4
been found as inclusions in coprolites (e.g. Poinar and Boucot 2006; Hugot et al.
2014; Chin 2021), the coprolites of many other vertebrates should be similarly
examined for the presence of cestode eggs in a manner comparable to what has been
achieved with subfossils and other archaeological remains (Gonçalves et al. 2003).
1.1.2 Trematodes (Flukes)
There are 18,000 described species of trematodes (Cribb et al. 2001). Like cestodes,
trematodes are also internal parasites of vertebrates and have complex life-cycles
(Cribb et al. 2003; Galaktionov and Dobrovolskij 2003). While most species have
adult stages that live in the gastrointestinal tract of their definitive vertebrate host,
there are some families of trematodes that have evolved to occupy other parts of the
host’s body such as the circulatory system, eyes, lungs, liver, bladder, and the connective tissue and muscles (Cribb et al. 2003; Poulin 2005). Additionally, extant
trematodes in the Digenea group (which represents over 99.9% of all known living
species) have an asexual reproduction stage (Cribb et al. 2003; Galaktionov and
Dobrovolskij 2003) which results in the production of vast number of mobile larval
stages which infect the next host in the life-cycle. Depending on the family, this is
either a second intermediate host where they encyst, or the vertebrate definitive host
where they will develop into sexually mature adults. This asexual reproduction
stage in the intermediate host is absent in most other parasitic worms, and in most
extant trematodes, asexual reproduction occurs in a mollusc host, which is usually
a gastropod in most families (Cribb et al. 2003; Galaktionov and Dobrovolskij
2003). As with other parasitic worms, adult trematodes are soft-bodied internal
parasites that do not usually fossilise even under the most ideal preservation conditions. However, they do produce environmentally resistant eggs that can potentially
be fossilised with coprolites. So far, there has been one published example of fossilised trematode eggs, reported from dinosaur coprolites dating from the Early
Cretaceous (Poinar and Boucot 2006). Given that trematodes are known from all
major extant vertebrate groups (Cribb et al. 2003; Galaktionov and Dobrovolskij
2003; Littlewood et al. 2015), and that eggs from various lineages of trematode have
been reported in more recent quaternary fossils and subfossil coprolites (e.g. JouyAvantin et al. 1999; Le Bailly and Bouchet 2010; Wood et al. 2013) examination of
older coprolite samples may yield more trematode egg fossils.
A recent fossil find indicates that amber can preserve the larval stage of trematodes along with their host. Recently, Poinar et al. (2017) reported finding what
appears to be the metacercaria stage of a trematode from a lizard preserved in
Myanmar amber dated to the Early Cretaceous. The size and position of the fossil
metacercaria is strikingly similar to that of some extant trematodes which use lizards (particularly Anoles) as the second intermediate host in their life-cycle (Poinar
et al. 2017). This fossil provides some very useful insight into the evolutionary history of these trematodes, and its similarity in general morphology, host type, and
T. L. F. Leung
been found as inclusions in coprolites (e.g. Poinar and Boucot 2006; Hugot et al.
2014; Chin 2021), the coprolites of many other vertebrates should be similarly
examined for the presence of cestode eggs in a manner comparable to what has been
achieved with subfossils and other archaeological remains (Gonçalves et al. 2003).
1.1.2 Trematodes (Flukes)
There are 18,000 described species of trematodes (Cribb et al. 2001). Like cestodes,
trematodes are also internal parasites of vertebrates and have complex life-cycles
(Cribb et al. 2003; Galaktionov and Dobrovolskij 2003). While most species have
adult stages that live in the gastrointestinal tract of their definitive vertebrate host,
there are some families of trematodes that have evolved to occupy other parts of the
host’s body such as the circulatory system, eyes, lungs, liver, bladder, and the connective tissue and muscles (Cribb et al. 2003; Poulin 2005). Additionally, extant
trematodes in the Digenea group (which represents over 99.9% of all known living
species) have an asexual reproduction stage (Cribb et al. 2003; Galaktionov and
Dobrovolskij 2003) which results in the production of vast number of mobile larval
stages which infect the next host in the life-cycle. Depending on the family, this is
either a second intermediate host where they encyst, or the vertebrate definitive host
where they will develop into sexually mature adults. This asexual reproduction
stage in the intermediate host is absent in most other parasitic worms, and in most
extant trematodes, asexual reproduction occurs in a mollusc host, which is usually
a gastropod in most families (Cribb et al. 2003; Galaktionov and Dobrovolskij
2003). As with other parasitic worms, adult trematodes are soft-bodied internal
parasites that do not usually fossilise even under the most ideal preservation conditions. However, they do produce environmentally resistant eggs that can potentially
be fossilised with coprolites. So far, there has been one published example of fossilised trematode eggs, reported from dinosaur coprolites dating from the Early
Cretaceous (Poinar and Boucot 2006). Given that trematodes are known from all
major extant vertebrate groups (Cribb et al. 2003; Galaktionov and Dobrovolskij
2003; Littlewood et al. 2015), and that eggs from various lineages of trematode have
been reported in more recent quaternary fossils and subfossil coprolites (e.g. JouyAvantin et al. 1999; Le Bailly and Bouchet 2010; Wood et al. 2013) examination of
older coprolite samples may yield more trematode egg fossils.
A recent fossil find indicates that amber can preserve the larval stage of trematodes along with their host. Recently, Poinar et al. (2017) reported finding what
appears to be the metacercaria stage of a trematode from a lizard preserved in
Myanmar amber dated to the Early Cretaceous. The size and position of the fossil
metacercaria is strikingly similar to that of some extant trematodes which use lizards (particularly Anoles) as the second intermediate host in their life-cycle (Poinar
et al. 2017). This fossil provides some very useful insight into the evolutionary history of these trematodes, and its similarity in general morphology, host type, and
T. L. F. Leung