Wednesday, 28 July 2021

Why Brachiopods are actually pretty cool

Any casual fossil hunter will know his invertebrates. There are perennial classics like the trilobites and ammonites, familiar yet still beautiful forms like clamshells and conches, true artforms of nature such as crinoids and sea-urchins and strange patterns in stone, like corals and sponges. There however exists a type of creature which you are unlikely to have ever seen alive in person, you can commonly find in Paleozoic and Mesozoic strata, is older than dirt and most often you can even find completely intact. These creatures are the brachiopods and, despite their many charms, most people interested in fossils or fossil collecting usually give them nothing but disregard, either because they seem unassuming, boring or people have little idea what they are really dealing with when they find them. With this post I want to highlight some aspects of these creatures, which in my eyes make them attractive little critters, in some ways more than other invertebrates, both from an intellectual view and a practical one for fossil collecting.

One of a kind

Fig. 2: The generalized anatomy of a brachiopod

What are brachiopods? Most people, when they first see them, assume they are some type of clam, but the reality could not be further. Clams, oysters, scallops and so on belong to a group called Bivalvia, a class of the mollusc-phylum, alongside other classes like the cephalopods and snails. As the name says, they have two valves, one on the left and right side enclosing their body. This means when you find a living clam lying flat on the beach it is actually reclining on its side. Brachiopods are not molluscs and while they too have two shells, these are arranged completely differently. Whereas in most clams the shells are wing-like mirror-images of each other, in brachiopods they are differentiated and correspond to the top and bottom of the animal. In other words when you see a living brachiopod lying on the beach, it really is lying on its “belly” or “back”. The back-half of those two shells encompasses the soft body, the rest of the cavity makes room for the lophophores, coiling tentacle-like organs which the animals use for filtering the water for food. These tentacles are actually supported by an internal skeleton made of cartilage, similar to your ears and nose, which grows from the smaller dorsal shell at the top. In many groups (though not all), the bottom/ventral shell also has a hole at the back, where a tail-like organ, called the pedicle, grows. The brachiopods can use this as a sort of stalk to anchor themselves to the ground, though compared to the muscular foot of clams its movement is limited.

Fig. 3: The difference in symmetry between brachiopods and bivalves. As you can see, the main line of symmetry in brachiopods goes through the shells, while in bivalves it goes between the shells.

Surprisingly or unsurprisingly, no other group of animals on Earth follows this body plan, which is why brachiopods are grouped into their very own phylum Brachiopoda. This makes them anatomically and evolutionarily as distinct as are the whole of Mollusca, Arthropoda and Chordata. Their closest living relatives are animals called phoronids (horseshoe worms) and moss-animals/bryozoans, the latter of which is also commonly found in fossil form. Together these three form the clade Lophophorata, its members are united through the same type of filter-feeding tentacles, the lophophore. Indeed, phoronids are perhaps best imagined as brachiopods without shells (they instead form a chitinous tube) and bryozoans as miniature phoronids who exclusively live in honeycomb-patterend colonial shells. Where exactly the Lophophorata fit in the tree of life is a point of contention. Bilaterally symmetric animals are usually split into two groups: Deuterostomes are animals such as echinoderms (starfish, crinoids, sea-cucumbers) in which the anus is the first orifice to develop in the embryo, while in protostomes (to which animals such molluscs and arthropods belong) it is the mouth. Brachiopods oddly follow neither of these models, the first body orifice that develops in the embryo closes up later and the mouth and anus develop out of new cavities, which is very weird. This makes it difficult to classify them on an embryological basis. Due to some developmental and anatomical details it had long been suspected that brachiopods and other lophophorates were inside or somewhere adjacent to the deuterostomes, which would actually make them not too distant relatives of ours. Yes, chordates, including you and me, are deuterostomes, meaning every human was at one point of his development nothing but an anus (and some apparently never grow out of this phase). Most genetic tests however put them in the protostomes, inside a mega-clade called Lophotrochozoa, which also includes molluscs and annelid worms (such as the earthworms in your garden). Not all however, a few other molecular studies still support the deuterostome-hypothesis.

