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Conulariida

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Conulariida
Temporal range: Upper Cambrian–Triassic
Conulariid from the Mississippian (c. 360 to 325 mya) of Indiana; scale in mm.
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Cnidaria
Order: †Conulatae
Clade: †Conulariida
Genera

See text.

Conulariida is a poorly understood fossil group that has possible affinity with the Cnidaria. Their exact position as a taxon of extinct medusozoan cnidarians is highly speculative. Members of the Conulariida are commonly referred to as conulariids and appear in the fossil record from the mid or late Cambrian Period until the Triassic.[1]

Structure

The conulariids are fossils preserved as shell-like structures made up of rows of calcium phosphate rods, resembling an ice-cream cone with fourfold symmetry, usually four prominently-grooved corners.[2] New rods were added as the organism grew in length; the rod-based growth falsely gives the fossils a segmented appearance. Exceptional soft-part preservation has revealed that soft tentacles protruded from the wider end of the cone, and a holdfast from the pointed end attached the organisms to hard substrate. The prevailing reconstruction of the organism has it look superficially like a sea anemone sitting inside an angular, hard cone held perpendicular to the substrate. Conulariid shell is composed of francolite with carbonate ion concentration 8.1 wt%. The lattice parameters of conulariid apatite are a = 9.315(7) Å, c = 6.888(3) Å.[3] The fine structure of their shell comprises multiple lamellae of alternately organic-rich and organic-poor layers.[4]

Fossil record

Close-up of a conulariid from the Mississippian of Indiana; scale in mm.
Conularia milwaukeensis from the Middle Devonian of Wisconsin.

Setting aside for the moment the Precambrian Vendoconularia, which may or may not be a conulariid at all,[5] the Conulata fossil record begins with undeniable specimens in the Upper Cambrian (Hughes et al., 2000) and extends without significant break through numerous major mass extinctions. The Conulariids finally disappear from the fossil record during the Lower Triassic stage of the Triassic Period (~245 million years ago).

In North America, conulariids are generally more common in rocks of Ordovician and Carboniferous age.

Lifestyle

Conulariids apparently lived only in normal-marine waters, such as the oceans and inland seas. Fossils are commonly found in rocks representing offshore, even anoxic, marine bottom environments. This has led some scientists to infer that these animals may have drifted planktonically for some or all of their lives, ultimately being buried in the anoxic sediments beneath the oxic waters in which they lived. However, basic functional considerations (such as the great weight of the shell) make such interpretations difficult to maintain.

Phylogeny

About 20 genera and 150 species are known,[6] but except for local occurrences, Conulariids are relatively uncommon.

The conulariids were originally thought to be anthozoan cnidarians. However, the lack of septa or other features diagnostic of anthozoans led researchers to abandon this hypothesis. Ivantsov and Fedonkin (2002) posit that the conulariids were ancestrally tri-radially symmetrical, as typified with Vendoconularia, typical of the structure seen in Vendozooans.[7] Conulariids are, however, not generally thought to be a part of the Ediacaran biota as their fossil record starts at Upper Cambrian.

It is now also thought that the conulate trilobozoans derived their fourfold symmetry from a sixfold symmetry, as seen in Vendoconularia. This in turn, is thought to be originally derived from an ancestral disk-like trilobozoan three-fold symmetry.

Conulariids have generally been thought to be of Cnidarian affinity, occupying a position near the base of the Cnidarian family tree. However, since the 2010s, authors consider conulariids to be most closely related to the Scyphozoa, or the "true jellyfish". A possible arrangement is as relatives of the extant stalked jellyfish.[8] However, the nature of Conulariids and their phylogenetic relationships to other organisms remain poorly understood, and the supposed cnidarian affinity remains speculative.

Pearls

Conulariids produced pearls within their shells, similar to the way molluscs such as oysters, other pelecypods, and some gastropods do today. These pearls give a clue as to the internal anatomy of the conulariid animal. But due to their calcium phosphate composition, their crystal structure, and their extreme age, these pearls tend to be rather unattractive for use in or as decorative objects.[9]

List of genera

References

  1. ^ Ben M. Waggoner; David Smith (1994). "Study of conulariid and related phosphatic conical exoskeletons from the Prague Basin (Czech Republic)". In G. L. Albanesi; M. S. Beresi; S. H. Peralta (eds.). Ordovician from the Andes. Serie Correlación Geológica Nº 17. INSTITUTO SUPERIOR DE CORRELACIÓN GEOLÓGICA.
  2. ^ Waggoner, B.M. & D. Smith (1994): The Conulariida, Mystery fossils. University of California Museum of Paleontology web page [1]
  3. ^ Vinn, O.; Kirsimäe, K. (2015). "Alleged cnidarian Sphenothallus in the Late Ordovician of Baltica, its mineral composition and microstructure". Acta Palaeontologica Polonica. 60: 1001–1008. doi:10.4202/app.00049.2013.
  4. ^ Ford, Robert C.; Van Iten, Heyo; Clark, George R. (2016). "Microstructure and composition of the periderm of conulariids". Journal of Paleontology. 90 (3): 389. doi:10.1017/jpa.2016.63.
  5. ^ Van Iten, H.; De Moraes Leme, J.; Coelho Rodrigues, S.; Guimaraes Simoes, M. (2005). "Reinterpretation of a Conulariid-Like Fossil from the Vendian of Russia". Palaeontology. 48 (3): 619–622. doi:10.1111/j.1475-4983.2005.00471.x. hdl:11449/31655.
  6. ^ [2]
  7. ^ Ivantsov, A. Y.; Fedonkin, M. A. (2002). "Conulariid-like fossil from the Vendian of Russia: a metazoan clade across the Proterozoic/Palaeozoic boundary" (PDF). Palaeontology. 45 (6): 1119–1129. doi:10.1111/1475-4983.00283. Archived from the original (Free full text) on 2016-01-28. Retrieved 2015-08-29.
  8. ^ Iten & al. (2016-09-21). Goffredo, S.; Dubinsky, Z. (eds.). The cnidaria, past, present and future : the world of Medusa and her sisters. Springer. pp. 38–39. ISBN 978-3319313030.
  9. ^ Boucot, A.J. (2013). Evolutionary Paleobiology of Behavior and Coevolution. Elsevier. pp. 750 (page 69). ISBN 9781483290812.

Bibliography

  • Babcock, L. E.; Feldmann, R. М. (1986). "Devonian and Mississippian conulariids of North America. Part A. General description and Conularia". Annals of Carnegie Museum. 55: 349–410.
  • Babcock, L. E. (1991). "The enigma of conulariid affinities". In A. M. Simonetta; S. Conway Morris (eds.). The Early Evolution of Metazoa and the Significance of Problematic Taxa. Cambridge: Cambridge University Press. pp. 133–143.
  • Hughes, N. C.; Gunderson, G. D.; Weedon, M. J. (2000). "Late Cambrian conulariids from Wisconsin and Minnesota". Journal of Paleontology. 74 (5): 828–838. doi:10.1666/0022-3360(2000)074<0828:lccfwa>2.0.co;2. {{cite journal}}: Unknown parameter |last-author-amp= ignored (|name-list-style= suggested) (help)
  • Van Iten, H. (1991). "Evolutionary affinities of conulariids". In A. M. Simonetta; S. Conway Morris (eds.). The Early Evolution of Metazoa and the Significance of Problematic Taxa. Cambridge: Cambridge University Press. pp. 145–155.
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