298
When matching the character states presumed for single LCAs with the phylogenetic tree in Fig. 8.7, it is the most parsimonious to postulate a shift from free-living
to an epibiotic (epizoic) or ectoparasitic lifestyle on mandibulate arthropods for the
branch uniting Seisonidea and Acanthocephala. Presumably, a second shift in the
way of living followed on the acanthocephalan stem line, namely towards an endoparasitic lifestyle with mandibulates as hosts. In a third step, the acanthocephalan
life cycle was expanded by upward-inclusion of gnathostomes as hosts (Conway
Morris and Crompton 1982; Herlyn et al. 2003; Wey-Fabrizius et al. 2014; Sielaff
et al. 2016). If the steps towards the two-host cycle of crown-Acanthocephala
involved several host species or only a single one, each, has to remain unanswered.
Nonetheless, several up to many individuals of at least one host species should have
been the ground on which each of the evolutionary steps took place. In this sense, at
least, the use of plural seems appropriate in respect to acanthocephalan hosts.
The stepwise establishment of a two-host cycle should have taken place in an
aquatic environment. This is indeed very likely considering that extant chaetognaths, gnathostomulids, micrognathozoans, monogononts, bdelloids, and seisonids live in aquatic environments and many acanthocephalans use aquatic hosts. In
turn, terrestrial life cycles as in extant archiacanthocephalans and some palaeacanthocephalan species should reflect secondary changes (Near et al. 1998). Yet, it is
less clear whether the LCA of crown-acanthocephalans used marine or freshwater
species as hosts since both alternatives occur in extant thorny-headed worms and
their closer phylogenetic relatives (Ax 2001; Petrochenko 1956, 1958; Kristensen
and Funch 2000; Fontaneto and de Smet 2015; Sterrer and Sørensen 2015). However,
if seisonids retained not only an epibiotic/ectoparasitic lifestyle from their LCA
with acanthocephalans but also the exploitation of marine crustaceans, acanthocephalan evolution should also have begun in a marine environment.
The establishment of the acanthocephalan two-host cycle was evidently accompanied by the evolution of several morphological novelties such as an invertible
proboscis, a particular muscular apparatus suspending the cerebral ganglion,
absence of an alimentary tract, a re-organized tegument and specialties in the reproductive systems of both sexes (Dezfuli et al. 2001; Near 2002; Herlyn and Röhrig
2003; Herlyn and Taraschewski 2017, etc.). Early stem-acanthocephalans, however,
should have displayed the plesiomorphic alternatives. In particular, they presumably
did not reach the body sizes known from extant species (see, e.g., compilation in
Petrochenko 1956, 1958). According to body sizes in extant gnathostomulids,
micrognathozoans, monogononts, bdelloids and seisonids, adults of ancient thornyheaded worms have presumably measured in the range of less than one to a few
limiting their fossilisation potential. Subsequent increases of body size along with
the evolution of gigantism in “invertebrate” host lineages can not be ruled out (Klug
et al. 2015). However, if evolution took such a path it unlikely happened in the stem
line of crown-acanthocephalans. With respect to this lineage, the presumed upwardinclusion of gnathostomes into the life cycle more likely paved the way for larger
body sizes. In extension of this argument, it was found for extant acanthocephalans
that size is positively correlated with body mass of their vertebrate hosts (Poulin
et al. 2003). Another positive correlate was the temperature regime imposed by the
H. Herlyn
When matching the character states presumed for single LCAs with the phylogenetic tree in Fig. 8.7, it is the most parsimonious to postulate a shift from free-living
to an epibiotic (epizoic) or ectoparasitic lifestyle on mandibulate arthropods for the
branch uniting Seisonidea and Acanthocephala. Presumably, a second shift in the
way of living followed on the acanthocephalan stem line, namely towards an endoparasitic lifestyle with mandibulates as hosts. In a third step, the acanthocephalan
life cycle was expanded by upward-inclusion of gnathostomes as hosts (Conway
Morris and Crompton 1982; Herlyn et al. 2003; Wey-Fabrizius et al. 2014; Sielaff
et al. 2016). If the steps towards the two-host cycle of crown-Acanthocephala
involved several host species or only a single one, each, has to remain unanswered.
Nonetheless, several up to many individuals of at least one host species should have
been the ground on which each of the evolutionary steps took place. In this sense, at
least, the use of plural seems appropriate in respect to acanthocephalan hosts.
The stepwise establishment of a two-host cycle should have taken place in an
aquatic environment. This is indeed very likely considering that extant chaetognaths, gnathostomulids, micrognathozoans, monogononts, bdelloids, and seisonids live in aquatic environments and many acanthocephalans use aquatic hosts. In
turn, terrestrial life cycles as in extant archiacanthocephalans and some palaeacanthocephalan species should reflect secondary changes (Near et al. 1998). Yet, it is
less clear whether the LCA of crown-acanthocephalans used marine or freshwater
species as hosts since both alternatives occur in extant thorny-headed worms and
their closer phylogenetic relatives (Ax 2001; Petrochenko 1956, 1958; Kristensen
and Funch 2000; Fontaneto and de Smet 2015; Sterrer and Sørensen 2015). However,
if seisonids retained not only an epibiotic/ectoparasitic lifestyle from their LCA
with acanthocephalans but also the exploitation of marine crustaceans, acanthocephalan evolution should also have begun in a marine environment.
The establishment of the acanthocephalan two-host cycle was evidently accompanied by the evolution of several morphological novelties such as an invertible
proboscis, a particular muscular apparatus suspending the cerebral ganglion,
absence of an alimentary tract, a re-organized tegument and specialties in the reproductive systems of both sexes (Dezfuli et al. 2001; Near 2002; Herlyn and Röhrig
2003; Herlyn and Taraschewski 2017, etc.). Early stem-acanthocephalans, however,
should have displayed the plesiomorphic alternatives. In particular, they presumably
did not reach the body sizes known from extant species (see, e.g., compilation in
Petrochenko 1956, 1958). According to body sizes in extant gnathostomulids,
micrognathozoans, monogononts, bdelloids and seisonids, adults of ancient thornyheaded worms have presumably measured in the range of less than one to a few
limiting their fossilisation potential. Subsequent increases of body size along with
the evolution of gigantism in “invertebrate” host lineages can not be ruled out (Klug
et al. 2015). However, if evolution took such a path it unlikely happened in the stem
line of crown-acanthocephalans. With respect to this lineage, the presumed upwardinclusion of gnathostomes into the life cycle more likely paved the way for larger
body sizes. In extension of this argument, it was found for extant acanthocephalans
that size is positively correlated with body mass of their vertebrate hosts (Poulin
et al. 2003). Another positive correlate was the temperature regime imposed by the
H. Herlyn