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uring the jurassic, sauropod
dinosaurs rose to predominance among
vertebrate herbivores, in terms of both species di-
versity and biomass (e.g., Romer 1966; McIntosh
1990). Their perceived decline on northern land-
masses during the Cretaceous has been linked to
the evolution of tooth batteries in ornithischian
herbivores (e.g., Lull and Wright 1942; Ostrom
1961; Bakker 1978; Lucas and Hunt 1989). On
southern landmasses, in contrast, sauropod di-
versity increased during the Cretaceous
(Weishampel 1990; Hunt et al. 1994), and a
newly discovered southern sauropod, the reb-
bachisaurid Nigersaurus taqueti, is now known to
have evolved a complex tooth battery (Sereno et al.
1999).
Rebbachisaurids are a poorly known sauro-
pod clade, reported thus far only from Cretaceous
rocks in South America (Calvo and Salgado
1995; Bonaparte 1996; Apestiguía et al. 2001;
Lamanna et al. 2001), Africa (Lavocat 1954;
Taquet 1976; Sereno et al. 1999), and Europe
(Dalla Vecchia 1998; Pereda-Suberbiola et al.
2003). Many of these are fragmentary finds,
leaving much of the skeletal anatomy of this
group in question, especially the skull. In this
chapter, we describe the tooth-bearing bones
and dental battery of Nigersaurus taqueti and pro-
vide an initial cranial reconstruction. We outline
the feeding specializations common to diplo-
docoids and how these were modified within
rebbachisaurids.
Institutional Abbreviation: MNN, Musée
National du Niger, Niamey.
SYSTEMATIC PALEONTOLOGY
SAUROPODA MARSH 1878
EUSAUROPODA UPCHURCH 1995
NEOSAUROPODA BONAPARTE 1986
DIPLODOCOIDEA MARSH 1884
REBBACHISAURIDAE BONAPARTE 1997
NIGERSAURUS SERENO ET AL. 1999
TYPE SPECIES: Nigersaurus taqueti Sereno et
al. 1999.
AGE: Early Cretaceous (Aptian–Albian).
DISCUSSION: Nigersaurus taqueti is the most
common sauropod and one of the most common
species recovered in the rich vertebrate fauna
described from Gadoufaoua, Niger Republic
157
five
Structure and Evolution
of a Sauropod Tooth Battery
Paul C. Sereno and Jeffrey A. Wilson
D
(Taquet 1976; Sereno et al. 1998, 1999, 2001,
2003; Larsson and Gado 2000). Nevertheless,
because the skull and skeleton are delicately con-
structed and highly pneumatic, there are no com-
plete skulls and only a few partially articulated
skeletons. Nigersaurus taqueti was named and
identified as a rebbachisaurid by Sereno et al.
(1999). An earlier report from the same beds of a
dicraeosaurid allied with titanosaurs (Taquet
1976:53) very likely pertains to the same species.
NIGERSAURUS TAQUETI SERENO ET AL.1999
(FIGS. 5.1, 5.3–5.8)
HOLOTYPE:MNN GDF512, partial disarticu-
lated skull and partially articulated neck pre-
served in close association on 1 m2of sandstone
outcrop. Sereno et al. (1999:1346) also list a
“scapula, forelimbs, and hind limbs” as part of
the holotype. These and other bones, which we
regard as referable to this species, were found
at some distance from the skull and neck and
cannot be reliably associated with the holotypic
specimen. The partial skeleton described by
Taquet (1976) also was discovered in the vicin-
ity of the holotype and may pertain to the same
species.
LOCALITY AND HORIZON:Gadoufaoua (16°
27´N, 9°8´´E), eastern edge of the Ténéré Desert,
Niger Republic; Elrhaz Formation.
REFERRED MATERIAL: MNN GDF513, worn
crown. Additional skeletal and dental material
is described elsewhere.
REVISED DIAGNOSIS:Rebbachisaurid sauro-
pod characterized by five accessory fenestrae in
the jugal, surangular, and angular; tooth num-
ber increased to 20 and to 34 in the maxilla and
dentary, respectively; tooth replacement
increased to as many as 10–12 in a single col-
umn; premaxilla and dentary lacking alveolar
septa; maxilla with oval (vertically elongated)
replacement foramina; extension of the dentary
tooth row lateral to the sagittal plane of the
158 STRUCTURE AND EVOLUTION OF A SAUROPOD TOOTH BATTERY
FIGURE 5.1. Preliminary reconstruction of the skull of Nigersaurus taqueti based on MNN GDF512. (A) Skull in left lateral
view. (B) Left lower jaw in dorsal view with tooth batteries removed. 1–5, accessory cranial fenestrae; a, angular; altr, alveolar
trough; antfe, antorbital fenestra; ar, articular, br, buccal ridge; cp, coronoid process; d, dentary; d1–d34, replacement foram-
ina for dentary tooth positions 1–34; emf, external mandibular fenestra; en, external naris; f, frontal, fo, foramen; j, jugal; ltf,
laterotemporal fenestra; m, maxilla; mc, Meckel’s canal; nf, narial fossa; pm, premaxilla; po, postorbital; popr, paraoccipital
process; ps, parasphenoid; q, quadrate; qfo, quadrate fossa; qj, quadratojugal; sa, surangular; saf, surangular foramen, so,
supraoccipital; sq, squamosal; sym, symphyseal surface; vc, vascular canal.
lower jaw; subcircular mandibular symphysis;
crowns with prominent mesial (medial) and
distal (lateral) ridges; scapula with prominent
rugosity on the medial aspect of the proximal
end of the blade.
DESCRIPTION
SKULL AND DENTITION
The skull and neck of the holotypic specimen
of Nigersaurus taqueti were found in close asso-
ciation. Most of the dorsal skull roof is pre-
served (fig. 5.1A). The braincase is intact, with
the proximal end of the stapes in place in the
fenestra ovalis. The quadrate is the only palatal
bone preserved. All of these bones, with the
exception of the frontal and braincase, are
composed of thin laminae or narrow struts
and are extremely delicate. Five unique acces-
sory fenestrae are present, two in the jugal and
three in the surangular and angular (figs.
5.1–5.5). These accessory openings are bor-
dered by bone that tapers gradually in width to
a paper-thin edge that has a smooth margin. It
is highly unlikely, therefore, that they repre-
sent lesions or some other kind of bone
pathology, like the healed openings reported in
aged individuals of other dinosaurian species
(e.g., Brochu 2003).
