Welcome to the third (and final) article in my little series on the dinosaur(s) once known as Iguanodon. As we’ve seen in the previous parts, Iguanodon of traditional usage – Iguanodon sensu lato – has recently been blasted into numerous separate genera. As we’ll see here, while some of these taxonomic changes are likely to persist, others may well not.

In the previous article, we looked at the British Wealden iguanodontians recently taken out of Iguanodon and named Barilium, Sellacoxa and Kukufeldia. All are from the Valanginian-aged part of the Wealden known as the Hastings Group [the composite image at left combines Greg Paul’s illustrations of (at right, top to bottom) I. bernissartensis, Dollodon and Mantellisaurus, the skull of Dakotadon (at top left), and David Norman’s skeletal reconstruction of the Mantel-piece (read on)].

The Barilium, Sellacoxa and Kukufeldia specimens are far from the only reasonably good Hastings Group iguanodontian specimens that are known. In 1889, Richard Lydekker named I. fittoni (from the Wadhurst Clay Formation) for an ilium, a few vertebrae and a partial ischium. Again, this specimen possesses a number of features that aren’t seen in other iguanodontians (the blade of its ilium is narrow and compressed dorsally, and some of its sacral vertebrae have ventral keels, for example). Moreover, it doesn’t share any special features that might link it with I. bernissartensis (Norman 2010). It would seem, then, that it also needs to be taken out of Iguanodon and given a new name.

Norman (2010) proposed that a number of additional Wadhurst Clay Formation iguanodontian remains – including vertebrae, a femur, a lower jaw, and a remarkably robust forelimb [shown here, from Richard Owen’s 1874 description] – can all be referred to I. fittoni (some of these remains have been given the name I. hollingtoniensis, published in 1889). The practise of referring non-overlapping remains to any given species is problematical: it might seem reasonable to assume that these elements all belong to the same animal, but you can’t know this in the absence of associated remains. And there’s always the problem that someone will come along and take one of your referred elements, declare that it belongs to an additional taxon, and name it (you mean, like Kukufeldia?).

Among the specimens referred to I. fittoni by Norman (2010) are several vertebrae with particularly tall, narrow neural spines. It is this distinctive anatomy that explains the new name chosen by David Norman for this species: it’s now Hypselospinus. Like the holotype vertebrae of Hypselospinus, these vertebrae have sub-cylindrical centra and prominent posterior chevron facets, so their referral to the same taxon is (hopefully) secure. Again, Carpenter & Ishida (2010) seem to have been unaware of Norman’s plans to use the name Hypselospinus, and have published Wadhurstia for the same taxon. It instantly becomes another objective junior synonym.

And yet more and more…

I mentioned early on that Iguanodon sensu lato has been reported from North America. Remains attributed to Iguanodon sensu lato have been on record since 1975 when the species I. ottingeri was named for some scrappy remains that can’t really be diagnosed. More recently, Weishampel & Bjork (1989) named I. lakotaensis for a partial skull from the Barremian Lakota Formation of South Dakota. Somewhat ironically, they described it as "the first indisputable remains of Iguanodon from North America". This is ironic because I. lakotaensis doesn’t look much like Iguanodon at all (that is, like I. bernissartensis) and falls well away from it in phylogenetic analyses. Paul (2008) argued that it definitely deserves generic distinction and hence a new name, and he renamed it Dakotadon lakotaensis [its reconstructed skull is shown here, from Paul (2008)].

Yet another new species has recently been pulled out of Iguanodon sensu lato: Carpenter & Ishida (2010) recognise Proplanicoxa galtoni for an Isle of Wight, Wessex Formation pelvis. This specimen had originally been referred to Vectisaurus valdensis but later placed in Iguanodon (Norman 1990). The name Vectisaurus is not presently in use as the type specimen is regarded as indeterminate. Yet again, it’s the ilium that’s particularly distinctive: its postacetabular region isn’t vertically aligned as it ‘should’ be, but slants outwards towards the horizontal. This condition could be regarded as antecedent to the fully horizontal postacetabular region of the North American iguanodontian Planicoxa, hence Carpenter & Ishida’s (2010) choice of generic name for this proposed taxon.

