Yesterday, Biblaridion published a YouTube review of The Book, in part based on an interview with me. I guess most readers will know about Biblaridion because of his large and long-running video series called 'Alien biospheres', in which he methodically discusses the subtype of speculative biology that deals with life elsewhere than Earth. He has a whopping 155,000 followers, so I was very happy that he wanted to pay attention to the Furaha project. We have met several times in person at TetZooCons and DinoCon.
I supplied Biblaridion with various images, most of which had not been published before. However, these unpublshed images are not new ones in The Book, because people should have a reason to buy that. I made a few new animations for Biblaridion, mostly of rusp skeletons. Rusps do not have a fixed length because of how the 'rings' in each segment are connected to neighbouring rings. Those connections functionally resemble springs at four connection points, so the rusp skeleton as a whole resembles a harmonica a bit.
Here is a video showing 'torsion' of the rusp skeleton. Biblaridion shows another one depicting lengthening of the rusp skeleton. But before that one causes any misunderstandings I hasten to add that rusps do not become significantly longer or shorter. The mean distance between neighbouring segments stays the same, so if the left margins of two segments approach one another, the right ones will tend to move apart. The reason for this is that the overall volume of the beastie does not change and neither does the circumference of the segments. Think of an earthworm that becomes thinner when it becomes longer. If hat worm, for some reason, could not become thinner, it couldn't lengthen either. I exaggerated the harmonica effect in the animation for Biblaridion's video to clarify how the skeleton works, not as an illustration of how rusps would actually move. Rusp lengthening is however large enough to make it difficult for students at the Institute of Furahan Biology to measure rusp length...
As an aside, human vertebral colmns do not have fixed lengths either. Our intervertebral discs become flatter during running or during the day and longer at night, adding up to a one or two cm difference.
For those new to my blog, please have a look. The search function allows you to find items on many SpecBio subjects.
Furthermore, there's my YouTube channel and an Instagram channel. If you wish to know how to order The Book, look here. If you order directly from Crowood and mention 'TCP10', you get a 10% discount. I'll also ask the publisher when the book becomes directly availble in the USA and elsewhere.
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Addition 29 January: I asked the publisher; the UK distributor may not deliver to Canada and the USA; the distributor to do that may only do so after six months (the book was published on 3 December, so that would be early June then. Blackwell's apparently has free shipping for a while, but I don't know for how long..
https://blackwells.co.uk/bookshop/product/Wildlife-on-the-Planet-Furaha-by-Gert-Van-Dijk/9780719845710
15 comments:
I've watched the YouTube video. On how many legs are best for large animals, an argument could be made that there have been in the history of life pretty big arthropods. I'm not an invertebrate guy, but it doesn't seem to me that large size was accompanied by any obvious reduction in the number of limbs.
On the leg front my personal suspicion is that given enough time walking limb counts will go down as a byproduct of repeated instances of centaurism. Look at hexapods(of the Earthly, as opposed to Furahan variety, heh), for example. Among their closest relatives are remipedes, which resemble aquatic centipedes with dozens of oar-like legs, and yet these have been repeatedly centaurized and specialized into mouthparts, antennae, gonopods, raptorial graspers, and other organs, to the point that there are insects today like mantisflies which only walk on 4.
On the other hand, at large sizes, I highly doubt that any accessory eyes beyond the necessary two would persist, at least not at the same complexity as the primary pair. Most damningly for multi-eyed spec evo aliens in my opinion is a very recent paper demonstrating a second pair of potentially fully-functional camera eyes in very early vertebrates, which the authors believe degenerated into the heat-sensing parietal eye of many fish, amphibians, and reptiles(which was also fully lost on multiple occasions, most notably by mammals, archosaurs, and turtles).
Davide; I agree. Overall, is there a noticeable trend towards fewer legs in any clade? I can't think of one, which suggests that regardless of whether you start with four, six, eight or many legs, that number seems to stick. There can be occasional examples of animals walking on fewer legs, but no systematic overall trends.
Idle 1: In my response to Davide I mentioned occasional examples, and I think that mantises are an excellent example of useful centaurism (they are also a very useful proof that animals of their fairly small size can get around with just four legs). But we don't see centaurism happen so often that the percentage of clades with just four walking legs within the overall insect clade is on the increase, as far as I know.
