Irregularia

 Aboral view of Late Palaeocene Togocyamus seefriedi, from here.

Belongs within: Euechinoidea.
Contains: Holectypoida, Cassiduloida, Oligopygidae, Neolampadidae, Clypeasteroida, Disasteroida, Holasteroida, Asterostomatidae, Toxasteridae, Hemiasterina, Micrasterina.

The Atelostomata is recognised as one of the major groups of irregular echinoids, in which the ancestral pentaradial symmetry has been altered towards a bilateral symmetry with the anus moved towards or onto the adoral surface. Many live buried in sand or muddy substrates. In members of the Atelostomata, the lantern is absent in adults and food is obtained by gathering organic particles contained in the sediment (by gathering with the tube feet and/or ingesting the sediment directly). In the other recognised grouping of irregular echinoids, the Gnathostomata, the lantern and jaws are retained. Major subgroups of irregular echinoids include the Spatangoida, the heart urchins, which are adapted for living in (often relatively deep) burrows and have the periproct located towards the posterior end of the test (Fischer 1966).

The Hemiasterina are a lineage of heart urchins known from the Lower Cretaceous to the present, characterised by the presence of a peripetalous fasciole (Fischer 1966).

Echinoids: regularly irregular
Published 9 March 2019

In manufacturing, one of the most desired qualities is regularity. Success is achieved by ensuring that each unit matches the last, that its qualities remain predictable and reliable. In evolution, by contrast, the opposite is often true: embracing irregularity may allow a lineage to expand in directions not previously available. For evidence, just look at the success of the irregular echinoids.

Echinoneus cyclostomus, one of the few living holectypoid urchins, copyright Philippe Bourjon.

The Echinoidea, sea urchins, are commonly divided between regular and irregular forms. In regular echinoids, representing the ancestral type for the class, the mouth and anus are positioned at opposite points on the test. The mouth sits squarely in the centre of the animal’s underside (the oral surface) while the anus sits at the centre of the upper (aboral) surface. The five ambulacra, the lines of small plates in the test from which the tube feet emerge, are more or less evenly arranged around the superficially radially symmetrical test. Irregular echinoids, in contrast, have the anus more or less displaced from the midpoint of the test. In the earliest irregular echinoids, this displacement might be relatively slight: the periproct (the membrane through which the anus opens, usually covered in echinoids with an array of small plates) was still found at the centre of the aboral surface but was enlarged and/or stretched towards one end of the test (Saucède et al. 2007). In more derived forms, the periproct has moved more significantly, potentially being found on the side of the test or even on the oral surface near the mouth.

Front view of heart urchin Spatangus purpureus, copyright Roberto Pillon.

This displacement of the anus indicates a directionality to the test that isn’t found in regular echinoids. A number of other changes have associated it in the evolution of echinoids, such as reduction of the size of the spines covering the test and an increased directionality in their axes of movement. The mouth may also become displaced towards the front of the test, and the test as a whole may become more bilateral in its overall shape. The jaws become modified or, in a couple of groups, lost entirely. All these alterations add up to indicate a distinct change in lifestyle between regular and irregular echinoids. Whereas regular echinoids roam the surface of sea bottom, using their powerful jaws to graze directly on algae or scavenge on animal carcasses, irregular echinoids are deposit feeders that tend to live at least partially buried in the sidement. They may swallow large amounts of sediment and digest organic matter mixed therein, or gather up organic particles with their tube feet and/or by means of mucous strands transported in ciliary grooves. Burrowing is achieved by movement of the spines or by using the tube feet to pass sand grains above the aboral surface. In the shallow-burrowing heart urchin Spatangus purpureus, an array of longer spines on the aboral surface are used to keep a funnel open between the buried urchin and the surface, allowing water to carry oxygen to it. Echinocardium cordatum, which burrows as deep as 18 cm beneath the substrate surface, maintains an opening to the surface by means of elongate tube feet (Durham 1966).

One of the most irregular of irregular echinoids, the deep-sea Pourtalesia miranda, from Oliver (2016). The enlarged insert shows a symbiotic bivalve Syssitomya pourtalesiana.

The change in lifestyle was certainly a successful one: nearly 60% of living echinoids are irregular. The earliest irregular echinoids appeared in the early Jurassic, with recent analyses agreeing that they represent a monophyletic group (Saucède et al. 2007; Kroh & Smith 2010). Nevertheless, a certain degree of parallelism in adaptations appears to have been occurred. Living irregular echinoids can be divided between two clades: one is relictual, containing only two genera in the order Holectypoida, whereas the remaining species belong to the larger clade Microstomata. The earliest known members of the holectypoid lineage retained strong jaws even after they evolved the ability to burrow in sediment. In contrast, the earliest known member of the Microstomata retained large spines, indicating a non-burrowing lifestyle, but already possessed the adaptations for a particulate diet (Saucède et al. 2007). With time, both lineages developed the feature that they lacked, adding them together for a winning combination.

