[Bears Armed]

I have added this group of animals to the fauna of the regional map, following a poll in which a clear majority of the voters supported this (even if we don’t count the four voting nations that aren’t [yet] actually on the map, or the one puppet that voted after its player’s main nation had already also done so, that’s still 12-to-1 in favour …) See: https://www.nationstates.net/page=poll/p=153911

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Do we still have Trilobites in the IDU's world?

The Free Bears of Bears Armed wrote: In RL the Trilobites (https://en.wikipedia.org/wiki/Trilobite) became extinct shortly before the rise of the Dinosaurs, but the Horseshoe Crab [of comparable antiquity] still survives. Might we still have living Trilobites in our world, somewhere in the almost-unexplored deeps or even in the shallows alongside -- or replacing -- the Horseshoe Crab?

No: Legionas.
Yes, only in the deeps: Laeral, Ruskayn, Res Publica Melitensis, Gladysynthia, North Cross, Lauchenoiria, and Comhar.
Yes, deeps and shallows (alongside Horseshoe Crab): Zamastan, Libertas Omnium Maximus, Christos, Gardavasque, Agury, Sunemia, Rio Palito, Birnir, and Ponoxien.
Yes, deeps and shallows (replaces Horsehoe Crab): ---
Yes, shallows only (alongside Horseshoe Crab): ---
Yes, shallows only (replaces Horseshoe Crab): Arcadia Dazeber.

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Obviously it’s still up to each nation’s own player to decide whether Trilobites are present in that nation’s own territorial waters, but it seems reasonable to presume that (at the least) those who voted in favour of there being trilobites in the “shallows” will have them present.
Looking at the pattern of votes cast, this suggests so far that our surviving Trilobites’ collective range centres on the Olympic Ocean (although not along the western coasts of Hesperia?), with outlying populations around both Libertas Omnium Maximus and Arcadia-Dazeber, but that they might be absent from most of the Tenebric Ocean’s shores. Decisions by additional players might change this, of course. I am working on IC explanations for this distribution.

[Bears Armed]

Relationships

Trilobites have a body-plan which places them clearly in the Phylum Arthropoda: A segmented body with jointed limbs, a chitinous exoskeleton, and compound eyes. Thus they are related to the Horseshoe Crabs, Crustaceans, Arachnids (Spiders, Scorpions, Mites & Ticks, etc.), Insects, Centipedes & Millipedes, and several other groups that you are less likely to have heard of. They might even be Crustaceans, although apparently the current consensus among palaeontologists says not. If they branched off from the Arthropods’ basal stocks before the ancestors of the various groups that survive in RL diverged from each other then some taxonomists (favouring a ‘crown group’ model of classification) would place them outside (but as a ‘sister-group’ to) the Arthropoda proper, in a wider assemblage labelled ‘Panarthropoda’: Here in the IDU, however, their own survival into modern times would simply extend the scope of the ‘crown group’ and thus of the Arthropoda proper.
The distinctive three-lobed body-plan that all Trilobites share is no more “unusual” than the body-plans of various Crustacean groups, but there is one feature in which they differ from all other animals! This is the fact that the lenses in their eyes are composed not of organic materials, as in all of those others, but of Calcite crystals — one per individual segment in each compound eye — instead.

[Bears Armed]

RL history

The earliest fossils of Trilobites that have been discovered so far date from the Cambrian Period (c. 540—485 million years ago), which is the earliest stage in history from which many fossils that can clearly be assigned to surviving phylums have been found. They did not appear right at the beginning of this time, which is known mostly for its “small shelly fauna”, but were present and diversifying well before its mid-point. However, the earliest known forms were already clearly recognisable as fully-complete Trilobites in body-plan, diversified, and geographically widespread, which obviously suggests an earlier evolutionary history for which direct evidence has not yet been found: Maybe the earlier stages simply took place within a limited geographical range from which no fossiliferous rocks have yet been found (Maybe, even, IC they originated in the IDU’s world and then spread to RL Earth? ); maybe those proto-Trilobites, like some other Arthropods, had only relatively “soft” exoskeletons rather than the mineral-reinforced ones of the later forms (and of some Crustaceans), so that remains surviving to be fossilised was much rarer; maybe, perhaps because of that softer casing, those proto-Trilobites were much smaller than the later “true” ones and so any fossils that did manage to form would be harder to spot…