This raises another matter. Many arthropod mouth parts seem to have started as limbs, and some lineages have way more legs than others. There must have been a period (the Cambrian probably) in which such matters were very fluid; afterwards changes happened only as slower evolution, and in that phase I can't think of any evidence for gradual reduction of the number of legs.
Idle 2. I just searched for that: here is an article form New Scientist: https://www.newscientist.com/article/2512668-our-earliest-vertebrate-ancestors-may-have-had-four-eyes/
I haven't read the full paper yet. It's the Cambrian again! I like the idea that the discovery that early Earth vertebrates may have had four eyes came AFTER the publication of the Furaha book, because now vertebrates take after Furahan scalates, not the other way around ;-)
But I wouldn't dare conclude from that that this design was inherently inferior to two eyes. We have no idea whether four-eyes succumbed because four eyes are bad, because another trait did them in, or by accident. Not having any blind spots seems a good idea to me...
to Gert: yeah, I was about to send a link of the new find: https://youtu.be/Whr3jRYZwVs?si=vOPMFxLNMWkydgk2 with a good illustration at 5:50
also - Congratulations!
(that can never be said enough times)
to Idle Speculation - and yet most clades of arthropods *don't* have four limbs, even after repeated centaurizations. (though your argument applies to the numerous gills and gill arches of early vertabrates too - we lost most of them, but I don't know of anyone who would say that two gills are inherently superior to six)
Keenir/Gert: Gill arches are an interesting point of comparison! I never intended to imply that fewer limbs were inherently superior and I personally dislike that line of thinking, but I can understand how you would read my comment that way. What I was envisioning with my comments on centaurism was less that its rate among different clades would increase or that "centaur" clades would outcompete others and moreso that with time they would form a greater percentage of biodiversity as they radiated and their competitors went extinct, not due to any inherent superiority so much as ecological happenstance. This isn't necessarily a linear or consistent process; the Cenozoic saw quadrupedal mammals assume megafaunal niches after the disappearance of dinosaurs, which were ancestrally bipeds and continued to be in many clades, just something I could imagine happening since structures tend to specialize and reduce in number over time, and a trend I'd expect to see even moreso with groups with 3 or more pairs of walking legs ancestrally since they wouldn't have to sacrifice nearly as much in stability as a bipedal tetrapod to reap the same benefits of a freed-up pair of limbs.
Gert the Second: On the point of eyes I think that additional pairs provide diminishing returns for their energy requirements(at least at large sizes; arthropods get away with it constantly since things work a bit different at their typical scales tmk). A single pair of strategically-placed eyes can already cover an incredibly wide field of view when stationary as seen in e.g. horses, and being able to rotate them and/or move them around on a neck or eyestalks lets you cover even more. If an organism had non-rotating(say, tubular as seen in owls) eyes mounted at fairly wide distances from each other on an immobile structure like a cephalothorax I reckon multiple pairs would give you a lot more bang for your proverbial buck. A consequence of that setup I think would be interesting to explore would be having multiple zones of stereoscopic vision where the fields of view of each pair of eyes overlap. Maybe you could have an ankylosaur-like animal with a weaponized tail and larger rear eyes with improved depth perception to aim it, for example.
to Idle Speculation: ah, thank you for the clarification...I think we may (or may not) need to specify, at least from time to time, whether we mean locomotor limbs or not...a cheetah and ostrich both have four limbs, but an ostrich can only use two for display and cooling purposes, whereas a cheetah has to sit and pant like a dog to cool off. So yes to your earlier statement, a mantisfly has an advantage over, say a "typical" hexapod of its size (maybe an ant or stick insect) because of its centaurization...but the proto-mantisfly had the legs to spare to go down that developmental road: it still has at minimum four-legged stability post-centaurization...as opposed to humans and clubbing ibises https://planetfuraha.blogspot.com/2011/01/bird-with-club-and-some-other-smashers.html
to Idle Speculation: interesting idea about eyes and returns...but what comes to my mind aren't spiders with multiple eyes...its things like the Four-Eyed Fish. https://en.wikipedia.org/wiki/Four-eyed_fish
to All: I think Gert's right that we only have a sample size of 1...but if anything demonstrates that there is no inherently superior number of walking legs, its the fact that millipedes are born with one body segment and one pair of legs, and add to that over their lifetime. :D
ps: though just because that early "fish" had four eyes, doesn't mean it was related to us; no more than the Great White Shark or the Opah are close relatives of mammals, because they are warm-blooded.