Paleontologist-friendly

Despite their lacking popularity, brachiopods make for almost perfect specimens to collect, analyse and exhibit, largely thanks to their anatomy. The primary layer of a brachiopod’s shell is made up of calcite, which is the most stable and durable of the carbonate minerals, and also used by echinoderms for their internal skeleton. By default, this makes them more likely to be preserved than the shells of ammonites and some bivalves, which often incorporate the more likely to disintegrate mineral aragonite. More importantly, when you find a brachiopod you will almost always find the complete, three-dimensional animal, in complete contrast to most bivalves. Ever wonder why at the beach or the fossil record, you often find only one half of a clam’s shell? It is because bivalves possess ligaments at the hinge, which open up the shell when in resting position, while they need muscles to actually close it. When a bivalve dies, the ligaments shorten and naturally split apart the two shell-halves only to be finally separated with further decomposition. This condition is almost completely reversed in brachiopods. They need muscles to open the shell and to close it. Thus, when a brachiopod dies, the shell will stay closed and the animal is very likely to fossilize with both shells still attached to each other. This also helps shield and preserve the otherwise delicate arm-skeleton on the inside of the shell, which you could then theoretically access by carefully sawing the specimen open. This all makes it very easy to identify the proper classification. The often smooth, bulbous shapes of brachiopods also significantly help with preparating the fossils, as it makes it easy to split them apart from the surrounding rock. Depending on the condition, nature may have already done the job for you, the most beautiful brachiopods I found at the eroding Holderbank quarry I could pick up from the rubble like pebbles, many intact and with only a bit of dirt attached.

Fig. 4: Just a few of the Late-Jurassic brachiopod-fossils I found myself in the Tafeljura, some more intact than others. The smooth-shelled forms are terebratulids (probably of the genus Argovithyris), the rilled ones rhynchonellids (probably Lacunosella). The only common name that brachiopods have is “lamp-shells” as their form vaguely resembles the shape of an ancient oil-lamp. Out of experience I can tell you that rubbing a brachiopod will not grant you three wishes, just odd looks from the other people at the quarry.

Outside of pure fossil-hunting and collecting, brachiopods are also of significant use in biostratigraphy. The fact they are so easily and often preserved, that they are preserved in such good conditions and therefore are easy to identify and the fact they used to be so widespread all combine to make individual brachiopod-species the perfect index-fossils, which helps geologists identify which time period a stratum came from.

Mysterious Origins

Brachiopods are older than dirt. This is a statement one can technically make on a factual level, since the first stable paleosoils only start appearing in the Devonian. The earliest fossils of definitive crown-group brachiopods of the genus Aldanotreta first start appearing in the early Cambrian, at the beginning of the Tommotian (or Cambrian Stage 2) 529 million years ago as part of the Small Shelly Fauna. This makes them older than the appearance of the first trilobites(!) and the famous lifeforms found in the Sirius Passet and Burgess Shale Lagerstätten. The Small Shelly Fauna is often little known to the general public, as it is not as weird as the Ediacaran biota which preceded it and not as spectacular as the Burgess Shale organisms which came after it, but it is of vital importance for understanding the origin of various animal groups. As the name suggests, the SSF can be generally described as an assemblage of various small animal-shells, sometimes complete but most often fragmentary. While correctly classifying the members of this fauna is sometimes difficult, confident assessments can still be made and thanks to its surprisingly continuous record, studying the SSF allows one to accurately time the pace at which the Cambrian radiation occurred. The first forms of the SSF already start appearing in the late Ediacaran with creatures such as Namacalathus, Sinotubulites and Cloudina, whose exact affinities are unknown, though they likely belong to cnidarians and diploblastic worms. Fascinatingly, Cloudina’s nested shells often preserve boreholes, which appear to have been selectively made by other animals to feed on the organism’s insides. This makes it possibly the oldest record of one animal predating on another and this first occurrence of predators is the likely reason why so many small shelled animals suddenly started appearing in the first place. As the arms-race continued, the Cambrian began and the enigmatic forms of the Ediacaran died out, we find various new sclerites and partial shells appearing in the SSF, including those of echinoderms but also weird forms like halkieriids, chancelloriids, hyoliths and tommotiids. As the Early Cambrian goes on, we also find small shells and sclerites of early molluscs, brachiopods, stem-arthropods and other modern groups. By the beginning of the Middle Cambrian most enigmatic forms die out and the fauna is superseded by the first appearance of trilobites and other famous Cambrian lifeforms of larger size. By the Ordovician most of what used to be the SFF is now only made up of the small shells of gastropods (snails).

