Erketu ellisoni, a long-necked sauropod from Bor Guvé (Dornogov Aimag, Mongolia)

2006·1.3 MB·English

by  Ksepka, Daniel T

x T AMERICAN MUSEUM
rsovitates
PUBLISHED BY THE AMERICAN MUSEUM OF NATURAL HISTORY
CENTRAL PARK WEST AT 79TH STREET, NEW YORK, NY 10024
Number 3508, 16 pp., 12 figures, 3 tables March 16, 2006
Erketu ellisoni, a Long-Necked Sauropod from Bor
Guve (Dornogov Aimag, Mongolia)
DANIEL T. KSEPKA1 AND MARK A. NORELL2
ABSTRACT
The first specimen of the new sauropod Erketu ellisoni, from the Lower Cretaceous of the eastern
Gobi of Dornogov, Mongolia, is described here. The specimen comprises a well-preserved
articulated anterior cervical series, an articulated lower hindlimb, and a sternal plate. This
sauropod displays a unique combination of features including low, bifid neural spines, elongate
cervical centra, and crescent-shaped sternal plates. Computed tomography imaging reveals the
vertebrae were extensively invaded with pneumatic camellae. The holotype individual of Erketu
was of modest mass relative to other neosauropods, but had an extremely elongate neck.
Phylogenetic analysis indicates Erketu is a member of the Somphospondyli and may belong to
a more exclusive clade therein.
INTRODUCTION American Museum of Natural History
Central Asiatic Expeditions at the Lower
The 2002 American Museum of Natural Cretaceous Ooshiin Nuur locality (Osborn,
History-Mongolian Academy of Sciences field 1924). These teeth were assigned to
expedition discovered a new locality, Bor Asiatosaurus mongoliensis, a name now con¬
Guve, in Dornogov Aimag, Mongolia (fig. 1). sidered a nomen dubium (McIntosh, 1990).
Several sauropod cervical vertebrae were The Central Asiatic Expeditions also collected
found eroding out of the surface at this a basioccipital, three cervical vertebrae, and
locality, and excavation revealed the rest of several teeth of the poorly known sauropod
the anterior cervical series preserved in artic¬ Monoglosaurus haplodon from the Lower
ulation directly in front of these elements. Cretaceous Hu Khung Ulan locality
Additional postcranial elements were uncov¬ (Gilmore, 1933). Several important sauropod
ered as the cervicals were being collected. specimens have since been recovered from
Despite exhaustive efforts, no skull was found Upper Cretaceous deposits, including the
in the vicinity of the atlas. nearly complete postcranial skeleton of
The first sauropod remains described from Opisthocoelicaudia (Borsuk-Bialynicka, 1977)
Mongolia were two teeth collected by the and skull of Nemegtosaurus, collected by the
division of Paleontology, American Museum of Natural History ([email protected]).
2Division of Paleontology, American Museum of Natural History ([email protected]).
Copyright © American Museum of Natural History 2006 ISSN 0003-0082
2 AMERICAN MUSEUM NOVITATES NO. 3508
Polish-Mongolian expeditions (Nowinski, being described (Koppelhus, in prep.).
1971), and the skull of Quesitosaurus, col¬ Indistinguishable fossil fruits have been col¬
lected by the Soviet-Mongolian expeditions lected from the stratigraphically lower Khara
(Kurzanov and Bannikov, 1983). With the Khuutal beds, which crop out to the east.
exception of these discoveries, sauropod ma¬ In describing the morphology of the verte¬
terial from Mongolia has been scarce and brae, we use the terminology and abbrevia¬
fragmentary. The discovery of Erketu provides tions for vertebral lamina proposed by Wilson
an important addition to Mongolian sauropod (1999) and the terminology for pneumatic
diversity. features proposed by Wedel et al. (2000).
