Alamosaurus
Name:
Alamosaurus
(Alamo lizard).
Phonetic: Al-ah-moe-sore-us.
Named By: Charles W. Gilmore - 1922.
Classification: Chordata, Reptilia, Dinosauria,
Saurischia, Sauropodomorpha, Sauropoda, Titanosauria,
Saltasauridae, Opisthocoelicaudiinae.
Species: A. sanjuanensis (type).
Diet: Herbivore.
Size: Estimated around 20-24 meters long,
based
upon comparison between more complete juvenile remains and partial
adult remains. Some isolated remains suggest larger.
Known locations: USA, New Mexico - Ojo Alamo
Formation (possibly part of the Kirtland Formation, Utah -
North Horn Formation and Texas - Black Peaks Formation, El
Picacho Formation and Javelina Formation.
Time period: Maastrichtian of the Cretaceous.
Fossil representation: Specimens of many
individuals, although often of incomplete skeletons. Juvenile
specimens are also known. Skulls are still unknown.
The
one ‘fact’ that is often presented for Alamosaurus
is that it is
named after the Alamo, the famous mission in San Antonia that was the
site for one of the most important battles in the Texas Revolution.
However this is completely false and the truth of the matter is that
the first Alamosaurus remains which were found in
New Mexico were so
named because they came from the Ojo Alamo Formation.
Alamosaurus
was a reasonably large titanosaur,
the most common group of more
advanced sauropod dinosaurs that were the most common during the
Cretaceous. A complete adult specimen of Alamosaurus
continues to
prove elusive, although this is a common problem for most large
dinosaurs in general. Juveniles however are often more complete
because their smaller bodies are more easily covered by sediment, and
comparison of juvenile fossils with those of adults has led to
estimates of around twenty meters long, give or take a meter or two,
for adult Alamosaurus. A skull of Alamosaurus
is also currently
unknown, although associated teeth are quite slender, possibly for
snipping at tree tops. Alamosaurus also seems to
have lacked the
osteoderm armour (bony plates that grew in the skin) that is known
to be present in many other titanosaurs since out of all the known
Alamosaurus remains, no osteoderms have yet been
found.
More
importantly, the presence of Alamosaurus in the
south western
portions of the United States is proof that the sauropods had not
disappeared completely from North America during the Cretaceous.
Sauropods however are usually associated with dryer and higher up
landscapes, and these areas do not seem to have been that abundant,
due mainly to the Western interior Seaway submerging most of the
central portions of North America. As such it is quite conceivable
that the distribution of sauropods like with most animals is down to
the availability of suitable habitat.
How
Alamosaurus came to live within the United States at
the end of the
Cretaceous is a question that has caused some confusion for some since
for a long time the popular conception of sauropods was that they
mostly disappeared from North America at the end of the Jurassic.
This idea is based upon the observation that fossils of sauropods were
more common in North America at the end of the Jurassic, but seem to
have been replaced by ornithischians like hadrosaurs
and later on
ceratopsian
dinosaurs during the Cretaceous. However the saying that
absence of evidence is not necessarily evidence of absence is commonly
used in palaeontology, and slowly but surely Cretaceous age sauropods
are slowly but surely being discovered in North America with examples
including Astrodon, Sauroposeidon
and Cedarosaurus
amongst others.
Slowly, Cretaceous sauropod (or rather more specifically
titanosaur) genera are coming together to help complete a picture of
the dinosaurian fauna of North American ecosystems where titanosaur
sauropods are actually still quite common, although still perhaps not
as common as their late Jurassic ancestors.
There
are too alternative theories concerning how Alamosaurus
came to live in
North America. One is a direct Asian origin where Alamosaurus
or
immediate ancestors to it crossed the Bering Strait land bridge and
spread down to the south-western regions where the habitat may have
remained more suitable for a stable and permanent population. This
is in part supported by the placement of Alamosaurus
within the
Opisthocoelicaudiinae group of titanosaurs, the type genus of which,
Opisthocoelicaudia,
is currently only known from Asia. However
this group also sits within the Saltasauridae and Alamosaurus
is seen
to have a lot of similarities with the type genus of this group,
Saltasaurus.
This has led to speculation of a South American origin
for Alamosaurus, although the main problem here
is that South America
is thought to have been separated from North America during the
Cretaceous by ocean. Additionally other strictly South American
dinosaurs such as the abelisaur theropods are still not known in North
America, and vice versa, North American dinosaurs like tyrannosaurs
are not known in South America.
Altogether
the arguments for a South American origin are not that convincing,
but one possibility that may explain these similarities is convergent
evolution. This is simply where two animals (or in this case
dinosaurs) that are separated by time and/or geography find
themselves in the same survival conditions and so physically change to
carry near identical adaptations for coping with their survival
requirements. This principal has been observed countless times, and
where the animals share a common ancestor and similar biology, the
similarity is just the more likely to occur.
Dinosaur
art and popular fiction often depicts life and death struggles between
large theropods such as tyrannosaurs and long necked sauropods. As
stated above this was long thought unlikely to have happened in North
America, but Alamosaurus did live in North
America at a time that
saw large tyrannosaurs
such as Daspletosaurus
and Bistahieversor
roaming the land in search of prey. A fully grown Alamosaurus
may
still have been too much of a challenge for these dinosaurs, but a
smaller juvenile would have certainly been within their predatory
scope. Other dinosaurs that Alamosaurus shared
its habitat with
include the ceratopsian dinosaur Torosaurus and the
hadrosaur
Edmontosaurus. Additionally it would have also
been possible to see
the giant pterosaur Quetzalcoatlus
soaring through the skies of the
time.
Further reading
- A new sauropod dinosaur from the Ojo Alamo Formation of New Mexico. -
Smithsonian Miscellaneous Collections 72(34):1-9 - Charles W. Gilmore -
1922.
- A juvenile specimen of the sauropod Alamosaurus sanjuanensis from the
Upper Cretaceous of Big Bend National Park, Texas. - Journal of
Palaeontology. 76(1): 156-172. - T. M. Lehman & A. B. Coulson -
2002.
- First isotopic (U-PB) age for the Late Creatceous Alamosaurus
vertebrate fauna of West Texas and its significance as a link between
two faunal provinces. - Journal of Vertebrate Paleontology. 26:
922-928. - T. M. Lehman, F. W. McDowell & J. N. Connelly -
2006.
- The First Giant Titanosaurian Sauropod from the Upper Cretaceous of
North America. - Acta Palaeontologica Polonica. 56 (4): 685. - D. W.
Fowler & R. M. Sullivan - 2011.
- Osteoderms of the titanosaur sauropod dinosaur Alamosaurus
sanjuanensis Gilmore, 1922. - Journal of Vertebrate
Paleontology. 35
(1). - M. T. Carrano, M. D’Emic - 2015.
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