300
Hagadorn 2010). Consequently, ancient seisonids may already have lived on
phyllocarids in the Paleozoic, and they might have retained this host usage from
their LCA with acanthocephalans, just as they presumably kept the epibiotic/ectoparasitic lifestyle. If so, first stem-acanthocephalans should have exploited phyllocarids as well. But parasitization of other Paleozoic “invertebrates” by early
acanthocephalans can also not be ruled out. Among them might have been species
of Trilobita, the potential sister group of crown-Mandibulata (Scholtz and
Edgecombe 2006).
Whether only phyllocarids or (also) other mandibulates or even trilobites were
exploited in the beginning of acanthocephalan evolution will probably remain elusive. It will also be difficult if not impossible to assess more precisely when the
presumed shift from an ecto- to an endoparasitic lifestyle might have taken place in
acanthocephalan evolution. However, the establishment of a two-host cycle should
post-date the emergence of fish-like gnathostomes in the Middle Ordovician
(Sansom et al. 2015; also Janvier 2003) or later (Friedman and Sallan 2012; Brazeau
and Friedman 2015; Klug et al. 2017). In case that early acanthocephalans also used
fish-like vertebrates without jaws as definitive hosts, the two-host cycle could have
been established even earlier. Such possibility can not be ruled out since extant
acanthocephalans were occasionally reported from lampreys (Petromyzontida)
(Petrochenko 1956; Conway Morris and Crompton 1982). In any case, acanthocephalan diversity might subsequently have increased along with the diversification
of crown-Gnathostomata upon extinction of placoderms in the Upper Devonian
(Trinajstic et al. 2007; Sansom et al. 2015). When conquering new hosts, acanthocephalans may have benefited from generally less tight bonds to definitive than
intermediate hosts (Conway Morris and Crompton 1982; Parker et al. 2015).
The emergence of the individual gnathostome taxa used as hosts provides an
approximate orientation for the earliest possible origin of individual lineages within
Acanthocephala. Thereby, life cycles involving tetrapods should have evolved from
cycles with fish-like gnathostomes (Near 2002). Although not necessarily representing a suitable model for such transitions, it is worth noting that some of the extant
acanthocephalan species exploit an aquatic intermediate host and a terrestrial definitive host (e.g., Dezfuli and Giari 1999). Either way, most of the extant species
retained the ancestral condition of an aquatic cycle. Thus, life cycles in extant eoacanthocephalans involve sharks (Elasmobranchii, Selachii) and ray-finned fishes
(Actinopterygii), in particular bowfin (Amiiformes) and teleost fishes, besides turtles (Petrochenko 1956; Near et al. 1998). Polyacanthocephalans also have aquatic
life cycles, with teleost fishes and caimans (Crocodilia) serving as gnathostome
hosts (Amin 1987; Echi et al. 2015). Since Polyacanthocephala either has a nested
position inside Eoacanthocephala or is sister to Eoacanthocephala (Verweyen et al.
2011; Echi et al. 2015; Gazi et al. 2016), the LCAs of both taxa might already have
used fish-like gnathostomes in the Middle Ordovician or later (see above, for references). Under the renewed assumption that the exploitation of fish-like gnathostomes represents the ancestral state, the origin of the palaecanthocephalan stem line
could also go back to the Paleozoic. In fact, extant palaeacanthocephalans infect
diverse fish-like gnathostomes, especially sharks and rays (Elasmobranchii,
H. Herlyn
Hagadorn 2010). Consequently, ancient seisonids may already have lived on
phyllocarids in the Paleozoic, and they might have retained this host usage from
their LCA with acanthocephalans, just as they presumably kept the epibiotic/ectoparasitic lifestyle. If so, first stem-acanthocephalans should have exploited phyllocarids as well. But parasitization of other Paleozoic “invertebrates” by early
acanthocephalans can also not be ruled out. Among them might have been species
of Trilobita, the potential sister group of crown-Mandibulata (Scholtz and
Edgecombe 2006).
Whether only phyllocarids or (also) other mandibulates or even trilobites were
exploited in the beginning of acanthocephalan evolution will probably remain elusive. It will also be difficult if not impossible to assess more precisely when the
presumed shift from an ecto- to an endoparasitic lifestyle might have taken place in
acanthocephalan evolution. However, the establishment of a two-host cycle should
post-date the emergence of fish-like gnathostomes in the Middle Ordovician
(Sansom et al. 2015; also Janvier 2003) or later (Friedman and Sallan 2012; Brazeau
and Friedman 2015; Klug et al. 2017). In case that early acanthocephalans also used
fish-like vertebrates without jaws as definitive hosts, the two-host cycle could have
been established even earlier. Such possibility can not be ruled out since extant
acanthocephalans were occasionally reported from lampreys (Petromyzontida)
(Petrochenko 1956; Conway Morris and Crompton 1982). In any case, acanthocephalan diversity might subsequently have increased along with the diversification
of crown-Gnathostomata upon extinction of placoderms in the Upper Devonian
(Trinajstic et al. 2007; Sansom et al. 2015). When conquering new hosts, acanthocephalans may have benefited from generally less tight bonds to definitive than
intermediate hosts (Conway Morris and Crompton 1982; Parker et al. 2015).
The emergence of the individual gnathostome taxa used as hosts provides an
approximate orientation for the earliest possible origin of individual lineages within
Acanthocephala. Thereby, life cycles involving tetrapods should have evolved from
cycles with fish-like gnathostomes (Near 2002). Although not necessarily representing a suitable model for such transitions, it is worth noting that some of the extant
acanthocephalan species exploit an aquatic intermediate host and a terrestrial definitive host (e.g., Dezfuli and Giari 1999). Either way, most of the extant species
retained the ancestral condition of an aquatic cycle. Thus, life cycles in extant eoacanthocephalans involve sharks (Elasmobranchii, Selachii) and ray-finned fishes
(Actinopterygii), in particular bowfin (Amiiformes) and teleost fishes, besides turtles (Petrochenko 1956; Near et al. 1998). Polyacanthocephalans also have aquatic
life cycles, with teleost fishes and caimans (Crocodilia) serving as gnathostome
hosts (Amin 1987; Echi et al. 2015). Since Polyacanthocephala either has a nested
position inside Eoacanthocephala or is sister to Eoacanthocephala (Verweyen et al.
2011; Echi et al. 2015; Gazi et al. 2016), the LCAs of both taxa might already have
used fish-like gnathostomes in the Middle Ordovician or later (see above, for references). Under the renewed assumption that the exploitation of fish-like gnathostomes represents the ancestral state, the origin of the palaecanthocephalan stem line
could also go back to the Paleozoic. In fact, extant palaeacanthocephalans infect
diverse fish-like gnathostomes, especially sharks and rays (Elasmobranchii,
H. Herlyn