Tree of Life


Subphylum Chelicerata

Phylum Arthropoda


Chelicerata Fossils

eurypteridSubphylum Chelicerata comprises a major group within Phylum Arthropoda, including such animals Xiphosuraas the arachnids (e.g., spiders and scopions), the extinct eurypterids, and the extant horseshoe crabs that are considered to be living fossils. These arthropods and their ancestral and extinct forms were and are mainly predators. Chelicerates are now predominently terrestrial animals, with most marine chelicerates, including all of the eurypterids, now extinct.

Their name comes from their chelicerae, pointed appendages that grasp food, that differ from the Acarinachewing mandibles of most arthropods. Being unable to ingest solid food, most Chelicerata either drink blood or spit or otherwise inject digestive enzymes into their prey, and feed on the fluidized result. Like all arthropods, chelicerates' bodies and appendages are covered with a tough cuticle primarily composed of chitin and proteins that chemically harden. Because this exoskeleton cannot stretch, chelicerates have to molt in order to grow. Thus, they have to molt the old, and await hardening of the new, during which time they have reduced mobility and are otherwise more defenseless.

Subphylum Chelicerata Classification

Chelicerata Class Arachnida Acarina (ticks and mites)
Amblypygi (whip scorpions)
Araneae - Spiders (note 3)
Opiliones - Phalangids, harvestmen or daddy longlegs (note 4)
Palpigradi - microwhip scorpion
Pseudoscorpionida - Pseudoscorpions (note 5)
Ricinulei - hooded tickspiders
Scorpiones - Scorpions (note 6)
Solifugae - camel spiders
Thelyphonida - vinegaroons
Trigonotarbida - Extinct (note 2)
Class Merostomata Order Xiphosura - Horseshoe crabs (note 8)
Order Eurypterida (eurypterids) (note 9)
Class Pycnogonida Sea spiders
  1. Arachnids, which are named for the mythological figure Arachne, are mainly terrestrial arthropods comprising some 70,000 extant species including the spiders, scorpions, harvestmen, ticks, and mites. The arachnids are easily differentiated from insects by their eight legs instead of six. The leg-like pedipalpi of some arachnids are adapted for sensing, capturing prey or reproducing.
  2. The Trigonotarbida were spider-like arthropods, and possibly the oldest terrestrial arthropods. Despite having eight legs, they are believed to have been an evolutionary offshoot of the true spiders that arose about the same time in the Devonian.
  3. note 3: Spiders are arthropods that produce silk, have eight legs and no wings. The order is divided into two sub-orders: the Opisthothelae (which include the infraorders Mygalomorphae (trapdoor and tarantula spiders) and Araneomorphae (the modern spiders), and the Mesothelae, which contains the Family Liphistiidae, primitive burrowing spiders from Asia.
  4. Note 4: Opiliones are commonly called harvestmen or daddy long legs, have eight legs like spiders, except they are exceptionally long. The order is mainly omnivorous.
  5. Note 5: Pseudoscorpions are tiny, scorpion-like arthropods with flat, pear-shaped bodies and eight legs. They have two very long pedipalps, or pincers, that resemble a scorpion's claws, but unlike scorpion's have a short and rounded abdomen without tail and stinger. The pincer contains a venom gland and duct and the poison is used to capture and immobilize their tiny prey.
  6. Note 6: Scorpion remains found in coal deposits from the Carboniferous Period have no demonstrable difference and extant species, suggesting no evolutionary need for change for hundreds of millions of years. All species of scorpion possess venom that in most species is not dangerous to humans.
  7. Mites and ticks were among the first land arthropods; the oldest known so far are Late Devonian. However, because of their small size, the Acari are uncommon in the fossil record. A few are known from the Carboniferous, and more from Tertiary amber.
  8. Note 8: The horseshoe crab has a long fossil record from the lower ordovician or latest Cambrian, predating flying insects and dinosaurs, during which time they have undergone little change.
  9. Note 9: The eurypterids were among the largest and most fearsome marine predators of the Paleozoic. Varying in size from some 10 centimeters up to two meters in length, they were also the largest arthropods that ever lived. They arose in the Ordovician, and went extinct in the Permian. There fossil records includes all the continents.