Hyolitha
| Hyolitha Temporal range: Lower Cambrian–Upper Permian[1] |
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| Hyolithes cerops, Spence Shale, Idaho (Middle Cambrian) | |
Scientific classification  |
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| Kingdom: | Animalia |
| Phylum: | Mollusca (?) |
| Class: | †Hyolitha Marek, 1963 |
| Orders | |
Hyolitha are enigmatic animals with small conical shells known from the Palaeozoic Era.
Contents
Morphology[edit]
The calcareous – probably aragonitic[2] – shells have a cover (operculum) and two curved supports known as helens, all of which grew by marginal accretion.[2] Most are one to four centimeters in length and are triangular or elliptical in cross section. Some species have rings or stripes.
Shell[edit]
The orthothecid shell has an internal layer with a microstructure of transverse bundles, and an external layer comprising longitudinal bundles.[2]
Helens[edit]
Helens are long structures that taper as they logarithmically coil gently in a ventral direction.[2]
The helens were calcareous, with an organic component, and had an organic-rich central core surrounded by concentric laminae of calcite. They grew by the addition of new material at their base, on the cavity side, leaving growth lines.[2] They were originally described by Walcott as separate fossils under the genus name Helenia, (Walcott's wife was named Helena and his daughter Helen); Bruce Runnegar adopted the name helen when they were recognized as part of the hyolith organism.[2]
Operculum[edit]
The operculum closes perfectly over the aperture of the shell, leaving two gaps through which the helens can protrude.[2] It comprises two parts: the cardinal shield, a flat region at the top of the shell; and the conical shield, the bottom part, which is more conical.[2] The inside of the shell bears a number of protrusions, notably the dorsal cardinal processes and the radially-arranged clavicles.[2]
Taxonomy[edit]
The hyoliths are divided into two orders, the Hyolithida and the Orthothecida.
Hyolitha have dorso-ventrally differentiated opercula, with the ventral surface of the shell extending forwards to form a shelf termed the ligula.[2]
The Orthothecida are somewhat more problematic, and probably contain a number of non-hyoliths simply because they are so difficult to identify with confidence, especially if their operculum is absent.[2] They have a straight (planar) opening, sometimes with a notch on the bottom side, and sealed with an operculum that has no ligula, clavicles, furrow or rooflets.[3]
Hyptiotheca is an unusual hyolithid, in that it lacks clavicles.[3]
Orthothecids fall into two groups: one, the orcothecida sensu stricto,[4] is kidney or heart shaped in cross-section due to a longitudinal groove on its ventral surface, and its opercula bear cardinal processes; the other has a rounded cross-section and often lacks cardinal processes, making them difficult to distinguish from other cornet-shaped calcareous organisms.[3] All were sessile and benthic; some may have been filter feeders.[4]
Phylogenetic position[edit]
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.[5] Their grade of organization is considered to be of the 'mollusc-annelid-sipunculid' level,[6] consistent with a Lophotrochozoan affinity, and comparison is usually drawn with the molluscs or sipunculids.[6][7] Older studies (predating the Lophotrochozoan concept) consider hyoliths to represent a stem lineage of the clade containing (Mollusca + Annelida + Arthropoda).[8]
Ecology[edit]
They were probably benthic (bottom-dwellers).
In the Cambrian, their global distribution shows no sign of provinciality, suggesting a long-lived planktonic larval life stage; but by the Ordovician distinct assemblages were becoming evident.[3]
Occurrence[edit]
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.[1][9]
References[edit]
- ^ a b Malinky, J. M. (2009). "Permian Hyolithida from Australia: The Last of the Hyoliths?". Journal of Paleontology 83: 147–152. doi:10.1666/08-094R.1.
- ^ a b c d e f g h i j k Mus, M. M.; Bergström, J. (2007). "Skeletal Microstructure of Helens, Lateral Spines of Hyolithids". Palaeontology 50 (5): 1231. doi:10.1111/j.1475-4983.2007.00700.x.
- ^ a b c d Malinky, J.M.; Skovsted, C.B (2004). "Hyoliths and small shelly fossils from the Lower Cambrian of North−East Greenland". Acta Palaeontologica Polonica 49 (4): 551–578.
- ^ a b Malinky, J. M. (2009). "First Occurrence of Orthotheca Novák, 1886 (Hyolitha, Early Devonian) in North America". Journal of Paleontology 83: 588–596. doi:10.1666/08-164R.1.
- ^ Malinky, j. 2009 "Permian Hyolithida from Australia: The Last of the Hyoliths?" Journal of Paleontology 83(1):147-152.
- ^ a b Runnegar, B. (1980). "Hyolitha: Status of the phylum". Lethaia 13: 21. doi:10.1111/j.1502-3931.1980.tb01025.x.
- ^ Kouchinsky, A. V. (2000). "Skeletal microstructures of hyoliths from the Early Cambrian of Siberia". Alcheringa: an Australasian Journal of Palaeontology 24 (2): 65–81. doi:10.1080/03115510008619525.
- ^ Runnegar, B.; Pojeta, J.; Morris, N. J.; Taylor, J. D.; Taylor, M. E.; McClung, G. (1975). "Biology of the Hyolitha". Lethaia 8 (2): 181. doi:10.1111/j.1502-3931.1975.tb01311.x.
- ^ Steiner, M.; Li, G.; Qian, Y.; Zhu, M.; Erdtmann, B. D. (2007). "Neoproterozoic to Early Cambrian small shelly fossil assemblages and a revised biostratigraphic correlation of the Yangtze Platform (China)". Palaeogeography, Palaeoclimatology, Palaeoecology 254: 67. doi:10.1016/j.palaeo.2007.03.046.
