299
host. In particular, adult acanthocephalans were found to grow to larger body sizes
in endothermic than ectothermic hosts. It is to be said, however, that both effects
largely disappeared when correlation analyses included a correction against possible phylogenetic bias.
8.8 Acanthocephala and Gnathifera: Fossil Report
and Time Line
The following estimates of the time line of gnathiferan and especially acanthocephalan evolution have as a condition that the phylogenetic and temporal assignment of
fossils is at least approximately correct. This especially applies to the dating of the
earliest appearance of potential hosts (for a discussion, see De Baets and Littlewood
2015; Warnock and Engelstädter 2021). In Gnathifera, another obstacle is that fossil
evidence is comparably sparse. In fact, there seems to be only one report on fossilized eggs attributed to Acanthocephala, from the Upper Cretaceous (see above). All
other ancient acanthocephalan eggs are remains of several hundred to about
12,000 years (Table 8.1). In addition, only few fossil wheel animals have been found
so far, i.e., a monogonont from Eocene North Maslin Sands in South Australia
(Southcott and Lange 1971) and bdelloids in Dominican amber from the
Miocene (Poinar and Ricci 1992; Waggoner and Poinar 1993; Iturralde-Vinent and
MacPhee 1996). However, Eocene and Miocene fossils almost certainly do not shed
light on the emergence of Rotifera-Acanthocephala. In fact, if the already- mentioned
Cambrian species I. fellatus really belongs to Gnathifera, the stem line of RotiferaAcanthocephala may go back to the Early Palaeozoic (Cong et al. 2017). An Early
Paleozoic origin of Gnathifera and Rotifera-Acanthocephala would receive additional
confirmation if arrow worms (Chaetognatha) really are sister to or occupy a nested
position within Gnathifera (Fig. 8.7; Fröbius and Funch 2016; Marlétaz et al. 2019;
Vinther and Parry 2019). Thus, well preserved fossils of arrow worms are documented
from the Cambrian Chengjiang Lagerstätte in China and from the Burgess Shale in
Canada (Shu et al. 2017; Briggs and Caron 2017), and a potential stem chaetognath,
Amiskwia sagittiformis, also lived in the Cambrian (Vinther and Parry 2019).
Due to the lack of direct evidence, time estimates regarding acanthocephalan
evolution have to rely on the appearance of mandibulate and gnathostome hosts in
the fossil record (compare De Baets et al. 2015). In particular, the presumed ancestors of Seisonidea and Acanthocephala should not have lived on mandibulates prior
to the emergence of such hosts in the Cambrian (Daley et al. 2018). Probably, these
hosts had a crustacean-like appearance or were crustaceans (see Zhang and Pratt
2012; Harvey et al. 2012). It might even be possible to narrow down the spectrum
of first hosts to single taxa within Crustacea. Thus, extant seisonids live on
Phyllocarida (Crustacea), especially on Leptostraca (Fontaneto and de Smet 2015;
see also Sørensen et al. 2005), which emerged in the Permian according to the
present knowledge. However, leptostracans very much resemble CambrianCarboniferous phyllocarids collectively called Archaeostraca (Collette and
8 Thorny-Headed Worms (Acanthocephala): Jaw-Less Members of Jaw-Bearing…
host. In particular, adult acanthocephalans were found to grow to larger body sizes
in endothermic than ectothermic hosts. It is to be said, however, that both effects
largely disappeared when correlation analyses included a correction against possible phylogenetic bias.
8.8 Acanthocephala and Gnathifera: Fossil Report
and Time Line
The following estimates of the time line of gnathiferan and especially acanthocephalan evolution have as a condition that the phylogenetic and temporal assignment of
fossils is at least approximately correct. This especially applies to the dating of the
earliest appearance of potential hosts (for a discussion, see De Baets and Littlewood
2015; Warnock and Engelstädter 2021). In Gnathifera, another obstacle is that fossil
evidence is comparably sparse. In fact, there seems to be only one report on fossilized eggs attributed to Acanthocephala, from the Upper Cretaceous (see above). All
other ancient acanthocephalan eggs are remains of several hundred to about
12,000 years (Table 8.1). In addition, only few fossil wheel animals have been found
so far, i.e., a monogonont from Eocene North Maslin Sands in South Australia
(Southcott and Lange 1971) and bdelloids in Dominican amber from the
Miocene (Poinar and Ricci 1992; Waggoner and Poinar 1993; Iturralde-Vinent and
MacPhee 1996). However, Eocene and Miocene fossils almost certainly do not shed
light on the emergence of Rotifera-Acanthocephala. In fact, if the already- mentioned
Cambrian species I. fellatus really belongs to Gnathifera, the stem line of RotiferaAcanthocephala may go back to the Early Palaeozoic (Cong et al. 2017). An Early
Paleozoic origin of Gnathifera and Rotifera-Acanthocephala would receive additional
confirmation if arrow worms (Chaetognatha) really are sister to or occupy a nested
position within Gnathifera (Fig. 8.7; Fröbius and Funch 2016; Marlétaz et al. 2019;
Vinther and Parry 2019). Thus, well preserved fossils of arrow worms are documented
from the Cambrian Chengjiang Lagerstätte in China and from the Burgess Shale in
Canada (Shu et al. 2017; Briggs and Caron 2017), and a potential stem chaetognath,
Amiskwia sagittiformis, also lived in the Cambrian (Vinther and Parry 2019).
Due to the lack of direct evidence, time estimates regarding acanthocephalan
evolution have to rely on the appearance of mandibulate and gnathostome hosts in
the fossil record (compare De Baets et al. 2015). In particular, the presumed ancestors of Seisonidea and Acanthocephala should not have lived on mandibulates prior
to the emergence of such hosts in the Cambrian (Daley et al. 2018). Probably, these
hosts had a crustacean-like appearance or were crustaceans (see Zhang and Pratt
2012; Harvey et al. 2012). It might even be possible to narrow down the spectrum
of first hosts to single taxa within Crustacea. Thus, extant seisonids live on
Phyllocarida (Crustacea), especially on Leptostraca (Fontaneto and de Smet 2015;
see also Sørensen et al. 2005), which emerged in the Permian according to the
present knowledge. However, leptostracans very much resemble CambrianCarboniferous phyllocarids collectively called Archaeostraca (Collette and
8 Thorny-Headed Worms (Acanthocephala): Jaw-Less Members of Jaw-Bearing…