Here's a DFSORT job that will do what you asked for. I assumed your input file has RECFM=FB and LRECL=80, but the job can be changed appropriately for other attributes.
//S1 EXEC PGM=ICEMAN
//SYSOUT DD SYSOUT=*
//SORTIN DD DSN=... input file (FB/80)
//SORTOUT DD SYSOUT=*
//SYSIN DD *
* Add seqnum after end of records.
* Sort descending on seqnum.
* Remove seqnum.
If you're not familiar with DFSORT and DFSORT's ICETOOL, I'd suggest reading through "z/OS DFSORT: Getting Started". It's an excellent tutorial, with lots of examples, that will show you how to use DFSORT, DFSORT's ICETOOL and DFSORT Symbols. You can access it online, along with all of the other DFSORT books, from: