Ouranosaurus
Name:
Ouranosaurus
(Brave lizard).
Phonetic: Or-an-o-sor-us.
Named By: Philippe Taquet - 1976.
Classification: Chordata, Reptilia, Dinosauria,
Ornithischia, Ornithopoda, Styracosterna, Hadrosauriformes.
Species: O. nigeriensis (type).
Diet: Herbivore.
Size: 7 to 8 meters long, 67 centimetre
skull.
Known locations: Africa, Niger - Echkar
Formation. Specimens also known from other locations in Africa.
Time period: Aptian to Cenomanian of the Cretaceous.
Fossil representation: 2 almost complete
individuals.
Ouranosaurus
similarities and differences to Iguanodon
Although
initially classed as an iguanodontid dinosaur, subsequent studies of
Ouranosaurus fossils have revealed it to be a form
of basal
hadrosaur.
Despite this, Ouranosaurus still
bears some features
that are similar to the more famous Iguanodon,
specifically the
forelimb. The hands of the forelimbs still have five digits with the
three central digits being the most robust and arranged to support the
weight of Ouranosaurus when it was in a quadrupedal
posture. The
inner digit is a single thumb spike and the outer is more flexible than
the weight bearing digits.
However
the forelimbs of Ouranosaurus was roughly just over
half the length of
the hind limbs, making them proportionately shorter than those of
Iguanodon. Also not only was the thumb spike
smaller, but the outer
digit was reduced and underdeveloped. In Iguanodon
this fifth digit
is thought to have been prehensile and used for wrap around and pull
down vegetation so that Iguanodon could feed on a
greater abundance of
plant material. Ouranosaurus's fifth digit
however was no way near as
flexible and while it could still potentially be used, it would not
have been as capable as Iguanodons.
At
first impression it may seem strange that a herbivorous dinosaur like
Ouranosaurus would lose an adaptation that should
have helped it to
feed upon plants. What needs to be done however is to look at the
bigger picture, or in this case the whole forelimb, and
environment. Ouranosaurus is thought to have
lived in lowland areas
such as river deltas that were unlikely to have high growing
vegetation, but a large amount of rapidly growing reeds and low
plants. With most if not all of the available food being nearer the
ground, Ouranosaurus would have spent most of its
time on all fours
in a quadrupedal posture, and the shorter forelimbs would have
reduced the distance between its mouth and the food it was eating.
Also the lack of tall growing vegetation would mean that Ouranosaurus
had no need to pull plants down to its mouth which resulted in the
fifth digit becoming less flexible. An additional benefit of this
digit becoming inflexible may have even been to reduce the ground
pressure of the forelimbs as Ouranosaurus walked on
softer water logged
ground, meaning that they did not stick in as much. This all comes
together to suggest that rather than being a generalist browser like
Iguanodon, Ouranosaurus was a
low browsing specialist. Further
support for this theory can also be inferred by the presence of the
sauropod Nigersaurus,
also from North Africa that has a very
specialised skull and mouth for browsing on low vegetation.
Ouranosaurus’s
Beak, Teeth & Skull
Ouranosaurus
was on its way to becoming a duck billed hadrosaurid, and this can be
seen by the broad beak that in life would have been covered with
keratin. This beak would have been very good at pulling up the soft
and leafy plants that grew on the edges of water systems, and the
broad edge would have also allowed for the pulling of multiple small
plants at the same time.
The
beak was not the only food processing mechanism and roughly two fifths
back from the beak the teeth started (the toothless gap between the
end of the teeth and beak is sometimes called the diastema). These
were arranged in rows for small gaps between them where small
replacement teeth were emerging to fill the gaps, thus forming a
constant line of teeth. The presence of teeth suggests that some of
the plant material that Ouranosaurus was eating was
tougher than just
leafy fronds, and may have been the stems and perhaps even roots as
well. It is also possible that Ouranosaurus may
have sometimes fed
upon tougher plants when it was unable to feed from softer vegetation.
