Biology:Deuteropoda

Deuteropoda is a clade of arthropods whose members are distinguished from more basal stem-group arthropods like radiodonts by an anatomical reorganization of the head region (which is proposed to include a differentiated, titular "deutocerebral" first appendage pair, a multisegmented head, and a hypostome/labrum complex), and by bearing pairs of segmented biramous (two branched) limbs.

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

Diagram of primitive deuteropod Isoxys curvirostratus showing upward-curling grasping first appendage as well as two branched (biramous) limbs consisting of an inner endopod and outer exopod, as is characteristic of Deuteropoda.

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 (part of the mouth) and a differentiated first pair of appendages innervated by a part of the brain called the deutocerebrum (making them "deutocerebral"). In contrast, lobopodians (part of the "lower stem group") and onychophorans have a pair of pre-ocular (located in front of the eyes) appendages innervated by a part of the brain called the protocerebrum (making them "protocerebral"), which has been proposed by a number of authors to have evolved to be the labrum of modern living arthropods, and to be homologous (derived from the same ancestral structure) to the segmented frontal appendages of radiodonts, which these authors argue was protocerebral. In living arthropods, the deutocerebral appendage corresponds to the first pair of antennae in mandibulates 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 generally 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 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 were two-segmented rather than single segmented and that their frontal appendages were in fact deutocerebral, or that the first appendages of early deuteropodans were in fact protocerebral.

Many early primitive deuteropod first appendages like those in Kylinxia and isoxyids 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 frontal appendage of radiodonts, to which they show strong structural similarities, or that they were convergently evolved separately.

Members of Deuteropoda lack the oral cone mouth present in radiodonts, but instead ancestrally possess a posteriorly facing mouth on the underside of the head, which is at least sometimes covered by a hypostome. Another distinctive characteristic ancestrally present in deuteropodans (with the exception of the radiodont-like Oreinorema) but not more basal stem-arthropods is the presence of two-branched (biramous) segmented (arthropodized) limbs 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 exception of Erratus and isoxyids, and secondarily some modern arthropods) also have a segmented and jointed (arthrodized) trunk (main body section),

Phylogeny

The cladogram below is a possible panarthropod phylogeny, taking into account the differentiation between "lower stem group arthropods" (Total group Euarthropoda) and "upper stem group arthropods" (Deuteropoda), based on a phylogenetic analysis published in 2026. Not all parts of this cladogram are universally accepted, as some other studies have found a closer relationship to Chelicerata than to Mandibulata for Artiopoda, and for Megacheira to be outside crown-group Arthropoda.

Panarthropoda

† "Lobopodia" 80 px 90 px 50 px (paraphyletic, ancestral to tardigrades, onychophorans and arthropods)   Total group Euarthropoda   †Megadictyon 80 px         †Kerygmachela 80 px       †Pambdelurion 80 px       †Opabiniidae 70 px       † Radiodonta (e.g Anomalocaris) 70 px   Deuteropoda   †Erratus 70 px       †Sunella 70px       †Isoxyidae 70px + "Great appendage bivalved forms" 70px         †Fengzhengia 80 px     †Kylinxia 70px         †Megacheira 80px     Pan-Chelicerata 70px 70px         †Kiisortoqia 70px     †Bushizheia 70px   Antennulata   †Artiopoda (including Trilobita) 70px   Mandibulata   †Fuxianhuiida 70px       †Hymenocarina 70px       Myriapoda 70px     Pancrustacea 70px 70px

References

Wikidata ☰ Q112480276 entry