301
Batoidea), sturgeons (Acipenseriformes), and bowfin and teleost fishes in addition
to Amphibia, Sauropsida and Mammalia (Petrochenko 1956, 1958; Near et al.
1998). As mentioned above, the taxon Archiacanthocephala is specific because the
extant species have life cycles with terrestrial mammals and birds as definitive hosts
(e.g., Near et al. 1998). If this definitive host spectrum reflects an association of
early archiacanthocephalans with stem-Amniota, the transition to the usage of terrestrial definitive hosts should not have occurred much earlier than about 346–358
million years ago, which approximately marks the divergence of Amphibia and
Amniota. However, if early archiacanthocephalans had originally exploited solely
mammals and later on additionally conquered birds, the transition to a terrestrial
cycle should postdate the divergence of Sauropsida and Mammalia 297–326 million
years ago (split estimates according to timetree.org). The age of fossil archiacanthocephalan eggs obtained from an Upper Cretaceous coprolite (Cardia et al. 2019) is
in line with both possibilities.
Above estimates on the emergence of individual taxa contrast to some degree
with phylogenetic reconstructions suggesting a closer relationship of
Eoacanthocephala (inclusively Polyacanthocephala) to Palaeacanthocephala than
Archiacanthocephala (e.g., García-Varela and Nadler 2005; Verweyen et al. 2011),
thus giving the following tree: (Archiacanthocephala, (Palaeacanthocephala,
Eoacanthocephala)). In fact, such a phylogeny implicates that the stem line of archiacanthocephalans goes back further in time than the stem lines of palaeacanthocephalans and eoacanthocephalans. However, archiacanthocephalans could have
obtained an aquatic life cycle long before a terrestrial cycle was established. In any
case, some inconsistency between the distribution of traits and phylogenetic trees is
not uncommon in acanthocephalan research: The presence or absence of lateral
sensory organs (see above), for example, can only be aligned with the aforementioned phylogeny if one assumes their secondary loss within Palaeacanthocephala
(Weber et al. 2013). The complex situation is also reflected in the naming of the
three major acanthocephalan taxa: The prefixes palae- (old, ancient or primitive),
eo- (earliest), and archi- (primary) all express the view that the respective taxon
combines to a remarkable extent ancient characters (compare Meyer 1932; Van
Cleave 1936).
8.9 Cambroclavida: Microfossils of Questionable
Acanthocephalan Affiliation
Certain Cambrian sclerites were repeatedly regarded as acanthocephalan remains
(Qian and Yin 1984; Amin 2013). These fossils are collectively referred to as
Cambroclavida and actually are of unclear phylogenetic affiliation (Clausen and
Álvaro 2006; Kouchinsky et al. 2012). The microfossils were discovered in periGondwanan deposits of China and Europe, amongst others (e.g., Elicki and Wotte
2003). Part of them is remotely reminiscent of the hooks of extant acanthocephalans
(Fig. 8.5): They show hook-like recesses that emerge from a basis that might appear
8 Thorny-Headed Worms (Acanthocephala): Jaw-Less Members of Jaw-Bearing…
Batoidea), sturgeons (Acipenseriformes), and bowfin and teleost fishes in addition
to Amphibia, Sauropsida and Mammalia (Petrochenko 1956, 1958; Near et al.
1998). As mentioned above, the taxon Archiacanthocephala is specific because the
extant species have life cycles with terrestrial mammals and birds as definitive hosts
(e.g., Near et al. 1998). If this definitive host spectrum reflects an association of
early archiacanthocephalans with stem-Amniota, the transition to the usage of terrestrial definitive hosts should not have occurred much earlier than about 346–358
million years ago, which approximately marks the divergence of Amphibia and
Amniota. However, if early archiacanthocephalans had originally exploited solely
mammals and later on additionally conquered birds, the transition to a terrestrial
cycle should postdate the divergence of Sauropsida and Mammalia 297–326 million
years ago (split estimates according to timetree.org). The age of fossil archiacanthocephalan eggs obtained from an Upper Cretaceous coprolite (Cardia et al. 2019) is
in line with both possibilities.
Above estimates on the emergence of individual taxa contrast to some degree
with phylogenetic reconstructions suggesting a closer relationship of
Eoacanthocephala (inclusively Polyacanthocephala) to Palaeacanthocephala than
Archiacanthocephala (e.g., García-Varela and Nadler 2005; Verweyen et al. 2011),
thus giving the following tree: (Archiacanthocephala, (Palaeacanthocephala,
Eoacanthocephala)). In fact, such a phylogeny implicates that the stem line of archiacanthocephalans goes back further in time than the stem lines of palaeacanthocephalans and eoacanthocephalans. However, archiacanthocephalans could have
obtained an aquatic life cycle long before a terrestrial cycle was established. In any
case, some inconsistency between the distribution of traits and phylogenetic trees is
not uncommon in acanthocephalan research: The presence or absence of lateral
sensory organs (see above), for example, can only be aligned with the aforementioned phylogeny if one assumes their secondary loss within Palaeacanthocephala
(Weber et al. 2013). The complex situation is also reflected in the naming of the
three major acanthocephalan taxa: The prefixes palae- (old, ancient or primitive),
eo- (earliest), and archi- (primary) all express the view that the respective taxon
combines to a remarkable extent ancient characters (compare Meyer 1932; Van
Cleave 1936).
8.9 Cambroclavida: Microfossils of Questionable
Acanthocephalan Affiliation
Certain Cambrian sclerites were repeatedly regarded as acanthocephalan remains
(Qian and Yin 1984; Amin 2013). These fossils are collectively referred to as
Cambroclavida and actually are of unclear phylogenetic affiliation (Clausen and
Álvaro 2006; Kouchinsky et al. 2012). The microfossils were discovered in periGondwanan deposits of China and Europe, amongst others (e.g., Elicki and Wotte
2003). Part of them is remotely reminiscent of the hooks of extant acanthocephalans
(Fig. 8.5): They show hook-like recesses that emerge from a basis that might appear
8 Thorny-Headed Worms (Acanthocephala): Jaw-Less Members of Jaw-Bearing…