Belongs within: Irregularia.
Sunken petals
Published 27 July 2024
The irregular echinoids are a diverse group in which the radial symmetry of the ancestral sea urchins has become modified to some degree into a secondary bilaterality. This change in symmetry was associated with a trend from the exposed surface-grazing habits of their ancestors towards a more concealed life buried beneath the sediment. However, though burial may bring protection from potential predators, it also entails the need to keep oneself from suffocation. As a result, a typical feature of irregular urchins is enlarged ambulacral pores arranged on the surface of the test in a petal-like pattern (called, funnily enough, a ‘petal’). From these enlarged pores emerge correspondingly enlarged tube-feet that keep the surface of the test clear of sediment and/or maintain a burrow allowing water to flow around the urchin. However, there is one group of irregular urchins recognisable by its lack of strongly distinct petals: the Neolampadidae.
The neolampadids are not a well-known family of urchins. Most are found in deep waters, sometimes over 1200 metres down (Durham & Wagner 1966). The first fossil neolampadids were not identified until 1963, a remarkably late debut for the usually quite fossiligenic echinoids. The adapical ambulacral pores are simple or absent, though an incipient floscelle (a rosette-like pattern of swollen and sunken plates) may be present around the mouth. From two to four genital pores may be present.
Because of the lack of petals, some authors have thought that neolampadids might be primitive for irregular urchins. However, the current consensus is that neolampadids have gone through a pattern of paedomorphosis from petaliferous progenitors (Souto et al. 2019). How could they have lost such a seemingly vital feature? The answer may lie in their deep-sea habits. Much of the deep-sea bottom is covered in a deep, flocculent ooze, the product of a continual rain of deceased micro-organisms from the surface waters far above. Burrowing in this ooze is simply not a going concern, as no amount of frantic tube-foot activity is going to keep the fluid sludge from caving in. As a result, the deep-sea neolampadids are probably no longer burrowers but have returned to life on the sediment surface. And with no need for enlarged burrowing tube feet, the petals become redundant and just withered on the test.
Systematics of Neolampadidae
<==Neolampadidae [Neolampadina, Neolampadoida]
|--Neolampas Agassiz 1869DW66
| `--*N. rostellata Agassiz 1869DW66
|--Tropholampas Clark 1923DW66
| `--*T. loveni (Studer 1880) [=Catopygus loveni]DW66
|--Anochanus Grube 1868DW66
| `--*A. sinensis Grube 1868DW66
|--Aphanophora de Meijere 1902DW66
| |--*A. echinobrissoides de Meijere 1902DW66
| `--A. bassoris Holmes 1995MM22
|--Nannolampas Mortensen 1948DW66
| `--*N. tenera (de Meijere 1902) [=Neolampas tenera]DW66
|--Notolampas Philip 1963DW66
| `--*N. flosculus Philip 1963DW66
`--Pisolampas Philip 1963DW66
`--*P. concinna Philip 1963DW66
*Type species of generic name indicated
References
[DW66] Duncan, J. W., & C. D. Wagner. 1966. Neolampadoids. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt U. Echinodermata 3 vol. 2 pp. U628–U630. The Geological Society of America, Inc., and The University of Kansas Press.
[MM22] McNamara, K. J., & S. Martin. 2022. Middle Eocene echinoids from the western Eucla Basin, Western Australia. Records of the Western Australian Museum 37: 31–56.
Souto, C., R. Mooi, L. Martins, C. Menegola & C. R. Marshall. 2019. Homoplasy and extinction: the phylogeny of cassidulid echinoids (Echinodermata). Zoological Journal of the Linnean Society 187: 622–660.