The deposits at Bor Guve appear to Computed tomography (CT) scans were per¬
represent a floodplain environment. Inter- formed at the Stony Brook University
bedded grey siltsones and channel sandstones Hospital Radiology Department, using a GE
dominate. Lag deposits have yielded turtle Light Speed 16 scanner.
remains and dinosaur teeth, and sauropod and
theropod remains have been recovered from ABBREVIATIONS
the siltsones. Among the theropod material
uncovered at this site are elements from Institutional: AMNH, American Muse¬
a maniraptoran slightly larger than um of Natural History, New York, New
Deinonychus and an Allosaurus-sized predato¬ York, USA; CM, Carnagie Museum of
ry theropod. Fossil fruits (fig. 2) from Natural History, Pittsburgh, Pennsylvania,
a probable angiosperm plant of unknown USA; CCG, Chengdu College of Geology,
affinity are locally abundant at the locality. Sichuan, China; HM, Humbolt Museum,
These fruits bear a superficial resemblance to Berlin, Germany; IGM, Geological Institute
extant Abelmoschus esculentus (okra). The of the Mongolian Academy of Sciences, Ulaan
fruits are not, to our knowledge, assignable Baatar, Mongolia; OMNH, Oklahoma
to any known taxon, and are currently Museum of Natural History, Norman,
2006 KSEPKA AND NORELL: SAUROPOD FROM MONGOLIA 3
Fig. 2. Fossil fruits from Bor Guve. Scale bar = 1 cm.
Oklahoma, USA; PMU, Paleontological of Mick Ellison, for his contributions to
Museum, Uppsala, Sweden; ZDM, Zigong ongoing AMNH dinosaur research.
Dinosaur Museum, Zigong, China. Diagnosis: Referable to Titanosauriformes
Anatomical: ap, accessory anterior pro¬ based on elongate cervical vertebrae with
cess of prezygodiapophyseal lamina; ax, axis; camellae and referable within Titanosauri¬
cml, camella; cr, cervical rib; ic, intercentrum formes to Somphospondyli based on reduced
of atlas; mtp, metapophysis; np, neurapophysis neural arch lamination. Differentiated from
of atlas; pcdl, posterior centrodiapophyseal all other Titanosauriformes in which cervical
lamina; pcpl, posterior centroparapophyseal vertebrae are known by combination of
lamina; pdol, postzygodiapophyseal lamina; extremely elongated (El indices of anterior
pfm, pneumatic foramen; pfs, pneumatic fossa; cervicals exceeding 5.0) cervical centra and
prdl, prezygodiapophyseal lamina; prz, pre- bifurcate anterior cervical neural spines.
zygapophysis; tb, tubercle; td, torus dorsalis, Type Locality and Horizon: Bor Guve:
tp, transverse process. late Early Cretaceous. Stratigraphically, the
beds at this locality lie below the Tsaagan
Tsonch beds (a unit that contains I guano don
SYSTEMATIC PALEONTOLOGY orientalis) and above the Khara Khuutul beds,
both believed to be of late Early Cretaceous
SAUROPODA MARSH, 1878 age (Shuvalov, 2000). The lack of materials
suitable for radiometric dating leaves the exact
NEOSAUROPODA BONAPARTE, 1986
age of this locality uncertain.
MACRON ARIA WILSON AND SERENO, 1998
DESCRIPTION
TITANOSAURIFORMES SALGADO, CORIA, AND
Axial Skeleton
CALVO, 1997
SOMPHOSPONDYLI WILSON AND SERENO, 1998 The atlas is complete and articulated with
the axis (fig. 3). The intercentrum is cresecen-
Erketu ellisoni new taxon tic in anterior view, though deformation has
Holotype: IGM 100/1803: articulated cer¬ compressed the dorsal apices inward, causing
vical series including complete first through it to appear more circular. In lateral view, the
fifth cervical vertebrae, partial sixth cervical intercentrum is subrectangular. A roughened
vertebra, right sternal plate, articulated right articular surface is present near the poster-
tibia, fibula, astragalus, and calcaneum. oventral corner of the lateral face. The wing¬
Etymology: Erketu: In Mongolian sha- like neurapophyses embrace the neural spine
manistic tradition, there are 99 Tengri (de¬ of the axis. The fusion of the neurapophyses to
ities). Erketu Tengri is the Mighty Tengri, the intercentrum and closure of all other
a creator-god who called Yesiigei, the father neurocentral sutures indicates IGM 100/1803
of Chingis Khan, into being, ellisoni: In honor represents an adult individual.
4 AMERICAN MUSEUM NOVITATES NO. 3508
Fig. 3. Atlas and axis of IGM 100/1803 in (a) right lateral, (b) left lateral, (c) dorsal, (d) ventral, (e)
anterior, and (f) posterior view. Note that the cervical rib has been detached in (c)-(f). For abbreviations, see
text. Scale bar for (a), (b), (c), and (d) = 5 cm.
