/data/animals/animal[@id = /data/code/record[field[@id = ( '60' ) ]]/@id]

  The above '60' is in correct context of /data/animals/animal (it is
  the value of id).. do I lose context on the right hand side of this
  equation? 

The context on the right of the = is the same as that on the left,
namely the field node, which is why the @id to the left of the = selects
id attributes of field.

so

  /data/animals/animal[@id = /data/code/record[field[@id = (
  preceding-sibling::animal[1]/@id  ) ]]/@id]

will always compare @id to the empty set as the field element has no
sibling element called animal.

Too late to really check, but I suspect that you can't do this with a
single xpath if you don't allow yourself any variables or the current()
function (eg as provided by xslt).

David

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