Abstract
Mandibular mechanisms in Geophilomorpha are revised based on three-dimensional reconstructions of the mandibulo-tentorial complex and its muscular equipment in Dicellophilus carniolensis (Placodesmata) and Hydroschendyla submarina (Adesmata). Tentorial structure compares closely in the two species and homologies can be proposed for the 14/17 muscles that attach to the tentorium. Both species retain homologues of muscles that in other Pleurostigmophora are traditionally thought to cause swinging movements of the tentorium that complement the mobility of the mandibles. Although the original set of tentorial muscles is simplified in Geophilomorpha, the arrangement of the preserved homologues conforms to a system of six degrees of freedom of movement, as in non-geophilomorph Pleurostigmophora. A simplification of the mandibular muscles is confirmed for Geophilomorpha, but our results reject absence of muscles that in other Pleurostigmophora primarily support see-saw movements of the mandibles. In the construction of the tentorium, paralabial sclerites seem to be involved in neither Placodesmata nor Adesmata, and we propose their loss in Geophilomorpha as a whole. Current insights on the tentorial skeleton and its musculature permit two alternative conclusions on their transformation in Geophilomorpha: either tentorial mobility is primarily maintained in both Placodesmata and Adesmata (contrary to Manton’s arguments for immobility), or the traditional assumption of the tentorium as being mobile is a misinterpretation for Pleurostigmophora as a whole.
Highlights
The tentorium of myriapods is a cuticular formation of the head with a distinct composition of exoskeletal bars around the mouth opening and endoskeletal processes (Koch 2003)
In the construction of the tentorium, paralabial sclerites seem to be involved in neither Placodesmata nor Adesmata, and we propose their loss in Geophilomorpha as a whole
Anatomical transformations considered by her as to correlate with burrowing life habits include (i) alteration of the tentorial skeleton, especially absence of a transverse bar; (ii) absence of a “coclypeus”, “whose movements are not needed” (p. 340); (iii) absence of all tentorial muscles (T2-T10 fide Manton) effecting the swing of the tentorium in non-geophilomorph representatives of the Pleurostigmophora; and (iv) simplification of the mandibular musculature, including absence of a transverse mandibular tendon, under restriction of mandible movements to those affected by the mandibular muscles alone
Summary
The tentorium of myriapods is a cuticular formation of the head with a distinct composition of exoskeletal bars around the mouth opening and endoskeletal processes (Koch 2003). In the context of myriapod phylogeny (e.g., Edgecombe 2011), ambiguity remains with regard to Manton’s (1964) conclusion that the myriapod tentorium is primarily mobile, and that its “swinging” movements correlate with the presence of a mobile mandibular gnathal lobe that lacks a mandibular muscle for its abduction. Recent comparative studies of the peristomatic structures in a broader sampling of geophilomorphs by Koch and Edgecombe (2012) challenged this assumption They revealed anatomical evidence in support of the view that at least in the sister group to all other geophilomorphs, the Placodesmata, the mobility of the tentorium is maintained, its loss being potentially synapomorphic for adesmatan Geophilomorpha only. Some of the evolutionary transformations of the tentorium in Adesmata as advocated by Koch and Edgecombe (2012) were contradicted by Bonato et al (2014) within the scope of their analyses of geophilomorph phylogeny
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