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Hyolitha (Hyoliths)

      Hyolitha are enigmatic animals with small conical shells known from the Palaeozoic Era. The calcareous shells have a cover (operculum) and two curved supports known as helens. Most are one to four centimeters in length and are triangular or elliptical in cross section. Some species have rings or stripes. Because hyoliths are extinct and do not obviously resemble any extant group, it is unclear which living group they are most closely related to. They may be molluscs; authors who suggest that they deserve their own phylum do not comment on the position of this phylum in the tree of life.Fossil traces showing a twisted, looped, intestine bear some resemblance to the gut of sipunculan worms. Despite the fact that hyolithid shells are common as fossils, little is known about their ancestry, internal structures, and life mode. They were probably benthic (bottom-dwellers). The first hyolith fossils appeared about 540 million years ago in the Purella antiqua Zone of the Nemakit-Daldynian Stage of Siberia and in its analogue the Paragloborilus subglobosus–Purella squamulosa Zone of the Meishucunian Stage of China. Hyolith abundance and diversity attain a maximum in the Cambrian, followed by a progressive decline up to their Permian extinction.

Source: http://en.wikipedia.org/wiki/Hyolithid
Img. 94:
Hyolitha Marek, 1963 VOD197
Hyolitha Marek, 1963 VOD310
Hyolitha Marek, 1963 VOD19
Hyolitha Marek, 1963 VOD215
Hyolitha Marek, 1963 VOD190
Hyolitha Marek, 1963 VOD22
Hyolitha Marek, 1963 VOD4
Hyolitha Marek, 1963 VOD32
Hyolitha Marek, 1963 VOD84
Hyolithus parens Barrande, 1867 XB568
Hyolithus XB553
Hyolithus KP803
Hyolithus XB592
Hyolithus XB565
Hyolithus XB588
Hyolithus XB590
Hyolithus MŠ2684
Hyolithus KP1064
Jincelites CW207
Jincelites vogeliValent et al. (2009), fig. 3 A, B MŠ3084

Virtual museum of the Czech Geological Survey, www.geology.cz, (C) Czech Geological Survey, 2011, v.0.99 [13.12.2011]