Size and sex

The Bernissart discoveries are notable for comprising two types of Iguanodon. One (Iguanodon bernissartensis - quite literally 'the Iguanodon that lived in Bernissart') is large and robustly built, and represented by more than 35 skeletons; the other (Iguanodon atherfieldensis, formerly called I. mantelli - literally 'Mantell's Iguanodon') is smaller and more delicately built (approximately 6 metres in length) and represented by only two skeletons.

These specimens were regarded as distinct species until they were reassessed in the 1920s, by Francis Baron Nopcsa, a nobleman from Transylvania and a palaeontologist. The discovery of two quite similar types of dinosaur that evidently lived in the same place, at the same time, prompted him to ask the simple and yet obvious question: are they males and females of the same species? Nopcsa attempted to determine sexual differences in a number of fossil species. In the case of the Iguanodon from Bernissart he concluded that the smaller and rarer species was the male and the larger and more numerous species was the female. He observed, perfectly reasonably, that it is often the case that female reptiles are larger than males. The biological reason for this is that females often have to grow large numbers of thick-shelled eggs; these drain considerable resources from the body before they are laid.

M While this seems quite a reasonable supposition, it is in fact | very difficult to prove scientifically. Apart from size, which is £ surprisingly variable among reptiles as a whole and not nearly as consistent a feature as Nopcsa would have had us believe, the features used to distinguish the sexes among living reptiles are most commonly found in the soft anatomy of the sex organs themselves, coloration of the skin, or behaviour. This is particularly unfortunate because only very rarely do fossils ever preserve such features.

The most valuable evidence would be the discovery of soft anatomical fossils of the sexual organs of Iguanodon - unfortunately, this is an extremely unlikely event. And, since we can never know their true biology and behaviour, we have to be a little cautious and also realistic. For the present, it is safer to record the differences (we may have our own suspicions, perhaps), but simply leave it at that.

A careful study of the more abundant large Iguanodon from Bernissart revealed that a few were smaller than the average.

Measuring the proportions of each of these skeletons revealed an unexpected growth change. Smaller, presumably immature specimens had shorter arms than would have been expected. The comparatively short-armed juveniles may well have been more adept bipedal runners, but as large adult size and stature was achieved they may have become progressively more accustomed to moving around on all fours. This also fits with the observation of an intersternal ossification in only larger, presumably adult, individuals, which spent more of their time on all fours compared to smaller, younger individuals.

0 0

Post a comment