The snout is proportionately much shorter
and the dental arcade is less prognathous than
in diplodocids or dicraeosaurids (figs. 5.1, 5.2).
As mentioned above, the external naris is not
retracted above the orbit in Nigersaurus. A
statement to the contrary—that the external
naris is positioned as in diplodocids—was
made on the basis of the maxilla before the pre-
maxilla was exposed (Sereno et al. 1999:1344).
Although the nasal and thus the posterior mar-
gin of the external naris are not known, the
anterior portion of the border is far anterior to
that in any known diplodocid. The external
naris is large, laterally facing, separated in the
midline from its opposite by a vertical premax-
illa–nasal septum, and surrounded by a more
pronounced narial fossa. As in other diplo-
docoids, the jaw articulation and laterotemporal
fenestra are shifted anteriorly under the orbit
(figs. 5.1, 5.2). The supratemporal fenestra is
very reduced or absent altogether, in contrast to
those in other diplodocoids (Holland 1924) or
other sauropods.
The transversely expanded form of the alveo-
lar ramus of the dentary is unique among
dinosaurs (fig. 5.1B). No other dinosaur has a
tooth row that extends lateral to the longitudinal
plane of the lower jaw. The maximal width
across the anterior end of the paired dentaries
slightly exceeds the length of the entire lower
jaw. Despite the gaping alveolar trough that
housed hundreds of slender teeth packed
together as a tooth battery, the dentary and other
bones of the lower jaw in Nigersaurus are as
lightly built as those in Diplodocus and much
more slender than those in Camarasaurus (figs.
5.1, 5.2). The description below is limited to the
tooth-bearing bones and the teeth.
PREMAXILLA
The premaxilla is a slender bone, the ventral
third of which houses a battery of teeth aligned
in four columns (fig. 5.3, table 5.1). The dorsal
two-thirds of the premaxilla extends as a thin
lamina appressed to its opposite in the mid-
line. In lateral view, these two parts of the pre-
maxilla meet at an angle of approximately 30º,
greater than that in Diplodocus (figs. 5.1A,
5.2B). Four replacement foramina and a con-
necting groove for the dental lamina are visi-
ble on the posterior aspect of the bone. Unlike
those in other diplodocoids, the alveolar mar-
gin and space within the premaxilla housing
replacement teeth are not divided by bony
septa into discrete alveoli. Rather, they are
developed as an open trough, the arched ante-
rior wall of which is thin and extends more
than 1 cm farther ventrally than the posterior
wall (figs. 5.3, 5.4).
Although the active, or functioning, teeth
have fallen from the trough, a battery of replace-
ment teeth is visible in computed tomography
STRUCTURE AND EVOLUTION OF A SAUROPOD TOOTH BATTERY 159
FIGURE 5.2. Skull reconstructions in lateral view of (A) the basal neosauropod Camarasaurus lentus and
(B) the diplodocid Diplodocus longus (from Wilson and Sereno 1998). a, angular; amf, anterior maxillary fora-
men; aofe, antorbital fenestra; ar, articular; asf, anterior surangular foramen; bo, basioccipital; d, dentary; f,
frontal; j, jugal; l, lacrimal; lf, lacrimal foramen; m, maxilla; n, nasal; nf, narial fossa; p, parietal; paofe, prean-
torbital fenestra; pl, palatine; pm, premaxilla; po, postorbital; popr, paraoccipital processes; pra, prearticular;
prf, prefrontal; ps, parasphenoid; psf, posterior surangular foramen; pt, pterygoid; q, quadrate; qj, quadratoju-
gal; sa, surangular; sf, surangular foramen; sq, squamosal; so, supraoccipital; v, vomer.
FIGURE 5.3. (A) Stereopair and (B) matching drawing of the right premaxilla of Nigersaurus taqueti (MNN GDF512) in
lateral view. am, articular surface for the maxilla; altr, alveolar trough; en, external naris; fo, foramen; nf, narial fossa;
plpr, palatal processes. Scale bars equal 5 cm.
TABLE 5.1
Measurements of the Tooth-Bearing Bones and Teeth of Nigersaurus taqueti
Premaxilla
Maximum length 209
Maximum width of maxillary articular surface, midheight 18
Alveolar trough, anteroposterior width 10
Posterodorsal ramus, length 77
Maxilla
Alveolar ramus
Preserved width 94
Depth (to ventral rim of antorbital fenestra) 73
Replacement foramen, height 11
Replacement foramen, width 3
Dentary
Alveolar ramus, width 115
Alveolar trough, anteroposterior width of medial end 12
Symphysis
Dorsoventral height 21
Anteroposterior depth 23
Teeth
Maxillary tooth
Crown height (25)
Basal crown width 4
Dentary tooth
Crown height (20)
Basal crown width 3
NOTE: Measurements are for the holotypic skull of Nigersaurus taqueti (MNN GDF512). Paren-
theses indicate estimated measurement. Measurements are in millimeters.
FIGURE 5.4. High-resolution
computed tomographic sec-
tions through the right premax-
illa of Nigersaurus taqueti
(MNN GDF512) in (A) longitu-
dinal and (B) cross-sectional
views. The packing pattern of
the tooth battery and the asym-
metrical distribution of enamel
on individual crowns are visi-
ble. altr, alveolar trough; mt,
mature tooth; nt, new tooth; rf,
replacement foramen.
(CT) scans in the upper half of the alveolar ramus
(fig. 5.4B). The premaxillary teeth are self-sup-
porting and in mutual contact along the lengths
of their crowns, which presumably erupted and
wore as a unit. The teeth originate just anterior to
the replacement foramina. As the teeth increase
in length, their roots extend into the upper half of
the alveolar ramus before migrating ventrally
toward the alveolar trough (fig. 5.4A).
MAXILLA
The ventral one-fifth of the maxilla houses a
battery of teeth arranged in columns internal to
each oval replacement foramen (Sereno et al.
1999:fig.2D; fig. 5.5, table 5.1). The dorsal four-
fifths of the bone is developed proximally as a
thin plate and distally as a narrow and delicate
strut that divides the external naris and antor-
bital fenestra (fig. 5.1). As in the premaxilla, the
alveolar margin is developed as an undivided
trough, as seen in posterior view, with the labial
(anterior) wall extending approximately 1 cm
farther ventrally than the lingual (posterior)
wall. Unlike the premaxilla and dentary, bony
septa are present within the body of the alveolar
ramus. These septa join the posterior and ante-
rior walls, enclosing crypts for each replacement
series that are approximately twice as deep labi-
olingually (anteroposteriorly) as mesiodistally
(transversely).
