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The near passerine bird order Cuculiformes traditionally included three families as below:
However, the taxonomy of this group is now controversial. The Sibley-Ahlquist taxonomy raises the Musophagidae to an order Musophagiformes which may or may not be warranted. The Hoatzin continues to be enigmatic; it is probably best regarded as a distinct, monotypic order, especially if the turacos are also considered one. The coucals and anis are sometimes considered subfamilies of the Cuculidae, or otherwise assigned to families of their own, the Centropodidae and Crotophagidae respectively.
The fossil record of this group is much debated. The unique anatomy of Cuculidae (in the old sense) often allows for the easy identification even of isolated bones. Cuckoos of prehistoric genera are known since the Late Eocene, some 40-35 million years ago. Turaco fossils are known from much later only.
A number of other Paleogene fossils have at times been assigned to the Cuculiformes, but they are not usually included here anymore in more recent treatments: The Late Paleocene to mid-Eocene Gracilitarsidae were at first believed to be cuculiforms but this is not generally accepted today. They remain mysterious though, due to their apomorphic anatomy, and might be close to the equally enigmatic Sylphornithidae known from the mid-Eocene to the Early Oligocene. From the Early Eocene, Parvicuculus and Procuculus might be Cypselomorphae of a hitherto unrecognized lineage. Foro might be allied with the Hoatzin; it had been touted as a "missing link" between this bird and the Cuculiformes proper but as noted above, it is just impossible to closely ally the Hoatzin with any extant birds.
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