Wednesday, 1 September 2021

Trying to sculpt a dinosaur (Sauropelta)

This blog and the Furaha project are about speculative biology, so there is no reason to talk about dinosaurs. Then again, you could argue that there are so many unknowns in palaeontology that it is to a large extent a speculative discipline. 

Anyway, I decided to sculpt a dinosaur with polymer clay. Why polymer clay? It is perfectly possible to sculpt digitally, with more detail, and cleaner too. I do so habitually to help me decide on shapes and perspective. And if I would want a physical object, I can order a 3D print. But physical sculpting has its own rewards, and sometimes I would like to have a nice statuette of a rusp or another Furahan animal. 

Why a dinosaur and not a rusp? In the distant past I had used 'Fimo' and 'Das' to sculpt animals, but I was never satisfied with the ease of use or the level of detail. Still, I kept an eye on what people did with Sculpey and similar materials. There are people on YouTube, such as 'Kayakasaurus', who make very nice dinosaurs in polymer clay. I thought that my first attempt would probably not be very good, so I chose something I did not care about that much, which may sound strange. But anyway, that is why I chose a dinosaur. Perhaps foolishly, I chose a material that looked interesting but for which there were not many didactic video's: Cosclay. That is a polymer clay that after baking was supposed to stay flexible, allowing very thin parts without risk of breaking. The video shows that this was indeed the case.

 


The video above shows the result of that experiment. I have also uploaded a version with better resolution to YouTube. 

I will leave it to you whether the attempt was successful or not. Among the things I learned was that I need to have much better control over details, and that means I need a working environment with better light and preferably a magnifying glass. 

The working method was copied from YouTube videos. I started with a nice skeletal drawing by Gregory Paul, from his excellent book 'The Princeton Field Guide on Dinosaurs'. In spite of the title, it is not a Field Guide; how many field guides preferentially show animal skeletons? 

Click to enlarge; copyright Gregory S. Paul

Here is a drawing of a Sauropelta skeleton from that book. I used the trick of printing a scanned image on the desired size, so I would end up with a roughly 1:16 scale model: the model is about 25 cm, half of which is tail. 


 

Click to enlarge; Carpenter K. Can J Earth Sci 1984; 21: 1491-1498

I also had a quick look at some published papers on Sauropelta. Above you see drawings from a formal 1984 paper. Compare the posture and the length of the spikes to the drawing by Paul. The fossils may not have changed much, but the reconstructions certainly do. What I like about the 1984 paper is the emphasis on how close the left and right feet are to one another. I tried to do that in the model, but probably still used too large a distance. 

Click to enlarge; Brown et al Current Biology 2017; 27: 2514-2521
 

And here is a recent drawing of the Sauropelta skin bones that form the large back shield (the 'pelta') . I only found it after I had already baked the final model, so the shield bones on my version are much too big. An accurate dinosaur model would require a large amount of study. Looking at other reconstructions makes me think that most reconstructions rest more on speculation than on science. Mine included! 

But at least I think I am ready for a rusp sculpt now. But first, more painting, and perhaps a stegosaur sculpt, just to be certain.

