EVOLUTION OF THE ECHINODERMS — FELL 463 



FOSSIL SOMASTEROIDS 



This evidence, as is now apparent, was first placed in our hands in 

 1951, though its full significance was not apparent till quite recently. 

 Figure 5, A-F, shows the lower Ordovician sea star Chinianaster^ of 

 which the late W. K. Spencer published the first detailed description 

 in that year. Spencer recognized it as the type of an entirely un- 

 known kind of sea star, evidently antecedent to both ophiuroids and 

 asteroids, and characterized by the frondlike structure of the arm, in 

 which transverse rows of rodlike ossicles, which he called virgalia, lay 

 on either side of a double axial series of ambulacral ossicles. 



Figure 5, G, shows Villebrunaster^ another of these ancient sea stars 

 and, like Chinianaster^ from the lower Paleozoic of Europe. Spencer, 

 as a paleontologist, was well aware of the fact that fossils indicate 

 that ophiuroids and asteroids had a common ancestry, and so, in 

 common with other echinoderm paleontologists, he employed a classi- 

 fication in which both groups are placed together in one class called 

 Asterozoa or Stelleroidea. Until 1900 the same classification had also 

 been used by zoologists, and Lankester's Treatise published in that 

 year had retained it ; for it was not mitil the next decade that embryo- 

 logical theories came to the fore. Spencer, who was not an embry- 

 ologist, recognized, of course, that the animals he had before him 

 were members of the Asterozoa, though they obviously could not be 

 referred to either of the known subclasses Asteroidea and Ophiur- 

 oidea. He therefore proposed for their reception a third subclass, 

 the Somasteroidea, antecedent to the other two subclasses. You will 

 see from the illustrations that both genera show a broad, petal-shaped 

 arm, extremely flattened and leaf like, with each adjacent pair of arms 

 separated by a deep interradial cleft, the disk region quite small, and 

 the mouthparts obviously veiy simple and derived from the adjoining 

 elements in the base of the arm skeleton. 



Spencer also recognized another kind of somasteroid, shown in 

 figure 5, I, the genus Archegonaster. It too had the characteristic 

 transverse rows of virgalia, but was more starfishlike in appearance, 

 because the arms were fused broadly at the base, and the outermost 

 virgalia had been converted into a marginal series of large plates 

 defining the outline of the animal. Also the innermost virgalia, ad- 

 joining the axial ambulacral ossicles, carried spines and evidently 

 corresponded to the plates called adambulacral plates in modern star- 

 fishes. Spencer thus concluded that starfishes had been derived from 

 somasteroids, and although he did not say so, he left it to be inferred 

 that ophiuroids also must have arisen from somasteroids. He believed 

 that somasteroids must have had ciliary food grooves between the 

 rows of virgalia, and that their mode of nutrition would have re- 

 sembled that of pelmatozoans. 



