pseudothumb

plural pseudothumbs

1. A digit on a vertebrate that resembles but is not a thumb.

   The extra digit is a "pseudothumb” containing a sheathed spine that can be pushed out.

   2016, Tim Halliday, The Book of Frogs, page 542

   2 more examples

   But a panda's forepaws are specially adapted for manipulating bamboo stems, with a modified elongated wrist bone forming the pseudothmb covered by a fleshy pad.

   2008, Heather Angel, Panda

   There are also at least two examples of more complex ectopic digits. The first is in the aye-aye, a Madagascan lemur, which has a distinct pseudothumb on its forelimb consisting of a bony part that is an expanded radial sesamoid and a dense cartilaginous extension (Hartstone-Rose et al., 2019). This pseudothumb receives attachments from muscles that facilitate abduction, adduction, and opposition, which together facilitate dexterity.

   2021, Jonathan Bard, Evolution: The Origins and Mechanisms of Diversity, page 257