To cite package 'strap' in a publication use:

Bell MA, Lloyd GT (2015). “strap: an R package for plotting phylogenies against stratigraphy and assessing their stratigraphic congruence.” Palaeontology, 58, 379-389. doi:10.1111/pala.12142.

Best practice is to also cite the packge version number you used (found with 'library(help = strap)').

Corresponding BibTeX entry:

  @Article{,
    title = {strap: an R package for plotting phylogenies against
      stratigraphy and assessing their stratigraphic congruence},
    author = {Mark A. Bell and Graeme T. Lloyd},
    journal = {Palaeontology},
    year = {2015},
    volume = {58},
    pages = {379-389},
    doi = {10.1111/pala.12142},
  }