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Wednesday, 15 January 2014

Baryonyx Begins

Baryonyx walkeri, the famous, much discussed spinosaur from Lower Cretaceous Britain. A number of goniopholidids skulk in the foreground. Prints of this image can be obtained from my print store.
I assume that most people want to read new stuff when they're browsing online, and I try to take the same attitude to my own blogging. I'd much rather offer fresh perspectives or cover rarely-discussed issues than simply rehashing the same tried and tested material that we've all picked up from elsewhere. This explains why the image above, showing the famous Lower Cretaceous British spinosaurid Baryonyx walkeri, has sat on my hard-drive since November. Drafted for a project which needed a picture of a theropod, it's been collecting dust for ages while I've been wondering what to say about it here. Problem is, we all know too much about Baryonyx. It's well known that the 1986 description of Baryonyx provided the first real look anyone had ever had into spinosaur anatomy; that spinosaurids have kinda weird, superficially-crocodile like skulls which permitted feeding on a variety of prey which included fish and other dinosaurs; that the forelimb anatomy of this group, formed of robust arms and large claws, is particularly interesting... The list of things we all know about Baryonyx goes on. Is there anything left to say about this animal which hasn't been poured over dozens of times before?

Hopefully, this
There may be one component of Baryonyx palaeontology which, while hardly unknown, at least doesn't get mentioned too often. When considering the discovery of Baryonyx, we imagine the story starting in 1986 or perhaps, in 1983, when the holotype specimen was found. Eric Buffetaut's (2010) research into early spinosaurid discoveries suggested this tale warrants a prequel however, one which started a whopping 160 years before Baryonyx was found. As with other British dinosaurs such as Iguanodon and Cetiosaurus, Baryonyx also has a long history which dates back to the first dinosaur discoveries, and it even stars A-list 19th century palaeontologist celebrities.

Our story begins around 1820, when Gideon Mantell recovered, or was given, a series of conical, sharp teeth from the same Tilgate Forest quarries which would later yield the first bones of Iguanodon. These teeth came in three flavours, all variations on a conical shape with differing degrees of slenderness and curvature. They all clearly belonged to carnivorous animals. Of interest here are teeth that Mantell characterised as being paritcularly slender, laterally compressed, with carinae on the anterior and posterior margins and distinct grooves on the lingual and labial surface. To Mantell, the teeth resembled those of long-snouted crocodiles or perhaps, at the suggestion of his mentor in comparative anatomy, William Clift, monitor lizards. Mantell first put these throughts into print in 1822 and in several other publications throughout the 1820s, favouring their crocodilian identification and making repeated positive comparisons to long-snouted Crocodyliformes such as gharials and the extinct teleosaurs.

Cuvier's (1824) illustration of Mantell's Wealden 'crocodile' tooth, left, compared to the actual specimen. From Buffetaut (2010). Image borrowed from here.
Mantell was not the first to illustrate these strange Wealden teeth however. This accolade belongs to Baron Georges Cuvier who, having been sent teeth for analysis by Mantell, proceeded to illustrate them in 1824 (above). Cuvier was not above illustrating unusual fossil specimens even when his peers had more claim to the prestige of publishing them first, having also done this with Iguanodon material. Cuvier's 1824 illustration and discussion of Mantell’s alleged crocodyliform teeth agreed with Mantell’s identification, also positively comparing them with the teeth of animals we would ultimately call teleosaurs. Far from being miffed with Cuvier partially scooping his discovery, Mantell seemed chuffed that he and Cuvier were in agreement in documents published in the late 1820s. Perhaps this came as a relief after Mantell's Iguanodon teeth were identified those of a rhinoceros by Cuvier: having one of the leading comparative anatomists in the world shoot your ideas down - even if he's wrong - can't do much for one's self esteem.

A few years later saw entry of Sir Richard Owen into discussions of Mantell’s alleged crocodyliform Wealden teeth. In Owen’s seminal Odontography, a major overview of animal teeth published from 1840-1845, he named them Crocodilius (Suchosaurus) cultridens. Like Cuvier and Mantell, Owen also made several favourable comparisons between the Wealden teeth and other fossil Crocodyliformes, including teleosaurs*. In a contemporary work, Report on British Fossil Reptiles (1842), Owen noted some subtle differences between the cross-sectional shape and carinae position of the Wealden teeth and those of crocodiles, although he maintained a crocodyliform referral for the teeth at this time. Later, Owen seemed less certain about this identification, noting in an 1878 publication that his subgenus Suchosaurus had a ‘nearer affinity or transition to the Dinosaurian order than does any of the Mesozoic Crocodilia’. This was the first clear indication that all was not as seemed with Mantell's early Wealden discovery. However, an 1884 Owen manuscript which basically rehashed his 1878 discussion of Suchosaurus, contained none of the doubt he expressed in 1878. Clearly, Owen had changed his mind about the similarity of the Mantell's 'crocodile' teeth to those of dinosaurs.

