Dinosaurs are an incredibly diverse lineage of animals, having existed for over 230 million
years and adapting to fill many different kinds of niches across our planet. They made it through
all sorts of mass extinction events - even surviving today as the 10,000 living bird
species currently inhabiting the Earth. Many mysteries still remain when it comes
to prehistoric dinosaurs, though, and a lot of debate and controversy surrounds
various aspects of dinosaur palaeobiology and evolutionary relationships.
One of the most interesting dinosaur mysteries that still hasn’t been entirely resolved is the
issue of the Megaraptors - or, more properly, the megaraptorans - a group of carnivorous theropods
that lived during the Cretaceous period. This group of predatory dinosaurs were fairly
diverse, containing quite a few species, although the exact relationships between
groups of megaraptorans - and even which ones should be included as
megaraptorans themselves - are debated. These animals ranged in size from about 4
metres in length, such as the relatively basal Fukuiraptor that lived during the
Early Cretaceous in Japan, all the way up to the massive and very recently described Maip
macrothorax - named in April of 2022 based on material from Argentina dating to the very
end of the Cretaceous - which is estimated to have reached about 9 or 10 metres in length.
One of the most notable features of these dinosaurs are the large and robust forelimbs,
with particularly big and fearsome-looking claws on the ends of the inner two fingers
of each hand and a much smaller claw on the third digit. In fact, the heavily built arms
and hands of these theropods were the subject of a recent study that aimed to reconstruct the
musculature of the limbs, in order to try and gain a clearer understanding of what
exactly these animals were using them for. When looking at the arm bones, palaeontologists
found that a lot of the muscle attachment sites seen on the fossils are notably well-developed,
suggesting an important use of the arms and hands in the lives and
palaeobiology of the Megaraptorans. They even found that during the evolution
of the group, going from more basal members such as Fukuiraptor to derived
forms - particularly Australovenator from Australia and Megaraptor itself
from Argentina - there was a notable increase in the size of these muscle attachment points,
so clearly they were becoming more specialised and more reliant on the use of their forelimbs.
The muscle attachment sites on the bones making up the pectoral girdle,
too, show a similar enlargement and trend towards becoming more pronounced in later species.
Looking at Australovenator in particular, from the Late Cretaceous of Queensland,
which is known from partial forelimbs, it’s been established that these dinosaurs were definitely
up to something with their unusual arms. By creating digital models of the arm and hand
bones in order to see how they connected to one another and how far they could move relative to
each element, palaeontologists in 2015 discovered that Australovenator had an extraordinary range
of motion for its arms and wrists, as well as its fingers which bore the massive claws.
All of this evidence has therefore been taken to indicate that the Megaraptorans
were likely employing the use of these highly flexible, strong arms and hands that were
capable of complex movements to capture their prey - however, the exact details of how they
were doing this and what specific prey capture methods they had are yet to be understood.
Whatever it was, though, it was definitely something interesting, and seemingly very
different to other groups of giant carnivorous theropods, which mostly show a trend of decreasing
arm size and more reliance on their jaws - made famous by the tyrannosaurids.
Megaraptor itself was first named back in 1998, and was subsequently classified in a variety
of different positions among other theropod dinosaurs, even as a spinosaur at one point.
The group - technically called a clade - known as Megaraptora was actually only named in 2009,
when palaeontologists took a look at a grouping of carnivorous theropod dinosaurs called the
allosauroids and attempted to resolve some of the classification issues within these animals.