Consensus and controversy: to the future

If by now you’ve struggled to keep up, we have (1) Iguanodon bernissartensis (the only species definitely included in Iguanodon), (2) Mongolian, big-nosed Altirhinus kurzanovi, (3) Purbeck Owenodon hoggii, (4) Isle of Wight Mantellisaurus atherfieldensis, (5) controversial Belgian Dollodon bampingi, (6) Wadhurst Clay Barilium dawsoni, (7) Barilium’s objective synonym Torilion dawsoni, (8) Brickenden’s jaw-based species Kukufeldia tilgatensis, (9) tall-spined Hypselospinus fittoni, (10) Hypselospinus’s objective synonym Wadhurstia, (11) Dawson’s it-might-be-another-Barilium Sellacoxa pauli, (12) South Dakota’s Dakotadon lakotaensis, and (13) Isle of Wight, shelf-hipped Proplanicoxa galtoni.

There are other species included in Iguanodon sensu lato (including I. ottingeri, I. orientalis and I. major), plus there’s the ‘original’ species I. anglicus, but they’re generally regarded as nomina dubia, or as synonyms of other species. And you might not be surprised to hear that even this isn’t the end of it all: a few controversies and unresolved issues mean that there might be even more taxa, and more name changes, yet to come.

One of the most famous British Iguanodon specimens – the ‘Maidstone specimen’ or ‘Mantel-piece’, discovered in 1834 and obtained by William H. Bensted – is also one of the youngest, coming from the Aptian Lower Greensand Formation. Long thought (incorrectly) to be the type specimen for Iguanodon mantelli (this species name actually goes to the material already named I. anglicus), it was referred to Mantellisaurus atherfieldensis by Norman (1993). However, in some details it’s quite different from the holotype of this species: again, the ilium seems telling, as that of the Mantel-piece is particularly deep, dorsally arched, and with only a short, sub-triangular postacetabular process. Rather than resembling the ilium of M. atherfieldensis, this ilium is – suggest Carpenter & Ishida (2010) – more like that of the Asian iguanodontians Equijubus from the Albian of China and Fukuisaurus from the Hauterivian of Japan. What Carpenter & Ishida (2010) imply is that the Mantel-piece might be yet another new genus.

I mentioned earlier that Dollodon – named by Greg Paul for the gracile, long-armed iguanodontian from Bernissart – has proved controversial. David Norman has already argued that the features alleged to make it unique are all invalid, and that Dollodon can in fact be included in one of the previously recognised taxa (I assume Mantellisaurus). His full argument has yet to be published and is due to appear soon, and I know that other authors are due to re-examine this putative taxon in other publications. Carpenter & Ishida (2010), however, did follow Paul (2008) in regarding Dollodon as valid. But they argue that it’s synonymous with Iguanodon seelyi, a species named in 1882 from the Wessex Formation on the Isle of Wight, and they therefore used the newly combined name Dollodon seelyi.

However, I. seelyi wasn’t just named for an ilium, but also for a partial foot, a femur and some other remains. These bones – the femur in particular – show that I. seelyi was a large and massively robust iguanodontian. This makes it very different from IRSNB 1551 (the Dollodon bampingi holotype) and very much like I. bernissartensis. Accordingly, the idea that I. seelyi is the same thing as I. bernissartensis seems well supported, and Carpenter & Ishida’s (2010) proposal that I. seelyi and D. bampingi might be the same thing is almost certainly incorrect. Their newly formulated name – Dollodon seelyi – should therefore be consigned to history.

Given that an incredible 12 new generic names have now been proposed for species previously included in Iguanodon, and with murmurings of more on the way, it’s obvious that we’re in the middle of a massive stage of revisionism. It’s also evident that Iguanodon sensu lato really does house substantial genus-level diversity that’s been obscured by a conservative taxonomy: everyone who works on these animals is in agreement that taxa like Owenodon, Barilium and Hypselospinus at least deserve to be separated out.

Why all the social inertia?

An interesting question to ask is: if Iguanodon sensu lato really does house all this diversity, why did nobody do anything about it before? The answer mostly concerns the phenomenon known as social inertia. If certain influential people say that something is so, there’s a tendency for others to follow. In terms of its impact on the classification of animals, this phenomenon is certainly not unique to dinosaurs, or to palaeontology: the classification of living amphibians, snakes and birds has been notoriously conservative for the same reason.

Carpenter & Ishida (2010) argue that the British Museum’s prolific Richard Lydekker – well known for his production of highly influential catalogues of fossil vertebrates – was almost single-handedly responsible for promoting the view that Iguanodon should house such disparate animals as Barilium and I. bernissartensis. Lydekker’s ‘super inclusive’ view of Iguanodon was almost universally adopted, even though Lydekker himself noted that animals like Barilium and Hypselospinus were ‘intermediate’ between Camptosaurus and I. bernissartensis (Lydekker 1888).