Although all worn maxillary teeth have
fallen from the alveolar trough, columns of
replacement teeth are visible in the two most
mesial (medial) tooth positions (Fig. 5.5). As in
the premaxilla and dentary, the teeth originate
just labially (anteriorly) to each replacement fora-
men, extend with growth dorsally and ventrally
within the alveolar ramus, and then migrate in
series toward the alveolar trough.
DENTARY
The dentary has a very unusual structure in
Nigersaurus taqueti (figs. 5.1, 5.6, table 5.1). Other
rebbachisaurids may eventually be shown to
share some or all of these apomorphies. Several
aspects of the dentary, however, currently have
no parallel among reptiles.
In dorsal view, the T-shaped bone is divided
into a mandibular ramus, oriented anteroposte-
riorly, and an alveolar ramus, oriented trans-
versely (figs. 5.1B, 5.6). The posterior end of the
mandibular ramus, as in other dinosaurs, is ori-
ented in a parasagittal plane. It is divided into a
slender dorsal process, which contacts the suran-
gular, and a broader, tongue-shaped ventral
process. The anterior end of the mandibular
ramus, unlike that in any other sauropod, is
dorsoventrally compressed and flares in trans-
verse width before joining the alveolar ramus
(fig. 5.1B). As a consequence, this portion of the
mandibular ramus appears unusually slender in
lateral view (fig. 5.1A). In dorsal view, this portion
of the mandibular ramus is subtriangular with a
concave dorsal surface. Laterally it is bounded by
a sharp, upturned edge that joins the lateral
extremity of the alveolar ramus. A large oval fora-
men opens in the center of this portion of the
mandibular ramus. This foramen lies dorsal to a
sizable vascular canal that passes anteriorly
within the dentary toward the alveolar trough.
Shallow grooves also exit the foramen and pass
anteriorly toward the row of replacement foram-
ina (Sereno et al. 1999:fig. 2C; fig. 5.6).
The breadth of the alveolar ramus exceeds
the length of the mandibular ramus, a
remarkable proportion that results in lower
jaws that are as broad as they are long (figs.
5.1B, 5.6, table 5.1). The extraordinary width of
the anterior end of the lower jaws is the result
of the lateral extension of the alveolar ramus,
approximately 30% of which is positioned lat-
eral to the sagittal plane of the mandibular
ramus. The medial portion of the alveolar
ramus is subcylindrical. The mandibular sym-
physis, as a result, is subcircular (table 5.1).
The symphysial surface is smooth and gently
concave, suggesting that a small amount of
flexion about the sagittal plane may have been
possible.
The alveolar trough is nearly straight in
dorsal view (fig. 5.1B). It narrows in width
toward its lateral end. Other changes toward
the lateral side of the trough include a decrease
in the size of the replacement foramina and a
STRUCTURE AND EVOLUTION OF A SAUROPOD TOOTH BATTERY 163
FIGURE 5.5. (A) Stereopair and (B) matching drawing of the left maxilla of Nigersaurus taqueti (MNN GDF512) in poste-
rior view with two replacement tooth columns exposed by erosion. al, alveolus; antfe, antorbital fenestra; apm, articular
surface for the premaxilla; apl, articular surface for the palatine; m1– m20, maxillary tooth positions 1–20; mfr, missing
fragment; rf, replacement foramen; sf, subnarial foramen. Scale bars equal 5 cm.
FIGURE 5.6. (A) Stereopair and (B) matching drawing of the left dentary of Nigersaurus taqueti (MNN GDF512) in dorsal
view. altr, alveolar trough; br, buccal ridge; d1–d34, replacement foramina for dentary tooth positions 1–34; mc, Meckel’s
canal; mfr, missing fragment; pdpr, posterodorsal process; pvpr, posteroventral process; sym, symphysis; t, tooth; vc, vascu-
lar canal. Scale bars equal 5 cm.
decrease in the distance between the replace-
ment foramina and the edge of the alveolar
trough. This gradual decrease in the width of
the trough and the size and position of the
replacement foramina suggests that the
replacement rate and/or tooth size decreased
toward the lateral end of the tooth row. Several
loose teeth are preserved in the bottom of the
alveolar trough and confirm the gradual
decrease in tooth size laterally along the bat-
tery. There are 34 replacement foramina and,
thus, 34 columns of teeth, which is 20 more
than are typically present in the dentary of the
diplodocid Diplodocus (Holland 1924; fig.
5.2B). As there are no septa at any depth within
the alveolar trough, dentary teeth would have
erupted and migrated toward the functioning
wear surface as a self-supporting dental battery.
No other sauropod has a dentary with an alveo-
lar ramus of similar form lacking septa.
TEETH
The upper and lower teeth are virtually identi-
cal in form (Sereno et al. 1999: fig. 2C; fig.
5.7). These similarities are based on the pre-
maxillary, maxillary, and dentary teeth in situ
in MNN GDF513. Upper and lower teeth differ
only in size; dentary teeth are smaller, perhaps
by as much as 20%–30%. This is difficult to
estimate more accurately because the avail-
able dentary teeth, a few of which remain
within the alveolar trough, are of unknown
maturity. A similar size differential, with
upper teeth larger and longer than the lowers,
has been observed in Diplodocus (Holland
1924:389) and the titanosaur Nemegtosaurus
(Wilson 2005: 305).
The numbers of premaxillary, maxillary, and
dentary tooth positions are 4, at least 24, and
34, respectively. Maxillary and dentary tooth
counts are approximately double those in other
diplodocoids such as Diplodocus (Holland,
1924; fig. 5.2B). Although the maxillary tooth
row is incomplete laterally, there are probably
only a few missing tooth positions, because the
length of the combined premaxillary and maxil-
lary tooth rows in MNN GDF513 is only slightly
less than that for the dentary. The higher num-
ber of tooth positions in the dentary (34 versus
approximately 30 for the premaxilla plus
166 STRUCTURE AND EVOLUTION OF A SAUROPOD TOOTH BATTERY
FIGURE 5.7. Isolated tooth of Nigersaurus taqueti (MNN GDF513) in (A) lingual (internal), (B) mesial or distal (medial or
lateral), and (C) labial (external) views. de, dentine; e, enamel; ee, enamel edge; ewf, external wear facet; iwf, internal wear
facet. Scale bar equals 1 cm.
maxilla) is consistent with the smaller size of
the dentary teeth.
