#hawkinsite

[mirtadraws]

OK so I participated in an Asimov-themed fanzine months ago and then forgot to upload the thing I made. You can read the finished zine and/or participate in the next edition (Foundation TV series themed) here: @theasimovzineproject !!

I decided to do a creature design: the "Hawkinsite" alien from Asimov's short story "Hostess". It was described as a kind of bovid centaur with so many joints in the first set of limbs.

When I tried drawing it, I didn't know how to approach it, without knowing how this creature acquired their strange limbs. So I decided to draw the evolutionary history of it first, which you can see in turqoise. There's a few poses in red, demonstrating how it might move, and there's detail of its front limbs in red too.

I did quite a bit of research on the evolution of tetrapods, read a few papers and consulted with a biologist, which was all overkill but it was interesting.

I had a few different paths that I could take. The one I chose was to start with a real extinct fish called (yes) Coccosteus, which was special because it had six limbs, with the last pair being their claspers (that's right). I might go into detail about my research & design process in another post.

For now, here's some closeups:

[mirtadraws]

Hawkinsite process

I'll try to systematize my notes & thought process while coming up with this speculative evolution design based on a short story by Isaac Asimov. The sources are linked in the text, and the image sources are embedded as a link in the image.

This is a Long Post, so it is under the cut:

This is what I was aiming for -

The Final Form of the alien from "Hawkins' Planet", taken from Asimov's short story "Hostess":

Oxygen atmosphere with traces of hydrogen cyanide (HCN) (breathes oxygen with traces of cyanide. When on Earth, carries cyanide in a small container with a straw)

Higher gravity then Earth (legs are close together to withstand higher g)

Blood is colourless

Don’t sleep, but do something more like meditation (disassociate their mind from their body)

Intelligent + telepathic ability

Never stop growing, but the rate of growth slows down with time

Biological immortality

Vegetarians

Described as an ungulate, ruminant, bovine-looking, but that’s all convergent evolution, not common ancestry

Hexapod, resembling a centaur. 2 pairs of legs and 1 pair of “arms”

“Arms” have 4 fingers, no thumbs, but the fingers are placed in a kind of circle around the “wrist” joint; 5 ball joints in each finger, vestigial hooves on fingers

Skin is dry, hard, “ridged”, hot (endotherms), glistening (no fur)

Large mouth, no lips, no incisors, “bovine-looking” face

The Problem

I want to design a land-dwelling, endothermic creature with 3 pairs of limbs. In this hypothetical tree of life, instead of Tetrapoda there is Hexapoda and instead of Tetrapodomorpha is Hexapodomorpha. What is a plausible way of doing that? How far in the cladogram do I need to go to introduce that first change?

Evolution of the "stem hexapod"

Option A - Placodermi Claspers - This is the option I chose

Coccosteus, a placoderm from Middle Devonian (~380Ma)

“Unlike the claspers of modern sharks and rays that are a part of the paired pelvic fins, the claspers and female basal plates in placoderms were not at all connected to that fin. Instead they developed as an extra pair of limbs further down the body. This discovery implies that the first vertebrates did not conform to the typical tetrapod-like pattern of having just two pairs of paired limbs (arms, or pectoral fins, and legs, or pelvic fins) that typifies the basic jawed vertebrate body pattern from fishes through to mammals.”

"As the bony placoderm claspers were not rigidly fixed to the pelvic fin, they would have been able to rotate forwards, so the earliest sexual mating position was likely to have been a missionary one.”

“The claspers in placoderms can now be regarded as a serial homologue or gradual development of the other paired appendages - an extra pair of legs, so to speak.

The mechanism for how this happens is clear. Certain homeobox (hox) genes control the repetition of structures such as limbs (as in the sonic hedgehog gene) and the position of them on the body axis (the hox genes tbx4, tbx5).”

Source: The first vertebrate sexual organs evolved as an extra pair of legs

After this, the development goes the same way as it did with tetrapods, just with an extra pair of limbs: the creatures acquire a bony skeleton, then lobed fins, then venture onto land. The 3 pairs of limbs all have different morphologies. The function and morphology of the different pairs of limbs can change multiple times, if necessary.

Alternative B - Early Gnathostomata

Start with a basal form of Gnathostomata from the late Ordovician. The creature has jaws and one pair of limbs (pectoral fins, which are just a thin membrane). It looks like some kind of Antiarchi placoderm. The fins are located behind the head armour; the spikes are not limbs, just bony protrusions.

At some point, the descendants of this creature develop pelvic fins - but instead of 1 pair, they get 2 pairs of pelvic fins. Because of reasons (?), I suppose it has to do with hox genes. I don’t know if this is plausible.

Same evolutionary path as Option A: get a jaw, then bones, then lobed fins. The 3 pairs of limbs would have 2 different morphologies: 2 pairs of pelvic limbs, and 1 pair of pectoral limbs.

