Arthropods are about as alien to us as another highly evolved bilateral animal can be. That’s undoubtedly the reason why their looks and features are often adopted into science fiction for creating odd-looking extraterrestrial beings. The differences between them and us -vertebrates- seem like night and day! But despite their humble body sizes, arthropods have been extremely successful in populating our biosphere, reaching a greater total biomass than vertebrates, most of which is fish anyway.
So given the enormous success of this particular body plan, it stands to reason that creatures similar to our arthropods could enjoy the same success in alien biospheres. That is, if they were to evolve into the same overall design in the first place. It would, in any case, be a very interesting exercise in speculative zoology to design and tweak such “alternate arthropods” to evolve in different ways. But before we can conceive of alternate arthropods we must first understand what led to their genesis here on Earth. How does one cook up an “arthropod” in the first place? Let’s find out!
It was the mid 80s when a team of paleontologists caused a sensation. Derek Briggs and Harry Whittington had reappraised century-old fossil finds and pieced together a being from the Early Cambrian unlike anything living today. Its name: Anomalocaris
Especially remarkable was its mouth that appeared to work like the diaphragm of a camera. It seemed a radical alternative to the vertically opposed jaws of vertebrates and the laterally projected jaws of many arthropods. Other alien-looking creatures were examined by the scientists too. Together with their colleague Simon Conway Morris, the scientists struggled to wrap their heads around the apparent weirdness of this brave new world.
There was Opabinia : a segmented and finned animal with on its head a hose-like appendage ending in a grasping claw. And the elongate Hallucigenia was reconstructed to be walking over the sea floor on paired, stilt-like legs, while sporting a row of flexible tentacles along the back. These tentacles were thought to pass food forwards towards the blob-shaped head. An interpretation clearly inspired by sea urchins.
Animal phyla already known to science were also represented, though some of these are nowadays rather obscure and only count very few species. For instance, the so-called proboscis worms called Priapulids were highly numerous and perturbed sediments all over the globe, like Ottoia over here. Their rumpled bodies are covered in radially arranged scales and hook-like teeth line an extensible feeding apparatus called the proboscis. There were also a number of remarkable creatures, like Aysheaia for instance, that had many lobe-feet and strikingly resembled modern-day velvet worms called onychophorans. Grouped together as Lobopodians these are widely regarded to be the closest known relatives to arthropods together with the hardy, microscopic tardigrades.
Early arthropods were present already too, like the unique Marella,Yohoia and Leanchoilia. Only known as fossils, these don’t really fit into any known lineages known today. Even the ubiquitous trilobites that dominated the bottom fauna during the early Paleozoic Era, stood on their own, forming a distinct sub-phylum. Nevertheless, members of more well-known arthropod subphyla were found too! The impressive Sanctacaris looked like an early form of the Chelicerates that covers modern-day spiders, scorpions and mites. And forms like Canadaspis appeared to be an early, but fully-formed Crustacean. So, it was already clear back then that, despite the many strange new creatures, the Early Cambrian did already have the familiar arthropods as well as less familiar ones.
Arthropod Origins Revealed
But what are arthropods and how are they related to other groups?
The hallmark of arthropods is of course their visibly segmented body.
There’s another group that has this as a defining feature, namely the ubiquitous earthworms, ragworms and leeches belonging to the Annelida or ringed worms. Polychaetes like ragworms even have many paired appendages that function as limbs, making these creatures look very similar to the aforementioned onychophorans. And for a long time, annelids were believed to be an intermediate stage towards arthropods.
However, in 1997 a molecular study gathered life forms as diverse as nematodes, priapulids, kinorhynchs, onychophorans, and arthropods, into a major animal group called the Ecdysozoa. What was long assumed to be a convergent feature of certain invertebrates, turned out to be a characteristic tying these seemingly disparate groups together, namely: A tough cuticle that encases the body and which is regularly shed to allow for growth. Or in other words: A specific kind of molting, called ECDYSIS.
The Ecdysozoan cuticle consists of chitin, which is similar in structural function to the keratin that hairs and nails are made of. As essentially non-living structures, the only way to renew these is by getting rid of the old.
And this particular constraint to the arthropod body plan, has very deep roots indeed. Recent paleontological insights have started to paint a picture in which the many oddities of the Early Cambrian can now be lined up in a lucid evolutionary progression.
For starters, let’s take another look at Hallucigenia: Closer scrutiny revealed that, not only was it reconstructed upside down, but also the wrong way around! The strange spikes actually adorned its back and the tentacles appeared to be paired limbs. The new picture that emerges is that of a creature with the build of a Lobopodian. With the head region now identified, evidence has been uncovered of a so-called oral cone.
And this was the final piece of the puzzle falling in its place.
The Oral Cone is a special anatomical structure found in several different ecdysozoans, with one or more concentric rings of spikes encircling the mouth. The spikes usually function as teeth or jaws by grabbing and grinding edible objects. In priapulids, many teeth-like scales line their throats, which can be turned inside out into an elongate proboscis reaching forwards, which can then be retracted again. Early so-called Palaeoscolecids like Scathascolex over here, display a similar pattern: An annulated body covered in small scales, while hooked teeth encircled the mouth from which a proboscis extended lined with smaller teeth.
As a specialization of the primitive structure in life forms like Scathascolex, the oral cone appears to be yet another ecdysozoan homology. It can be found back as various incarnations in almost all the different groups involved. Nematodes, tardigrades, priapulids, kinorhynchs all have corresponding circum-oral structures used for feeding. And apparently, the same goes for Hallucigenia as well, and in retrospect even for Anomalocaris and its diaphragm-like mouth! The evolutionary scenario that’s now taking shape seems to have gone as follows:
Early benthic bilaterians lived as bottom feeders probably very much like acoelomate flatworms with a slug-like body and a ventral mouth for grazing substrates. Certain lineages started tunneling: First through the algal mats and then deeper into the sediments below, which induced the mouth being moved to the front, and an anus at the end.