Crown shape and structure are uniform
among upper and lower teeth. Unworn crowns
are gently curved along their length and taper
gradually to a narrow, rounded tip, which
shows subtle wrinkles on its thickly enameled
labial (external) surface. Teeth in place in the
premaxilla and maxilla and near their natural
position in the dentary confirm that the crowns
are convex labially (anteriorly) in both the upper
and the lower jaws.
At midlength, the crown has a trapezoidal
cross section, with smooth enamel and a longi-
tudinal groove on each side that accommodates
the edges of adjacent crowns (Sereno et al.
1999:fig. 2D; fig. 5.7). Toward its tip, the crown
has an oval cross section. The enamel is approx-
imately eight times thicker on the labial (exter-
nal), as opposed to the lingual (internal), side of
the crown in both upper and lower teeth.
Whereas other diplodocids and some titanosauri-
ans have similar-shaped, narrow, cylindrical
crowns, markedly asymmetrical enamel has not
been reported previously among sauropods and
is absent in other diplodocoids (Dicraeosaurus
[Janensch 1935–1936:pl. 12, fig. 16], Diplodocus
[Holland 1924:fig. 6].
TOOTH WEAR
Although wear facets are not present in teeth
preserved in the holotypic jaw bones of
Nigersaurus taqueti, worn teeth referable to this
species have been recovered from many sites in
the Gadoufaoua beds and show a distinctive
pattern of wear. There is little reason to doubt
the reference of these teeth to Nigersaurus
taqueti, despite the presence of an unnamed
titanosaurian in the same horizons; the crowns
have narrow cylindrical proportions and highly
asymmetrical enamel.
Unlike the teeth in nearly all other
sauropods, those in Nigersaurus taqueti have a
pair of wear facets located on opposite sides of
the crown. The first—a labial (external) facet—
is typical of dicraeosaurids and diplodocids; it
cuts the crown at a high angle and appears to be
the product of nonocclusal abrasion (Holland
1924; Upchurch and Barrett 2000; fig. 5.7B, C).
The elevated, rounded rim of thick enamel sug-
gests that the wear facet was produced by
ingested plant matter rather than the harder
enameled edge of an opposing crown. The
second—a lingual (internal) facet—resembles
the internal facet on some narrow-crowned
titanosaurians; it cuts the crown at a low angle
and appears to be the product of occlusal abra-
sion (fig. 5.7B, C).
TOOTH BATTERY
STRUCTURE
Archosaur teeth, primitively, are anchored by
alveoli that are separated from adjacent teeth by
bony septa. They erupt en echelon along the jaw
rather than in unison as a single unit. In most
dinosaurian herbivores, wear facets develop
from tooth-to-tooth occlusion at several points
along the tooth row rather than developing as a
continuous wear surface.
A tooth battery, by contrast, is here defined as
a tooth composite composed of self-supporting
teeth that erupt and wear in unison. The teeth
are supported by adjacent teeth and erupt as a
unit with wear surfaces that are continuous
from one tooth to the next. In Nigersaurus part
of the dentition fulfills these criteria—the pre-
maxilla and dentary tooth rows. These bones do
not have alveoli, intervening septa, or even well-
developed grooves to guide erupting tooth
columns. Rather, the teeth are packed into an
open alveolar trough.
Although all functioning (worn) teeth have
fallen away from the jaws of the holotypic spec-
imen, the tooth battery is intact within the body
of the premaxilla (fig. 5.3), as visualized in cross
and longitudinal sections with high-resolution
CT (fig. 5.4). Embryonic teeth migrate into the
alveolar trough via replacement foramina. As
they grow in length and diameter, they migrate
deeper into the body of the premaxilla before
passing ventrally out of the alveolar trough (fig.
5.4A). There appear to be as many as 10 teeth at
a single tooth position, from embryonic tooth to
STRUCTURE AND EVOLUTION OF A SAUROPOD TOOTH BATTERY 167
wearing crown. The four tooth columns in the
premaxilla are arranged en echelon so that
the widest portion of a given crown contacts the
more tapered portion of adjacent crowns, with
the prominent edge of enamel lodged in a
groove on the side of adjacent crowns (fig. 5.4B).
Less is known about the structure of the
tooth battery of the dentary, although it was
probably very similar. The crowns have the
same shape, structure, and orientation. The
position of the replacement foramina near the
margin of the alveolar trough indicates that
embryonic teeth began their trajectory closer to
the open end of the trough and grew deep into
the alveolar trough before emerging at the
functional end of the tooth battery. The most
distal (lateral) tooth columns in the dental bat-
tery of the dentary must have angled anterolat-
erally as shown by the extension of the trough
lateral to the lateral most replacement foramen
(fig. 5.1B).
The body of the maxilla has septa separating
columns of teeth (fig. 5.5). Closer to the ventral
end of the maxilla, the septa give way to an open
trough with a groove for each tooth column on
anterior and posterior walls. As in the premax-
illa, there appear to have been at least 10 teeth
to a column medially and fewer laterally; an
incomplete sequence of 8 teeth is visible in the
first column (fig. 5.5). The maxillary teeth, pre-
sumably, emerged from the alveolar trough as a
self-supported tooth battery.
FUNCTION
Exactly how the tooth batteries of Nigersaurus
functioned to produce the wear facets evident on
isolated teeth remains unsolved, despite knowl-
edge of the general structure of the tooth batter-
ies. What we can outline at this point are several
observations that provide some insight into the
mystery.
We know that teeth in lower and upper tooth
rows do not match one-to-one given their dif-
fering sizes and numbers. We also know that
both upper and lower tooth batteries were con-
structed in a similar manner, with the convex,
thickly enameled crown surface facing labially
(anteriorly). Many mammals, such as rodents,
have analogous anteriorly positioned, self-
sharpening lower and upper incisors with lin-
gual (internal) wear facets (Taylor and Butcher
1951). As in Nigersaurus, both lower and upper
incisors are externally convex, with thickened
enamel on their labial (external) sides and wear
facets facing lingually (internally). This is the
closest analogy to the structure and orientation
of the individual teeth in Nigersaurus. Mam-
malian incisors like these, however, are much
more robust and are sharpened in a manner
very different from that in any dinosaur. The
chisel-shaped edge is maintained by breakage
(preferential chipping of the softer dentine) and
by tooth-to-tooth abrasion. Unlike Nigersaurus,
the facets on lower and upper incisors are not
symmetrical (the uppers typically have higher-
angle, stepped facets), and the shearing break-
age that keeps the leading edge sharp rarely pro-
duces facets that are uniformly planar.