The axis is opisthocoelous with well-de¬ Young and Zhou, 1972). Most titanosauri-
veloped pneumatic fossae. The anterior half of forms, including Brachiosaurus (Janesch, 1950:
the axial centrum develops a strong ventral fig. 14), Euhelopus (Wiman, 1929: pi. 3, fig. 3),
keel. The cotyle is subcircular, with nearly and Saltasaurus (Powell, 1992) have more
equal height and width. The short transverse dor sally projected axial neural spines.
process has a ventrally directed distal The third (fig. 4) and succeeding cervical
flange. The anteriorly displaced parapophyses vertebrae have opisthocoelous centra with
bear single-headed ribs that extend signifi¬ well-developed pneumatic fossae and variably
cantly beyond the posterior border of the developed laminae. The most striking feature
axial centrum, though the total length is of the cervical vertebrae is their elongation.
uncertain due to breakage. The axial neural The elongation index (El, sensu Wedel et al.,
spine barely projects dorsal to the level of the 2000: centrum length/cotyle height) exceeds
postzygapophyses. The low axial neural those of all other sauropods for which
spine of Erketu is similar to that of the comparable data are available (table 1). It
unnamed titanosaur from Peiropolis, Brazil, should be noted that because the cotyle is
referred to as the “Series A” taxon (Powell, taller than wide in Erketu, calculating El as
2003) and the eusauropod Mamenchisaurus centrum length/cotyle width (sensu Upchurch,
sinocandorum (Russell and Zheng, 1993; no¬ 1998) would increase the values for this taxon
tably, the axial neural projects well dorsal both absolutely and relative to most other
to the postzygapophyses in the supposedly sauropods. Sauroposeidon pro teles (Wedel
cogeneric Mamenchisaurus houchuanensis; et al., 2000) and Omeisaurus junghsiensis
2006 KSEPKA AND NORELL: SAUROPOD FROM MONGOLIA 5
Fig. 4. Cervical 3 of IGM 100/1803 in (a) left lateral, (b) right lateral, (c) dorsal, (d) anterior, and (e)
posterior view. For abbreviations, see text. Scale bar for (a), (b), and (c) = 5 cm.
(Young, 1939) also possess tremendously uncertain position, precluding direct compar¬
elongate vertebrae. However, only mid-cervi- ison with Erketu.
cals are preserved for S. pro teles, and in O. As in Euhelopus and the primitive Asian
junghsiensis the preserved cervicals have suf¬ eusauropods Shunosaurus, Omeisaurus, and
fered distortion of the cotyles and are of Mamenchisaurus, the postaxial centra are
6 AMERICAN MUSEUM NOVITATES NO. 3508
TABLE 1
Elongation Indices and Centrum Lengths from Various Sauropods
Values for Erketu are from this study, values for Mamenchisaurus are from Young and Zhou (1972), values
for Omeisaurus tianfuensis are from He et al. (1988), and all other values are from Wedel et al. (2000). For
Erketu ellisoni, C5 is dorsoventrally compressed, causing an artificially high El value of 8.5. If cotyle width is
used intead of height, a value of 5.8 is calculated; the true value would be slightly greater.