Fig. 5: The strange Cambrian organism Halkieria. Take special notice of the odd, brachiopod-like shells imbedded at the front and after.

Like with many of the Cambrian invertebrates, the discussion around what sort of organism the brachiopods evolved out of is an interesting one. Traditionally brachiopods had been linked to an organism named Halkieria and its close relatives. Halkieriids can be best imagined as flat slugs covered in a chain-mail-armor made up of multiple mineralized scales called sclerites. These already appeared at the very start of the Cambrian and survived even past the Middle Cambrian extinction of most of the SSF. Perhaps they were related to the slug-like Kimberella from the Ediacaran. In addition to their sclerites, halkieriids also possessed two shell-plates at both the head- and tail-end of the body, which may have been used for muscle-attachment or for protecting gills. In their structure these shell-plates look suspiciously, and I mean very suspiciously, like the two valves of brachiopods, in addition to also having been made of calcite. Some halkieriids are curiously also found in a protective rolled-up position, similar to pillbugs. The classic idea of brachiopod-origins, the “brachiopod-fold-hypothesis” has therefore been that some halkieriids became sessile by permanently folding their bodies along the mid-section into this sort of pillbug-shape and then grew compacter until the two shells at the end of the body met in the middle to form a hinge. What may support this is that the larvae of some modern brachiopods go through a stage which resembles such a fold. Unfortunately, this hypothesis does not entirely explain where many key-features of brachiopods come from, such as the lophophores or the stalk. Furthermore, the halkieriids possessed various features not found in brachiopods, such as a muscular foot and in one member possibly even a radula, which would put them a lot closer to true molluscs. Though even more bizarrely, the sclerites of halkieriids are nearly identical to those found in chancelloriids, which are radially symmetric lifeforms often interpreted as being early Cambrian sponges.

Fig. 6: Shell-parts of Eccentrotheca, a possible close relative of phoronids and brachiopods.

A better alternative opens up if one assumes that brachiopods evolved out of an animal similar to other lophophorates. In this light their most obvious ancestors become the tommotiids, which basically looked like phoronids with tube-shaped shells. Among these animals one can even find fossils which show the transitional stages from a phoronid-like creature towards the typical brachiopod body-plan. Eccentrotheca from Australia is an early Cambrian tommotiid found with a cone-shaped shell. The pointy end had a hole through which a stalk probably grew to attach the organism to the ground, while the other end was broad and open to allow the lophophores to filter the water. At this end the animal curiously had additionally sclerites, which may have been the beginning of the closing shell. After this came Micrina and Paterimitra, considered stem-group brachiopods, which already had the two shells, though where still somewhat cone-shaped and could not fully enclose their bodies.

Fig. 7: Paterimitra, a stem-brachiopod.

