The Mystery of Megaraptor & the Megaraptorans

Generally, paleontologists have a good understanding of the evolutionary relationships between different dinosaurs.

Then there are the enigmatic Megaraptorans.

The tale of these mysterious predators began in 1998 with the announcement of an otherworldly claw. Described by paleontologist Fernando Novas, the specimen – discovered in the Portezuelo Formation of Argentina’s Rio Neuquén basin – superficially resembles a Dromaeosaur (raptor) foot claw but is much larger. At 33.9 centimetres long (~40 cm when alive with a horny sheath), this titanic claw is over 50% larger than the next largest Dromaeosaur, signifying it belonged to a monstrous new species[i]. To commemorate this mega-sized raptor, Novas dubbed this new taxa Megaraptor. Creative, I know!

Yet even from the beginning, Megaraptor’s connection to the Dromaeosaurs was superficial. While the claw did resemble a raptor, the rest of the skeleton – comprised of bones from the hand and foot – was more akin to theropods like Allosaurus and Torvosaurus. Herein lies the beginnings of the Megaraptor controversy; was it a giant raptor and part of the Coelurosaur family, which includes groups such as raptors, Tyrannosaurids, and birds? Or, was it part of a more ancient lineage of theropods, the Carnosaurs, which includes species like Allosaurus and Spinosaurus? And where exactly did it lie in either family?

This question has intrigued paleontologists ever since. In 2004, a more complete specimen of Megaraptor was discovered, establishing that the killer claw was surprisingly part of its hand, not its foot[ii]. In fact, the hands – which are amongst the largest for a theropod dinosaur – possessed not one but two massive claws. This discovery clarified that Megaraptor was not a Dromaeosaur but did little else to settle the confusion. At the time, paleontologists ruled out Megaraptor being a Coelurosaur or Abelisaurid. They also didn’t have enough evidence to firmly place it within Spinosauridae or Carcharodontosauridae, the two most prominent lineages of Carnosauria. In other words: nobody knew what to make of this bizarre taxa!

In the decade afterwards, numerous species of Megaraptorans emerged from across the planet. In Japan, Fukuiraptor provided a glimpse at the early evolution of Megaraptorans but had traits found in both Carnosaurs and Coelurosaurs. In 2009, the mid-Cretaceous genus Australovenator was described from Northeastern Australia. Australovenator was initially believed to be an Allosaurid or a Carcharodontosaurid; for fans of Walking with Dinosaurs, you may recognize it as the “polar Allosaur” featured in the program! Around the same time, two late-Cretaceous species – Aerosteon and Orkoraptor – were described from South America. Despite the plethora of new species, none possessed the necessary diagnostic traits to push the needle for understanding their phylogeny.

However, a pattern of dispersion did emerge. Megaraptorans evolved at the beginning of the Cretaceous in Asia, then dispersed south. First to Australia in the mid-Cretaceous, then South America through Antarctica during the mid-to-late Cretaceous. As the Cretaceous progressed, the family grew immensely, beginning with the 4-meter-long Fukuiraptor and ending with the newly discovered 10-meter giant Maip. Unfortunately, this does little to establish their placement on the theropod family tree. Both Coelurosaurs and Carnosaurs were present in Asia during the Early Cretaceous, meaning early genera like Fukuiraptor could have descended from either.  

Different paleontologists offered potential relationships towards the end of the 2000s. Multiple studies concluded that Megaraptorans were related to Carcharodontosaurids[iii], and one book by Thomas Holtz suggested they may have been Spinosaurids[iv]. Most paleontologists placed them next to the Allosaurids and Carcharodontosaurids due mainly to their resemblance and a few anatomical similarities. Not exactly the best criteria!

Then, in 2013, Fernando Novas returned with a controversial new proposal: not only were the Megaraptorans Coelurosaurs, but they were basal Tyrannosauroids – the sister lineage of the family that leads to Tyrannosaurus rex. Novas based his hypothesis on numerous traits shared between Megaraptorans and Tyrannosauroids, mostly related to their skull, hips, and legs[v]. In this model, the Megaraptorans diverged from the other Tyrannosauroids in the early Cretaceous, dispersing southwards and becoming top predators in the Late Cretaceous.

This placement has gained support in the years since. In 2014, an analysis of a juvenile Megaraptor specimen supported them being on the Tyrannosauroid branch based on numerous anatomical traits[vi]. Amongst these were D-shaped premaxillary teeth, often used as a diagnostic trait of the Tyrannosauroids. A 2022 study on the Tyrannosauroid Eotyrannus also found the Megaraptorans to be nested with Tyrannosauroidea, lending more evidence to the presence of a second lineage of tyrant dinosaurs[vii].

