meaning - Who is the authority -- scientists, or linguists -- on the definitions of everyday words referring to types of animals?

For instance, biologists these days like to say that the word "dinosaur" is inclusive of modern birds, since birds are descended from dinosaurs. This is consistant with biologists' tendency to categorize things monophyletically -- that is, if it is descended from something, it "is" that something. A bird is descended from a dinosaur, so it must be a dinosaur.

Linguists (such as those employed by dictionaries e.g. Webster and Oxford) tend to be more pragmatic -- concerned more about communication and usefulness rather than strictly adhering to monophyletic categories. They consider "dinosaur" to only refer to those animals that lived in the mesozoic period....that is, they explicitly exclude modern birds from the definition, because when most people use the term "dinosaur," they are not talking about birds.

This is similar to the way we exclude tetrapods (amphibians, reptiles and mammals) from the definition of "fish", even though tetrapods are descended from fish. That is, "fish" is, by definition, a paraphyletic category. Other examples are "monkey" and "ape", which, by definition, are paraphyletic and therefore exclude humans.

So who is the authority on the definition of these words? And if biologists are, isn't it being inconsistant to define the word "fish" to exclude a subset of its decendants, while refusing to do the same with the word "dinosaur"?

Answer

It is you and I, the everyday users of the living language, who unwittingly "define" the contemporary semantics of a term.

Usage varies with time, with geography and with context.

One thing to remember is that scientific definitions are comparatively static -- they stay rigidly far, far longer than those in colloquial language, for a purpose -- so scientists can communicate without ambiguity.

Dictionaries try to resolve the dichotomy to some extent by labeling the terms as 'technical', 'formal', 'informal', 'colloquial', 'slang' and so on.

di·no·saur /ˈdīnəˌsôr/ Noun
For a scientist,

A fossil reptile of the Mesozoic era, often reaching an enormous size. …

For us, mere mortals, it sometimes also means

A person or thing that is outdated or has become obsolete because of failure to adapt to changing circumstances.

I don't know if the word really means 'bird' for anyone, except in the appropriate technical context.

The fossil record indicates that birds evolved from theropod dinosaurs during the Jurassic Period and, consequently, they are considered a subgroup of dinosaurs by many paleontologists. (WP) [emphasis mine]