The notion of the ‘super inclusive’ Iguanodon was maintained throughout the 20th century, mostly because little work was published on the species concerned. Within recent decades, however, Iguanodon expert David Norman has also maintained Lydekker’s view across his body of published work (e.g., Norman 1987, 2004); at least, until this year (Norman 2010). There are indications from some of Norman’s papers (e.g., Norman & Barrett 2002) that he always intended to publish a revised taxonomy for Iguanodon. But the fact that he didn’t do this until very recently made it inevitable that other authors would start giving new genera to the various species concerned (the fact that they did is hardly a surprise: it’s been on record since 1991 at least that Greg Paul wanted to split Iguanodon sensu lato into separate genera). You might well argue that Norman simply never found the time to properly publish on this issue – a situation I very much sympathise with – and, as you can see from the accompanying photo [from here], Norman’s ‘take home’ message is that you should publish your conclusions sooner rather than later.

It only remains to be said that several of the alleged taxa discussed here are controversial, and people are going to be arguing about them for a long time. The fact that some of these putative taxa are mostly based on the shape of the ilium does worry me a little [look at all the variation in iguanodontian ilium shape shown here; from Carpenter & Ishida (2010)]. Ilium shape does seem to be useful at some level (indeed, some iguanodontian specimens thought to belong to the same species do have near-identical ilia), but the tremendous variation seen in Iguanodon sensu lato still hints at the possibility that ilium shape was inherently variable within species and genera. Indeed, observations made from other tetrapod species already lead me to suspect that the shape of limb girdle bones is somewhat ‘plastic’ and influenced by the developmental history of the individual. We await more work: it will be interesting to see how this pans out.


Carpenter, K. & Ishida, Y. 2010. Early and “Middle” Cretaceous iguanodonts in time and space. Journal of Iberian Geology 36, 145-164.

Lydekker, R. 1888. Catalogue of the fossil Reptilia and Amphibia in the British Museum (Natural History). Part 1, Ornithosauria, Crocodylia, Dinosauria, Squamata, Rhynchocephalia, and Proterosauria. British Museum (Natural History), London.

Norman, D. B. 1987. Wealden dinosaur biostratigraphy. In Currie, P.M. & Koster, E.H. (eds) Fourth Symposium on Mesozoic Terrestrial Ecosystems. Boxtree Books (Drumheller, Alberta), pp. 165-170.

- . 1990. A review of Vectisaurus valdensis, with comments on the family Iguanodontidae. In Carpenter, K. & Currie, P. J. (eds) Dinosaur Systematics: Approaches and Perspectives (Cambridge University Press, Cambridge), pp. 147-161.

- . 1993. Gideon Mantell’s “Mantel-piece”: the earliest well-preserved ornithischian dinosaur. Modern Geology 18, 225-245.

- . 2004. Basal Iguanodontia. In Weishampel, D. B., Dodson, P. & Osmólska, H. (eds) The Dinosauria, Second Edition. University of California Press (Berkeley), pp. 413-437.

- . 2010. A taxonomy of iguanodontian (Dinosauria: Ornithopoda) from the lower Wealden Group (Cretaceous: Valanginian) of southern England. Zootaxa 2489, 47-66.

- . & Barrett, P. M. 2002. Ornithischian dinosaurs from the Lower Cretaceous (Berriasian) of England. Special Papers in Palaeontology 68, 161-189.

Paul, G. S. 2008. A revised taxonomy of the iguanodont dinosaur genera and species. Cretaceous Research 29, 192-216.

Weishampel, D. B. & Bjork, P. R. 1989. The first indisputable remains of Iguanodon (Ornithischia: Ornithopoda) from North America: Iguanodon lakotaensis, sp. nov. Journal of Vertebrate Paleontology 9, 56-66.

About the Author: Darren Naish is a British palaeozoologist and writer who mostly works on Cretaceous dinosaurs and pterosaurs. He blogs about animals – living and extinct – at Tetrapod Zoology, and has just published a compilation of Tet Zoo blog articles in book form (tetrapods are four-limbed vertebrates: amphibians, reptiles, birds and mammals). Other recent books include The Great Dinosaur Discoveries and Dinosaurs Life Size. He is based at the University of Portsmouth, U.K.

The views expressed are those of the author and are not necessarily those of Scientific American.