These early burrowers first of all needed to be able to push sediment particles away. A network of crosswise muscle fibers gave the necessary hydrostatic firmness and mobility. Greater body size and coarser sediments increased the risk of external damage. This was then mitigated by the excretion of chitin leading to a tougher integument.
In order to increase grip, the exterior first crumpled into ridges, which were then adorned by regularly placed bumps, that would eventually turn into scales and cuticular hooks. From the previous video, we already learned that reaction-diffusion dynamics would be able to easily generate such body patterning developmentally.
The next step is continuing this pattern into the mouth cavity as internal throat teeth. Turning the now toothed throat inside-out yields a grabbing proboscis, and with a ring of elongate teeth around the mouth food can be shoved inside: The oral cone. This Priapulid-like stage is probably the ancestral state of the Ecdysozoa.
An intermediate stage can be imagined with more prominent scales arranged in two rows along the back and two rows along the belly. The back rows then develop into dorsal plates sporting defensive spines and the ventral rows grow out into lobes, corresponding to the Lobopodian body plan. This is probably related to a shift in lifestyle from digging to crawling over surfaces again.
The lobe legs then develop a gill branch enabling a more active lifestyle and even swimming. Dorsal plates will extend over the delicate gills to protect them, arriving at proto-arthropods like Opabinia and Anomalocaris.
Finally, the entire exterior hardens into segments, and we’d have a true arthropod!
Parallels & Alternatives
So on Earth, the road towards the arthropod body plan was initiated by a digging mode of life. This gave the need for a tough integument with traction knobs and anchoring hooks that eventually turned into outer plating and paired legs, respectively. This development was coupled with an elongation of the body into a wormlike form, as well as a serial pattern for repeating vital organs over the length of the body.
Now, bilateral animaloids adapted for digging through sediments will inevitably develop on other earth-like planets too, leading to similarly tough and worm-like creatures. [infaunal]
So the question is:
From this universal starting point,
will general properties of planetary benthic environments likewise lead to the convergent evolution of arthropod-like animaloids?
Fortunately, we know of several parallel developments on our own planet already. The annelids mentioned earlier have a highly similar body plan yet are not directly related. Annelida are more closely related to molluscs and joined in a major clade called the: Lophotrochozoa
Likewise having tough skin, segmentation, traction hooks and even lobe feet, annelids show how a digging lifestyle leads to very similar lifeforms. Some forms even sport hardened jaws reminiscent of those of arthropods. However, despite their chitinous skin, annelids don’t show strong trends towards developing an exoskeleton, relying more heavily on their hydrostatic skeleton instead. So-called scale worms do carry alternating and overlapping plates and an extinct group of annelids called Machaeridia did have more extensive outer plating. It’s not unthinkable that further evolution of this design could also yield arthropodoid animals.
Another notable example are the Apodida, which are burrowing sea cucumbers. To provide traction, their elongate bodies are embellished with repetitive bumps arranged in the typical pentaradial pattern of echinoderms. Echinoderms also have calcified elements, called ossicles, located just beneath the skin, which can make for a sturdy skeletal structure consisting of interlocking plates. However, in sea cucumbers, the ossicles are tiny and in Apodida even microscopic. Yet, it’s easy to imagine this lineage developing hardened, external scales that could eventually turn into a arthropod-like exoskeleton. This would make for a way to start an arthropodoid lineage with radically different innards.
The Arthropod Recipe
All external skeletons, armour platings or shells started out in evolution as small, closely-knit elements like scales or teeth that eventually fused into wholesale plates or shells. The transition to the Cambrian is characterised by the so-called small shelly fauna which denotes a period when different lineages of animals were starting to form skeletons. Arthropods, echinoderms, vertebrates, annelids and molluscs each represent different solutions to adopting some sort of skeleton for protection, support and muscle attachment. We’ll dive more deeply into skeletal structures in another video.
Simply put, external skeletons seem to be the simplest and quickest solution early in evolution. Just encrust parts of your skin with excretions of toughened matter and you’re good to go. Internal skeletons need to be made of more dynamic tissues to allow for growth, and this requires higher levels of complexity. For many animal lineages, there is a trend for external skeletons to become reduced and eventually supplanted by newly evolved internal skeletal structures, or disappear altogether. And this also goes for vertebrates that also had external plating early in their evolution. There must be special reasons for sticking with an exoskeleton, and for arthropods it seems to be that their body plan is literally boxed in by their Ecdysozoan legacy of molting.
So this is basically the Arthropod Recipe:
- An originally elongate, vermiform body plan
- Constrained by a tough, external integument
- Showing serial repetition of organ systems and body parts, including…
- Limbs that started as outgrowths of external structures.
The resulting arthropodoid archetype is characterised by an exoskeleton divided up into segments in order to be able to bend, with each segment bearing one or more sets of limbs. The resulting arthropodoid archetype is characterised by an exoskeleton divided up into segments in order to be able to bend, with each segment bearing one or more sets of limbs.
Now, under Earth-like gravity conditions, arthropodoids will likely suffer similar constraints to body size, only alleviated somewhat under water, like for giant spider crabs. In another video, I will speculate on specific ways that these could be overcome.
But even as they are here and now, Earth’s arthropods have proven the extreme versatility of a segmented body plan. As repetitive body slices, segments can be adjusted individually and in groups as to allow for many different adaptations. The evolutionary trend involved in specializing body regions is called Tagmosis and in the next video, we will take a closer look at its multitude of possibilities.