Monday, 16 August 2021

How global warming changes the future's past in the Furaha Universe

'The Book' starts with some explanations about the planet Furaha, how humanity got there, why anyone would choose to go there, as well as some other background material. The image above gives the reader an idea how large the planet and its two small moons are in comparison to Earth and its large moon. A variety of techniques were used to make it. First of all, for something like this a 'digital elevation map' (DEM) is needed, showing elevation on a grid of longitude by latitude. Such maps are easy to find, for Earth that is, and Matlab's mapping toolbox offers many ways to play with maps. Obviously, I had to make my own DEM for Furaha. The intermediate stage is to make a nice digital map in which colours represent elevation. Second, take a suitable 3D-program and set up a scene with spheres in it. I still use my 2014 copy of Vue Infinite to do so. Then you just wrap the maps around the spheres, set up some light, and you get a nice picture comparing two globes, one representing Furaha, one Earth. However, I increasingly felt that the image I used for Earth might not be appropriate for the future. Mind you, I have never settled on a period in which the future history of Furaha is to take place. Is it the 24th century, like Star Trek? Sometime earlier or later? I do not know and do not care too much. However, the Institute of Furaha Biology is at least some 200 years old, so at least two centuries are needed for that. Moreover, a 'Faster-Than-Light' drive, or an 'Around-Light' drive for that matter, does not seem around the corner, so perhaps we need to give that some time as well. Overall, we are probably looking at at least four centuries. Would the overall contours of the continents look different? The recent report on global warming makes me wonder how much sea level might rise. Humanity is causing global warming, and it is too late to prevent all of it, but part of it can be prevented. If people act in time and wisely, that is. Will they, or will it be a case of too little, too late? The Book is a science fiction project, so the choices are fictional. In the Furaha setting, in that far future, wrong choices were made. All of the Greenland ice melted, and all Antarctic ice, and so the sea rose. By 70 meters. I found that number on the internet as a rough estimate of what would happen if all the ice melted. Wikipedia tells me that Greenland is good for a 7.4 meter 'sea-level equivalent', and Antarctica supplies 58.3 meters, so together they account for 65.7 meters. That is still a sizeable amount.
I took the elevation data and adapted them to reflect a 70 m rise. I then had to edit some areas manually, because the program found some inland areas that it now regarded as 'sea', even though those areas had no access to the oceans. Large parts of the UK are gone, as well as the Netherlands, Denmark, Bangladesh, etc. No current coastal city would escape unharmed. I have no idea how such a change would affect the climate, pollution of the seas and myriads of other matters. In the Furaha universe civilisation endured, because otherwise I would have nothing to write about. But I think these fictional future people would look back to their past, our future, with perhaps some new hard-won wisdom. The citizens of the Furahan Institute of Biology passed a unanimous vote to change the formal scientific name of mankind. They abandoned the old familiar name 'Homo sapiens', meaning 'wise man', and changed it to 'Homo semisapiens': man who is half wise. They felt that wisdom must have two parts: the first consists of the ability to think, and the second requires that those thoughts are acted on. According to the Institute, humanity passed the first criterion, but failed the second.

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!  



Friday, 11 June 2021

‘Wadudu Castles’, or Furahan 'termite mounds'

I needed a background for a painting.

Every chapter in The Book at present starts with a double page showing a painting supposedly made by someone in the Furaha Universe. These paintings are all about humans on Furaha. Some are portraits of dignitaries, whose illustrious career earned them the privilege of forever having their countenance grace the Halls of Academia. Or, of course, until someone else comes along with even more vanity and money. Such portraits do not need much in the way of background; a generous helping of 'Van Dyke Brown' will do. But other paintings show Field Work, often in a much-romanticised version, and those are the ones that need a background showing some Furahan Biology. This one starts with wadudu. 

Click to enlarge; copyright Gert van Dijk

‘Wadudu’, as astute readers may remember, are small Furahan animals, somewhat resembling insects. If not, see here and here. Some wadudu may be communal species and build their own living arrangements, like termite mounds. Some early doodles of possible wadudu castles are shown above: these are ‘Mark I Castles’. The sketches show roof-like outcroppings or ridges along their surface, with a vague idea that they could protect the structure against the sun, or perhaps against rainfall or even snow. Termites are limited to hot regions, but there is no reason to apply that rule to wadudu castles too. 

Click to enlarge; copyright Gert van Dijk

I made a quick colour sketch of one design, shown above. This ‘Mark II Castle’ is not that different from a 'Mark I'; it has an elongated shape and is bilaterally symmetrical. The narrow side points towards the sun during the hottest times of the day, to limit the surface area that is directly insolated. The rootlike structures running along the ground are hollow cylinders that function as highways allowing the beasties inside easy access to wherever they need to go. Different castles may be connected by such cylinders, both on the ground and below it, allowing communities to link up and form even larger superorganisms. The overhanging roofs were smooth and white, to reflect sunlight and to protect the castle from heating up too much. 

