11
until they were finally recognised as highly-derived crustaceans (Lavrov et al.
2004). Extant pentastomids are respiratory tract parasites of tetrapods, mostly reptiles (Riley 1986), but the oldest known fossils of pentastomids are larval stages
dating back from the Late Cambrian (Walossek and Müller 1994; Walossek et al.
1994; Klompmaker and Boxshall 2015), over 100 million years before the appearance of tetrapods, therefore it is unclear what those pentastomids parasitised (assuming that they were parasites at all). It has been suggested that those early pentastomids
might have been parasites of basal chordates and led a more simple life-cycle than
extant pentastomida (Sanders and Lee 2010), living in the gill cavities of their hosts
(Walossek and Müller 1994).
A more recently described fossil suggests that the original hosts for pentastomids
were in fact crustaceans (Siveter et al. 2015) and that they colonised vertebrate hosts
much later in their evolutionary history. Invavita piratica lived during the midSilurian period and unlike the isolate larval microfossils found by Walossek and
Müller (1994), the I. piratica specimen was interpreted as an adult which was found
attached to its host—an ostracod (Siveter et al. 2015). This indicates that pentastomids might have originated as ectoparasites of crustaceans and incorporated vertebrates host and became internal parasites much later in their evolutionary history
(but see Haug et al. 2021). So how and when did this switch in host and mode of
parasitism occur?
Leung (2017) suggested it might have occurred through a process of “upward
incorporation” (Parker et al. 2015)—the original crustacean host of pentastomids
may have also been prey for the jawed fish that began appearing in the ocean during
that period (Friedman and Sallan 2012; Klug et al. 2017), and what might have
begun as an way of surviving their host’s predation by transferring to the vertebrate
predator subsequently became an obligate part of their life-cycle. In this scenario, as
tetrapods evolved from those jawed fish and began colonising terrestrial biomes, the
pentastomids coevolved with them, by which point they had evolved to live as internal parasites which allow them to survive in hosts which lived in terrestrial biomes.
Further fossils of pentastomids can test the validity of this hypothesis, as well as
provide more insights into how and when pentastomids transitioned from being
ectoparasites of crustaceans to endoparasites of tetrapods, and how this might relate
to the colonisation of land by tetrapods.
1.1.9 Ticks
There is abundant evidence for non-avian dinosaurs, pterosaurs, and non- mammalian
synapsids being covered in fine integumentary features (Dhouailly et al. 2017)—
which seems to be an ideal niche for ectoparasitic arthropods. Ticks are common
ectoparasites of terrestrial vertebrates (Klompen et al. 1996), and while molecular
phylogeny analysis placed the origin of ticks to the late Carboniferous (Mans et al.
2012), the oldest confidently identified fossil ticks are from the Cretaceous (e.g.
Poinar and Buckley 2008; Estrada-Peña and de la Fuente 2018; Dunlop 2021).
1 Parasites of Fossil Vertebrates: What We Know and What Can We Expect…
until they were finally recognised as highly-derived crustaceans (Lavrov et al.
2004). Extant pentastomids are respiratory tract parasites of tetrapods, mostly reptiles (Riley 1986), but the oldest known fossils of pentastomids are larval stages
dating back from the Late Cambrian (Walossek and Müller 1994; Walossek et al.
1994; Klompmaker and Boxshall 2015), over 100 million years before the appearance of tetrapods, therefore it is unclear what those pentastomids parasitised (assuming that they were parasites at all). It has been suggested that those early pentastomids
might have been parasites of basal chordates and led a more simple life-cycle than
extant pentastomida (Sanders and Lee 2010), living in the gill cavities of their hosts
(Walossek and Müller 1994).
A more recently described fossil suggests that the original hosts for pentastomids
were in fact crustaceans (Siveter et al. 2015) and that they colonised vertebrate hosts
much later in their evolutionary history. Invavita piratica lived during the midSilurian period and unlike the isolate larval microfossils found by Walossek and
Müller (1994), the I. piratica specimen was interpreted as an adult which was found
attached to its host—an ostracod (Siveter et al. 2015). This indicates that pentastomids might have originated as ectoparasites of crustaceans and incorporated vertebrates host and became internal parasites much later in their evolutionary history
(but see Haug et al. 2021). So how and when did this switch in host and mode of
parasitism occur?
Leung (2017) suggested it might have occurred through a process of “upward
incorporation” (Parker et al. 2015)—the original crustacean host of pentastomids
may have also been prey for the jawed fish that began appearing in the ocean during
that period (Friedman and Sallan 2012; Klug et al. 2017), and what might have
begun as an way of surviving their host’s predation by transferring to the vertebrate
predator subsequently became an obligate part of their life-cycle. In this scenario, as
tetrapods evolved from those jawed fish and began colonising terrestrial biomes, the
pentastomids coevolved with them, by which point they had evolved to live as internal parasites which allow them to survive in hosts which lived in terrestrial biomes.
Further fossils of pentastomids can test the validity of this hypothesis, as well as
provide more insights into how and when pentastomids transitioned from being
ectoparasites of crustaceans to endoparasites of tetrapods, and how this might relate
to the colonisation of land by tetrapods.
1.1.9 Ticks
There is abundant evidence for non-avian dinosaurs, pterosaurs, and non- mammalian
synapsids being covered in fine integumentary features (Dhouailly et al. 2017)—
which seems to be an ideal niche for ectoparasitic arthropods. Ticks are common
ectoparasites of terrestrial vertebrates (Klompen et al. 1996), and while molecular
phylogeny analysis placed the origin of ticks to the late Carboniferous (Mans et al.
2012), the oldest confidently identified fossil ticks are from the Cretaceous (e.g.
Poinar and Buckley 2008; Estrada-Peña and de la Fuente 2018; Dunlop 2021).
1 Parasites of Fossil Vertebrates: What We Know and What Can We Expect…