Origin and earliest fossil records of hexapods

The sparse fossil record of mid-Palaeozoic hexapods, the pattern of colonization of land by Palaeozoic plant and arthropod groups, and temporal constraints on hexapod origins imposed by hypotheses of arthropod phylogeny on hexapod origins all suggest that the earliest hexapods probably appeared during the Late Silurian. Although often used as a synonym for 'Insecta', the term 'Hexa-poda' designates the more inclusive clade that consists of the Collembola (springtails) and Protura (proturans), collectively termed the Parainsecta, as well as its sister-group, the Insecta, which encompasses all other hexapods.

Early hexapod phylogeny

Phylogenetic evidence for the derivation of hexapods from another arthropod clade is still in an exploratory phase. Currently there are five major hypotheses of the origin of the Hexapoda (Fig. 1.3.9.1), but each is sufficiently unstable that differences in methodological approach, changes in included taxa or characters, and consideration of the fossil record can result in significant topological shifts within cladograms. This field is rapidly changing, and evidence linking the genetics and developmental biology of major arthropod clades is being used to test major phylogenetic schemes.

One of the oldest views of arthropod phylogeny is diphyly, with two superclades: the Schizoramia, consist-

■Trilobita "Chelicerata ■Crustacea "Myriapoda "Hexapoda

Trilobita Chelicerata Crustacea -Myriapoda Hexapoda ■ Onychophora

-Chelicerata -Crustacea -Myriapoda Hexapoda

Chelicerata "Myriapoda Crustacea

Hexapoda

- Myriapoda Onychophora Chelicerata Crustacea

Hexapoda

Schizoramia

Mandibulata

Uniramia

Atelocerata ing of Chelicerata + Trilobita; and its sister-group, the Mandibulata, comprising the Crustacea and Hexapoda + Myriapoda (Fig. 1.3.9.1a). By contrast, Sydney Manton did not recognize these relationships, arguing instead for separate origins of the Trilobita, Crustacea, Chelicer-ata, and a Uniramia clade defined as Onychophora and

Fig. 1.3.9.1 Summaries of five hypotheses indicating the relationships of hexapods (in bold) to other major arthropod groups. (a) The hypothesis of arthropod diphyly, with Schizoramia and Mandibulata as distinct clades. (b) The hypothesis of extensive arthropod polyphyly, showing the traditional Uniramia as a distinct clade. (c) The hypothesis of arthropod monophyly, featuring the Mandibulata and its subgroup, the Atelocerata, as distinct clades. (d) One hypothesis of arthropod monophyly and a Crustacea + Hexapoda clade. (e) Another hypothesis of arthropod monophyly and a Crustacea + Hexapoda clade, with Onychophora included as arthropods. Branch lengths are arbitrary; dashed lines indicate nodes that are either theoretically inferred or not statistically valid. This is not an exhaustive list; see Fortey and Thomas (1997) for additional details.

Hexapoda+Myriapoda (Fig. 1.3.9.1b). Since her thesis was presented, arthropod polyphyly and the concept of the 'Uniramia' has largely been abandoned. Variations of two other major hypotheses receive most support from recent biomolecular and morphological studies. The first is the mandibulate/atelocerate hypothesis, consisting of a Crustacea and Hexapoda+Myriapoda clade that is a sister group to the Chelicerata, with or without inclusion of the Trilobita (Fig. 1.3.9.1c). The second, most recent, hypothesis posits a monophyletic Crustacea + Hexapoda clade that is a sister-group to other major arthropod clades, namely the Chelicerata+Myriapoda (Fig. 1.3.9.1d) or the Chelicerata + Onychophora with Myri-apoda occurring as a basalmost clade (Fig. 1.3.9.1e). (See contributions in Fortey and Thomas (1997) for differing perspectives regarding the position of the Hexapoda within the Arthropoda.)

Within the Hexapoda, the position of the Parainsecta has always been controversial, particularly as the unique entognathous mouthparts and head sensory organs of the Collembola + Protura have frequently been cited as distinct from the Insecta. There is limited evidence that the Parainsecta may be separately derived from bran-chiopod crustaceans, a result that may explain why internal analyses of hexapods frequently result in an unresolved trichotomy involving the Parainsecta, the Diplura (which has variously been considered as parain-sectan or insectan), and the Insecta. Nevertheless, the Insecta is generally regarded as monophyletic, and is united by several secure apomorphies. The most important evolutionary developments within the Insecta (Kukalovâ-Peck 1991; Fig. 1.3.9.2) were: (1) the transformation of the monocondylous mandible of the Archaeognatha into a robust mandible articulating with the head capsule by two condyles (Dicondylia); (2) the acquisition of wings (Pterygota); (3) the origin of holometabolous development (Holometabola), in which a larval stage became separated from its morpholo-

Palaeoptera

Apterygota

O LU

Neoptera

Monocondylia Dicondylia

Entognatha Ectognatha

Pterygota

diversity and abundance, presenting more questions than answers (Shear and Kukalova-Peck 1990). Hexa-pods are absent as fossils during a 55 million year interval from the Middle Devonian to the latest Early Carboniferous, during which time the major features of winged insects (Pterygota) must have evolved, based on the spectacular diversity of approximately 10 pterygote insect orders that appear suddenly at an interval straddling the Early to Late Carboniferous boundary (Brauckmann et al. 1995).

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