Amargasaurus
Name:
Amargasaurus
(Amarga lizard / bitter lizard).
Phonetic: A-mar-gah-sore-us.
Named By: Leonardo Salgado & Jos�
Bonaparte - 1991.
Classification: Chordata, Reptilia, Dinosauria,
Saurischia, Sauropodomorpha, Sauropoda, Diplodocoidea,
Dicraeosauridae.
Species: A. cazaui (type).
Diet: Herbivore.
Size: Holotype individuals neck length about 2.4
meters long. Total body
length unknown due to most of the tail being missing.
Known locations: Argentina, Neuqu�n province -
La Amarga Formation.
Time period: Barremian to Aptian of the Cretaceous.
Fossil representation: Quite well preserved
individual that includes a partial skull, left fore and aft limb,
left ilium, right shoulder, all cervical (neck), dorsal
(back), sacral (sacrum) and some caudal (tail) vertebrae.
Amargasaurus
has easily become one of the most popular dinosaurs thanks to the
elongated neural spines of the neck vertebrae that are immediately
apparent from even a casual glance at the skeleton of this dinosaur.
Technically known as hemispinous processes, these spines rise up
from the top of the vertebrae and are bifurcated, which means they
split into two separate spines from a combined base on top of the
vertebra.
Since
the discovery of Amargasaurus there has been a lot
of theory and
speculation as to how the enlarged neural spines affected the
appearance of this dinosaur. One of the first main ideas was that the
spines supported a double skin sail that started just behind the head
and along the neck and sometimes the body too. This sail was
conceived as being mainly for display but possibly also
thermoregulation. Sometimes depicted as rising all the way to the tip
of the spines and at other times only part way up the main problem
with the skin sail idea was that it would have resulted in a neck that
was probably very rigid, and an actual encumbrance to feeding and
drinking.
The
second main idea that became even more popular was that there was no
skin sail, the neural spines would have risen up just by themselves.
Being bone the spines would have had to have a keratinous covering
to protect them from exposure to the elements and physical damage.
They were still possibly used for display but in this configuration
possibly also as weapons to help Amargasaurus
defend itself from
predators. It was also considered that the spines may have rattled
against one another so that they not only formed a visual display,
but an audial display too. However problems with the weapon and
contact ideas can be pointed out as the spines are directly connected
to the neck, an area that can easily incur mortal injury if damaged.
Either the spines were strong enough to be used with physical force,
which increased the likelihood of damage to the vertebrae and spinal
column, or the spikes were designed to break before that happened,
but that weakness would make them pretty useless as weapons.
However, it now seems that all of the above are moot points because
as we are about to read there is now a third theory.
In
2022 a scientific paper (Cerda et al) was
published providing
what was the most in-depth study of the cervical vertebrae that had
been done up to that point. This study looked at everything from the
structure to the microanatomy of the bones. One of the key things to
immediately come out of this study was there was no evidence of
keratinous covering for these neural spines. This was concluded not
only from micro analysis of bone which was highly vascularized, but
by the presence of growth marks. Even more telling was evidence for
the presence of Sharpey's fibres connecting to the spines so that a
whole system of interspinous ligaments may have connected the spines.
These ligaments also seem to have gone all the way to the ends of the
neural spines. What this all means is that the spines of Amargasaurus
probably didn’t stand proud of the body, and in all likelihood
supported a soft tissue growth that rose up from the back of the neck
and down towards the body.
Why
Amargasaurus had such a growth to its neck can never
be known with
absolute certainty. There has long been debate to how sauropod
dinosaurs breathed and how they got air along their necks and into
their bodies. Maybe the growth above the neck of Amargasaurus
helped
facilitate the transfer of oxygen rich air along the neck, but if
this is the case then why are these structures not commonly seen in
other sauropod types and certainly not to this degree of
specialisation? Perhaps the most likely function of the neck is
display, allowing Amargasaurus to easily
recognise others of their
kind as well as attracting mates.
