"Why no hexapods ? " Topic

Hi guys

just a strange thought occured to me. In sci-fi and fantasy, there are often 6,8 or more legged creatures. On Earth, we have octopus and squid in the sea, but on land everything bigger than a dinner plate has 4 legs or 2 legs+2 arms/wings. Obviously insects went with 6, arachnids 8 and centipedes etc with loads, but due to respitory set-up they have been small since the carboniferous. All higher life on land has gone with the 4 limb design. Is everything bigger than a bug evolved from the same mud crawling fish that ended up with 4 limb buds and it went from there ?

Phil

We're all, mammals, reptiles, birds, amphibians, descendants of the same vertebrate species that explored the far, drier, reaches.

There's some other stuff out there, but probably not what you'd call higher life.
(The invertebrates who have more specialised breathing apparatus, which under present circumstances (lower amount of oxygen in the air) are limited to small sizes could probably have evolved variants that function on a bigger scale. But why bother. Size doesn't matter, in the end.)

As to why it started a lot earlier, but most of the other species were competed out. There were many more and varied forms of life back then.

It's a lot like unbridled capitalism. You start out with lots and lots of small independent diners, trying out and serving lots of different dishes, but competition leaves but a few big chains serving the same dishes in the same manner.

We vertebrates are the McDonalds of life.

(Well, except that by many methods of measuring, like number of species, individuals, biomass, and perhaps leaving an imprint on Earth, we're not that successful.)

All tetrapods share an ancestor, which branched away from the lobe-finned fish.

Fun fact is that they developed quad features long before they emerged to dry land, as it was a handy method of moving about the sea bed vegetation. Other fishes "levitate" in the water and are vulnerable if they stray too close to the bottom.

Tetrapods were not the first creatures to emerge on land, Arthropods did it long before them, as it allowed them to lay eggs in a mostly predatorless environment.

Tetrapods are indeed a tiny minority, but their body structure is very efficient and they have proven to be very adaptable to most environments (land, sea and air) and because of their size they are among to most visible land animals, but Arthropods have us badly outnumbered.

Insects have 6 legs, but are structurally ten-limbed. Some species have lost obvious wings, but two pairs of wings are part of the default for insects.

The few true hexapods on Earth are radials, if I recall correctly.

This is a subject that greatly interests me, though I am by no means super well informed.

My understanding is that the upper size limits of any particular body plan has to do with the ability to extract food from the environment and the limits of the energy available from it. Also, according to my favourite paleontologist, Peter Ward in Out of Thin Air (Google synopsis and reviews) the level of oxygen in the air also has considerable effect on size limits in creatures' body plans.

There's evidence of widely varying oxygen levels in prehistory and some of the troughs correspond with known mass extinctions. Animals that evolve to thrive in the low oxygen periods (say early dinosaurs in the Triassic) just explode when oxygen levels peak and their sizes increase drastically (again dinosaurs in the late Triassic and most of the rest of the Mesozoic). There's fossil evidence of spiders with individual legs of 18 inches in length! (that always makes me shudder to think of it) during higher oxygen phases in the late Paleozoic before the major end of Paleozoic extinction, where oxygen levels plummeted (the Permian mass extinction).

Apparently the issue with arthropods ad their multi-limbed body plans has to do with their respiration system and how they rely on oxygen permeating through their interior. The oxygen levels have a big effect on this. With more oxygen in the air, the body can support a larger mass (IE, the spiders of today versus the <screech!> 18" legged monsters of the late Paleozoic – dang can you imagine a more than 3' wide spider scuttling towards you? D&D in real life!!!!).

Anyway, I've gone on about Ward's oxygen theory for the past two paragraphs – I read his book some years ago when his theory was new and I don't know if it has held it's own in the often hostile world of science. But it sure as hell is interesting!
--
Tim

Thanks guys

thats what I thought, but it is alway good to ask the hivemind.

Phil

There are the echinoderms, such as Starfish:
Radials, as per gypseycomet's comment
larger than a dinnerplate;
but obviously: sea-life, not land.

