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Wednesday, May 2, 2007

Centipede Evolution

Centipede Evolution

Centipedes have a very sparse fossil record, since they are delicate and easily destroyed after death, and they live in habitats which only rarely produce fossils.

The arthropods, the group to which centipedes belong, are evolutionary descendents of the very ancient segmented worms known as the Annelida. These animals were constructed in a series of rings or segments, each externally identical to those before and after it, with a mouth at one end and an anal opening at the other. Each segment contained a number of sensory bristles or hairs. The annelids are one of the earliest of the multicellular organisms, and fossil traces of these animals date all the way back to pre-cambrian times, over half a billion years ago. The earliest segmented worms lived with such other ancient animals as jellyfish and sponges. Modern annelids include the common earthworm or nightcrawler, known by the scientific name Lumbricus terrestris.

One group of annelids underwent a process of evolution which resulted in the birth of the arthropods. Each body segment developed a pair of walking legs. The first six segments of the annelid ancestor were fused together to produce a head, which contained a number of simple eyes. The appendages in these fused segments were modified to form mouthparts. Although the earliest arthropods were marine animals, two separate groups left the sea between 450 and 420 million years ago, to become the ancestors of today's myriapods and scorpions.

One branch of these early arthropods are known as uniramians, which are distinguished by having only one branch for each leg. Modern descendents of the uniramians include the centipedes, millipedes, and the insects. The earliest known fossil myriapod consists of a number of scutigeromoprh legs from the late Silurian period, approximately 400 million years ago. Some earlier fossils may represent burrows formed by early myriapods. The earliest recognizable centipedes appear in the Devonian and Carboniferous period. It is a virtual certainty that centipedelike organisms were one of the first terrestrial animals, and that earlier specimens of fossil centipedes will be found as the fossil record becomes more complete.

The scutigeromorphs and the lithobiomorphs have many anatomical traits in common. Both have 15 pairs of legs, and both add new segments to their body length as they get older. It is probable that these two orders diverged from each other only recently in evolutionary terms.

There are almost 3,000 species of centipedes living on earth today. It is estimated that about 6,000 species remain to be described by science.

1 comment:

James Remington Orozco-Newton said...

You sir have helped me out a lot. Thanks.