We strongly suspect that the low-angle, lin-
gual (internal) wear facet was produced by tooth-
to-tooth abrasion, because the facet is extremely
flat and cuts smoothly across the external mar-
gin of enamel. The high-angle labial (external)
wear facet, in contrast, appears to have been pro-
duced by tooth-to-plant abrasion, because the
facet is concave, with a rounded, polished rim of
enamel along its trailing labial (external) edge
(fig. 5.7).
Because all of the isolated teeth from Niger,
England (Naish and Martill 2001), and Brazil
(Kellner 1996) show a similar pattern of wear—
with the low-angle facet first to appear—it is
likely that both lower and upper teeth are rep-
resented and wear in a similar manner. Yet it is
not clear how this is functionally possible.
Hadrosaurid and ceratopsian tooth batteries
have opposing, mirrored patterns of tooth wear:
the enamel is thickest on opposing sides of the
crown (medially in maxillary teeth, laterally in
dentary teeth); the crowns curve in opposite
directions (laterally in maxillary teeth, medially
in dentary teeth); and the wear facets occur in
opposing orientations (medially or ventromedi-
ally in maxillary teeth, laterally or dorsolaterally
168 STRUCTURE AND EVOLUTION OF A SAUROPOD TOOTH BATTERY
in dentary teeth). Although an isolated ornithis-
chian tooth from a tooth battery may be difficult
to assign to either the upper or the lower tooth
row (when upper and lower crowns have a sim-
ilar shape and ornamentation), these teeth have
mirrored positions when they are found in
place. One possible explanation for the uniform
pattern of wear in Nigersaurus is that the lingual
(internal), low-angle facet is produced by the
lower crowns passing lingually (internally) to
the upper crowns (the usual tetrapod condition)
but that the lower crowns are worn away and
eventually obliterated in the process. If this
were true, the entire sample of isolated teeth of
Nigersaurus and related taxa found now on sev-
eral continents would include only premaxillary
and maxillary teeth from the upper tooth bat-
teries. If not, and if we are correct that the low-
angle facet is produced by tooth-to-tooth oc-
clusion, there must have existed an occlusal
mechanism unlike any described to date among
tetrapods—a mechanism capable of producing
low-angle, lingual (internal) wear facets on both
lower and upper crowns.
Another conundrum involves the cutting
edge of the crown. In ornithischians with tooth
batteries and in mammalian herbivores, thick-
ened enamel is always positioned along the cut-
ting edge. This is not the case in Nigersaurus.
Not only are the lower and upper teeth oriented
with their curvature and thickened enamel on
the same (external) side, but the leading wedge-
shaped edge of the crown is formed entirely of
dentine. It is difficult to understand how this
edge, which is perfectly straight, is maintained
in the course of wear without the protection of
enamel (fig. 5.7). Even if one envisages adjacent
crowns, it is hard to understand how the soft
leading edge of dentine would not be concave
from wear rather than straight.
A final observation suggests that the tooth
battery of Nigersaurus is quite different from
that in ornithischians. To produce an elongate,
low-angle wear facet on the lingual (internal)
side of the crown (fig. 5.7A), most of this side of
the crown must have been exposed. The suc-
ceeding replacement crown, in other words,
must have been positioned at a good distance
from the cutting edge of the functional crown.
Only one crown in each tooth column could
have been functional. In ornithischian denti-
tions, higher-angle wear facets allow more than
a single crown in the same tooth column to par-
ticipate in the active, cutting surface of the den-
tal battery.
In the future, we plan to digitally define and
prototype the intact portion of the tooth battery
within the premaxilla in the hope that it will
shed further light on how the battery func-
tioned during mastication.
DISCUSSION
COMPARISONS
SKULL
In general proportions, the skull of Nigersaurus
taqueti bears little resemblance to that of
diplodocids (figs. 5.1, 5.2B). It has a more abbre-
viate, less prognathous snout; the depth of the
cranium is approximately 90% of its length (as
measured from the snout to the quadrate
condyle). This cranial proportion is even more
abbreviate than that of Camarasaurus (fig. 5.2A)
and Jobaria (Sereno et al. 1999), which have a
cranial depth between 50% and 60% of its
length. The external naris in Nigersaurus is large
and parasagittal in position as in Camarasaurus
and Jobaria. In diplodocids, in contrast, the exter-
nal naris is smaller, dorsally facing, and retracted
to a position anterodorsal to the orbit (the condi-
tion in dicraeosaurids is as yet unknown). The lat-
erotemporal fenestra in Nigersaurus is propor-
tionately elongate and extends anteriorly as far as
the antorbital fenestra and external nares, farther
than in any sauropod described to date. There is
no development of a preantorbital fenestra as
occurs in other neosauropods (fig. 5.2B). The der-
mal bones of the skull roof in Nigersaurus are
remarkably slender and delicate compared to
those of other sauropods including diplodocids.
The posterodorsal ramus of the maxilla, which
separates the external naris and antorbital fossa,
is reduced to an extremely delicate, strap-shaped
STRUCTURE AND EVOLUTION OF A SAUROPOD TOOTH BATTERY 169
lamina 1 mm thick and a few millimeters wide
(fig. 5.1).
The lower jaw is easy to distinguish from that
in diplodocids (figs. 5.1, 5.2B). The coronoid
process on the surangular is prominent and
deep in lateral view, more closely resembling
that in the titanosaurian Rapetosaurus (Curry
Rogers and Forster 2004) than the low profile
jaws of diplodocids (fig. 5.2B). In Nigersaurus the
teeth are restricted to the transverse portion of
the anterior end of the skull, and at least the
lower tooth row extends lateral to the parasagittal
plane of the lower jaw. In both of these attrib-
utes, Nigersaurus is unique among dinosaurs.
Diplodocids show an incipient condition in these
regards; all but the lateral extremities of the tooth
rows are positioned along the anterior, trans-
verse margin of the skull, and the dentary tooth
row flares just beyond the parasagittal plane of
the posterior portion of the lower jaw.
A dentary from Upper Cretaceous rocks in
South America referred to Antarctosaurus wich-
mannianus (Huene 1929:69, pl. 29, fig. 5) is sim-
ilar to that in Nigersaurus taqueti in the extreme
breadth of the transverse portion of the ramus
and the concomitant increase in tooth count.