The result of this study was that 6 problematic theropod taxa - namely Aerosteon, Australovenator,
Chilantaisaurus, Fukuiraptor, Megaraptor and Orkoraptor were found to be relatives
of a dinosaur found in England called Neovenator. Neovenator is known to be a definite member of the
Allosauroids - a large superfamily of carnivorous theropods including the famous Allosaurus and
its relatives, plus the Carcharodontosaurs such as Acrocanthosaurus, Giganotosaurus,
Carcharodontosaurus and many others. The researchers in 2009 therefore named
two new clades of theropod dinosaurs to account for this newly-found relationship. The larger,
more inclusive grouping was named Neovenatoridae, and included all of the 6 problematic taxa
plus Neovenator itself - and then a more inclusive group within this one was named,
too, comprising Aerosteon, Australovenator, Fukuiraptor, Orkoraptor and Megaraptor. This
clade was named Megaraptora, and for the first time the relationship between these mysterious
dinosaurs was officially recognised. This had some important implications for the evolution
of the Allosauroids, too, as Orkoraptor is known from fossils dating to the very end of the age
of dinosaurs so the inclusion of this dinosaur seemed to disprove the idea that Allosauroids went
extinct before the end of the Mesozoic Era. Later ideas and debates
But, this is palaeontology - and specifically, large carnivorous dinosaur palaeontology - so
of course things were not going to remain quite so straightforward,
and controversy soon followed. In 2013, a very different hypothesis of
Megaraptoran relationships was published. Palaeontologists in Argentina performed a
large review of Patagonian theropods from the Cretaceous, finding some surprising evidence
that supported the placement of the Megaraptorans within the Coelurosaurs - another major clade
of theropod dinosaurs including Ornithomimosaurs, Oviraptorosaurs,
Therizinosaurs, Dromaeosaurs, Tyrannosaurs, and the birds, as well as others.
Even more surprisingly, the Megaraptorans were found to be very close relatives of
the Tyrannosaurids, and it was argued that the anatomical characters of the bones used
to show previously that the Megaraptors were allosauroids are actually quite problematic
since these features are widespread among many different lineages of theropods.
The paper also created a new family within the Megaraptoran clade itself, named Megaraptoridae,
to include all of the species that come from the southern supercontinent of Gondwana. At the time
of publication, this included Megaraptor, Aerosteon and Orkoraptor from Argentina,
and Australovenator from Australia - though others have since been named.
2013 also saw the description of another large carnivorous dinosaur that has a
problematic history of classification itself - Siats meekerorum. Siats comes from Utah,
and dates to the Late Cretaceous about 98 million years ago. When it was described in 2013,
it was classified as a Megaraptoran, and Megaraptors were again considered to be
included within Neovenatoridae. This was therefore reported as the first evidence
of Megaraptorans being present in North America, and seemed to show some interesting things about
the patterns of how different lineages of large theropods replaced one another in North
America - before the Tyrannosaurs properly took over later in the Cretaceous.
We’ll come back to Siats again, but in the time since it was first named it’s been classified all
over the place, as an allosauroid, some kind of coelurosaur, and as a tyrannosauroid.
Then, in 2014, researchers in Argentina again published some more good evidence
for a Megaraptoran affinity with coelurosaurs - and specifically the tyrannosauroids again.
They described the partial skeleton of a juvenile Megaraptor individual, which preserved part of the
skull of this animal and shows all sorts of features that are very similar to the
tyrant dinosaurs. These features include very similar openings on the premaxilla of the skull,
teeth at the front of the upper jaw with a D-shaped cross-section, and very similar
structures on the roof of the skull. In their evolutionary analysis of the
position of Megaraptors in this study, the authors found strong support for them being
nested deep within tyrannosauroidea - the superfamily of all tyrannosaurs - and they
even recovered Eotyrannus from the Isle of Wight in the UK as a megaraptoran as well,
seemingly showing that these dinosaurs were also present in Europe - though more recent studies
have since taken Eotyrannus out of the group. Moving forwards to 2016, this year saw the
description of another new Megaraptoran, named Murusraptor barrosaensis and coming
from the Late Cretaceous of Argentina. The classification of Megaraptorans was of
course discussed in this paper, and the two main competing hypotheses - that they’re neovenatorid
allosauroids or that they’re tyrannosauroids - were both considered, but neither one was really
committed to more than the other. Murusraptor was added to the known Argentinian radiation
of Megaraptorans, though, along with Megaraptor, Aerosteon and Orkoraptor.
2016 also saw the description of yet another new Megaraptoran from - you guessed it - Argentina,
named as Aoniraptor libertatem. This dinosaur is represented by a series
of fossilised vertebrae, but they show enough similarities to other Megaraptorans
to allow the classification of this dinosaur as such. Additionally, Aoniraptor shows more
features with similarities to the anatomy of tyrannosaurs, especially in the tail vertebrae,
again providing support for Megaraptors as a lineage of tyrannosauroids.
