When claws became hoofs and hoofs became claws

The cladogram
represented by the large reptile tree (LRT) shows that at more than one time within the clade Mammalia, sharp, clawed unguals became broad, often hoofed, unguals. And then several times broad unguals once again became sharp claws. Here’s the pattern:

  1. Small basal mammals all had sharp claws. So did basal monotremes, marsupials and placentals.
  2. Ernanodon, among the marsupials, had broad, round unguals
  3. Toxodon, among the marsupials, had short, anteriorly concave unguals, perhaps supporting hoofs.
  4. Basal placentals, including Carnivora and Glires, had sharp claws. Primates, like Proconsul, developed fingertip pads surrounding their unguals with only the keratinous nails exposed.
  5. Alcidedorbignya, among pantodont placentals (basal herbivores), had proto–hoofs (tips nearly as wide as bases). In the same clade, so did Pantolambda.
  6. Bayrlambda, among basal xenarthrans, had short unguals, barely longer than wide.
  7. Paramylodon, a ground sloth, had long sharp claws, but Glyptodon, a sister had truncated unguals, more like hoofs.
  8. Bradypus, a tree sloth, had long, curved, sharp claws. So did Orycteropus, Dasypus and kin.
  9. Titanoides, a basal phenacodont had sharp claws.
  10. Phenacodus, another basal phenacodont, had round hoof-like unguals. Many subsequent large mammalian herbivores had similar hoof-like unguals, except…
  11. Mysticete whales, like Balaenoptera, had long sharp unguals hidden within their flippers.
  12. Homalodotherium had sharp claws, as did Protypotherium, but
  13. Miocochilius had hoofs and it was a sister to those two.
  14. Dusisiren the sea cow, had sharp unguals beneath the mitten of its flipper, but…
  15. Procavia, the rock hyrax and Elephas had tiny unguals.
  16. The remaining mammals, like Sus, the pig, are all ungulates and they all have hoofs, except…
  17. Chalicotherium, which apparently had claw-like hoofs.

So you can see,
the sharp claw – broad hoof morphology comes and goes within the clade Mammalia, according to the LRT, with only one clade developing padded unguals with keratinous nails.

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