 



Click to enlarge; From Claggett et al. J Struct Eng 2018; 144: 02518001; DOI: 10.1061/(ASCE)ST.1943-541X.0002043

The wadudu would have to produce different materials for that to work, so I had a look at Earth's termites. It turns out that termite mounds can have very different shapes (see above). The paper I copied the images from (Claggett et al DOI: 10.1061/(ASCE)ST.1943-541X.0002043) differentiated between conical, dome, cathedral, mushroom and meridian or compass mounds. The differences depend on differing circumstances and requirements to control temperature, humidity and ventilation. 

Click to enlarge; Oberst et al. J R Soc 2021; 18: 20200957;  https://doi.org/10.1098/rsif.2020.0957

It runs out that termites indeed alter the soil they use to build their mounds with. Termites use grains of sand or other materials and saliva to form 'boluses' that are then cemented together. They may use coarse granules in the centre of walls, and finer ones for wall surfaces. The termites can even mix clay in the boluses to provide water-resistant walls. With all this going on, it would not be a stretch to provide Furahan wadudu with equal or better capabilities to produce different materials, from watertight layers to highly reflective ones. That opens up some really interesting possibilities, so I think I will return to wadudu castles in the future.
 

The bilateral symmetry of the Mark II may not have been optimal. For most places on Earth, the sun will never be directly overhead. The axial tilt of Earth ensures that the sun will generally be towards the South if you are on the Northern hemisphere, and vice versa. This means that a sun shade does not have to be symmetrical in the North-South direction. The smart way would be build a shade that is angled in accordance with the direction of the sun, something that will depend on the latitude (and the rotation of the planet's axis). This is very similar to the way solar panels are positioned. Solar panels absorb light, but the shades should reflect as much of it as possible. They should be smooth and as reflective as possible; a glaring white would be nice. Depending on the climate, the nights might be cold, and in that case part of the castle could welcome the sun's rays early in the morning. That part could even be dark so the castle can warm up. The structure at the opposite end would catch the last rays of the setting sun, so you might think that the castle could do with some warming up before the night. Possibly; I assumed that the castle would warm up during the day regardless of the shades, so the roofs may need to protect against the evening sun too. The roofs could bend down towards the direction of the setting sun to do just that.  

Click to enlarge; copyright Gert van Dijk



Click to enlarge; copyright Gert van Dijk

I sculpted such a Mark III castle in ZBrush: here it is, shown from several points of view to highlight its asymmetrical shape. Instead of one roof, it has several. Imagine that it is about 3 meters high. The roofs should be white, whereas the rest of the structure will probably have the colour of the sand it is built from. 



The videos above show such a castle. Unfortunately, I could not colour it correctly for the video. I wished to show how it looks while the sun moves from sunrise to sunset in one smooth movement, but that did not work well. I had defined a few positions for the sun, but the software treated them like stops on a train ride: the sun now reaches a place, stops ('all aboard!') , and departs for the next place. I should do it again, but the point was to produce a painting, not a video. And I can now paint that background.
    
By now you will probably be wondering what the castle builders look like. Actually, I don't know yet. I haven't yet decided whether they the castles are built by wadudu, spidrids, or by another group altogether. Shouldn't I have decided that before I designed the castles? I think not: I doubt that anyone could deduce termite anatomy by studying a termite mound, other than that the builders are small, can move grains of sand around and stick them together. That'll do for now.









Thursday, 29 April 2021

Zoom interview with Dougal Dixon tomorrow

 I received, courtesy of Dougal Dixon, an invitation from Oscar Salguero, a book curator from New York, to view a Zoom presentation given tomorrow by Dougal. Mr Salguero assured me anyone can register for free. Because time is short I will just pass on the information.

 

 "Mr. Dixon will give a live presentation highlighting some of his most important works on speculative zoology from the last 40 years. This will be presented via zoom and it's sponsored by the Center for Book Arts in NY."

