Name:
Spinosaurus
(Spine lizard).
Phonetic: Spine-oh-sore-us.
Named By: Ernst Stromer - 1915.
Synonyms: Sigilmassasaurus?
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: Estimates highly variable amongst sources and range
anywhere between 12.6 and 18 meters total body length. Skull length
estimated between 1.5 and 1.75 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 begin to 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, this version of Spinosaurus
has
also been envisioned
as a quadrupedal one, particularly in the 1970s and 1980s.
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,
Ichthyovenator
is known from Laos, while 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.
From
the 1990s to 2014, Spinosaurus was commonly
portrayed as a very
long but comparatively lightly built theropod dinosaur, that aside
from the skull and sail/hump on the back, was roughly similar to
other large theropod dinosaurs in build. Then in September 2014 a
new study written by Nizar Ibrahim, Paul C. Sereno, Cristiano Dal
Sasso, Simone Maganuco, Matteo Fabbri, David M. Martill,
Samir Zouhri, Nathan Myhrvold and Dawid A. Iurino, was published
along with great fanfare by National Geographic including a full size
model reconstruction. This ‘new’ Spinosaurus
still had a
crocodile-like skull and a sail on its back, but this time
Spinosaurus had been rebuilt as an obligatory
quadrupedal dinosaur.
This
new reconstruction came about from the process of all the old confirmed
Spinosaurus fossils being rebuilt three
dimensionally in computer
software, and then augmented with new fossil finds that had taken
place over only a few years before. The reconstruction was still
missing several other pieces, but other spinosaurid dinosaurs were
studied and by comparison scaled and added to fill the gaps where
possible. The key difference between this reconstruction and older
ones is that the legs in the 2014 Ibrahim et al reconstruction are
significantly shorter than they were previously envisioned and unlikely
to be capable of long term bipedal locomotion. The pelvis was also
noted as having a reduced size, meaning less attachment space for
large leg muscles, and meaning that the rear legs were less able to
support the weight of the body. Also, the researchers plotted the
centre of gravity of the body and found that it was well forward from
the hips, different from the overwhelming majority of other large
theropods, and by extension meaning that Spinosaurus
would have had
to support its body with its arms when on land.
The
‘new 2014' Spinosaurus quickly filled the
internet with news
stories not only because it was so different from before but because
for at least twenty years before Spinosaurus had
fast become one of the
more popular dinosaurs in fiction such as films and games. However
many have questioned the accuracy of the Ibrahim et al reconstruction.
The key problem that most people have is that this reconstruction is a
composite made up of numerous Spinosaurus
individuals as well as
missing pieces filled in by comparison to other spinosaurid
dinosaurs, with some even going so far as to state that the new
reconstruction is only a ‘chimera’ and cannot be validated without
a substantially more complete specimen of Spinosaurus,
though as
history tells us, that so far has been exceedingly frustrating to
find.
The
other main issue that some people have is the size of the rear legs,
notably from the vast reduction in size to what they have been used to
seeing. This begs the question however, if Spinosaurus
did have
notably small hind limbs, then what about other spinosaurid
theropods? At the time of writing the only two spinosaurid genera
known to have reasonably complete hind limb remains are Baryonyx
and
Suchomimus (sometimes considered by some to be
synonymous with
Baryonyx), which seem to be closer to other more
classic theropod
leg proportions. However, both of these are thought to be based
upon juvenile and subadult individuals respectively, so could it be
that younger spinosaurids had proportionately long legs that reduced in
length as they reached adulthood. A certain body part not growing at
the same rate as the rest of the body is actually very common, and
the precedent of leg length proportions reducing with age in large
theropod dinosaurs does exist, and is particularly observable in some
of the large tyrannosaurs.
In Albertosaurus
for example, a genus
where most of the life stages are known, there is a clearly
observable trend where as individuals get older, larger and
specifically heavier, the length of the hind limbs significantly
shortens.
Another
idea for shorter legs in Spinosaurus than other
spinosaurids could be
that Spinosaurus simply represents a more advanced
and hence more
specialised form. So far Baryonyx is only known
from the earlier
stages of the Early Cretaceous, and Suchomimus
slightly later in the
Aptian stage of the Cretaceous. Spinosaurus
fossils however range
between the last stage of the Early Cretaceous, the Albian, to
the first stage of the Late Cretaceous, the Cenomanian. This means
that Spinosaurus would have had many millions of
years more opportunity
to develop and stabilise a reduction in the rear legs.
As
a genus though Spinosaurus has always been in a
state of flux, and we
may yet only be one more discovery away from having to do yet another
revision to how we reconstruct this dinosaur.
Tail reconstruction
In
2020 a new paper by Ibrahim et al was
published, this time
describing an almost complete tail of a Spinosaurus.
The vertebrae of
this tail showed highly developed neural spines as well as deep
chevrons on the underside. These extended features applied to all of
the vertebrae right down to the end of the tail, making the whole
tail into a broad paddle shape. Furthermore, the tail of
Spinosaurus was not stiff, it was actually very
flexible and capable
of a lateral, side to side movement. This means than in theory
Spinosaurus could have propelled itself through the
water with its
tail.