Also in 2016, another very interesting and potentially quite important dinosaur was described
that we need to throw into this mix. Gualicho shinyae was named from fossils found in
- unsurprisingly - Argentina, and is a fascinating example of apparent convergent evolution with the
tyrannosaurids since it possesses just two fingers on its hands, with the third digit reduced to a
splint, just like in the tyrant dinosaurs. Gualicho was classified as a Neovenatorid in
this initial description, and was found to be a close relative of the African Deltadromeus
(however the evolutionary position of that dinosaur could be a whole other video on
its own). The paper also found Megaraptorans to be positioned back within Neovenatoridae,
as in those earlier studies. Since 2016, though, Gualicho has also
been recovered as a coelurosaurian, along with the Megaraptorans.
2016 was quite an action-packed year for Megaraptor research,
as another paper published that year also described some more megaraptoran material
from the Early Cretaceous of Australia. This specimen, which comprises various fragments
of the skeleton, remains unnamed and is simply referred to by its accession
number LRF 100–106. However, it actually gives us some pretty important information
about the origin of the Megaraptorid family. Not only does this material represent the largest
theropod so far identified in Australia, but it shows that Australia was likely
involved in faunal interchanges between other Gondwanan continents and that Megaraptoridae
itself may actually have originated and first begun to radiate in Australia, before spreading
to South America. Megaraptorans as a whole, though, were found to have first arisen in Asia,
as indicated by Fukuiraptor from Japan - the most basal Megaraptoran known at that point.
Heading into 2018, the description of a Megaraptoran named Tratayenia rosalesi
was published - and there are no prizes for guessing where this one was found
(it was Argentina). Tratayenia was placed as another member of the
more inclusive family of southern Megaraptorans, Megaraptoridae, and found to be quite similar
to other Argentinian representatives - but still distinct enough to be a new species.
Tratayenia also turned out to be the largest carnivorous dinosaur in the particular geologic
formation it was found in, showing that - after the extinction of the carcharodontosaurids
here - Megaraptorans were the ones to take over as the apex predators of South America.
Interestingly, though, according to the phylogenetic analysis of the evolutionary
relationships done in this paper, Megaraptorans were positioned as an early-branching lineage of
coelurosaurian dinosaurs, but not as members of the tyrannosauroids - supporting another,
third hypothesis of Megaraptorans as basal coelurosaurs that are
not included within the tyrannosaurs. And that brings us to 2019, when an interesting
new Megaraptoran named Phuwiangvenator yaemniyomi was described - this time actually from the Early
Cretaceous of Thailand. This new species was found to not be a member of the Megaraptoidae
family and was actually positioned as the most basal representative of Megaraptora,
even more basal than the slightly younger Fukuiraptor from Japan. This seems to show,
then, that the Megaraptors most likely first originated in Asia, since the most basal
forms come from formations here. Another interesting conclusion drawn
by the palaeontologists on this paper is that the Megaraptors seem to have become
fairly geographically widespread early on in their evolution, with slightly younger but
still Early Cretaceous-aged Megaraptorans known from Australia and South America, too - however
the Megaraptors then became highly provincial later on in Cretaceous,
with the majority known from South America. In terms of Megaraptoran classification,
the controversy was again discussed in the paper naming Phuwiangvenator and the clade
was recovered as belonging to Coelurosauria, but not being especially closely related to
the tyrannosaurs. Then, we come to 2022,
when a very exciting paper was published describing the largest Megaraptoran found
to date - the enormous Maip macrothorax, from the very latest Cretaceous of Argentina.
Maip comes from a formation dating to the final Stage of the Cretaceous period, and is so far the
only relatively complete Megaraptoran from this time. It again groups with the other,
previously-named South American Megaraptorids and some key anatomical differences with the
Megaraptorans known from Asia and Australia were also identified - such as the South American
forms being much larger in overall body size and far more robust in their proportions.
Maip itself is particularly notable for its very wide chest, measuring about 1.2 metres
across and giving this species a particularly bulky appearance for a Megaraptor. The animal
likely measured about 10 metres long, and it seems the reason this species was able to grow
to such a large size was due to the extinction of the Carcharodontosaurid dinosaurs in South
America near the start of the Late Cretaceous - the lineage of theropods including Giganotosaurus,
Mapusaurus and others. The spinosaurids also went extinct at a similar time, too.
