367
(velvet worms) or tardigradans (water bears), as they already possess pivot joints
which are absent in onychophorans and tardigradans, but still lack the strict sclerotisation of the body surface.
On the contrary, molecular data suggest a position of Pentastomida deep among
sclerotised arthropodans, inside Euarthropoda, inside Eucrustacea as sistergroup to
Branchiura (e.g., Regier et al. 2010; Oakley et al. 2013; Giribet and Edgecombe
2013). Interestingly also a morphological view has supported this interpretation
(Wingstrand 1972; Storch and Jamieson 1992). Unfortunately, this phylogenetic
interpretation was based on two problematic aspects: a simplified sperm morphology shared by both groups and the fact that both are parasites (see Walossek and
Müller 1994; Chesunov 2002).
If Pentastomida would represent indeed an ingroup of Eucrustacea we would
face a severe problem: They would represent the only case in which all crustacean
characters would have been lost in all life stages. Even highly modified parasitic
crustaceans that “dissolve” their entire morphology as parasites (see above,
Thecostraca) retain clear crustacean type characters in the one or other life phase
(the softness of representatives of Pentastomida could, of course, be explained
also with such a reduction interpretation). We should therefore still be very cautious with accepting pentastomidans as crustaceans. The branch leading to the
modern forms and also that of Branchiura is extremely long, the effect of the
occurrence of artefacts based on such branches is well known (e.g., De Baets and
Littlewood 2015).
10.7.3 Fossil Representatives
Larval forms of pentastomidans have been reported from Cambrian and Ordovician
deposits in exceptional Orsten-type preservation (Andres 1989; Walossek and
Müller 1994; Waloszek et al. 2006; Castellani et al. 2011). Although some of these
forms appear very unusual (e.g., Heymonsicambria taylori; Walossek et al. 1994)
and thus their interpretation as pentastomidans has been questioned, others
(“hammer- type” larvae) share an astonishing amount of characters with modern
pentastomidans, making alternative interpretations difficult.
A younger supposed occurrence is less easy to be interpreted. A small Silurian
fossil attached to an ostracodan crustacean has been interpreted as a pentastomidan
(Siveter et al. 2015). Although also this type of preservation (“Herefordshire-type”)
must be called exceptional, it is to a certain degree limited. The factual spatial resolution is about 100 μm, as the voxel size is about 20 μm. Hence not too many characters can be resolved that would support interpretation as a pentastomidan besides
five structures in the front. The fossil could potentially also represent a derived parasitic copepodan crustacean or even a non-arthropodan structure as no distinct
arthropodan features appear to be present.
10 Evolutionary History of Crustaceans as Parasites
(velvet worms) or tardigradans (water bears), as they already possess pivot joints
which are absent in onychophorans and tardigradans, but still lack the strict sclerotisation of the body surface.
On the contrary, molecular data suggest a position of Pentastomida deep among
sclerotised arthropodans, inside Euarthropoda, inside Eucrustacea as sistergroup to
Branchiura (e.g., Regier et al. 2010; Oakley et al. 2013; Giribet and Edgecombe
2013). Interestingly also a morphological view has supported this interpretation
(Wingstrand 1972; Storch and Jamieson 1992). Unfortunately, this phylogenetic
interpretation was based on two problematic aspects: a simplified sperm morphology shared by both groups and the fact that both are parasites (see Walossek and
Müller 1994; Chesunov 2002).
If Pentastomida would represent indeed an ingroup of Eucrustacea we would
face a severe problem: They would represent the only case in which all crustacean
characters would have been lost in all life stages. Even highly modified parasitic
crustaceans that “dissolve” their entire morphology as parasites (see above,
Thecostraca) retain clear crustacean type characters in the one or other life phase
(the softness of representatives of Pentastomida could, of course, be explained
also with such a reduction interpretation). We should therefore still be very cautious with accepting pentastomidans as crustaceans. The branch leading to the
modern forms and also that of Branchiura is extremely long, the effect of the
occurrence of artefacts based on such branches is well known (e.g., De Baets and
Littlewood 2015).
10.7.3 Fossil Representatives
Larval forms of pentastomidans have been reported from Cambrian and Ordovician
deposits in exceptional Orsten-type preservation (Andres 1989; Walossek and
Müller 1994; Waloszek et al. 2006; Castellani et al. 2011). Although some of these
forms appear very unusual (e.g., Heymonsicambria taylori; Walossek et al. 1994)
and thus their interpretation as pentastomidans has been questioned, others
(“hammer- type” larvae) share an astonishing amount of characters with modern
pentastomidans, making alternative interpretations difficult.
A younger supposed occurrence is less easy to be interpreted. A small Silurian
fossil attached to an ostracodan crustacean has been interpreted as a pentastomidan
(Siveter et al. 2015). Although also this type of preservation (“Herefordshire-type”)
must be called exceptional, it is to a certain degree limited. The factual spatial resolution is about 100 μm, as the voxel size is about 20 μm. Hence not too many characters can be resolved that would support interpretation as a pentastomidan besides
five structures in the front. The fossil could potentially also represent a derived parasitic copepodan crustacean or even a non-arthropodan structure as no distinct
arthropodan features appear to be present.
10 Evolutionary History of Crustaceans as Parasites