Belongs within: Coelenterata.
Contains: Psammocorallia, Cyclozoa, Hyolithellida, Myxozoa, Medusozoa, Anthozoa.
The Cnidaria are an aquatic, mostly marine, group of invertebrates including corals, jellyfish and related taxa. The cnidarian body form is ancestrally radial with a single gut opening surrounded by tentacles; the individual zooid may be sessile (a polyp) or free-living (a medusa). Cnidarians are also characterised by the presence of stinging cells called nematocysts or cnidoblasts. Both these features may be modified to some degree in derived subgroups.
Among living cnidarians, recent authors have agreed on a basal division between the Anthozoa and Medusozoa with the parasitic Myxozoa closer to Medusozoa. The Middle Cambrian fossil Thaumaptilon walcotti bears a superficial resemblance to modern colonial anthozoans (particularly pennatulaceans) but may lie outside the cnidarian crown group (Shu et al. 2006). The frond bears dark spots that have been interpreted as indicating zooids but have a scattered, closely packed distribution at odds with modern anthozoans (Budd & Jensen 2017). Other possible cnidarians of the early Cambrian include the Hydroconozoa, small, sessile organisms with a cup-like calcareous skeleton. Xianguangia sinica is a polyp-like form with feather-like tentacles from the lower Cambrian Chengjiang Formation of China (Ou et al. 2017).
Conical problematica
Published 15 November 2010
Scattered throughout the fossil record are little mysteries, organisms whose remains have been preserved but which are not obviously relatable to any more familiar group. Either their remains are too simple to preserve much evidence of their affinities, or they are too distinct from other organisms for their affinities to be clear, or both, or some other reason. Unless they are particularly common or otherwise significant, most of these problematica are probably doomed to remain so. Case in point:
The figures above show Asymmetroconus splendidus, described by Korde in 1975 from the Albian (early Cretaceous) of the Crimea. The photos are of thin sections of the fossils; the complete skeleton would have probably been shaped rather like a wine goblet. The largest specimens of Asymmetroconus were just under 8 mm in height. In the same paper, Korde described a number of similar fossils aged from the Albian to the Danian (earliest Palaeocene), assigning them all to the new order Asymmetroconida. Korde attributed the asymmetroconidans to the Hydroconozoa, a group of similar fossils he had himself described previously from the early Cambrian. Hydroconozoa have generally been assigned to the Cnidaria, though their exact position therein remains obscure. Asymmetroconida resembled hydroconozoans in being small and goblet-shaped, with a conical interior to the cup and a basal globular hollow below the point of the cone. However, they differed from Cambrian hydroconozoans in their skeletal microstructure and in the asymmetry of the cup, with one side much thicker than the other. Rozanov & Zhuravlev (1992) later dismissed the idea of Mesozoic hydroconozoans, stating simply that structures described as such had ‘little in common with this group’. No alternative identification of the Asymmetroconida has ever been proposed, and they do not appear to have been properly studied since Korde’s original description.
Systematics of Cnidaria
<==Cnidaria [Porinae]
|--Thaumaptilon walcottiSCM06, BJ17
|--Xianguangia sinica Chen & Erdtmann 1991 (see below for synonymy)OH17
|--HydroconozoaEL11
| |--Gastroconus [Gastroconidae]NS93
| | `--G. venustus Korde 1963NS93
| |--Hydroconus [Hydroconidae]NS93
| | `--H. mirabilis Korde 1963NS93
| `--Dasyconus [Dasyconidae]NS93
| `--D. porosus Korde 1963NS93
`--+--+--MyxozoaJ-GP07
| `--MedusozoaJ-GP07
`--AnthozoaJ-GP07
Cnidaria incertae sedis:
PsammocoryneM62
Bellonella rigidaGR98
‘Cymodocea’ Lamouroux 1816 non Rafinesque 1814BR05
Toxorchis kellneriB26
‘Ptolemaia’ McCutcheon & Wilson 1961 non Osborn 1908V66
Branchiocerianthus imperatorT95
PsammocoralliaG79
CyclozoaG79
Planomedusites Sokolov 1972G79
`--*P. grandis Sokolov 1972G79
Cosmetira pilodellaD56
HyolithellidaV06
Archotuba Hou et al. 1999C12
`--A. conoidalis Hou et al. 1999SHB15
Pteroides griseumPP64
Sarcodyctyon catenatumPP64
Thanmostoma cidaritisPP64
Brachiocerianthus norvegicusPP64
Eolympia pediculataOH17
Archisaccophyllia kunmingensisOH17
ByssusG-C95
|--B. botryoidesG-C96
`--B. velutinaG-C95
Eohydiania wenganenseC12
Sinocyclocylicus guizhouensisC12
Cambrorhytium Conway Morris & Robison 1988C12
`--C. fragile (Walcott 1911) [=Selkirkia fragilis]SHB15
Angela Lesson 1843K18
Sphenothallus Hall 1847MW04
Xianguangia sinica Chen & Erdtmann 1991 [incl. Galeaplumosus abilus Hou et al. 2011, Chengjiangopenna wangii Shu & Conway Morris 2006]OH17
*Type species of generic name indicated
References
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[M62] Monniot, F. 1962. Recherches sur les graviers a Amphioxus de la région de Banyuls-sur-Mer. Vie et Milieu 13: 231–322.
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[OH17] Ou, Q., J. Han, Z. Zhang, D. Shu, G. Sun & G. Mayer. 2017. Three Cambrian fossils assembled into an extinct body plan of cnidarian affinity. Proceedings of the National Academy of Sciences of the USA 114 (33): 8835–8840.
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