Last common ancestor Based on the distribution of shared
plesiomorphic features in extant and fossil taxa, the
last common ancestor of all arthropods is inferred to have been as a modular organism with each module covered by its own
sclerite (armor plate) and bearing a pair of biramous
limbs. However, whether the ancestral limb was
uniramous or biramous is far from a settled debate. This Ur-arthropod had a
ventral mouth, pre-oral antennae and
dorsal eyes at the front of the body. It was assumed to have been a non-discriminatory
sediment feeder, processing whatever sediment came its way for food,
Fossil record '', one of the puzzling arthropods from the
Burgess Shale It has been proposed that the
Ediacaran animals
Parvancorina and
Spriggina, from around , 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 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 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. '' may be a key transitional fossil between stem-arthropods and true arthropods. 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. They have remained almost entirely aquatic, possibly because they never developed
excretory systems that conserve water. and terrestrial tracks from about appear to have been made by arthropods. Arthropods possessed attributes that were easily
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 . The oldest known
arachnid is the
trigonotarbid Palaeotarbus jerami, from about in the Silurian period.
Attercopus fimbriunguis, from 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 . 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 . 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 , 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 is the
springtail Rhyniella, from about in the Devonian period, and the
palaeodictyopteran
Delitzschala bitterfeldensis, from about in the Carboniferous period, respectively.
External phylogeny is closely related to arthropods Further analysis and discoveries in the 1990s reversed this view, and led to acceptance that arthropods are
monophyletic, in other words they are inferred to share a common ancestor that was itself an arthropod. For example,
Graham Budd's analyses of
Kerygmachela in 1993 and of
Opabinia in 1996 convinced him that these animals were similar to onychophorans and to various Early Cambrian "
lobopods", and he presented an "evolutionary family tree" that showed these as "aunts" and "cousins" of all arthropods. These changes made the scope of the term "arthropod" unclear, and Claus Nielsen proposed that the wider group should be labelled "
Panarthropoda" ("all the arthropods") while the animals with jointed limbs and hardened cuticles should be called "Euarthropoda" ("true arthropods"). A contrary view was presented in 2003, when Jan Bergström and
Hou Xian-guang argued that, if arthropods were a "sister-group" to any of the anomalocarids, they must have lost and then re-evolved features that were well-developed in the anomalocarids. The earliest known arthropods ate mud in order to extract food particles from it, and possessed variable numbers of segments with unspecialized appendages that functioned as both gills and legs. Anomalocarids were, by the standards of the time, huge and sophisticated predators with specialized mouths and grasping appendages, fixed numbers of segments some of which were specialized, tail fins, and gills that were very different from those of arthropods. In 2006, they suggested that arthropods were more closely related to
lobopods and
tardigrades than to anomalocarids. In 2014, it was found that tardigrades were more closely related to arthropods than velvet worms. Higher up the "family tree", the
Annelida have traditionally been considered the closest relatives of the Panarthropoda, since both groups have segmented bodies, and the combination of these groups was labelled
Articulata. There had been competing proposals that arthropods were closely related to other groups such as
nematodes,
priapulids and
tardigrades, but these remained minority views because it was difficult to specify in detail the relationships between these groups. In the 1990s,
molecular phylogenetic analyses of
DNA sequences produced a coherent scheme showing arthropods as members of a
superphylum labelled Ecdysozoa ("animals that moult"), which contained nematodes, priapulids and tardigrades but excluded annelids. This was backed up by studies of the anatomy and development of these animals, which showed that many of the features that supported the Articulata hypothesis showed significant differences between annelids and the earliest Panarthropods in their details, and some were hardly present at all in arthropods. This hypothesis groups annelids with molluscs and
brachiopods in another superphylum,
Lophotrochozoa. If the Ecdysozoa hypothesis is correct, then segmentation of arthropods and annelids either has evolved
convergently or has been inherited from a much older ancestor and subsequently lost in several other lineages, such as the non-arthropod members of the Ecdysozoa.
Internal phylogeny Early arthropods Aside from the four major living groups (
crustaceans,
chelicerates,
myriapods and
hexapods), a number of fossil forms, mostly from the early Cambrian period, are difficult to place taxonomically, either from lack of obvious affinity to any of the main groups or from clear affinity to several of them.
