Feature of foot, toe pear shaped, on more than one unknown fossil (13)

'fossil' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.

'feature of foot toe pear shaped on more than one unknown' is the wordplay.
I cannot quite see how this works, but
'feature' could be 'arch' (I have seen 'architectural feature' mean 'arch' so perhaps 'feature' could also mean 'arch') and 'arch' is found in the answer.
'of' could be 'o' and 'o' is present in the answer.
'one' could be 'a' (a thing is one thing) and 'a' is found in the answer.
'unknown' could be 'y' (algebra) and 'y' is found within the answer.
This explanation may well be incorrect...

Can you help me to learn more?