Remembering Iguanodon

Retrosaur Iguanodon, c. 1854. Based, of course, on the sublime work of Benjamin Waterhouse Hawkins.

Space year 2014 marks the 189th anniversary of the naming of a dinosaur icon, Iguanodon. The major beats of the discovery and research history of this Lower Cretaceous herbivore are so well-established within palaeontological lore that most readers will need little reminder of it here. We all know that Iguanodon was first known from large, iguana-like teeth found in southern England in 1822, supposedly by Gideon Mantell's wife, which we all also know is widely considered an embellished tale: the teeth were probably found by Mantell himself or quarrymen. Equally familiar is Mantell's naming of Iguanodon in 1825 with the first specific name given to this genus, anglicus, added by Friedrich Holl in 1829. As the second dinosaur to be named, Iguanodon was part of the trio of dinosaur genera used by Richard Owen to name Dinosauria in 1842 and was reconstructed alongside its cousins, Megalosaurus and Hylaeosaurus, by Richard Owen and Benjamin Waterhouse Hawkins as an awesome dinosaurian rhino in 1854. Discoveries of more complete Iguanodon remains, first in Britain and then in the coal mines of Bernissart, Belgium, led to a reconsideration of this bauplan. The most extensive work on this front was conducted by Louis Dollo in the 1880s, who took the complete Iguanodon skeletons from Bernissart - among the first complete dinosaurs known from anywhere in the world at that time - and created the famous 'kangaroo' posture for Iguanodon, broken tails and all, which dominated reconstructions of this animal for the next century. It was not until the 1980s that Iguanodon adopted the appearance of the facultatively bipedal, horizontally-backed ornithopod we know today. So far, so familiar.

Undoubtedly, Iguanodon is a 'classic' dinosaur, and has been a mainstay of popular dinosaur literature for decades. Other dinosaur species named in the early 1800s have not enjoyed the same treatment (Thecodontosaurus, Ceitiosaurus and Hylaeosaurus for instance, are not household names), so its popularity is not just a result of it being one of the first dinosaurs known. Most of us can probably remember a key Iguanodon depiction from our childhood dinosaur books, magazines or films - or from a Love in the Time of Chasmosaurs vintage palaeoart post if you're not yet through puberty - with it stood upright and, of course, giving an irrepressible thumbs-up with its famous thumb spike. These Mesozoic Fonzies, which diehards always knew came in big (I. bernissartensis) and small (I. atherfieldensis) flavours, wouldn't stop manually approving their surroundings even when being attacked by passing generic 'megalosaurs'. Final revisions to its anatomy - an aloft tail and quadrupedal stance - have been drifting into popular depictions for years now, replacing MesoFonz with a heavyset herbivore often depicted living in herds and browsing at different levels. While its lack or truly bizarre anatomy or ferocity may have prevented Iguanodon from ever being the most famous of dinosaur species, there's little doubt that it's held a long-term place in palaeo-pop culture.

All good things...
At least, until recently. If the internet palaeo scene is anything to go by, Iguanodon seems to be sliding down the popularity pole at the moment. It just doesn't seem to be the topic of much conversation any more, or even artwork. Feathered theropods, weird sauropods, horned dinosaurs and even hadrosaurs - boring old hadrosaurs - seem to have stolen the limelight. Perhaps this is because our taxonomic and palaeobiological perceptions of many prehistoric animals have radically changed in recent years whereas Iguanodon, frankly, has remained rather static. It's a bit too familiar. Dinosaur palaeontology has changed radically in the last few decades, but it's changed around Iguanodon, which has done little more than tip forward a little since the 1980s. Discussions about feathers, postures, weird soft-tissue details and whatnot have passed it by entirely, and even a relatively recent shake-up of its taxonomy, where the Cretaceous-straddling, globe-spanning monster-Iguanodon genus was carved up into multiple genera spread across time and space (see Darren Naish's Scientific American articles here, here and here for details) did little to revive public interest in one of our longest serving and best-known dinosaurs. Iguanodon seems to be a dinosaurian washed-up Golden Age movie star: once great, now rarely mentioned, and only wheeled for nostalgia.

