Although it was one of the first dinosaurs to be scientifically described during the early 19th century, the theropod Megalosaurus remains one of the most enigmatic (and problematic) large dinosaurs known. Even though an entire family, the Megalosauridae (established by Huxley in 1869), bears the name of this famous dinosaur, the group has come to be seen as a taxonomic wastebasket with no real meaning. Indeed, a new paper in the journal Palaeontology advocates dropping the family, at least until better material can be found from Megalosaurus.
According to the new paper by Benson, Barrett, Powell, and Norman, the only fossil material that we can truly say represents Megalosaurus is the lower jaw (dentary) that allowed William Buckland to identify the dinosaur as an immense carnivore, not unlike a giant terrestrial crocodile. The rest of the material, even the fossil femur tip described by Robert Plot in his Natural History of Oxfordshire, cannot be attributed to Megalosaurus with any degree of certainty, at least not until a more complete skeleton is found for comparison. This means that previous analysis going back to the earliest days of paleontology were essentially working with a fossil chimera; many parts of the post-cranial skeleton attributed to Megalosaurus may have belonged to other species of theropods.
The recognized lack of Megalosaurus material creates a bit of a problem, then. The dentary, which is distinct enough to continue to bear the name Megalosaurus bucklandii (not M. bucklandi or M. conybeari), cannot be compared with material from other theropods as the dentary is often missing from species that may be closely related. According to the authors of the paper, the family Megalosauridae should be discontinued as Megalosaurus cannot be effectively compared with other genera placed within the family, and the status of the family representing basal members of the Spinosauroidea (Megalosauridae + Spinosauridae) isn't upheld.
If paleontologists accept the recommendations of the authors there could be quite a bit of taxonomic reshuffling. Only Megalosaurus could be said to be a "megalosaurid," and the rest of the genera associated with it (i.e. Afrovenator, Torvosaurus, Eustreptospondylus, Piatnitzkysaurus, Poekilopleuron, and Dubreuillosaurus) will require re-analysis. Perhaps a more complete skeleton of Megalosaurus will someday be found to help resolve some of these issues, but for now one of the cornerstones of paleontology will remain mysterious.
BENSON, R.B., BARRETT, P.M., POWELL, H.P., NORMAN, D.B. (2008). THE TAXONOMIC STATUS OF MEGALOSAURUS BUCKLANDII (DINOSAURIA, THEROPODA) FROM THE MIDDLE JURASSIC OF OXFORDSHIRE, UK. Palaeontology, 51(2), 419-424. DOI: 10.1111/j.1475-4983.2008.00751.x
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To be fair, most of the taxa you mention (Afroventaor, Torvosaurus, etc.) aren't really in the Megalosauridae, but are there just until some better grouping can be attained. Indeed, most of them are just plain poorly known. Megalosauridae may simply be transferred to Torvosauridae under Sereno's "Spinosauroidea," which wouldn't result in a lot of reappraisal. Aside from Afrovenator and Torvosaurus, the "Megalosauridae" probably doesn't even constitute a natural group, but instead may represent a stepwise progression toward Avetheropoda, much like Prosauropoda isn't a natural group, but consists of a bunch of dead-ends moving toward Sauropoda.
It is interesting, though, that only Buckland's dentary can be assigned to the genus. Sadly, it is not surprising.
I don't really know what to think of this. On the one hand, it seems a bit extreme calling for the termination of an entire branch of Theropoda just because its namesake isn't as well known as we once thought (and hey, "Torvosauridae" sounds pretty cool anyway, IMO). On the other hand, I think a massive reassessment of the group has been a long time coming regardless of what happens to Megalosaurus. I'm no expert, but it seemed that a lot of specimens were just arbitrarily tacked on to Megalosauridae simply because they were too derived to be ceratosaurs and too basal to be avetheropods (especially Spinosauridae).
Ultimately, without some groundbreaking new fossil evidence coming to light, I don't expect a whole lot to come of a mass reassessment at this point beyond Megalosauridae maybe becoming Torvosauridae and the exact placement of torvosaurs and spinosaurs within getting shuffled around. But I do think at least some inspection of the branch at the moment is worth the effort with how it's been treated over the years; it'll take more fossil evidence to really understand what's going on, but we shouldn't just sit around and wait for a miracle skeleton or two before at least extending the effort to better understand what we currently have.
It doesn't seem too surprising that one of the earliest-described dinosaurs is chimeric, considering how little was known at the point in history when Megalosaurus was described. It's great to see that the original material is being restudied.
Do the researchers offer up any suggestions as to what the rest of the material can be referred to? I've heard rumblings of a Stonesfield abelisauroiod, so I wonder if there could be a connection.