Monday, 19 July 2021

The great hexapod revolution and Furahan Fishes' evolution

 In the past I had remarked that I was trying to solve two evolutionary puzzles concerning hexapods, the last major animal group needed to finish The Book. Well, those puzzles were solved, so I am now busy with the Great Hexapod Revolution.  I worked on the puzzles off and on, and realised that there should really at least be a sprinkling of plants, small insect-like creatures and mixomorphs. These expanded The Book from 100 to 130 pages. I guess that number means I can safely lower the number the hexapod paintings to keep the book manageable.        

The 'revolution' means that there will be changes to the anatomy of just about every hexapod I ever painted. I will therefore revisit some old paintings and give them a makeover. The process also deciding which characteristics should be included and which had to go. Once I had a list of useful characteristics for terrestrial hexapods, the next problems was of course how they actually evolved.

That meant I went back to the drawing board for Furahan 'Fishes'. (I know that 'fish' can be singular as well as plural, but the English language also had 'fishes', in particular when multiple species are meant,  and Furahan biologists used the term in that meaning. Blame them, not me. )

Anyway, for those who are not up to date with Furahan cladistics, there are six groups of Furahan 'Fishes', numbered I to VI, for which example species had already been painted. The anatomy of Fishes I to III needed a bit of tweaking, and I did not like the paintings much anymore.

Click to enlarge; copyright Gert van Dijk

This was the previous, now discarded, image showing Fishes I. The shape is well visible, and the major Fishes I characteristics are there for all to see: two lateral membranes, no jaws, four eyes, and some respiratory openings along the bottom. As an illustration of these traits, it works. But it looked too schematic and a bit boring: a living animal will have peculiarities common to its species or even to it being an individual, and those were completely absent.


Click to enlarge; copyright Gert van Dijk

So here is the new picture showing Fishes I: there is a background to make the image more appealing, and the animal has more individuality, I think. I leafed through my cephalopod books and was inspired by iridescence and partial transparency of some species. I wondered whether I could pull that off, and think I succeeded reasonably well. Close observers will see that there are now openings on the back of the animal too; well, that is because Fishes I now come with four respiratory canals. It's part of the revolution...

I have never shown much in the way of evolutionary trees, and the ones I did were not meant to be included in the book. However, I thought that I should perhaps include one or two cladograms in The Book, so I made a table showing characteristics of the six groups of Fishes to help with a cladogram of Fishes I to VI.  

Click to enlarge; copyright Gert van Dijk
Here it is. It has the 'diagonal form' you often see in biology books. When I first encountered cladograms, this diagonal representation really confused me. If you start at the bottom, you reach most species by making some sharp turns, but there is one route up that involved just one straight line. Perhaps it was my strong preference for visual matters, but the relation between the two species connected by that straight unbroken line seemed much stronger than any route that involved zigzagging. But that is not true at all: the fact that there is a split (a 'node') is meaningful, not at which angle the lines depart from the node. It turns out I was not the only one who tended to interpret such diagrams the wrong way: students learning biology have more trouble with diagonal than with 'bracket' diagrams. Well, stop making diagonal cladograms!


Click to enlarge: from TR Gregory, Evo Edu Outreach 2008; I: 121-137

That point is well made in the diagram above. The source is free and very readable. If anyone else also has trouble with cladograms, dendograms or phylogenetic trees, including remembering the differences between them, I recommend this paper: it lists the 10 most common misperceptions of such trees. It is very clear. But I cannot help thinking that if there are no less than 10 common misperceptions, there are probably even more uncommon misperceptions, and then I start wondering if there is no easier way to teach evolutionary relationships.


Click to enlarge; copyright Gert van Dijk

So here is a very similar tree but now every line running up to a node is vertical before it reaches that node, and the two resulting descendant lineages depart from the node in a symmetrical way. It is much more intuitive, I think!

