I’ve done this song and dance before, but one big problem you have here is that there is no good cladistic definition of ‘reptile’, in the popular sense, which does not violate the rules of cladistics. What you want is a grouping that unites lepidosaurs (including euryapsids), archosaurs other than birds (and perhaps dinosaurs and pterosaurs), anapsids, and non-mammalian synapsids. This ends up being a pseudoclade of the amniotes with two and perhaps three crown groups omitted.
If this were a one-off problem, it would be possible to describe it in that way and say that it’s a concession to popular usage, like ‘duck’ includes all Anseridae except Anser and Cygnus. But it’s a repetitive problem. Sarcopterygii is a group worth discussing together without including all Tetrapoda in the mix. Holostei have common synapomorphies worth discussing together even when the Teleosts are omitted. There are a lot of groups that represent significant advances which are overshadowed by the successes of their own crown groups. And discussing them in cladistics becomes very awkward.
Also remember that the Linnaean “King Philip Came Over” hierarchy does have value in sorting out relative closeness of relationship, where “clade” can mean anything from “all eukaryotes” to “members of Boa species from the Atlantic piedmont of northern South America”
But that’s the problem: if you can’t count generations even for humans, how can you presume to count them for the entirety of Sauropsida? It’s not possible, even in theory. So, even if you want to define “relatedness” as “the number of generations removed from a common ancestor”, it’s impossible to quantify in that way; it’s a useless definition. That’s why scientists don’t use that definition.
No, that’s not the assertion made. The assertion made is that all are equally related to us. Again, we cannot count generational distances. So what are we left with? Temporally, all extant reptiles are equidistant from the ancestral sauropsid: about 320 million years. All extant mammals would be about as far removed from the ancestral synapsid, and thus all extant reptiles and all extant mammals are pretty much equally removed from the common ancestor of the two groups.
Cladistically, relatedness is determined by shared characters. Strictly speaking, a cladogram is just a hypothesis for a phylogeny. When comparing organisms, we can look at which characters (anything from morphology to physiology to genetics) are shared by groups, and given a large enough sampling of species, infer which characters are ancestral and which are derived. Cladistic relatedness is determined based on shared derived characters (synapomorphies) between groups. And, cladistically, we share with any given extant reptile only those characters which are common to all amniotes. All other characters possessed by an reptile you care to choose are unique to them, or to the reptilian clade to which they belong, and thus do not factor into any determination of how closely related they might be to us. So, again, we are left with the determination that we are equally related to allextant reptiles.
You can certainly argue for generational distance as a determinant, but it’s nothing more than wishful thinking (and nonscientific) to do so. It is impossible, now or in the future, to measure.
I’m not going to beat this to death, I do understand your point. But I think someone familiar with the reptilian line could give us a good idea of which reptiles seem least changed from our common ancestor. And the mammalian side doesn’t matter. All humans are effectively equidistant within the level of precision for any answer that could be provided. Yes, reptiles as a group are the same distance from humans, but individual reptiles aren’t equidistant from the common ancestor, and reasonable guesses could be made which one(s) are closer genetically to the common ancestor. If you need to use the term ‘genetic similarity’, fine, but it would help answer the OP. I don’t have the knowledge of the details within the family* of reptiles, but perhaps you, colibri, blake, or others do.
*‘family’ in the colloquial sense. I don’t have time to look up the proper taxonomic description at the moment.
Biologists tend to use terms like “non-avian reptiles” or “non-avian dinosaurs” or “non-human apes” when they want to discuss the members of a clade that retained a lot of ancestral traits and exclude some particular highly derived branches of the clade.
I think I can see what is being asked. Let’s take another example that is a bit simpler than reptiles vs mammals.
Humans are apes, yes? Humans are most closely related to chimps, and after that, gorillas. That means, a chimp’s closest relative is not a gorilla, but a human. And when we ask whether a chimp or a human is more closely related to a gorilla, the answer is that they are both equally related to a gorilla.
However, the chimp and gorilla share lots of ancestral conditions, while humans have a lot of derived traits. So chimps and gorillas are both knuckle-walkers because the common ancestor of chimps, gorillas and humans was a knuckle-walker, but humans evolved a new trait of bipedalism. Same with human hairlessness, and so on.
So Tripolar seems to be arguing that, since there are so many conserved traits between a chimp and a gorilla, it makes sense to say that a gorilla is more related to a chimp than it is to a human.
But now look at the problem. This relationship can’t be symmetrical. A gorilla is more related to a chimp than it is to a human, but for the concept of relation to make any sense a chimp has to be more related to a human than a gorilla.
You can say that a chimp is similar to a gorilla without ruffling any feathers. They are similar in lots of ways. It often makes sense to think about the features of non-human apes, because humans are so often a special case due to all our derived features. But it doesn’t help anything to argue that gorillas are more closely related to chimps than they are to humans, because it is not true.
Number of generations isn’t really the single key determinant of genetic variation anyway, for two reasons:
Most mutations occur in sperm cells and, IIRC, the number of mutations is roughly proportional to number of mitosis cycles a sperm underwent during its germ cell ancestry. Thus a 40-year old father will pass more mutations to his children than a 20-year old father. I think. Experts?
Genetic change is hardly constant anyway, but is accelerated by bottlenecks forcing or encouraging selection.
