8
can help the parasite complete its life-cycle as they serve as a bridge between
arthropods and the acanthocephalan’s final host, which are often macropredatory
vertebrates which do not usually prey upon small arthropods (Nickol 1985;
Kennedy 2006).
The adult stages of acanthocephalans are soft bodied and thus unlikely to be
preserved as fossils, and while theoretically the hooks on the proboscis may potentially become fossilised under ideal circumstances, much like what has been discussed above for tapeworms, flukes, and roundworms, the stage most likely to be
found as fossils would be eggs embedded in coprolites produced by their vertebrate
hosts (Herlyn 2021).
Presently, the oldest known fossil example of acanthocephalans are fossilised
eggs which were found as inclusions in coprolites that have been attributed to Late
Cretaceous crocodyliformes (Cardia et al. 2019b). Much like the fossil pinworm
eggs discussed above, they provide us with some interesting insight into the ecology
of their host, since the presence of acanthocephalan eggs indicates the crocodyliformes that produced the coprolite had either consumed arthropods and/or vertebrate animals. This allows us to conclude that the coprolites most likely originated
from one of the species of carnivorous (or at least omnivorous) crocodyliformes that
had been found in that formation.
Furthermore fossils of acanthocephalans may also provide information on the
evolution of Acanthocephala as a whole and their pattern of host-usage (Herlyn
2021). Presently, Acanthocephala is divided into four distinct classes, and each of
those classes infect different suite of vertebrate animals from different environments
as their final hosts (Verweyen et al. 2011). Finding evidence of acanthocephalan
infection in fossil vertebrates, especially in lineages that left no living relatives, may
help shed light how these parasites had evolved to infect the type of hosts that they
do today.
1.1.5 Monogeneans
Monogeneans are a major group of parasitic flatworms which mostly live as ectoparasite of fish, with approximately 3000–4000 described species (Whittington
1998). Being soft-bodied animals they are highly unlikely leave behind fossils,
however their attachment organ—the haptor—has microscopic, scleritised hooks
which can potentially be fossilised under certain conditions (Littlewood and
Donovan 2003). Fossilised monogenean hooks have previously been found associated with placoderms and acanthodians (Upeniece 2001, 2011), and their positioning on the fins, gill region, and abdomen of the host is comparable to that of extant
monogeneans (Whittington 1998). Given their presence on those two phylogenetic
distinct groups of extinct fish and the wide range of extant fish lineages that host
monogeneans, it can be expected that most fossil fish from the Devonian period
onward would have been parasitised by monogeneans.
T. L. F. Leung
can help the parasite complete its life-cycle as they serve as a bridge between
arthropods and the acanthocephalan’s final host, which are often macropredatory
vertebrates which do not usually prey upon small arthropods (Nickol 1985;
Kennedy 2006).
The adult stages of acanthocephalans are soft bodied and thus unlikely to be
preserved as fossils, and while theoretically the hooks on the proboscis may potentially become fossilised under ideal circumstances, much like what has been discussed above for tapeworms, flukes, and roundworms, the stage most likely to be
found as fossils would be eggs embedded in coprolites produced by their vertebrate
hosts (Herlyn 2021).
Presently, the oldest known fossil example of acanthocephalans are fossilised
eggs which were found as inclusions in coprolites that have been attributed to Late
Cretaceous crocodyliformes (Cardia et al. 2019b). Much like the fossil pinworm
eggs discussed above, they provide us with some interesting insight into the ecology
of their host, since the presence of acanthocephalan eggs indicates the crocodyliformes that produced the coprolite had either consumed arthropods and/or vertebrate animals. This allows us to conclude that the coprolites most likely originated
from one of the species of carnivorous (or at least omnivorous) crocodyliformes that
had been found in that formation.
Furthermore fossils of acanthocephalans may also provide information on the
evolution of Acanthocephala as a whole and their pattern of host-usage (Herlyn
2021). Presently, Acanthocephala is divided into four distinct classes, and each of
those classes infect different suite of vertebrate animals from different environments
as their final hosts (Verweyen et al. 2011). Finding evidence of acanthocephalan
infection in fossil vertebrates, especially in lineages that left no living relatives, may
help shed light how these parasites had evolved to infect the type of hosts that they
do today.
1.1.5 Monogeneans
Monogeneans are a major group of parasitic flatworms which mostly live as ectoparasite of fish, with approximately 3000–4000 described species (Whittington
1998). Being soft-bodied animals they are highly unlikely leave behind fossils,
however their attachment organ—the haptor—has microscopic, scleritised hooks
which can potentially be fossilised under certain conditions (Littlewood and
Donovan 2003). Fossilised monogenean hooks have previously been found associated with placoderms and acanthodians (Upeniece 2001, 2011), and their positioning on the fins, gill region, and abdomen of the host is comparable to that of extant
monogeneans (Whittington 1998). Given their presence on those two phylogenetic
distinct groups of extinct fish and the wide range of extant fish lineages that host
monogeneans, it can be expected that most fossil fish from the Devonian period
onward would have been parasitised by monogeneans.
T. L. F. Leung