from a mainstream paper:
The re-evolution of a coiled shell, or any complex character, is considered unlikely or impossible (Dollo’s law) because the loss of the character is followed by the loss of the genetic architecture and developmental mechanisms that underlie that character. Here, we quantify the level of coiling in calyptraeids, a family of mostly uncoiled limpets, and show that coiled shells have re-evolved at least once within this family. These results are the first demonstration, to our knowledge, of the re-evolution of coiling in a gastropod, and show that the developmental features underlying coiling have not been lost during 20-100 Myr of uncoiled evolutionary history. This is the first example of the re-evolution of a complex character via a change in developmental timing (heterochrony) rather than a change in location of gene expression (heterotopy).
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1691546/
To be honest, i understand, why Mr. Dollo believed that a complex trait can’t be re-evolved, Mr. Dollo obviously did not believe in miracles …
some more examples where (allegedly) complex traits re-evolved:
A family of lizards has achieved something very unexpected, evolving to give birth to live young, before going back to egg laying. Most remarkably, the zoologists who observed this think it is possible they rediscovered laying eggs multiple times.
https://www.iflscience.com/plants-and-animals/lizards-reevolve-to-lay-eggs-after-having-already-evolved-to-give-birth-to-live-young-/
or another one:
Can complex traits be re-evolved by lineages that have lost them? Phylogenetic study now suggests that wings may indeed have reappeared several times within the ancestrally wingless stick insects.
https://www.ncbi.nlm.nih.gov/pubmed/12781152