There are at least 24 teeth in the dentary, with the
majority (approximately 18) located in the broad,
transverse portion of the ramus. The dentary of
this South American form, however, is not as
derived as that in Nigersaurus. The teeth are set in
sockets rather than an undivided alveolar trough,
the tooth row is L-shaped rather than straight and
restricted to the transverse portion of the ramus,
and the symphysial surface is narrow rather than
circular. Despite these similarities, the phyloge-
netic affinity of Antarctosaurus is not yet resolved.
There is a possibility that this taxon, which was
found together with titanosaur cranial and post-
cranial remains, may have acquired these fea-
tures convergently with Nigersaurus.
TEETH AND TOOTH WEAR
Teeth that closely match those of Nigersaurus
taqueti in form, structure, and wear have
been described recently from slightly older
Barremian-age beds on the Isle of Wight (Naish
and Martill 2001:pl. 36) and from the Upper
Cretaceous Bauru Group in Brazil (Kellner
1996:fig. 7). The crowns are narrow and sub-
cylindrical and, at midlength, have a trapezoidal
cross section. The size, form, and angle of the
pair of wear facets are exactly like those
described above for Nigersaurus taqueti. As
noted by Kellner (1996:619), the low-angle lin-
gual (internal) facet is the first to appear and is
always more elongate than the labial (external)
facet, as preserved in progressively worn
crowns. The enamel may have an asymmetri-
cal distribution on the crown, but this needs
firsthand verification. Kellner provisionally
regarded these teeth as titanosaurian, because
of the predominance of titanosaurian postcra-
nial bones from the same beds. It is very prob-
able, however, that these Brazilian teeth belong
to a rebbachisaurid diplodocoid that lived on
South America in the Late Cretaceous. (e.g.,
Limaysaurus; Salgado et al. 2004).
In other diplodocoids, the teeth are larger
relative to the jaw bones and anchored in indi-
vidual alveoli (fig. 2B; Dicraeosaurus [Janensch
1935–1936:figs. 107, 111]; fig. 2B). The crowns
have a circular cross section, symmetrical
enamel, and a single low-angle, labial (external)
wear facet (Holland 1924). Despite some varia-
tion, isolated teeth of Dicraeosaurus show the
same external facet (Janensch 1935–1936:pl. 12,
figs. 23, 25). This facet is characterized by a
rounded lip of enamel along its trailing (external)
edge (fig. 5.7; ewf) and scratches that course
across the dentine from the internal to the exter-
nal sides (Fiorillo 1991; Calvo 1994b; Upchurch
and Barrett 2000). How this wear facet formed
has remained a mystery ever since Holland
(1924) summarized early speculation, which
included scraping cycad trunks (Holland),
procuring fish hiding in stream beds (Tornier),
and munching on freshwater bivalves (Sternfeld).
Because Holland (1924:fig. 4) depicted “Diplod-
ocus seizing a mussel,” that hypothesis gained
the upper hand, although Holland maintained
no personal preference. More recent proposals
include stripping leaves from branches (Dodson
1990; Barrett and Upchurch 1994) and raking
170 STRUCTURE AND EVOLUTION OF A SAUROPOD TOOTH BATTERY
bark (Bakker 1986). Nigersaurus adds a new wrin-
kle. Here we see the same kind of external facet
in teeth set into a tooth battery. These batteries, in
turn, are housed in a skull with a very different
shape and even more delicate construction than
in diplodocids (fig. 5.2).
Some titanosaurians have narrow-crowned
teeth that bear a general resemblance to those of
Nigersaurus taqueti, but these teeth have more
robust proportions (e.g., Kellner and Mader
1997:fig.2; Rapetosaurus [Curry Rogers and
Forster 2004:fig. 32]). The internal (lingual)
facet—the only one present—cuts the crown at
a low angle. An external (labial) facet has
never been described outside Diplodocoidea.
Nemegtosaurus, which has internal V-shaped and
apical wear facets (Nowinski 1971; Wilson
2005), has continued to confuse discussions of
tooth form and masticatory style among
diplodocoids (e.g., Upchurch and Barrett 2000).
Nemegtosaurus and the conspecific, or closely
allied, Quaesitosaurus from the Late Cretaceous
of Asia are better understood as titanosaurians
rather than diplodocoids (Calvo 1994a; Wilson
and Sereno 1998; Wilson 2002, 2005).
EVOLUTION OF A SAUROPOD TOOTH BATTERY
Nigersaurus taqueti must be placed within the
context of diplodocoid phylogeny to better under-
stand how its novel tooth battery evolved
(fig. 5.9). Diplodocoids include diplodocids,
dicraeosaurids, and rebbachisaurids. Diplodocids
are best known from the Late Jurassic of North
America and include Diplodocus, Apatosaurus,
STRUCTURE AND EVOLUTION OF A SAUROPOD TOOTH BATTERY 171
FIGURE 5.8. Fi fth cervical vertebra and co-ossified rib of Nigersaurus taqueti (MNN GDF512) in lateral view. asp, anterior
spine; ns, neural spine; pl, pleurocoel; poz, postzygapophysis; prz, prezygapophysis; rs, rib shaft. Scale bar equals 5 cm.
Barosaurus, and others (Hatcher 1901; Holland
1924; Berman and McIntosh 1978; Ostrom
and McIntosh 1999). Dicraeosaurids include
Dicraeosaurus (Janensch 1935–1936) from the
Late Jurassic of Africa and the long-spined
Amargasaurus from the Early Cretaceous of South
America. Rebbachisaurids include Limaysaurus
(Calvo and Salgado 1995; Salgado et al. 2004)
from the Early Cretaceous of South America
and Nigersaurus (Sereno et al. 1999) and
Rebbachisaurus (Lavocat 1954) from the mid- and
Late Cretaceous of Africa. Rebbachisaurid teeth
and other fragmentary remains indicate that the
group was also present in Europe and persisted
to the end of the Cretaceous at least in South
America. As a sister taxon to dicraeosaurids and
diplodocids, rebbachisaurids must have diverged
from other diplodocoids before the end of the
Late Jurassic, although no trace of the group has
yet been found from this period (fig. 5.9).
