Somasteroidea

Somasteroidea Temporal range: Early Ordovician–Late Devonian PreꞒ Ꞓ O S D C P T J K Pg N Tremadocian to Famennian
Villebrunaster thorali , a basal member of the Somasteroidea
Scientific classification
Kingdom:	Animalia
Phylum:	Echinodermata
Subphylum:	Asterozoa
Order:	† Somasteroidea Spencer, 1951 [1]
Families
† Archegonasteridae Spencer, 1951 [1] † Archophiactinidae? Spencer, 1927 [2] † Catervaparmasteridae Blake, 2000 [3] † Chinianasteridae Spencer, 1951 [1] † Helianthasteridae? Owen, 1965 [4] † Villebrunasteridae Fell, 1963 [5]

The Somasteroidea, or Stomasteroidea, is an extinct order of asterozoan echinoderms first defined in 1951 by W. K. Spencer. ^[1] Their first appearance in the fossil record was in the Early Ordovician (Tremadocian) and they had become extinct by the Late Devonian (Famennian). They are similar to the asteroids in that their bodies are flattened dorsoventrally and they have five petaloid arms with broad bases. The ambulacral plates in somasteroids are simple and unspecialized, and the arms were thought to be not flexible and were unable to assist in feeding, but the oral mouth parts were more complex. ^[6]

Relationship to other Asterozoan classes

Villebrunaster thorali (Somasteroidea)

Chiniaster (Somasteroidea)

Stuertzaster (Stenuroidea)

Differences in virgals between somasteroids and stenuroids; note that in somasteroids the number of virgals per ambulacral varies based on the width of the arm, while the number of virgals (labeled as inner and outer laterals) in stenuroids is the same along the entire length of the arm.

Most authors consider Somasteroidea to be the basal stock from which the other three classes evolved, ^{[7] [8]} but an argument in favor of monophyly and a position closer to stenuroids and ophiuroids than to asteroids has also been made. ^[9]

Somasteroids are "more or less petaloid," with arm shape reflecting virgal series lengths. ^[10] Somasteroids have been described as being more rigid in shape than derived asterozoans, although this apparent structure could be exaggerated by tissue changes at the time of death. ^[11]

References

1. W. K. Spencer. (1951). Early Palaeozoic starfish. Philosophical Transactions of the Royal Society of London, series B 235(B623):87-129
2. W. K. Spencer. (1927). A monograph of the British Palaeozoic Asterozoa. Monograph of the Palaeontographical Society, London 1925(7):325-388
3. D. B. Blake. (2000). An Archegonaster -like somasteroid (Echinodermata) from Pomeroy, co. Tyrone, Northern Ireland. Irish Journal of Earth Sciences 18:89-99
4. H. G. Owen. (1965). A monograph of the British Palaeozoic Asterozoa. Table of contents, supplement and index. Monograph of the Palaeontographical Society, London 1964:541-583
5. H. B. Fell. (1963). A new family and genus of Somasteroidea. Transactions of the Royal Society of New Zealand, Zoology 3(13):143-146
6. Somasteroidea Tree of Life Web Project. Retrieved 2011-09-29.
7. Mooi, Rich; David, Bruno (June 2000). "What a new model of skeletal homologies tells us about asteroid evolution". American Zoologist. 40 (3): 326–339. doi:10.1668/0003-1569(2000)040[0326:WANMOS]2.0.CO;2.
8. Blake, Daniel B.; Guensburg, Thomas E. (2015). "The class Somasteroidea (Echinodermata, Asterozoa): morphology and occurrence". Journal of Paleontology. 89 (3): 465–486. doi:10.1017/jpa.2015.22.
9. Dean Shackleton, Juliette (2005). "Skeletal homologies, phylogeny and classification of the earliest asterozoan echinoderms". Journal of Systematic Palaeontology. 3 (1): 29–114. doi:10.1017/S1477201905001525.
10. Blake, Daniel B.; Hotchkiss, Frederick H.C. (2022). "Origin of the subphylum Asterozoa and redescription of a Moroccan Ordovician somasteroid". Geobios. 72–73: 22–36. doi: 10.1016/j.geobios.2022.07.002 .
11. Blake, Daniel B. (2024). "A review of the class Stenuroidea (Echinodermata: Asterozoa)". Bulletins of American Paleontology. 409. doi:10.32857/bap.2024.409.01.