If this placement is accurate, the implications are immense. Not only would the Tyrannosauroids have been far more dispersed than previously imagined (given that all previous members are from the Northern hemisphere), but also far more diverse. While northern species like T. rex had tiny arms and a devastating bite, their southern cousins were the opposite, using their massive claws to slash prey despite a (comparatively) underwhelming bite force. Mastering one evolutionary pathway is hard enough, yet the Tyrannosauroids appear to have perfected two!

This possibility may also explain the presence of miscellaneous Tyrannosauroid fossils in the southern hemisphere. The Australian genera Timimus, which was once thought to be an Ornithomimid and now a Tyrannosauroid, might be a species of Megaraptoran. Ditto for the Brazilian Santanaraptor, though it has been suggested it was a basal Coelurosaur and not part of Tyrannosauroidea or Megaraptora[viii]. Regardless, the presence of Tyrannosauroid fossils in southern locations can now be explained away by the Megaraptorans.

It should be noted that the Megaraptoran connection to the Tyrannosauroids is still tentative. Some paleontologists still place them somewhere within Carnosauria, while others believe they could be an offshoot of primitive Coelurosaurs. It may be unlikely, but they may be neither Coelurosaur nor Carnosaur, instead coming from somewhere in between. The uncertainty of the Megaraptorans is most evident in the 2019 book Dinosaur Facts and Figures: the Theropods and Other Dinosauriformes, where the Megaraptorans are the only theropod lineage to have a question mark beside their placement on the family tree. If that doesn’t speak volumes, I don’t know what does!

There are a few reasons why uncertainty still exists. Crucially, a complete Megaraptoran skeleton has not been discovered, preventing paleontologists from getting a thorough glimpse at this enigmatic family. Second, the known specimens lack the diagnostic characteristics of any essential theropod family. Sure, one species may have the D-shaped teeth of Tyrannosauroids, but others have plenty of traits more akin to Carcharodontosaurids. Until more complete specimens are discovered, the true nature of these giant-clawed killers will remain a mystery.

The Megaraptorans have proved one thing: that even in the golden age of paleontology, some dinosaurs will remain tantalizingly elusive.

Thank you for reading this article! If you want to know about another problematic theropod – the legendary Spinosaurus – I suggest you read “Why are Paleontologists Obsessed with Spinosaurus?” here at Max’s Blogosaurus!

I do not take credit for any images found in this article. Header image courtesy of @weird_dog_thing on twitter.

Works Cited:

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[i] Novas, Fernando E. “Megaraptor Namunhuaiquii, Gen. et Sp. Nov., a Large-Clawed, Late Cretaceous Theropod from Patagonia.” Journal of Vertebrate Paleontology, vol. 18, no. 1, 1998, pp. 4–9, https://doi.org/10.1080/02724634.1998.10011030.

[ii] Calvo, Jorge, et al. “Phylogenetic Status of Megaraptor Namunhuaiquii Novas Based on a New Specimen from Neuquén, Patagonia, Argentina.” Ameghiniana, vol. 41, no. 4, Dec. 2004.

[iii] Benson, Roger B., et al. “A New Clade of Archaic Large-Bodied Predatory Dinosaurs (Theropoda: Allosauroidea) That Survived to the Latest Mesozoic.” Naturwissenschaften, vol. 97, no. 1, 2009, pp. 71–78, https://doi.org/10.1007/s00114-009-0614-x.

[iv] Holtz, Thomas R., and Luis V. Rey. Dinosaurs: The Most Complete, up-to-Date Encyclopedia for Dinosaur Lovers of All Ages. Random House, 2007.

[v] Novas, Fernando E., et al. “Evolution of the Carnivorous Dinosaurs during the Cretaceous: The Evidence from Patagonia.” Cretaceous Research, vol. 45, 2013, pp. 174–215, https://doi.org/10.1016/j.cretres.2013.04.001.

[vi] Porfiri, Juan D., et al. “Juvenile Specimen of Megaraptor (Dinosauria, Theropoda) Sheds Light about Tyrannosauroid Radiation.” Cretaceous Research, vol. 51, 2014, pp. 35–55, https://doi.org/10.1016/j.cretres.2014.04.007.

[vii] Naish, Darren, and Andrea Cau. “The Osteology and Affinities of Eotyrannus Lengi, a Tyrannosauroid Theropod from the Wealden Supergroup of Southern England.” PeerJ, vol. 10, 2022, https://doi.org/10.7717/peerj.12727.

[viii] Delcourt, Rafael, and Orlando Nelson Grillo. “Tyrannosauroids from the Southern Hemisphere: Implications for Biogeography, Evolution, and Taxonomy.” Palaeogeography, Palaeoclimatology, Palaeoecology, vol. 511, 2018, pp. 379–387, https://doi.org/10.1016/j.palaeo.2018.09.003.