Although the teeth of Nigersaurus are partic-
ularly slender, all diplodocoids are characterized
by proportionately narrow, subcylindrical
crowns that are only weakly expanded and
flattened (fig. 5.9, node 1). The rate of tooth
replacement, in addition, appears to have been
accelerated across the group, although this is
most apparent in Nigersaurus. In this regard, we
172 STRUCTURE AND EVOLUTION OF A SAUROPOD TOOTH BATTERY
REBBACHISAURIDAE
MACRONARIA
CRETACEOUS
L
E
L
Maa
Cmp
San
Cen
Alb
Apt
Brm
Hau
Vlg
Ber
Tth
159
144
121
99
84
71
65
DIPLODOCIDAE
3
DIPLODOCOIDEA
1
2
DIPLODOCIDAE
DICRAEOSAURIDAE
+
DICRAEOSAURIDAE
DIPLODOCIDAE
4
REBBACHISAURIDAE
srm ewf
iwf
ewf
15
2
ns
34 db 1
FIGURE 5.9. Phylogram showing the relationships and recorded durations of the three principal diplodocoid clades (Reb-
bachisauridae, Dicraeosauridae, Diplodocidae) and some of their feeding specializations. Node 1 (Diplodocoidea): slender
cylindrical crowns, low-angle external (labial) wear facet, teeth restricted anteriorly in subrectangular muzzle. Node 2 (Di-
craeosauidae + Diplodocidae): bifid cervical neural spines. Node 3 (Diplodocidae): nares retracted, 15 cervical vertebrae. Node 4
(Rebbachisauridae): low-angle, internal wear facet, increase in tooth number, asymmetrical enamel, dental batteries in some
taxa. 1–34, dentary teeth 1 through 34; 2–15, cervical vertebrae 2–15; db, dental battery; en, external naris; ewf, external wear
facet; iwf, internal wear facet; ns, neural spine; srm, subrectangular muzzle.
assume that the number of teeth in a single col-
umn is related to the replacement rate.
Diplodocus, for example, has as many as six
teeth in a single column in the maxilla (Marsh
1884:pl. 4, fig. 3). Worn diplodocoid teeth have
a characteristic external, low-angle, wear facet
on both upper and lower teeth (fig. 5.7 ewf)
that must have formed from tooth-to-plant
abrasion (contra Calvo 1994b; Barrett and
Upchurch 1994; Upchurch and Barrett 2000).
Most diplodocoid teeth are positioned in a
transverse row along the squared anterior mar-
gin of the snout (fig. 5.9, node 1).
Nigersaurus has further developed a number
of these features (fig. 5.9, node 2). Tooth size
decreases relative to the size of the jaw bones,
and tooth number increases to 30 or more in
lower and upper tooth rows. In Nigersaurus the
teeth are restricted to the anterior margin of the
snout and extend lateral to the sagittal plane of
the lower jaw, features unknown elsewhere
among dinosaurs. More teeth are present in a
single column, and the rate of replacement, pre-
sumably, has increased as well. Most of the teeth
in the jaws are pressed so close to one another
that there are no intervening septa and, for the
first time among sauropods, batteries of self-
supporting teeth erupt as a single unit. Although
still a functional conundrum, an additional low-
angle wear facet appears on the lingual (internal)
side of the active crowns in both lower and upper
tooth batteries.
Dicraeosaurids and diplodocids evolved
other features that likely impacted food procure-
ment but are not present in Nigersaurus or other
rebbachisaurids. These include a reduction in
the number of teeth, a projecting “chin” on the
dentary, a relative lengthening of the snout,
retraction of the external nares, and an increase
in the length and number of cervical vertebrae
(fig. 5.9, nodes 3, 4). In dicraeosaurids and
diplodocids, in addition, the resting curvature of
the neck arches anteroventrally (Janensch 1929;
Stevens and Parrish 1999). This is not the case
in Nigersaurus, as exemplified by the fifth cervi-
cal vertebra (fig. 5.8).
COMPARISON TO ORNITHISCHIAN
TOOTH BATTERIES
Tooth batteries evolved twice among ornithis-
chian dinosaurs from a hypothetical common
ancestor with a simple dentition characterized
by a suite of ornithischian features related to
herbivory—a predentary, cheek embayments on
the dentary and maxilla, asymmetrical enamel
in dentary and maxillary crowns, and wear
facets from tooth-to-tooth occlusion on the buc-
cal (lateral) and lingual (medial) sides of dentary
and maxillary teeth, respectively. Evolution of
tooth-supported batteries occurred first among
ornithopods, with initial changes toward a den-
tition occurring before the close of the Jurassic,
and later among ceratopsian dinosaurs, where
all structural changes occurred during the Late
Cretaceous (Sereno 1997, 2000).
The fully developed tooth batteries in
hadrosaurids and ceratopsids are structurally
very similar, evolved in comparable stages, and
involved an increase in body size of approxi-
mately an order of magnitude (Sereno 1997: fig.
7; 2000: fig. 25.9). Independent but similar
structural changes include a relative decrease in
tooth size and increase in tooth columns and
replacement rate, the loss of alveolar septa,
restriction of the enamel to opposing sides of the
crown in maxillary versus dentary teeth, an
increase in the prominence of a ridge on the
enameled side of the crown, adjustment of the
crown shape for efficient packing, an increase in
the volume of supporting bone in the maxilla
and dentary, a reduction of postdentary elements
in the lower jaw, and the development of a coro-
noid process with an expanded process for mus-
cular attachment.
The circumstances surrounding the evolu-
tion of tooth batteries in rebbachisaurid
sauropods bear a few similarities to and many
striking differences from those in ornithischi-
ans. Similarities include the reduction of tooth
size, increase in number of tooth columns,
increase in replacement rate (or at least the
number of teeth per column), loss of alveolar
septa between tooth columns, thickened
STRUCTURE AND EVOLUTION OF A SAUROPOD TOOTH BATTERY 173
enamel on one side of the crown and near-loss
of enamel on the other, and adjustment of crown
shape for efficient packing. These similarities,
thus, are confined to the size, shape, number,
and rate of replacement of the teeth and the
asymmetrical distribution of enamel.
Fundamental differences begin with timing.
Available rebbachisaurid fossils suggest that a
tooth battery among sauropods had evolved some-
time during the Early Cretaceous; the teeth from
the Isle of Wight (Naish and Martill 2001) look
very similar to those of Nigersaurus and are
Barremian (ca. 125 Ma; middle Early Cretaceous)
in age. This postdates the establishment of tooth-
supported dentitions in ornithopods (Late
Jurassic) but predates the appearance of ceratop-
sian tooth batteries (Late Cretaceous). Another
basic difference involves body size. There was no
increase in body size among sauropods concomi-
tant with the development of tooth batteries.
Although Rebbachisaurus ranks among the largest
of sauropods, Nigersaurus ranks among the small-
est, with a body length of approximately 15 m.