Wingless, that is 'true' hexapods only include the arthropod orders
springtails, diplurans, and proturans
Not Bigger than a Dinnerplate, no.
Springtails: en.wikipedia.org/wiki/Springtail

Diplurans: en.wikipedia.org/wiki/Diplura

en.wikipedia.org/wiki/Protura

Different from Insects due to the arrangement of mouth-parts
(Internal vs External {eg mandibles})
I don't recall if they have a developmental stage w/ wings (re gypsycomet)
but I don't believe so (if memory serves)…

It is, however: Theoretically Very Possible:

Arthropleura

A bit larger than your average dinnerplate, unless you are related to Pantagruel and Gargantua.

Patrick, probably from the carboniferous, when higher oxygen levels would support larger creatures without lungs, allowing for huge insects and big gribbleys like the one above.

Hallucigenia, a fossil found in the Cambrian Burgess Shale, has seven pairs of clawed "tentacles" and seven pairs of spines. It's only a couple of cm long though, and it's found in assemblages of known marine creatures.

Highly speculative reconstruction here:

link

An image of one of the fossils here:

link

It's just as likely that the fossil is simply part of a larger animal that we've not found yet. The fossil anomalocaris from the same deposit was originally thought to be a type of shrimp but turned out to be the mouthparts or of something much bigger.

It's history, if it's one thing we have learned from taxonomy, it's you can't get rid of your past.

Humans are primates, becasue we evolved from primates, humans evolved from monkeys, so technicaly we are still monkeys,

But you can go further, since we evolved from lobe fined fish, we are still lobe fined fish.

Humans evolved from tetrapods, so even tho we walk on two legs, we are still tetrapods, and so is our skeleton, thats why humans often get back problems, our skeleton is evolved to walk on fours, we aren't actualy ment to walk on two, this means we put strains on our back that a dog or cat or horse dosn't.

Hell even snakes are tetrapods even tho they have no legs at all. You can't get rid of your ansestry.

Of and on a side note, you do get decapods that are quite alot bigger then a dinner plate and lives on land

The coconut crab, can weigh 4kg, and is the size of a small dog.

Humans are primates, becasue we evolved from primates, humans evolved from monkeys, so technicaly we are still monkeys

I think you mean apes, Gunfreak. We are a little farther removed from monkeys.

I'm not sure the Earth ever had enough atmospheric O2 to diffuse through the chitin successfully, to allow something like the creature in Patrick's picture to exist.

Arthropleura fossils have been found up to 2.5m long, and tracks that are thought to belong to them exist preserved in would have been shallow mud.
link

The pictured example is probably a bit longer and wider than evidence we've seen, but Arthropleura did get pretty big.

"I think you mean apes, Gunfreak. We are a little farther removed from monkeys."

Nope monkeys, yes we are apes, but apes is a subset of old world monkey, so while all apes are monkeys not all monkeys are apes.

Back in the early Jurassic when I was a student, one of my lecturers described a theory that fish are most efficient when they swim if they manage to curve their body into a complete sine wave (S shaped curve, so with 2 bends in it). Or a multiple of that – 2 sine waves (4 bends), 3 sine waves, etc. One and a half sine waves (3 bends) was less efficient.

On top of this was the fins – it turned out that the fish with a sine wave curve was most stable in the water with 4 fins (2 pairs of 2) for steering. As those 4 fins are the ones that became legs in the land vertebrates, the lecturer speculated that if vertebrate fish had evolved on other planets, their land living descendents would always have legs in multiples of four: 4, 8, 12 and so on.

Meanwhile, giant insects in prehistory… as well as the high oxygen levels, there was the fact that they didn't have much in the way of competition back then. Vertebrates hadn't figured out how to eat land plants so the only big herbivores on land were the arthropods, such as the millipedes like Arthropleura. And since all the first land vertebrates were amphibians, they were probably tied to water to a greater extent than the arthropods were, so they might not have been able to colonise drier areas of the forests.

Once the amphibians had evolved a toad-like lifestyle that can stay away from water for a long time, the giant insects were in trouble. Especially if the hungry carnivorous amphibians could now get to the safe place you go to shed your skin!