Spinosaurus
Name:
Spinosaurus
(Spine lizard).
Phonetic: Spine-oh-sore-us.
Named By: Ernst Stromer - 1915.
Classification: Chordata, Reptilia, Dinosauria,
Saurischia, Theropoda, Megalosauroidea, Spinosauridae,
Spinosaurinae.
Species: S. aegyptiacus (type), S.
maroccanus is a possible second, although many consider
this species
as being the same as the type species.
Diet: Piscivore/Carnivore.
Size: Estimated between 12.6 to 18 meters
long.
Known locations: North Africa, particularly Egypt
- Bahariya Formation, and Morocco - Kem Kem Beds.
Time period: Albian to Cenomanian of the Cretaceous.
Fossil representation: To date at least six partial
specimens of the skull, mandible, neural spines and other
fragmentary post cranial remains. Teeth however are considerably more
common.
Discovery &
Reconstruction
Spinosaurus
as we know it today did not come into existence until the 1990’s,
something that is quite surprising when you think that it was first
described in 1915. The reason for this is that there has been no
choice but to study only the most partial of remains, a study that
was frustrated even further in World War Two when the Munich museum
which housed the first Spinosaurus remains was
destroyed by a bombing
raid which also obliterated the Spinosaurus
holotype specimens.
The
holotype was a collection of partial remains that were recovered from
the Bahariya Formation of Egypt in 1912. These remains included a
small number of ribs, gastralia (belly ribs), vertebrae,
teeth, dentaries (the part of the lower jaw that holds the teeth)
left maxilla and of course some of the now famous neural spines.
This material was enough to convince Stromer that he was dealing with
a new theropod dinosaur, but the unprecedented nature of the find
combined with the understanding of the day still resulted in an
inaccurate first reconstruction.
The
main area of fault was the skull in that due to the lack of good skull
material Spinosaurus was given a more classic
carnosaur skull that
resembled something like that of an Allosaurus.
Also further material
attributed to Spinosaurus by Stromer in 1934
now appears to have
belonged to another theropod such as Carcharodontosaurus
that was
active in North Africa around the same time as Spinosaurus.
However
in his defence Stromer did consider the new material of hind limbs and
vertebrae to belong to something else which is why he named the
specimens ‘Spinosaurus B’.
For
the largest part of the twentieth century Spinosaurus
was frequently
depicted in dinosaur books as a sail backed carnosaur that if it were
not for the sail would be like any other large theropod. This
depiction also existed well into the first decade of the twenty-first
century despite new discoveries proving otherwise. While usually
depicted as a bipedal predator, Spinosaurus has
also been envisioned
as a quadrupedal one. Today Spinosaurus is
thought to have been
primarily bipedal, but also possibly quadrupedal when hunting and
feeding.
New
Spinosaurus material of vertebrae and partial
dentaries from the Kem
Kem Beds of Morocco were described by Dale Russel in 1996, but it
was a partial snout described by Russel and Torquet in 1998 that
hinted at the real nature of Spinosaurus. This
new material combined
with drawings of the Spinosaurus holotype specimen
allowed for the
material to be compared to two other dinosaurs that were discovered
late in the twentieth century. In England in 1983 a dinosaur
was found that would be named Baryonyx
in 1986, and this was a
theropod that had an unusual crocodile-like skull. Then in 1998
another new dinosaur was discovered in the Elrhaz Formation of Niger
that was very similar to Baryonyx save for tall
neural spines on its
back vertebrae. This dinosaur also had a crocodile like skull that
resulted in it being named Suchomimus
(crocodile mimic).
The
comparison of the new Spinosaurus material was a
comparatively easy one
as even though the snout material was incomplete, it was so similar
to Baryonyx and Suchomimus that
it was almost certain that Spinosaurus
was a similar type of dinosaur as these two. Today Spinosaurus
skeletons on display in museums are based upon these other two more
complete dinosaurs with some changes worked in to reflect the slight
differences in the known Spinosaurus material,
new examples of which
continue to be infrequently recovered from Africa.
Today
these dinosaurs all sit within their own group and since Spinosaurus
was the first one named the group is called the Spinosauridae. Aside
from Baryonyx and Suchomimus,
others such as Irritator
and Oxalaia
are known from Brazil as well as other more dubious entries which may
probably be synonymous with others. The fact that spinosaruids are
known from Brazil and North Africa is also proof that the two
continents were still joined by land bridges perhaps as late as the
Early Cretaceous.
Estimating the size of Spinosaurus
Spinosaurus
is now often regarded as the biggest known meat eating theropod
dinosaur (herbivores like large sauropods were of course bigger),
however the
actual size is really just an estimate extrapolated from an educated
guess. What is clear is that Spinosaurus was a
very large animal but
herein lies the problem as the larger animals get, the less complete
their remains tend to be because it takes so much more material to bury
them and protect the body from scavengers and as well as the full
ravages of nature. The more an animal is exposed upon death means the
less complete long term remains like fossils will be.