The gossip magazines would have a field day.

Behind the scenes, however, Iguanodon or, more correctly, 'iguanodonts' are becoming more and more interesting. Far from large, bland and overly-familiar ornithopods, the modern concept of iguanodonts comprises several distinct Lower Cretaceous species with markedly different bauplans which created complex herbivore communities. Their anatomy varied in many aspects other than simply size - even their famous thumb spikes are actually quite disparate - and functionality must have been equally diverse. The very evolution of iguanodonts is also more complex than we thought: rather than forming a clear group of ornithopods, iguanodont taxa seemingly comprise a messy, not-fully-understood bush of species on the ornithopod branch trunk leading to true hadrosaurs (e.g. McDonald 2012a). Thus, there is no truly correct term for a group comprising Iguanodon and its close relatives: 'iguanodont' is used here in a vernacular sense. In short, it seems that iguanodonts have fallen off the popular radar just as they're getting more interesting and worthy of attention

Iguanodonts: the undiscovered country
At the heart of this newfound complexity is the aforementioned reappraisal of iguanodont diversity. It's worth stressing that the charge to slay the waste basket monstergenus Iguanodon, started by Norman and Barrett (2002) and followed by the likes of Paul (2008), Norman (2010), Carpenter and Ishida (2010), Naish and Martill (2008), McDonald et al. (2010), McDonald (2012a, b) and others, was not a case of splitting minor taxonomic hairs. Unlike the differences which separate many fossil animals, most taxa pulled from Iguanodon are characterised by radically different morphology which would be obvious even in life. In Britain alone, the handful of species recognised as various members of Iguanodon may now comprise as many as nine genera (not counting objective synonyms). It's well known that Iguanodon is now monospecific, containing only the giant species I. bernissartensis. In the UK at least, this is principally known from the Wessex Sub-basin of the Wealden Supergroup of the Isle of Wight, although it also occurs in the Weald Sub-basin of Surrey, Sussex and Kent (below). It was joined in both basins by Mantellisaurus, the smaller iguanodont once called Iguanodon atherfieldensis and, in the Wessex, by two other possible taxa: Proplanicoxa galtoni and Dollodon bampingi. All but Proplanicoxa galtoni are known from elsewhere in Europe, which cannot be said for other British iguanodonts Barilium dawsoni*, Hypselospinus fittoni, Sellacoxa pauli and Kukufeldia tilgatensis from the Weald Sub-basin, also of the Wealden Supergroup of Sussex and Surrey. These animals are geologically older than the more familiar Iguanodon and Mantellisaurus and, for now at least, do not seem to overlap stratigraphically. A further genus, Owenodon hoggi, has been named for "Iguanodon" material from the British Purbeck Group. A number of other Asian and North American genera have also been pulled from Iguanodon, but the British record seems unusually diverse and implies that multiple iguanodonts existed in the same basins. Admittedly, exactly how many European iguanodont taxa are valid remains uncertain - there are arguments for it being over-split and overly-conservative - but even a relatively cautious assessment suggests several iguanodont faunas evolved in ancient Britain.

*Fascinating aside: according to Norman (2011a, b) there's a good chance that the original Iguanodon teeth belong to Barilium. There's not much we can do about this now - after years of confusion over what Iguanodon is, the name has been irreversibly transferred to I. bernissartensis. While most agree this was one appropriate cause of action to take - most of us have always thought of this species as the 'classic' Iguanodon - there are lots of niggles and issues with the choice of bernissartensis as the surrogate type species of Iguanodon. The similarity of the original 'I. anglicus' teeth to Barilium is just another hangover from the excessive lumping that Iguanodon experienced in its first 180 years of recognition.

