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Undifferentiated PD axes are highlighted in green. The telopodite is highlighted blue, the protopodite is highlighted in yellow, the exopodite is white. (A) Monopodial limb with annulations represented by the lobopodian Aysheaia. (B) Simple biramous limb divided into exopodite and an undifferentiated PD axis in proarthropods such as Shankouia zhenghei, Fuxianghuia protensa, Chengjiangcaris longiformis. (C) Multisegmented limb with the PD axis differentiated into a segmented protopodite and telopodite as represented by Canadapsis laevigata. Endites are present on the medial surface. Other examples of undifferentiated and multisegmented biramous limbs were present in the megacheirans (possible stem lineage Chelicerates) Leachoilia, Fortiforceps, and Tanglangia. More differentiated multisegmented limbs were present in fossils like Ercaia miuscule, Cindarella eucalla, and Saperion glumaceum. Biramous limbs with differentiation of the P/D axis into clear protopodite and telopodite divisions are present in the trilobite like Naroia and Euarthropods (Chen, 2009). (D) Differentiated limb with an unsegmented protopodite with a single well-developed endite as represented by the trilobite-like Naraoia¸and possibly the ancestral state of all arthropod post-antennal appendages. The unsegmented protopodite is attached to a shaft (sh).
The mandible is a post-antennulary appendage and it follows that it has evolved from a biramous limb. The ancestral mandible was likely to be a biramous limb, with two palps, and a gnathal edge (a modified endite) present on the base of the appendage. This gnathal edge would have probably had an incisor and molar processes.
Evolution of simple biramous limbs
The gradual evolution of biramous limbs from a simple unsegmented lobe-like monopodial limb is evident in the Cambrian fossil record and examples of each stage of limb development are present in different Cambrian arthropods, which is shown in fig.1.2 (Boxshall, 2004; Waloszek et al., 2007; Chen, 2009). The evolution of the arthropod limb progresses from simple lobes or annulated rod-like limbs present in lobopodians or tardipolypods through stem-lineage groups to crown group Euarthropoda, the true arthropods, in possession of differentiated biramous limbs (Waloszek et al., 2007; Chen, 2009; Edgecombe, 2010).
The lobopodians are stem lineage panarthropods, and may well be the ancestors to all arthropods and onychophorans. The lobopodians possess monopodial limbs (see fig.1.2A) that are homologous to all appendages of extant arthropods1 (Waloszek et al., 2007; Budd and Telford, 2009; Chen, 2009). Members of one extant clade, the Onychophora, resemble Cambrian lobopodians and possess lobopodial limbs, however, onychophorans also possess numerous derived characters, such as terrestialisation and appendages such as jaws (not mandibles) and slime papillae (Budd and Telford 2009).
An intermediate stage of arthropod limb evolution from monopodial limbs to the biramous limb is present in some stem lineage arthropods (Waloszek et al., 2007;
Chen, 2009; Edgecombe, 2010). Some of these stem lineage arthropods gave rise to the euarthropods which include the mandibulate arthropods, chelicerates and extinct taxa such as a trilobites, naraoiids and megacheirans. The limbs on numerous stem lineage arthropods are simple biramous limbs that consist of a simple segmented rod and a flap which are homologous to the telopodite and exopodite respectively. These limbs are repeated on each segment and are undifferentiated (see fig. 1.2B).
The final stage of biramous limb evolution involves the differentiation of the proximal and distal axis of the limb and the development of endites (see fig. 1.2C,D).
The differentiation of the proximal part of the arthropod biramous limb with the addition of endites was probably associated with a change in diet and was a key event in the evolution of the Euarthropoda (Chen, 2009).
Endites on the protopodite
The protopodite is the proximal part of the biramous limb that is attached to the body. The protopodite is defined as the proximal part of the limb to which the telopodite and exopodite are attached. The primitive state of the protopodite of the biramous limb is considered to be unsegmented with a solitary endite. Numerous arthopod taxa possess a non-segmented protopodite with one endite (see fig.1.2D).
Examples of single endites on undivided protopodites include all post-antennal limbs of trilobites, the prosomal limbs of horse shoe crabs (Xiphosurans), the pedipalps of These monopodial limbs are posterior to antennae that are of uncertain homology, but the base of these antennae may be homologous to a structure present in all euarthropods called the Labrum.
spiders, and the first two legs of scorpions and this is considered to be the primitive state.
Mandibulate arthropod appendages that have endites include the mandible, naupliar antennae of crustaceans, the post-mandibular limbs of cephalocarid and branchiopod crustaceans, and the maxillae of hexapods, chilopods and symphylans and crown group crustaceans. There are also many appendages that possess multiple endites on each segment, for example in the Cambrian fossil crustacean Rehbachiella kinnekullensis there are up to nine endites on the unsegmented protopodite of the trunk limbs and four present on the maxilla (Walossek 1993). There are up to 6 endites present on extant branchiopods. There are usually two endites present on the maxilla of Hexapods called the lacinia and galea (Boxshall, 2004).
Mandibulate arthropod protopodites are often segmented which indicates that the protopodite has added segments to the primitive protopodite. It also means that the endites of chelicerates and trilobites are not homologous to the endites of the mandibulate arthropods. In order to conclude about the serial homology of the mandible to other appendages of mandibulate arthropods, it is necessary to identify which segment the mandibular gnathal edge is attached to, so that it can be compared to the segments of other appendages.
