13
integumentary features, it is possible that the evolution of integumentary structures
in synapsids and dinosaurs might have provided suitable environments for the diversification of many ectoparasitic insect groups (Leung 2017), in a manner comparable to how angiosperm plants had coevolved with many herbivorous and pollinator
insects (Labandeira et al. 1994; Grimaldi 1999). Of those, the insect orders that have
evolved to parasitise vertebrates, the most successful groups are the lice (orders
Phthiraptera) and the fleas (order Siphonaptera), both of which are composed
wholly of ectoparasitic species, and both are common parasites of birds and
mammals.
But while homeothermic terrestrial vertebrates had been interpreted to exist at
least since the Triassic (Padian and Sues 2015), as mentioned above, both lice and
fleas only began appearing in the fossil record after the K-Pg mass extinction event.
Zhu et al. (2015) found that fleas diversified on mammals before they also colonised
birds. A recent study on the molecular phylogeny of lice indicates that they radiated
on birds and mammals after the K-Pg event, of which one lineage was exclusive to
birds, and that the rapid diversification of birds and mammals after K-Pg was associated with the evolutionary radiation of their lice (Johnson et al. 2018). At the same
time, another recent study indicates that there was a K-Pg mass extinction of stem
birds, subsequently followed by a radiation of crown birds (Field et al. 2018).
Together, these provide evidence that modern lice and fleas diversified alongside
lineages of birds and mammals that survived the K-Pg extinction event. Much like
other fossil insects, amber would be the most promising material to investigate for
ectoparasitic insects, especially amber fossils that include some integumentary
material or parts of the body from a potential host, as discussed above in relation to
fossil ticks.
1.2 A Note of Caution Regarding Fossil Parasites
As discussed above, despite their diversity and abundance on extant birds and mammals, truly definitive fossils of lice and fleas did not appear until the Early Eocene
(Wappler et al. 2004) and Miocene (Dittmar et al. 2015) respectively. While there
have been some reports of “giant Jurassic fleas” from the mid-Mesozoic (Gao et al.
2012), their status as fleas or even as ectoparasites appears questionable. Dittmar
et al. (2015, 2016) have pointed out a number of problems with the interpretation of
those fossil insects as hematophagous ectoparasites. Indeed, that seems to be a
recurring problem in the literature on various fossil insects which have been interpreted as having a parasitic life-style based on nothing more than conjecture (as
discussed in Leung 2017). For example, the Strashilidae, a family of Jurassic insects,
was initially interpreted as ectoparasites of pterosaurs or dinosaurs based on the
morphology of mouthpart and hindlimbs (Ponomarenko 1976). This assumption
was carried over in subsequent published studies on this family of insects (Rasnitsyn
1992; Vršanský et al. 2010) until newer fossil specimens led to a re-evaluation of its
lifestyle and revealed it to be a sexually dimorphic aquatic insect (Huang et al.
1 Parasites of Fossil Vertebrates: What We Know and What Can We Expect…
integumentary features, it is possible that the evolution of integumentary structures
in synapsids and dinosaurs might have provided suitable environments for the diversification of many ectoparasitic insect groups (Leung 2017), in a manner comparable to how angiosperm plants had coevolved with many herbivorous and pollinator
insects (Labandeira et al. 1994; Grimaldi 1999). Of those, the insect orders that have
evolved to parasitise vertebrates, the most successful groups are the lice (orders
Phthiraptera) and the fleas (order Siphonaptera), both of which are composed
wholly of ectoparasitic species, and both are common parasites of birds and
mammals.
But while homeothermic terrestrial vertebrates had been interpreted to exist at
least since the Triassic (Padian and Sues 2015), as mentioned above, both lice and
fleas only began appearing in the fossil record after the K-Pg mass extinction event.
Zhu et al. (2015) found that fleas diversified on mammals before they also colonised
birds. A recent study on the molecular phylogeny of lice indicates that they radiated
on birds and mammals after the K-Pg event, of which one lineage was exclusive to
birds, and that the rapid diversification of birds and mammals after K-Pg was associated with the evolutionary radiation of their lice (Johnson et al. 2018). At the same
time, another recent study indicates that there was a K-Pg mass extinction of stem
birds, subsequently followed by a radiation of crown birds (Field et al. 2018).
Together, these provide evidence that modern lice and fleas diversified alongside
lineages of birds and mammals that survived the K-Pg extinction event. Much like
other fossil insects, amber would be the most promising material to investigate for
ectoparasitic insects, especially amber fossils that include some integumentary
material or parts of the body from a potential host, as discussed above in relation to
fossil ticks.
1.2 A Note of Caution Regarding Fossil Parasites
As discussed above, despite their diversity and abundance on extant birds and mammals, truly definitive fossils of lice and fleas did not appear until the Early Eocene
(Wappler et al. 2004) and Miocene (Dittmar et al. 2015) respectively. While there
have been some reports of “giant Jurassic fleas” from the mid-Mesozoic (Gao et al.
2012), their status as fleas or even as ectoparasites appears questionable. Dittmar
et al. (2015, 2016) have pointed out a number of problems with the interpretation of
those fossil insects as hematophagous ectoparasites. Indeed, that seems to be a
recurring problem in the literature on various fossil insects which have been interpreted as having a parasitic life-style based on nothing more than conjecture (as
discussed in Leung 2017). For example, the Strashilidae, a family of Jurassic insects,
was initially interpreted as ectoparasites of pterosaurs or dinosaurs based on the
morphology of mouthpart and hindlimbs (Ponomarenko 1976). This assumption
was carried over in subsequent published studies on this family of insects (Rasnitsyn
1992; Vršanský et al. 2010) until newer fossil specimens led to a re-evaluation of its
lifestyle and revealed it to be a sexually dimorphic aquatic insect (Huang et al.
1 Parasites of Fossil Vertebrates: What We Know and What Can We Expect…