The first molluscs ‐ small animals

Haszprunar, Gerhard

doi:10.1080/11250009209386641

Cited by 71 publications

(40 citation statements)

References 76 publications

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“…In this scenario the most basal annelid should have the body entirely surrounded by chaetae. This pattern would be congruent with the pattern of spicules and hooks present in aplacophoran molluscs (HASZPRUNAR 1992;SCHELTEMA et al 1994) and sipunculans (STEPHEN & EDMUNDS 1972;RICE 1993;CUTLER 1994). Unfortunately for this hypothesis, chaetae surrounding the body are only found in derived forms of terrestrial Oligochaeta.…”

Section: Parapodia and Lobopodia: Homologous Locomotory Appendagessupporting

confidence: 69%

“…The presence of homeobox genes in Platyhelminthes is not incongruent with this scenario. A repetition of structures may be found also in the scoleces of cestode flatworms, in Mollusca Linnaeus, 1758(HASZPRUNAR 1992SCHELTEMA et al 1994), and apparently also in the Precambrian metazoan Dickinsonia Sprigg, 1947(RUNNEGAR 1982. A repetition of structures thus seems to precede both the origin of the mesodermic body cavities and the origin of the mesodermic metameres.…”

Section: Mollusk-like Cleavagementioning

confidence: 92%

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Polychaeta, Annelida, and Articulata are not monophyletic: articulating the Metameria (Metazoa, Coelomata)

et al. 2003

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Polychaetes are metameric worms recognized for having parapodia, chaetae, and nuchal organs. Some authors have extended the Annelida to include Pogonophora, Echiura, and Clitellata. These suggestions are insufficient to generate a monophyletic group. They do not take into account two very large and important clades that in a cladistic analysis at a higher level are shown to be nested within the Annelida: the Ecdysozoa (arthropods and related taxa) and Enterocoela (deuterostomes and related taxa). Evolutionary histories of most characters across metazoan phyla are still very poorly known. Metameres and coeloms have been considered homoplastic in the literature, and yet the homeobox genes responsible for the expression of metamerism and of paired appendages, at least, are very largely distributed among the Metazoa. A phylogenetic analysis was performed for the ingroups of Polychaeta, including Clitellata, Enterocoela, and Ecdysozoa as terminal taxa. The remaining non-metameric phyla Platyhelminthes, Nemertea, Mollusca, and Sipuncula were included to root the tree within the Bilateria. Empirical data was obtained from the literature and run with the software Hennig86 with two comparative interpretations of a priori hypotheses of primary homology: one with negative characters (coding losses) and another considering only positive characters (without assumptions about losses). The most relevant conclusions are: (1) Annelida and Polychaeta are non-monophyletic, even when including Echiura, Clitellata, and Pogonophora; (2) Articulata, as traditionally circumscribed for Annelida and Arthropoda, is also not monophyletic; (3) Metameria becomes monophyletic only when Ecdysozoa and Enterocoela are included in addition to the traditional annelid taxa; (4) Ecdysozoa are the sister group of Aphrodita; (5) Clitellata are related to deposit-feeding sedentary polychaetes (scolecids), and Questidae represent their sister group; (6) Owenia plus Enterocoela form a monophyletic group related to the tubicolous polychaetes

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Section: Parapodia and Lobopodia: Homologous Locomotory Appendagessupporting

confidence: 69%

Section: Mollusk-like Cleavagementioning

confidence: 92%

Polychaeta, Annelida, and Articulata are not monophyletic: articulating the Metameria (Metazoa, Coelomata)

et al. 2003

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“…And because gonads are also offshoots of the kidney rudiments (Moor 1983), the number of these structures may also have been affected. Schaefer and Haszprunar (1997b) reached a similar conclusion regarding the ventilator function of the gills based on the lack of a distinct respiratory zone on the monoplacophorans gills and comparisons to them to gill and ventilator configurations in other mollusks (see Haszprunar 1992). Whether or not the larger Paleozoic monoplacophorans had primarily respiratory gills or ventilators is not known, but this question might be explored using models that incorporate gas diffusion measurements with volume to surface area ratios.…”

