Criterial features

Today’s Scenes From a Multiverse:

At issue here is what features are criterial for classifying a creature as a mammal.

Here’s a list of features common to all mammals; the asterisk marks features that distinguish mammals as a group from reptiles, birds, and amphibians:

warm-blooded (endothermic);
vertebrate;
*with sweat glands;
*including mammary glands, those specialized to supply milk to young;
*with hair made of keratin;
with a 4-chambered heart;
*with 3 middle ear bones;
with a diaphragm, a muscle separating the thoracic cavity from the abdominal category;
*with a lower jaw made of a single bone, the dentary;
with teeth that are replaced only once in a lifetime;
(most) viviparous rather than oviparous, giving birth to live young rather than laying eggs

Of course, in cartoon worlds almost all creatures, spiders included, can speak (in fact, engage in argument), and some of them can handle money.

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