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up a course of weight-training will deposit extra skeletal bone in § order to cope with the increased load, especially if this training § regime is continued over time.

In particular areas of the body, where large muscles exert forces on the skeleton, the scarring on bones can be quite distinctive, even in fossils; this creates a crude map that allows some of the original musculature to be reconstructed (Figure 24). Such reconstructions are based on the known muscular arrangements seen in related living animals, tempered by allowances for the anatomical differences or novelties seen in the fossil animal that is being investigated.

Although far from scientifically ideal, an example of this kind of approach when trying to understand the musculature of Iguanodon is to use as a starting point information from two of the nearest living relatives of dinosaurs: birds and crocodiles. Clearly neither of

24. Dinosaur muscle reconstruction

these types of animal represent, at all accurately, the anatomy of Iguanodon: birds are highly modified for flight, have no teeth, have a minuscule tail and unusually modified hips and leg muscles; crocodiles, though more conventionally reptilian in shape, are highly specialized as aquatic predators. Despite these real problems, they provide a general framework or template - termed the 'extant phylogenetic bracket', or EPB - for reconstruction that can be supplemented by the finer details of the anatomy of Iguanodon.

The latter includes the general evidence from the overall physical

The extant phylogenetic bracket (EPB)

By creating a phylogenetic tree of the nearest relatives of dinosaurs, it is clear that crocodiles evolved before dinosaurs appeared and that birds evolved after the earliest dinosaurs. Dinosaurs are therefore sandwiched evolutionarily between living crocodiles and living birds.

Anatomical features shared by both living birds and crocodiles should also be present in dinosaurs because they are quite literally 'bracketed' by these living creatures. Sometimes this type of approach can help to deduce biological features among extinct groups even when there is no clear

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