During the Paleozoic brachiopods were the most successful and widespread filter-feeders and reef-builders, reaching their peak in the Devonian, and could be found in a variety of marine ecosystems. Today they are but a shadow of their former selves, most of them only live in deep-sea environments and thus are rarely ever seen by humans unless they go directly looking for them. The lack of post-Paleozoic success is largely linked to the success of bivalve molluscs, as well as the Permian-Triassic mass extinction event. During said event large parts of Earth’s ocean (back then there was only one due to the supercontinent Pangea) not only became anoxic but also highly acidic due to carbon-emissions by massive volcanism, causing death and decline to almost all shelled animals, since their calcareous shells easily dissolved under these conditions (thanks to human carbon emissions the same process is unfortunately repeating today). While bivalves were hit just as hard as brachiopods at the time, they managed to steadily increase in abundance in the subsequent periods, while brachiopods were never able to recover their former diversity. Why exactly it happened that way is not clear, though it seems increasingly unlikely that bivalves directly outcompeted brachiopods, as the two did not share exact same lifestyles and trace-fossils attest that Mesozoic shell-crackers such as placodont reptiles, heavy-toothed fish and starfish actually preferred eating clams over brachiopods. Even today most marine predators tend to avoid eating brachiopods, either because they have chemical defences, taste awfully or just do not have enough meat to them to justify the effort (Cohen et al. 2006).

Hidden Depths

A probable reason why many people think of brachiopods as boring is that they seemingly lack in diversity. Once they have seen one, they think they have seen all of them. Bivalves occupy many niches and possess various body-shapes adapted to digging, filter-feeding, reef-building, microcarnivory and in the case of scallops even swimming. In contrast most brachiopods seem to only be sessile filter-feeders. However even they have developed a truly fascinating variety of morphologies and lifestyles, which one can see both in the fossil record and modern day.

Fig. 8: A Cambrian hyolith, illustrated by Emiliano Troco, standing on its stilts in order to filter the water free from the murky filth on the ground.

The oldest example of aberrant groups may be the hyoliths, which are either stem-brachiopods or basal lophophorates. Hyoliths were truly alien-looking animals which also first appeared in the Cambrian and survived all the way to the Permian mass extinction. They might be best described as brachiopods trying to be… well, actually I cannot really find a good analogy for them. Like crown-group brachiopods they had a main ventral shell in which the body sat and an operculum, a smaller shell attached with a hinge, on which the lophophores grew and with which they could close the shell. The big difference is that the ventral shell did not have a pedicle, but was instead much larger than the operculum and cone-shaped like the shell of an orthocone, likely meaning that the animal lied flat on the seafloor. Most bizarrely, out of the mantle grew two large spikes or stilts, which could elevate the body a bit from the ground. It is thought that through muscles the hyoliths could rotate and move these spikes and that way actually crawl across the seafloor if they wanted to.

Fig. 9: An aquarium with a few Lingula anatina. The name derives from the tongue-shape of the shell. Take note of the sensory setae at the shell-margins.

Higher up on the family tree are the inarticulate brachiopods, which unlike all other brachiopods do not have a mechanical shell-hinge. Inarticulates are still alive today in the form of Lingula and its close relatives, which have used this to their advantage. The lack of a hinge allows Lingula to horizontally scrape its shells and its setae against each other in a sort of shearing-motion, which the brachiopod can then use to dig itself underground like many bivalves do, but most other brachiopods cannot. Various other quirks also set them apart from regular brachiopods. Their pedicle is muscular, dextrous and flexible, acting more like a tail or tentacle than the static stalks of the other brachiopods. Because humans can and will eat anything that is not immediately toxic to them, their fleshy stalks have made Lingula the only brachiopod to be commercially fished and consumed. Linguloids are also the only brachiopods which use oxygen-carrying proteins in their blood, though it is haemerythrin instead of the hemoglobin that we use. Instead of calcite, linguloids build their shells out of apatite, which is the same material vertebrates use for our bones. Under observed aquarium conditions, Lingula has actually shown quite complex behaviour: They will use their pedicle like the tentacle of an octopus to probe for a suitable spot, then use it to arch their whole body until the shell-tips point directly downward onto the spot, then use the aforementioned shearing-motion to dig themselves a burrow head-first into the ground, then lift themselves out again with their pedicle, rotate and then finally settle tail-first into their new home (Cohen et al. 2006). That is quite calculated for an animal which does not have a brain. Actually, many aspects of how brachiopods sense the world are mysterious to us, in part because researchers simply lack interest to investigate them. For example, adult brachiopods lack eyes or any similar sensory organs, but, for reasons we cannot understand so far, are able to sense when a dark shadow (for example of a large fish or human diver) looms over them, causing them to instinctively close their shells.

