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It is possible that myriapod fossils were not abundant due to the rarity of myriapod species or the myriapod habitat not being conducive to fossil preservation (Shear and Edgecombe, 2010). Another explanation of this ghost lineage is that stem lineage myriapods were crustaceamorphs, their body plans sharing a significant number of crustacean characters which have been lost in the stem lineage to the myriapods. Some primitive characters have certainly been lost in myriapods such as the palps on the mandibles, the exopodites on all limbs, and the second antennae.
Fig.1.6. The crustaceamorph Martinssonia elongata, a possible stem lineage representative of Mandibulata. figure is adapted from Muller and Waloszek (1986). (A) SEM of Martinssonia larva.
Anterior is to the left, dorsal is up. (B) Schematic of (A). (C) SEM of the second appendage and third appendage. (D) Schematic of the second appendage, the first post-antennulary appendage homologous to the second antenna of Crustaceans. (E) Schematic of the third appendage, the second postantennulary appendage homologous to the mandible. (F) the fourth appendage, third post-antennal appendage homologous to the first maxilla of mandibulates. Martinssonia shows features of a possible stem lineage mandibulate such as relatively undifferentiated serially homology biramous limbs. There is an enlarged proximal part with endites on the medial side. The second post-antennal appendage (homologous to the mandibular segment) does not possess a characteristic mandibulate gnathal edge, which is required to define it as a true mandibulate. The postantennal limbs have three endites, the most proximal of which are more developed and form a moveable plate-like structure. There is no typical mandibular gnathal edge present. The proximal endite on the second postantennal limb, homologous to the mandibular limb, is more developed than the endites of other limbs and could represent the developing mandibular endite. Abbreviations are as follows: telopodite (en), exopodite (ex), protopodite (pro) basis (ba), shaft (sh), proximal endite (pce), endite spines (esp), antenna (atl), second (apII), third (apIII), fourth (apIV), fifth (apV), sixth (apVI), and seventh appendages (apVII).
These losses have also occurred in the hexapod lineage, leading to the idea that these may be convergent losses due to adaptations to a terrestial environment.
Therefore, the apparent lack of stem lineage mandibulate or myriapod fossils may actually be due to mis-interpretation of Cambrian crustaceamorph fossils. As crustaceamorph fossils share most characters with crustaceans there is a bias favouring the placing of these crustaceamorph fossils as crustaceans. Some of them may in fact be stem lineage mandibulates or myriapods. Identification of a myriapod fossil could help in re-constructing the primitive mandible by comparing the mandible present on ancestral myriapods to the mandibles of ancestral crustacean species.
Stem lineage Mandibulata
If Mandibulata is accepted with the mandibular gnathal edge as a synapomorphic character of this group, a well-defined mandible (complete with incisor and molar processes) should be present in stem lineage mandibulates.4 (Kraus, 2001;
Edgecombe et al., 2003). If crustaceamorph fossils are lacking true mandibular appendages, then they cannot be either crustaceans or crown group mandibulates.
One crustaceamorph fossil that could, however, be interpreted as a member of the stem lineage leading to Mandibulata5 is Martinssonia elongata (Muller and Waloszek, 1986) (see fig. 1.6).
The proximal endite on the second postantennal limb of Martinssonia, homologous to the mandibular limb, is thought to represent the developing mandibular endite in the stem lineage to Mandibulata (Edgecombe, 2010; Haug et al., 2010; Rota-Stabelli et al., 2011). The appendages on the segments homologous to the mandibular and maxillary segments, the second (see fig. 1.6E) and third (see fig. 1.6F) post-antenullary appendages of Martinssonia, are similar to one another. A reasonable hypothesis is that the mandible evolved through a biramous maxilla like appendage. The primitive maxilla and maxilla-like precursor of the mandible would have probably shared similar developmental pathways as they are serially homologous appendages. To understand the evolution of the mandible may therefore be helped by an understanding of maxilla development, and subsequent understanding of how the mandible is differentiated from a maxillary appendage.
Contrary to this view, Waloszek et al. on the C.O.R.E. website have stated: “a mandible was NOT present in the beginning of the evolution of Crustacea, so any names referring to a mandible, such as Mandibulata, are misleading or wrong” http://www.core-orsten-research.de/ Other possible stem lineage mandibulates include: Oelandicaris, Cambrocaris, Cambropachycope, Goticaris, Henningsmoenicaris scutula, Tanazios dokeron, Apankura machu and Phospatocopida (Siveter et al, 2001; Vaccari et al. 2004; Siveter et al. 2007; Zhang et al. 2007; Edgecombe, 2009)
Chelicerate evolution and stem lineage fossils
The only extant clade of non-mandibulate arthropods is the Chelicerata, comprised of arachnids (spiders and scorpions), xiphosurans (horseshoe crabs) and pycnogonids (sea spiders).
Identification of the stem lineage of Chelicerates is also problematic due to fossils lacking characters only present in crown group chelicerates (autapomorphies). There are, however, numerous Cambrian fossils that are hypothesized to be part of the stem lineage to Chelicerata.6 These taxa are characterized by the presence of biramous limbs, like stem lineage arthropods as well as possessing similarities to two characters which are diagnostic for members of the chelicerate crown group: the chelicerae (pincer-like appendages that may have evolved from antennal appendages) and the prosoma associated with six pairs of appendages. The presence of structures that may represent precursors of these two characters on particular Cambrian fossils, such as the megacheiran great appendage (which has been interpreted as a precursor to the chelicerae), favours their placement on the stem lineage to Chelicerata.
