Osteology and relationships of Byronosaurus jaffei (Theropoda, Troodontidae)

32 Pages·2003·4.6 MB·English

by  Makovicky, Peter J.

AMERICAN MUSEUM
Novitates
PUBLISHED BY THE AMERICAN MUSEUM OF NATURAL HISTORY
CENTRAL PARK WEST AT 79TH STREET, NEW YORK, NY 10024
Number 3402, 32 pp., 21 figures, | table March 27, 2003
Osteology and Relationships of
Byronosaurus jaffei (Theropoda: Troodontidae)
PETER J. MAKOVICKY,' MARK A. NORELL,? JAMES M. CLARK,* AND
TIMOTHY ROWE +
ABSTRACT
The troodontid Byronosaurus jaffei is known from two specimens from adjacent localities
in the Nemegt basin, Omnégov Aimag, Mongolia. These specimens are composed of well-
preserved cranial material and fragmentary postcrania. All of these elements are described
here. Byronosaurus jaffei is included in a comprehensive phylogenetic analysis of Coeluro-
sauria to ascertain its relationships. Several interesting characters of Byronosaurus jaffei have
implications both for theropod relationships and for understanding patterns of variation within
coelurosaurian theropods. These include the position of a foramen that marks the exit of the
supra-alveolar canal (which we suggest is homologous with the subnarial foramen), the flat-
tened internarial bar, the unusual interfenestral bar, and the unserrated teeth. Additionally, the
well-preserved braincase allows detailed comparison with other troodontid taxa.
INTRODUCTION described, most from Central Asia (Osm6l-
ska and Barsbold, 1990; Russell and Dong,
The remains of troodontid dinosaurs are 1993). Here we provide a detailed descrip-
extremely rare, and only nine taxa have been tion of Byronosaurus jaffei, a troodontid
' Assistant Curator, Department of Geology, The Field Museum, Roosevelt Rd. at Lake Shore Drive, Chicago IL
60605. Research Associate, Division of Paleontology, American Museum of Natural History. e-mail: pmakovicky @
fieldmuseum.org
? Chairman, Division of Paleontology, American Museum of Natural History. e-mail: [email protected]
3 Ronald S. Weintraub Associate Professor Department of Biological Sciences, George Washington University,
Washington, DC 20052. Research Associate, Division of Paleontology, American Museum of Natural History. e-mail:
jclark @ gwu.edu
4J. Nalle Gregory Regents Professor of Geology, and Director, Vertebrate Paleontology Laboratory, Department
of Geological Sciences, The University of Texas at Austin, TX 78712. e-mail: [email protected]
Copyright © American Museum of Natural History 2003 ISSN 0003-0082
2 AMERICAN MUSEUM NOVITATES NO. 3402
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olotype locality looking north. The arrow signifies the point of discovery of IGM 100/
from the Djadokhta Formation at Ukhaa Tol- groups will be considered in more detail else-
god that was named and briefly described by where.
Norell et al. (2000). Only one other troodon-
tid, Saurornithoides mongoliensis (Osborn,
MATERIAL
1924), has been reported from Djadokhta, or
Djadoktha-type, beds in Asia (Currie and
LocaLity: The holotype specimen (GM
Dong, 2001a). Although troodontids are rare,
100/983) was discovered at the sublocality
well-preserved braincases are known for sev-
called “‘Ankylosaur Flats” in 1993 by Mi-
eral taxa (Osborn, 1924; Barsbold, 1974;
chael Novacek (fig. 1). Additional remains of
Currie, 1985; Currie and Zhao, 1993; Russell
this specimen were collected during the 1994
and Dong, 1993, Xu et al., 2002), and have
and 1995 field seasons. Unlike many of the
provided important phylogenetic insights
fossils at Ukhaa Tolgod, the specimen was
into this clade.