Evolution of the biramous limb into all arthropod appendages.
A characteristic of arthropods is the possession of a wide range of specialized limbs and appendages adapted for a plethora of different functions. The arthropod body plan with its basic composition of a series of repeating segments, each bearing a pair of appendages which can evolve as a module separate from other segments, has most likely facilitated the evolution of this group to fit such a wide variety of habitats (Williams and Nagy, 2001; Giorgianni and Patel, 2005).
Fig.1.3. Evolution of post-antennal arthropod appendages from the ancestral biramous limb. The telopodite is highlighted blue, the protopodite is highlighted in yellow, the exopodite is highlighted white. The ancestral biramous limb (A,B) evolved into the gnathal appendages such as the mandible with palp (C), mandible without a palp (D), maxilla (E), gnathochilarium (F) or trunk appendages such as the biramous limb (G), uniramous limb (H) and phyllopodous limb (I). The proximal endite from which the mandibular gnathal edge may have evolved (present in some Cambrian fossils such as Martinssonia) is highlighted in black in (B).
All post-antennal appendages such as the mouthparts and trunk limbs of Euarthropoda evolved from the ancestral biramous limb (see fig.1.3). Appendages near the anterior of the organism are often adapted to form sensory appendages and mouthparts such as the mandibles, maxillae, chelicerae, labial and second antenna (fig.1.3B-F). Sensory specialization is evident in the palps, endites, and the abundance of sensory organs on these appendages. Evolution of these appendages is often associated with incorporation of their segments into head ‘tagma’ (an aggregation of segments into a distinct unit), for example into the heads of mandibulates.
Appendages of the trunk, by contrast, are often adapted to particular modes of locomotion. For example, uniramous limbs have evolved in numerous terrestial arthropods such as the myriapods, insects and isopods (woodlice) (fig. 1.3H). In these lineages, the expodite has been lost (likely due to convergent evolution). There are no biramous limbs present in myriapods or hexapods. The biramous limb is often retained in taxa that reside in aquatic environments (See fig.1.3G.). One form of biramous limb phyllopodous limbs - is paddle-like and has evolved for swimming in taxa like the branchiopods (See fig.1.3I). Appendages that are near the posterior of the animal are often lost, for example the abdomen of insects, and the opisthosoma of chelicerates typically lack appendages (Boxshall, 2004; Angelini and Kaufman, 2005; Giorgianni and Patel, 2005).
Evolution of the biramous limb into the mandible.
The mandible is an evolutionary novelty that permitted numerous additional means of capturing prey or processing food using a biting or cutting tooth (pars incisiva), and a grinding or crushing molar process (pars molaris). The mandible, like all post-antennulary appendages, evolved from a biramous limb with endites present on the base of the limb. A possible scheme of mandible evolution is illustrated in fig.1.7.
The mandible gnathal edge has probably evolved from a biramous limb by modification of the proximal endite on the protopodite (fig. 1.7A,B), reduction of the size of the telopodite and exopodite (fig. 1.7C,F) and changes in protopodite structure resulting from adaptations to functioning as a biting/chewing mouthpart. Numerous lineages have lost the exopodite (fig.1.7F) and in many cases the telopodite too (fig.1.7D,E,G,H).
One structure that the gnathal edge may have evolved from is the proximal endite as seen on the protopodite of extinct species such as Martinssonia and Henningsmoenicaris that is larger and more developed than other endites (Muller and Waloszek, 1986; Boxshall, 2004; Waloszek et al., 2007; Edgecombe, 2010; Rota-Stabelli et al., 2011). The proximal endite of Martinssonia is shown in fig. 1.6C,D and 1.7A.
1.3 Mandible diversity
The mandible is the anterior-most of the gnathal appendages (mandible, maxilla and labial appendages) in the gnathocephalon (see fig. 1.1C), the posterior head region of mandibulates. The labrum forms an upper lip that, together with the mandibles and hypopharynx, forms a chewing chamber (Bucher and Wimmer, 2005;
Scholtz and Edgecombe, 2006; Edgecombe, 2010). The pair of mandibular appendages flank the mouth-opening and sit underneath the labrum which forms the roof of the mouth. The similar arrangement of the mandible relative to other appendages on homologous segments suggests that the mandible is homologous in all mandibulates (Wägele, 1993; Bitsch, 1994; Edgecombe, 2010). The mandible is followed by more posterior manipulative and sensory appendages called maxillary appendages or maxillae. Some maxillary appendages have fused at the ventral midline, for example the gnathochilarium of diplopods and the labial appendages of hexapods.
The primitive function of the mandible is a biting chewing appendage that can also be used to manipulate food. The biting edge consists of two parts that are common to the majority of mandibles, an incisor process (the pars incisiva) and a molar process (the pars molaris), which are shown in fig.1.1. This type of mandible is present in the majority of taxa within the Mandibulata. All millipedes and centipedes and the majority of crustaceans (if not all) are in possession of this type of mandible.
The insects display considerably more diverse functional adaptations of the mandible and the gnathal appendages in general than other members of Mandibulata.
While the biting/chewing mandible still remains in the majority of insect taxa 2, such as the orders Orthoptera, Coleoptera, Hymenoptera and numerous basal orders relative to the Holometabola, it has adapted to numerous functions in different taxa.