Alternative C - Pectoral Fins Split

Prionotus evolans, the striped sea robin

“Recent studies have shown that the fin rays of fish and fingers of vertebrates, including humans, share a common developmental expression origin (Nakamura et al., 2016; Yano and Tamura, 2013). While this does not indicate homology, our initial observations revealed that the walking rays flex in one direction while resisting flexion in all other directions, much like a digit.”

Source: Walking on chains: the morphology and mechanics behind the fin ray derived limbs of sea-robins

The pectoral fins split into 2 parts: one part retains its original form and function, and the other part is composed of several rays that have been modified to serve as extra pairs of limbs.

My opinion: Not plausible. The disappearance of rays seems to be crucial for the development of legs:

“Two major innovations—the loss of the fin rays (a major fin component of extant and extinct ray- and lobe-finned fish) and the emergence an autopod with digits—led to the modern tetrapod limb.”

Source: Making Fins from Limbs

“We propose that the loss of both actinodins and actinotrichia during evolution may have led to the loss of lepidotrichia and may have contributed to the fin-to-limb transition.”

Source: Loss of fish actinotrichia proteins and the fin-to-limb transition

Also, legs that might evolve from ray-finned fish would probably be weaker than their Sarcopterygii counterparts, which contradicts the basic premise of this creature (more legs help it withstand higher gravity on its planet)*

*that's one of the reasons I kept it's "arms" so thin in the creature's final form, to not be too heavy

Alternative D - Acanthodii (least plausible)

Brochoadmones is an extinct genus of Acanthodii.

Possibly closer related to Chondrichthyes, possibly basal to both Chondrichthyes and Osteichthyes. Apparently, the 6 pairs of ventral appendages are “finlets”, not real fins, although I’m not sure what the difference is. Perhaps the finlets even had some mobility. Finlets are present today in tuna fish, but they are not paired. They are also present in bichirs - dorsal and not paired.

Could the pairs of “finlets” in Brochoadmones have evolved into proper limbs? Perhaps 2 or more pairs even fused together, to form some kind of limb that has 2 bones such as the radius and ulna? The finlets of Brochoadmones are membranes covered in scales, without rays, and they are supported by bony, rigid spines (probably derived from scales). This is probably the least plausible option. I’m not sure exactly why, but the “finlets” seem like a product of the skin, and not really limbs, or suitable to create limbs from.

Evolution milestones:

Osteichthyes Guiyu Oneiros squamation body plan -ganoid scales -swim bladder -opercula -rooted teeth -maybe a few leftover dorsal plates behind the head -no spines -ray fins

Sarcopterygii & Tetrapodomorpha ("Hexapodomorpha") -lobed fins -2 dorsal fins -symmetrical tail -Rhipidistia -> Tetrapodomorpha -swim bladder becomes lungs -reference: Eusthenopteron: earliest evidence of bone marrow, enamel teeth, humerus (with a shoulder joint), ulna, radius, fibula, tibia, internal nostrils (he can breathe with mouth closed now!) -reference: Panderichthys -flattened head, eyes and nostrils moving to top of head to breathe air, loss of dorsal and anal fins, propulsion principally by paired fins, humerus starting to become L-shaped (to enable muscles for different movements), pelvis is still weak and not weight-bearing, change to external fertilisation; claspers are aiding with locomotion (in my fictional alien, not in real Panderichthys) -reference: Tiktaalik: pectoral girdle separated from the skull (beginnings of having a neck), wrist capable of flexing (can support weight on land), pelvis becomes larger, and pelvic fins are almost as large as pectorals, still has scales -reference: Ichthyostega: locomotion more like a seal, not a salamander; weight-bearing front limbs, large pectoral girdle and pelvis, pelvis connected to rest of skeleton, still have an operculum, large legs with 7 digits, spends time mostly on land, fin rays small and present only on the tail, small scales

Tetrapods ("Hexapods") -loss of gills and fin rays, and lateral line -adaptation of ears for air -stronger backbone & sturdier limbs -loss of digits -early Carboniferous -Anthracosauria = Reptiliomorpha basically -reference: Diasparactus -Tetrapoda, Reptiliomorpha, closely related to Amniota

Amniota: -development of the amniote egg -internal fertilization -skin becomes water-impenetrable -truly terrestrial -a basal Synapsid -reference: Asaphestera

Therapsida -limbs under the body -Cynodontia -differentiated teeth -still lays eggs -endothermic, mabye had fur

Mammaliaformes -lower jaw is 1 bone - the other bones that used to comprise the jaw migrate upwards to become part of the auditory system -Megazostrodon -small, insectivorous -nocturnal bottleneck (bigger brain) -probably laid eggs like monotremes

Eutheria -placental mammals -can have wobbly noses :> -viviparous -larger, herbivorous, specialized climber

Artiodactyla -even-toed ungulate -reference: Diacodexis

Bovidae -Eotragus