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© 2010 Dinosauria International Ten Sleep Report Series No. 1
SYSTEMATIC PALEONTOLOGY
Order SAURISCHIA
Suborder SAUROPODOMORPHA
Family DIPLODOCIDAE
Genus: Amphicoelias Cope 1877
Type species: Amphicoelias altus Cope
Included species: Amphicoelias altus, A. “brontodiplodocus”
and A. emilieae
Type Locality: Morrison Formation, Quarry 12, Garden
Park, Colorado
Synonyms: Apatosaurus Marsh, Barosaurus Marsh,
Diplodocus Marsh, Seismosaurus Gillette, Eobrontosaurus
Filla and Redman, Supersaurus Jensen, and Suuwassea
Harris and Dodson
Emended diagnosis: Amphicoelias differs from all other
diplodocids in the extreme microcephalomorph condition
of the skull in relation to the length of the axial skeleton in
mature individuals. In comparison to other sauropods, the
skull in Amphicoelias has proportionately the longest rostrum
relative to the post-orbital length. Thus, the premaxillae are
caudally elongated and the anterior-posterior length of the
nasal and frontal bones are reduced and receded, more so than
in other sauropods. The Amphicoelias skull is characterized
by a rostral crest formed by the caudal processes of the
premaxillae (autapomorphy), and with the premaxillae
and maxillae transversely extended and flattened forming
a duck billed shaped rostrum. Presence of the external
narial openings located posterior-dorsal almost above orbits
(synapomorphy). Amphicoelias differs from all sauropods
in the development of an interfenestra bridge dividing the
preantorbital from the antorbital fenestrae (autapomorphy).
In Amphicoelias, Nigersaurus, and Tornieria the preantorbital
fenestrae are positioned immediately behind the last
maxillary tooth. There are two preantorbital openings
present in the maxilla of Nemegtosaurus distinguishing it from
Amphicoelias Pterygoid bones in Amphicoelias are sagitally
positioned at a steep angle in comparison to other known
sauropod skulls. Amphicoelias shares with the diplodocid
genera Tornieria, and Dicraeosaurus, the following derived
sauropod characters: occipital condyle ventrally positioned;
quadrates rostroventrally inclined; basipterygoid processes
slender, elongate and oriented rostroventrally; and jaws
behind preantorbital fenestra are without teeth.
As in all diplodocids the dentition is homodont, with both
crown and roots shaped almost into long slender cylindrical
rods. Upper and lower teeth are replaced in crowded
overlapping successional rows. The crowns in unworn teeth
are slightly compressed labially and taper towards the apex.
The teeth are spaced apart, and project forward in contrast
to the unique transversely arranged dental battery present in
Nigersaurus. The mandible is long and slender with ventrally
expanded symphysis, and as in all sauropods the articular
joint is positioned lower than level of its dentition.
Axial skeleton: There are fifteen cervical, ten dorsal, five
sacral, and 80 plus caudal vertebrae. Based on cervical
and caudal vertebral lengths, Amphicoelias represents the
longest sauropod in regards to absolute body length. There
are fifteen cervical vertebrae, which in length of centrum
increase from C 1- 14, and then reducing at C 15. Hence,
the longest cervical and longest presacral vertebra is C 14.
Neural spine bifurcation in cervicals start at C 3 in A. altus
and C 9 in A. "brontodiplodocus" C 14-15 represent modified
dorsal vertebra which are autapomorphies of Amphicoelias
C 2- 15 are opisthocoelous.
There are ten rib bearing dorsals, the anterior four are
opisthocoelous while the posterior ones are amphicoelous.
The 1 st dorsal has the longest centrum. The neural spine
of the tenth, or last free dorsal, is off alignment from its
long axis, therefore, is not positioned perpendicular to the
centrum as in the other preceding dorsals. There are no
complete presacral series in other recognized diplodocids to
compare with these derived Amphicoelias characters.
Sacral vertebrae can fuse in juveniles half the size of adults.
The sacrum in adults is normally comprised of one dorsal, two
sacral and two caudal vertebrae. Posterior dorsals, sacrum,
and anterior caudal vertebrae all have comparatively well
developed taller neural spines. The sacral vertebrae centra
have ventral keels, which are less pronounced in males.
Amphicoelias is readily distinguished from other sauropods
in the caudal vertebrae not only in its combined absolute
length but also in the unique and complex morphology.
Complete caudal vertebrae are not known in closely
related taxa (Amargasaurus, Dicraeosaurus, Nigersaurus, and
Rebbachisaurus). Thus, the following combined characters
are considered autapomorphy of Amphicoelias. Except
for the biconvex centra of the whiptail caudals, all centra
are procoelous. Anterior caudals are short, broad, and
possess pleurocoels (suppressed or absent in males) with
dorsoventrally expanded transverse processes. The centra
of anterior caudals gradually decrease in size posteriorly.
At a mid-point, they increase in length and robustness
from Cd 16-20 (hypertrophy) before decreasing again in
size where they meet the supernumerary whiplash series.
The approximately forty or so supernumerary caudals are
subequal in length, elongate, cylindrical, slender and simple
in structure. The location of hypertrophic transitional series
is variable. Caudal fusion is also variable depending on the
individual and age, and can occur singly or in sets of two.
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