ART. 11 SOME BURGESS SHALE FOSSILS HUTCHINSON 11 



Anostracan. If Opdbinia is derived from a primitive Anostracan 

 with a few postpedigerous segments, most of which have been lost 

 posteriorly, Rochdalia seems to have carried the reduction still 

 further. 



Of the remaining Burgess Shale species considered as Anostraca 

 by Henriksen, namely Yohoia tenuis and Bidenfia diffiGilis^ both are 

 clearly much more remote from the living Anostraca than is 

 Ofdlnnia. Both genera have but 12 post-cephalic segments, so ex- 

 ceeding the latter in their reduction of the body segmentation. 

 Bidentia is very inadequately known from Walcott's account, and 

 Yohoia is almost equally in need of further study. The pedunculate 

 eyes of the latter probably indicate its anostracan affinities, though 

 as Henriksen points out the genus differs from all known Anostraca 

 in the peculiar pleural expansions of its first eight segments. It is 

 worth pointing out, however, that the Lipostracan Lepidocaris has 

 very distinct jointed pleura. Henriksen places Yohoia in a new 

 famil3% the Yohoidae; if Bidentia is ever better known the same 

 course will probabh- be necessary. For the present we may con- 

 clude that these forms represented highly specialized and aberrant 

 marine Anostraca. 



Ecological Considerations. — As has been pointed out the modern 

 Branchiopoda are essentially organisms of seasonal waters or have 

 clearly been derived from such. The marine Anostraca of the early 

 Paleozoic represent a more specialized series of morphological types 

 than the living representatives of the order. Opahinia with its fan- 

 like arrangement of the posterior appendages must have presented a 

 more caridoid appearance than a modern Anostracan though was 

 probably sufficiently like the latter in form to have sw^um dorsal' 

 side downward. Moreover, if Yohoia is rightly referred to the group 

 there is definite evidence from the position in which the latter animal 

 is fossilized (Walcott, 1912, pi. 29, figs. 7, 8, and 12) that the charac- 

 teristic flexure of the body of the higher Crustacea had developed in 

 the Anostraca. It is clear, therefore, that at a remote period a 

 development of tlie Anostraca occurred in the sea giving forms whicli 

 were ecologically comparable to various types of higher Crustacea 

 that have replaced them. This group of marine Anostraca charac- 

 terized by a reduction in the segmentation probably invaded inland 

 waters, for Rochdalia was presumably a fresh-water form. The 

 morphologically primitive modern Anostraca have become special- 

 ized in their life history for existence in dry regions and have 

 suffered no competition from similar developments by more advanced 

 Crustacea of other groups that have replaced the marine branch of 

 the order. It is highly jDrobable that other living orders of Branch- 

 iopoda shared in the marine development of the group in curly 