Fig. 10: An old illustration of a dissected Lingula to give you a better understanding of how the lophophores sit inside the body of an average brachiopod. Extinct forms such as the spiriferids had in contrast rather ridiculous spiral shapes resembling radiators.

Lingula is often regarded as a “living fossil”, as brachiopods of pretty much the exact same morphology have existed since the Ordovician period. In fact this animal was among those which made Darwin first coin the term. It has however become doubtful if these really are of the same lineage or that Lingula has remained unchanged since then, it appears more likely that these are separate brachiopod-lineages which arrived on the same burrowing-adaptations, as it is really not all that difficult to evolve the Lingula-shape from the basic brachiopod body plan (Emig 2008). This is called parallelism and it describes the phenomenon of two closely related lineages convergently arriving on the same shape, because they inherited the same initial body plan and all the evolutionary potential that came with it. Parallelism is arguably not a true example of convergent evolution, as the ability itself to evolve into a certain form becomes an inherited trait in these cases. Many things people commonly think of as examples of convergent evolution, such as carcinization in crustaceans or feliforms repeatedly evolving saber-teeth, are actually parallelisms.

Fig. 11: Reconstruction of Late Paleozoic productids and their strange ventral spikes.

Among the articulate brachiopods one of the most successful and strangest forms was the order Strophomenida, who had their heyday in the Ordovician and later again in the Permian, but are now unfortunately extinct. Whereas most modern brachiopods are biconcave (both shells bow outward), the Ordovician strophomenids were concavo-convex, meaning the shells nested into each other like a pair of stacked bowls. Even stranger was the Permian sub-group Productida. Productids entirely lacked the pedicle and instead attached themselves to the seafloor through a unique method: The ventral shell grew long, curved spikes with which the organism rooted itself into the sand, like the foundations of a beach-house. The Devonian productid Gigantoproductus with a size of 30 cm is the largest brachiopod known, but instead of spikes it used strange, wing-like protrusions in its shell for more stability in the water-currents.

Fig. 12: Reconstruction of Leptodus and close relatives.

Strangest among the productids was probably the Middle Permian Leptodus. This animal’s dorsal shell was highly reduced and fern-shaped, meaning that even when it closed onto the ventral shell, the soft mantle of the animal inside was still exposed through windows. Two interpretations for this exist. The first is that the dorsal shell had, for unknown purposes, actually become incorporated into the body, as a sort of internal skeleton, similar to what happened to the shells of coeloid cephalopods. A One-Valved Brachiopod, if you will. The more likely interpretation however is that Leptodus was the brachiopod-answer to corals and rudist bivalves: The animal had managed to symbiotically incorporate zooxanthellae (planktonic algae) into its skin and was partially living off photosynthesis through its partners. The fan-shaped shell offered light-exposure in exchange for minimized protection.

Fig. 13: Cretaceous Pygope. The sediment-filled hole in the middle would have been hollow in life.

