I was recently reading an academic paper on Amdo Tibetan phonetics and the author uses IPA vowel diacritics that look like "less-than" and "greater-than" signs.
Here is a picture so you know what I'm talking about:
These are narrow phonetic transcriptions in IPA and the greater-than and less-than signs are being used as vowel diacritics, but I cannot find these diacritics listed on the IPA chart, nor can I find them on Wikipedia's list of nonstandard IPA symbols.
As you can see, the author uses the standard diacritic + to indicate "advanced", so these symbols cannot mean advanced/retracted.
Amdo Tibetan is not a tonal language, so these symbols cannot be related to tones.
The paper is an analysis of the pronunciation of single words and it does not mention prosody, so I can't imagine that the diacritics are prosodic in nature.
What do these less-than and greater-than vowel diacritics mean?