not articulated and was found as a loose as-
Few phylogenetic studies within Troodon-
sociation of bones. Nevertheless, the rostral
tidae have been undertaken, and broader
fragment and the braincase were found in
studies that include troodontids and their pur-
place adjacent to one another in life position,
ported closest relatives (dromaeosaurids
separated by a zone of weathering. The re-
[Gauthier, 1986; Sereno, 1999], birds [H oltz,
maining parts of the skeleton were collected
2001]) have not taken into account the vari-
ation among troodontid taxa and have been as eroded pieces below the skull. Later ex-
hindered by fragmentary material and inad- amination of the specimen revealed that re-
equate descriptions. Here we expand our spe- mains of a second individual were inter-
cies-level phylogenetic analysis of the group mixed with IGM 100/983. This individual
(Norell et al., 2000) and examine the evi- (IGM _ 100/987) is significantly larger than
dence for monophyly of this group and more IGM_ 100/983, and consequently its bones
exclusive clades within it. The evidence for can easily be separated. This specimen lacks
the relationships of troodontids to other troodontid apomorphies, and is referred to
2003 MAKOVICKY ET AL.: BYRONOSAURUS JAFFEI 3
the Ornithomimidae (Makovicky and Norell, DESCRIPTION
1998).
The skull of IGM 100/983 is preserved in
REFERRED SPECIMEN: A second specimen
two parts. The anterior section comprises the
(IGM_ 100/984) was collected on July 15,
rostrum and articulated dentaries that have
1996 at the “‘Bolor’s Hill” locality approxi-
been transversely compressed although most
mately 6 km west of the main Ukhaa Tolgod
bones appear uncrushed (figs. 2, 3). It was
exposure. While extremely fragmentary, the
found in two pieces that have been glued to-
specimen preserves some important features
gether, and the glue fills some large gaps
not present on the type. The specimen con-
along the contact. The anterior section ends
sists of a rostrum in six pieces. It is referred
at the anterior margin of the orbits. The
to Byronosaurus jaffei on the basis of super-
braincase was preserved separately and has
numary teeth all lacking serrations, distinc-
not been crushed. The top of the braincase is
tive lacrimals with a lateral shelf that over- missing, exposing an eroded endocast, and
hangs the anterodorsal corner of the orbit, some delicate outer parts of the basicranium
and a pair of passages through the interfe- are incomplete.
nestral bar, which is not recessed from the IGM 100/984 is very fragmentary. It was
plane of the rostrum. preserved as eight separate nodules, only
three of which contain recognizable ele-
ments. One contains the premaxillae and the
DIAGNOSIS
anterior ends of the maxillae (fig. 4), a sec-
Byronosaurus jaffei exhibits the following ond contains a section of the rostrum at the
troodontid synapomorphies (Xu et al., 2002): level of the interfenestral bar, and a third
contains a section of skull roof at the preor-
numerous teeth, close packing of dentary
dentition near the rostral tip of the lower jaw, bital bar preserving the lacrimals and the
presence of a distinct groove for the neuro- posterior part of the nasals.
vascular foramina on the dentary, and a dor- Our description is primarily based on IGM
100/983, supplemented by IGM_ 100/984.
soventrally flattened internarial bar.
Measurements of the holotype IGM 100/983
Byronosaurus jaffei is part of a clade com-
are tabulated in appendix 2. Comparisons
prising all troodontids except the basal taxon
with Saurornithoides junior are based on
Sinovenator changi. This clade is ambigu-
Barsbold (1974, 1983) and are supplemented
ously diagnosed by: braincase displaying a
by our own observations. Comparisons with
large pneumatic sinus (the “lateral depres-
Troodon formosus are based on Currie (1985)
sion’’ ventral and anteroventral to the brain-
and Currie and Zhao (1993), and those with
case; presence of a subotic recess; extensive
Sinornithoides youngi are based on Russell
anterior process of the lacrimal forming the
and Dong (1993) and our examination of the
dorsal border of the antorbital fenestra; and
specimen. Comparisons with Saurornithoides
maxillary participation in the posterior mar-
mongoliensis are based on our own observa-
gin of the nares. Ambiguity in the optimi-
tions of the type (AMNH 6174). Other com-
zation of these characters is due to missing
parisons are referenced herein.
