Lizards aren’t mammals…
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Sorry… as I read these fascinating theories and thoughts — the whys and why nots — I keep thinking of another animal.
But, It has legs. But, these animals also have characteristic of mammals, birds, and reptiles. How did monotremes even happen?
Feel free to ignore post if this is too far off on a tangent …
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The characteristics they share with birds and reptiles - laying eggs - were inherited from the common ancestor that all those lineages shared (they did not evolve those traits independently). The first mammals all laid eggs: one branch continued to lay eggs and evolved into monotremes, while another branch evolved to not lay eggs. That non-egg laying branch of mammals was super successful and diversified into all the other mammal lineages alive today, while the only monotremes that survived were echidna and platypus. The reason that echidna and platypus survived while all other egg-laying mammals went extinct is probably lost in time, but it might have helped that there were no placental mammals in Australia to eat/outcompete them.
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Maybe there are limbless mammals but they are just very good at hiding. Whenever people cycle past them, they just assume they are bits of old rope…
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Yes… Does the venomous trait also go back to a very early common ancestor? I understand platypus males are the only venomous mammals.
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Platypuses aren’t the only venomous mammals—solenodons, some shrews, and slow lorises have venomous bites. However, platypuses are the only living mammals with venomous spurs. There is apparently some fossil evidence (I’m not sure how strong) that many early mammals had venomous spurs, which would indicate that venomous spurs were an ancestral trait of mammals lost in all modern groups except the platypus.
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OH I thought it said ANIMALS ugh Im sorry I feel like a 
As you all think about this question, watch out for any spandrels creeping into your reasoning…
“Spandrels” is evolutionary biologist shorthand referring to the classic opinion piece “The spandrels of San Marco and the Panglossian paradigm: a critique of the adaptationist programme” published in 1979 by Stephen Jay Gould and Richard Lewontin.
Here’s a free text version:
https://faculty.washington.edu/lynnhank/GouldLewontin.pdf
Here are two paywalled links to the full version:
https://www.jstor.org/stable/77447
https://royalsocietypublishing.org/doi/abs/10.1098/rspb.1979.0086
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That’s a conversation-stopper ;-). Does anyone talk anymore about spandrels or pendentives or whatever that squishy idea has morphed into these days? I’d like to see examples in regards to evolutionary limb loss.
Yes… the venomous trait… yes! One of our ancestors, euchambersia; possibly had venom.
I’m a big fan of both Gould and Lewontin. I haven’t followed what this idea has morphed into, but I agree with the overall thinking. Some animals, like mammals, I believe lack the genetic propensity to completely lose things like limbs. As pointed out, whales still have the basic skeletal structure of a land mammal, albeit in a highly modified form. The modification was the result of either a random genetic modification or ‘advantage’. A hopeful monster, if you will. But disadvantage also plays a part. Humans have a small finger that is rarely used. Yet it is no real disadvantage to us, so I really doubt it will vanish in the population. Limbless lizards likely arose from a mutation that posed no disadvantage to the phenotype. A legless mammal would be at a disadvantage, and may not have the ability to lose limbs like reptiles do.
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I think some of the factors may hide in why animals become limbless?
About 150 million years ago, snakes roamed about on well-developed legs. Now researchers say a trio of mutations in a genetic switch are why those legs eventually disappeared.
It said that ‘sonic hedgehog’ gene enhancer which helps to grow limbs does not work properly, and it also said the snake had limbed ancestry.
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The field of evolutionary developmental biology (evo-devo) is fascinating and I certainly haven’t kept up with a lot of the research and hypotheses contained in it. Didn’t even exist when I was in school. But basically it largely boils down to small changes in regulator genes that affect development of an individual organism can have big effects on morphology. It doesn’t take much to turn off genes that make limbs or that affect how large the limbs get if they are grown at all. Same for many other morphological features, some of which may be repurposed for other functions (e.g., external gills in aquatic insects modified into wings). Small tweaks in the genome and changes in timing when genes are expressed can have big effects on what sort of organism emerges following development.
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Euchambersia isn’t our direct ancestor; it’s a member of a side branch off of mammal ancestors called Therocephalia. Other therocephalians show no signs of having been venomous, so the venomous bite of Euchambersia was probably a trait unique to it, not a trait of direct mammal ancestors. Cynodonts—the direct ancestors of mammals—don’t show any sign of venom. As such, venomous spurs were probably a trait that first appeared right around the origin of Mammalia and were subsequently lost in most lineages, if the hypothesis that they were present in many early mammals is confirmed. Shrews are another convergent origin of venom.
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There are subterranean mammals and mammals that live in environments where shedding heat is more important than retaining it. There are even mammals with various degrees of relaxed homothermy. So I’m not sure one can say it is impossible for a very long skinny mammal to evolve for thermoregulation reasons. But certainly the way most mammals live would preclude it.
Yes, not direct.
Huh! This discussion makes me imagine an adapted underground mammal, hairless and limbless, maybe reduced eyes. Probably eats roots and hunts worms and insects. Big teeth of course. Nurses offspring during temperate seasons; hibernates or estivates as needed.
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“The Trouble With Tribbles”, was one of my favorite episodes from Star Trek the original series.
: )
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That just sounds like a Naked Mole Rat without limbs.
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Well… WOW! I did not know about such. Almost like I imagined. They do not use their limbs for digging. Their teeth are their digging tool, with 1/4 their muscle mass in their jaws. And, their teeth, nose, and skin are the important sensory organs. I wonder if their evolutionary path might take them along the lines of becoming limbless in a number of eons?
They sound like fascinating animals with their insect/termite like social organization.
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