During the remainder of the Cambrian and through most of the following Ordovician (c. 485—434 million years ago) the Trilobites remained a major component of the marine fauna and different forms developed for a wide range of lifestyles: Most walked along or swam close to the sea-bed, as predators, scavengers, ‘deposit feeders’ (which “mined” the seabed for smaller organisms & organic debris), or filter-feeders; some swam higher up, apparently filter-feeding and maybe actively hunting in the plankton; it has been suggested that one family had become parasitic on some types of ‘sessile’ (i.e. fixed-in-place) animals, although this is still disputed; and one family is even thought to have developed a symbiotic relationship with sulphur-using bacteria which supplied them with nutrition. However, the end of the Ordovician featured a ‘mass extinction’ event: There was an Ice Age, which lowered sea-levels (as well as global temperatures) and thus not only drained wide expanses of the continental shelves over which the Trilobites had lived but also — probably — brought oxygen-poor water from the deeps close to the surface. Trilobites were among the [numerous] groups of animals that suffered quite heavy losses during this stage, but survived it although with their phylogenetic and ecological diversities significantly reduced: form then on, it seems that most if not all of them were restricted to primarily-scavenging lifestyles. They declined further during the subsequent Silurian and Devonian Periods (c.434—419 MYA; and c. 419—386 MYA), presumably due to the evolution of more effective competitors and/or predators, and a series of extinction events during the second half of the latter (possibly linked to another Ice Age?) reduced them to just a single Order whose surviving lineages seem to have been restricted to moderately deep water. This remnant stock then managed to persist through the Carboniferous Period (or ’Mississippian’ & ‘Pennsylvanian’ Periods; c. 386—291 MYA, either way…) and Permian Period (c.291—252 MYA)… but the end of the Permian was marked by a ‘mass extinction event — this time apparently linked to a rise in global temperature, rather than to a serious drop — which was so severe that some palaeontologists call it “The Great Dying”! No fossil Trilobites have ever been found for which dates later than this are considered appropriate, and so they are presumed to have been among the numerous groups that it exterminated completely. The overall pattern of changes in faunas (and to some extent in marine ‘floras’, as well, although less so among the land-based plants…) was so extreme that this event is used to mark the transition from a Palaeozoic Era (i.e. “Era of Ancient Life”) to a Mesozoic Era (i.e. “Era of Middle Life”; also popularly called “The Age of Dinosaurs”).

[Bears Armed]

Survival in the IDU

Our world’s different pattern of continents presumably indicates a different history of ‘plate tectonic’ events going back a long way, quite possibly even back past the beginning of the Cambrian Period. This being the case, our world’s history of ‘mass extinction’ events could plausibly have been different as well, even if external factors such as the ones that might help to start Ice Ages — or major meteoric impacts — were the same here as in RL.
I am not going to suggest that all of our world’s ‘mass extinctions’ were less severe than those of RL Earth, nor to deny that ecosystems here might then also have had to cope with one or two additional such events which RL ecosystems did not. Nevertheless, it seems reasonable to me to presume that these differences also led to some differences in the compositions of the post-extinction ecosystems… although some of those differences would probably later have been evened-out by some extinct groups being replaced — and possibly some surviving groups being out-competed, as well — by members of groups that had survived in one or more RL-like Earths and “leaked” across to here. (I had already mentioned that idea elsewhere in this sub-forum, in the specific context of the mass-extinction at the end of the Mesozoic Era and the subsequent developments of — especially — our Mammal and Bird faunas…).
Unless any of you insist on changing this, I think that our modern-day Trilobites would belong to two or three separate Orders, one or two of which had also existed in RL but one of which would be endemic.

The Proetida were the last order of Trilobites to appear on RL Earth, and were also the order to which the last surviving Trilobites on RL Earth belonged. They seem to have lasted that long because they had become adapted to deeper waters, where there was less competition: As some other groups of animals that followed such a pattern have survived on RL Earth until today, even though all of their close relatives living in shallower water have long been extinct, it doesn’t stretch probability too far to suggest that these could also have done so. Thus, this order would contain our deep-sea species.

