303
postulated first epibiotic or ectoparasitic association with mandibulates in the common stem line of seisonids and acanthocephalans can not have occurred prior to the
emergence of jawed arthropods in the Cambrian (Daley et al. 2018). Such a onehost cycle was probably passed on to the seisonid and acanthocephalan lineages,
followed by a change from living on to living in mandibulates in acanthocephalan evolution (Herlyn et al. 2003; Wey-Fabrizius et al. 2014; Sielaff et al. 2016).
Likewise, the presumed upward-inclusion of gnathostomes into the acanthocephalan life cycle should not have occurred prior to the emergence of corresponding
hosts in the Middle Ordovician (Sansom et al. 2015; also Janvier 2003) or later
(Brazeau and Friedman 2015; Klug et al. 2017). Although we cannot be sure
whether evolution has taken the path outlined, the following appears to be more
certain: The LCA of crown-acanthocephalans probably showed an obligate twohost cycle involving mandibulates and gnathostomes as intermediate and definitive
hosts, respectively (Fig. 8.1). Extensions of this two-host cycle by paratenic and
second definitive hosts could have occurred subsequently.
The presumed one-host-cycle in early acanthocephalan evolution implicates that
adult worms should have differed considerably with respect to morphology, when
compared to the adults in extant species. In particular, early acanthocephalans
should not have grown to body sizes as known from extant species. A marked
increase in body size rather followed the upward-inclusion of gnathostomes as
hosts. Several other evolutionary novelties should also have evolved along with the
two-host cycle. Especially, metamorphosis of the larval stage inside the mandibulate intermediate host (acanthor) to a young adult (acanthella) is obviously a developmental correlate of the two-host cycle (compare Meyer 1932). A hooked proboscis
and a muscular apparatus suspending the cerebral ganglion (receptacle and
receptacle- surrounding muscle) likely evolved in the same context. Likewise, traits
that are related to an increase in fecundity (large testes, fragmented ovaries, uterine
bell, etc.) should have emerged in the stem line of crown-acanthocephalans, along
with the establishment of a two-host cycle (Herlyn and Röhrig 2003; Poulin and
Morand 2000; Parker et al. 2015). However, there might also be characters in extant
acanthocephalans that already existed in the supposed one-host stage (Sielaff et al.
2016). In particular, a digestive tract might then already have been lacking as suggested by its absence in all developmental stages of the extant species (compare
Near et al. 1998; Wey-Fabrizius et al. 2014). Correspondingly, morphological and
physiological changes that enable nutrient uptake via the tegument at least in part
occurred prior to the establishment of a two-host cycle (Mauer et al. 2020).
Eggs are the only free propagules in the life cycles of the extant acanthocephalan
species (Figs. 8.1 and 8.2). They are also the sole ancient remains of acanthocephalans known to date. This probably reflects their enhanced preservability due to the
incorporation of keratin and, depending on the taxon, chitin (Whitfield 1973; Peters
et al. 1991; Taraschewski and Peters 1992; Taraschewski et al. 1992). The ancient
eggs discovered so far have most likely an archiacanthocephalan origin, as suggested by their size and the increased thickness and structure of their shells (Table 8.1
and references therein). In most of the cases, the eggs were retrieved from human,
carnivoran and xenarthran coprolites of several hundred to about 12,000 years.
However, there seems to be no reason why gnathostome vertebrates feeding on
8 Thorny-Headed Worms (Acanthocephala): Jaw-Less Members of Jaw-Bearing…
postulated first epibiotic or ectoparasitic association with mandibulates in the common stem line of seisonids and acanthocephalans can not have occurred prior to the
emergence of jawed arthropods in the Cambrian (Daley et al. 2018). Such a onehost cycle was probably passed on to the seisonid and acanthocephalan lineages,
followed by a change from living on to living in mandibulates in acanthocephalan evolution (Herlyn et al. 2003; Wey-Fabrizius et al. 2014; Sielaff et al. 2016).
Likewise, the presumed upward-inclusion of gnathostomes into the acanthocephalan life cycle should not have occurred prior to the emergence of corresponding
hosts in the Middle Ordovician (Sansom et al. 2015; also Janvier 2003) or later
(Brazeau and Friedman 2015; Klug et al. 2017). Although we cannot be sure
whether evolution has taken the path outlined, the following appears to be more
certain: The LCA of crown-acanthocephalans probably showed an obligate twohost cycle involving mandibulates and gnathostomes as intermediate and definitive
hosts, respectively (Fig. 8.1). Extensions of this two-host cycle by paratenic and
second definitive hosts could have occurred subsequently.
The presumed one-host-cycle in early acanthocephalan evolution implicates that
adult worms should have differed considerably with respect to morphology, when
compared to the adults in extant species. In particular, early acanthocephalans
should not have grown to body sizes as known from extant species. A marked
increase in body size rather followed the upward-inclusion of gnathostomes as
hosts. Several other evolutionary novelties should also have evolved along with the
two-host cycle. Especially, metamorphosis of the larval stage inside the mandibulate intermediate host (acanthor) to a young adult (acanthella) is obviously a developmental correlate of the two-host cycle (compare Meyer 1932). A hooked proboscis
and a muscular apparatus suspending the cerebral ganglion (receptacle and
receptacle- surrounding muscle) likely evolved in the same context. Likewise, traits
that are related to an increase in fecundity (large testes, fragmented ovaries, uterine
bell, etc.) should have emerged in the stem line of crown-acanthocephalans, along
with the establishment of a two-host cycle (Herlyn and Röhrig 2003; Poulin and
Morand 2000; Parker et al. 2015). However, there might also be characters in extant
acanthocephalans that already existed in the supposed one-host stage (Sielaff et al.
2016). In particular, a digestive tract might then already have been lacking as suggested by its absence in all developmental stages of the extant species (compare
Near et al. 1998; Wey-Fabrizius et al. 2014). Correspondingly, morphological and
physiological changes that enable nutrient uptake via the tegument at least in part
occurred prior to the establishment of a two-host cycle (Mauer et al. 2020).
Eggs are the only free propagules in the life cycles of the extant acanthocephalan
species (Figs. 8.1 and 8.2). They are also the sole ancient remains of acanthocephalans known to date. This probably reflects their enhanced preservability due to the
incorporation of keratin and, depending on the taxon, chitin (Whitfield 1973; Peters
et al. 1991; Taraschewski and Peters 1992; Taraschewski et al. 1992). The ancient
eggs discovered so far have most likely an archiacanthocephalan origin, as suggested by their size and the increased thickness and structure of their shells (Table 8.1
and references therein). In most of the cases, the eggs were retrieved from human,
carnivoran and xenarthran coprolites of several hundred to about 12,000 years.
However, there seems to be no reason why gnathostome vertebrates feeding on
8 Thorny-Headed Worms (Acanthocephala): Jaw-Less Members of Jaw-Bearing…