Some of you will have noted that, according to the cladogram, Fishes IV and VI have a more recent common ancestor than either does with Fishes V. In other words, Fishes IV and VI are closer related than IV is with V or V is with with VI. Oh dear! Shall I keep that in, and blame it on a mistake of early Furahan Biologists? Or I could just exchange the labels 'V' and 'IV'? Or I could go back to names I came up when I thought I might still need not just names for the Species and Genus, But also for the Familia, Ordo, Classis and Regnum (the Latin names of the groups of the old Linnaean system): Fishes I would revert to Clavifluitati, II to Gnatha, and III to Penpinnata. I will think of something.

Anyway, onwards with the Great Hexapod Revolution!  


Sockmonkey said...

One of my first thoughts on seeing the more "natural" version of that particular cloakfish was if it could undulate it's fins perpendicular to the body as well as parallel.
Being able to jerk sideways to dodge or hide without having to turn the body would be useful.
It would also enable it to circle something of interest while keeping all of it's eyes on it.

On another angle, imagine them having tiny rough scales on the underside of their wings so that amphibious cloakfish could grip and climb the Furahan version of mangroves.

Sigmund Nastrazzurro said...

Sockmonkey: many animals with undulating fins can perform neat tricks, such as moving all of the fins downwards at once, which propels the entire animals upwards. If one fin was kept stiff and the other did such a push movement, the animal would be shifted sideways. Or if waves on one end travel forward and backward on the other end, the animal turns around where it is. In short, such fins offer much manoeuvrability. I like the scales idea.

It is interesting you saw them as cloakfish, although I meant them as something else entirely. There might be a common ancestor there somewhere...

Unknown said...

Anthony Docimo saying:

imho, keep the numbers!

Maybe keep both the original Fishes I image and the cephalopod-inspired Fishes I replacement -- perhaps a comment like "At first, everyone decried the theoretical image of a species which had all the defining aspects of the Fishes I -- and then *this* was discovered, being stunningly similar."

*reads clade info* Zero Fishes (Fishes 0?) are like hagfish? or lancets? its a neat development either way.

maybe some Furahan scientists angle their clades, others use horizontal, still others vertical?

Unknown said...

ps: VERY glad to hear that you've come to (a) decision(s) regarding the legs and other variable matters regarding the Furahan fauna. kudos!

-anthony docimo

Sigmund Nastrazzurro said...

- For the time being I will keep the numbers. Furahan science is filled with big egos and other human weaknesses anyway, so a few mistakes here that have become part of history help to illustrate that it is all 'just done by humans', to paraphrase a Dutch saying.

- I like the idea of using the old image as a prediction that has later proven to be true. I will experiment with giving it an antique look.

- 'Zero Fish', or indeed 'Fishes 0', are indeed similar to lancelets.

- In view of my opinion that science is a very human endeavour, it is regrettably inescapable that some Furahan scientists will persist in using a cladogram style that is certain to confuse some of their students ;-)

- As for the decisions, what I did was go over all the evidence regarding leg shapes, including zigzagging, zagzigging, the number of joints and whether legs could 'flip' (meaning they zigzag moving forwards and zagzig when moveng backwards, and the somewhat boring conclusion was that there is not that much mechanical leeway to come up with a really weird movement. So the basic aspects of the legs will look familiar. The jaws, less so...

Unknown said...

from Anthony C Docimo:

had a thought. perhaps, instead of using the zig-zag of vert. legs, use the more arch- or bow- structure of spider legs. start out splayed, like vert.s -- and then rotate them forwards or backwards. just from playing with this in my mind, I suspect it'll be more stable if they face backwards (with the bonus of the organism being able to lean further forwards to grab food)

Sigmund Nastrazzurro said...

Anthony, let's see if I understand you correctly. I think you wish to have successive joints bend in the same direction, rather than having the direction switch for successive joints. It is the next bit I do not see in my mind's eye yet: would such a leg rotate around a vertical axis running through the hip joint?