The Lichida were among the orders that became extinct during the latter half of the Devonian period on RL Earth. Their members were characteristically spiny, which is why they’re distinct enough from all the others for me to consider them worth using. I don’t yet have any particular theory about how & why they would have survived here when so many others do not, so would suggest that if we do use them then it as the stock for an area isolated from the world’s “main” Trilobite populations — perhaps the coastal waters of Libertas Omnium Maximus, as that nation was among those voting for the presence of trilobites “in the shallows” but the only other nations near there who voted at all were against that possibility — with their origin a mystery (and possibly due, although nobody IC would know this, to [fairly recent?] interdimensional “leakage” from a version of Earth that is still in its Ordovician or Devonian period?)

The Neotrilobita — unless I find a better name for them — would then be our endemic order, containing primarily species resident in the shallower waters around [some of the] shores of the Olympic Ocean. As my ideas about the region’s geological history involve that ocean only having formed as a former super-continent (including all four of our present main land-mases) broke up from the mid-Mesozoic onwards, my suggestion for their evolution & survival is that their ancestors had actually managed to spread — unlike any Trilobites known of so far on RL Earth — successfully through the brackish water of estuaries into the fresh waters further inland: This let them survive the sea-level changes of the later Devonian and even the end-Permian mass extinction. During the Mesozoic they then expanded back downstream into the shallow seas (or, at least, salty “lakes”) that existed at times in various places “within” the super-continent, and thence into the Olympic Ocean as that opened up as well. Any last freshwater Trilobites that managed to survive until the end of the Mesozoic were probably wiped out in the ‘K-Pg Event’ (i.e. “Death of the Dinosaurs”), although some limited re-colonisation of that environment might have taken place more recently. I will presume that they and Crustaceans such as those that fill [probably similar] ecological roles today on RL Earth are ecologically balanced enough against each other for both groups to exist here, although in some cases conditions in certain areas may favour one or the other.
Arcadia Dazeber voted in favour of trilobites in shallow waters, but Legionas to their north voted against us having any surviving Trilobites at all: If those views are matched by their decisions now about having Trilobites in their own nations’ coastal waters, and if no linking populations around the southern end of that continent are approved by players there, then possibly the ancestors of Arcadia Dazeber’s stocks “returned to the sea” locally in the late Mesozoic, separately from those that populated the Olympic Ocean, and have been unable to spread further because (? reason still to be determined…).

The basic ecological role filled by these coastal species would be as scavengers & sometimes predators, although the latter would be probably limited to smaller and relatively slow-moving (or even sessile) prey, without heavy armour of its own, as is also the case for many types of Crab or Lobster, but I have some ideas about various more-specialised lifestyles & forms that would also be possible.
Bearing in mind the “origin story” that I now suggest for our main group of Trilobites, and having done some more research into the RL ones, I am dropping my suggestion that some species might — like the Horseshoe Crabs — crawl ashore to lay their eggs in beach sands. However, if we do go with my suggestion of a ‘freshwater’ stage in the evolution of the endemic order Neotrilobita, there might be species whose adults normally live in brackish estuaries or swamps but migrate upriver to reproduce and whose young spend some time developing in freshwater before moving back downstream…

[Bears Armed]

Notes
Unfortunately I will not be unable to provide ‘scientific’ names for as high a proportion of the endemic Trilobite species & larger groups than I have been doing for the region’s endemic Mammals and Birds. My knowledge of this particular group and of “Taxonomists’ Latin” simply isn’t good enough to interpret much of the specialised terminology used in naming the class’s RL members and to recombine this appropriately.

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Remember that the Trilobites which survived in the IDU’s home reality have apparently had around 250 million years longer in which to evolve than any of the group’s lineages managed in RL where their recorded existence lasted for “only” around 270 million years (although it must really have been a bit longer than that, with the earlier fossils not yet found…). This allows for some increased diversity in form & lifestyle here, although the roles of swimming predator or planktonic filter-feeder have almost certainly been lost permanently toother types of animal such as ‘Fish’ of various kinds. There are still basic limits, however, on factors such as maximum size or land-dwelling capabilities.

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Evidence from RL fossils demonstrates that some types of Trilobite were able to curl up into balls, to protect their less-armoured legs & undersides from attack. It therefore seems likely that some of the species living in our region today can also do so, but this would not be the case for all of them: Lifestyle, threat levels, thickness of armour, and number of body-segments, would presumably be factors to consider.

It is presumed that Trilobites generally had planktonic larvae, but the possibility that females of some types retained their eggs & perhaps also larvae in ‘brood pouches’ inside the front part of their carapace has also been suggested: This would probably be most likely for species living in the most “difficult” environments.

