A Jurassic giant with its mouthparts showing

A giant Jurassic dipluran shows that hidden mouthparts evolved more than once.

Look at those mandibles.

The mandibles of Sinojapyx giganteus are robust and thick, armed with strongly curved, coarse teeth, and part of them sits outside the head capsule. In living diplurans, the mandibles are enclosed by the head capsule and only extend a little when feeding. The mandibles of this Jurassic giant are structurally closer to the ectognathous condition of other hexapods: they are not fully hidden by the head. That single detail turns "is entognathy homologous?", a purely theoretical question, into an empirical one that a fossil can test. Together with the stout, symmetrical, smooth-edged pincers, it lets us sketch how the animal lived: much like a centipede or scorpion today, it probably lurked in the leaf litter or soil crevices of Daohugou's warm, humid forests, ambushing small invertebrates with its mandibles and pincers and sitting at the top of the soil food web. The transitional shape of the cerci, on their way from sensory appendages to efficient capturing weapons, is written in that symmetrical, toothless structure.

The next question I would most like answered: did entognathy evolve independently in the three groups under the same selective pressures?

If Diplura, Collembola, and Protura each "pulled" their mouthparts into the head capsule during their own move into underground life, did they face similar environmental pressures, feeding modes, and developmental paths? Or are there several different ways of building a hidden mouth, all of which simply look alike from the outside? To answer that, we need more fossils like Sinojapyx giganteus, especially earlier members of Diplura, to pin down when and in what form each episode of "entognathisation" began. That is one reason our team keeps working in Daohugou and in older deposits.