November 6. Iguanodon

Reconstruction of skeleton by Marsh (public domain)

Iguanodon was one of the dinosaurs originally used by paleontologist Richard Owen back in the 1840s to define the Dinosauria. They were found in the Lower Cretaceous rocks of Belgium. Iguanodon probably descended from bipedal herbivores a couple meters long that date to the late Jurassic. Iguanodons grew to ten or 12 meters, about 35 feet long, and they too were bipedal herbivores, but their arms or front feet were strong enough for them to operate as quadrupeds as well. They are probably ancestral to the duck-billed hadrosaurs that became common in late Cretaceous time.

Iguanodon has had a checkered history. The first fossils, described in 1825, have since been reassigned to a different genus, and technically there is only one species in the genus Iguanodon. Subsequent fossil discoveries included some nearly complete specimens, so its anatomy can be analyzed with confidence. In 1878, at least 38 iguanodon individuals were found in a coal mine at Bernissart, Belgium, a town that gives its name to the sole iguanodon species. Their presence in the coal beds initially created considerable confusion, because the coals were Carboniferous in age. Dinosaurs were definitely anomalous in Carboniferous rocks, so what was up? Careful mapping of the rocks in the coal mine revealed the presence of an ancient 300-foot ravine, a channel cut into the Carboniferous beds. The interpretation is that a herd of grazing iguanodons stampeded over the edge of the ravine, falling to their deaths surrounded by cliffs of Carboniferous coal. If you want to imagine that they were being chased by some big carnivorous predator, you can. The ravine was later filled in with Cretaceous sediments, and it was recognized that the fossils were actually within those age-appropriate rocks rather than in the Carboniferous coal.

Thumb claw photo by en:user:Ballista,
used under Creative Commons license

Iguanodon’s hand had a long, sharp thumb-like spike, which may have been used for defense or for cracking big seeds – it was a herbivore. Its teeth – the name means “iguana tooth” – confirm its herbivorous nature. They were abundant at least locally in northern Europe and Britain, but iguanodons existed for only a geologically short period of time –around a million years or so – at about 125 million years ago, at the boundary between the Barremian and Aptian stages of the early Cretaceous.  

—Richard I. Gibson

Reconstruction of skeleton by Marsh (public domain)

Thumb claw photo by en:user:Ballista, used under Creative Commons license