Hyolitha Fossil range: Cambrian–Permian[1] PreЄ Є O S D C P T J K Pg N
Hyolithes cerops, Spence Shale, Idaho (Middle Cambrian)
Scientific classification
Kingdom:	Animalia
Subkingdom:	Metazoa
Superphylum:	Lophotrochozoa
Phylum:	Mollusca?[2]
Class:	Hyolitha

Hyoliths from the Middle Ordovician of northern Estonia; these are internal molds.

Hyolitha are enigmatic animals with small conical shells known from the Palaeozoic Era.

[edit] Shell morphology

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.

[edit] Taxonomy & ecology

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.^[1] Fossil traces showing a twisted, looped, intestine bear some resemblance to the gut of sipunculan worms.^[3]

Despite the fact that hyolithid shells are quite common as fossils, next to nothing is known about their ancestry and internal structures. They were obviously benthic (bottom-dwellers), and there is some evidence that they were carnivores.^{[citation needed]}

[edit] Fossil record

The first hyolith fossils appeared about 540 million years ago in the Purella antiqua Zone of the Nemakit-Daldynian Stage of the Siberia and in its analogue the Paragloborilus subglobosus–Purella squamulosa Zone of the Meishucunian Stage of the China. Hyolith abundance and diversity attain a maximum in the Cambrian, followed by a progressive decline up to their Permian extinction.^[4]

[edit] References

[edit] External links

• Introduction to the Sipuncula, by UCMP

Paleontology portal