The 1892 RLDS edition reinterpreted the phrase up on as the single preposition upon. The RLDS text has subsequently maintained this reading. The LDS text has maintained the earliest form, up on. Typically we expect the preposition upon with the verb or noun heap:
Most pertinent, however, is the fact that when the verb heap is used in the passive to describe the piling up of corpses, we have not only upon but also up with it:
These last two examples suggest that the phrase up on in Alma 2:38 could be an error for up upon; that is, the original text here may have read “their bones have been found and have been heaped up upon the earth”. Such a conjecture would mean that early on in the transmission of the text either up was lost (and upon was reinterpreted as up on) or upon was replaced by on (leading directly to up on).
As noted in the nearby discussion regarding up upon in Alma 2:15, there are four clear examples in the history of the text where the up of up upon was lost, including one by scribe 2 of 𝓟 in 3 Nephi 20:2. On the other hand, there is also evidence in the textual history for the tendency to mix up upon and on, especially since these two pronouns are generally synonymous:
Most of the mix-ups occurred in typesetting the editions and show the tendency for the archaic upon to be replaced by on. There are two instances that involve scribes (in Helaman 5:26 and 3 Nephi 20:2). The second of these shows scribe 2 of 𝓟 replacing upon with on, which is what could have happened in Alma 2:38.
Yet we need to remember that upon and on are generally synonymous, so if up upon can occur in the text, so can up on, as in the following two examples:
These two examples thus argue that in Alma 2:38 the reading “their bones ... have been heaped up on the earth” is acceptable. For this reason, the critical text will not emend up on to up upon here. Since “heaped up upon” can occur, so can “heaped up on”.
Summary: Retain in Alma 2:38 the earliest reading with up on (“their bones ... have been heaped up on the earth”); although up on could be an error for up upon, the use of on instead of upon is possible.