
presenceinShanagashileofthreeoftheclade’s phy—the lateral lamina of the ventral ramus
six unambiguous synapomorphies: a solid of the maxilla reduced to a small triangular
quadrate (char. 52.0—unknown in Shanag exposure (char. 244.1). Shanag ashile remains
ashile); dentary with subparallel dorsal and distinguishable from Sinornithosaurus millenii
ventral edges (char. 70.1); large maxillary and due to its lack of a promaxillary fenestra.
dentary teeth, less than 25 in dentary (char. Prior to the discovery of IGM 100/1119, the
84.0); anterior cervical centra level with or only theropod material from the Early
shorter than posterior extent of neural arch Cretaceous O¨o¨sh deposits were undiagnostic.
(char. 96.0—unknown in Shanag ashile); ex- Shanag ashile marks the first theropod taxon
tremelylongextensionoftheprezygapophyses from the deposits referable to a specific thero-
of the distal caudal vertebrae (char. 120.1— pod clade—the Dromaeosauridae. The discov-
unknown in Shanag ashile); and dorsal dis- eryofIGM100/1119isimportantasitextends
placement of maxillary fenestra (char. 237.1). our knowledge of Early Cretaceous dromaeo-
Giventhelarge percentageofmissingdatafor saurids. The best known Early Cretaceous
IGM100/1119,Shanagashilewasrecoveredin dromaeosaurids—Sinornithosaurus and Micro-
numerous placements within the large clade raptor—are known from the Jehol beds of
more derived than Unenlagiinae clade (i.e., China. The presence Shanag ashile in the
Sinornithosaurus+Velociraptor),resultinginan Jehol–contemporaneous O¨o¨sh deposits of
unresolved strict consensus. The Adams con- Mongolia suggests a wider diversity of Early
sensus depicted in figure 5 shows this lack of Cretaceous dromaeosaurids that show a
resolution is due to the labile position of broader distribution of character states and
IGM 100/1119 and to a lesser extent, avianlikecranialfeatures.
10 AMERICAN MUSEUMNOVITATES NO. 3557
Fig. 5. Phylogenetic placement of IGM 100/1119. A. Strict consensus of 552 most parsimonious
reconstructionsofcoelurosaurianinterrelationshipsfoundinourphylogeneticanalysisof251charactersand
56 coelurosaurian taxa. The new taxon is indicated in bold. B. Adams consensus topology of non-
unenlagiine dromaeosaurids recovered in from thisanalysis.
ACKNOWLEDGMENTS field crew of the 1999 field season for their
hardwork,ChrisBrochuforprovidingimages
Collection and study of this specimen was of FMNH PR2081, and Peter Makovicky,
supportedbyNSFgrantsDEB-9300700,DEB Jack Conrad, and Sterling Nesbitt for helpful
0608003, and ATOL 0228693. We thank the discussions. Amy Davidson prepared the