The guideline at WP:FAUNA#Monotypic taxa is pretty clear about this: "All other fauna, including all other fossil fauna, should always be represented at the lowest-ranked taxon which the article inclusively covers. Examples: Confuciusornithidae, not Confuciusornithiformes; Bullockia maldonadoi, not Bullockia." For whatever reason, this is unfortunately opposite to the recommendation of WP:FLORA: "A genus with a single species is treated at the article for the genus."