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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
Hyolitha
Img. 94:
Nowakia acuaria KOS2946
Nowakia acuaria KOS2935
Nowakia acuaria KOS2599
Nowakia acuaria KOS2959
Nowakia acuaria KOS637
Nowakia acuaria KOS2944
Nowakia acuaria KOS2483
Nowakia acuaria KOS2754
Nowakia pragensis pragensisLukeš et al. (2008) PL2741
Nowakia sororculaLukeš et al. (2008) PL2726
Nowakia sororculaLukeš et al. (2008) PL1308
Nowakia KOS2430
Nowakia KOS820
Orthotheca intermedia p4716
Orthotheca intermedia p6163
Orthotheca intermedia p5281
Orthotheca intermediafoto XB552
Orthotheca intermedia p1094
Orthotheca intermedia BB451
Orthotheca interstrialis p3657

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