Do parallel SELECTs make a transaction?

While running under READ COMMITTED (the pessimistic type, not RCSI), I've just seen an UPDATE's IX lock deadlock with a SELECT's shared lock on the PAGE-level. The SELECT ran with a parallel execution plan. Neither explicitly used a transaction. I understand that UPDATEs implicitly create transactions. Does this suggest that parallel SELECTs also create implicit transactions? Is this not true of non-parallel SELECTs? I'm finding this rather confusing, because none of the textbook examples of deadlocks show two transaction-less queries deadlocking.