With those other giant terrestrial carnivores out of the way, South America at the very end
of the Cretaceous seems to have become the land of the Megaraptors, with these dinosaurs
growing to ever-increasing sizes and taking advantage of those newly-opened niches. The
major radiation of Megaraptoran diversity in South America also appears to have coincided
with the extinction of those other theropods, too, providing more evidence to explain the
success of the Megaraptors around this time. The paper naming Maip also managed to better
refine some of the internal relationships within Megaraptora as well. The Asian and
Australian representatives were again shown to be the more basal members,
leading up to the more derived South American clade. Within this South American clade, though,
a more inclusive clade (called ‘Clade A’) was defined that includes Megaraptor, Murusraptor
and the less inclusive ‘Clade B’, which contains Orkoraptor, Tratayenia, Aerosteon and Maip.
The significance of these two South American groupings is that they appear to reinforce
the timings of the Megaraptorid radiation, with Megaraptor and Murusraptor coming from
earlier in the Cretaceous and reaching generally smaller sizes compared to the members of Clade B,
which lived later and grew even bigger. But what about the paper’s findings on where
Megaraptorans sit among theropods? Well, the authors once again supported
a position as coelurosaurians, but instead of being members of the tyrannosauroids,
they found Megaraptora to be the sister clade to Tyrannosauroidea - meaning they
are the closest relatives to each other. So, the paper concludes that, unlike the situation
at the end of the Cretaceous in North America, where the tyrannosaurids took over the apex
predator niche after the carcharodontosaurids disappeared, in South America it was the
Megaraptors that occupied these trophic positions, alongside a diverse lineup of
other theropods such as the abelisaurids - famously including Carnotaurus - and the
Unenlagiines, such as Austroraptor. And that leads us to another fairly recent
and interesting development in the Megaraptoran story, when in 2022 an extensive and detailed
study of the Early Cretaceous-aged tyrannosauroid Eotyrannus, from the Isle of Wight in England,
was published. This research thoroughly described all the known material for this dinosaur, and then
also investigated its evolutionary relationships among theropods, finding some significant
support for Megaraptorans to again be included within the Tyrannosauroidea superfamily.
It supports the Megaraptorans as being a second wave of large-bodied tyrannosauroids that evolved
towards the end of the Early Cretaceous, and even suggests that the arrival of this lineage
may have actually delayed the radiation of the tyrannosaurids - the group leading to T. rex
itself. This is because Siats, which we met earlier, was again found to be a basal member
of the Megaraptorans - so it potentially represents a lineage of Megaraptorans that
invaded North America and ended up suppressing the rise of the tyrannosaurids until much later.
The previous interpretation of Eotyrannus as a Megaraptor that was made in 2014,
and which I mentioned earlier, was not supported by this study,
however. Eotyrannus, therefore, is positioned as a more basally-branching tyrannosauroid,
with Megaraptorans splitting off later. The placement of Megaraptorans as a branch of
tyrannosauroids - which appears to be better and better supported every year - has many
fascinating implications for the evolution of one of the most famous dinosaur lineages. Not only
did they potentially have a major influence on the evolution of later tyrannosaurids,
but it’s very interesting that the Megaraptorans seem to have evolved more specialisations for
prey capture and manipulation with their forelimbs - whereas the tyrannosaurids
went down a very different route, undergoing a trend towards smaller forelimbs and prioritising
prey capture with their robust jaws. So, there we have an overview of the Megaraptorans
and the recent history of study on this fascinating group of theropods. Obviously, this is
still a changing and very fluid subject, and I’m certain more studies will be coming out soon that
make parts of this video outdated - but this is our understanding of these dinosaurs so far.
It’s amazing how much our understanding of this pretty significant lineage of dinosaurs
has changed over the last decade or so, and to think of these animals as another branch of
tyrannosauroids is honestly very exciting, showing that these dinosaurs actually led to not just one,
but two separate lineages of apex predators. I really cannot wait to see what other discoveries
are made about the Megaraptorans, and what new species must be out there for us to find.
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