(post 6)
Order: Proetida
Reference: https://en.wikipedia.org/wiki/Proetida

The adult forms of these animals live on the sea-bed, almost entirely below the levels to which light penetrates from the surface, so would have little use for eyesight (unless luminous organisms are extremely common down there…). However, if their larvae are planktonic then those would still find functional eyes useful and so it is possible that the adults of some species do retain functional eyes, although probably not very large or complex ones. (However, again, these are probably the most likely group of Trilobites to retain their eggs in ‘brood pouches’ instead of having planktonic larvae…) These are All species probably possess good senses of smell, for finding food, and some might have evolved a way to track the movements of reasonably-sized creatures nearby through the pressure waves that these cause in the water.
The ancestral forms that lived through the end-Permian mass extinction event here might have been adapted to cope with relatively low oxygen levels, but the extent to which this trait has been retained by modern types is likely to vary.

Suggested possible types

Type One
The adults of this type walk across the sea-bed, mainly scavenging organic scraps for their food but possibly also taking occasional live prey (e.g. worms, sea cucumbers, & other relatively small seabed-feeding organisms, or injured fish that fall into their reach…) as well. Their largest members could reasonably reach lengths of 12-20 inches [25-50cm], as the largest Trilobite fossils found in RL so far were around 16 inches [40cm] and the largest examples found of their current RL ecological counterparts — a marine genus of ‘Isopod’ Crustacean (Bathynomus), related to the terrestrial Woodlice (‘Sow Bugs’, ‘Pill Bugs’) — have been almost 20 inches in length. Slightly larger forms might be possible, in areas where food is relatively abundant (due to, for example, sunken Whale carcasses…), but any oceanographers’ claims to have sighted ones over three feet or so in length should definitely remain unsubstantiated by IC evidence for now: Their plausibility would decrease with increasing size, due not only to food supply limitations but also to biomechanical constraints and to the shortage of suitable cover for shelter from attack while moulting. Waters at those levels are normally quite well-oxygenated by now, so any ancestral adaption for low oxygen levels has quite possibly been lost. If we use those Isopods for comparison then the largest types would probably live in the ‘bathyal zone’ down to maximum depths of around 1.25 miles (2 kilometres), but there could be related species of about half those lengths on the ‘continental slope’ — and perhaps even parts of the continental shelf — above them. Obviously those living regularly closer to the surface would be the likeliest to have retained reasonably good eyesight.
The limits on their rages are probably more-or-less comparable to those for the RL ‘Giant Isopods’. The shallowest waters in which any of these have been found so far were only around 200 feet [22 metres] deep, but that was exceptional and depths of at least 560 feet [170 metres] seem to be more typical. The deepest level at which any of them have been found was around 8’200 feet [2.5 km] down, although the same species involved in that has also been found as close as 1’000 feet [300 metres] to the surface, but 7’000 feet [2.14km] seem to be a more typical limit and most examples of the best-known species are found — possibly due to food supply — at depths of “only” around 1200-2400 feet [365-730 metres].
As the difference in sizes between these and “typical” trilobites is much less than the equivalent difference among the Isopods the label “Giant” is probably used only for the very largest members of this group.
(We could have ‘Giant Deepsea Isopods’ like those RL ones here as well, if people want, but probably in different areas to these trilobites: Maybe, basically, one Ocean for each group?)

Type Two
The adults of these would be significantly smaller than those of Type One, and would be better-adapted for active swimming. They would prey mainly on ‘Sea Cucumbers’ (Echinoderm’ animals, thus related to Starfish & Sea Urchins), which are often relatively abundant at those levels and are ‘deposit feeders’ that extract micro-organisms & organic debris from ingested seabed material: The swimming ability is for moving longer distances to seek their specific prey, instead of just wandering around in the hope that enough food turns up as the larger type presumably does most of the time.
(Potential food supplies at that level are probably too low to support viable populations of Trilobites that deposit-feed directly…)

