177
(Boucot and Poinar 2010; De Baets and Littlewood 2015; Leung 2017; De Baets
et al. 2021) but are comparatively rare (e.g. nematodes protruding from insects;
Poinar 2015). Unfortunately, mussel-fish interactions still lack such evidence. This
might however be a matter of time—entirely soft-bodied parasites or their sclerotized hooks have been found associated with gills of their fish hosts as well as shipworm with a strongly reduced shells (Cressey and Patterson 1973; Cressey and
Boxshall 1989; Upeniece 2001; De Baets et al. 2015; Robin et al. 2018). The fossilization potential might depend on the microenvironment building in the gills
during decay (e.g. decaying tissue provides phosphates, drop of pH secures stability of calcium phosphate in common phosphatization of soft tissues) as well as on
microbial activity (Martill and Harper 1990; Briggs and Kear 1993; Briggs et al.
1993; Wilby and Whyte 1995; Briggs and Wilby 1996; Wilby and Briggs 1997;
Briggs 2003; Briggs and McMahon 2016; Purnell et al. 2018). According to Purnell
et al. (2018) processes of decay, maturation, and mineralization are controlled by
diverse factors that vary spatially and temporally in the way they act during these
processes. As a result, characters of extraordinarily preserved fossils may not
entirely correspond to what simple experimental decay data predict (there is a need
to better understand the fossilization process); also the effects of processes of preservation may be selective to tissue types and may impose on the results of simple
decay experiments. Various traits (size/morphology/habitat) could however be
indicative/suggestive of a parasitic lifestyle.
5.5.1 Parental Care
The fossil record of unionoids ranges back to the Triassic, however little is known
about their early evolution, because their shells are of rather generalized morphology (e.g. McMichael 1957; Good 1998; Watters 2001; Skawina and Dzik 2011; Van
Damme et al. 2015). During this time all the present continents formed the supercontinent of Pangaea (Dubiel et al. 1991; Seton et al. 2012)—this suggests a possible worldwide dispersal of the early unionoids. In the Late Triassic of Poland, fossil
unionoids occur that preserved characters important for unionoid taxonomy: characteristic juvenile sculpture on the shell (post-larval), muscle attachment scars and
phosphatized gills (Skawina and Dzik 2011, Skawina 2010). This is the oldest data
on the anatomy of the organ, which is today crucial for unionoid reproduction
although it isn’t necessary for developing a parasitic life mode. The Late Triassic
unionoids had filibranch gill anatomy, but no evidence is available regarding possible brooding of larvae in these bivalves (Skawina and Dzik 2011). Although today
in freshwater habitats bivalves commonly exhibit parental care (a convergence in
Unionida, Sphaeridae, Cyrenidae and some Dreissenidae; Graf and O’Foighil 2000;
Morton and Puljas 2012; Graf 2013; Mansur et al. 2019), none of these bivalves
have filibranch grade gills. All of them brood their offspring in modified eulamellibranch demibranchs (Unionida, Cyrenidae, Congeria [Dreissenidae]), or first within
the demibranchs, then in the mantle cavity (Rheodreissena [Dreissenidae]).
5 Evolutionary History of Bivalves as Parasites
(Boucot and Poinar 2010; De Baets and Littlewood 2015; Leung 2017; De Baets
et al. 2021) but are comparatively rare (e.g. nematodes protruding from insects;
Poinar 2015). Unfortunately, mussel-fish interactions still lack such evidence. This
might however be a matter of time—entirely soft-bodied parasites or their sclerotized hooks have been found associated with gills of their fish hosts as well as shipworm with a strongly reduced shells (Cressey and Patterson 1973; Cressey and
Boxshall 1989; Upeniece 2001; De Baets et al. 2015; Robin et al. 2018). The fossilization potential might depend on the microenvironment building in the gills
during decay (e.g. decaying tissue provides phosphates, drop of pH secures stability of calcium phosphate in common phosphatization of soft tissues) as well as on
microbial activity (Martill and Harper 1990; Briggs and Kear 1993; Briggs et al.
1993; Wilby and Whyte 1995; Briggs and Wilby 1996; Wilby and Briggs 1997;
Briggs 2003; Briggs and McMahon 2016; Purnell et al. 2018). According to Purnell
et al. (2018) processes of decay, maturation, and mineralization are controlled by
diverse factors that vary spatially and temporally in the way they act during these
processes. As a result, characters of extraordinarily preserved fossils may not
entirely correspond to what simple experimental decay data predict (there is a need
to better understand the fossilization process); also the effects of processes of preservation may be selective to tissue types and may impose on the results of simple
decay experiments. Various traits (size/morphology/habitat) could however be
indicative/suggestive of a parasitic lifestyle.
5.5.1 Parental Care
The fossil record of unionoids ranges back to the Triassic, however little is known
about their early evolution, because their shells are of rather generalized morphology (e.g. McMichael 1957; Good 1998; Watters 2001; Skawina and Dzik 2011; Van
Damme et al. 2015). During this time all the present continents formed the supercontinent of Pangaea (Dubiel et al. 1991; Seton et al. 2012)—this suggests a possible worldwide dispersal of the early unionoids. In the Late Triassic of Poland, fossil
unionoids occur that preserved characters important for unionoid taxonomy: characteristic juvenile sculpture on the shell (post-larval), muscle attachment scars and
phosphatized gills (Skawina and Dzik 2011, Skawina 2010). This is the oldest data
on the anatomy of the organ, which is today crucial for unionoid reproduction
although it isn’t necessary for developing a parasitic life mode. The Late Triassic
unionoids had filibranch gill anatomy, but no evidence is available regarding possible brooding of larvae in these bivalves (Skawina and Dzik 2011). Although today
in freshwater habitats bivalves commonly exhibit parental care (a convergence in
Unionida, Sphaeridae, Cyrenidae and some Dreissenidae; Graf and O’Foighil 2000;
Morton and Puljas 2012; Graf 2013; Mansur et al. 2019), none of these bivalves
have filibranch grade gills. All of them brood their offspring in modified eulamellibranch demibranchs (Unionida, Cyrenidae, Congeria [Dreissenidae]), or first within
the demibranchs, then in the mantle cavity (Rheodreissena [Dreissenidae]).
5 Evolutionary History of Bivalves as Parasites