ADVERTISEMENT
  About the SA Blog Network













Guest Blog

Guest Blog


Commentary invited by editors of Scientific American
Guest Blog HomeAboutContact

The explosion of Iguanodon , part 2: Iguanodontians of the Hastings Group


Email   PrintPrint



Iguanodon of tradition (or Iguanodon sensu lato, if you will) was a huge, sprawling monster, containing numerous species spread across about 40 million years of geological history. Welcome to the second article in this series (part 1 here). In the previous article we looked at the Purbeck Limestone iguanodontian Owenodon – originally named as a species of Iguanodon in 1874 – and at the two Isle of Wight forms named by Greg Paul – Mantellisaurus and Dollodon – both of which were, also, long included in Iguanodon sensu lato.

Most of the iguanodontians conventionally included in Iguanodon – including Gideon Mantell’s original material, and the more recently described forms from the Isle of Wight – come from a Lower Cretaceous British unit called the Wealden Supergroup, or from its continental equivalents [the rather confusing terminology of the Wealden is shown below; note that the system I show here is not used universally. The image above shows an assortment of English iguanodontian fossils and their places of discovery; from Naish & Martill (2008)]. Iguanodontians like the ones we looked at last time – Iguanodon bernissartensis, Mantellisaurus and Dollodon – are all from the ‘upper part’ of the Wealden and are mostly from the Barremian stage of the Lower Cretaceous (that is, they’re about 130 million years old).

But several other species included in Iguanodon sensu lato are from the ‘lower part’ of the Wealden, and are hence about 10 million years older (they’re mostly from the Valanginian stage of the Lower Cretaceous). They’re pretty different from Iguanodon bernissartensis: Norman & Barrett (2002) noted that their reappraisal of Owenodon "calls into question the affinities of other lower Wealden taxa" while Naish & Martill (2008, p. 617) said of these species that "it can be doubted that they are congeneric with [I. bernissartensis]".

One of these ‘lower Wealden’ animals is Iguanodon dawsoni, named by Richard Lydekker in 1888 for vertebrae and pelvic and hindlimb bones from the Wadhurst Clay Formation of Hastings, East Sussex. These bones (which belong to a single individual) represent a large, heavily built iguanodontian. Its ilium is distinctive, being dorsally convex in profile and with a thick dorsal margin. The postacetabular part of the ilium is simple, looking like a posterior extension of the body of the blade (this contrasts with many younger iguanodontians, where the postacetabular part possesses a dorsally inclined component).

These features – and others – make I. dawsoni a very different animal from I. bernissartensis; it’s so different that there’s little doubt that it does indeed deserve to be extracted from Iguanodon sensu lato and given its own name. David Norman (2010) recently did this, and used the new name Barilium (meaning ‘heavy ilium’) for the species. Iguanodon dawsoni is, therefore, now Barilium dawsoni.

A second new generic name for Barilium has just been published by Carpenter & Ishida (2010): these authors term it Torilion (meaning ‘rounded ilium’: a reference to that simple postacetabular portion). Given that Norman has been using the name Barilium at conferences since early 2009, I’m a bit surprised to see Carpenter and Ishida publishing a second one (charitably, I can only assume that they don’t keep up with the world of dinosaur gossip). Whatever, Torilion immediately becomes an objective junior synonym of Barilium.

The Old Roar Quarry specimen

A second Valanginian specimen has conventionally been identified as another specimen of Barilium: it’s a large pelvis, preserved in articulation with the sacral vertebrae and some of the dorsals [the specimen is shown below: from Carpenter & Ishida (2010), used with permission]. This is the Old Roar Quarry specimen (properly NHMUK R3788), discovered at St. Leonards-on-Sea, East Sussex. In view of its spectacular size and articulated nature, I find it surprising that it’s been hardly mentioned in the literature: Blows (1998) incorporated it into a schematic reconstruction of Barilium, but other than that there’s nothing until it was discussed and illustrated by Naish (2008) and Naish & Martill (2008).

My opinion on the Old Roar Quarry specimen has always been that it really doesn’t look much like the Barilium holotype at all: the dorsal margin of its ilium has a distinctive sinuous curvature, the preacetabular process is strongly down-turned (that of the Barilium holotype is approximately straight), and the postacetabular process projects posterodorsally, among other things. In view of these differences, Naish & Martill (2008) noted that the inclusion of the Old Roar Quarry specimen in Barilium “is almost certainly incorrect and the true affinities of this specimen remain to be determined” (p. 618). Finally, two additional opinions on the specimen have both been published though, strangely, neither cites Naish & Martill (2008) (what, you mean Journal of the Geological Society of London is an obscure publication?).