Type Three
The adults of these would live around the seabed’s ‘hydrothermal vents’ in tectonic ‘rifts’, where the geothermally-heated water can reach very high temperatures. Like the other animals living in those environments they would rely directly or indirectly for food on the chemosynthetic Bacteria which thrive in the mineral-rich waters that rise from within the Earth’s crust there, but their larvae would possibly still be planktonic. Forms adapted for living around vents rather than just seeps would obviously have to have evolved greater tolerance for high temperatures, and probably have some kind of temperature-checking sense (such as eyes adapted to see in infra-red?) as well. Those environments seem to support few significant predators in RL, so the local Trilobites’ armour might have become reduced in thickness and/or in mineralisation. Adaption for relatively low oxygen levels would probably be useful here, as might a greater tolerance for hydrogen sulphide.
Subtype ‘Three A’ simply scrapes bacteria from surfaces, and might have some of its legs adapted for this purpose.
Subtype ‘Three B’ has some of its legs adapted to form a ‘net’ or ‘sieve’ that filters floating bacteria out of the water, if[/]i the currents around the vents & seeps actually allow adequate concentrations of that food-supply [i](which I need to check before confirming that this variety is actually possible…).
Subtype ‘Three C’ actually carries some of the Bacteria as symbiotes, living in pouches of some kind in its own structure, however the RL fossil Trilobites that are thought to have had symbiotic bacteria managed this (which one source says was in specialised structures attached to their gills). It might also consume some bacteria scraped from surfaces, to supplement the symbiotes’ efforts.
Subtype ‘Three D’ takes a more indirect route to that foodstuff: It is an external parasite on the ‘Giant Tube Worms’ (‘Vestimentifera’) that also live at some such sites and that themselves rely on symbiotic bacteria for their nourishment. The adults of this variety would probably be smaller than those of any of the other subtypes listed here.


Type Four, subtypes ‘Four A’ to ‘Four D’: These are similar to Type Three, but live around the cooler ‘methane seeps’ instead and therefore haven’t evolved as great a tolerance for heat: Some of them might also colonise whale carcasses, probably arriving there after major scavengers such as the members of Type One have finished. (Type Three is probably descended from earlier members of Type Four.)


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More References
https://en.wikipedia.org/wiki/Giant_isopod
https://en.wikipedia.org/wiki/Whale_fall
https://en.wikipedia.org/wiki/Sea_cucumber
https://en.wikipedia.org/wiki/Hydrothermal_vent
https://en.wikipedia.org/wiki/Cold_seep
(Vestimentifera =) https://en.wikipedia.org/wiki/Siboglinidae

[Bears Armed]

Order: Neotrilobita
The adults of these Trilobites generally live in significantly shallower waters than do the surviving Proetida, quite often in coastal areas, with some of them possibly adapted to the brackish waters of estuaries & coastal swamps or perhaps even then (unlike any RL ones identified from fossils so far) — from brackish-water forms — for living in fresh water instead: Those adaptations could have arisen separately in several different lines. Food is more abundant at these levels than in the depths, but so are both competitors and predators as well, which would have encouraged evolution into a wider range of types than for their deep-sea relatives.
Suggested possible adaptations

