3
1.1.1 Cestodes (Tapeworms)
There are approximately 6000 described species of cestodes (Stunkard 1983). Better
known as tapeworms, cestodes are found in all major groups of extant vertebrates
(Littlewood et al. 2015). They have a complex life-cycle that is deeply linked with
predator-prey relationships (see Mackiewicz 1988 for details). The adult cestode
lives in the intestine of a vertebrate animals where it produces eggs via sexual reproduction. These eggs are released into the environment where they are ingested by an
animal which functions as the intermediate host where the larva can grow and
develop. This intermediate host is usually an arthropod, though for some families of
cestodes, the intermediate host is a vertebrate animal, and may involve sequentially
infecting two or more intermediate hosts. The life-cycle is completed when the
infected intermediate host is eaten by the definitive vertebrate host (Mackiewicz 1988).
Being soft-bodied internal parasites, adult cestodes are not readily fossilised
(Littlewood and Donovan 2003). The only known fossils for cestodes come in the
form of their fossilised eggs (Zangerl and Case 1976; Dentzien-Dias et al. 2013; De
Baets et al. 2015). While the sclerotized hooks on the cestode’s scolex, the attachment organ anterior of the parasite, can potentially be fossilised under the right
conditions, and putative fossils of such hooks in fossil cestode eggs have been
reported by Dentzien-Dias et al. (2013). While Upeniece (2011) and De Baets et al.
(2015) have suggested some fossil hook circlets associated with Devonian placoderm and acanthodians fossils might be cestode hooks, such fossilised hooks would
be situated deep within the host’s body and are unlikely to be found from an external
examination of a fossil specimen. Upeniece (2001, 2011) has previously documented microscopic hooks present on placoderm fossils and suggested they might
be cestode or monogenean hooks. Based on the arrangements of the hooks and
where they were distributed on the placoderm, Leung (2017) suggested that it is
more likely that they belong to monogeneans (see also De Baets et al. 2015, 2021a)—a
group of ectoparasitic flatworms that will be discussed later in this chapter.
So far, there have been two published examples of fossilised cestode eggs, both
of them from elasmobranch hosts (Zangerl and Case 1976; Dentzien-Dias et al.
2013), however, given the taxonomic range of extant vertebrates that act as definitive hosts for cestodes (Littlewood et al. 2015), there is a strong possibility that
fossilised cestode eggs may be present in the coprolites or lower gastrointestinal
tract of fossil vertebrates other than elasmobranchs. The findings of fossilised cestode eggs in elasmobranch coprolites supports molecular phylogeny studies which
points to a very long coevolutionary history between elasmobranchs and many cestode lineages (Olson et al. 2010; Caira and Jensen 2014; Caira et al. 2014). Indeed
a significant number of extant cestode species (28%) parasitise elasmobranch hosts
(Littlewood et al. 2015), and the lineages that infect tetrapod and teleost hosts are
nested within elasmobranch-infecting lineages (Caira et al. 2014). Fossils of cestode eggs can provide additional insight into when cestodes began colonising nonelasmobranch hosts, and act as useful calibration points for molecular phylogeny
studies (De Baets et al. 2021a). Given fossils of other helminth eggs have previously
1 Parasites of Fossil Vertebrates: What We Know and What Can We Expect…
1.1.1 Cestodes (Tapeworms)
There are approximately 6000 described species of cestodes (Stunkard 1983). Better
known as tapeworms, cestodes are found in all major groups of extant vertebrates
(Littlewood et al. 2015). They have a complex life-cycle that is deeply linked with
predator-prey relationships (see Mackiewicz 1988 for details). The adult cestode
lives in the intestine of a vertebrate animals where it produces eggs via sexual reproduction. These eggs are released into the environment where they are ingested by an
animal which functions as the intermediate host where the larva can grow and
develop. This intermediate host is usually an arthropod, though for some families of
cestodes, the intermediate host is a vertebrate animal, and may involve sequentially
infecting two or more intermediate hosts. The life-cycle is completed when the
infected intermediate host is eaten by the definitive vertebrate host (Mackiewicz 1988).
Being soft-bodied internal parasites, adult cestodes are not readily fossilised
(Littlewood and Donovan 2003). The only known fossils for cestodes come in the
form of their fossilised eggs (Zangerl and Case 1976; Dentzien-Dias et al. 2013; De
Baets et al. 2015). While the sclerotized hooks on the cestode’s scolex, the attachment organ anterior of the parasite, can potentially be fossilised under the right
conditions, and putative fossils of such hooks in fossil cestode eggs have been
reported by Dentzien-Dias et al. (2013). While Upeniece (2011) and De Baets et al.
(2015) have suggested some fossil hook circlets associated with Devonian placoderm and acanthodians fossils might be cestode hooks, such fossilised hooks would
be situated deep within the host’s body and are unlikely to be found from an external
examination of a fossil specimen. Upeniece (2001, 2011) has previously documented microscopic hooks present on placoderm fossils and suggested they might
be cestode or monogenean hooks. Based on the arrangements of the hooks and
where they were distributed on the placoderm, Leung (2017) suggested that it is
more likely that they belong to monogeneans (see also De Baets et al. 2015, 2021a)—a
group of ectoparasitic flatworms that will be discussed later in this chapter.
So far, there have been two published examples of fossilised cestode eggs, both
of them from elasmobranch hosts (Zangerl and Case 1976; Dentzien-Dias et al.
2013), however, given the taxonomic range of extant vertebrates that act as definitive hosts for cestodes (Littlewood et al. 2015), there is a strong possibility that
fossilised cestode eggs may be present in the coprolites or lower gastrointestinal
tract of fossil vertebrates other than elasmobranchs. The findings of fossilised cestode eggs in elasmobranch coprolites supports molecular phylogeny studies which
points to a very long coevolutionary history between elasmobranchs and many cestode lineages (Olson et al. 2010; Caira and Jensen 2014; Caira et al. 2014). Indeed
a significant number of extant cestode species (28%) parasitise elasmobranch hosts
(Littlewood et al. 2015), and the lineages that infect tetrapod and teleost hosts are
nested within elasmobranch-infecting lineages (Caira et al. 2014). Fossils of cestode eggs can provide additional insight into when cestodes began colonising nonelasmobranch hosts, and act as useful calibration points for molecular phylogeny
studies (De Baets et al. 2021a). Given fossils of other helminth eggs have previously
1 Parasites of Fossil Vertebrates: What We Know and What Can We Expect…