*The story becomes somewhat complicated here by the referral of a vertebra to Suchosaurus, which was suggested as early as 1888 to represent an iguanodont and remains that way today. However, this was not immediately accepted by 19th century naturalists and it is not is not always clear if they are discussing the vertebra or teeth when considering the affinities of Suchosaurus.

Most 19th century palaeontologists followed Owen’s lead in considering Suchosaurus a crocodile-like animal, but others were not convinced. Felix Plieninger (1846) and John Hulke (1979) both suggested that Suchosaurus had greater affinity with Dinosauria than other reptiles. Plieninger was definitely basing his discussion on the Suchosaurus teeth, although it's not clear whether Hulke is considering these or the vertebra also referred to this taxon. Other authors, such as Heinrich Georg Bronn (1849) considered Suchosaurus a reptile of uncertain identity, at least for a while (he later agreed with Owen). The mystery surrounding Suchosaurus was more-or-less left there however, as these were among the last discussions Suchosaurus was to receive in palaeontological literature. Although a second Suchosaurus species was named in 1897-1898 for a Portuguese tooth, the taxon fell out of regular use in discussions of Wealden animals – even its Crocodyliformes – in the 20th century.

Back to the future
Fast forward to the 1980s and, to everyone’s delight, Baryonyx walkeri was discovered and described from a fairly complete skeleton found in Surrey. Among the many surprises associated with its discovery were its slender, laterally compressed and grooved teeth, a distinctive dentition that saw any similar isolated tooth from the Wealden being allocated to Baryonyx (e.g. Martill and Hutt 1996 - see below). It didn’t take long the penny to drop: by 2003 it was realised that the teeth of Baryonyx were very, very similar to those of Suchosaurus (Milner 2003). After nearly 200 years, the real identity of Suchosaurus was revealed: a spinosaurid theropod. This meant that, far from being relatively new discoveries for the Wealden, spinosaurs were actually among the very first animals to be documented from the British Lower Cretaceous. Hats must be tipped to the likes of Plieninger and Hulke and, to a lesser extent, Owen and Bronn, for their insightful taxonomic comments on the very fragmentary material they had to work with. Each of them saw, to greater and lesser extents, past the crocodile-like appearance of the Suchosaurus teeth to suggest they may be crocodile-like dinosaurian reptiles. That's pretty good going considering what they had to work with.
Isolated teeth attributed to baryonychine theropods from the Wessex Formation, Isle of Wight. From Martill and Hutt (1996).
Unsurprisingly, several authors have mentioned the likelihood that Suchosaurus and Baryonyx are one and the same (e.g. Buffetaut 2010; Mateus et al. 2011). Although their teeth do differ in subtleties of carinae and groove development, it has long been noted (even by Mantell way back in the 1820s) that such particulars of dentition are readily worn away in life or taphonomy. It is therefore possible, maybe probable, that Suchosaurus and Baryonyx are synonyms, the teeth of the former simply being worn versions of the latter. If so, the animal we know as Baryonyx was actually one of the first dinosaurs ever found. But if it's likely that Baryonyx and Suchosaurus are the same animal, why are we still talking about Baryonyx instead of resurrecting Suchosaurus as the Wealden spinosaurid genus? There are several reasons. Firstly, the dentition of Baryonyx is not unique among baryonychines, creating the (admittedly unlikely) possibility of Suchosaurus being synonymous with another baryonynchine taxon. A second possibility, that Suchosaurus is a second Wealden baryonychine, presents another problem. But most important is our third reason: the type material of Suchosaurus possesses no real defining features, giving us nothing to diagnose this genus with. This echoes the situation of many ‘classic’ dinosaur species of course, and palaeontologists sometimes work taxonomic magic to transfer their name to other, diagnostic specimens. There seems little reason to do this for Suchosaurus however, it being a fairly obscure and under-discussed animal for much of the 20th century. Unlike Iguanodon or Allosaurus, which have received name transfer treatment in the past, Suchosaurus is not a familiar animal with a poor type specimen, so it isn’t too criminal to refer the type specimen to Spinosauridae indet. and let the name Suchosaurus slip into the nomen dubium realm of obscurity.