Still
with a smaller estimate of just over twelve and a half meters,
Spinosaurus would have been comparable to Tyrannosaurus,
and only
just smaller than Giganotosaurus
(it needs to be remembered that even
though Giganotosaurus has a size estimate of
thirteen meters,
it would still only be marginally larger than the largest known
Tyrannosaurus). Comparison to other smaller
spinosaurids that were
consequently scaled up to the same size as the Spinosaurus
material
points to sizes that approach the larger length estimate as indeed
being possible. For the time being at least, Spinosaurus
is likely
to remain the longest theropod.
Length
however is but one measure of size, and often it is not the length of
the animal that is important but the weight. Gauging the weight of an
animal from bones is considerably more difficult that just measuring
the length because so many factors need to be considered. Different
kinds of tissue can be denser than others resulting in different
weights even though they take up the same space (for example muscle
weighs more that fatty tissue). You also need to look at how the
tissue is distributed as many 'larger' animals are often only large
when viewed from one angle and can be very thin in their actual build.
No
clear size estimate for Spinosaurus currently
exists as estimates are
usually based between individuals interpretation on comparison
techniques, the results of which can only be as accurate as the
comparison techniques and data used. Spinosaurus
is sometimes
cited as being between from just under twelve to twenty-one metric
tons. Comparison to other large theropods is problematic not just
because of the broad range of estimates for Spinosaurus,
but also
because the amount that other theropods weighed is a constant subject
of debate. While Spinosaurus is often cited as
being heavier than
other large theropods, it often ends up being lighter too.
The lifestyle of Spinosaurus
Although
Spinosaurus had been a mainstay of dinosaur books
since as early as
the 1970’s, the wider public were not introduced to its current
form until the release of the 2001 film Jurassic Park III.
In
both this film and earlier depictions where is had a more 'classic'
theropod skull, Spinosaurus was a predator
larger and more fearsome
than even a Tyrannosaurus rex, and would spend
its time chasing and
killing other dinosaurs like Ouranosaurus.
The
reality however may in fact be very different. To reveal the nature
of Spinosaurus, you first need to look at the
skull elements, not
only the most well-known parts but it’s the skull that often gives
the best indication of lifestyle for any predator. Usually theropods
have relatively short and high snouts to house such body parts as
biting muscles and nasal cavities so that they can hunt by scent.
Spinosaurus however had a comparatively long and
narrow snout like a
crocodile. The tip of the snout has a recessed dip in the premaxilla
which the tip of the rounded lower jaw matches and fits into. This
adaptation is seen in some other animals such as crocodiles and serves
to increase grip upon smaller prey, particularly slippery prey such
as fish.
The
teeth of Spinosaurus are neither serrated and
flattened for slicing,
or strongly built for crunching bone. They are however narrow,
sharp and numerous like they are sometimes seen in crocodiles as well
as piscivorous fish eating pterosaurs.
The arrangement of the forward
teeth of the upper jaw is such that the largest are on either
sides of the premaxilla notch and point towards the rounded tip of the
lower jaw. The teeth on this rounded lower jaw tip point upwards into
the curvature of the snout notch. Oxygen isotope analysis of
Spinosaurus teeth has also revealed that they were
exposed to aquatic
environments for long periods.
Another
further special adaptation are the nostrils which are high up just in
front of the eyes. This is very unusual in itself for a carnivorous
animal because as an unofficial rule carnivores have their nostrils in
the front of their snouts, to not only allow for scents to be more
accurately analysed through a larger nasal cavity, but also to easily
smell the meat that they are eating. The fact that the nostrils are
so high strongly suggests that the more usual placement was not
possible due to how Spinosaurus lived and behaved.
The
final piece of currently available evidence is the actual construction
of the snout. A 2009 study by C. Dal Sasso, S. Maganuco and
A Cioffi focused upon what were small passages called foramina that
lead towards the same cavity inside the snout. These are taken to
have been pressure sensitive receptors that when dipped into the water
revealed the motions of passing fish that created pressure waves as
they swam through the water, allowing Spinosaurus
to not only know
when a fish was nearby, but when it would be at its closest for a
strike.
All
together these adaptations point to Spinosaurus
being a very
specialised predator that hunted for fish from the side of rivers.
The long and narrow snout meant that Spinosaurus
could dip its
pressure sensitive nose into the water while having a large area for
surface capture. The higher nostrils meant that Spinosaurus
could
comfortably breathe while its snout was dipped in the water, although
a possible weakness here could be a reduced nasal cavity that meant
Spinosaurus could not process smells as well as
other large theropods
that had larger nasal cavities. Because the teeth were angled to
follow the contours of their opposite jaws they would have provided the
maximum amount of available grip on a slippery and struggling fish.
One
of the most accurate depictions of this lifestyle was in the 2011
BBC series Planet Dinosaur, which depicted Spinosaurus
as a large
and specialised carnivore that primarily focused upon hunting fish like
Onchopristis,
yet would also supplement its diet by scavenging
carrion. It should be remembered that as a meat-eater Spinosaurus
would not have passed up the opportunity for a free meal, perhaps
using its more massive size to intimidate smaller theropods like
Rugops,
or even terrestrial crocodiles like Kaprosuchus
from a
carcass. If active at the same time as one another then Spinosaurus
may have even gone after the kills of giant crocodiles like Sarcosuchus
which would have been living in the same ecosystem.
