The Origins and Evolution of the Acanthocephala

Molecular evidence for Acanthocephala as a subtaxon of Rotifera

“…A topology with Acanthocephala and Platyhelminthes as sister taxa required 49 additional substitutions, and a topology representing Rotifera as a monophyletic group with Acanthocephala as the sister taxon required the addition of 28 more substitutions. The results strongly support the hypothesis that acanthocephalans share an immediate common ancestor with bdelloid rotifers, and are inconsistent with previous proposals concerning which extant group is most closely related to acanthocephalans (VanCleave 1941;Conway Morris and Crompton 1982).…”

Phylogenetic Relationships of the Acanthocephala Inferred from 18S Ribosomal DNA Sequences

“…Previous workers (Conway Morris and Crompton, 1982) proposed a Cambrian origin for the Acanthocephala; the conservative nature of the arthropod intermediate host group utilized by acanthocephalans (Table 1) in combination with the similarities in trees between major clades of acanthocephalans and their arthropod hosts is consistent with such an interpretation. However, the hypothesis that acanthocephalans and priapulids form a sister group (Conway Morris and Crompton, 1982) is not supported by morphological or molecular phylogenies (Garey et al, 1996;Lorenzen, 1985;Winnepenninckx et al, 1995), thus invalidating the suggestion that Burgess Shale (Cambrian) fossil priapulids (e.g., Ancalagon minor) represent an ancestral form of acanthocephalans (Conway Morris and Crompton, 1982). Given substantial rate variation in 18S rDNA sequences among metazoan taxa, other genes will be needed to address relative times of divergence using molecular data.…”

“…All archiacanthocephalans have eight uninucleate cement glands with giant nuclei. In contrast, the Eoacanthocephala have a single cement gland with multiple giant nuclei, and the Palaeacanthocephala have multiple cement glands (2-8) with ''fragmented'' nuclei (Bullock, 1969;Conway Morris and Crompton, 1982;Van Cleave, 1952). The nesting of the Acanthocephala within the Rotifera and the similarity of morphology, anatomical position, and function of the pedal glands in rotifers and cement glands in acanthocephalans provide reasons to hypothesize that these structures are homologous.…”

“…Structural characters provided the basis for interpreting systematic groupings of acanthocephalans; however, using these data to develop phylogenetic hypotheses has been hampered by a paucity of informative characters, morphological and ecological divergence among extant acanthocephalan groups, and an inability to polarize character states (Bullock, 1969;Conway Morris and Crompton, 1982). With no known fossil record and conflicting hypotheses concerning free-living sister taxa (outgroups), determination of shared derived character states among major acanthocephalan groups has been complicated (Conway Morris and Crompton, 1982). To date, relationships among the major lineages have not been sufficiently resolved.…”

First description of an apical epidermis cone in Paratenuisentis ambiguus (Acanthocephala: Eoacanthocephala) and its phylogenetic implications

“…3A, D±I). An epidermis cone can also be seen in a generalized sketch of the eoacanthocephalan proboscis in Conway Morris and Crompton (1982), (Fig. 3J).…”