Systematics of Irregularia
IrregulariaSG93
    |--EognathostomataSG93
    |    |--HolectypoidaU78
    |    `--Pygasteridae [Pygasteroida]SG93
    |         |--Plesiechinus Pomel 1883Fe66
    |         |    |--*P. macrostoma (Wright 1861) [=Pygaster macrostoma]Fe66
    |         |    |--P. hawkinsi Jesionek-Szymanska 1970SG93
    |         |    |--P. ornatusFe66
    |         |    `--P. reynesi (Desor 1868)SG93
    |         `--Pygaster Agassiz 1836 [incl. Echinoclypus Pomel 1869, Macropygus Desor 1857, Megapygus Hawkins 1912]Fe66
    |              |--*P. semisulcatus (Phillips 1829) [=Clypeus semisulcatus]Fe66
    |              |--P. trigeriFe66
    |              |--P. truncatus Agassiz 1840SG93 [=*Macropygus truncatusFe66]
    |              `--P. umbrella Hawkins 1912 [=*Megapygus umbrella]Fe66
    `--MicrostomataSG93
         |--MenopygidaeSG93
         |    |--Infraclypeus Gauthier 1875SG93, WD66
         |    |    `--*I. thalebensis Gauthier 1875WD66
         |    `--Menopygus Pomel 1883 [incl. Pyrinodia Pomel 1883, Pyrenodia (l. c.)]WD66
         |         |--*M. nodoti (Cotteau 1859) [=Galeropygus nodoti]WD66
         |         |--M. baugieri (Cotteau 1873)SG93
         |         `--‘Desorella’ guerangeri Cotteau 1862 [=*Pyrinodia guerangeri]WD66
         |--Neognathostomata [Conoclyparia, Conoclypina]SG93
         |    |--CassiduloidaSG93
         |    |--OligopygidaeSG93
         |    |--NeolampadidaeSG93
         |    |--+--ClypeasteroidaSG93
         |    |  `--Togocyamus Oppenheim 1915M89
         |    |       |--*T. seefriedi (Oppenheim 1915) [=Echinocyamus (*Togocyamus) seefriedi]M89
         |    |       `--T. alloiteaui Roman & Gorodiski 1959SG93
         |    `--GaleropygidaeSG93
         |         |--Laticlypus giganteus Szőrény 1966SG93
         |         |--Hyboclypus Agassiz 1839 (see below for synonymy)K66
         |         |    |--*H. gibberulus Agassiz 1839K66
         |         |    `--H. caudatus Wright 1851 [=*Aulacopygus caudatus]K66
         |         `--Galeropygus Cotteau 1856 [=Galeopygus (l. c.); incl. Ressopygus Pomel 1883]K66
         |              |--*G. agariciformis (Wright 1851) [=Hyboclypus agariciformis]K66
         |              |--‘Clypeus’ constantini Cotteau 1873 [=*Ressopygus constantini]K66
         |              `--G. lacroixi Lambert 1925SG93
         `--AtelostomataSG93
              |--DisasteroidaSG93
              |--HolasteroidaSG93
              `--Spatangoida [Amphisternata]SG93
                   |  i. s.: Barnumia Cooke 1953Fi66
                   |           `--*B. browni Cooke 1953Fi66
                   |         Cestobrissus Lambert 1912Fi66
                   |           `--*C. lorioli Lambert 1912Fi66
                   |         Cottreaucorys Lambert 1920Fi66
                   |           `--*C. blayaci (Cotteau 1909) [=Homoeaster blayaci]Fi66
                   |         Enichaster de Loriol 1882Fi66
                   |           `--*E. oblongus de Loriol 1882Fi66
                   |         Gonzalezaster Sánchez Roig 1952Fi66
                   |           `--*G. lamberti (Sánchez Roig 1949) [=Nudobrissus lamberti]Fi66
                   |         Homoeopetalus Arnold & Clark 1934Fi66
                   |           `--*H. axiologus Arnold & Clark 1934Fi66
                   |         Mazzettia Lambert & Thiéry 1915 (see below for synonymy)Fi66
                   |           `--*M. pareti (Manzoni 1878) [=Maretia pareti, *Manzonia pareti]Fi66
                   |         Niponaster Lambert 1920Fi66
                   |           `--*N. hokkaidensis Lambert 1920Fi66
                   |         Nudobrissus Lambert 1920 [=Dictyaster Stefanini 1908 non Wood-Mason & Alcock 1891]Fi66
                   |           `--*N. malatinus (Mazzetti 1885) [=Pericosmus malatinus, *Dictyaster malatinus]Fi66
                   |         Pusillaster Lambert 1920Fi66
                   |           `--*P. dallonii Lambert 1920Fi66
                   |         Royasendia Airaghi 1901Fi66
                   |           `--*R. canavarii Airaghi 1901Fi66
                   |         Amygdala Gray 1825 (n. d.)A66
                   |         Brissoides Leske 1778 (n. d.)A66
                   |         Cardiopatagus Pomel 1883 (n. d.)A66
                   |         ‘Cassis’ Parkinson 1811 (n. d.) non Scopoli 1777A66
                   |         Neopatagus Sánchez-Roig 1953 (n. d.)A66
                   |         Pleraster Quenstedt 1874 (n. d.)A66
                   |--AsterostomatidaeFi66
                   |--ToxasteridaeFi66
                   |--HemiasterinaFi66
                   `--MicrasterinaFi66
Irregularia incertae sedis:
  Desorella Cotteau 1855 (see below for synonymy)WD66
    |--*D. elata (Desor 1847) [=Hyboclypus elatus, *Desoria elata]WD66
    `--‘Dysaster’ semiglobosus Desory 1842 [=*Pachyclypus semiglobosus]WD66
  Galeroclypeus Cotteau 1873WD66
    `--*G. peroni Cotteau 1873WD66
  Loriolella Fucini 1904 [incl. Pseudopygaster Hawkins 1922]WD66
    |--*L. ludovici (Meneghini 1867) [=Cidaris ludovici]WD66
    `--L. eos (Hawkins 1922) [=*Pseudopygaster eos]WD66
  Eodiadema Duncan 1889Fe66 [EodiadematidaeSG93]
    |--*E. granulatum Wilson 1889Fe66
    |--E. bechei (Wright 1860)SG93
    `--E. pusillum Lambert 1990SG93