So there we have it, then: Baryonyx and Wealden spinosaurids have, from a certain point of view, been known for as long as any other dinosaur you care to mention, and some folks had an inkling of spinosaurid's superficially crocodile-like morphology even when they were only known from teeth. For more on this story and the early discoveries of spinosaurids, be sure to check out Buffetaut (2010), Mateus et al. (2011). To find out happened to the rest of the 'crocodile' teeth handled by Mantell in the 1820s, read Salisbury and Naish (2011). Further, exciting news on the developing science of Wealden spinosaurs can be found in this near-recent blog post over at Mark Wildman's Saurian.

References
  • Bronn, H. G. (1849). Index Palaeontologicus, 2. Abetheilung. Schweizerbart, Stuttgart.
  • Cuvier, G. (1824). Recherche sur les ossemens fossiles, tome V, 2ème partie. Dufour et E. d'Ocagne, Paris.
  • Hulke, J. W. (1879). Vectisaurus valdensis, a new Wealden dinosaur. Quarterly Journal of the Geological Society, 35(1-4), 421-424.
  • Mantell, G. A., & Mantell, M. A. (1822). The fossils of the South Downs; or illustrations of the geology of Sussex. Lupton Relfe, London.
  • Martill, D. M., & Hutt, S. (1996). Possible baryonychid dinosaur teeth from the Wessex Formation (Lower Cretaceous, Barremian) of the isle of Wight, England. Proceedings of the Geologists' Association, 107(2), 81-84.
  • Mateus, O., Araújo, R., Natário, C., & Castanhinha, R. (2011). A new specimen of the theropod dinosaur Baryonyx from the early Cretaceous of Portugal and taxonomic validity of Suchosaurus. Zootaxa, 2827, 54-68.
  • Milner, A. C. (2003). Fish-eating theropods: a short review of the systematics, biology and palaeobiology of spinosaurs. Journadas Internacionales sobre paleontologiá de Dinosaurios y su Entoro, 2, 129-138.
  • Owen, R. (1840–1845). Odontography. Hippolyte Bailliere, London.
  • Owen, R. (1842). Report on British fossil reptiles. Part II. Reports of the meetings of the British Association for the Advancement of Science, 11, 61-204.
  • Plieninger, T. (1846). Über ein neues Sauriergenus und die Einreihung der Saurier mit flachen, schneidenden Zähnen in eine Familie. Jahreshefte des Vereins für vaterländische Naturkunde in Württemberg, 2(1), 148-154.
  • Salisbury, S. W. & Naish, D. (2011). Crocodilians. In Batten, D. J. (ed.) English Wealden Fossils. The Palaeontological Association (London), pp. 305-369.

Friday, 10 January 2014

The abundant, apocalypse-surviving, rocket-propelled Valdosaurus: Britain's most interesting dinosaur?

Two Wealden dryosaurids Valdosaurus canaliculatus, argued below to be striking and exciting animals that we should all be enthused about, shown here milling about a Lower Cretaceous woodland, not doing very much. Prints of this image are available here.
The wide interest palaeontology receives is largely because many fossil animals are spectacularly charismatic, their popularity enhanced by large body size and terrific anatomical features. Not all fossil animals are well adorned with attention-grabbing anatomy, of course. Some animals, even those belonging to particularly famous groups like marine reptiles, pterosaurs or dinosaurs, are devoid of immediately-striking features and proportions, and may be so 'average' that they rarely receive dedicated attention outside of technical literature. Step forward the most middling, most average and downright beigest of all ornithopods, the dryosaurids.
Mounted skeleton of the Jurassic dryosaurid Dysalotosaurus lettowvorbecki in the Humboldt Museum's excellent dinosaur hall. Photo by Masur, from Wikimedia Commons. Beige.

The Jurassic-Cretaceous clade Dryosauridae contains animals which, at first glance, represent the mean of dinosaur extremes. Most of these animals, early offshoots of the ornithopod group Iguanodontia, are small or medium-sized, at 3-4 m long. They have fairly short, unremarkable ornithopod skulls and small grasping hands which are, well, quite a lot like those of other small-ish ornithopods. They possess no horns, claws or even particularly arresting teeth, their jaw tips being entirely toothless and cheek teeth, as with most ornithopods, being leaf-shaped and coarsely serrated. This restricted them to a diet of leaves and other herbage, and there's not even any strong hints of omnviory. In short, they aren't especially cute, aren't really spectacular, and, to look at one, it's hard to find a truly distinctive, memorable feature. Perhaps this is why the only time they're given decent palaeoartistic renditions is when they're being pursued by a more striking theropod, and are relegated to bit-parts and cameo roles in most dinosaur books and palaeodocumentaries.

