You study a group of ammonites (an extinct lineage of cephalopods) that lived during the Mesozoic. ...

You study a group of ammonites (an extinct lineage of cephalopods) that lived during the Mesozoic. You measure the shell diameter of different fossil ammonite species.
 
  When you combine your data within each of six different time periods you obtain the following average shell sizes and corresponding standard errors. What rule does this data conform to?
 
  Time interval Mean shell diameter (+/ standard error)
  245214 MYA 15 mm (+/ 5 mm)
  215184 MYA 21 mm (+/ 7 mm)
  185154 MYA 36 mm (+/ 12 mm)
  155124 MYA 63 mm (+/ 21 mm)
  12594 MYA 72 mm (+/ 24 mm)
  9565 MYA 84 mm (+/ 28 mm)
 
  a. Biogenetic rule c. Cope's rule
  b. Allen's rule d. Bergmann's rule