Anyway, OP may be asking what living reptile most resembles the ancient proto-sauropsid (or common ancestor of proto-sauropsid and proto-synapsid). Any “artist sketches” of it? Wikipedia has a picture of Nyctiphruretus which seems not too dissimilar to the geckoes inhabiting my house, though Wiki states that that extinct species is closer to turtles…
Even that is not an easy thing to answer. Turtles, for example, are one of - if not the - most ancient of extant reptile lineages. Yet, they are also very highly derived (no other animal has its shoulder girdle within its ribcage, for example). Lacertilian lizards maintain the ancestral “lizardy” form, but have derived skeletal features. Plus, they’ve “only” been around since the Mesozoic.
Trying to guess it by morphology is probably pretty futile though. Things can look similar and be very.much more distantly related than they are to things that look different.
There is a common ancestor of chimps and humans which is more closely related to the tree shrew than we are. Please stop pointing out the ambiguous cases and try concentrating on the cases that would give a better ‘guess’ at the answer.
Yes, but this is a much better way to answer the question instead of saying all reptiles are equally related to humans. I think you could provide something of a better answer with your knowledge of these animals. As I’ve repeatedly stated, the precise answer is unknown because of the lack of information. But I started this with a guess, labelled as such, which you pointed out was not a good guess. Now I’m guessing you can make a better guess. For instance, I’m guessing an ostrich would not be the answer, and you could probably eliminate a lot of other descendents of that common ancestor based on the degree of measureable change.
This is the answer. I think everybody is a bit obtusely dancing around the point that tripolar etc. are not asking about relatedness in the biological/cladistic sense. they’re asking in the laymans sense, that is, what is the most basal and/or plesiomorphic reptile.
The most basal reptile group are the turtles. While there is controversy about their exact placement in the tree of life, conventional thinking is that they split from other reptiles shortly after all reptiles split with the mammal lineage. The most basal turtles are snapping turtles, IIRC. Therefore, the living reptiles closest to the origin of mammals in a purely diagrammatic sense (i.e. looking at a cladogram’s branching order) are snapping turtles. Meaningless? Yes, for all practical purposes, but that’s the question that was being asked.
The most plesiomorphic reptile would be generic lizards. Tuatara are actually pretty derived in some ways, but the ‘lizard’ body plan is shared by both the first stem-reptiles and the first stem-mammals.
As an aside…
Actually, what we think of as “crocodilians” (aquatic ambush predator types of thing,s that general body plan) actually POST-date the first dinosaurs by a significant timespan. Recognizably croc-like pseudosuchians didn’t appear until the late Jurassic at least, and the modern crocodile group (Crocodilidae) didn’t show up until the late Cretaceous. The idea that crocs are living fossils unchanged since the dawn of the dinosaurs is complete nonsense. Prior to this, the ancestors of crocodiles were a hugely diverse group on par with the dinosaurs: there were herbivorous armored forms, bipedal runners, big terrestrial predatory forms, even a whole group of fully marine, mosasaur-like types. Modern crocs aren’t primitive, they’re actually an extremely specialized group adapted for shallow-water ambush predatory life.
To put it another way, the common ancestor of crocs and dinosaurs would have looked more like a dinosaur than a croc.
I’ll go out on a limb here and say that possible he was referring to our Cretaceous “tree-shrewish” ancestors, not the modern day tree shrews. If that’s the case, I would think our simian ancestor would be somewhat closer related by a few million years. Not that much compared to 65 million or so, but still measurable.
And if that’s not what he meant, then nevermind. . . .
Just to correct myself above, modern crocodilids (that is crocodiles, alligators, and gharials) and their common ancestor with each other did not appear until the Eocene. How’s that for living fossils: the group including modern ducks and chickens is older
But look at what you are saying: We have this group, A, which split off from groups B, C and D before B, C, and D split into their respective branches. You are claiming that A is closer to other outside groups than B, C and D. But why not just lump B, C and D together and say they are just as related to the outsiders? There is nothing objectively correct about lumping all turtles together just because the look alike. As was already noted, chimps and gorillas look much more similar to each other than either does to humans, and yet chimps are more closely related to humans than they are to gorillas.
You are letting phenotype trump genotype, which doesn’t make sense when you are talking about relatedness.
That’s not what I’m claiming at all. I specifically claimed that, after B, C, and d split from A, B was the first to differentiate from C and D. That’s all, and I believe that’s the answer to the question that was being asked.
The OP asked which is more closely related, but not being hip to phylogenetic terminology, I don’t believe that’s what he meant to ask. He was obviously asking “which is most basal among B, C, and D.” But everyone else seems to have used it as an excuse to act more knowledgeable about the cladistic revolution than thou.
But that is not what relatedness means, even in a layman’s terminology.
Imagine those groups were human families. Would you still claim that members of clan B were more closely related to outsiders than members of either clan C or D are?
Genotype doesn’t help much, either, in this case. Morphologically, turtles are typically regarded as basal(-ish) reptiles, largely because of their anapsid skulls. But recent molecular analyses unite turtles with crocodiles!
So it’s still not as easy as finding “the most basal” extant reptile and calling it a day.
Thanks, that is a useful view of the situation. And even if the answer is still undetermined as a result of that information, it paints a better picture of the situation.
You surmised correctly. But I didn’t type it that way, and Alessan’s response would be correct based on what I did type.
As I said, I don’t think the OP asking about ‘relatedness’ is really what he meant to say. I read the resulting discussion as trying to get at what is closest to the common ancestor of both groups. You’re right that this is not “relatedness,” but someone not familiar with phylogenetic nomenclature will not know to to ask “which is most basal” or which is closest to the common ancestor" or “which is most plesiomorphic.” all of those are valid questions.
and say that possible he was referring to our Cretaceous “tree-shrewish” ancestors, not the modern day tree shrews. If that’s the case, I would think our simian ancestor would be somewhat closer related by a few million years. Not that much compared to 65 million or so, but still measurable.