most people think of sangheili as some kind alien-reptiles However,i think that they are actually closer to synapsids than to reptiles.

1.Presence of a single temporal fenestra: This is a hole in the skull behind the eye socket, which is characteristic of synapsids while reptiles have two fenestra.

2.Heterodont dentition: unlike reptiles sangheili have teeth that are differentiated in shape and size to perform different functions like incisors, canines, and molars unlike reptiles which have more pointy teeth.

3.Lower jaw composed of a single bone which seperated into a some kind of mandible-like mouth

4.Secondary palate: Synapsids have a secondary palate, which separates the nasal cavity from the mouth cavity.

1. their feet are hooved: another unreptilian feauture. there are no true hooved reptiles.

2. limb posture: sangheili have a upright horse-like posture. Most reptiles have a sprawling limb posture.

3. Sangheili are likely endothermic: atleast from what i read in Legacy of Onyx , the sangheili seem to be warmblooded.(reptiles are ectothermic )

so in conclusion, the sangheili are more like space dimetrodons rather than space lizards.