Vertebra
C2 C3 C4 C5 C6 C7 C8
Centrum length (mm)
Apatosaurus lousisae (CM 3018) 190 280 370 — 440 — 485
Brachiosaurus brancai (HM SII) — 420 663 810 900 930 973
Camarasaurus supremus (AMNH 235 265 310 395 — 350 605
5761)
Diplodocus carnegii (CM 84) 163 243 289 372 442 485 512
Erketu ellisoni (IGM 100/1803) 160 268 387 489 — — —
Euhelopus zdanskyi (PMU.R233) 94 130 222 234 238 260 262
Mamenchisaurus hochuanensis 160 215 320 415 480 580 590
(CCG V 20401)
Omeisaurus tianfuensis (ZDM 170 241 368 495 595 670 673
T5701)
Sauroposeidon proteles (OMNH — — — — 1220 1230 1250
53062)
Elongation index
Apatosaurus lousisae (CM 3018) 2.2 3.0 3.3 — 3.2 2.9
Brachiosaurus brancai (HM SII) — 3.5 4.3 5.4 5.0 4.4 4.0
Camarasaurus supremus (AMNH 1.9 2.5 2.3 2.3 — 3.1 3.5
5761)
Diplodocus carnegii (CM 84) 3.1 4.1 3.3 4.7 4.7 4.9 4.3
Erketu ellisoni (IGM 100/1803) 3.4 4.5 5.5 X — —
Euhelopus zdanskyi (PMU.R233) 2.5 2.7 5.4 3.6 3.2 3.2 2.8
Mamenchisaurus hochuanensis 2.0 2.5 2.6 2.8 2.9 2.9 2.7
(CCG V 20401)
Omeisaurus tianfuensis (ZDM 2.7 3.0 3.8 4.9 3.6 X X
T5701)
Sauroposeidon proteles (OMNH — — — — 6.1 5.6 4.6
53062)
significantly higher than wide at the cotyle pneumatic fossa in the anterior cervical
(centrum six is wider than high due to vertebrae. However, this lamina is better
dorsoventral deformation). The opposite con- developed in the sixth cervical vertebra, in-
dition is developed in the basal titanosaur dicative of an expected increase in complexity
Phuwiangosaurus, which has transversely ex¬ of the fossae moving posterior in the vertebral
panded cervical vertebral centra (Martin et al., column. The anterior pneumatic fossa extends
1994). The lateral surface of each cervical ventrally onto the dorsal surface of the
centra in Erketu is marked by a large pneu¬ parapophyses on the third and fourth cervi-
matic fossa that reduces the centum to a thin cals, where the parapophyses are preserved.
median septum. The posterior centrodiapo- Additional excavations are present on the
physeal and posterior centroparapophyseal centra and neural spines. The distribution of
lamina are strongly developed along the area these minor excavations, of pneumatic foram¬
of the pneumatic fossa, but are weak near the ina in the pneumatic fossa, and of similar
posterior border of the centrum. A weakly features elsewhere on the vertebrae is highly
developed lamina divides the large lateral variable and may even differ between the two
pneumatic fossa from a smaller anterior sides of an individual vertebra. The posterior
2006 KSEPKA AND NORELL: SAUROPOD FROM MONGOLIA 7
centroparapophyseal laminae make the ven¬ The delicately tapering, double-headed ribs
tral surface of the anterior half of the centrum are completely fused to the diapophyses and
concave. Posteriorly, the ventral surface is parapophyses. An anterior projection of the rib
slightly convex. extends to the anterior limit of the centrum. The
The neural arches of cervical vertebrae 3-5 main body of the most complete rib extends
are characterized by low, gently curving posteriorly to overlap at least one succeeding
profiles. The prezygapophyses extend anterior centrum, but it is incomplete at the posterior
of the condyle as in most sauropods. The end and may have been significantly longer.
prezygodiapophyseal lamina is unremarkable Computed tomography imaging reveals de¬
in cervical three. In the succeeding cervicals, tails of the internal structure of the fourth
however, it is hypertrophied and extends far cervical vertebra (fig. 4). The centrum is
anterior of the articular facet as a thin plate, reduced to a median plate by the pneumatic
oriented perpendicular to the articular plane fossa at its midpoint. Anterior, posterior, and
(fig. 5). The primitive Saharan eusauropod dorsal to the pneumatic fossa, the centrum is
Jobaria also possesses a similar morphology filled with pneumatic chambers. The small
(Sereno et al., 1999: “accessory anterior pro¬ size, lack of regular branching pattern, and
cess”). The postzygapophyses extend only thinness of the septae dividing these chambers
slightly beyond the posterior border of the identify them as camellae. The presence of
centrum. The postzygodiapophyseal lamina is camellae extends into the diapophyses and
poorly developed and fades from a sharp parapophyses. The camellate structure of the
lamina to a low, rounded ridge a short latter is particularly highly developed. The
distance posterior to the diapophysis. The internal structure of the cotyle and zygapoph-
pronounced torus dosalis extends posterior to yses is unclear, possibly as an artifact of
the postzygapophyseal articular facet in cervi¬ scanning resolution, though it appears the
cals four and five, though not in cervical three. zygapophyses are at least somewhat pneuma¬
The intrapostzygapophyseal lamina connect tized. A large foramen above the diapophysis
to the dorsal surface of the neural canal via of the fourth vertebrae appears to communi¬
a vertical strut. The neural arch laminae are cate with the neural canal (see fig. 7d).
