Circular RNAs (circRNAs) are found across eukaryotes and can function in 
post-transcriptional gene regulation. Their biogenesis through a circle-forming 
backsplicing reaction is facilitated by reverse-complementary repetitive 
sequences promoting pre-mRNA folding. Orthologous genes from which circRNAs 
arise, overall contain more strongly conserved splice sites and exons than other 
genes, yet it remains unclear to what extent this conservation reflects 
purifying selection acting on the circRNAs themselves. Our analyses of circRNA 
repertoires from five species representing three mammalian lineages (marsupials, 
eutherians: rodents, primates) reveal that surprisingly few circRNAs arise from 
orthologous exonic loci across all species. Even the circRNAs from orthologous 
loci are associated with young, recently active and species-specific 
transposable elements, rather than with common, ancient transposon integration 
events. These observations suggest that many circRNAs emerged convergently 
during evolution - as a byproduct of splicing in orthologs prone to transposon 
insertion. Overall, our findings argue against widespread functional circRNA 
conservation.