289
present (Fig. 8.4a). Thus, cement and cement apparatus might reflect increased
competition between males, as does the enlargement of the usually two testes
(Fig. 8.3b; Poulin and Morand 2000). In any case, a capping structure at the posterior ending of a fossil intestinal parasite would be an indication of a female
acanthocephalan.
8.4 Soft Tissue, Functional Morphology and the Ideal Fossil
Fossils from the lower Cambrian of China and Canada show that the preservation of
worms can be so excellent that besides contour and appendages also internal structures like the digestive tract appear to be visible (Hu et al. 2008; Briggs and Caron
2017; Shu et al. 2017). The mentioned fossils are reminiscent of priapulids and
might have dwelled the sediment, which buried them later on. Still, soft tissue preservation is not restricted to Cambrian sediment dwellers but also occurred in epibiotic or ectoparasitic helminths. A first example is Inquicus fellatus, a parasite from
the Cambrian of China, which lived on priapulids or priapulid-like worms (Cong
et al. 2017). Another example is a special find amongst the already-mentioned
remains of fish ectoparasites from the Upper Devonian of Latvia: The corresponding fossil shows not only a circlet of six hooks reminiscent of a monogenean opisthaptor, but also the contour of the hook-bearing soft tissue (Upeniece 2001, 2011;
De Baets et al. 2015). But when soft-tissue preservation is rare in ectoparasites, it
should be even rarer in intestinal parasites.
8.4.1 Outer Contour and Tegument
Under the premise that preservation of intestinal parasites is unlikely, the tegument
or epidermis should not be less suitable for fossilization in acanthocephalans than
other helminths (compare Littlewood and Donovan 2003). Whether the tegument
experiences sclerotization (Taraschewski et al. 1989) or not, it is very resistant to
mechanical destruction and enzymatic decomposition (personal observation; but
see Reinhard 1990). This toughness is probably due to the syncytial organization of
the tegument and a presumably proteinaceous lamina underneath its distal plasma
membrane, a character complex shared by acanthocephalans and their closer phylogenetic relatives (Ahlrichs 1997; Herlyn and Ehlers 2001; Near 2002). Provided
that fossilization of the acanthocephalan tegument occurred, the outer contour of the
hypothetical remains might show an increase in body diameter from neck to metasoma as it is common in the extant species (Figs. 8.3 and 8.4; e.g., Petrochenko
1956, 1958). Furthermore, if a larger bulbous differentiation shines through at the
proboscis apex, this could be an intrusion of the tegument ((apical) tegument or
epidermis cone), as it is characteristic of the monophylum including polyacanthocephalans and eoacanthocephalans (Figs. 8.4b and 8.5b; Herlyn 2001; Amin 2013;
8 Thorny-Headed Worms (Acanthocephala): Jaw-Less Members of Jaw-Bearing…
present (Fig. 8.4a). Thus, cement and cement apparatus might reflect increased
competition between males, as does the enlargement of the usually two testes
(Fig. 8.3b; Poulin and Morand 2000). In any case, a capping structure at the posterior ending of a fossil intestinal parasite would be an indication of a female
acanthocephalan.
8.4 Soft Tissue, Functional Morphology and the Ideal Fossil
Fossils from the lower Cambrian of China and Canada show that the preservation of
worms can be so excellent that besides contour and appendages also internal structures like the digestive tract appear to be visible (Hu et al. 2008; Briggs and Caron
2017; Shu et al. 2017). The mentioned fossils are reminiscent of priapulids and
might have dwelled the sediment, which buried them later on. Still, soft tissue preservation is not restricted to Cambrian sediment dwellers but also occurred in epibiotic or ectoparasitic helminths. A first example is Inquicus fellatus, a parasite from
the Cambrian of China, which lived on priapulids or priapulid-like worms (Cong
et al. 2017). Another example is a special find amongst the already-mentioned
remains of fish ectoparasites from the Upper Devonian of Latvia: The corresponding fossil shows not only a circlet of six hooks reminiscent of a monogenean opisthaptor, but also the contour of the hook-bearing soft tissue (Upeniece 2001, 2011;
De Baets et al. 2015). But when soft-tissue preservation is rare in ectoparasites, it
should be even rarer in intestinal parasites.
8.4.1 Outer Contour and Tegument
Under the premise that preservation of intestinal parasites is unlikely, the tegument
or epidermis should not be less suitable for fossilization in acanthocephalans than
other helminths (compare Littlewood and Donovan 2003). Whether the tegument
experiences sclerotization (Taraschewski et al. 1989) or not, it is very resistant to
mechanical destruction and enzymatic decomposition (personal observation; but
see Reinhard 1990). This toughness is probably due to the syncytial organization of
the tegument and a presumably proteinaceous lamina underneath its distal plasma
membrane, a character complex shared by acanthocephalans and their closer phylogenetic relatives (Ahlrichs 1997; Herlyn and Ehlers 2001; Near 2002). Provided
that fossilization of the acanthocephalan tegument occurred, the outer contour of the
hypothetical remains might show an increase in body diameter from neck to metasoma as it is common in the extant species (Figs. 8.3 and 8.4; e.g., Petrochenko
1956, 1958). Furthermore, if a larger bulbous differentiation shines through at the
proboscis apex, this could be an intrusion of the tegument ((apical) tegument or
epidermis cone), as it is characteristic of the monophylum including polyacanthocephalans and eoacanthocephalans (Figs. 8.4b and 8.5b; Herlyn 2001; Amin 2013;
8 Thorny-Headed Worms (Acanthocephala): Jaw-Less Members of Jaw-Bearing…