Ashtanga yoga is not necessarily Patanjali yoga. Ashtanga yoga is more ancient than Patanjali and goes back to Hiranyagarbha who is the original expounder of the yoga school. This is explained in the Mahabharata. Kapila Muni also explains Ashtanga yoga to his mother Devahuti in the Srimad Bhagavatam. Patanjali is only one teacher who elaborated on this system. Actually Patanjali expounded on two systems of yoga, Ashtanga yoga (yama, niyama, asana, pranayama, pratyahara, dharana, dhyana, samadhi) and Kriya yoga (tapah, svadhyAya and Ishvara praNidhAna) which is a subset of the niyamas, but on its own it is a complete system as well. Furthermore, Patanjali says that Ishvara praNidhAna on its own can lead to samAdhi. It is the most potent element of yoga. Note that the Valmiki Ramayana starts with the words tapaH and svadhyAya and that the topic of the Ramayana is Ishvara praNidhAna. Some sources also speak of a six limbed yoga. Modern people are too quick to equate Ashtanga yoga with Patanjali, while Ashtanga yoga itself is much older.