Bone Analysis Confirms: ‘Little Iguanodon’ is a Separate Species



Mantellisaurus atherfieldensis, 'the smaller iguanodon', and in the background the bigger Iguanodon bernissartensis. (Photo: RBINS)
Bone Analysis Confirms: ‘Little Iguanodon’ is a Separate Species
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Reinout Verbeke

Inside the glass cage of the Bernissart Iguanodons in the Museum of Natural Sciences, one much smaller specimen stands out. But its bone structure is of an adult animal, an analysis by Koen Stein (Vrije Universiteit Brussel) concludes. That means the specimen belongs to a different genus and species than the larger iguanodons it is surrounded by.

Stein drilled a cylinder-shaped sample out of the fossil bones of the ‘little iguanodon’. The analysis of the internal structure confirms that he/she was full-grown and therefore had to belong to another species than the bigger Iguanodon bernissartensis. ‘Adult dinosaurs have a different, less porous bone structure than young specimens. They also have a lot less blood vessels.’

You Name It

After the discovery of around thirty 125 million year old iguanodons in a coal mine in Bernissart (Hainaut province, Belgium) between 1878 en 1881, George Boulenger thought the smaller specimen belonged to Iguanodon mantelli. According to another hypothesis it was a juvenile version of Iguanodon bernissartensis. A young animal can change drastically during its growth. But in the 1980s, the British researcher David Norman did another attempt to determine the animal. He connected the smaller animal to another species, Iguanodon arherfieldensis.

In 2008, palaeontologist Gregory Paul moved the little dinosaurs to another genus: Dollodon, Dollodon bampingi to be precise. But, according to other scientists, there is no difference between Dollodon bampingi and Mantellisaurus arherfieldensis, like the specimen in the Natural History Museum in London. Whichever group (taxon) it belongs to, one thing is certain: it is not Iguanodon bernissartensis.

Murder Case

Stein did his analysis as part of the ColdCase project, the current investigation into the cause of death of the Bernissart iguanodons, led by palaeontologist Pascal Godefroit (RBINS). Since their discovery, there has been lots of speculation about their death: drowned during a sudden rise of the water, fell of a cliff, lost a battle to natural enemies, succumbed to drought and got stuck in a swamp ... The 1980s brought the theory that they each died separately and naturally, and over the years slid into a subsidence or 'cran'. That appears to be partly true.


The fact that all found specimens – also the smaller ones – were adults, can be a clue. On a paleontological site where animals died gradually and ‘passively’, there are usually around 80 percent young animals, because they are more vulnerable. There were no young iguanodons, which points to a fast and collective death. The palaeontologists suspect that the dinosaurs were poisoned by swamp gases: hydrogen sulphide (H2S), a so-called ‘silent killer’. The layers of clay where the iguanodons were found, contain a lot of pyrite or iron disulphide (FeS2), which is formed by red iron oxide (Fe2O3) and the toxic gas H2S. H2S is toxic even in small concentrations.

‘Also the sediments within the bones contain high amounts of pyrite’, Stein says. ‘Bacteria or algae in the swamp could have produced the toxic gas’. The position in which the iguanodons were found – on their side and with their heads tilted back – also points in the direction of poisoning. The skeletons were found in different layers, so they probably died during different events.   


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