Centipede - Evolution

Evolution

The fossil record of centipedes extends back to 430 million years ago, during the Late Silurian. They belong to the subphylum Myriapoda which includes Diplopoda, Symphyla, and Pauropoda. The oldest known fossil land animal, Pneumodesmus newmani, is a myriapod. Being among the earliest terrestrial animals, centipedes were one of the first to fill a fundamental niche as ground level generalist predators in detrital food webs. Today, centipedes are abundant and exist in many harsh habitats.

Within the myriapods, centipedes are believed to be the first of the extant classes to branch from the last common ancestor. There are five orders of centipedes: Craterostigmomorpha, Geophilomorpha, Lithobiomorpha, Scolopendromorpha, and Scutigeromorpha. These orders are united into the clade Chilopoda by the following synapomorphies:

1. The first post-cephalic appendage is modified to venom claws.
2. The embryonic cuticle on second maxilliped has an egg tooth.
3. The trochanter–prefemur joint is fixed.
4. There is a spiral ridge on the nucleus of the spermatozoon.

Chilopoda is then split into two clades: the Notostigmomorpha including the Scutigeromorpha and the Pleurostigmomorpha including the other four orders. The main difference is that the Notostigmomorpha have their spiracles located mid-dorsally. It was previously believed that Chilopoda was split into Anamorpha (Lithobiomorpha and Scutigeromorpha) and Epimorpha (Geophilomorpha and Scolopendromorpha), based on developmental modes, with the relationship of Craterostigmomorpha being uncertain. Recent phylogenetic analyses using combined molecular and morphological characters supports the previous phylogeny. Epimorpha still exists as a monophyletic group within the Pleurostigmomorpha, but Anamorpha is paraphyletic.

Geophilomorph centipedes have been used to argue for the developmental constraint of evolution: that the evolvability of a trait, the number of segments in the case of geophilomorph centipedes, was constrained by the mode of development. The geophilomorph centipedes have variable segment numbers within species, yet as with all centipedes they always have an odd number of pairs of legs. In this taxon, the number of segments ranges from 27 to 191 but is never an even number.