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PALEONTOLOGICAL SOCIETY PAPERS, V. 8, 2002decline in specialized reef-dwelling fish predators.This is questionable since most known Devoniangnathostome fish fossils are not associated withreefs, but occur rather in open marine settings. Onemight, alternatively, suggest that crinoids were ableto adjust, up to a point, to the increased pressureof grazing by sharks, holocephalans, and otherfishes. The rise in spinosity and plate thickness mayhave been effective temporarily in preventingdecimation by predators.Surprisingly, all crinoid subclasses exhibitdecreased spinosity in the late Paleozoic, followinga Lower Carboniferous high (Signor and Brett,1984). Waters and Maples (1991) suggest thatpredators were able to “keep up” with the armamentsof their prey and that spinose plates becameineffective as a defensive strategy; smaller size andcompactness of the calyx may then have been moreeffective strategies. This trend toward smaller sizesmay have other meanings, such as declining foodresources, although no correlations are obvious.A majority of the common Devonianplatyceratid host crinoids were spiny, and nearlyall spiny crinoids were at least occasional hosts ofplatyceratids. In contrast, none of the commonOrdovician or Silurian crinoid or cystoid hosts werespiny. Obviously, the spines were not a deterrentto platyceratids. Arthroacantha, the most widelycited host genus (with populations showing up to70% individual infestation by Platyceras)possessed both movable spines on the calyx andaxillary spines on the arms. Intriguingly, Platycerasdumosum, one of the common symbionts, alsopossessed long spines. Brett (in press) suggests thatthe development of spines in crinoids was an antipredatoryadaptation mediated by the presence ofgastropods. Even if crinoids were not tasty prey(as has been suggested by some modern studies:Meyer and Ausich, 1983), gastropods may havebeen. If gastropod-bearing crinoids were frequently“targeted” by durophagous predators, they mayhave experienced a higher selection pressure toevolve spines (as did the gastropods themselves)than did non-host crinoids.Life Habit Changes.—Vertebrates typicallyshow a pattern of decreasing skeletal armor duringthe late Paleozoic. Early agnathan “ostracoderms”and placoderms were heavily encased in dermalbone. Dermal bone may have served non-defensivefunctions, such as areas for muscle attachment andphosphate sinks. However, it is also probable thatthis armor deflected predatory attack, especiallyfrom contemporary invertebrate predators.Ironically, it may be the preference of these earlyvertebrates for marginal marine environments thatfostered escalation, as these environments werealso home to large predaceous eurypterids.Subsequently, the rise of gnathostome fishesmust have placed additional predation pressure onother vertebrates. It is perhaps surprising that someof the largest predators of the Devonian—thearthrodires—had heavily armored heads. This mayreflect the evolution of still more effective, fasterswimmingsharks, or it may merely reflect anothertype of adaptation—possibly for phosphateexchange—unrelated to predation. In any case,heavy dermal armor was largely lost with theextinction of placoderms. The successful predatorsof the later Paleozoic probably reduced armor asan adaptation for increased maneuverability andrapid swimming. This apparently was a highlysuccessful tradeoff. The appearance of varied finspines in sharks and peculiar spine and brush“headgear” in the stethacanthids (Fig. 6.5) mayrepresent anti-predatory or sexually selectiveadaptations (Zangerl, 1981).Life habit changes among Paleozoic organismsinclude the development of endobenthic andcemented modes of life. Semi-endofaunal(frequently termed quasi-infaunal in earlierliterature) habits were adopted by many orthide andstrophomenide brachiopods as early as EarlyOrdovician time, but the proportion of semiendobenthicbrachiopods increased in the latePaleozoic with the rise to dominance of productidesand chonetid brachiopods (Thayer, 1983) (Figs. 11,13). This change coincides with the middle to latePaleozoic revolution of predators.Bottjer (1985) related increasing intensity ofpredation to the progressive occupation by bivalvesof successively deeper endobenthic tiers (Ausichand Bottjer, 1982). Endobyssate and shallow-110