Norman’s (2010) take on the specimen is that it belongs to Barilium dawsoni, and that its unusual shape “reflects probable pre-burial desiccation and post-mortem distortion” (p. 54). I find this difficult to accept, given that both sides of the Old Roar Quarry specimen exhibit the same unusual form, and given that its unique features look ‘natural’ and not the result of post-mortem distortion (but I’m happy to be wrong… we’ll come back to the subject of ilium variability later on). A second opinion is provided by Carpenter & Ishida (2010) who argue that the specimen is distinct, unique and diagnosable. This decision leads them to regard it as a new taxon, and they name it Sellacoxa pauli (Sellacoxa = ‘saddle hips’, a reference to the dorsally concave margin of the ilium). Yikes, another new Wealden iguanodontian (and we’re not finished yet).

As an interesting historical aside, the Sellacoxa holotype is (in my opinion) particularly interesting because of its alleged implication in the Piltdown man saga. On being obtained by Charles Dawson in 1909, it was sent directly to Arthur Smith Woodward at the British Museum (Natural History) [Dawson shown here, from Wikipedia]. This was unbeknownst to William Ruskin Butterfield, the then-curator of the Hastings Museum. When learning of the loss to Hastings of a good specimen, Butterfield was unhappy, and it’s on this basis that Esbroeck (1972) proposed that Butterfield went on to make a fool of Dawson by perpetrating the Piltdown hoax. Walsh (1996) described this theory as “freewheeling speculation” (p. 96); it lacks supportive evidence and is full of holes (Spencer 1990).

Captain Brickenden’s lower jaw

Among the many isolated iguanodontian fragments described from the English Wealden by Mantell and his contemporaries was a large (52 cm long), well preserved right dentary, again assumed to belong to Iguanodon sensu lato. Discovered at Cuckfield in West Sussex, it was presented to Mantell by Captain Lambart Brickenden in 1848 and has frequently been mentioned in both the technical and popular literature on early dinosaur discoveries.

Norman (2010) suggested that Brickenden’s dentary might belong to Barilium. However, while Barilium is (as we’ve seen above) from the Wadhurst Clay Formation, the Brickenden dentary is from another unit: the somewhat younger Grinstead Clay Formation. Furthermore, it possesses at least one distinctive feature all its own: a row of four small foramina that extend across part of the bone’s anterolateral region [you can see them below – in C – where they’re marked in red. This figure is from McDonald et al. (2010)]. The bone possesses some other unusual features that, in combination, aren’t seen in other iguanodontians: the ramus is mostly straight and the coronoid process is vertical and has a slight anterior expansion, for example (McDonald et al. 2010).

This all means that this specimen can’t be confidently referred to any named taxon, and yet again there’s no evidence linking it specifically to Iguanodon bernissartensis, so it shouldn’t be included in Iguanodon at all. Accordingly, Andrew McDonald and colleagues recently named it Kukufeldia tilgatensis (McDonald et al. 2010). The generic name is derived from an old name for Cuckfield (Kukufeld). Incidentally, the Kukufeldia dentary is really quite different from some Iguanodon sensu lato lower jaws reported from the Wadhurst Clay Formation. These Wadhurst Clay specimens are currently the subject of a fair bit of controversy, and competing opinions exist on how they should be interpreted.

The Barilium, Sellacoxa and Kukufeldia holotype specimens are far from the only reasonably good Hastings Group iguanodontian specimens: we’ll be looking at more of them in the next article.

References:

Blows, W. T. 1998. A review of Lower and Middle Cretaceous dinosaurs of England. New Mexico Museum of Natural History 14, 29-38.

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

Esbroeck, G. van. 1972. Pleine Lumière sur l’Imposture de Piltdown. Cedre, Paris.

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. 2008. Conan-Doyle [sic], Piltdown, and the dinosaur in the well: obscure Wealden dinosaurs and the stories behind them. In Moody, R., Buffetaut, E., Martill, D. & Naish, D. (eds) Dinosaurs and Other Extinct Saurians: A Historical Perspective. Abstracts of Meeting Held on the 6-7 May 2008. Geological Society of London, London, pp. 8-9.

- . & 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, London 165, 613-623.

Norman, D. B. 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.

Spencer, F. 1990. Piltdown: A Scientific Forgery. British Museum (Natural History) (London) & Oxford University Press (Oxford).

Walsh, J. E. 1996. Unravelling Piltdown: The Science Fraud of the Century and Its Solution. Random House (London).

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, UK.

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






Add Comment

Add a Comment
You must sign in or register as a ScientificAmerican.com member to submit a comment.

More from Scientific American

Scientific American MIND iPad

Give a Gift & Get a Gift - Free!

Give a 1 year subscription as low as $14.99

Subscribe Now >>

X

Email this Article

X