This question has been puzzled me for years.
Google was not my friend. 
I also tried calling a university’s “answer line”. The student who took my question responded with, “Humans are mammals. Mammals are not related to reptiles.” :rolleyes:
O, Collected Wisdom of The Dope, what say you? TIA!
All living reptiles are equally close relatives of humans.
I’ll guess crocodiles.
First off, nothing like a modern mammal was ever closely related to a reptiles. Their were many intermediate forms before we got mammals from a common ancestor to both reptiles and mammals.
But crocodiles seem to have maintained many similarities to very ancient ancestors that were around at the time that some group of reptilians took the turn down the road to eventually becoming mammals.
I’d say pretty much what the other guy said. Reptiles belong to a separate clade from mammals, so the really isn’t a “most closely related” among them, relative to us. Questions of relative relatedness really only work when comparing taxa within a clade, not across clades. So, reptiles, in general, are more closely related to us (in that Sauropsida - which contains reptiles as we know them - is a sister clade to Synapsida - whih contains us - within Amniota), than are, say, insects, but that’s about the best you can say.
Crocodiles are fairly derived, as reptiles go. They are more closely related to birds than to any other reptile.
Thanks for the prompt replies.
But I guess I haven’t explained myself clearly enough. If I understand evolutionary theory correctly (and it’s possible I do not), all life on earth descended from one living form/entity/creature. Therefore, every living thing on earth is related to each other, to some degree.
So, which species of reptile, currently in existence, is the one most closely related (however distant that relationship is) to h.s.s.? ETA: if the species cannot be identified, then genus or clade or something similar would be nice to know.
Now tell me where I’ve misunderstood the concept or have made an incorrect assumption. 
Think of the diversity of life as branches on a bush. All the branches eventually meet in the same root, so the entire bush is related. But at some point long ago, the branch that led to mammals diverged from the branch that led reptiles. Everything that is on the tips of one branch is obviously equally related to what is on the tips of the other branch. No specific tip is closer than any other specific tip.
Jeff Goldblum
You’ve misunderstood the concept. Reptiles and mammals are two different branches (clades). Beyond the branching point, all members of one branch are equally closely related to all members of the other branch.
Say you have five first cousins. You are asking which of your first cousins is more closely related to you. They are all equally related to you. Your grandparents are the branching point. The families of each of their offspring, including your parents, represent new branches/clades, all equally related to one another.
I would repeat my earlier statement. Reptiles and mammals are related in that they are both amniotes (and tetrapods, and vertebrates, etc). After that, they diverge substantially, to the point where none are “more closely related” to us than any other.
If I understand you correctly, you’re referring to ancient crocodilians being the ancestors of birds, and not any modern reptiles. But the crocodilians and modern reptiles would have had a common ancestor, along with the ancestors of mammals. I’m just guessing that the ancient crocs were more closely related to those ancient ancestors of mammals than any modern reptile is, and modern crocs closer to their ancient ancestors. None of this is going to answer the OP without some sort of definition of how to measure the closeness of the relationships. You have to go back through time and for both humans and modern reptiles measure the differences to the common ancestor. I doubt we can clearly identify that common ancestor, or have a way of accurately measuring those differences (although possibly DNA will reveal the answer some day).
Chart of the ancestry of mammals here
What Colibri just said.
To be super pedantic, we are most closely related to the reptile lineage with the longest generation length.
But we are not comparing first cousins. You would consider yourself more closely related to your first cousin than your seventh cousin. From some common ancestor there was some number of genetic changes that resulted in modern humans. For each modern reptile there will be a different number of genetic changes to that common ancestor. The reptile with the least number of genetic changes could be considered the most closely related to humans.
Nope, I’m saying crocodiles as a group, being archosaurs, like birds, are more closely related to them than to any non-archosaur group (be that group reptile or mammal). The earliest crocodiles show up around the same time as the dinosaurs, during the Triassic. There were many reptiles that are older (e.g., turtles).
That’s not how you measure relatedness. You might say that a reptile that retains more ancestral traits is more similar to ancestral mammals, and therefore more similar to modern mammals. And a reptile with lots of derived traits is more different than ancestral reptiles, and therefore more different than ancestral mammals, and therefore more different than modern mammals.
But that’s not really true. Aren’t birds (which are a specialized type of reptile with lots and lots of derived traits) more similar in a lot of ways to mammals than either are to, say, turtles? Except, all the ways that birds are similar to mammals are because we share similar derived characteristics. If we went back in time to look at the most recent common ancestor of birds and mammals, we’d find a creature that wasn’t at all mammal-like or bird-like. It would look like a very standard stem reptile.
You may be interested in the Richard Dawkins book, The Ancestor’s Tale, which traces human evolution back as far as possible and shows the points at which our ancestry converges with that of other living things. Our closest reptile relatives were various mammal-like reptiles, but these are extinct today. All living reptiles and birds are equidistant in their relationship to us.
The question does have an answer when applied to fish: Our closest piscine relatives are the six species of lungfish. The coelacanth is also more closely related to us than it is to most other fish.
Ok, I get what your saying. I was misinformed, and under the impression that crocs predated dinosaurs by a long time. You’re implying that some modern reptile is more closely related to the common ancestor of reptiles and mammals. Any idea if that reptile could be identified?
As for crocodiles and birds, the theory that birds evolved from proto-crocodyllians is pretty much completely discredited. The similarity between early birds and theropod dinosaurs is so profound that there’s no question that birds are specialized dinosaurs. Maybe someday we’ll find a crocodyllian fossil that has bird-like traits, but that bird-like croc wouldn’t be more closely related to modern birds than any other crocodyllian.
However, crocodiles are more closely related to dinosaurs than they are to any other reptiles. In a family tree, crocodiles and dinosaurs would be first cousins, while the family of croc/dinos and lizards would be second cousins, and the family of croc/dino/lizards and turtles would be third cousins.
So, crocodiles are more closely related to birds than they are related to turtles, even though crocodiles share a lot of ancestral traits with turtles, and birds have a lot of highly derived traits that they don’t share with crocodiles.
The problem here seems to be the definition of “related”. All reptiles are equally related to us in the same sense that all your seventh cousins are equally related to you – they all share the same common ancestor with you. You’re using “related” in a slightly different sense… by your definition, a seventh cousin who happened to end up with more of your genes (i.e., retaining more of the same genes of the common ancestor as you retained) would be “more closely related” to you than a seventh cousin who happened to end up with fewer of your genes.