Desorella Cotteau 1855 [=Desoria Cotteau 1855 nec Nicolet 1841 nec Gray 1851; incl. Pachyclypus Desor 1858, Pachyclypeus (l. c.)]WD66

Hyboclypus Agassiz 1839 [=Hyboclipus (l. c.), Hyboclybus (l. c.), Hyboclypeus (l. c.), Hybodyhus (l. c.); incl. Aulacopygus Pomel 1883]K66

Mazzettia Lambert & Thiéry 1915 [=Manzonia Pomel 1883 nec Garov. 1866 (ICBN) nec Brusina 1870]Fi66

*Type species of generic name indicated

References

[A66] Anon. 1966. Doubtful nominal genera of echinoids. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt U. Echinodermata 3 vol. 2 pp. U633. The Geological Society of America, Inc., and The University of Kansas Press.

Durham, J. W. 1966. Echinoids—ecology and paleoecology. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt U. Echinodermata 3 vol. 2 pp. U257–U265. The Geological Society of America, Inc., and The University of Kansas Press.

[Fe66] Fell, H. B. 1966. Diadematacea. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt U. Echinodermata 3 vol. 1 pp. U340–U366a. The Geological Society of America, Inc., and The University of Kansas Press.

[Fi66] Fischer, A. G. 1966. Spatangoids. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt U. Echinodermata 3 vol. 2 pp. U543–U628. The Geological Society of America, Inc., and The University of Kansas Press.

[K66] Kier, P. M. 1966. Cassiduloids. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt U. Echinodermata 3 vol. 2 pp. U492–U523. The Geological Society of America, Inc., and The University of Kansas Press.

Kroh, A., & A. B. Smith. 2010. The phylogeny and classification of post-Palaeozoic echinoids. Journal of Systematic Palaeontology 8 (2): 147–212.

[M89] Mooi, R. 1989. Living and fossil genera of the Clypeasteroida (Echinoidea: Echinodermata): an illustrated key and annotated checklist. Smithsonian Contributions to Zoology 488: 1–51.

Saucède, T., R. Mooi & B. David. 2007. Phylogeny and origin of Jurassic irregular echinoids (Echinodermata: Echinoidea). Geological Magazine 144 (2): 333–359.

[SG93] Simms, M. J., A. S. Gale, P. Gilliland, E. P. F. Rose & G. D. Sevastopulo. 1993. Echinodermata. In: Benton, M. J. (ed.) The Fossil Record 2 pp. 491–528. Chapman & Hall: London.

[U78] Ubaghs, G. 1978. Classification of the echinoderms. In: Moore, R. C., & C. Teichert (eds) Treatise on Invertebrate Paleontology pt T. Echinodermata 2. Crinoidea vol. 1 p. T359–T367. The Geological Society of America, Inc.: Boulder (Colorado), and The University of Kansas: Lawrence (Kansas).

[WD66] Wagner, C. D., & J. W. Durham. 1966. Gnathostomata or Atelostomata. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt U. Echinodermata 3 vol. 2 pp. U631–U632. The Geological Society of America, Inc., and The University of Kansas Press.

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