Marrella was the first one to be recognized as significantly different from the well-known groups. • The
"Giant" or "Siberiid Lobopodians", such as
Jianshanopodia,
Siberion and
Megadictyon, are the most basal
grade in the total-group Arthropoda. • The
"Gilled Lobopodians", such as
Kerygmachela,
Pambdelurion and
Opabinia, are the second most basal grade. • The
Radiodonta, which traditionally known as anomalocaridids come in third position, and are thought to be
monophyletic. • A possible "upper stem-group" assemblage of more uncertain position The clade is defined by important changes to the structure of the head region such as the appearance of a differentiated
deutocerebral appendage pair, which excludes more basal taxa like radiodonts and "gilled lobopodians". The placement of the
Artiopoda (which contains the extinct trilobites and similar forms) is also a frequent subject of dispute. The main hypotheses position them in the clade
Arachnomorpha with the Chelicerates. However, one of the newer hypotheses is that the chelicerae have originated from the same pair of appendages that evolved into antennae in the ancestors of
Mandibulata, which would place trilobites, which had antennae, closer to Mandibulata than Chelicerata, in the clade
Antennulata. The
fuxianhuiids, usually suggested to be stem-group arthropods, have been suggested to be Mandibulates in some recent studies. }}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}|label1=
Panarthropoda}}List of arthropod groups and genera († denotes extinct taxa) • "
Dinocaridida"
† (generally considered
paraphyletic, sometimes treated as
lobopodians) •
Kerygmachelidae † •
Parvibellus † (possible
"Siberiid Lobopodian") •
Opabiniidae † •
Agnostida (possibly trilobites) † •
Nektaspida † •
Erratus † •
Fengzhengia † •
Isoxyida † •
Kiisortoqia † •
Bradoriida † •
Megacheira † (possibly paraphyletic, alternatively placed as stem-chelicerates) •
Chelicerata •
Chasmataspidida † •
Eurypterida † •
Arachnida •
Phosphatocopina (possible stem mandibulate) † •
Mandibulata •
Hymenocarina † •
Euthycarcinoidea † •
Thylacocephala? † •
Myriapoda •
Pancrustacea •
Oligostraca •
Ostracoda •
Mystacocarida •
Ichthyostraca •
Multicrustacea •
Cyclida † •
Thecostraca •
Tantulocarida •
Copepoda •
Malacostraca •
Allotriocarida •
Cephalocarida •
Branchiopoda •
Remipedia •
Hexapoda •
Collembola •
Protura •
Diplura •
Insecta •
Incertae sedis •
Aaveqaspis † •
Arthrogyrinus † •
Bennettarthra † •
Burgessia † •
Cambropachycopidae † •
Cambropodus † •
Camptophyllia † •
Chuandianella † •
Keurbos † •
Notchia † •
Papiliomaris † •
Parioscorpio † •
Pleuralata † •
Rhynimonstrum † •
Sarotrocercus † •
Strabopida † •
Sunellidae † •
Wingertshellicus † •
Zhenghecaris †
Living arthropods The phylum Arthropoda is typically
subdivided into four
subphyla, of which one is
extinct: •
Artiopods are an extinct group of formerly numerous marine arthropods. They were reduced to a handful of orders in the
Late Devonian extinction, then became extinct in the
Permian–Triassic extinction event. They contain groups such as the
trilobites,
nektaspids,
aglaspidids, and the
cheloniellids among others. •
Chelicerates comprise the marine
sea spiders and
horseshoe crabs, along with the terrestrial
arachnids such as
mites,
harvestmen,
spiders,
scorpions and related organisms characterized by the presence of
chelicerae,
appendages just above/in front of the
mouthparts. Chelicerae appear in scorpions and horseshoe crabs as tiny
claws that they use in feeding, but those of spiders have developed as
fangs that inject
venom. •
Myriapods comprise
millipedes,
centipedes,
pauropods and
symphylans, characterized by having numerous
body segments each of which bearing one or two pairs of legs (or in a few cases being legless). All members are exclusively terrestrial. •
Pancrustaceans comprise
ostracods,
barnacles,
copepods,
malacostracans,
cephalocaridans,
branchiopods,
remipedes and
hexapods. Most groups are primarily
aquatic (two notable exceptions being
woodlice and hexapods, which are both purely
terrestrial) and are characterized by having
biramous appendages. The most abundant group of pancrustaceans are the terrestrial hexapods, which comprise
insects,
diplurans,
springtails, and
proturans, with six
thoracic legs. The
phylogeny of the major extant arthropod groups has been an area of considerable interest and dispute. Recent studies strongly suggest that Crustacea, as traditionally defined, is
paraphyletic, with Hexapoda having evolved from within it, so that Crustacea and Hexapoda form a clade,
Pancrustacea. The position of
Myriapoda,
Chelicerata and Pancrustacea remains unclear . In some studies, Myriapoda is grouped with Chelicerata (forming
Myriochelata); in other studies, Myriapoda is grouped with Pancrustacea (forming
Mandibulata), or Myriapoda may be sister to Chelicerata plus Pancrustacea. as well as the estimated timing for some of the clades: }} == Interaction with humans ==