Gischler et al.—New Pleistocene callianassid crustacean microcoprolite
and Evans, 1983; Neal et al., 2008; Netto et al., 2017). The observations of Moore (1932) and Shinn (1968) of modern crus- tacean fecal pellets suggest that the producer of the Pleistocene Palaxius could have been a callianassid. Crescentic canals like those in P. floridanus n. isp. were found inmodern pellets of other callianassid crustaceans, such as Axius, according to Moore (1932, pl. 1, fig. 7). Canals of fecal pellets of the modern Neotrypaea (Callianassa) californiensis exhibit ten crescentic canals in cross- section (Powell, 1974, fig. 14), although in different orientation than the Pleistocene specimens described here. The fecal pellets of the modern Callichirus (Callianassa) major have twenty-six crescentic internal canals (Pryor, 1975). As a consequence of these observations, Becker and Chamberlain (2006) have con- sidered callianassids as producers of Palaxius ichnofossils. This hypothesis is supported by the association of Palaxius with callianassid body fossils in an Eocene methane-seep deposit (Peckmann et al., 2007). Because of the ubiquity of callinassids in modern environments of the Florida platform, it seems curious that P. floridanus n. isp. has not been encountered more fre- quently. An explanation could be diagenetic micritization, a common phenomenon in shallow marine carbonates (Purdy, 1968; Reid and Macintyre, 1998) that has potentially obliterated the original canal structures leaving only texturally inconspicuous fecal pellets in this environment. Another explanation could be the fact that both the Key Largo and theMiami limestones underwent additional diagenetic alteration, especially in the meteoric environment, that has caused widespread recrystallization and neomorphism (Stanley, 1966; Robinson, 1967; Evans and Ginsburg, 1987;Multer et al., 2002).
Conclusions
The new Crustacean microcoprolite Palaxius floridanus n. isp. is described from late Pleistocene shallow-water limestones of south Florida. The producer of the ichnofossil presumably was a callia- nassid crustacean, which inhabited moderately agitated environ- ments behind the south Florida platform margin. P. floridanus n. isp. resembles P. decemlunulatus from the Oligocene. Late Jurassic and late Cretaceous forms reminiscent of P. floridanus n. isp. and previously assigned to P. decemlunulatus belong to a different ichnogenus and ichnospecies, respectively.
Acknowledgments
The senior author is grateful to R.N. Ginsburg who introduced him to the geology of south Florida, and to H.G. Multer who involved him in the project analyzing his drill cores from the south Florida area. H.A. Curran and an anonymous reviewer made thoughtful comments that improved this paper.
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