It has been shown that the first leg segment of extant Chelicerates is homologous to the mandibular segment of Mandibulata based upon the anterior expression of Hox genes such as Dfd (Damen et al., 1998; Telford and Thomas, 1998b).
Therefore, as the mandible has evolved from a leg appendage, studying the development of the first leg appendage of chelicerates (particularly less derived forms present in the Xiphosura) could aid our understanding of mandible evolution.
Understanding of non-mandibulate appendages present in chelicerates and trilobites is important to identify the ancestral condition of the mandibular appendage.
It is also important for determining homology of the protopodite segments and endites between mandibulates and non-mandibulate groups.
For example possible stem lineage chelicerates include Sidneiya, Sanctacaris vacata, and Aglaspis spinifer Boxshall, G. A. (2004) 'The evolution of arthropod limbs', Biol Rev Camb Philos Soc 79(2): 253-300, Budd, G. E. and Telford, M.
J. (2009) 'The origin and evolution of arthropods', Nature 457(7231): 812-7, Chen, J.-Y. (2009) 'The sudden appearance of diverse animal body plansduring the Cambrian explosion', Int J Dev Biol 53(5-6): 733-51.. Also, the great appendage arthropods (megacheirans) are often considered as a stem group of the lineage leading to Chelicerata (designated Prochelicerata by Chen), for example, Haikoucaris, Fortiforceps, Leanchoilia, Yohoia.
1.6 Serial homology of the mandible and maxilla
In this section I will present a hypothetical scheme of mandible (fig.1.7) and maxilla (fig.1.8) evolution from a primitive biramous limb with a two-segmented protopodite (fig.1.7A,B and fig. 1.8A). In this scenario, the mandibular gnathal edge is hypothesized to be present on the proximal-most segment of the protopodite, the coxa. I will also present an alternative hypothesis which proposes that the mandibular biting edge is present on a more distal segment of the protopodite (see fig.1.9A).
Serial homology of two structures means that the two structures within the same organism share a common ancestry. If a structure is serially homologous to another structure, then identical genes have been used to pattern both the different structures and it is for this reason that the two structures look identical.
Serial homology requires that the same genes or gene regulatory network has patterned both of the structures giving rise to their morphological similarity.
Otherwise, in the unlikely scenario that those similar structures were patterned by different genes in the common ancestor, it would be an example of convergent evolution of those structures within the same organism.
To demonstrate serial homology between the mandible and maxilla protopodite requires showing that the same developmental genes are used to pattern those structures in the ancestral state. To suggest ancestral patterning mechanisms convincingly requires showing that in numerous representative taxa, the genetic mechanisms that are responsible for patterning the mandibular and maxillary protopodite are shared. It also requires an understanding of the evolutionary history of mandible and maxilla diversification from a common, shared, serially homologous biramous limb structure. There is significant diversity of protopodite morphology across arthropod taxa, such as the degree of segmentation of the protopodite and in the numbers of endites associated with each segment. This has implications for attempts to homologize the mandibular protopodite to the protopodites of other appendages.
Fig.1.7. Hypothetical mandible evolution from an ancestral biramous limb. The mandibular gnathal edge is located on the proximal protopodite segment (gnathal edge indicated with an arrowhead). The telopodite is highlighted blue, the protopodite is highlighted in yellow, the exopodite is highlighted white. (A) Second post-antennal limb of Martinssonia elongata. The limb is biramous, maxilla-like with a developed proximal endite. The unsegmented protopodite is attached to a shaft. (B,C) Hypothetical primitive mandibles derived from the mandible of a Cambrian fossil Maxillopod Bredocaris (B) and the biramous copepod mandible (C). The hypothetical plesiomorphic mandible (B,C) evolved from a maxillalike precuror (A) with reduction of the two palps and reduction of the basis on the telopodite (C). The exopodite palp is lost in numerous crustacean mandibles (F). The telopodite palp is independently lost in numerous pancrustacean mandibles (G). The mandible is structurally modified to possess two articulations and perpendicularly orientated gnathobase in the Dicondylia (H) Myriapod mandibles have lost both palps and the gnathal edge is present on a gnathal lobe (D-E). In this evolutionary framework, the diplopod mandible (E) has evolved an additional protopodite segment. Subcoxa (scx), coxa (cx), basis (ba), shaft (sh), mandible base (mb), proximal endite (pe) and gnathal lobe (gl) are indicated.
Morphological definition of the protopodite
The gnathobasic mandible gnathal edge is a modified endite which is located on the protopodite. Morphologically, the protopodite is defined as the segments at the base of the biramous limb to which the two branches - the telopodite and exopodite attach. This structure is the biramous limb which is primitive for Arthropoda and present in stem lineage arthropods shown in fig. 1.2D (Boxshall, 2004; Waloszek et al., 2007; Chen, 2009).
Definition of the protopodite on morphological grounds is more difficult for uniramous appendages as there is no exopodite so it is hard to define where the protopodite ends and the telopodite begins. To distinguish the protopodite and telopodite in uniramous limbs requires additional criteria and comparisons to other biramous limbs. The protopodite can also be defined by studying musculature patterns. The protopodite is also characterized by the presence of endites (Snodgrass, 1935; Boxshall, 1998).
The primitive euarthropod protopodite is generally considered to be unsegmented (Boxshall, 2004). In the primitive biramous limb, the telopodite and exopodite are attached to an unsegmented protopodite. This is the case for chelicerates, trilobites and stem lineage euarthropods. Mandibulate arthropods often have up to three segments present on the protopodite, which are called the subcoxa, coxa and basis.
Mandibular protopodite evolution