Abstract:
The remarkable climatic and biotic changes caused by the late Paleocene thermal maximum (circa. 55.0 Ma) were preceded by a prolonged period of gradual warming. It was during this earlier portion of the late Paleocene (55.0-61.0 Ma) that the planktic foraminiferal genus Morozovella originated and diversified. From a macroevolutionary standpoint, the morozovellid radiation constitutes a critical phase in the recovery of tropical planktic foraminifera from the Cretaceous/Tertiary extinction event--it represents the first time that morphological diversity approached pre-extinction levels. In general, the stable isotope signatures of the morozovellids are most analogous to those of modern planktic foraminifera that inhabit shallow depths well above the oceanic thermocline (i.e. mixed layer). Consequently, the morozovellids are typically regarded as having occupied a shallow depth habitat. This generalization, however, downplays the role of hydrographic stratification and depth ecology in the evolution of morozovellid species. Study of morozovellid stable isotope signals from the Indian Ocean (ODP Hole 758A) reveals taxonomic patterns of depth-habitat segregation that are suggestive of ecological divergence during speciation. At the crux of this diverisification is a cladogenetic event that gave rise to two main morozovellid groups, the sinistrally-coiled M. velascoensis and dextrally-coiled M. subbotinae lineages. Both multi- and single-specimen oxygen isotope data indicate that early representatives of the M. velascoensis and M. subbotinae lineages evolved from their common ancestor (M. angulata) by invading different parts of the mixed layer, the former migrated to shallower depths and the latter to deeper depths. Furthermore, these two sister lineages followed different ecological pathways during their evolution. Speciation in the M. velascoensis lineage entailed episodic excursions from the shallow, near-surface into deeper parts of the mixed layer, while evolution in the M. subbotinae lineage proceeded at depth in the mixed layer with less vertical migration.