This came as a pleasant surprise. I was going through some shells collected from years ago, and when I was done, I saw that some sand has fallen off, probably from inside a gastropod. Among the sand was this ammonite, only 2mm in length. I don't know which shell it came off, or where it may be from. Most of my shells are from Hong Kong, but I have also collected in several other East Asian countries.
I have read that Sulciferites hongkongensis, Coroniceras sp., and Arietites sp. have been found in Hong Kong, but don't know whether there are other species. I also found this passage from page 427 of "Biostratigraphy of China" on Google Books which might be relevant:
Might it be possible to narrow down the ID or age from the pictures? Thanks in advance!