Megaladapidae is one of the families of primates that is often argued over by taxonomists. Everyone has an opinion in taxonomy. I have decided to put it as a separate family because honestly, sportive lemurs are pretty cool. Megaladapids were said to be extinct because the taxonomic group originally covered all of the giant lemurs the size of gorillas that use to roam Madagascar. But like most megafauna, it was killed off by humans.
Now the reason I have put sportive lemurs into the family megaladapidae (against the wishes of wikepedia, apparently) is because it is thought that the sportive lemur filled the niche left behind by the extinct megaladapids. Other people like to put it into the family lepilemuridae, which I will admit is fun to say, but in my opinion you can’t beat belting out the word megaladapidae. I know, sometimes I make taxonomic decisions for completely selfish reasons.
The reason the only living megaladapid is so different is that it is completely nocturnal, unlike most lemurs. Not only that, but like many nocturnal animals the megaladapids are completely solitary and very territorial against same-sex intruders.