Probably was a 767-300. From a distance a 763 could be confused for a 777. AA1413 DFW-LAS operated as a 763 today and landed at 0800 local time.
http://flightaware.com/live/flight/AAL1413
Equipment subsitutions like this are somewhat rare, but not shockingly so. An equipment substitution to a 777 on a domestic flight that is not hub-to-hub would be exceedingly rare.