Apart from the inarticulate ones only a few brachiopod-orders survived into the Mesozoic and modern day, but even among these can be found weird fossil forms. One of the most outstanding are the Jurassic Pygopidae, in which both shells split in half to separately house one of the lophophores. In many pygopids the shell thus forms a triangular hole in the middle, just for the two halves to fuse again at the end. The particular shape of this fusion is what gave the group its name, Pygopidae can roughly be translated as “butt-faces”. Also worth mentioning is Early Cretaceous Peregrinella. While it outwardly looks like typical rhynchonellid brachiopods, it stands out due to its large size (10 cm), still unattained by modern brachiopods, and its calcified arm-skeleton. The sediments it is found in show that Peregrinella lived around deep-sea methane-vents and it is theorized that it may have attained its weird quirks through living in symbiosis with chemosynthetic bacteria. Similar explanations have also been used for other giant brachiopods like Gigantoproductus mentioned earlier.

Let us have some fun

An underappreciated way of highlighting and generating interest in the often strange anatomy of living animals is to show their potential through little exercises in speculative biology, which, if you know me, I am a big enthusiast of. Now I will present you a few excerpts from a small project of mine I started working on and off a few years ago. The creatures here are not direct descendants of brachiopods, but extraterrestrials that came from an ancestor with very similar anatomy. While I have already thought of a scientific name for them, for simplicity’s sake we will just call them xenobrachiopods for now. Please excuse my very limited drawing- and photoshopping-skills, I have never been a good artist and just hope the sketches get the ideas across I want to convey.

The atmosphere of planet [REDACTED] is unnaturally dense thanks to multiple periods of constant volcanic outgassing. Despite being quite a stretch removed from its parent sun, the intense greenhouse gases create balmy temperatures and the air is constantly laden with water vapor and misty clouds. This, combined with a Mars-like gravity, has created perfect conditions which allow small to microscopic animals to freely float in the atmosphere as aeroplankton. Part of this plankton are the larvae of the Shell-Tree, a primitive xenobrachiopod. Hatching from their airborne eggs as free-floating organisms, the tree’s larvae eventually grow too large to stay in the air and sink to the ground. Should they happen to land in a suitable spot, the pedicle takes deep roots into the ground and permanently hardens into a biomineralized, armored stalk, which grows until resembling a large tree trunk. The top this trunk holds the only lightly mineralized valves. The lophophores of the shell-tree have grown into long, feather-like fans, which it uses to pick up moisture as well as to feed on the aeroplankton that it once used to be a part of. Statocysts alert the shell-tree of large trembling in the ground and changes in air-pressure, indicating large predators or incoming storms, making it retract its fans and close the shell. The individual here is a still juvenile pioneer, growing at the outskirts of a desert which is gradually becoming more habitable as the climate of the planet changes. Soon it will be joined by other shell-trees and vendobiont-like photosynthesizers, eventually forming a dense forest.

A few hundred million years before the young shell-tree hatched, back in the primordial soup, a close cousin took a very different route to life. Back then a new phylum of predatory animals arose on [REDACTED], the trichordates, starfish-like animals who with their three strong mesoskeletal arms were able to pry open the shells of any valved animal. The ancestors of the Blimpshell first reacted to this new threat by evolving a more flexible, muscular pedicle and adapting to a burrowing lifestyle similar to Lingula. Unfortunately, the trichordates eventually learned how to burrow themselves, so this was not of much use anymore. A few of these xenobrachiopods responded by making it impossible to pry their shells open: Both valves became permanently fused to each other like the plastron and carapace of a tortoise. The adductor muscles became repurposed to control the pedicle and the lophophores. Only three to four hole-shaped openings remained at the front of the shell to allow the animal to stick out its lophophores into the water. Eventually even stranger things happened. To probe the ground for food, the arm-skeleton of the front-most lophophore fused with the mouth to form a mobile, bony proboscis, which outwardly looked similar to the siphon of clams but functioned more like the neck of vertebrates. Part of the former lophophore became repurposed into an efficient gill-set, with four to six spiracles at the front of the proboscis pumping water onto them. In a herbivorous subgroup two hardened plates developed at the front of the proboscis to scrape algae off rocks. These eventually evolved into a beak with a very cephalopod-like jaw-joint. Some could grow beaks large enough to make piecemeal out of the once dreaded trichordates. As the proboscis increasingly took over the role of eating and breathing, the other lophophores could adapt to new roles, in one clade evolving into fleshy fins. While these were very flimsy, in structure similar to the fins of a lungfish, they could get the job done of heaving the xenobrachiopod across the seafloor. The pedicle meanwhile became a tail, with comb-like setae to swim away from predators in quick bursts.

