## 5 Discussion
### Middle Eocene Planktic Forminiferal Paleoecology  
Trophic and life strategies inferred for Eocene taxa derive from several studies based on large data set obtained from diverse latitudes, biogeographic distribution, environmental preferences as extensively discussed in numerous papers (e.g., [18; 61] and reference therein), thus planktic foraminiferal paleoecology is rather well known. The stable isotope data available for most of species and genera allow us to decipher their habitat in the water column and whether they were symbiont-bearing or not. Therefore, the compositional and abundance changes of the planktonic foraminifera assemblages across the MECO interval from the Baskil section allow us to interpret the environmental modifications recorded. The genera _Acarinina_ and _Morozovella_ were mixed-layer taxa that dominated the tropical and subtropical assemblages in late Paleocene to middle Eocene (with _Morozovella_ preceding _Morozovella_) and became extinct close to the Bartonian/Priabonian boundary (e.g., [79]). Even though photosymbionts are not preserved in the fossil record, indirect evidence for their presence in planktic foraminifera of the past can be deciphered through the stable isotope composition of differently sized specimens (e.g., [62; 80; 81; 82; 83; 84; 85; 86; 87; 88]). In fact, larger specimens support greater dinoflagellate symbiont density and enhanced photosynthetic activity remove preferentially the lighter \({}^{12}\)C isotope during photosynthesis, leaving adjacent water enriched in \({}^{13}\)C, thus \(\delta^{13}\)C signature is higher with increasing test size (e.g., [89]). Such a relationship has provided supporting evidence of photosynthetic activity in _Morozovelloides_ and _Acarinina_ (e.g., [87; 90; 91]). By contrast, the absence of test size-\(\delta^{13}\)C gradients in _Subbottina_ specimens is consistent with symbiotic ecology (e.g., [61; 62; 84; 92; 93; 94]). The differences in the mean \(\delta^{13}\)C values for _Morozovelloides_ and _Acarinina_ with respect to _Subbottina_ give evidence for different depth habitats. The relatively high \(\delta^{13}\)C values for the two former genera suggest a mixed-layer habitat, while relatively low \(\delta^{13}\)C values for the latter suggest a thermocline habitat (e.g., [61; 95; 96] and references therein). We kept specimens of _Subbottina senti_ separate from other subbotinids, because this taxon likely occupied a different habitat [61; 62; 92; 95]. Specifically, _S. senti_ is considered a mixed-layer form that sank to middle mixed-layer or deeper depths during gametogenesis (e.g., [61; 62; 92]). Turborotaliids display stable isotope signatures compatible with a sub-surface, possibly upper thermocline habitat (e.g., [92]), whereas _Paragloborotalia_, _Catapsydrax_, _Globorotaloides_ and _Turborotalita carcoselleensis_ are considered as deep thermocline dwellers (e.g., [61; 92]). _Globigerinatheka_ is a symbiont-bearing mixed layer dweller [62; 92; 97], but stable isotope values commonly indicate calcification in deeper waters [74; 98; 99; 100; 101] suggesting that the genus had late-stage calcification, with CaCO\({}_{3}\) crusts forming deeper in the water column [92; 100]. The _Globigerinatheka_ isotope records thus reflect conditions between the mixed layer and the thermocline [95; 98; 99], as also indicated by boron isotope data [102]. The genus _Hantkennia_ is recognized as having inhabited the Oxygen Minimum Zone (OMZ) [61; 103]. The general inferred stable isotope paleobiology and distribution of the biserial _Chiloguembelina_ ascribe it as low-oxygen tolerant, meso- to eutrophic thermocline dweller thriving in stressed environmental conditions; _Pseudohatsigerina_ as well is known as opportunist taxon (e.g., [18; 94; 104; 105; 106; 107]). However, even though we are fairly well informed on middle Eocene planktonic foraminifera ecology, the interpretation of changes in assemblages is not always straightforward due to the multiple factors influencing their successful or decline in marine environment.