7
phylogenetically affiliated with and may have evolved from insect-infected lineages
(Blaxter and Koutsovoulos 2015). Furthermore, many vertebrate-infecting nematodes use insects and other arthropods as intermediate hosts (Anderson 2000), thus
much like the fossilised trematode metacercariae described by Poinar et al. (2017),
some amber fossils may contain insects (and other small animals) which hosted the
larval stage of vertebrate-infecting nematodes. Furthermore, some biting insects can
act as vectors for parasitic nematodes—another superfamily of nematodes that commonly parasitises tetrapod vertebrates are the Filarioidea. Also known as filarials,
the adult stage lives in the lymphatic or cardiovascular system of their vertebrate
host, producing larval stages known as microfilarials which use blood-feeding
arthropods as vectors to transmit to new hosts (Anderson 2000). Presently, there are
three known species of filarials which have been described from fossils, all of which
have been placed in the Cascofilaria genus; C. baltica which was found associated
with a blackfly in Eocene age Baltic amber, C. dominicana which was found associated with a female mosquito in Dominican amber, and C. parvus which was also
found associated with a mosquito host (Poinar 2015). The morphology of those
fossil microfilarials resemble extant filarial species that infect mammals and
amphibians (Poinar 2015). It is worth pointing out that there is some evidence to
suggest human-infecting (and other mammal-infecting) filarial worms might have
originated from bird-infecting filarials about 17–25 million years ago (Suh et al.
2016; Suh 2021), which overlaps with the age of Dominican amber which have
preserved remains of tropical birds including their feathers and egg shells (Poinar
2010). Thus, parasites which are found in biting insects that are preserved in such
amber deposits can provide vital insight into the origin and evolution of vectortransmitted parasites such as filarial nematodes.
1.1.4 Acanthocephalans (Thorny-Headed Worms)
Acanthocephalans, commonly known as thorny-headed worms, are a group of internal parasites with complex life cycles. There are about 1150 known living species,
and all major groups of extant vertebrates have been found to host acanthocephalans, with the adult stage of this parasite living in the gastrointestinal tract of various
vertebrate animals including fish, amphibians, reptiles, mammals, and birds
(Kennedy 2006). The adult worm anchors itself to the gut wall of the vertebrate host
using an eversible proboscis which is covered with hooks and spines (Miller and
Dunagan 1985).
The typical life cycle of acanthocephalan involves two hosts, with the adult
living in a vertebrate host, producing eggs which are released into the environment, and an arthropod intermediate host which become infected by acanthocephalan larvae when they ingest the parasite’s eggs (Kennedy 2006). However,
many species also incorporate additional vertebrates to act as paratenic (transport
host) (e.g. Sinisalo and Valtonen 2003; Médoc et al. 2011)—the acanthocephalan
larvae do not undergo further development in those hosts, however paratenic hosts
1 Parasites of Fossil Vertebrates: What We Know and What Can We Expect…
phylogenetically affiliated with and may have evolved from insect-infected lineages
(Blaxter and Koutsovoulos 2015). Furthermore, many vertebrate-infecting nematodes use insects and other arthropods as intermediate hosts (Anderson 2000), thus
much like the fossilised trematode metacercariae described by Poinar et al. (2017),
some amber fossils may contain insects (and other small animals) which hosted the
larval stage of vertebrate-infecting nematodes. Furthermore, some biting insects can
act as vectors for parasitic nematodes—another superfamily of nematodes that commonly parasitises tetrapod vertebrates are the Filarioidea. Also known as filarials,
the adult stage lives in the lymphatic or cardiovascular system of their vertebrate
host, producing larval stages known as microfilarials which use blood-feeding
arthropods as vectors to transmit to new hosts (Anderson 2000). Presently, there are
three known species of filarials which have been described from fossils, all of which
have been placed in the Cascofilaria genus; C. baltica which was found associated
with a blackfly in Eocene age Baltic amber, C. dominicana which was found associated with a female mosquito in Dominican amber, and C. parvus which was also
found associated with a mosquito host (Poinar 2015). The morphology of those
fossil microfilarials resemble extant filarial species that infect mammals and
amphibians (Poinar 2015). It is worth pointing out that there is some evidence to
suggest human-infecting (and other mammal-infecting) filarial worms might have
originated from bird-infecting filarials about 17–25 million years ago (Suh et al.
2016; Suh 2021), which overlaps with the age of Dominican amber which have
preserved remains of tropical birds including their feathers and egg shells (Poinar
2010). Thus, parasites which are found in biting insects that are preserved in such
amber deposits can provide vital insight into the origin and evolution of vectortransmitted parasites such as filarial nematodes.
1.1.4 Acanthocephalans (Thorny-Headed Worms)
Acanthocephalans, commonly known as thorny-headed worms, are a group of internal parasites with complex life cycles. There are about 1150 known living species,
and all major groups of extant vertebrates have been found to host acanthocephalans, with the adult stage of this parasite living in the gastrointestinal tract of various
vertebrate animals including fish, amphibians, reptiles, mammals, and birds
(Kennedy 2006). The adult worm anchors itself to the gut wall of the vertebrate host
using an eversible proboscis which is covered with hooks and spines (Miller and
Dunagan 1985).
The typical life cycle of acanthocephalan involves two hosts, with the adult
living in a vertebrate host, producing eggs which are released into the environment, and an arthropod intermediate host which become infected by acanthocephalan larvae when they ingest the parasite’s eggs (Kennedy 2006). However,
many species also incorporate additional vertebrates to act as paratenic (transport
host) (e.g. Sinisalo and Valtonen 2003; Médoc et al. 2011)—the acanthocephalan
larvae do not undergo further development in those hosts, however paratenic hosts
1 Parasites of Fossil Vertebrates: What We Know and What Can We Expect…