Recognizing homologies

Biologists use a few criteria to help them decide whether a shared morphological character (such as the presence of four limbs) is likely to be a homology:

Bat, bird, mouse, and crocodile forelimbs

Same basic structure
The same bones (though differently shaped) support the limbs of mice and crocodiles. In the illustration of forelimbs at left, homologous bones are colored alike.

Hind limb/pelvis attachment in crocodile

Same relationship to other features
The limb bones are connected to the skeleton in similar ways in different tetrapods. The joint between the femur and the pelvis has a ball-and-socket structure which is typical of tetrapods, as shown in the crocodile to the left.

Same development
The limbs of all tetrapods develop from limb buds in similar ways (see below).

These criteria help biologists tentatively identify homologous morphological characters that are likely to be reliable indicators of shared ancestry.

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Crocodile hip image courtesy of Dave Smith, UCMP.
Alligator limb bud image courtesy of the U.S. Geological Survey/Florida Integrated Science Center; photo by James Basto.
Mouse limb bud photo courtesy of the Embryo Images: Normal & Abnormal Mammalian Development website.