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The mandible has evolved from a primitive biramous limb (fig.1.7). The primitive mandibular palps have clearly been lost independently several times in myriapods (fig.1.7D,E), insects (fig.1.7.G,H) and in certain crustacean taxa. However, the evolution of the mandibular protopodite is less obvious. In the majority of mandibles, the gnathal edge (derived from an endite) is present on the most proximal segment, which is called the coxa (Boxshall, 2004). The only clear exception is the diplopod (millipede) mandible which has a more proximal segment (fig.1.7E).
Boxshall argues that the primitive ancestral mandible protopodite was a two segmented protopodite. The two segments are called the coxa and basis and are present in crustacean mandibles (fig.1.7B,C,F). The crustacean mandibular gnathal edge is present on the more proximal segment, the coxa, whilst the two palps (exopodite and telopodite) attach to the distal basis segment (see fig.1.7B,C). A coxa and basis were present in the possible stem lineage mandibulate Phosphatocopida (Siveter et al., 2001).
The segmented myriapod mandible The diplopod mandible protopodite consists of two segments, a proximal cardo and distal stipes (subcoxa and coxa) shown in fig.1.7E and fig.1.8C. The incisor and molar processes are present on the distal segment of the protopodite, the stipes (or coxa), on a movable gnathal lobe. Both the segmented mandible and the movable gnathal lobe of Diplopoda are considered derived structures (Boxshall, 2004;
Edgecombe, 2004). Alternatively, the diplopod mandible could represent the primitive state of the mandible if the ancestral mandible had a subcoxal segment as Machida and Kukalova-Peck have hypothesized (Kukalova-Peck, 1998; Machida, 2000; Haas et al., 2001).
There is a resemblance of the movable gnathal lobe of the diplopod mandible (fig. 1.9C) to the proximal endite of putative stem lineage mandibulates such as Martinssonia (fig.1.6C). The proximal endite is present on several appendages including the third and fourth post-antennulary segments (mandibular and maxillary) (Muller and Waloszek, 1986; Waloszek et al., 2007).
The diplopod mandible (fig.1.9C) has some similarities to the protopodite of the insect maxilla (fig. 1.9D). Both appendages are monocondylic and have two protopodal segments, a cardo and stipes. The gnathal lobe of the diplopod mandible is connected by a muscle to the stipes. There is a similar muscle connecting one of the maxillary endites, the lacinia, to the stipes that may suggest homology (Snodgrass, 1935; Haas et al., 2001).
Evolution of the Maxilla
In order to homologize the mandible to the maxilla, it is necessary to understand how the maxilla has evolved from the primitive biramous limb from which the mandible has also evolved. However, it is difficult to construct a hypothesis of maxilla evolution from an ancestral biramous limb because of the diversity of maxillary appendages. Despite this it is useful to try to construct a hypothetical scheme in order to highlight the implications of trying to homologize the mandible to the maxilla (see fig. 1.8).
Fig.1.8. Hypothetical maxilla evolution from an ancestral biramous limb. The telopodite is highlighted blue, the protopodite is highlighted in yellow, the exopodite is highlighted white. The precoxa (pcx) or subcoxa (scx), coxa (cx), basis (ba), shaft (sh), proximal endite (pe), cardo (ca), stipes (st), lacinia endite (la), galea endite (ga) are indicated. (A) Third post-antennal limb of Martinssonia elongata homologous to the maxilla. The limb is biramous, maxilla-like with several endites, two on the basis, the most proximal endite is more developed. The unsegmented protopodite (ba) is attached to a shaft (sh). (B) Hypothetical ancestral pancrustacean maxilla adapted from Boxshall (Boxshall, 1998). The protopodite has two segments, a coxa and basis with two endites on each present in crustacean taxa such as Cephalocarida, Branchiura and Malacostraca. (C) Crustacean maxilla with three protopodite segments.
The coxa of B is divided into a precoxa/subcoxa and coxa present in crustacean taxa such as Copepoda, Ostracoda, Mystacocarida and Remipeda. (D) Maxilla of Archaeognathan hexapod Pedetontus unimaculatus. The exopodite has been lost and the protopodite has three segments: precoxa/subcoxa, coxa and basis. The lacinia and galea endites are present on two separate segments unlike the maxilla in E. (E) Typical insect maxilla with a two segmented protopodite with a lacinia and galea endite on the stipes. Boxshall homologises these segments to D as follows: The precoxa/subcoxa is homologous to the cardo. The coxa and basis are fused and are homologous to the stipes. (F) The second chilopod maxilla has lost the endite. The coxa of the maxilla is fused to the sternite (s). both coxae of the maxillae are fused to the sternite to form the coxosternite. The second maxilla is actually homologous to the fourth post-antennal limb of Martinssonia which resembles the third post-antennal appendage shown in A. (G) Diplopod gnathochilarium consists of the protopodite of a pair of maxillae fused at the ventral midline that have lost both telopodites.
Maxillules (homologous to the maxilla of both hexapods and myriapods) of crown group crustaceans are diverse in structure. Some crustaceans (like malacostracans) have two segmented maxillules (coxa and basis) with one endite associated with each segment or two endites associated with each segment (see fig.1.8B). Other crustaceans have three segmented maxillules (a precoxa, coxa and basis) with endites on each segment (see fig.1.8C). The basis has one or two endites present. Cambrian arthropods such as Rehbachiella and Martinssonia also have three or four endites present on the protopodite of the fourth post-antennal appendage (Muller, 1983; Muller and Waloszek, 1986).
Insect maxillae (fig. 1.8E) have two segments, a cardo and stipes, with two endites, a lacinia and galea, attached to the stipes. Archaeognathan hexapods have three segmented protopodites with two endites, the lacinia and galea, present on the two distal-most segments (Kukalova-Peck, 1998; Machida, 2000).
