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Examples include:

  • A snake does not have legs, but is still considered a tetrapod as it is a descendant of animals with four legs
  • Bdelloidea (class of rotifera) can only reproduce asexually, but is still considered an animal as it has ancestors that reproduced sexually
  • Star fishes have pentaradial symmetry, yet they are considered bilateralians as their ancestors were bilaterally symmetrical
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  • $\begingroup$ I've changed the headline from What is the term for a missing feature in a descendant that was present in an ancestor and defines a clade? because it could be read as saying the feature defines the clade. Since clades are defined by an ancestral organism and not a particular feature, I changed it to read in an ancestor which defines a clade?, which I presume was intended. $\endgroup$
    – mgkrebbs
    Commented Feb 11 at 19:40
  • $\begingroup$ The earliest snakes still had limbs: nationalgeographic.com/science/article/… $\endgroup$
    – kmm
    Commented Feb 11 at 20:31
  • $\begingroup$ Loss-based apomorphy or apomorphy? $\endgroup$
    – BigMistake
    Commented Feb 11 at 21:56
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    $\begingroup$ Secondary loss? $\endgroup$
    – BigMistake
    Commented Feb 11 at 22:45

2 Answers 2

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Maybe you're thinking of a vestigial trait? In snakes, legs are considered "vestigial"; some snakes retain remnants of legs and pelvis (https://www.amnh.org/exhibitions/darwin/evolution-today/how-do-we-know-living-things-are-related/vestigial-organs).

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Similarly to BigMistake's proposition, the term "secondarily reduced" is being used in "The Invertebrate Tree of Life" by Giribet and Edgecombe to describe a feature that has vanished. It is not specific to clade-defining features, but seems good enough.

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