Table 2.

Higher taxa assigned to each of the three physiological groups discussed in the text

Physiologically “unbuffered” taxa	Physiologically “buffered” taxa	Physiologically indeterminate taxa
Calcareous forams	Textulariina	Archaeocyatha
Calcareous sponges, p, m	Radiolaria	Conularida
Anthozoa (corals), P, m	Siliceous sponges, p, M	Problematica
Stenolaemata, P, m	Ctenostomata, M	Inarticulata (brachiopoda), C
Cheilostomata, M	Prosobranchia, P, M	Tergomya, C
Articulata (brachiopoda), P	Opisthobranchia, P, M	Hyolitha, C
Epifaunal bivalvia, P, M	Infaunal bivalvia, P, M	Helcionellida
Ammonoidea, P, m	Nautiloidea, P, m	Mollusca incertae sedis
Belemnitida, P, m	Polychaeta, C, p, M	Rostroconchia
Ostracoda, P	Eurypterida, c, p	Arthropoda incertae sedis
Rhombifera, p	Malacostraca, c, M	Trilobita, C
Diploporita, p	Conodontophorida, c, p	Archaeocopida (ostracoda), P
Blastoidea, p	Agnathan chordates
Crinoidea, P	Placodermi and acanthodii
Echinoidea, M	Chondrichthyes, M
Graptolithina, c, p	Osteichthyes, M

Details of criteria for assignment available in ref. 21. Letters following taxon names identify the categories to which their containing classes were assigned in Sepkoski's 1981 analysis (13), as noted in Table 1. Boldface indicates the highest loadings. No letter means the group was not explicitly designated in Sepkoski's analysis.