Overlooked reversals in vertebrate evolution

Some of the following reversals were covered in earlier posts. 
Others premiere here. Still others remain overlooked or forgotten for the moment. Come back over time to see additions as they spring to mind.

Reversals are distinct from convergent traits
in that reversals also appear in ancestral taxa (in the LRT) while convergent traits do not.

  1. Platypus electrosensory bill — sturgeon, paddlefish, shark electrosensory rostrum
  2. Odontocete simple cone teeth — basal bony fish through pelycosaur simple cone teeth
  3. Multituberculate reappearance of post-dentary bones — cynodont jaw joint
  4. Prohalecites gill cover (operculum) — sturgeon, paddlefish, ratfish gill cover
  5. Minjinia, a disc-head, jawless placoderm — drepanaspid jawless anapsid
  6. Echinosorex, the moonrat and member of Glires lacks gnawing incisors — tree shrews that are not members of Glires also lack gnawing incisors
  7. Fins (dorsal, pectoral, pelvic, caudal) in ichthyosaurs, odontocetes, mysticetes, and other tetrapods — Similar fins in basal tetrapods (= fish)
  8. Loss of hind limbs in snakes, whales, amphisbaenids — lampreys, Birkenia
  9. Acanthostega radius twice as long as the ulna — as in pre-tetrapod lobefin fish
  10. Ossified skin (bony plates) in ankylosaurs, glyptodonts, etc. — Ossified scales in basal vertebrates (= fish)
  11. Flightless birds — flightless pre-bird theropods
  12. Loss of a flexible neck in mysticetes — basal vertebrates and tetrapods
  13. Heterodont dentition in Longisquama + pterosaurs and, by convergence in cynodonts —as in Hybodus, a shark basal to bony fish
  14. New world vulture overall morphology (pigeon sisters) — Old world vultures (hawk sisters)
  15. Long-legged shore birds (stilts, Himantopus, derived from short-legged Eocypselus) — long-legged basal birds like seriemas (Cariama).
  16. Atavistic tails in humans — monkeys, lemurs
  17. Extensible ventral mouth parts in skates — as in pre-gnathostome sturgeons
  18. Gill openings ventral to pectoral fins in skates and rays — as in osteostracans
  19. Separation of the gill basket from the neurocranium in sturgeons — as in Birkenia
  20. Fusion of the gill basket with the neurocranium in osteostracoderms — as in Arandaspis.
  21. Extended rostrum in pachycormiforms — as in Chondrosteus and sharks.
Figure 1. Skulls of pterosaur ancestors from Huehuecuetzpalli through Macrocnemus, Cosesaurus, Longisquama and the pterosaur Bergamodactylus.

Figure 1. Skulls of pterosaur ancestors from Huehuecuetzpalli through Macrocnemus, Cosesaurus, Longisquama and the pterosaur Bergamodactylus. Note the heterodont dentition (#13). These taxa were omitted from the recent pterosaur precursor paper by Ezcurra et al. 2020, by Hone and Benton 2007, 2008 and every other cladogram (Kellner 2003, Unwin 2003) over the last twenty years.

Patagopteryx: turning back the genetic clock on pedal digit 1?

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.

Figure 1. The two-toed ostrich (Struthio) nests with the four-toed Patagopteryx, when all relatives have only three toes.

Figure 1. The two-toed ostrich (Struthio) nests with the four-toed Patagopteryx, when all relatives have only three toes. Note the pubic foot. 

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. 

Figure 2. Ventral view of the pes of Patagopteryx. Digits numbered. Pedal digits 1 and 5 are not present in other palaeognaths.

Figure 2. Ventral view of the pes of Patagopteryx. Digits numbered. Pedal digits 1 and 5 are not present in other palaeognaths.

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).

Figure 2. It's always valuable to see what the taxon looks like with scale bars. This is a tiny specimen, but rather completely known.

Figure 3. It’s always valuable to see what the taxon looks like with scale bars. This is a tiny specimen, but rather completely known. Pedal digit 1 may not be retroverted in this taxon. The proximal phalanxes may have been elevated here. 

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.

References
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.

wiki/Patagopteryx