10
same time since the two group share a close common ancestry. This is supported by the
recent discovery of some Middle to Late Jurassic fish fossils which are fossilised with
isopods attached to their body (Haug et al. 2021; Nagler et al. 2016, 2017).
But as mentioned above, apart from the external body surfaces, cymothoids are
also known to live in other parts of the host body such as the buccal cavity, gill
chambers, and body cavity (Smit et al. 2014). A recent molecular phylogeny study
indicates that the shift between living on the body surface and living in the buccal
cavity or gill chambers might have independently occurred a number of times within
the cymothoids (Hata et al. 2017). Furthermore, colonisation of the host body cavity
has independently evolved in two separate lineages of cymothoids (Hata et al.
2017). Therefore, we should also expect that aside from being present on the skin
and fins of fossil fish, there is also a possibility that some might be obscured within
the host’s mouth, gill chambers, and abdominal cavity, much like the parasitic copepod (Cressey and Patterson 1973) from the gill chamber of an Early Cretaceous fish
fossil. As mentioned in relation to that example, microCT and similar techniques
can potentially be used to examine well-preserved fish fossils for the presence of
parasitic isopods (Nagler et al. 2017). While they are mainly parasites of teleost fish,
cymothoids have also been documented from some elasmobranchs (Williams et al.
2010), and two disparate groups of extant marine reptiles—snakes (Saravanakumar
et al. 2012) and turtles (Júnior et al. 2015). In the case of turtles, they were found
around parts of the body that have thinner tissue and are more vascularized such as
eyelids (Júnior et al. 2015). Given their capacity to infect those two extant marine
reptile groups, it is possible that some Jurassic and Cretaceous marine reptiles may
have also served as viable hosts for these parasitic isopods, thus expanding the
range of potential hosts in the fossil record which may have hosted cymothoids.
However, a note of caution in regards to interpreting fossils of isopods as parasites when they are found in association with larger animals. Marine isopods are
opportunistic feeders that frequently scavenge on vertebrate carcasses, and fossils
of their scavenging activities have previously been found (Wilson et al. 2011;
Klompmaker and Boxshall 2015; Robin et al. 2019). Thus before concluding any
fossils of vertebrate-isopod association as a case of parasitism, one must ensure
there is evidence to indicate that the host was alive and engaged in extended interaction with the isopod(s) when they became fossilised. Another possible method of
establishing whether a fossil isopod was parasitic is by comparing its functional
morphology and with that extant parasitic isopods, as parasitic species have specialised feeding and attachment structures that distinguish them from their free-living
relatives (e.g. Nagler and Haug 2016).
1.1.8 Pentastomids (Tongue Worms)
Aside from copepods and isopods, another group of parasitic crustaceans which are
known from fossils are the pentastomids. Also known as “tongue worms” due to the
shape of the adult stage, the taxonomic affinity of pentastomids had been an enigma
T. L. F. Leung
same time since the two group share a close common ancestry. This is supported by the
recent discovery of some Middle to Late Jurassic fish fossils which are fossilised with
isopods attached to their body (Haug et al. 2021; Nagler et al. 2016, 2017).
But as mentioned above, apart from the external body surfaces, cymothoids are
also known to live in other parts of the host body such as the buccal cavity, gill
chambers, and body cavity (Smit et al. 2014). A recent molecular phylogeny study
indicates that the shift between living on the body surface and living in the buccal
cavity or gill chambers might have independently occurred a number of times within
the cymothoids (Hata et al. 2017). Furthermore, colonisation of the host body cavity
has independently evolved in two separate lineages of cymothoids (Hata et al.
2017). Therefore, we should also expect that aside from being present on the skin
and fins of fossil fish, there is also a possibility that some might be obscured within
the host’s mouth, gill chambers, and abdominal cavity, much like the parasitic copepod (Cressey and Patterson 1973) from the gill chamber of an Early Cretaceous fish
fossil. As mentioned in relation to that example, microCT and similar techniques
can potentially be used to examine well-preserved fish fossils for the presence of
parasitic isopods (Nagler et al. 2017). While they are mainly parasites of teleost fish,
cymothoids have also been documented from some elasmobranchs (Williams et al.
2010), and two disparate groups of extant marine reptiles—snakes (Saravanakumar
et al. 2012) and turtles (Júnior et al. 2015). In the case of turtles, they were found
around parts of the body that have thinner tissue and are more vascularized such as
eyelids (Júnior et al. 2015). Given their capacity to infect those two extant marine
reptile groups, it is possible that some Jurassic and Cretaceous marine reptiles may
have also served as viable hosts for these parasitic isopods, thus expanding the
range of potential hosts in the fossil record which may have hosted cymothoids.
However, a note of caution in regards to interpreting fossils of isopods as parasites when they are found in association with larger animals. Marine isopods are
opportunistic feeders that frequently scavenge on vertebrate carcasses, and fossils
of their scavenging activities have previously been found (Wilson et al. 2011;
Klompmaker and Boxshall 2015; Robin et al. 2019). Thus before concluding any
fossils of vertebrate-isopod association as a case of parasitism, one must ensure
there is evidence to indicate that the host was alive and engaged in extended interaction with the isopod(s) when they became fossilised. Another possible method of
establishing whether a fossil isopod was parasitic is by comparing its functional
morphology and with that extant parasitic isopods, as parasitic species have specialised feeding and attachment structures that distinguish them from their free-living
relatives (e.g. Nagler and Haug 2016).
1.1.8 Pentastomids (Tongue Worms)
Aside from copepods and isopods, another group of parasitic crustaceans which are
known from fossils are the pentastomids. Also known as “tongue worms” due to the
shape of the adult stage, the taxonomic affinity of pentastomids had been an enigma
T. L. F. Leung