EDIT

OccurrenceServiceImpl.findRootUnitDTOs(DerivedUnitDTO derivedUnitDTO, ..) hides derivatives of originals which are not on the path to the root.

At least the number of hidden derivatives at each level of the tree should be reported, adding a TypedEntityReference or just the UUID for each hidden derivative would make the DTO more usable.
From the user perspective in specific usecases more information is desirable:

Norbert: "...hinweis auftauchen würde in der Art wie "Another specimen of this field unit is treated under ""

AK: "... vielleicht auch so: " n more specimens treated under taxon x", dies kann auch mehrfach auftauchen, ..."

Finding the associated taxon requires more intensive computation, therefore we should obtain this information from the client side if it is really needed with a saparate requirest, maybe asynchroneously.
For now adding a list of TypedEntityReference or UUID would be sufficient.