Sunday, 6 September 2009
British Wildlife: I
Iguanodon anglicus Holl, 1829
Iguanodontidae; Ornithischia; Sauropsida; Chordata
Iguanodon is one of the most well known ornithopods (bird-footed dinosaurs, including 'duck bills' like the hadrosaurs). It was the second dinosaur genus to be officially named (after Megalosaurus) in 1825 by Gideon Mantell. He did, however, neglect to nominate a type species for the genus.
Iguanodon anglicus, the English Iguanodon, was nominated as the type, but the name remains a nomen dubium ('dubious name', meaning it is difficult to assign specimens to that name, due to a lack of unique features). All that was known of I. anglicus at the time was a single tooth.
The species I. bernissartensis Boulenger was chosen as the neotype, and it is probably the most well known species, in terms of number of specimens. Whole skeletons are recognised, many of which being found at the coal mines in Bernissart, Belgium, hence the specific name. This is also the reason why the bones of this species are very dark in colour.
Iguanodontid footprint cast
Compton Bay, Isle of Wight
It's hard to appreciate the sheer size of Iguanodon without seeing the whole skeleton, but take this cast of a footprint, taken in situ on the beach in Isle of Wight. A human foot of about size 10 would fit in the middle digit on that footprint. Just three toes can be seen - digits II, III and IV, relating to the middle three digits on a human hand. The famous thumb, consisting of a conical spike, does not make an imprint. The cast was made, completely naturally, by the sediment filling up the original impression made by the dinosaur, over time turning to limestone. The stratum (rock layer) in which the animal walked had been eroded by the waves, leaving the cast behind.
Dentary (lower jaw bone) of Iguanodon sp.
Dinosaur Isle museum, Isle of Wight
The name Iguanodon of course means 'iguana tooth'. The similarity was noticed by Mantell when naming the genus back in 1825 between the tooth of the new reptile and those of living iguanas, except the new tooth was much bigger. The animal then had to be huge too, and it was reconstructed as a giant lizard with a long tail, a sprawling build and a horn on its nose. This last detail was added because part of the specimen was a conical bone looking like a horn. Had the skeleton been articulated, like those at Bernissart, it would have been clear to Mantell and others that the bone belonged on the thumb. Not knowing that, he reconstructed the animal with the bone on its nose.
Ischnura elegans (Vander Linden, 1820)
Coenagrionidae; Odonata; Insecta; Arthropoda
There are no tetrapods or other vertebrates native to and currently extant in the UK. There are insects though, so I chose a pretty damselfly to illustrate the letter I. Damselflies make up the suborder Zygoptera ('yoke wing'), and are generally daintier than their more well known relatives, the dragonflies (order Anisoptera - 'unequal wing'). Both types lay their eggs in water, and the young, known as nymphs, lead a carnivorous lifestyle, eating animals many times their size, including fish and amphibians. Once metamorphosed, they become graceful fliers, remaining close to water. When it comes to mating time, the damselflies will form what is known as a mating wheel. This involves the male hooking the tip of his abdomen to the back of the female's head. This appears to the uninitiated to be how the male fertilises her eggs. Of course not, silly! The 'wheel' part involves the female hooking up the end of her abdomen to the front of the male's abdomen, where his genitalia are. The male will still be attached to the female's head for a while after mating, and this can be seen easily when looking out for the insects by the waterside in summer.
Diarsia mendica Fabricius, 1775
Noctuidae; Lepidoptera; Insecta; Arthropoda
Another insect for the letter I, this time a lepidopteran ('scaly wing'). Lepidopterans are better known as the butterflies and moths. The ingrailed clay is of the latter type, in the family of moths known as noctuids. I don't have much to say about this species unfortunately.
Next week: J; an agnathan, a corvid and an unusual woodpecker.