Hi Tofer here, I wanted to comment about limbs because of the conversation above. From what i understand (and i could be wrong) all dinosaurs came from a two legged ancestor and many of them went back to four limbs, but only the herbivores for the most part, i would guess that was for stability for things like ankylosaurs and triceratops because a more “tank-like” animal being pushed over very bad, so a stabler animal is better. Then i would look at arthraplura which had alot of limbs and was a megafauna, so if i had to guess animals specialised for larger herbivory neiches have a tendency to favour more limbs. although as a rule of thumb, because animals like the the hadrosaurs, and kangaroos show this is not always the case, and birds have not found issue with such neiches with a bodyplan that would struggle at best to readapt a quadrepedal locomotive style.
I hope i did not make a complete fool of myself with my mediocre knowledge, and have a nice day
Tofer: I agree and would like to point out a few things that actually add to your argument. Hadrosaurs seem to have been obligate quadrupeds as adults, to the point of having a horse-like hoof covering their forefeet as preserved in fossil "mummies". Additionally, birds do seem to have a cap on their maximum size, although that likely comes down to their leg anatomy and the fact that their knees bear most of the weight instead of the hips.
Keenir: The thing about four-eyed fish that I think makes them a bit different from the classical multi-eyed spec creature is that the different sets of eyes aren't redundant since they're each adapted for a medium with a very different index of refraction.
to Idle Speculation: not sure how that makes them different from organisms that actually have more than two non-bifircated eyes. look at, say, jumping spiders, where different pairs of eyes have different distances they can see clearly.
Very nice discussion! Thank you all.
Idle (30 January): (on the utility of centaurism) I understand your point better now, I think. You raise a very believable point when you say that -more-than-four-legged animals might easier free two legs from locomotion than four-legged ones, because the latter face a much larger stability challenge.
(on the number of eyes). It is true that with just two eyes you can already cover a large part of the environment, is they are places just right to achieve that goal. Essentially that means that no body part should obstruct the view; the easiest way is to protrude the eyes on the side of the head or on stalks. Buit as you said, more eyes will do that with a less exposed position AND give you stereoscopy, even 360 degrees if need be.
As for your 'ankylosaur-like animal with a weaponised tail and larger rear eyes', if you move those eyes to the rear end of the animal itself rather than the rear of the head, you are basically describing a rusp...
Keenir (30 January) very nice point about millipedes gaining segments through life! What does that say about the mechanical importance of the number of legs. Probably that the number doesn't really matter that much for them!
Tofer (30 January). You have a point there: large herbivores then to have a large gut so they (well, their bacteria) have all the time they need to process enough food to make it all work. You can run around on two walking legs with a large gut (geese!) but four walking legs would certainly help.
Idle (31 January): I would think that it's having to fly that tends to limit large size in birds, rather than leg anatomy. That's new to me; aren't bird legs much like theropod legs that could grow very large?
Idle and Keenir: Interesting. On the one hand you can have multiple pairs of eyes that have clearly different functions (under or above water, different parts of the spectrum, wide angle versus telescope eyes, light-sensitive versus high-resolution, etc.) On the other hand you could have multiple pairs that all function in the same way but are pointed in different directions. But, biology being as messy as it is, you probably immediately get both aspects at the same time, destroying that neat division....
That messiness usually has to do with multiple simultaneous demands, and I think that that holds for the number of legs too. Obvious functional demands are weight-bearing, speed, stability, energy requirements for growth, repair and daily activity, and redundancy. Most commenters in Biblaridion's video come up with one such demand, but the demands form a multi-dimensional space. In such a space, different numbers of legs might fit in different spots where they form a local optimisation. But animals seem unable to simply add new pairs of legs, so many of those local optima are out of reach and will stay that way.
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