data and multiple placements of several troo- The face and braincase of the type speci-
dontid taxa, including Byronosaurus jaffei,
men (IGM 100/983) were scanned separately
Sinornithoides youngi, and an unnamed in May 1997 at The University of Texas
Mongolian taxon represented by IGM 100/ High-resolution X-ray Computed Tomogra-
44.
phy Facility. Both were scanned on the high-
Byronosaurus jaffei can be distinguished resolution subsystem, based on a Feinfocus
from all other troodontids by the possession microfocal X-ray tube and an image-inten-
of several derived characters, including teeth sifier detector system. The face was scanned
lacking serrations, an interfenestral bar that with X-ray energy set at 120 kV, 0.200 mA,
is not recessed from the plane of the maxilla, at 190% offset in an air wedge, with a
and a shallow groove along the buccal mar- source-object distance of 85 mm. Slice thick-
gin of the maxilla. ness is 0.25 mm, and 562 consecutive slices
- AMERICAN MUSEUM NOVITATES NO. 3402
Fig. 2. The rostrum of the holotype specimen IGM 100/983 in right lateral and dorsal views. Ab-
breviations are listed in appendix 1.
2003 MAKOVICKY ET AL.: BYRONOSAURUS JAFFEI 5
Fig. 3. The rostrum of the holotype specimen IGM 100/983 in left lateral and ventral view. Abbre-
viations are listed in appendix 1.
6 AMERICAN MUSEUM NOVITATES NO. 3402
Fig. 4. Stereopair of the ventral surface of the premaxillary area in the referred specimen IGM 100/984.
were generated. For each slice, 1200 views bears four teeth (fig. 4), as is primitive for
were taken with two samples per view. Re- theropods. The nasal process is gently arched
constructed 512 x 512 pixel imagery was and dorsally flattened and triangular in cross
exported as 8-bit TIF files. The braincase section (fig. 2), and it forms the anterodorsal
was scanned with X-ray energy set at 120 margin of the external narial opening. It is
kV, 0.400 mA, at 190% offset in an air thin anteriorly and increases in breadth in the
wedge, with a source-object distance of 45 area where it divides the nasals along the
mm. Slice thickness is 0.20 mm, and 150 midline, as in Saurornithoides junior. The
consectutive slices were generated. For each maxillary process is short and heavily pitted
slice, 1200 views were taken with two sam- by small foramina on its small facial surface.
ples per view, and a field of view of approx-
Medially, the surface of the maxillary pro-
imately 36.3 mm. Reconstructed 1024 xX cess arches to form the floor of the naris (fig.
1024 pixel imagery was exported as 8 bit 2). There is no posterior process as in dro-
TIF files. Original CT datasets were archived
maeosaurids that excludes the maxilla from
on CD-ROM at the University of Texas and
participation in the margin of the narial open-
the American Museum of Natural History.
ing (figs. 2, 3). Although obscured, the suture
Animated serial section movies and three di-
with the maxilla is apparently very simple.
mensional volumetric reconstructions are
MAXILLA: The maxilla forms most of the
also viewable on the Internet at www.
lateral surface of the skull. Anteriorly, it
DigiMorph.org.
forms much of the floor of the large naris.
The maxilla contacts the premaxilla along a
THE ROSTRUM
Straight suture anteriorly in this chamber
PREMAXILLA: Paired premaxillae form the (figs. 4, 5). Just posterior to this suture, and
anterior margin of the skull. Each premaxilla before the maxilla rises from the narial floor
2003 MAKOVICKY ET AL.: BYRONOSAURUS JAFFEI a
Fig. 5. Detail of the narial region of the ho-
lotype specimen IGM 100/983 in left lateral view.
Abbreviations are listed in appendix 1.
In Troodon formosus the maxillary fenestra
is backed by an unperforated osseous wall.
