A little while ago I did some research looking at limb reduction in terrestrial vertebrates. It turns out, nearly all modern & fossil examples are one of two types, a long tailed “grass swimming” type and a short tailed “burrowing” type. These are examples of morphologic attractors like you describe - ThreadSky

spyyyder.bsky.social • 7 days ago

A little while ago I did some research looking at limb reduction in terrestrial vertebrates. It turns out, nearly all modern & fossil examples are one of two types, a long tailed “grass swimming” type and a short tailed “burrowing” type. These are examples of morphologic attractors like you describe

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spyyyder.bsky.social•7 days ago

Another interesting fact is what sorts of animals tended to develop these “morphotypes”; in the case of terrestrial limb reduction, it’s only amphibians, stem reptiles, and lizards (with 26 instances!). No mammals or archosaurs developed this morphotype, which suggests that certain traits …

spyyyder.bsky.social•7 days ago

… seem to predispose a lineage towards or away from certain body plans. My hunch here is that it has to do with size or metabolism, with both archosaurs and stem mammals being larger on average and with higher metabolic rates. I imagine that in the case of 6 limbs, similar patterns would apply!

polyducks.bsky.social•7 days ago

It also seems that insects have leg lengths which vary depending on the surrounding parts, with beetles having long back legs which avoid the middle legs and hug the abdomen. Lifestyle, body shape, and balance would influence leg layout. (I am so linguistically out of my depth between you two)

lorenschmidt.bsky.social•7 days ago

it's ok, i'm not a specialist i just like looking things up and then confusing everyone by using a weird hodgepodge of technical terms and stuff i make up haha.

lorenschmidt.bsky.social•7 days ago

yeah you're right! a lot of beetles seem to have really large rear legs. i'd guess they're often specialized for pushing forward but i have not read about this.

i've studied insect gait a little and there are multiple gaits, but it seems like the foundational one is moving in two sets of three.

lorenschmidt.bsky.social•7 days ago

parallel to a canter in a horse. so you do get collision issues! and i'm sure long back legs do help a lot with that, that's such an interesting idea.

lorenschmidt.bsky.social•7 days ago

i've never researched that! to clarify, is this the sort of process things like legless lizards, caecelians and snakes go through? i love 'grass swimming' as a type.

spyyyder.bsky.social•7 days ago

Yes, exactly! Those are the most known examples, and certainly the ones with the highest degree of limb minimization. The figure below shows the two “ecomorphs” (sry, not morphotypes) described in an excellent paper (cited in alt text). Caecilians are the burrowing type, for example.

spyyyder.bsky.social•7 days ago

Also, just to make sure: are you aware that caecilians make a milk-like substance to feed their young? It’s one of the weirdest and best facts, imo, so I’d be remiss not to bring it up.

lorenschmidt.bsky.social•7 days ago

i'm reminded of some research into size limits in dinosaurs. i read a paper which says that there are some semiarbitrary plumbing particulars (heart, respiration, and the routing of some blood vessels iirc?) which make it hard for mammals to get as large as sauropods.

spyyyder.bsky.social•7 days ago

Yeah, although I’m not the most up to date on dinosaur stuff, I believe the current thinking is that all dinosaurs share the more efficient breathing cycle that birds do, with unidirectional lungs, as well as a high metabolic rate, which helped some of them grow so large.

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