The
possibility also remains that Spinosaurus may have
hunted land
animals, although no fossil evidence is known that strongly supports
this. In South America a pterosaur bone was found with a spinosaurid
tooth stuck into it, and recovery of the related Baryonyx
revealed
the presence of Iguanodon
bones inside of the area
that its gut would
have been. Still these may have been cases of scavenging rather than
attempted hunting. Baryonyx also revealed the
partially digested
remains of the fish Lepidotes,
further supporting the fish
specialisation hypothesis.
Because
Spinosaurus disappears from the fossil record well
before the end of
the dinosaurs sixty-five million years ago, it must have succumbed to
something else other than the established extinction theories that
ended the dinosaurs once and for all. Perhaps the easiest explanation
for its demise is that it simply became far too specialised, and when
the ecosystem it was living in changed to be drier the rivers systems
dried up, removing the prey source that Spinosaurus
was best equipped
to deal with. In the face of competition with more generalist
theropods, Spinosaurus just could not compete
with their success and
was eventually driven to extinction.
Sail or hump, and more
importantly why?
The
key features of Spinosaurus are the high neural
spines of the dorsal
vertebrae that were the inspiration for the name, the largest of
which of the original material was one hundred and sixty-five
centimetres long. The actual construction that resulted from these
spines however is one of the key subjects of debate with the two main
camps being 'sail' and 'hump' (a rare third is that the spines
stuck out by themselves but the majority of palaeontologists consider
this very unlikely).
A
sail would have given Spinosaurus an appearance
similar to the famous
but much older Dimetrodon.
The sail itself would have been a membrane
of skin and thin tissue that would have been held high off the back for
maximum exposure. However the spines themselves seem incredibly
strong and robust just for the purpose of supporting a skin sail, and
this leads into the hump theory. A hump probably would not have been
a very musculature structure but composed more of fatty tissues that
may have been used for food storage as well as weighing less than the
same proportionate amount of muscle.
The
only thing that inspires even greater debate about whether Spinosaurus
had a sail or hump is just what it was there for. Why did Spinosaurus
have to be so different, not just from other theropods, but the
other spinosaurids where the vertebrae are known to have much smaller
neural spines. Returning to the above theory of a hump of fatty
tissue would suggest that the humps primary use would be store fat when
Spinosaurus was able to gorge itself on a plentiful
supply. Going
with the fish specialisation, Spinosaurus's prey
may have been
seasonal with fish swimming upstream to spawn, but being relatively
sparse throughout the rest of the year. Spinosaurus
may have stored
extra food as fat so that it could continue into the leaner times of
the year where prey was less frequent when it may have had to
supplement its diet by scavenging. Fish would also probably not be
constantly active in the same water system and Spinosaurus
may have had
travel quite a distance when searching for fish. This concept has
also been proposed for Acrocanthosaurus,
another theropod dinosaur
with slightly enlarged neural spines that was active in the Aptian to
Albian stages of North America. This may have been an adaptation to
the climate as Suchomimus which is also from North
Africa had a similar
but smaller growth on its back where as Baryonyx
which is known from
England did not have any neural spine growth at all (all though there
is speculation that the Baryonyx holotype is of a
juvenile dinosaur).
Another
and more controversial theory is that of thermoregulation. By pumping
blood up into either the sail or hump, Spinosaurus
could expose its
blood to the warmth of the sun’s rays increasing its body temperature
so that it could become more active. Also if too warm it may have
relied upon a prevailing wind to cool its blood so that it did not
overheat. The problem with this theory is that it automatically
assumes that Spinosaurus was cold blooded and
relied upon basking in
the sun. There have been many studies done that suggest dinosaurs
were potentially warm-blooded even if the exact method was not
identical to mammalian methods of maintaining a warm-blooded
metabolism. As a very large dinosaur Spinosaurus
may have been
subjected to the effects of gigantothermy where an animal is so massive
that its own body insulates its internal parts from the outside cold.
An
in between theory is that since Spinosaurus
presumably spent a lot of
time in the water waiting to strike at fish it may have been chilled by
the very water it was standing in. By exposing its sail/hump to the
sun it could possibly warm its blood enough to counter the waters
cooling effect. However this would not explain why others like
Baryonyx and Suchomimus did not
do the same, unless size of the
animal is a determining factor.
The
most popular theory, which is a failsafe option for any unknown
growth, is that the sail/hump was for the purpose of display. This
would be a characteristic where the most complete, and possibly even
the most colourful sail/hump was the best, and the individual it
belonged to was more likely to pass on its genes to the next generation
of Spinosaurus. This could in part also connect
with the fat hump
theory in that a well fed Spinosaurus would have a
larger and fatter
hump that would show others of its kind how successful a predator it
was, proving that it was more worthy of reproducing.
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