286
(Testudines), stem-group representatives of birds commonly referred to as “dinosaurs”, mammals and other taxa, besides remains of Crocodyliformes. Nonetheless,
it may also be that the Cretaceous creature ingested one or more acanthocephalan
developmental stages when preying on a crustacean or other intermediate host.
Either way, the finding demonstrates that acanthocephalan eggs can fossilize, and
that details of eggshell composition and acanthor morphology remain discernible
under advantageous conditions. With such excellent preservation, the find reminds
of putative hooklets inside the egg of a tapeworm (Platyhelminthes, Cestoda) discovered in a 270 million year old shark coprolite (Dentzien-Dias et al. 2013).
Likewise, eggs of presumably cestode origin have been reported from rectum content of a Carboniferous shark fossil (Zangerl and Case 1976). Perhaps, acanthocephalan eggs will once be discovered in coprolites of similar or even higher age.
8.3.2 Hooks
While trunk and neck bear spines in some of the extant acanthocephalan species and
not in others, the proboscis is almost always armed with recurved hooks (Figs. 8.4
and 8.5a, b). These hooks are extracellular differentiations consisting of protein (see
Miller and Dunagan 1985), with roots resting in the basement membrane (basal
lamina) underlying the tegument (also integument, epidermis, cutis) (Fig. 8.6a, b).
However, this anchoring might actually be somewhat flexible due to the probable
discontinuity of the fibers constituting hook roots and basement membrane
(Fig. 8.6b; Taraschewski et al. 1989). In histological preparations, hook roots and
shafts show a peripheral rind of condensed material and a core of less densely
woven fibers (see labelled hook in Fig. 8.6a). This structuring seems to be reflected
in a radial gradient of element abundances as revealed by X-ray analysis. Presumably,
the incorporation of substances like calcium, sulfur, and phosphorus contributes to
the stiffening of the hooks, which reportedly proceeds in anterio-posterior direction
along the proboscis (Taraschewski 1989a, b; Amin and Heckmann 2017). In any
case, the sclerotization should increase the chances that acanthocephalan hooks are
contained in the fossil record (compare Littlewood and Donovan 2003).
Although exceptional preservation conditions are required, fossilization of
“invertebrate” hooks is possible. This is exemplified by fossil hooks in abdominal
and gill regions of Devonian remains from Latvia assigned to Placodermi and
Acanthodii (Upeniece 2001, 2011; De Baets et al. 2015). Their probable location on
the surface of the fish fossils, their appearance and partially also their circular arrangement suggest that the hooks once belonged to ectoparasitic monogeneans (Platyhelminthes, Monogenea) (Upeniece 2011; Leung 2017; De Baets et al.
2021), and not acanthocephalans. Still, the size of the fossil hook-like remains
(0.02–0.40 mm; Upeniece 2011) is in a range known from extant acanthocephalans
(Figs. 8.5a and 8.6a). However, when likewise small monogenean hooks can fossilize, why have no fossil hooks of acanthocephalans been discovered so far? The most
likely reason is that acanthocephalans live inside their gnathostome hosts—and
H. Herlyn
(Testudines), stem-group representatives of birds commonly referred to as “dinosaurs”, mammals and other taxa, besides remains of Crocodyliformes. Nonetheless,
it may also be that the Cretaceous creature ingested one or more acanthocephalan
developmental stages when preying on a crustacean or other intermediate host.
Either way, the finding demonstrates that acanthocephalan eggs can fossilize, and
that details of eggshell composition and acanthor morphology remain discernible
under advantageous conditions. With such excellent preservation, the find reminds
of putative hooklets inside the egg of a tapeworm (Platyhelminthes, Cestoda) discovered in a 270 million year old shark coprolite (Dentzien-Dias et al. 2013).
Likewise, eggs of presumably cestode origin have been reported from rectum content of a Carboniferous shark fossil (Zangerl and Case 1976). Perhaps, acanthocephalan eggs will once be discovered in coprolites of similar or even higher age.
8.3.2 Hooks
While trunk and neck bear spines in some of the extant acanthocephalan species and
not in others, the proboscis is almost always armed with recurved hooks (Figs. 8.4
and 8.5a, b). These hooks are extracellular differentiations consisting of protein (see
Miller and Dunagan 1985), with roots resting in the basement membrane (basal
lamina) underlying the tegument (also integument, epidermis, cutis) (Fig. 8.6a, b).
However, this anchoring might actually be somewhat flexible due to the probable
discontinuity of the fibers constituting hook roots and basement membrane
(Fig. 8.6b; Taraschewski et al. 1989). In histological preparations, hook roots and
shafts show a peripheral rind of condensed material and a core of less densely
woven fibers (see labelled hook in Fig. 8.6a). This structuring seems to be reflected
in a radial gradient of element abundances as revealed by X-ray analysis. Presumably,
the incorporation of substances like calcium, sulfur, and phosphorus contributes to
the stiffening of the hooks, which reportedly proceeds in anterio-posterior direction
along the proboscis (Taraschewski 1989a, b; Amin and Heckmann 2017). In any
case, the sclerotization should increase the chances that acanthocephalan hooks are
contained in the fossil record (compare Littlewood and Donovan 2003).
Although exceptional preservation conditions are required, fossilization of
“invertebrate” hooks is possible. This is exemplified by fossil hooks in abdominal
and gill regions of Devonian remains from Latvia assigned to Placodermi and
Acanthodii (Upeniece 2001, 2011; De Baets et al. 2015). Their probable location on
the surface of the fish fossils, their appearance and partially also their circular arrangement suggest that the hooks once belonged to ectoparasitic monogeneans (Platyhelminthes, Monogenea) (Upeniece 2011; Leung 2017; De Baets et al.
2021), and not acanthocephalans. Still, the size of the fossil hook-like remains
(0.02–0.40 mm; Upeniece 2011) is in a range known from extant acanthocephalans
(Figs. 8.5a and 8.6a). However, when likewise small monogenean hooks can fossilize, why have no fossil hooks of acanthocephalans been discovered so far? The most
likely reason is that acanthocephalans live inside their gnathostome hosts—and
H. Herlyn