Section: Monoplacophorans As Living Fossilsmentioning

confidence: 86%

“…As pointed out by Haszprunar (1992) and Schaefer and Haszprunar (1997a), the lack of organ symmetry in the monoplacophorans has important implications for arguments regarding arguments for metamerism in this group. Instead of a linear series of body units, each with similar if not identical features, there is an almost haphazard association of shell muscles, gills, auricles, kidneys, and gonads in the monoplacophorans (Wingstrand 1985;Haszprunar 1992;Haszprunar and Schaefer 1997a; Table 1). Wingstrand (1985:47) was aware of this and discussed the potential role of the mesoderm in producing this pattern and also identified the critical question-is monoplacophoran "metamerism" pleisiomorphic (primitive) or autapomorphic (uniquely derived)?…”

Section: Monoplacophorans As Living Fossilsmentioning

confidence: 99%

“…6). Lowenstam (1978) filmed and photographed living monoplacophorans showing the gills vibrating, apparently to move water through the pallial groove-vibrating gills are unknown in other mollusks where ciliary action, sometimes assisted by muscular contractions, moves water over the gill surfaces (Haszprunar 1992). In a subsequent paper, Lowenstam (1978) mentioned that movements of the shell were accompanied by an acceleration of gill beating, but did not further elaborate on this atypical function for these supposed respiratory structures.…”

Section: Monoplacophorans As Living Fossilsmentioning

confidence: 99%

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Monoplacophorans and the Origin and Relationships of Mollusks

Lindberg

2009

Evo Edu Outreach

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The story of the discovery and study of the Monoplacophora (or Tryblidia) and how they have contributed to our understanding of the evolution of the Mollusca highlights the importance of integrating data from the fossil record with the study of living forms. Monoplacophora were common in the early Paleozoic and were thought to have become extinct during the Devonian Period, approximately 375 Mya. In the mid 1950s, they were recovered from abyssal depths off of Costa Rica and were immediately heralded as a "living fossil." The living specimens confirmed that some of the organs (kidneys, heart, and gills) were repeated serially, just like the shell muscles that had been observed in fossil specimens. This supported the hypothesis that they were closely related to other segmented organisms such as annelids and arthropods. Today, there are 29 described living species and a growing body of work examining their anatomy, phylogeny, and ecology. Additional fossil specimens have also been discovered, and what was once thought to be a possible missing link between annelid worms and mollusks now appears to be a highly specialized branch of the molluscan tree that tells us little about the ancestral mollusk condition. However, some assumptions and generalizations from those early days still remain-such as the abyssal nature of the living species. A large part of the evolutionary history of the lineage remains to be discovered and will likely prove more complicated and interesting than afforded by the living fossil designation.

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Mollusca (Molluscs)

Haszprunar¹

2020

Encyclopedia of Life Sciences

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Mollusca are comprised by about 120 000 species (80% Gastropoda, 15% Bivalvia) and thus is the second most diverse animal phylum occurring in all major environments except the aerospace. It is also the leading phylum concerning endangered and vulnerable species. Mollusca is bilateral (or secondarily asymmetrical) Protostomia (Lophotrochozoa). Though quite distinct, their diversity in all aspects of morphology, physiology, behaviour, genetics and ecology is remarkable. Most species are equipped with external spicules or shell (plates) and a foot, a mantle cavity with gills, a ciliary or muscular gliding sole (foot), a visceral portion with heart and excretory organs and alimentary tract with rasping tongue (radula). The body cavity is mesenchymate with a gonopericaridal coelomatic system. Development starts with a spiral cleavage, mostly followed by (often modified) a trochophore‐like larva. Most freshwater and terrestrial taxa show direct development, however. Both the phylogenetic position of the Mollusca among the Lophotrochozoa and the internal relationships of the eight extant classes still are under debate. Key Concepts Mollusca is the second most diverse phylum. Gastropoda (limpets, snails, whelks, winkles, slugs) comprise 80%, Bivalvia (mussels, scallops, clams, cockles), about 15% of the diversity of extant Mollusca. There is also a huge body of fossil record throughout all ages. Many continental molluscan species are critically endangered. Mollusca combines a quite strict body plan with tremendous variability in all aspects. The relationships between molluscs and mankind may be positive (food, jewellery and art, research models) or but negative (marine or agricultural pests, vectors of severe diseases).

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The first molluscs ‐ small animals

Cited by 71 publications

References 76 publications

Polychaeta, Annelida, and Articulata are not monophyletic: articulating the Metameria (Metazoa, Coelomata)

Polychaeta, Annelida, and Articulata are not monophyletic: articulating the Metameria (Metazoa, Coelomata)

Monoplacophorans and the Origin and Relationships of Mollusks

Mollusca (Molluscs)

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