I find it hard, if not impossible to tell different taxa of Copiapoa seedlings apart - most probably due to lack of experience - they all look alike, or rather, their differences are no greater than can be found in seedlings from the same fruit. 

Benjy Oliver however has much more experience of raising Copiapoa from seed and believes that he can tell most species apart at a very early age. I must try to get him to put into words which features he uses to distinguish them.

One interesting observation he made is that for a considerable period, all Copiapoa seedlings are the same size, irrespective of the size which they can reach as mature plants. Again, I must get some more documented information on the subject: what taxa? up to what age?