 Friday, April 30, from 1 - 2:30pm EST

That should make 19:00 -21:30 hrs CET, and 18:00-20:30 hrs for people in the UK  

 Here is the event (click on "Register" button to receive the zoom link):

https://centerforbookarts.org/calendar/talk/speculative-zoology-fictitious-examples-of-factual-processes

Facebook page:

https://www.facebook.com/events/300204934901648

 ----------------------------------------------------------------------------------------------

 After the event

 This is just a short update for those who are curious about Dougal’s work.

In the talk, Dougal mentioned that there would be a 40-year anniversary edition of ‘After Man’, that should be published before the end of 2021. I understood that it will have more material than the books published so far, including sketches.

Another interesting bit of information was that there were Japanese model kits of some of the creatures featuring in ‘After Man’. Dougal showed a box with content of an unassembled kit of the Night Stalker. I was curious and wondered if I could find out more about those kits (secretly hoping I might still order one from some forgotten Japanese warehouse).

 


Well, I found two entries on a site of a company where you can indeed order such things, but the items were sold out. There were a few images, and I chose the one above, because it showed the contents of the box.

 



The two images above are were posted by a collector on a site called ‘dinotoyblog’. Interesting, aren't they? Alas, I found no hidden stores of such models...

 

Thursday, 22 April 2021

Terra Ultima by Raoul Deleo

 A friend called my attention to an art project she had read about in her newspaper, and she thought I would like it. I certainly did and decided to write about it, even though it is not a typical speculative biology project. The project is much more about the art than about science, and has a strong poetic quality. But foremost, I think, it is work of extraordinary quality.

The project is called 'Terra Ultima'. There is a nice site showing quite a few paintings, as well as a Youtube channel, with some videos showing the artist at work. The background story is that the artist, Raoul Deleo, has visited the continent of Terra Ultima several times and came back with the sketches of its wildlife to prove it. 


Here is a nice video showing the background story. Raoul lives only about 35 km from where I live, has been working on his project since at least 2015, but I managed to remain completely ignorant of his work until recently. I was just in time though, as a book about the project has just been published. In case you are interested you should know that at present it is in Dutch only, but Raoul told me that a French version is already in the works, and English-language publishers have already showed an interest. Let me just jump in and give you some examples.          


Click to enlarge; copyright Raoul Deleo

The 'Pallid Octopossum' (Octopossum leucostolum). I chose to start with this one, because terrestrial cephalopods are a Big Thing in speculative evolution. A long time ago, I argued that the tentacles of walking octopuses (yes, I know about the plural 'octopodes', but this sounds better) would turn into legs with joints (here is the first post of a sequence; to find the rest, just type in 'walking with tentacles' in the blog's search window). The arms need not evolve a skeleton if they mostly have to deal with tensile rather than compressive forces, so a brachiating animal might still have tentacles (or a cernuating one). This cute fellow's furry tentacles would fit the bill.

Before anyone starts complaining that this animal cannot possibly have evolved either from a cephalopod or a mammal, I agree. The combination of fur, external ears, a long probably sticky tongue on the one hand and tentacles with suckers on the other tells us that this animal is a composite.

Regular readers may remember that I had my doubts about the 'mix a monster' approach used in the film and game industries (examples here and here). At the time, I concluded that that was one of the defining differences between the worlds of 'speculative biology' and 'creature design': the first frowns upon approaches that are contrary to science, whereas the second couldn't care less, as long as the results looks cool and sells. I try to limit my protesting against a lack of scientific rigour only when people make unjustified claims about scientific quality (I am a scientist; that is what we do!).

Click to enlarge; copyright Raoul Deleo

A gecko! Sort of anyway (Erminogecko mollidactylus). It is a hexapod! There's fur, external ears again, three pairs of legs with four toes each, so this animal is also a composite. Have a close look at its potential prey. In an interesting twist, this creature has one pair of membranous insect-like wings, as well as two bird-like legs, and camera eyes, not compound ones. It is almost the reverse of the geckoid, so the illustrator is letting the viewer see that he knows what he is doing, and that the mixture is part of the fun.

