Why Does Ophicodon Have Immunity?

I’m honestly confuzzled as to why Ophicodon has Immunity. So far the only creatures with immunity sport some sort of sail (Dimetrodon, Monolometrodon, Postimetrodon, Trystonix ect.) With the exception of Indominus Rex and Ankylodon. Ophicodon doesn’t have any sail so why does it have immunity?

Short squat and secure on their stubby little legs perhaps? Just like the sails

Gallimimus and Ornithomimus are also Immune (also Monomimus but it’s a hybrid and judging by your post you see hybrids as exceptions). There doesn’t seem to be any rhyme or reason to it as far as I can make out.

Ophiacodon is in the same family with dimetrodon. Just without the sail and a little bit longer face