Iowa Code § 384.30: Execution.

General obligation bonds must be executed by the mayor and city clerk. If coupons are attached to the bonds, they must be executed with the original or facsimile signature of the clerk. A general obligation bond is valid and binding if it bears the signatures of the officers in office on the date of the execution of the bonds, notwithstanding that any or all such persons whose signatures appear thereon have ceased to be such officers prior to the delivery thereof.