You knew there was a 'but' coming
As usual, appearances can be deceiving. There's actually a lot more going on with these dinosaurian plain Janes than casual glances may suggest. If you stare at a skeleton of a dryosaurid long enough, the pronounced difference in their fore and aft proportions becomes inescapable. The head, neck, forelimbs and torso belong to much smaller dinosaurs than their legs and tail, which are proportionally much longer and more robust than we'd expect from their anterior skeleton. This enlarged posterior region does not seem to reflect expansion of the gut either, as it does in some other herbivorous dinosaur species. Rather, the pelvic bones betraying the depth of the belly are fairly shallow, suggesting the legs and tail are long for the sake of being long, not to accommodate increased body size or torso girth. Of further note are dryosaurid feet, which are not only surprisingly long and gracile but also possess a mere three toes, a contrast to the primitive ornithopod condition of four. Their femora have a very long, finger-like fourth trochanter (the variably shaped process on the posterior surface of dinosaur femora which anchored posterior hindlimb muscles) and a very large and prominent scar occurs where the famous caudofemoralis musculature anchored to the thigh. As in all dinosaurs, this muscle ran to the anterior portion of the tail (discussion here), the elongation and deepening of which permitted a great deal of room for its attachment.

Put together, these femoral and caudal features are clear indicators of large hindlimb retractor muscles and, along with the svelte body and long legs, betray the habit of very rapid running. Indeed, the cursorial features of dryosaurids are so marked that they were probably among the swiftest of all ornithischians, a trait which may explain their conspicuous lack of ornament, armour and weaponry compared to other dinosaurs. Such elaborations are not without use of course, but they are just extra weight for a running animal. Thus, rather than thinking of dryosaurid skeletons as boring, we should view them as the bones of lithe, streamlined, athletes. I wonder if we'd consider these animals dinosaurian Mr. Averages if we could see them in life, their running abilities likely making them very striking and conspicuous animals in many Jurassic and Cretaceous ecoystems.

Britain's Cretaceous dryosauid: Valdosaurus
Dryosauridae was a widespread group with fossils occurring in Africa, Europe, North America and New Zealand. One British dryosaurid, the Wealden Supergroup taxon Valdosaurus, has become particularly interesting in recent years. Bones ultimately referable to this genus were described from the Wessex Formation, Isle of Wight, as early as 1888 by Richard Lydekker. They were not recognised as being Dryosaurus-like until the 1970s however, the decade which also Valdosaurus canaliculatus receive its name (Galton 1977). A second Valdosaurus species was erected for Nigerian material by Galton and Taquet (1977), but has since been moved to its own genus, Elrhazosaurus. It took a while to appreciate that dryosaurids were a unique radiation of ornithopods rather than merely oversized versions of Hypsilophodon, with the first inclinations of this idea only appearing in the 1980s. Somewhat surprisingly, the monophyly of Dryosauridae was untested for many decades, but has recently been borne out under phylogenetic analysis (e.g. McDonald et al. 2010; Barrett et al. 2011). Alleged Valdosaurus material from Spain and Romania seemed to extend the range of this animal across Europe, but these referrals have not withstood scrutiny: Valdosaurus remains UK-only for the time-being.
Holotype femur of Valdosaurus compared to those of other small ornithopods. Note the size of the scar for M. caudofemoralis, 'p', in Valdosaurus. Figure and caption from Galton and Taquet 1977.
As usual for a Wealden dinosaur, Valdosaurus isn't known from a huge amount of material. Various bits and pieces have been referred to this taxon for decades but, like many other British dinosaurs, recent work has tightened up what can and cannot be referred to it (Barrett et al. 2011). We now consider Valdosaurus known primarily from hindlimb and pelvic remains, along with some vertebrae and portions of the pectoral girdle. These remains suggest Valdosaurus was a large dryosaurid with a body length of about 4 m, and a particularly well-distinguished, large caudofemoralis muscle scar on its femur (Barrett et al. 2011). Does this mean it had a particularly big set of hindlimb protractors and was especially fleet-footed? Maybe, but it's hard to say in the absence of more complete specimens. Still, assuming the rest of its proportions were like those of other dryosaurids, it was probably the fastest ground animal in the Wealden Supergroup and capable of outrunning any contemporary predator.

