Lower Devonian Chitinozoan Juveniles—Oldest Fossil Evidence of a Juvenile Stage in Protists, with an Interpretation of Their Ontogeny and Relationship to Allogromiid Foraminifera

Chitinozoans are a group of microfossils that have only recently been recognized as protists, on the basis of a discovery of chitinozoan juveniles in Lower Devonian (Siegennian) strata. The presence of pores, a laminated test wall, and an open mucron serving as a second aperture are evidences that chitinozoans were rhizopods. This is the oldest known juvenile stage in protists. It may also serve as evidence that the alternation of generation reproductive strategy, common in protists, existed as early as the Lower Devonian. An ontogenetic model of chitinozoans is here presented for the first time. A possible evolutionary pathway is proposed by which chitinozoans could have evolved into allogromiid foraminifera.