A. The eyes are enlarged & arranged to give almost 360-degree vision, and/or angled more upwards, for quicker detection of attackers by species living in particularly shallow &/or clear waters such as those typical of coral lagoons. This happened in some of the fossil trilobites known from RL.
B. The eyes are on stalks, perhaps as an adaptation for seeing in particularly turbid waters. This happened in some of the fossil trilobites known from RL.
C. The head-shield has its ends extending backwards in thinner spines for some distance, possibly to make things more difficult for predators. This happened in some of the fossil trilobites known from RL.
D. Defensive spines project from the armour. This happened in some of the fossil trilobites known from RL.
E. Some of the fossil trilobites known from RL also possessed quite large projections of various kinds, sometimes described as ‘horns’, from the fronts of their heads. These might be simple rods, rods with a flat plate at the end, Y-shaped, or with several tines like a fork. Palaeontologists have not yet agreed on the functions of these, but use by males in fights over mates has been suggested as has the possibility that they simply made the animals a more difficult shape for predators to handle. I suspect that they might also have been used sometimes in fights over resource-rich patches of territory, or even as levers for tipping over small stones in search of food.
F. One or two pairs of the foremost limbs are modified to scrape, slice, stab, or club, food.
G. Some of the limbs (probably towards the rear) are modified into flatter paddles, to allow bursts of swimming speed for escaping attackers (or to ambush prey?)
H. As ‘G’, but also with lobed outgrowths from the armour (also towards the rear) to help with this swimming.
I. Some of the limbs are modified to form a ‘net’ or ‘sieve’ that filters food (micro-organisms, organic debris) out of the water. The number of other competing (and well-adapted) filter-feeding animals probably limits the viability of this lifestyle for Trilobites today, but I have thought of — and will explain below, in the ‘Types’ — one situation in which it might still be viable.
J. The outer edge of the armour curls back towards the body, although thinner there, forming an open-bottomed cavity to protect the gills so that the animal can burrow into loose sand. The need to maintain a water supply to the gills would still limit this ability, so they wouldn’t be able to burrow deeply after prey or excavate sub-surface nests, but it could allow hiding just below the surface to escape predators and/or to ambush prey. Stalked eyes might also be useful for these species.
K. As above, but the inwards rim of that extension actually curls upwards to form a ‘trough’ holding a small amount of water that the gills can use if some situation leaves the animal temporarily ashore. This would be more useful for small Trilobites than for larger ones, due to the Square-Cube Law.
L. Ability to eat un-armoured Cnidarian, such as sea-anemones or soft corals, without their ‘cnidoblasts’ (stinging cells) triggering within the Trilobites’ guts: Those animals, being soft-bodied and sessile, would be fairly easy prey… and the crystalline lenses of Trilobites’ eyes already give them one protection that possible competitors in this role probably lack. This digestive ability is possessed by at least one group of RL animals.
M. Digestive system adapted to feed on algae, such as Kelp, perhaps with forelimbs modified for ‘shredding’ this food into small enough pieces. (This ability and the one above would be mutually exclusive.)
N. Adapted to live in brackish water, to live in fresh water, or perhaps even to migrate between those habitats (with the adults moving upstream to breed, and the young then spending some time feeding & developing in fresh water before they move back to the coasts). Stalked eyes might be useful for species living in muddy estuaries, or in swamps.
O. Tolerance evolved for relatively low Oxygen levels, useful for species living where the water is likely to become stagnant or eutrophication is likely such as swamps (particularly in the tropics).

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Suggested possible types

Type One, Subtype ‘One A’: The adults of these walk across or swim just above the seabed, looking for food. They normally live in shallower waters than the surviving Proetida inhabit, right up to the shoreline, but their ranges possibly extend deeper (although not to the ‘Bathyal Plain’) in any areas from which Proetida are absent. They are primarily scavengers but may also take some live prey (as with the ‘Type One’ Proetida, this is normally fairly smaller and/or slower than themselves, maybe even sessile, but might also include some already-injured animals of other types — such as fish — that fall into their reach) as well although well-armoured animals are usually beyond their capabilities. This is basically the same role that is filled in RL (and in places in the IDU, too, possibly even in places that have Trilobites as well) by Lobsters and “typical” Crabs, and they are probably comparable to those in their range of sizes. Species that favour the deeper and therefore darker sections of this group’s possible range might have adults with only small eyes, or with no eyes at all, but have planktonic larvae that can see as well as those of the other types.
Subtype ‘One B’: These are more active predators, probably with one or both of the possible modifications ‘F’ and ‘L’. Species that do not prey largely on Cnidaria might also have evolved a ‘lip’ around the front of their carapace that they can bring down to the ground to trap small prey within their grasp, a feature seen in RL in some of the Polyplacophoran Molluscs (‘Chitons’).
Subtype ‘One C’: This is a more herbivorous version, with modification ‘M’ and possibly some extra defences. It occurs in ‘kelp forests’, and other places where high concentrations of suitable algae occur regularly below the tideline. Stalked eyes (modification ‘B’) might be useful for peering around the edges of kelp fronds to check for danger without exposing themselves. Some types might have evolved to feed on ‘Seagrass’ as well, although that is a tougher material.

Type Two, Subtype ‘Two A’: This is at the lower end of the likely size-range, and has a slightly rounder cross-section to its body. It is a predator that feeds mainly on the larger types of Sponges and Sea-Squirts, getting past their defences by using one of the openings through which they obtain & then pass out the water that they filter for food, which means that they can then use their prey as protective cover as well as for food. Depending on the difference in sizes, the Trilobites might even function as parasites without killing their prey completely. They are normally better swimmers than most of their ‘Type One’ relatives.
Subtype ‘‘Two B’: The adults of these are very similar to, but possibly even smaller than, those of ‘Two A’. In their case, however, their normal prey is the larger varieties of the deposit-feeding ‘Sea-Cucumbers’ (which can grow larger here than they do in the deeps) and they enter these (as at least one species of fish does in RL) through one end or the other of the digestive passage. (Potential food supplies at that level are probably too low, due to the high level of competition, to support viable populations of Trilobites that deposit-feed directly…)

Type Three: These are probably descended from a ‘Two’ A lineage. They have the same prey but in their case it is the larvae — although at a fairly late stage in their development — that enter this, and even when they complete their metamorphosis they remain smaller than any other types’ adults. This means that their food-supply can last for long enough to let them reach sexual maturity within those shelters, and they then release their gametes into the open sea (through the same openings by which the filtered water leaves) without ever leaving cover themselves. Consequently the adults have only thin armour.

