Deuteropoda

The clade contains all living arthropods (i.e. chelicerates and mandibulates) as well as several fossil groups that share these characteristics (e.g. Fuxianhuiida, Megacheira, Isoxyida, and Artiopoda, the last of which includes the trilobites), while excluding other fossil groups that are more 'basal' or 'primitive' (e.g. radiodonts, opabiniids and lobopodians). Members of Deuteropoda that are basal to the last common ancestor of chelicerates and mandibulates (and thus lie outside the crown group of Arthropoda) are often referred to as "upper stem group arthropods".

Defining characteristics

Javier Ortega-Hernández in his 2016 study naming the clade, proposed that Deuteropoda is differentiated from more basal total group arthropods by the presence of a differentiated labrum (an often flap-like structure at the front of the mouth) and a differentiated first pair of appendages connected by nerves (innervated) to a part of the brain called the deutocerebrum (making them "deutocerebral"). In contrast onychophorans (velvet worms) and "gilled lobopodians" (which are more closely related to arthropods than to onychophorans) have a pair of enlarged pre-ocular (located in front of the eyes) appendages (which form the antennae in onychophorans and the frontal appendages of "gilled lobopodians"), innervated by a part of the brain called the protocerebrum (making them "protocerebral"). These appendages have been proposed by a number of authors to be homologous to (derived from the same ancestral structure) and to have ultimately evolved to be the labrum of modern living arthropods, as well as to the segmented frontal appendages of radiodonts, which these authors argue was protocerebral. In living arthropods, the deutocerebral first appendage corresponds to the first pair (or only pair in some taxa) of antennae in mandibulates (as well in the extinct artiopods like trilobites) and the chelicerae in chelicerates. All deuteropods are suggested to have a multi-segmented head with at least three segments including the protocerebrum, deutocerebrum and tritocerebrum (the head in non-deuteropodan stem-group arthropods including radiodonts is often suggested in contrast to have been composed of a single segment including the protocerebrum).

While the clade comprising Deuteropoda (though not necessarily the name itself) is well accepted, the characters Ortega-Hernández proposed as supporting Deuteropoda are somewhat controversial, with a number of authors arguing that the transformation of the head from radiodonts to early deuteropods is less radical than proposed, and argue that there is no clear evidence of a switch between a protocerebral head appendage in radiodonts to a deutocerebral one in Deuteropoda, with some authors arguing that brains of radiodonts (as well as possibly also "gilled lobopodians" like Kerygmachela) were at least two-segmented rather than single segmented and that their frontal appendages were in fact deutocerebral, or that the first head appendages of early "deuteropodans" like megacheirans were in fact protocerebral.

Many early primitive deuteropod first appendages like those in Kylinxia, isoxyids and megacheirans are segmented grasping appendages that are upward (dorsally)-curling, as opposed to downward curling like the frontal appendages of radiodonts, which may represent a shared ancestral trait for most or all of Deuteropoda. It is disputed whether or not the upward-curling grasping appendage of early deuteropods are homologous to the segmented frontal appendage of radiodonts (as well as ultimately the annulated, unsegmented frontal appendages of "gilled lobopodians" and siberiid lobopodians like Megadictyon, to which radiodont frontal appendages are widely agreed to be homologous), to which they show strong structural similarities, or that they were convergently evolved separately. These grasping appendages of early deuteropods are suggested to have evolved into the chelicerae of chelicerates and the antennae of mandibulates.

Members of Deuteropoda lack the oral cone mouth present in radiodonts (with the radiodont oral cone homologous to toothed, typically radially arranged mouthparts found in lobopodians and other ecdysozoans), but instead ancestrally possess a posteriorly facing mouth on the underside of the head, which is at least sometimes covered by a hypostome (a plate-like structure). Another distinctive characteristic ancestrally present in deuteropodans (with the possible exception of the radiodont-like Oreinorema) but not more basal stem-arthropods is the presence of two-branched (biramous) segmented (arthropodized) limbs divided into an inner endopod and outer exopod, running along the body posterior to the mouth, distinct from the swimming flaps present in radiodonts and "gilled lobopodians" and the lobopods of lobopodians, though the appendages of many modern arthropods are secondarily uniramous (unbranched). The segmentation of the body limbs may have evolved from the co-option of the genes causing the segmentation of the first head appendage in the Deuteropoda-Radiodonta common ancestor. Most deuteropods (with the notable exception of Erratus and isoxyids among Cambrian deuteropods) also have a clearly segmented and jointed (arthrodized) trunk (main body section), though this has been lost in some highly specialized parasitic lineages such as in rhizocephalans and pentastomids.