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.
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.
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 either from the side of
rivers, to even getting into the river and swimming amongst the fish.
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, though as a fish hunter, Spinosaurus
would have not needed a keen sense of smell anyway. 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 more 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
2014 reconstruction of Spinosaurus by Ibrahim et
al also supported
the idea that Spinosaurus would readily enter the
water and actually
swim about. Not only did they cover isotope analysis which confirmed
a great deal of exposure to aquatic environments, they also noted
that Spinosaurus had particularly dense bones.
This is a common
feature in animals that spend a lot of time swimming in the water as
the greater bone density helps the animal with buoyancy issues so that
it can actually swim under the surface is necessary. Ibrahim et al
also noted that the claws on the feet were flat-bottomed, which would
be a further aid in pushing against the water while swimming. This
also strongly suggests that the main swimming propulsion was provided
by the rear legs.
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 idea that the hump was for
thermoregulation is especially valid however if Spinosaurus
spent a lot of time swimming in the river systems of Cretaceous North
Africa.
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.
Further reading
- New remains of the enigmatic dinosaur Spinosaurus from the Cretaceous
of Morocco and the affinities between Spinosaurus and Baryonyx. - Neues
Jahrbuch f�r Geologie und Pal�ontologie, Monatshefte (2): 79–87. - E.
Buffetaut - 1989.
- Remarks on the Cretaceous theropod dinosaurs Spinosaurus and
Baryonyx. - Neues Jahrbuch f�r Geologie und Pal�ontologie, Monatshefte
(2): 88–96. - E. Buffetaut - 1992.
- Neural spine elongation in dinosaurs: sailbacks or buffalo-backs? -.
Journal of Paleontology 71 (6): 1124–1146. - J. B. Bailey - 1997.
- A new specimen of Spinosaurus (Dinosauria, Theropoda) from the Lower
Cretaceous of Tunisia, with remarks on the evolutionary history of the
Spinosauridae. - Bulletin de la Soci�t� G�ologique de France 173 (5):
415–421. - E. Buffetaut & M. Ouaja - 2002.
- Pterosaurs as part of a spinosaur diet. - Nature 430 (6995): 33. - E.
Buffetaut, D. Martill & F. Escuilli� - 2004.
- New information on the skull of the enigmatic theropod Spinosaurus,
with remarks on its sizes and affinities. - Journal of Vertebrate
Paleontology 25 (4): 888–896. - C. del Sasso, S. Maganuco, E. Buffetaut
& M. A. Mendez - 2005.
- New information regarding the holotype of Spinosaurus aegyptiacus
Stromer, 1915. - Journal of Paleontology 80 (2): 400–406. - J. B.
Smith, M. C. Lamanna, H. Mayr & K. J. Lacovara - 2006.
- A neurovascular cavity within the snout of the predatory dinosaur
Spinosaurus. - 1st International Congress on North African Vertebrate
Palaeontology. Mus�um national d'Histoire naturelle. - C. Dal Sasso, S.
Maganuco & A. Cioffi - 2009
- Oxygen isotope evidence for semi-aquatic habits among spinosaurid
theropods. - Geology 38 (2): 139–142. - R. Amiot, E. Buffetaut, C.
L�cuyer, X. Wang, L. Boudad, Z. Ding, F. Fourel, S. Hutt, F. Martineau,
A. Medeiros, J. Mo, L. Simon, V. Suteethorn, S. Sweetman, H. Tong, F.
Zhang & Z. Zhou - 2010.
- Fine sculptures on a tooth of Spinosaurus (Dinosauria, Theropoda)
from Morocco. - Bulletin of Gunma Museum of Natural History 14: 11–20.
- Y. Hasegawa, G. Tanaka, Y. Takakuwa & S. Koike - 2010.
- Semiaquatic adaptations in a giant predatory dinosaur - Science vol.
345, no.6204 - Nizar Ibrahim, Paul C. Sereno, Cristiano Dal Sasso,
Simone Maganuco, Matteo Fabbri, David M. Martill, Samir Zouhri, Nathan
Myhrvold & Dawid A. Iurino - 2014.
- A buoyancy, balance and stability challenge to the hypothesis of
a semi-aquatic Spinosaurus Stromer, 1915
(Dinosauria:
Theropoda). - PeerJ. 6: e5409. - D. M. Henderson
- 2018.
- Tail-propelled aquatic locomotion in a theropod dinosaur. -
Nature. - Nizar Ibrahim, Simone Maganuco, Cristiano Dal
Sasso, Matteo Fabbri, Marco Auditore, Gabriele Bindellini,
David M. Martill, Samir Zouhri, Diego A. Mattarelli, David
M. Unwin, Jasmina Wiemann, Davide Bonadonna, Ayoub Amane,
Juliana Jakubczak, Ulrich Joger, George V. Lauder &
Stephanie E. Pierce - 2020.
- Sigilmassasaurus is Spinosaurus:
a reappraisal of African
spinosaurines. - Cretaceous Research. 114: 104520. - R. S. H. Symth, N.
Ibrahim & D. M. Martilla - 2020.
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