Therefore, one of the most parsimonious reconstructions of maxilla evolution is that the primitive two segmented protopodite evolved into three segments in some crustaceans and primitive hexapods. The two segmented insect maxilla (fig. 1.8E) evolved from a three segmented maxilla present in archaeognathan hexapods (and crustaceans) (fig.1.8D). There are no endites present on the proximal segment of the insect maxilla. This is in contrast to the gnathal edge of the mandible which is present on the proximal segment of the protopodite.
Serial homology of the mandible to the maxilla
The hexapod maxilla (including the insect maxilla) is potentially a useful appendage to study in order to homologize mandibular structures to the primitive biramous limb. The typical hexapod maxilla has more similarities to the primitive biramous limb than the mandible. For example, there is often retention of a palp (the telopodite) and the endites are more similar to those present on biramous limbs.
Machida has hypothesized that the mandibular gnathal edge is homologous to the maxillary lacinia and galea. He based his conclusion on SEM studies on an archaeognathan hexapod Pedetontus unimaculatus (Machida, 2000). In the developing embryo, the mandibular appendage has two lobes, an outer and inner lobe. Machida argued that these structures related to the incisor process and molar process and are serially homologous to the galea and lacinia endites of the maxilla respectively. He also Fig. 1.9. Serial homology of the mandible to the maxilla. Figures A,B are adapted from Machida (2000).
Figures C,D are adapted from Snodgrass (1935). (A) Machida’s hypotheses of homology between the mandible to the maxilla. The subcoxa (asterisk) and coxa (star) of the mandible are proposed to be homologous to the cardo and stipes of the maxilla (black arrows). The molar (inner lobe) is homologous to the lacinia. The incisor process (outer lobe) is homologous to the galea (red arrows)(Machida, 2000).
The two endites, the molar and incisor processes, are present on the more distal coxal segment (star) which is homologous to the stipes. (B) Maxilla of archaeognathan hexapod Pedetontus unimaculatus.
The protopodite is divided into three segments: subcoxa, coxa and basis. The lacinia and galea are present on separated segments, unlike the mandible incisor and molar processes shown in A. If this maxilla is primitive, it means that the insect maxilla stipes (D) is likely to consist of two segments, coxa and basis, that have fused. (C) Diplopod mandible is divided into two segments, a cardo and stipes. A muscle attaching the gnathal lobe to the stipes is highlighted in red. (D) Generalized insect maxilla.
Muscle attaching the lacinia to the stipes is highlighted in red, which could be homologous to the highlighted muscle shown in C. The telopodite is highlighted blue, the protopodite is highlighted in yellow, the exopodite is highlighted white. The basis (ba), incisor process (in), molar process (mo), cardo (ca), stipes (st), lacinia endite (la), galea endite (ga) are indicated.
argued that the mandible consisted of two segments (a subcoxa and coxa) that are serially homologous to the cardo and stipes of the maxilla. Machida therefore interprets the mandible gnathal edge as consisting of two endites, and that the primitive mandible had a subcoxal segment (see fig.1.9A).
Machida’s hypothesis is a little simplistic. Evidence for his hypothesis is provided from comparison of the embryonic development of the mandible and maxilla from one species and does not take into account the diversity of maxillary appendage structures that are found in arthropods. Nor does he take into account the evolutionary history of the maxilla. Relating the maxilla to the mandible in terms of serial homology requires understanding the evolution of these two appendages from when they diverged from an identical serially homologous biramous limb over 500 million years ago (compare fig.1.7 to fig. 1.8).
Boxshall disagrees with two particular aspects of Machida’s hypothesis.
Machida homologizes the maxillary lacinia and galea to the mandibular molar and incisor processes. However, the archaeognathan maxilla has the lacinia and galea present on separate segments, a coxa and basis (see fig.1.9B) which suggests that the insect maxilla has evolved by fusion of the coxa and basis to form the stipes. This fusion resulted in the two endites being present on the same segment in the insect maxilla. However, there is no arthropod mandible that has the incisor and molar processes present on separate segments and so Machida’s hypothesis is considered unlikely.
Also, Machida hypothesizes that the mandible has a more proximal subcoxal segment and that the gnathal edge is present on a distal segment of the protopodite (fig.1.9A). However, in the majority of arthropods, the mandible gnathal edge is present on the proximal segment of the protopodite (see fig.1.7B-D,F-H) and not a distal segment as Machida hypothesized (Boxshall, 2004).
The subcoxa origin of the pleuron.
Hexapod legs have one obvious protopodite segment, the coxa. It is hypothesized that hexapods also have a subcoxa, an additional appendage segment, which is incorporated into the body wall to from the pleuron. This is the known as the subcoxal theory of the pleuron (Snodgrass, 1935; Boxshall, 2004). Pleural sclerites are also present in myriapods (Bäcker et al., 2008). The pleuron is likely to be an example of convergent evolution between Hexapoda and Myriapoda, to terrestialization by providing additional support to the basal joint of the walking legs. In numerous crustacean trunk limbs with a three segmented protopodite, the proximal segment, the precoxa, is incorporated into the body wall.
There is potential to homologize parts of proximal leg segments to the proximal protopodite segments of the gnathal appendages. If the pleural sclerites derive from a subcoxal segment (the subcoxal hypothesis), then there could be a homologous relationship between the subcoxa of the legs and the proximal protopodite segments of other appendages (Boxshall, 2004). In which case, the subcoxa could be homologous to the proximal segments of the gnathal appendages.
Hypotheses of embryonic mandible structure