178
Filibranch gills are present in both fossil (Klug et al. 2005) and extant trigoniids
(Tevesz 1975; Morton 1987), which is widely accepted as the sister group and
hypothesized ancestor to freshwater unionoids (Newell and Boyd 1975; Hoeh et al.
1998, 2001; Waller 1998; Giribet and Wheeler 2002; Graf and Cummings 2006;
Combosch et al. 2017; Lopes-Lima et al. 2017). Regarding anatomical similarities,
members of both orders share similar ctenidial ciliary patterns (Tevesz 1975) and
have chitinous gill supports, which are reinforced with calcium phosphate concretions (Ridewood 1903; Atkins 1938; Morton 1987; Silverman et al. 1987a, b, 1989;
Whyte 1991; Wilby and Whyte 1995; Hinzmann et al. 2015).
The presence of a veliger in Neotrigonia margaritacea (a member of the only
extant genus of trigoniids) has still not been confirmed but is presumed by most
authors (Tevesz 1975; O’Foighil and Graf 2000; Graf and Cummings 2006).
Atypical for marine bivalves (e.g. Mackie 1984)—Neotrigonia margaritacea likely
lacks a typical veliger larva and its prodissoconch morphology indicates a nonplanktotrophic pre-juvenile ontogeny (O’Foighil and Graf 2000), suggesting a
somewhat derived evolutionary status. O’Foighil (1986) noticed, that parental care
of young among bivalves is associated with a flattened prodissoconch. Neotrigonia
margaritacea has an inflated prodissoconch, which suggests that it is a broadcast
spawner, but it is unknown whether its pre-juvenile development is benthic or
pelagic (O’Foighil and Graf 2000).
Richard et al. (1991), followed by Watters (2001) and Haag (2012), noticed that
the presence of calcium concretions in mineralized gills of fossil unionoids from the
Jurassic onwards may suggest parental care, as today these concretions serve as a
source of calcium for glochidial shells (Silverman et al. 1985, 1987a, b; Machado
and Lopes-Lima 2011; Hinzmann et al. 2015). One may indicate that such structures are today present both in trigoniid and unionoid gills (e.g. Atkins 1938), as
well as are believed to exist also in fossil members of both orders, facilitating their
gill mineralization (documented by specimens with phosphatized gills from the
Triassic, Jurassic and Cretaceous; Richard et al. 1991; Simone and Mezzalira 1993;
Klug et al. 2005; Skawina 2010, Whyte 1991; Wilby and Whyte 1995; Skawina,
personal observations on Rhaetian Tihkia silesiaca, specimen ZPAL V33/305).
Both the trigoniids and the oldest unionoids share filibranch grade gill anatomy,
which lacks the tissue fusion of the gill filaments seen in eulamellibranch gills
(which should be preserved, if it was present, e.g. Simone and Mezzalira 1993;
Skawina 2010; Knight et al. 2014). In filibranch gills, filaments are connected
instead only through the association of ciliary discs (e.g. Ridewood 1903; Morton
1987) making incubation of larvae in suprabranchial cavity or in interlamellar
spaces serving as a marsupium impossible (Mackie 1984). Nevertheless Mackie
(1984) noticed that marine bivalves with filibranch gills may incubate embryos
within their mantle cavity (infrabranchial cavity, e.g. Philobrya munita; Morton
1978), which does not exclude a similar intermediate status in the early unionoids.
It is possible, that early unionoids established brooding not only for protection of
their larvae (e.g. against water currents, osmotic and nutrient poor environment,
Gray 1988; Graf and O’Foighil 2000; Graf and Cummings 2006; Graf 2013; Graf
et al. 2015), but also to ease embryonic shell formation by the close contact with
A. Skawina
Filibranch gills are present in both fossil (Klug et al. 2005) and extant trigoniids
(Tevesz 1975; Morton 1987), which is widely accepted as the sister group and
hypothesized ancestor to freshwater unionoids (Newell and Boyd 1975; Hoeh et al.
1998, 2001; Waller 1998; Giribet and Wheeler 2002; Graf and Cummings 2006;
Combosch et al. 2017; Lopes-Lima et al. 2017). Regarding anatomical similarities,
members of both orders share similar ctenidial ciliary patterns (Tevesz 1975) and
have chitinous gill supports, which are reinforced with calcium phosphate concretions (Ridewood 1903; Atkins 1938; Morton 1987; Silverman et al. 1987a, b, 1989;
Whyte 1991; Wilby and Whyte 1995; Hinzmann et al. 2015).
The presence of a veliger in Neotrigonia margaritacea (a member of the only
extant genus of trigoniids) has still not been confirmed but is presumed by most
authors (Tevesz 1975; O’Foighil and Graf 2000; Graf and Cummings 2006).
Atypical for marine bivalves (e.g. Mackie 1984)—Neotrigonia margaritacea likely
lacks a typical veliger larva and its prodissoconch morphology indicates a nonplanktotrophic pre-juvenile ontogeny (O’Foighil and Graf 2000), suggesting a
somewhat derived evolutionary status. O’Foighil (1986) noticed, that parental care
of young among bivalves is associated with a flattened prodissoconch. Neotrigonia
margaritacea has an inflated prodissoconch, which suggests that it is a broadcast
spawner, but it is unknown whether its pre-juvenile development is benthic or
pelagic (O’Foighil and Graf 2000).
Richard et al. (1991), followed by Watters (2001) and Haag (2012), noticed that
the presence of calcium concretions in mineralized gills of fossil unionoids from the
Jurassic onwards may suggest parental care, as today these concretions serve as a
source of calcium for glochidial shells (Silverman et al. 1985, 1987a, b; Machado
and Lopes-Lima 2011; Hinzmann et al. 2015). One may indicate that such structures are today present both in trigoniid and unionoid gills (e.g. Atkins 1938), as
well as are believed to exist also in fossil members of both orders, facilitating their
gill mineralization (documented by specimens with phosphatized gills from the
Triassic, Jurassic and Cretaceous; Richard et al. 1991; Simone and Mezzalira 1993;
Klug et al. 2005; Skawina 2010, Whyte 1991; Wilby and Whyte 1995; Skawina,
personal observations on Rhaetian Tihkia silesiaca, specimen ZPAL V33/305).
Both the trigoniids and the oldest unionoids share filibranch grade gill anatomy,
which lacks the tissue fusion of the gill filaments seen in eulamellibranch gills
(which should be preserved, if it was present, e.g. Simone and Mezzalira 1993;
Skawina 2010; Knight et al. 2014). In filibranch gills, filaments are connected
instead only through the association of ciliary discs (e.g. Ridewood 1903; Morton
1987) making incubation of larvae in suprabranchial cavity or in interlamellar
spaces serving as a marsupium impossible (Mackie 1984). Nevertheless Mackie
(1984) noticed that marine bivalves with filibranch gills may incubate embryos
within their mantle cavity (infrabranchial cavity, e.g. Philobrya munita; Morton
1978), which does not exclude a similar intermediate status in the early unionoids.
It is possible, that early unionoids established brooding not only for protection of
their larvae (e.g. against water currents, osmotic and nutrient poor environment,
Gray 1988; Graf and O’Foighil 2000; Graf and Cummings 2006; Graf 2013; Graf
et al. 2015), but also to ease embryonic shell formation by the close contact with
A. Skawina