Geology Rocks

dailyfossil:

Early Cambrian Trilobites - The first trilobites 

When: Appear in the early Cambrian ~ 526 million years ago 

Where: Globally

What: Trilobites are arthropods. Trilobites appear suddenly in the early Cambrian, and are almost immediately found worldwide. They are one of the major players in the Cambrian Explosion. Their apperance is so consistant that the presence of these animals defines the start of a subperiod within the Cambrian. The first records of trilobites include members of all nine orders (major subclades within Trilobita), meaning that they had to be present for some time before their first appearance in the fossil record. It is important to note that it is only shells that are preserved at first, so the lack of a record previously could be due to the previous lack of a mineralized external skeleton for trilobites. however, this interpretation has its own problems - as it implies the nine orders independently developed hard shells. Another possible explanation is that previous to 526 million years ago the water chemistry was such that preservation of the shells was impossible.  The photo and reconstruction above are probably of the early genus Paradoxides. This early trilobite, like most of the first ones to appear, was capable of swimming but mostly crawled along the bottom, searching for minute food particles in the surface sediments. 

As stated before trilobites are arthropods. This massive group includes animals with a hard external skeleton, such as lobsters, insects, spiders, etc.  The placement of trilobites in this group is not debated. What is much more contentious is their placement within the clade. At first they were placed as the sister-group to the crustaceans (lobsters and crabs, for example) but now it is thought that the horseshoe crab is their closest living relative, far removed from the rest of Arthropoda.