reduced, but not to the degree seen in A few camellae are exposed on the surface
Malawisaurus. In lateral view, the cervical on the dorsal side of the condylar neck of the
neural arches of Erketu resemble those of the fifth cervical (fig. 8). The bone at the neck of
“Series A” titanosaur and Malawisaurus more the condyle is very thin, as seen in the CT
closely than the higher and more steeply imagery. In several places, the outer layer of
angled neural arches of Euhelopus (Wiman, bone has collapsed or been worn away,
1929: pi. 3, fig. 3) and Brachiosaurus (Janesch, exposing camellae infilled with matrix. As
1950: figs. 17, 20, 23). exposed on the surface, the camellae are
The neural spines of the fourth and irregularly shaped, giving the bone surround¬
succeeding cervicals are bifurcated. The meta- ing them a honeycomb-like appearance. The
pophyses are very low and are thickened and exposed septae are approximately 1 mm thick.
rugose at their highest point. A small tubercle Larger camallae with thin septae are also
projects from each metapophysis at the exposed on the lateral surface of the damaged
anterior edge of the inflated portion (fig. 6). sixth vertebral centra.
There is no median tubercle between the
split neural spines, though this does not rule Appendicular Skeleton
out the presence of this feature farther
posterior in the cervical series. As mentioned The sternal plate (fig. 9) exhibits a strongly
above, pneumatic features of the neural spine concave lateral border, a feature seen in
are highly variable. Cervical four possesses Titanosauria as well as in the rebbachisaurid
a large, well-defined ovoid excavation that is Limayosaurus tessonei (Calvo and Salgado,
subdivided by a bony strut on the left 1995). The internal surface is concave poster¬
metapophysis, but no counterpart feature on iorly and nearly flat anteriorly. The external
the right. surface is gently convex posteriorly and nearly
AMERICAN MUSEUM NOVITATES NO. 3508
Fig. 5. Cervical 4 of IGM 100/1803 in (a) left lateral, (b) right lateral, (c) dorsal, (d) anterior, and (e)
posterior view. For abbreviations, see text. Scale bar for (a), (b), and (c) = 5 cm.
flat anteriorly. The sternal plate is greatly The right tibia, fibula, and astragalus
thickened at the anterolateral corner, but lacks (fig. 10) are preserved in articulation with
the anteroventral ridge developed in some only slight displacement. The hindlimb ele¬
titanosaurs (Sanz et al., 1999). ments bear a strong overall resemblance to
2006 KSEPKA AND NORELL: SAUROPOD FROM MONGOLIA
Fig. 6. Cervical 5 of IGM 100/1803 in (a) left lateral, (b) dorsal, and (c) right lateral view. This vertebra
has been deformed so that the centrum appears artificially dorsoventrally flattened and the right diapophysis
is visible ventrally in view a. For abbreviations, see text. Scale bar = 5 cm.
those of Gobititan shenzhouensis. The proximal The anterior and distal expansion of the tibia
face of the tibia is subcircular. The well- does not reach the degree seen in Opis-
developed cnemial crest projects laterally, thocoelicaudia (Borsuk-Bialynicka, 1977).
embracing the proximal end of the fibula. The fibula extends distal to the tibia. The
The greatest breadth at midshaft is in the fibula is broken near the distal end of the
anterolateral-posteromedial dimension, but at lateral trochanter, and the sigmoid appearance
the distal end the transverse breath is greatest. of the shaft is slightly exaggerated by this
10 AMERICAN MUSEUM NOVITATES NO. 3508
Fig. 7. Computed tomography images of cervical 4. (a) Lateral view of cervical 5 showing plane of
coronal sections. Scale bar = 5 cm. (b) Section through position 1. (c) Section through position 2. (d) Section
through position 3. For abbreviations, see text.
damage. The anterior and lateral trochanters more distally in Gobititan (You et al., 2003).
arise approximately one-third of the bone’s The anterior trochanter is short, whereas
length from the proximal end on the ante¬ the lateral trochanter extends past the mid¬
rolateral surface of the shaft, as in Opis- point of the shaft and has a slight postero-
thocoelocaudia. The trochanters are placed distal slant. The anteromedial edge of the shaft
Fig. 8. Dorsal view of the condyle of cervical 5. View (a) is unmodified; in view (b) the exposed camellae
are highlighted in dark gray.