Here we see a very rough sketch of the internal skeleton. The arm bases of the former lophophores now fill out the lateral opening to present a better site for muscle attachment. The proboscis attaches to the inside of the dorsal shell and can be retracted like the head of a turtle. The tail is omitted here, as it is not actually endoskeletal. Like the real life Lingula, the pedicle of the blimpshell’s ancestors had a chitinous cuticle, which in this form has evolved into a full-on exoskeleton of chitin-rings controlled by muscles which attach to the inside of the ventral shell. In some ways it resembles the tail of a crustacean or the mouth-arms of dinocaridids. Due to this nature, the tail is rarely preserved in fossils. The small bulges at the top and bottom of the front shells are viewing platforms for primitive eyes, allowing it to see around the proboscis. Instead of evolving its main sensory organs at the front of the proboscis as one would expect, the blimpshell simply adapted pre-existing structures from its ancestors. Said structures were aesthetes, similar to those of Earth’s chiton-snail, which are complex mineral-eyes firmly placed in the shell. The nervous system of the blimpshell is still quite decentralized, with multiple ganglia controlling the movement of the arms, tail, eyes and proboscis. The blimpshell and its relatives, despite their now gained mobility and ability, were still at the bottom of the food-chain. The oceans of their primitive world were ruled by large exoskeletal creatures looking like a mix between mythical sea-serpents and legless centipedes. 

A climate-induced catastrophe came eventually for these sea monsters and the new mobile xenobrachiopods could expand into new megafaunal niches, approximating perhaps the various placoderms of Earth’s Devonian. Eventually one group adapted to swampy mangroves and anoxic waters by adapting a buoyant system of pneumatic air sacs inside the shell-wall (which gave the blimpshell its name) into a sort of lung-system. The small bones of the fleshy fins fused to form long bones, joints, fingers and eventually fully formed arms with seven digits, which these animals could use to drag themselves across the landmasses. They came too late however, the terrestrial abode was already occupied by another clade of animals, such as the one eyeing our tree-climber here. This was a phylum of six-legged animals with an internal skeleton and even a backbone, making them the most similar to Earth’s vertebrates among the fauna of [REDACTED]. They were nonetheless strange in other ways: Their jaw was a complex pair of spider-like cheliceres, derived from former limbs, coupled with a long prehensile tongue. They had neither ears nor nostrils, instead smelling and hearing was accomplished by an array of antennae. Their eyes were multiple solid silicate discs. Breathing on land was accomplished through a unidirectional book-lung derived from a former jet-propulsion organ. Due to being the first lifeforms on the planet to evolve legs, the crustacean-like ancestors of the six-leggers were able to colonize the land long before the xenobrachiopods could. Smaller forms resembled arthropods and amphibians of various kinds, the large megafauna consisted of six-legged and kangaroo-like reptilian behemoths, which looked a bit like 1940s dinosaur reconstructions with insect-heads attached. The new terrestrial xenobrachiopods, such as this Terecuda could not compete and instead adapted to a life in the trees. The periostracum, the skin growing over the shell in the brachiopodous ancestors, has now grown thicker and a few muscles here and there now also attach to the outside of the shell, making the shell’s appearance similar to that of soft-shell turtles. The eyes on the shell have now become more complex and can move independently like a chameleon’s and a more complex brain is developing underneath them. The beak at the end of the proboscis has become a pseudo-skull, lacking eyes or a brain. What looks like a depression for an eye is actually a tympanum, a primitive ear derived by growing a membrane over one of the spiracles. The terecuda is an ectotherm that lays leathery-shelled eggs through its cloaca, though a few of its relatives are experimenting with elevated metabolisms.

