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Monoplacophora, meaning "bearing one plate", is a polyphyletic superclass of molluscs with a cap-like shell inhabiting deep sea environments. Extant representatives were not recognized as such until 1952; previously they were known only from the fossil record, and were thought to have become extinct 375 million years ago.
The Coelosclerithophorans are a polyphyletic group of organisms bearing hollow sclerites made of aragonite, and with a supposedly distinctive microstructure.
The evolution of the molluscs is the way in which the Mollusca, one of the largest groups of invertebrate animals, evolved. This phylum includes gastropods, bivalves, scaphopods, cephalopods, and several other groups. The fossil record of mollusks is relatively complete, and they are well represented in most fossil-bearing marine strata. Very early organisms which have dubiously been compared to molluscs include Kimberella and Odontogriphus.
Helcionellid or Helcionelliformes is an order of small fossil shells that are universally interpreted as molluscs, though no sources spell out why this taxonomic interpretation is preferred. These animals are first found about 540 to 530 million years ago in the late Nemakit-Daldynian age, which is the earliest part of the Cambrian period. A single species persisted to the Early Ordovician. These fossils are component of the small shelly fossils (SSF) assemblages.
Helcionelloida is an extinct group of ancient molluscs. These are the oldest known conchiferan molluscs, that is, they had a mineralised shell. Some members of this class were mistaken for Monoplacophorans. The class was erected by Peel in 1991.
Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks. Around 85,000 extant species of molluscs are recognized. The number of fossil species is estimated between 60,000 and 100,000 additional species. The proportion of undescribed species is very high. Many taxa remain poorly studied.
Knightoconus antarcticus is an extinct species of fossil monoplacophoran from the Cambrian Minaret Formation of Antarctica. It is thought to represent an ancestor to the cephalopods. It had a chambered conical shell, but lacked a siphuncle.
Anabarites is a problematic lower Cambrian genus, and is one of the small shelly fossils. It was abundant in the early Tommotian and is also found in the Nemakit-Daldynian. The fossils represent the triradially symmetrical mineralised tube in which the organism dwelt; it was sedentary. It is named after the Anabar region in Yakutia, Russia; its name does not imply 'heavy'.
Micropilina is a genus of monoplacophoran molluscs. They are very small, mostly deepwater animals which have a superficially limpet-like shell.
Khairkhaniidae is an extinct family of Paleozoic fossil molluscs of uncertain position. They are thought to belong to either the Gastropoda or Monoplacophora. They possess planispiral coiled shells with a columnar microstructure.
Aldanellidae is an extinct family of paleozoic molluscs that have been assigned to the Gastropod stem group but may also belong to a paraphyletic "Monoplacophora".
Yochelcionelloidea is an extinct superfamily of paleozoic molluscs of uncertain position. The earliest yochelcionellids are known from the Middle Tommotian, but they are most diverse from the Botomian through the early Middle Cambrian.
The Kirengellids are a group of problematic Cambrian fossil shells of marine organisms. The shells bear a number of paired muscle scars on the inner surface of the valve.
Anabarella is a species of bilaterally-flattened monoplacophoran mollusc, with a morphological similarity to the rostroconchs. Its shell preserves evidence of three mineralogical textures on its outer surface: it is polygonal near the crest of the shell, subsequently changing to both spiny and stepwise. Its internal microstructure is calcitic and semi-nacreous. Its name reflects its provenance from Anabar, Siberia. It has been interpreted as ancestral to the rostroconchs, and has been aligned to the Helcionellidae.
Fordilla is an extinct genus of early bivalves, one of two genera in the extinct family Fordillidae. The genus is known solely from Early Cambrian fossils found in North America, Greenland, Europe, the Middle East, and Asia. The genus currently contains three described species, Fordilla germanica, Fordilla sibirica, and the type species Fordilla troyensis.
Salterella is an enigmatic Cambrian genus with a small, conical, calcareous shell that appears to be septate, but is rather filled with stratified laminar deposits. The shell contains grains of sediment, which are obtained selectively by a manner also observed in foramanifera. The genus was established by Elkanah Billings in 1861, and was named after the English palaeontologist John William Salter.
The cephalopods have a long geological history, with the first nautiloids found in late Cambrian strata, and purported stem-group representatives present in the earliest Cambrian lagerstätten.
Stenothecoida is a taxon of bivalved fossils from the Early to middle Cambrian period. They look a bit like brachiopods or bivalve molluscs.
Watsonella is a genus of 'mollusc' known from early (Terreneuvian) Cambrian strata.
Bruce Norman Runnegar is an Australian-born paleontologist and professor at UCLA. His research centers on using the fossil record to determine how, where, and when life originated and evolved. He has published on a wide variety of topics, including the phylogeny of molluscs, Dickinsonia fossils and oxygen levels, and molecular clock techniques.
References
- 1 2 3 Yochelson, E. L. (1969). "Stenothecoida, A Proposed New Class of Cambrian Mollusca". Lethaia. 2: 49–62. doi:10.1111/j.1502-3931.1969.tb01250.x.
- ↑ Runegar & Jell. 1980. Alcheringa, 4(2): 111.
- 1 2 Runnegar, B.; Pojeta Jr, J. (Oct 1974). "Molluscan Phylogeny: the Paleontological Viewpoint". Science. 186 (4161): 311–317. Bibcode:1974Sci...186..311R. doi:10.1126/science.186.4161.311. JSTOR 1739764. PMID 17839855.
- ↑ Bouchet, Philippe; Rocroi, Jean-Pierre; Frýda, Jiri; Hausdorf, Bernard; Ponder, Winston; Valdés, Ángel & Warén, Anders (2005). "Classification and nomenclator of gastropod families". Malacologia . Hackenheim, Germany: ConchBooks. 47 (1–2): 1–397. ISBN 3-925919-72-4. ISSN 0076-2997.
- 1 2 Parkhaev P. Yu. 2001. Molluscs and siphonoconchs. In: Alexander E. M. et al. (eds.) The Cambrian biostratigraphy of the Stansbury basin, South Australia. Transactions of the Paleontological Institute, Russian Academy of Sciences, 282: 133–210, plates 24–54. Watsonellinae on the page 187.
- ↑ Budd, G. E.; Jensen, S. (2000). "A critical reappraisal of the fossil record of the bilaterian phyla". Biological Reviews of the Cambridge Philosophical Society. 75 (2): 253–95. doi:10.1111/j.1469-185X.1999.tb00046.x. PMID 10881389.
- ↑ (in Russian) Missarzhevsky. (after 10 July) 1989. Drevneishie skeletnye okamenelosti i stratigrafiia pogranichnykh tolshch Dokembriia i Kembiia. (English translation: Oldest skeletal fossils and stratigraphy of Precambrian and Cambrian boundary beds.) Trudy Geologicheskogo Instituta, Akademia Nauk SSSR, 443, 237 pp., 32 plates. Mellopegmidae is on the page 179.
- ↑ Grabau A. W. 1900. Palaeontology of the Cambrian terranes of the Boston Basin. Occasional Papers of the Boston Society of Natural History 4:601–694.
