Both of the traditions contain elements of the undivided school of Buddhism which could be called the "original teaching".
Both traditions also contain innovations, or teachings that were "revealed later", whichever way you want to put it. The Mahayana has a great diversity of innovative later sutras that come into the written canon of history considerably after the Pali Canon, the most famous of which, perhaps, is the Lotus Sutra.
It should be noted that the Mahayana teachings outrightly identify themselves as new, revelatory teachings, whereas Theravada are less likely to consider their teachings "new" in the same way. In Mahayana thought, the Lotus Sutra was taught by Gautama-Buddha toward the end of his life to a select number of apostles (a lot of which seem to be supernatural entities), possibly in a spiritual symbolic Pure Land wherein is found a 'perfect' prototype of Gṛdhrakūṭa
, or in English, the Holy Eagle Peak. (My own maybe-slightly-less-than-orthodox Buddhism is coming out here, instead of an objective approach, because as a dubious Mahayanist I believe the Lotus Sutra, and later Mahayana sutras, to be largely symbolic and mythic expoundings of the Dharma, versus the 'core' Pali teachings, which are more direct. I will try to return to being objective though in the interest of accurately disseminating information as best I can in regards to the theme of this thread.)
The Lotus Sutra was believed to have then been subsumed into the realm of the Nagas
(divine snakes: it is pertinent here to remember that snakes have always been symbolically associated with wisdom, secret-teachings, and knowledge, in both Eastern Indic traditions and Western Mesopotamian and Abrahamic traditions). The Lotus Sutra claims to be the revealed teachings of the Buddha, revealed from behind the veil of "wisdom" (i.e. the nagas
), to the Northerly Buddhist schools that would become the later Mahayana movement.
Similarly, the Southern School has a belief about the Buddhadharma of specifically the Abhidhamma being preached in the trāyastriṃśa
heaven by the Buddha, and later revealed to the earthly realms as the Abhidhamma via direct revelation through the apostle Śāriputra. Alas I do not know enough about the Abhidhamma and the background of it to elaborate on it as much as I did the Lotus Sutra.
Both traditions do
share a general "core teaching", and both canons (Mahayana and Pali) have this core teaching preserved, one way or another, through "original Buddhist" teachings that reference the same body of knowledge that the Pali Canon is a harmonization of. Mahayana Buddhists, if they are worth their salt at all, also revere the wisdom-teachings of the Pali Canon because they are the foundation
upon which all later innovation rests.
For an example of this, I would like to quote the user "Dhamma_Basti" from the thread "the pali texts are incoherent?":
The book in the link he provided gives us examples of parallel passages from a Northern-School tripitaka (specifically from the canon of the Mūlasarvāstivāda
school (who used Sanskrit as its language of Dharma-transmission, and who committed their inherited Buddhadharma to text quite a bit later than the Pali Canon) and the Pali Canon (who used Pali, as we all know).
Here is the section entitled Saṅghabhedavastu
from the Northern canon, in Sanskrit:
Kiṃ manyadhve bhikṣavo: rūpaṃ nityam vā ? Anityam idaṃ bhadanta. Yat punar anityaṃ duḥkham vā tan na vā duḥkham? Duḥkham idaṃ bhadanta. Yat punar anityaṃ duḥkhaṃ vipariṇāmadharmi, api nu tac chrutavān āryaśrāvaka ātmata upagacched etan mama, eso ’ham asmy, eṣa me ātmeti?
It is quite close to the Pali Canon's Mahāvagga
, the citation for which is given as "Vin I.14.5":
Taṃ kiṃ maññatha bhikkhave: rūpaṃ niccaṃ vā aniccaṃ vā ti? Aniccaṃ bhante. Yaṃ panāniccaṃ dukkhaṃ vā taṃ sukhaṃ vā ti? Dukkhaṃ bhante. Yaṃ panāniccaṃ dukkhaṃ vipariṇāmadhammaṃ, kallan nu taṃ samanupassituṃ: etaṃ mama, eso ’haṃ asmi, eso me attā ti?
As you can see, the two are very close, but there are still material discrepancies, small and large, between the two inherited traditions of Buddhadharma, in how they are communicated. Sometimes there are even outright contradictions between the Northern and Southern canons. The kinds of discrepancies you have between between the texts of the Northern and Southern schools (excluding, of course, the later Mahayana innovations) are generally the same sort of inconsistencies one encounters in different Gospel accounts in the Bible, in one Jesus says this, in the other Jesus says something slightly different, but generally the same. Or names of people and places things happened with switch around, but the lessons and teachings will be the same (or not).
Who inherited the "truer" tradition? Who can say. Although I will hand it to the Southern Schools/modern-day Theravada that they committed their canon to immutable text much before the Northern Schools did.