Having all segments in a leg form an arc instead of a zig-zag does have a disadvantage: joints in the middle of the leg will be a distance away in a horizontal plane from a vertical line running down from the hip joint. In a zig-zag leg, the joints will stay much closer to that vertical line. The distance to that line has a large effect on the forces needed to keep the joint in a specific position. That effect becomes larger with mass, meaning it is not a problem for insects and spiders, but it is for larger animals. This is why I chose to go with zigzag legs for larger animals.

anthony (not docimo) said...

"fishes" of course humans would call them that, i guess it's just unofficial naming like "jellyfish", "starfish" or "crayfish"

Sigmund Nastrazzurro said...

anthony (not docimo): exactly! Apparently, any marine animal has the right to be called a 'fish'.

Unknown said...

I'm trying to think of a something non-aquatic, yet still called a "fish" somewhere...and I can only think of one thing that is suffishently close.

-Anthony Docimo.

ps: yes, suf-fish-ently (sufficiently)...i apologize for the horrible play on word pronounciation in English.

Sigmund Nastrazzurro said...

Anthony: Here you are:

And that is indeed a horrible pun! I suppose you know that the word 'fish' can be used to illustrate the very weak relation between the sounds of English and the written word:

Unknown said...

egads, the one time I miss silverfish! (for both the "forgot" & the "poor aim" definitions)

yep, I know Ghoti. but its always nice to see.

-anthony docimo

Unknown said...

I realized that there is a group of animals that have limbs like forwards-facing spider-legs: horses. {certainly human fingers work like that, and its the same bones and tendons in horses, unless i'm wrong, which is very possible)

(i thought it {the discussion} was in this thread...god, my brain is abandoning me)

-anthony docimo

Sigmund Nastrazzurro said...

Anthony: well, it seems we start counting limb segments in horses at different places ;)

For me, the vertebrate limb starts with the first segment that is movable related to the trunk. For the foreleg, that is the scapula, and it is basically oriented with the top aft and the bottom end forwards. Next is the humerus, angling backwards. Compared to that, the forearm (ulna plus radius) angle forward again, so that is a zigzagzig to me. For the hind leg, you get the femur, angled forwards, the leg (tibia plus fibula), angled backwards, and the heel bone, angled forwards again. That would be a zigzagzig too. I see these as the three major segments, and think their purpose is to achieve a long stride, while minimising joint forces.

Beyond them, you get a lot of short bones, such as wrist bones and toes/fingers. These can often bend both ways, but they usually flex further than they extend. I think they have various roles; they adapt the leg to uneven ground; they allow for non-locomotor work, such a fighting, digging or spreading weight; and to some extent they help in achieving a suitable stride length.

Mind you, the above is my mixture of various sources and concepts; I have not seem a biomechanics textbook dividing the roles between segments.

Horses just mess up this nice scheme, by putting stride extension above all other duties ;-)

Unknown said...

Anthony Docimo {me} would like to apologize for being unclear and confusing in my most recent post-reply to you...I had intended to say (yet failed to type) that, like horse bones, these could be reasonably straight when not being flexed for propulsive purposes, that straightness holding the body upright.

Oh horses are definately messing with things, whether or not they are made by committee. :D

ps: for the purposes of this, I ignored the zig-zag of the legs themselves. :)

my name is inigo montoya said...

It would be interesting if multiple lineages of "fish" independently made the move onto dry land. I wonder if they'd coexist in different niches? Or would one end up outcompeting the other entirely?

After all non-tetrapod fish like mudskippers and walking catfish have become amphibious, though are by no means a dominant land species...

Sigmund Nastrazzurro said...

Inigo: I think it would depend on the niches they take once on land. If the niches differ enough, there is no competition. Apart for niche, a second factor would be 'aptitude': if two lineages of 'former fish' are about equally good at whatever it is they do, one will not drive the other to extinction (just yet).