There is probably a third school of 'design an animal', and this particular one is probably much older than creature design or speculative evolution: it is 'animal fantasy'. I guess its ancestry includes the fantastical animals bored mediaeval monks drew in the margins of their books, as well as the beings Hieronymus Bosch painted to make people scared of Hell.

Click to enlarge; copyright Raoul Deleo

A 'lieveheersbeertje' (Coccinellursus hexapedus). A 'lieve-heers-beest-je' is literally a 'Good-Lord's- beast-diminutive suffix'. That's Dutch for ladybug. A 'beer' is a bear, so the name is a nice pun combining 'ladybug' with 'bear'. I wonder what the translators will do with that; do any readers wish to take up the challenge?

The animal is again a composite hexapod. Fantasy animals mostly have to look good, and does this one look good! I really must work on my own fur painting skills. Have you noticed that the animals are all posed in such a way that their anatomy is well visible, without awkward foreshortening and with good lighting, so there are no shadows obscuring part of their shape or colour? That style reminded me of James Audubon.

Click to enlarge; copyright Raoul Deleo

Here I put together Raoul's 'Flamingo fawn' (Phoenicopterus cervocephalus, which conforms to 'deer-headed flamingo') next to Audubon's flamingo. Look at how the head is turned around, so the shape of the animal just fills the available space. Raoul confirmed that Audubon was a great inspiration, so that explains part of the atmosphere of these paintings. 

 

Click to enlarge; copyright Raoul Deleo

The 'sheen green harefly' (Libellula lagoformosa). An insect-mammal composite showing a wonderful mixture of forms and colours. I cannot help wondering how it would fly, with those four wings at its very top. I guess it might look a bit like a tetrapter in flight, although the harefly obviously has bilateral symmetry. 

Click to enlarge; copyright Raoul Deleo

The 'headwinged penguin' (Pygoscelis cephalopterix). Humour is one of the best ways to avoid people taking your work overly seriously, and Raoul's work is suffused with a mild humour. Penguins look charming to begin with, and equipping penguin heads with functional (!) wings puts the work into the proper perspective, I think.

Terra Ultima is a charming and very entertaining animal fantasy. Even if you prefer a diet of rigorous scientific speculative evolution, the Terra Ultima site is definitely worth a visit, if only to be inspired by the extremely skilful paintings.     

I have already ordered the book, and will keep you informed on this blog when there are versions on other languages.     
   





Sunday, 7 March 2021

Explaining and animating how cloakfish swim

In the two previous posts I wrote about parts of The Book that provide background information about how animals move on Furaha. There will be four double-page spreads showing such themes in The Book, about rusps, spidrids, tetrapters and cloakfish. 

I thought the cloakfish one would be easy, until I decided that it was high time I also made some progress towards a short CGI documentary I wrote about earlier, the one with cloakfish biodiversity as its main subject. That is a very big job and it is quite possible that I will fail on the programming side. But nothing ventured, nothing gained, so I went ahead and put some hours into Matlab programming. My strategy to design diverse cloakfish is to write editors that allow shapes to be designed with ease. The programmes then proceeds to make ‘3d meshes’, the basic working material for 3D design. How to get from meshes to nice photorealistic images is another story altogether. 

Click to enlarge; copyright Gert van Dijk

These are the editor screens. The user places control points here and there, which are then connected by a spline function. This results in nice smooth curves, useful for organic shapes. Panel A shows the body designer with a default shape. The inset shows a separate window controlling cross-sections of the beastie. By changing both shape and cross sections interesting forms can be produced. Panel B is the cloak editor. Apart from determining the shape of the cloak it also allows control over cloak movement, such as number of waves, wave amplitude, cloak curvature, thickness, etc. Panel C does something similar for the four front fins, and panel D shows the resulting output for the default shapes. 

Click to enlarge; copyright Gert van Dijk

With a few minutes’ worth of tinkering, you get this relatively slender cloakfish, probably a reasonably fast swimmer. 

Click to enlarge; copyright Gert van Dijk

Or this short and bulky ‘short-sleaved cloakfish’.