Simplified overview of British iguanodont distribution. The taxa listed here do not include recently named objective synonyms and includes several genera which some authors (e.g. Norman 2011a; McDonald 2012) would happily remove. I. anglicus, the original Iguanodon and nomen dubium, is included for interest only. Silhouettes provide very rough proxies for maximum taxon size to show the possible nature of iguanodont faunas, borrowed from Paul (2008). Hat tip to Bill Wimbledon for some pointers on Wealden chronostratigraphy.

Quite how these contemporary animals did not trip over each others ecological toes remains to be established. Some truth to the 'classic' view of Iguanodon species occurring in different size classes remains, with most newly recognised species equating to large- or medium-size dinosaurian herbivores. What is now very apparent, however, is that size is only one way in which these animals differ. The large iguanodont Barilium, for instance (below), is about the same length as I. bernissartensis (10-12 m) but is much more heavily built, with proportionally heavyset hips, shoulders, limb bones, a chunky anterior tail region and very tall neural spines along much of its back. While it's difficult to call I. bernissartensis a gracile creature, its bones are certainly more svelte than those of Barilium: its limbs are longer, its vertebrae lower, and its limb girdles less stocky. A similar story is echoed in the smaller iguanodonts which lived alongside the giants: Hypselospinus, contemporary of Barilium, was a relatively small (about 6 m long) but stocky species, with chunky limb bones and a thick shoulder girdle. By contrast, other 'small' iguanodonts - such as the 6- 7 m long Mantellisaurus and Dollodon - were rather gracile, with slender limbs and relatively delicate hands. Despite its robust body, Hypselospinus shared a relatively gracile jaw construction with other smaller iguanodonts. With many further differences in their fine anatomy, a clear message can be seen: iguanodonts were not merely resized variants of the same bauplan rolled out over the Lower Cretaceous. Quite how their different anatomies plugged into their palaeoecology and niche differentiation remains to be established, but its possible - maybe probable - that their anatomical differences reflect different foraging strategies, habitat preferences and routine predation responses. Perhaps the geologically younger, slender variants were quicker on their feet than their rotund forebears? Did the more robust species spent more time locomoting quadrupedally? No-one really knows at the moment, but there's clearly a lot of interesting things going on here and a lot of interesting research to be done.

Barilium dawsoni, a large and very robust iguanodont from the Valanginian of Sussex, caked in dried mud. This stunted pollex of this animal, which was probably quadrupedal most of the time, means it'd be hard-pressed to give a thumbs up even if it wanted to. A flock of 'Ashdown maniraptorans', tiny, poorly known theropods no larger than an Eurasian magpie, add scale (see Naish and Sweetman 2011 for details).

It would be remiss of us to not mention that the most famous iguanodont feature - their thumb spikes - are also far from uniform in size or construction. The function of the iguanodont pollex has long proved controversial, but a role in stabbing generic theropods in the neck is a common assumption. This long-held assumption is questioned by the range of morphologies associated with the pollex however. Most of us are familiar with the general construction of the iguanodont pollex thanks to oft-reproduced images of the Iguanodon hand, such as...

Left Iguanodon bernissartensis manus. Image from here.

Here, the pollex is conical and fairly large, but remains detached from the carpal block (iguanodont wrist bones fuse into a single unit with age). Thus, the pollex retains an ability to move somewhat. The pollex of Mantellisaurus is generally similar to that of Iguanodon, except that it is much, much smaller - probably far too small to be used as an effective predator deterrent. By contrast, the pollex of another small iguanodont, Hypselospinus, was proportionally large and robust, being about 40% as long as the forearm. Rather than being truly conical, the pollex of Hypselospinus was laterally compressed and tightly attached to the carpal block so little or no flexion was possible. The thumb of fatso Balirum was actually fused to the block itself, and is of further note for being incredibly short: Barilium would struggle to give a satisfactory 'thumbs up' to anyone. So again, we see evidence of diversity in these unassuming dinosaurs: pollex size, shape, flexion and reinforcement all vary across iguanodont taxa. We may take this as a sign that thumb spike function was also variable across iguanodonts, so there may not be one single explanation for their existence. The tight pollex articulations of some species seemingly make the pollex part of the antebrachial functional unit than the hand, and are strangely reminiscent of the carpometacarpal knobs and spurs of many birds (see - again - a TetZoo series on this topic, starting here). Alas, the function of many bird hand spurs are not well researched, but the general consensus - supported by direct evidence in many cases - is that they're primarily used in combat and aggressive behaviours, much of it intraspecific. In some cases, they may even be used to make noise when slapped against the flanks of their owners. Who knows: perhaps iguanodonts with tightly welded, inflexible thumb spikes used their pollices in a similar way. But what of species with flexible thumb spikes? Could they be used as weapons too? If so, how come the large pollex of Iguanodon was not fused to the carpus when the large thumb of Hypselospinus is? Did that make it a less effective weapon? And what was Mantellisaurus using that piddling little thumb spike for, if anything? Questions, questions, questions...

