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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 subglobosusPurella 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
Hyolitha
Img. 94:
Maxilites maximus XA230
Maxilites maximus XA229
Maxilites maximus YA1025
Maxilites maximus XB268
Maxilites maximus XB271
Maxilites maximus XB279
Maxilites maximus YA1012
Maxilites pauper p2020
Maxilites pauxillus M2196
Maxilites pauxillus YA19
Maxilites pauxillus M2197
Maxilites primus p3638
Maxilites primus M1192
Maxilites primus M1191
Maxilites primus p3687
Maxilites simplex p5274
Maxilites XA809
Maxilites VK205
Maxilites SZ44
Maxilites VL1528

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