Classification and likely relationships between the CrocERV groups and ERVs from other species. Maximum likelihood phylogenies were created from the RT domain of the crocodilian consensus sequences and a selection of sequences deposited in Repbase and published sequences. Part (a) is the entire RT tree, while (b) and (c) are expanded versions of ERV3 and 4, and ERV1 respectively. The complete version of (a) including sequence IDs is presented as Additional file 6: Figure S2. Symbols represent the taxa from which the sequences were derived. The numbers of CrocERV groups are shown outside of corresponding consensus sequences. Major ERV and retroviral groups, and the Gypsy elements, are indicated by brackets. Numbers within the phylogeny indicate aLRT values greater than 90%. The scale bar indicates branch length.