Valdosaurus: apocalypse survivor?
What makes Valdosaurus especially interesting and unique among Wealden dinosaurs is its longevity. The Wealden dinosaur fauna is essentially divided into two groups: a 'lower' and 'upper' assemblage. The 'upper' contains the most widely known taxa, animals such as Iguanodon, Polacanthus, Baryonyx, Hypsilophodon, Neovenator and the like. These animals occur in rocks dating to the Hauterivian and Barremian, about 133-125 Ma. The 'lower' fauna features slightly lesser known taxa: Barilium, Hylaeosaurus, Pelorosaurus and so on, all of which are of slightly older, late Berriasian-Valangian vintage (138-133 Ma). The transition between these faunas is not continuous, with a series of poorly-fossiliferous Hauterivian-aged strata - representing about 4 million years - occurring between more productive Wealden rocks. William Blows has proposed across several papers that this gap may not merely be an artefact of sampling (although this remains possible), but a low-level extinction event that wiped out the 'lower' fauna and allowed the 'upper' assemblage to repopulate (e.g. Blows 1998). Not all agree that this apocolyptic interpretation is the most likely explanation of the Hauterivian Wealden dinosaur gap - some argue that we just need to look harder for more bones. Whatever, something clearly caused a lot of upset for the Wealden dinosaur fauna in the Hauterivian, resulting in significant reorganisation of dinosaur faunas by late Hauterivian-Barremian stages: old taxa disappeared, and new ones - sometimes entirely different clades - replaced them.

How does Valdosaurus fit into this? Although classically part of the 'upper' Wealden assemblage, Valdosaurus is now known to occur in the 'lower' fauna (e.g. Blows 1998; Naish and Martill 2008; Barrett et al. 2011). This makes it the only dinosaur to span the Hauterivian gap, suggesting it lived through whatever turned out the older Wealden dinosaurs and brought in the new. It must be stressed that the older Valdosaurus remains are not terribly complete and may not be V. canaliculatus proper, but they are extremely similar to Valdosaurus remains from the 'upper' Wealden and at worst, they represent a very close relative, perhaps a direct ancestor. Of further interest here are recent reconsiderations of Valdosaurus/dryosaurid abundance within the Wealden. Dryosaurs were once considered rare in these deposits, but ongoing appraisals of British Lower Cretaceous dinosaurs suggest they were far more abundant than once realised, and populous enough to question the dominance of iguanodonts in Wealden herbivore palaeoecology (Barrett et al. 2011). Given that Valdosaurus represents a good chunk of the Wealden dryosaur material, we may take this as a sign that it was not only a long-lived taxon, but also one that had a strong foothold in Wealden habitats.

The longevity and abundance of Valdosaurus may see it being considered one of the greatest 'success stories' of Britain's Lower Cretaceous dinosaurs, and the implications of its success to Wealden palaeoecology undoubtedly make it one of the more interesting members of the assemblage. A key question - why did it persevere when all other species didn't? - now hangs over this unassuming animal's head. With work on the Wealden fauna constantly ongoing, it seems like a case of 'watch this space' to see just how our new perception of Wealden dryosaurids will influence broader interpretations of Britain's Lower Cretaceous dinosaurs. Exciting times indeed, then, and a far cry from the perception of Valdosaurus and dryosaurids as easily-forgotten, 'average' dinosaurs: at least one of them is, from a certain perspective, one of the most interesting dinosaurs in the UK.

References
  • Barrett, P. M., Butler, R. J., Twitchett, R. J., & Hutt, S. (2011). New material of Valdosaurus canaliculatus (Ornithischia: Ornithopoda) from the Lower Cretaceous of southern England. Special Papers in Palaeontology, 86: 131–163.
  • Blows, W. T. (1998). A review of Lower and Middle Cretaceous dinosaurs of England. New Mexico Museum of Natural History and Science Bulletin, 14, 29-38.
  • Galton, P. M., (1977). The Upper Jurassic dinosaur Dryosaurus and a Laurasia-Gondwana connection in the Upper Jurassic. Nature, 268(5617): 230-232
  • Galton, P. M., & Taquet, P. (1982). Valdosaurus, a hypsilophodontid dinosaur from the Lower Cretaceous of Europe and Africa. Geobios, 15(2), 147-159.
  • McDonald, A. T., Kirkland, J. I., DeBlieux, D. D., Madsen, S. K., Cavin, J., Milner, A. R., & Panzarin, L. (2010). New basal iguanodonts from the Cedar Mountain Formation of Utah and the evolution of thumb-spiked dinosaurs. PloS one, 5(11), e14075.
  • 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.

Thursday, 2 January 2014

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.