We’ve seen reversals before
in theropod dinosaurs. Remember Limusaurus re-developing a digit zero? And what a fuss it made with regard to theropod/bird finger homology? Nobody else considered the possibility that an ancient gene could switch on again and add a digit medial to digit one. But that was the best explanation after phylogenetic analysis.
Here’s an apparent new reversal
Patagopteryx (Alvarenga and Bonaparte 1992, Late Cretaceous) was a flightless bird that currently nests with the ostrich, Struthio, with which it shares a long list of traits (Fig. 1). Except it has at least one extra toe (Fig. 1). All currently tested palaeognath birds, including tinamous, have only three toes. Patagopteryx is the oldest known clade member, at 80 mya.
Did other palaeognaths lose pedal digit 1
several times by convergence? Or did Patagopteryx regrow pedal digit 1 in another case of reversal? Would it matter to your decision that pedal digit 1 is not reversed for perching, as it appears in slightly more distant ancestors, like Archaeornithura and Confuciusornis? Does it matter that Patagopteryx is at least 80 million years old?
The scores say the toe reappeared,
but I think the age of Patagopteryx trumps that. Pedal digit 1 was oriented anteriorly in Late Jurassic Solnhofen birds and Early Cretaceous neornithes, like Longicrusavis (Fig. 3).
The key takeaway here
is similar to what pilots say when they enter a cloud, “Trust your instruments.” In this case, trust your phylogenetic analysis. Sometimes autapomorphies pop up. Study them and, if you have to, accept them.
Alvarenga and Bonaparte 1992. A new flightless land bird from the Cretaceous of Patagonia; pp. 51–64 in K. E. Campbell (ed.), Papers in Avian Paleontology, Honoring Pierce Brodkorb. Natural History Museum of Los Angeles County, Science Series 36.
Chiappe LM 2002. Osteology of the flightless Patagopteryx deferrariisi from the late Cretaceous of Patagonia (Argentina) pp.281–316 in Mesozoic Birds, Above the Heads of Dinosaurs, Chapter: 13, Editors: Chiappe LM and Witmer LM, University of California Press.