In the Liji (禮運 21, 23) we find the following passages
故聖人作則,必以天地為本,以陰陽為端,以四時為柄,以日星為紀,月以為量,鬼神以為徒,五行以為質,禮義以為器,人情以為田,四靈以為畜。
何謂四靈?麟鳳龜龍,謂之四靈。故龍以為畜,故魚鮪不淰;鳳以為畜,故鳥不獝;麟以為畜,故獸不狘;龜以為畜,故人情不失。
The context is an explanation of how the sages used (or established) rituals to order human feelings/attitudes (治人情) and how rituals are part of a process of domestication, similar to the domestication of animals for food. My question is about the second passage.
The four "spiritual creatures" -- dragon, phoenix, qilin and tortoise -- are mentioned as emblematic for all domesticated animals of water, air, and land. Each one of them is linked to a special domain (dragon - water/fish, phoenix - air/birds, qilin - land/beasts), except for the tortoise, which is linked up with human dispositions. Why was this done? Does it have a specific significance? Are there traditional Chinese commentaries that mention this passage (or perhaps similar passages) and clarify why the tortoise was associated with 人情? Or should we say that this passage came about more as rhetorical sequencing (after all, the text is rather formulaic in terminology and phrasing), so that we shouldn't put much (or any) weight on this association (it just so happened that the 4 emblematic animals needed to be matched up with water/air/land, so there was a gap that needed to be filled for the 4th)?
It's probably not hard to invent an interpretation of this (for instance taking into account the use of tortoise plastrons in the old oracles, or other ceremonial uses of tortoises/turtles), but I'm mostly interested in knowing if there are references in traditional commentaries that discuss this association explicitly (if there are any).