There
was probably a limit to just how plants could be until Ouranosaurus
had
real difficulty in eating them. The jaws do not display strong muscle
attachments which suggests that Ouranosaurus had a
very weak bite
force. Herbivores usually do not have high bite forces unless
specially adapted to eat tough fibrous vegetation, and other
well-known dinosaurs such as Stegosaurus
are known to also have low
bite forces. The fact that Ouranosaurus had a low
bite force simply
means that it would have been more predisposed to eat softer
vegetation. The teeth may even have been used to mash the surface of
the plant material slightly so that digestive enzymes could more easily
extract nutrients from it.
The
nostrils of Ouranosaurus were placed high on the
snout, presumably so
that they did not get blocked by mud and dirt as Ouranosaurus
was
feeding. There are also two bony growths on either side of the skull
between the nasal and eye openings which may have been a display
feature or identifying characteristic of the species that allowed
Ouranosaurus to better recognise each other at close
range.
The Neural Spines of Ouranosaurus
The
most spectacular feature of Ouranosaurus is the
presence of tall neural
spines along the dorsal, sacral and caudal vertebrae. Popular
thinking has these spines supporting either a sail or hump that
rose up from the back of Ouranosaurus. The robust
nature of the
spines may lean towards the hump theory or at the very least a thick
and fleshy sail rather than a flap of skin. As always, asking what
kind of structure it was is but the first question, the second one
being what was it there for?
Thermoregulation,
the ability for an animal to have some control over its body
temperature, either cooling or warming, is a popular but
controversial theory. Herbivores tend not to need a high metabolism,
something that would be provided by a higher body temperature,
because they usually rely upon an extensive digestive system.
Another theory is that the growth was primarily a display device to
make individuals stand out to others of their kind, and may have been
brightly coloured, or at least differently to the rest of the body
for this purpose.
Another
option which is more associated with the hump theory is that it
provided food storage in times of abundant food so that Ouranosaurus
could better survive times when food got scarce. In can be hard to
conceive of a river delta that does not have some greenery, but
deltas only exist as long as water keeps flowing into them. Should
the rains fail one season, water levels would drop an even possibly
disappear. Also if Ouranosaurus lived in herds,
the herd may have
quickly depleted the available food in an area meaning that they would
have to keep traveling to find areas of fresh growth. Ouranosaurus
may have even travelled to drier areas further in land to lay eggs
where ground was more stable and less prone to flooding, and needed
to build up food reserves for the journey as well as caring over the
eggs and newly hatched juveniles, as seen in the dinosaur Maiasaura.
While the precise nurturing behaviour of Ouranosaurus
is still not
currently know, it’s not totally inconceivable that it may have
shown similar behaviour. The development of neural spines is also
strongly seen in the African predators Suchomimus
and Spinosaurus,
and this could also point to an adaptation for an ecological factor
such as an especially arid environment or the need to go prolonged
periods without feeding.
Potential Predators
It's
conceivable that Ouranosaurus, particularly
smaller juveniles, may
have come into contact with the spinosaurid
predator Suchomimus.
Although thought to be primarily fish eaters, bones of a juvenile
Iguanodon were found inside the remains of another
smaller spinosaurid
named Baryonyx
from England. Spinosaurids did frequent river deltas
that also seem to have been the main habitat of Ouranosaurus,
making
predation by spinosaurids possible while scavenging of dead
Ouranosaurus probable. The type genus of the
Carcharodontosauridae,
Carcharodontosaurus,
would have also been a
potential threat.
Ouranosaurus
also may have come into contact with giant crocodiles
like
Sarcosuchus.
The larger individuals of this genus approached lengths
of up to twelve meters, making them a very real threat to
Ouranosaurus, particularly not yet fully grown
juveniles and
subadults.
Further reading
- G�ologie et Pal�ontologie du Gisement de Gadoufaoua (Aptien du Niger)
[Geology and Paleontology of the Gadoufaoua Locality (Aptian of
Niger)]. - Cahiers de Pal�ontologie, Centre National de la Recherche
Scientifique, Paris 1-191. - Philippe Taquet - 1976.
- Neural spine elongation in dinosaurs: sailbacks or buffalo-backs? -
Journal of Paleontology 71: 1124-1146 - J. B. Bailey - 1997.
- The Venice specimen of Ouranosaurus nigeriensis (Dinosauria,
Ornithopoda). - PeerJ. 5: e3403. - F. Bertozzo, F. M. Dalla Vechia
& M. Fabbri - 2017.
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