Their time came eventually, as an asteroid hit [REDACTED] and caused mass extinction. The smaller six-leggers and some gracile, centaur-like grazers survived, but the time of the behemoths was over. In their place came creatures like the Deinobrachion above. Descended from a chameleon-like ancestor which already had an elevated high-walk, deinobrachion and its relatives evolved an upright gait with straight legs and in many ways converged on flightless birds and other theropods (though if you look closely its equivalent to a knee is functionally an elbow). The periostracum has grown even thicker and even developed a fuzzy pelt, similar to the hair found on the shells of some gastropods, which now covers up the sutures and fine details of the shell underneath. What the alien is doing here is a sort of intimidating mating dance to a potential partner just offscreen. Due to a meiotic quirk going back all the way to the primordial soup, nearly all animals on [REDACTED] are hermaphroditic and unable to develop differentiated sexes. In most primitive lifeforms both parties become pregnant after mating or they practice self-fertilization like plants do on Earth. More complex animals like this one instead have a love-life more similar to snails and flatworms in that the two partners have to determine (often violently) who has to get impregnated by the other and therefore expend their precious energy to produce and rear the offspring. Impressive horns, shields, domes and spikes have developed in some forms for the purpose of such mating-fights, though the deinobrachion keep it simple by just extending their “mating-tentacle” out of the cloaca and showing off their colourful dewlap to each other. The one with the most vibrant colours and the longest organ wins. Said organ is the little dongle you see here underneath the base of the neck in front of the ventral eyes. Like in their Earth-equivalents, the gut of the xenobrachiopods is U-shaped and once the fused carapace evolved, the cloaca had to share the same bony opening as the proboscis, though they are thankfully separated by skin and muscles.

Finally arriving back to the time of the shell-tree, we here see its arch-nemesis, the Tree-Cracker, which feeds on the insides and delicate fans of the former. Lizardine lips behind the hooked beak hide what are massive crushing teeth similar to those of placodont reptiles. The sharp beak is used to hack at any exposed muscles or ligaments of shell-trees in order to get them to open and if that fails the teeth simply crush through. With a fully elevated neck the tree-cracker stands about as tall as a giraffe, often higher, though it pales in comparison to its herbivorous relatives, which, thanks to the planet’s benevolent gravity and perforated shells similar to sea turtles, can reach dinosaur-like proportions, the largest species reaching a titanic size of [DATA EXPUNGED]. They are preyed on by large, bipedal carnivores, such as Tyrannoproductus. If size is not enough of a deterrent, the tree-cracker will present the predator with the long setae growing on its chitinous tail, which can sting like hell. A smaller relative is even able to absorb the toxins of the smaller shell-trees it eats and store them in the setae, which then function like dangerous syringes.

If you liked this and other articles, please consider supporting me on Patreon. I am thankful for any amount, even if it is just 1$, as it will help me at dedicating more time to this blog and related projects. Patrons also gain early access to the draft-versions of these posts.

Related Posts:

Literary Sources:

Papers:

Image Sources:

  • Fig. 2: Cohen 2013, p. 3.
  • Fig. 3: Prothero 2020, p. 93.
  • Fig. 4: Image taken by me.
  • Fig. 5: Wikimedia
  • Fig. 6: Skovstead et al. 2011.
  • Fig. 7: Skovstead et al. 2009.
  • Fig. 8: superterrane.tumblr.com
  • Fig. 9: Wikimedia
  • Fig. 10: Wikimedia
  • Fig. 11: Grant 1966.
  • Fig. 12: Williams 1953.
  • Fig. 13: Wikimedia
  • Rest taken or made by be.

 

This article was not written by alternate universe sapient brachiopod SCP-12969.