section_path: Evolutionary history/Fossil record
section_name: Fossil record
paragraph_text: Fossil record
  
  It has been proposed that the Ediacaran animals Parvancorina and Spriggina, from around 555 million years ago, were arthropods, but later study shows that their affinities of being origin of arthropods are not reliable. Small arthropods with bivalve-like shells have been found in Early Cambrian fossil beds dating 541 to 539 million years ago in China and Australia. The earliest Cambrian trilobite fossils are about 520 million years old, but the class was already quite diverse and worldwide, suggesting that they had been around for quite some time. In the Maotianshan shales, which date back to 518 million years ago, arthropods such as Kylinxia and Erratus have been found that seem to represent transitional fossils between stem (e.g. Radiodonta such as Anomalocaris) and true arthropods. Re-examination in the 1970s of the Burgess Shale fossils from about 505 million years ago identified many arthropods, some of which could not be assigned to any of the well-known groups, and thus intensified the debate about the Cambrian explosion. A fossil of Marrella from the Burgess Shale has provided the earliest clear evidence of moulting.
  
  The earliest fossil of likely pancrustacean larvae date from about 514 million years ago in the Cambrian, followed by unique taxa like Yicaris and Wujicaris. The purported pancrustacean/ crustacean affinity of some cambrian arthropods (e.g. Phosphatocopina, Bradoriida and Hymenocarine taxa like waptiids) were disputed by subsequent studies, as they might branch before the mandibulate crown-group. Within the pancrustacean crown-group, only Malacostraca, Branchiopoda and Pentastomida have Cambrian fossil records. Crustacean fossils are common from the Ordovician period onwards. They have remained almost entirely aquatic, possibly because they never developed excretory systems that conserve water.
  
  Arthropods provide the earliest identifiable fossils of land animals, from about 419 million years ago in the Late Silurian, and terrestrial tracks from about 450 million years ago appear to have been made by arthropods. Arthropods possessed attributes that were easy coopted for life on land; their existing jointed exoskeletons provided protection against desiccation, support against gravity and a means of locomotion that was not dependent on water. Around the same time the aquatic, scorpion-like eurypterids became the largest ever arthropods, some as long as 2.5 m (8 ft 2 in).
  
  The oldest known arachnid is the trigonotarbid Palaeotarbus jerami, from about 420 million years ago in the Silurian period. Attercopus fimbriunguis, from 386 million years ago in the Devonian period, bears the earliest known silk-producing spigots, but its lack of spinnerets means it was not one of the true spiders, which first appear in the Late Carboniferous over 299 million years ago. The Jurassic and Cretaceous periods provide a large number of fossil spiders, including representatives of many modern families. The oldest known scorpion is Dolichophonus, dated back to 436 million years ago. Lots of Silurian and Devonian scorpions were previously thought to be gill -breathing, hence the idea that scorpions were primitively aquatic and evolved air-breathing book lungs later on. However subsequent studies reveal most of them lacking reliable evidence for an aquatic lifestyle, while exceptional aquatic taxa (e.g. Waeringoscorpio) most likely derived from terrestrial scorpion ancestors.
  
  The oldest fossil record of hexapod is obscure, as most of the candidates are poorly preserved and their hexapod affinities had been disputed. An iconic example is the Devonian Rhyniognatha hirsti, dated at 396 to 407 million years ago, its mandibles are thought to be a type found only in winged insects, which suggests that the earliest insects appeared in the Silurian period. However later study shows that Rhyniognatha most likely represent a myriapod, not even a hexapod. The unequivocal oldest known hexapod and insect is the springtail Rhyniella, from about 410 million years ago in the Devonian period, and the palaeodictyopteran Delitzschala bitterfeldensis, from about 325 million years ago in the Carboniferous period, respectively. The Mazon Creek lagerstätten from the Late Carboniferous, about 300 million years ago, include about 200 species, some gigantic by modern standards, and indicate that insects had occupied their main modern ecological niches as herbivores, detritivores and insectivores. Social termites and ants first appear in the Early Cretaceous, and advanced social bees have been found in Late Cretaceous rocks but did not become abundant until the Middle Cenozoic.