Not sure... but I don't think it's ctenostome bryozoan, which is the obvious answer. I can see no evidence for swellings along the length of the branches, which I would expect to see.

What size are these? Could they be algal or bacterial etchings, rather than metazoan? I assume the surface is some kind of mollusc shell?

sorry for that, is a Dentalium shell, and the size of the traces is 0,2mm aprox.

Thanks for the answer

I think is a marine scaphopod molluscs in the family of Dentaliidae

yes, but i don't know about the traces!

This might be some sponge traces.

do you have any paper where i can see this same traces?

Talpina? 

Please use the paper Buatois et al. "Categories of architectural designs in trace fossils: A measur eof ichnodisparity". It is good!

Mere guessing, but my first impression was a branching alga like the Cambrian, Dalyia, or its equivalent in , I presume, your  Cretaceous locality. Second thought...common arthropods crawling about or clinging to the Dentalium, i.e. crabs, shrimp, appendages leaving scratch marks.

Thank you for the paper Ursula.

William, is a Miocene locality, thank you!

Some of the images are misleading and might lead to think of ctenostomes, but the close-ups look better. I can see stolon-like traces, but no zooid-like voids and I would not say ctenostomes bryozoans as my first choice. Boring sponges may be a good candidate.

Looks like either sponge borings or bryozoan incrustations