The orientations of the tooth batteries are
diametrically opposed. The tooth batteries have
an anteroposterior, rather than a transverse, ori-
entation in ornithischians and rebbachisaurids,
respectively. The ornithischian tooth battery is
located posteriorly within the jaws and used for
food processing; cropping is a function of an
expanded, toothless bill. The sauropod dental
battery, in contrast, is located anteriorly and may
have been used primarily in cropping.
The presence or absence of gastroliths as an
accessory means to break down plant matter may
be correlated with the aforementioned funda-
mental functional differences. The absence of
gastroliths among ornithischians with advanced
dentitions (euornithopods, neoceratopsians)
including dental batteries suggests that more effi-
cient oral processing of plant matter has replaced
gut-processing by gastroliths (Sereno 1997:473).
In sauropods, in contrast, the group in which
dental batteries evolved (diplodocoids) has the
greatest proven incidence of gastroliths, suggest-
ing that their derived dental features did not func-
tion primarily in the breakdown of plant matter.
Gastroliths are present in rebbachisaurids (Calvo
1994b), dicraeosaurids (Janensch 1929), and
diplodocids (Cannon 1906; Brown 1941; Gillette
1990). The absence of gastroliths among
nondiplodocoid sauropods is based on many
articulated skeletons of Shunosaurus,
Camarasaurus, Jobaria, and Opisthocoelicaudia.
Cedarosaurus, a macronarian sauropod of uncer-
tain affinity, is thus far the only nondiplodocoid
sauropod with gastroliths (Sanders et al. 2001).
The pattern of wear in the rebbachisaurid
tooth battery is completely different from that
in ornithischians, which uses thickened enamel
and tooth-to-tooth occlusion to form a self-
sharpening cutting margin. In rebbachisaurids,
only one of a pair of wear facets is formed by
tooth-to-tooth occlusion, the sharp leading edge
of worn crowns is formed in dentine rather
than enamel, the thickened enamel is located
on the same (labial) side of lower and upper
crowns, and lower and upper crowns appar-
ently have identical wear patterns.
The locus of the most rapid replacement and
wear is different in ornithischian and reb-
bachisaurid tooth batteries. In ornithischians,
the crown size, the number of teeth in a col-
umn, and the distance of the replacement fora-
men from the alveolar margin are all greatest in
the middle of the tooth battery. In Nigersaurus,
in contrast, the crown size, the number of teeth
in a column, and, to a lesser degree, the distance
of the replacement foramen from the alveolar
margin are all greatest toward the midline.
Finally, the tooth-bearing bones are con-
structed differently in ornithischians and
sauropods with dental batteries. In ornithischi-
ans with tooth batteries, the dentary, in particu-
lar, is robustly constructed, with a thick and
prominent coronoid process for attachment of
substantial adductor musculature. Postdentary
elements are greatly reduced in size. This pat-
tern of change in the lower jaw closely mirrors
changes that occurred earlier in the evolution
of the mammalian masticatory apparatus
(Allin 1975). In Nigersaurus, in contrast, the
tooth-bearing elements are constructed of thin
laminae, the dentary does not gain in relative
174 STRUCTURE AND EVOLUTION OF A SAUROPOD TOOTH BATTERY
length in the lower jaw, the coronoid process is
developed as a thin plate of bone, and the
supratemporal fenestra (the usual origin of
adductor musculature) is closed by approxima-
tion of surrounding bones.
CONCLUSIONS
Early in their evolution, sauropods adopted
tooth-to-tooth occlusion and, in consequence,
evolved lower and upper tooth rows of equivalent
length, characteristic patterns of wear facets, a
more substantial coronoid process, and a robust
mandibular symphysis (McIntosh 1990; Calvo
1994a; Wilson and Sereno 1998; Upchurch and
Barrett 2000). Among sauropods, it is now
apparent that diplodocoids evolved complex den-
titions during the Cretaceous, as exemplified by
the dental batteries of a recently named African
rebbachisaurid, Nigersaurus taqueti (Taquet 1976;
Sereno et al. 1999).
The dental battery on each side of the
upper and lower jaws is composed of more
than 30 columns of teeth that are packed into
a tight self-supporting unit in the premaxilla
and dentary. Individual teeth have slender rod-
shaped crowns characterized by highly asym-
metrical enamel. Dentary teeth are somewhat
smaller than but otherwise similar to premax-
illary and maxillary teeth. The crowns in both
lower and upper jaws have thickened enamel
on their convex labial (external) side. Wear
produces two stereotypical facets, the first
appearing as a low-angle, lingual (internal)
facet produced by tooth-to-tooth occlusion and
the second as a high-angle, labial (external)
facet produced by tooth-to-plant abrasion.
Both facets are well developed on crowns with
significant wear, resulting in a straight, sharp
apical wedge of dentine where the facets inter-
sect. How either of these facets was produced
remains a significant, and largely unanswered,
question.
Unlike the parasagittal dental batteries of
ornithischians, the dental battery in Nigersaurus
is oriented transversely and may have been used
for cropping rather than prolonged oral process-
ing. Although the rebbachisaurid dental battery
is preserved only in Nigersaurus, isolated teeth
from Lower Cretaceous horizons on the Isle of
Wight and rocks of Late Cretaceous age in Brazil
suggest that related forms with potentially a sim-
ilar degree of dental complexity were present on
other continents. It is highly unlikely, however,
that rebbachisaurids with dental batteries ever
achieved the taxonomic diversity of ornithischi-
ans with dental batteries (hadrosaurids, ceratop-
sids) in Late Cretaceous faunas of North America
and Asia.
Dental batteries evolved three times inde-
pendently within Dinosauria—in euornithopod
and neoceratopsian ornithischians and in reb-
bachisaurid sauropods. Fundamental functional
differences coupled with their diachronous
appearance suggest that dinosaurian dental bat-
teries did not evolve in response to a single envi-
ronmental cue, such as the rise of angiosperms
during the mid-Cretaceous (Sereno 1997: Sereno
1999; Barrett and Willis 2001).
ACKNOWLEDGMENTS
We thank members of the 1997 and 2000
expeditions for discovering the material, C.
Abraczinskas for drawing from specimens and
executing final drafts of reconstructions, E.
Dong, T. Keillor, and R. Masek for preparing
fossils, and B. Gado (Institut de Recherche en
Science Humaine), and I. Kouada (Ministère de
L’Enseignement Supérieur de la Recherche et
de la Technologie) for granting permission to
conduct fieldwork. This research was funded by
The David and Lucile Packard Foundation, the
National Geographic Society, the Comer
Science and Education Foundation, and Nathan
Myhrvold.
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