302
similar to the root of an acanthocephalan hook. On the other hand, the basis of the
cambroclavid sclerites is hollow (Fig. 8.5d), which is not the case in the roots of
acanthocephalan hooks (Fig. 8.6a, b). Moreover, the hooks of extant acanthocephalans are imbedded into the basement membrane underlying the tegument (Fig. 8.6a,
b). In addition, even if the fibers of basement membrane and hook roots should be
discontinuous (c.f. Taraschewski et al. 1989), it remains uncertain whether fossil
remains of acanthocephalan hooks would present as isolated units as it is usually the
case in cambroclavid microfossils. Furthermore, hooks of extant acanthocephalan
species are more or less recurved, while this is not the case in at least part of the
cambroclavid microfossils (Figs. 8.5 and 8.6a). Most of all, the size of cambroclavid
fossils does not accord with the expectation for early acanthocephalans, which
should have remained much smaller than the extant species (see above). For example, the maximum extension of Cambroclaves fossils can easily reach >500 μm
(Elicki and Wotte 2003). Recalling that early stem-acanthocephalans should have
measured in the range of one or few millimeters, hooks of cambroclavid dimension
would appear huge. Not least, the question arises why acanthocephalan hooks—if
cambroclavid microfossils are such—occur in peri-Gondwanan deposits, whereas
they have not been found in any other context so far. Thus, a closer affinity of cambroclavids to acanthocephalans seems unlikely at present.
8.10 Conclusions
Analyses of molecular and morphological data have shown that the
taxon Acanthocephala (thorny-headed worms) has a nested position inside
Gnathifera, a clade that also includes Gnathostomulida, Micrognathozoa and
Rotifera. Especially, Rotifera appears to be a paraphyletic assemblage as long as
Acanthocephala is excluded (Fig. 8.7). In addition, arrow worms (Chaetognatha)
seem to belong to the kinship of the Gnathifera. In support of this possibility, recent
studies suggest that arrow worms, for which Cambrian fossils are known (Shu et al.
2017; Briggs and Caron 2017), are either sister to Gnathifera or occupy a nested
position within the gnathiferan clade (Fröbius and Funch 2016; Marlétaz et al.
2019; Vinther and Parry 2019). Beyond that, Cambrian fossils have been attributed
to Gnathifera (Caron and Cheung 2019; Vinther and Parry 2019), some of which
might even have been epibionts or ectoparasites (Cong et al. 2017). Accordingly, a
Cambrian origin of Gnathifera is likely.
With regard to the Rotifera-Acanthocephala group, the temporal origin is less
clear. In fact, the oldest known fossils of Rotifera (inclusively Acanthocephala) or
Syndermata, as the group is also called, are acanthocephalan eggs from an Upper
Cretaceous coprolite (Cardia et al. 2019). Before this recently published finding,
only few remains of monogonont and bdelloid rotifers from the Eocene and Miocene
were known (Southcott and Lange 1971; Poinar and Ricci 1992; Waggoner and
Poinar 1993; Iturralde-Vinent and MacPhee 1996). On the other hand, estimates for
the appearance of mandibulates and gnathostomes enable rough time constraints for
the earliest possible associations with members of these taxa. In particular, the
H. Herlyn
similar to the root of an acanthocephalan hook. On the other hand, the basis of the
cambroclavid sclerites is hollow (Fig. 8.5d), which is not the case in the roots of
acanthocephalan hooks (Fig. 8.6a, b). Moreover, the hooks of extant acanthocephalans are imbedded into the basement membrane underlying the tegument (Fig. 8.6a,
b). In addition, even if the fibers of basement membrane and hook roots should be
discontinuous (c.f. Taraschewski et al. 1989), it remains uncertain whether fossil
remains of acanthocephalan hooks would present as isolated units as it is usually the
case in cambroclavid microfossils. Furthermore, hooks of extant acanthocephalan
species are more or less recurved, while this is not the case in at least part of the
cambroclavid microfossils (Figs. 8.5 and 8.6a). Most of all, the size of cambroclavid
fossils does not accord with the expectation for early acanthocephalans, which
should have remained much smaller than the extant species (see above). For example, the maximum extension of Cambroclaves fossils can easily reach >500 μm
(Elicki and Wotte 2003). Recalling that early stem-acanthocephalans should have
measured in the range of one or few millimeters, hooks of cambroclavid dimension
would appear huge. Not least, the question arises why acanthocephalan hooks—if
cambroclavid microfossils are such—occur in peri-Gondwanan deposits, whereas
they have not been found in any other context so far. Thus, a closer affinity of cambroclavids to acanthocephalans seems unlikely at present.
8.10 Conclusions
Analyses of molecular and morphological data have shown that the
taxon Acanthocephala (thorny-headed worms) has a nested position inside
Gnathifera, a clade that also includes Gnathostomulida, Micrognathozoa and
Rotifera. Especially, Rotifera appears to be a paraphyletic assemblage as long as
Acanthocephala is excluded (Fig. 8.7). In addition, arrow worms (Chaetognatha)
seem to belong to the kinship of the Gnathifera. In support of this possibility, recent
studies suggest that arrow worms, for which Cambrian fossils are known (Shu et al.
2017; Briggs and Caron 2017), are either sister to Gnathifera or occupy a nested
position within the gnathiferan clade (Fröbius and Funch 2016; Marlétaz et al.
2019; Vinther and Parry 2019). Beyond that, Cambrian fossils have been attributed
to Gnathifera (Caron and Cheung 2019; Vinther and Parry 2019), some of which
might even have been epibionts or ectoparasites (Cong et al. 2017). Accordingly, a
Cambrian origin of Gnathifera is likely.
With regard to the Rotifera-Acanthocephala group, the temporal origin is less
clear. In fact, the oldest known fossils of Rotifera (inclusively Acanthocephala) or
Syndermata, as the group is also called, are acanthocephalan eggs from an Upper
Cretaceous coprolite (Cardia et al. 2019). Before this recently published finding,
only few remains of monogonont and bdelloid rotifers from the Eocene and Miocene
were known (Southcott and Lange 1971; Poinar and Ricci 1992; Waggoner and
Poinar 1993; Iturralde-Vinent and MacPhee 1996). On the other hand, estimates for
the appearance of mandibulates and gnathostomes enable rough time constraints for
the earliest possible associations with members of these taxa. In particular, the
H. Herlyn