| Kulindroplax Temporal range: Wenlock PreꞒ Ꞓ O S D C P T J K Pg N | |
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Scientific classification 
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Mollusca |
| Class: | Aplacophora |
| Genus: | †Kulindroplax Sutton, Briggs, Siveter, Siveter and Sigwart, 2012 |
| Species: | †K. perissokomos |
| Binomial name | |
| †Kulindroplax perissokomos Sutton, Briggs, Siveter, Siveter and Sigwart, 2012 | |
Kulindroplax perissokomos is a Silurian mollusk, known from a single fossil from the Coalbrookdale Formation fauna of England. It lived during the Homerian Age (about 425 million years ago). It is considered a basal aplacophoran. Unlike all modern aplacophorans, which are shell-less, Kulindroplax has a chiton-like shell, and it is considered a transitional fossil in the evolution of molluscs.
The only known specimen, described in 2012, was conserved at the Oxford University Museum of Natural History.
Morphology
Kulindroplax is about 2 cm (0.79 in) wide and 4 cm (1.6 in) long. It is the first known mollusk showing an unambiguous combination of valves, or exterior shells, and a worm-like body. It bears seven similar, unarticulated valves, with a shorter head valve and a taller caudal one, lacking ornaments and also several densely packed, 1–2 mm long spicules. It has no discernible foot, and the radula is not preserved. A gill array is present, along with a respiratory cavity opening posteriorly. These features make it more reminiscent of caudofoveate aplacophorans.
While aculiferan mollusks usually bear eight dorsal valves (except for multiplacophorans and Acaenoplax), Kulindroplax bears only seven in a single chiton-like row.
Life habits
Kulindroplax seems adapted to moving through a substrate, with the spicules acting as "sediment ratchets". In contrast with some modern aplacophorans like the caudofoveates, which live within the sea bottom sediments, Kulindroplax probably crawled on the bottom, requiring a dorsal armour.
Evolutionary significance
Kulindroplax settles a 20-year-long dispute about the phylogeny of mollusks, namely the relationship of the worm-like, carnivorous Aplacophora within the group, in particular their relationship to the Polyplacophora. Aplacophorans have been historically variously treated as either a mollusk base-group, distant relatives of Cephalopoda or as a sister group to Polyplacophora in the clade Aculifera. Both molecular and fossil evidence appeared to support the last hypothesis. Including Kulindroplax in a phylogenetic matrix with other mollusks and mollusk-like fossil taxa allows to consistently resolve Aculifera as a sister group to Conchifera, in all variants of the analysis, thus bringing the fossil record in line with the recent molecular evidence.
While other fossil taxa like Acaenoplax and Phthipodochiton showed intermediate features between aplacophorans and polyplacophorans, no unambiguous fossil with an aplacophoran-like body and a polyplacophoran-like shell has been found before Kulindroplax.
Below is a cladogram of mollusk phylogeny according to Sutton et al., 2012. Taxa marked with † are extinct.
| ← Lophotrochozoa |
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References
- ^ Sutton, M. D.; Briggs, D. E. G.; Siveter, D. J.; Siveter, D. J.; Sigwart, J. D. (2012). "A Silurian armoured aplacophoran and implications for molluscan phylogeny". Nature. 490 (7418): 94–97. Bibcode:2012Natur.490...94S. doi:10.1038/nature11328. PMID 23038472. S2CID 4422386.
- ^ "Ancient mollusk tells a contrary story". Yale News. 3 October 2012. Retrieved 17 November 2012.
- ^ "Mollusc missing link revealed in 3D". Imperial College London. 3 October 2012. Retrieved 17 November 2012.
- Kocot, K. M.; Cannon, J. T.; Todt, C.; Citarella, M. R.; Kohn, A. B.; Meyer, A.; Santos, S. R.; Schander, C.; Moroz, L. L.; et al. (2011). "Phylogenomics reveals deep molluscan relationships". Nature. 477 (7365): 452–456. Bibcode:2011Natur.477..452K. doi:10.1038/nature10382. PMC 4024475. PMID 21892190.
- Sigwart, J. D.; Sutton, M. D. (Oct 2007). "Deep molluscan phylogeny: synthesis of palaeontological and neontological data". Proceedings of the Royal Society B: Biological Sciences. 274 (1624): 2413–2419. doi:10.1098/rspb.2007.0701. PMC 2274978. PMID 17652065. For a summary, see "The Mollusca". University of California Museum of Paleontology. Retrieved 2 October 2008.
- Scheltema, A.H. (February 1, 1993). "Aplacophora as Progenetic Aculiferans and the Coelomate Origin of Mollusks as the Sister Taxon of Sipuncula". The Biological Bulletin. 184 (1): 57–78. doi:10.2307/1542380. JSTOR 1542380. PMID 29300613.
- Vinther, J.; Sperling, E. A.; Briggs, D. E. G.; Peterson, K. J. (2011). "A molecular palaeobiological hypothesis for the origin of aplacophoran molluscs and their derivation from chiton-like ancestors". Proceedings of the Royal Society B: Biological Sciences. 279 (1732): 1259–1268. doi:10.1098/rspb.2011.1773. PMC 3282371. PMID 21976685.
- Smith, S. A.; Wilson, N. G.; Goetz, F. E.; Feehery, C.; Andrade, S. N. C. S.; Rouse, G. W.; Giribet, G.; Dunn, C. W. (2011). "Resolving the evolutionary relationships of molluscs with phylogenomic tools". Nature. 480 (7377): 364–367. Bibcode:2011Natur.480..364S. doi:10.1038/nature10526. PMID 22031330. S2CID 4321076.
| Taxon identifiers | |
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| Kulindroplax | |