(Brochu, personal commun.). Anteriorly, the
rostral end of the maxilla overhangs the max-
Fig. 4. Continued. illary fenestra to form a large pocket. The
maxillary fenestra is separated from the ant-
orbital fenestra by a large, very flat plate of
to form the posterodorsal margin of the naris, maxilla, here termed the interfenestral bar
lies a large, anteriorly directed foramen that (figs. 2, 7). Unlike Saurornithoides junior,
is completely enclosed by the maxilla (figs. Saurornithoides mongoliensis, or Sinorni-
5, 6). A similar foramen is observed in the thoides youngi (Currie and Dong, 2001a), the
alvarezsaurid Shuvuuia deserti (IGM _ 100/ interfenestral bar is not inset from the plane
977). This foramen enters into a long supra- of the maxilla.
alveolar canal that parallels the tooth row The main antorbital fenestra lacks an os-
(fig. 6). The maxilla rises from the narial seous floor, is larger than the anterior fenes-
floor in a gentle posterior slope. The lateral tra, is subtrapezoidal in shape, and narrows
surface below the narial chamber is covered dorsally. Anteriorly, the margin of the fenes-
with small pits and foramina (fig. 5). These tra is formed by the interfenestral bar, which
are most apparent as a row of large pits that is scalloped in two places (fig. 7). These
lie in a groove just dorsal and parallel to the emarginations lead to dorsal and ventral pas-
dentigerous margin. This row of pits runs sages through the interfenestral bar, which
posteriorly below the maxillary fenestra, de- are separated by an osseous wall (fig. 7). An
creasing in density posteriorly until they dis- erosional cross section of the snout of IGM
appear below the antorbital fenestra. 100/984 exposes the course of the two chan-
The lateral surface of the maxilla is per- nels through the interfenestral bar (fig. 8).
forated by two large, elongated antorbital The dorsal emargination marks the caudal
openings (figs. 2, 3). The maxillary fenestra end of the narial passage, which is visible
is elliptical and extends from below the ex- laterally as a swollen ridge on the dorsal part
ternal nares to the midway point between the of the osseous wall of the maxillary fenestra
maxillary-premaxillary suture and the pre- (figs. 9, 10). The ventral canal connects the
orbital bar. This fenestra has an osseous me- antorbital and maxillary fenestrae. The cau-
dial wall, which is perforated by a small hole dal entrance into the supra-alveolar canal,
in slightly different positions on each side, which transmits the innervation and blood
but these holes may be weathering artifacts. supply of the maxillary dentition, lies on the
8 AMERICAN MUSEUM NOVITATES NO. 3402
Fig. 6. Stereopair of the right dorso-oblique dorsal surface of the rostrum in the referred specimen
IGM 100/984. Notice the entrance to the supra-alveolar canal (fsac) which we interpret as the subnarial
foramen. Abbreviations are listed in appendix 1.
medial face of the interfenestral bar (fig. 9C). of tiny foramina within a shallow depression.
Below the antorbital fenestra, the maxilla is The posterior contact with the frontal is not
extremely thin, dorsoventrally concave, and preserved on the type specimen, but is visi-
devoid of the small foramina that are so con- ble on IGM 100/984. The nasals end just
spicuous anteriorly. posterior to the level of the preorbital bar. At
On the palate of IGM 100/984, longitudi- the level of the preorbital bar the nasals di-
nally extensive palatal shelves of the maxilla verge along the midline (fig. 13) to accom-
form the roof of the mouth (figs. 4, 9, 10). modate a missing anterior wedge of the fron-
These large palatal flanges extend posteriorly tal, as in Saurornithoides junior. Posterior to
from the contact with the premaxilla, and are the preorbital bar on the top of the skull the
joined by a median vomer at the border of nasals are flat, and together with the lacri-
the choanae (figs. 11, 12. Together these mals they form a flat surface above the an-
form a solid secondary palate on the roof of terior part of the orbits (fig. 13).