Click to enlarge; copyright Gert van Dijk

Or even this highly derived cloakfish, in which the cloaks are no longer ribbons, but shaped like penguin wings. They function in much the same way. 

 But the main point of this post was to show how the cloaks move. To do that, I had a look at the literature, and I found some papers on knifefish, but to my surprise more papers about artificial robot fish with similar fins. Apparently, people all over the world are working on robot fish, which is nice. Less nice is that some work at defence institutes, so what are they preparing for? Killer fish robots? Must we really? 

Click to enlarge; from: Liu, Curet. Swimming performance of a bio-inspired robotic vessel with undulating fin propulsion. Bioinspir. Biomim. 13 (2018) 056006 1748-3190/aacd26

Anyway, here is an example of robot ribbon fin design. The artificial cloak design followed exactly the same reasoning as my virtual cloakfish designs. Such papers make a distinction between ‘oscillation’ and ‘undulation’. If a cloak, ribbon or fin swings side-to-side as a whole, the word ‘oscillation’ is used, and when waves travel along the length of the cloak, it is ‘undulation’. But the distinction is not all that clear; it depends on the number of waves travelling along the cloak. If there is less than one wave on the cloak or fin at a time, then the movement largely concerns the fin as a whole, so the movement edges towards oscillation. Here is a YouTube film explaining the difference

Pure oscillatory sideways movement are useless, because they do not propel the beast forwards. The animation above shows such an almost pure sideways movement of the cloaks. The movement would push water away from the cloak, resulting in a force towards the attachment of the cloak (the ‘dagger’). In knifefish, with just one cloak underneath the body, this force ‘heaves’ the body up. With four cloaks, the dagger will not be going anywhere, so this is just a waste of energy. We want water to be pushed backwards. The cloaks push water backwards when its parts are at an angle to the direction of movement. These parts produce forwards thrust. 

Let’s equip our default cloakfish with exactly one wave per cloak. One half of that wave will be angled towards the left, and the other half towards the right. Both produce sideways forces, but these should cancel one another out. The robot designers reported that the robots swam nicely with just one wave per cloak. Mind you, the robots usually had just one cloak, like the knifefish. 

The animation also shows one other trick: if you look closely, you can see that the cloaks do not sway much at front, and the amplitude of the wave increases towards the back of the animal. I borrowed that from real biology, as at least rays and knifefish do this. 

Let’s now equip our knifefish with 2 waves per cloak. The parts of the wave that are useful for swimming are now at a steeper angle towards the direction of movement, nearing perpendicular to it. I thought that this should increase the propulsive force a lot, but work on the robot fish did not agree. The velocity did not increase much, but the robot was more stable, which is intriguing. Real knifefish have more than two fins on their ribbon fins at one time, so there must be an advantage in that. 

Click to enlarge; from: Blevins, Lauder. Rajiform locomotion: three-dimensional kinematics of the pectoral fin surface during swimming in the freshwater stingray Potamotrygon orbignyi.  The Journal of Experimental Biology 2012; 215, 3231-3241

This image is from a paper about ray fin movement. The fins as a whole move up and down (so they oscillate), while there are ripples along the edges of the fins (so they also undulate). Nature seems to like combinations better than separations. The edges of the fins curl up and down, so they do not move as if there are completely stiff rays in them. 

I decided to build that in too, so here is a ‘curly-cloaked cloakfish’. I like it. One odd thing about these cloakfish animations is that it is not immediately obvious how they work, when you see the movement. It is also not easy to find an angle, when you rotate the objects, from where it is easy to get an immediate overview just how the animal is built. That can be seen as a disadvantage, or, in reverse, as an advantage, because it underlines that we are looking at an alien shape. 

By now, cloakfish ‘evolution’ has progressed to allow a variety of body and cloak shapes. Shapes range from ‘long-sleeved’ cloakfish with long narrow ribbon fins with multiple waves along their surface, to very short star-shaped cloakfish with narrow wings that fly through the water. That is certainly enough material for two explanatory pages in The Book, and should be enough for a short documentary too. But that will depend on me improving my skills as regards merging and smoothing 3D meshes.