As
for the rest of the body Amargasaurus is typically
classed as a
dicraeosaurid sauropod,
a sub group of the diplodocid sauropods. Dicraeosaurids have long
thin whip
like tails, but necks that are actually quite short, at least when
compared to other sauropod dinosaurs. There is speculation that necks
that are enlarged above the vertebral column may actually be present in
other dicraeosaurid sauropods even if not to the size extent of
Amargasaurus. If correct then this would lend more
support to the
display idea for the neck shape. It would also possible explain why
dicraeosaurid have proportionately shorter necks, the extra growth
upwards would add weight and bulk, making a shorter neck much easier
to manage than a longer neck.
Studies
done on the partial skull remains of Amargasaurus
in 2014
(Carabajal et al) indicate that in life Amargasaurus
probably held
its head down as in the mouth pointing more towards the ground as
opposed to perfectly horizontal. This snippet of information was
revealed by modelling of the inner ear which revealed that if
horizontal registered as 0�, and vertical was 90�, then the
snout would point at the ground at about 65�. In the study
re-construction of the inner ear also revealed that Amargasaurus
would
have had poorer hearing than many other sauropod dinosaurs.
It
has also been noted in the 2014 study that the typical resting
posture of the head of Amargasaurus would place
it a little under
1 meter from the ground, but possibly rising as much as 2.7
metres off the ground when the neck was raised and the neural spines
compressed. However that information was established before the
2022 paper redefining the spines as being supported by ligaments,
and with that extra soft tissue it might mean that Amargasaurus
might not have been able to raise its head off the ground as much as
previously thought. A range of motion in the neck would still have
been possible, but to a less certain extent. Dicraeosaurid
sauropods are usually viewed as being very low browsers anyway, a
feeding niche that would mean that sauropod dinosaurs like
Amargasaurus could avoid competing directly with
the increasing types
of larger titanosaurs that were present in South America.
Other
interesting sauropods with hyper developed neural spines from the
Cretaceous of South America include Bajadasaurus
and Agustinia.
Further reading
- Un nuevo saur�podo Dicraeosauridae, Amargasaurus cazaui
gen. et sp.
nov., de la Formaci�n La Amarga, Neocomiano de la provincia del
Neuqu�n, Argentina [Amargasaurus cazaui gen. et sp.
nov., a new
dicraeosaurid sauropod from the La Amarga Formation, Neocomian of
Neuqu�n province, Argentina] - Ameghiniana 28(3-4):333-346 - L. Salgado
& J. F. Bonaparte - 1991.
- Cranial osteology of Amargasaurus cazaui Salgado
and Bonaparte
(Sauropoda, Dicraeosauridae) from the Neocomian of Patagonia -
Ameghiniana 29: 337-346. - L. Salgado & J. O. Calvo - 1992.
- Giants and bizarres: body size of some southern South American
Cretaceous dinosaurs - Historical Biology 16 (2-4): 71–83. - G. V.
Mazzetta, P. Christiansen & R. A. Farina - 2004.
- The sauropod diversity of the La Amarga Formation (Barremian),
Neuqu�n (Argentina) - Gondwana Research 12:533-546 - S. Apesteguia -
2007.
- Braincase, neuroanatomy, and neck posture of Amargasaurus
cazaui (Sauropoda, Dicraeosauridae) and its
implications for
understanding head posture in sauropods.- Journal of Vertebrate
Paleontology. 34 (4): 870–882. - Ariana Paulina
Carabajal, Jos� L. Carballido & Philip J. Currie -
2014.
- Osteohistology of the hyperelongate hemispinous processes of
Amargasaurus cazaui (Dinosauria: Sauropoda):
Implications for
soft tissue reconstruction and functional significance. - Journal
of Anatomy: 1–15. - Ignacio A. Cerda, Fernando E. Novas,
Jos� Luis Carballido & Leonardo Salgado - 2022.
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