The reversed hallux has long been an important aspect of the discussion of dinosaur habits, and more specifically the origin of flight though it might not seem so at first glance. The hallux is basically the technical term for what most people would call the big toe, that is, the first digit on the foot and its reversal (or otherwise) basically relates to the orientation it holds – does it point forwards with the other toes, or sideways, or has it reversed to point behind?
The reason this point is so often contested or debated lies in two simple truths. Firstly, birds with a reversed hallux are mostly perching birds – those that live in trees and perch on branches and by extension are generally decent fliers. Secondly, the orientation of the hallux is incredibly difficult to determine in flat 2-D fossils such as those we generally get for things like early birds and close relatives in the non-avian dinosaurs.
The result as you might have guessed is that palaeontologists are incredibly interested in reversed halluces since this may allow us to determine if early birds and (perhaps more importantly) their likely ancestors were hanging around (so to speak) in trees. However between the tendency for the hallux to come free from the rest of the toes and the fact that compression can lead to it lying in a misleading orientation on a slab means that reconstructing the original position accurately is difficult at best.
It will be no surprise therefore to learn that this has been controversial at best and for things like Archaeopteryx with multiple good, and conflicting, specimens know the debate has been long and remains (largely) unresolved. The picture here is of a birds from Liaoning that I saw recently and clearly shows the hallux facing behind on each foot with no obvious signs of dislocation. Convincing is this case, but unhelpfully this is rarely in the more interesting taxa.
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