Although thick stellar discs are detected in nearly all edge-on disc galaxies , their formation scenarios still remain a matter of debate .
Due to observational difficulties , there is a lack of information about their stellar populations .
Using the Russian 6-m telescope BTA we collected deep spectra of thick discs in three edge-on S0-a disc galaxies located in different environments : NGC 4111 in a dense group , NGC 4710 in the Virgo cluster , and NGC 5422 in a sparse group .
We see intermediate age ( 4 - 5 Gyr ) metal rich ( [ Fe/H ] \sim - 0.2 \dots 0.0 dex ) stellar populations in NGC 4111 and NGC 4710 .
On the other hand , NGC 5422 does not harbour young stars , its disc is thick and old ( 10 Gyr ) , without evidence for a second component , and its \alpha -element abundance suggests a 1.5 - 2 Gyr long formation epoch implying its formation at high redshift .
Our results suggest the diversity of thick disc formation scenarios .