The bit where I stop writing
In sum, while it would be silly to say that iguanodont science is undergoing anything like a revolution or renaissance, there's certainly a lot of tinkering going on and the results are exciting whatever your specific taste in palaeontology - taxonomic, functional, or palaeoecological. Granted, the outcome of these ongoing studies are not going to make newspaper headlines, but if you're interested in dinosaur palaeobiology - and you are if you've read this far - then this should be very cool, interesting stuff. If the apparent decline in public interest for iguanodonts is because many of us consider them overly-familiar, then we need to think about changing that attitude. Far from being 'done to death', after many decades of fairly static interpretation, iguanodont science is becoming more interesting than ever.

For an easy to access, relatively up to date and inexpensive look at a bunch of iguanodonts, you could do a lot worse than checking out Dave Norman's chapter on ornithopods in English Wealden Fossils (Norman, 2011b). Further brief musings on the decline of a dinosaur celebrity are provided in this post on Stegosaurus.

References

• Carpenter, K., & Ishida, Y. (2010). Early and “Middle” Cretaceous iguanodonts in time and space. Journal of Iberian Geology, 36(2), 145-164.
• Paul, G. S. (2008). A revised taxonomy of the iguanodont dinosaur genera and species. Cretaceous Research, 29(2), 192-216.
• McDonald, A. T. (2012a). Phylogeny of basal iguanodonts (Dinosauria: Ornithischia): an update. PloS one, 7(5), e36745.
• McDonald, A. T. (2012b). The status of Dollodon and other basal iguanodonts (Dinosauria: Ornithischia) from the Lower Cretaceous of Europe. Cretaceous Research, 33(1), 1-6.
• McDonald, A. T., Barrett, P. M., & Chapman, S. D. (2010). A new basal iguanodont (Dinosauria: Ornithischia) from the Wealden (Lower Cretaceous) of England. Zootaxa, 2569, 1-43.
• Naish, D., & Martill, D. M. (2008). Dinosaurs of Great Britain and the role of the Geological Society of London in their discovery: Ornithischia. Journal of the Geological Society, 165(3), 613-623.
• Naish, D., & Sweetman, S. C. (2011). A tiny maniraptoran dinosaur in the Lower Cretaceous Hastings Group: evidence from a new vertebrate-bearing locality in south-east England. Cretaceous Research, 32(4), 464-471.
• Norman, D. B. (2010). A taxonomy of iguanodontians (Dinosauria: Ornithopoda) from the lower Wealden Group (Cretaceous: Valanginian) of southern England. Zootaxa, (2489), 47-66.
• Norman, D. B. (2011a). On the osteology of the lower Wealden (Valanginian) ornithopod Barilium dawsoni (Iguanodontia: Styracosterna). Special Papers in Palaeontology, 86, 165-194.
• Norman, D. B. (2011b). Ornithopod dinosaurs. In: Batten, D. J. (ed.) English Wealden fossils. The Palaeontological Association (London), pp. 407-475.
• Norman, D. B., & Barrett, P. M. (2002). Ornithischian dinosaurs from the lower Cretaceous (Berriasian) of England. Special Papers in Palaeontology, 68, 161-190.