the mouth extending to the choanae, which LACRIMAL: The lacrimal forms the anterior
lie between the antorbital fenestrae. and anterodorsal border of the orbit. In lat-
NASAL: The nasal is a long bone that cov- eral view the lacrimal is T-shaped, as in dro-
ers the top of the rostrum for most of its maeosaurids; however, the anterior or supra-
length (fig. 2). Anteriorly, the nasal forms the antorbital fenestral process is much longer
posterolateral boundary of the large external than the supraorbital process (figs. 2, 3). Al-
naris. The nasal is wider anteriorly than pos- though poorly preserved, several important
teriorly. As preserved, anteriorly the nasal is features are apparent. The preorbital bar is a
laterally convex, forming a continuous tran- straplike process extending from the skull
sition from a vertical plane anteriorly to a roof to contact the jugal. The preorbital bar
horizontal plane posteriorly. This transition is is oriented nearly vertically; however, this
marked by a sharp angle, but the angle may orientation may be influenced by distortion.
be influenced by postmortem distortion. A Contact with the jugal is preserved only on
maxillary process extends posteroventrally, the right side, where a posterolaterally open-
while a long, attenuate premaxillary process ing foramen appears to be present, but this
extends anteriorly to form the posterodorsal area is broken (fig. 2). The posterior surface
border of the naris for half its length and con- is slightly convex in cross section. CT scans
tact the nasal process of the premaxilla me- reveal that the lacrimal duct exits the orbit
dially (fig. 5). This process gives the anterior just ventral to the contact between the ante-
surface of each nasal a concave appearance rior and posterior processes. Currie and Dong
in lateral view. The lateral edge of the nasal (2001a) suggested that a lacrimal duct is ab-
contacts the maxilla anteriorly and the lacri- sent in troodontids, but in Byronosaurus the
mal posteriorly along a straight suture. Par- lacrimal duct passes through the lacrimal and
allel and just medial to this suture lies a row opens into the posterodorsal corner of the
2003 MAKOVICKY ET AL.: BYRONOSAURUS JAFFEI 9
Fig. 6. Continued.
5mm
Fig. 8. Antero-ventro oblique view of the re-
ferred specimen (IGM 100/984) showing the os-
seous wall of the interantorbital canal (jac) sepa-
rating it from the nasal chamber just anterior to
the interfenestral bar. Abbreviations are listed in
appendix 1.
antorbital fossa, as observed in IGM 100/
984. The posterior, or supraorbital, process is
preserved only at its base on the type. On
IGM 100/984 the posterior process is appar-
ently completely preserved on the right lac-
rimal (fig. 13). In this specimen it extends to
the same level posteriorly as the nasals. The
posterior process forms a laterally expanded
Shelf above the orbit, as in Saurornithoides
junior, and it apparently meets the frontal
along a posteromedial-anterolateral diagonal
suture (fig. 13). Just posterior to the level of
the preorbital bar, the dorsal surface of the
lacrimal is perforated by a small anteriorly
directed foramen.
The anterior process is extensive and
forms most, if not all, of the dorsal border of
the posterior antorbital fenestra. At the pos-
Pismy], Postero-oblique view of the left side
terodorsal corner of the antorbital fenestra a
of the rostrum of IGM 100/983 showing the two
large pocket is formed by a buttress between
passages through the interfenestral bar. The upper
the preorbital and anterior processes of the
passage (en) connects with the nasal chamber. The
lower passage (iac) connects with the maxillary lacrimal. The lateral surface of this buttress
sinus. is extensive and flat, forming a wall to the
10 AMERICAN MUSEUM NOVITATES NO. 3402
Fig. 9. CT scans of the rostrum of IGM 100/983. Notice the osseous floor to the nasal cavity (fnc)
as it extends through the interfenestral bar in C, D, E, and EK Abbreviations are listed in appendix 1.
posterodorsal part of the antorbital fossa. A JUGAL: The jugal is extremely poorly rep-
small ridge on the anterior process overhangs resented by only a small strap of bone from
the pocket. The pocket that forms at the apex the anterior end, present on the right side of
of the buttress is smooth-walled and is IGM 100/983 (fig. 2). The preserved frag-
pierced at its apex by the lacrimal duct. ment of the jugal is a very thin bone that