Type Four’s adults are similar to those of ‘Two A’ in size, but (depending on ancestry) might not have the rounder cross-section. Their typical food source prey is Giant Clams or comparable large Bivalve Molluscs, with the Trilobite’s small size allowing it to get inside the shells of those without triggering a defensive closure. As with Type Two, the size differential means that it might be able to function as a parasite rather than as an actual predator.

Type Five probably descended from a member of ‘Type Five’, but is often a bit larger. This also lives within very large Bivalve Molluscs, using that location for shelter from predators, but does not attack them: Instead, it has some of its limbs adapted for filter-feeding (i.e. modification ‘I’) and simply competes with them for food… or, perhaps, mostly takes food particles of a different size than those which the Mollusc “prefers”. There might even be types of Bivalve that have evolved a tolerance for such Trilobites, letting them enter & leave freely even though they might have evolved to slightly larger sizes than the basic ‘Type Five’ because they also clear larger organic scraps — which might otherwise attract more dangerous animals, or high concentrations of potentially-pathogenic micro-organisms — from the immediate vicinity.

Type Six adults live on rocky shores, not only up to the tideline but also — at low tide — in rock-pools above this. They have a slightly flattened cross-section, making it easier for them to use cracks between or in rocks for protection from waves and the tides’ pull. (This also makes it easier for them to feed under the cover of any large Algae attached to the pools’ sides.) They might have the possible modification ‘K’, a trough around the lower rim of their armour holding water for the gills, allowing them to survive for a while if an exceptionally low tide leaves them stranded in a crevice that’s exposed to the air. (The difference in respiratory systems probably means that, unlike Crabs, no Trilobites could survive above the low-water mark — and escape the breaking waves, or the tides’ pull — by burying itself in the sand…) Subtypes ‘Six A’ would be a generalised scavenger/predator like ‘One A’, subtype ‘Six B’ a more active predator like ‘One B’, and ‘Six C’ a herbivore like ‘One C’. Their adults would be smaller than largest members of Type One, but probably [mostly] larger than most if not all members of Types Two/Three/Four. Some possible colloquial names for them could be ‘Shore-lice’ or ‘Rock-lice’ or ‘Pool-lice’, ‘Rock-crawlers’, or ‘Pool Bugs’…

Type Six is probably descended from members of Type Five. Its adults definitely have the possible modification ‘K’, and might escape the waves & tides either among rocks or among Mangrove roots & other swamp plants. They probably possess other modifications for life out of the water as well, such as stronger legs for walking, lightened armour (without the buoyancy that the water would provide to make carrying it easier), and perhaps slightly greater resistance to dehydration. Some might forage on beaches, some on mud-flats (like the ‘Mudskipper’ fish). They would probably be at a disadvantage compared to Crabs in most such situations, and would probably have to dip into water from time to time to refill their troughs, but there might be places where they could cope. Most would be fairly small, because this (due to the Square-Cube Law) would let them carry proportionately heavier armour, and go for longer on one load of water, than if they were larger. Ones more than couple of inches or so in length would probably be found only on isolated islands without native predators of any size, and would even then be limited to humid habitats, so that probably means oceanic islands of volcanic origin that are fairly close to the equator. Even then, these ‘Land-Prawns’ (to “borrow” a name from an SF series that I like..) would almost certainly have to be significantly smaller than the largest of RL’s Land Crabs, and would probably lose out to members of that group [for various reasons] if they arrive on the same islands.
In some humid & food-rich environments there might even be smallish members of this group that have adapted to live in — and forage ashore from — fresh water instead of salt: Tropical rainforests would seem the likeliest habitat for these.

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References
https://en.wikipedia.org/wiki/Cnidaria
https://en.wikipedia.org/wiki/Sea_anemone
(Soft Corals =) https://en.wikipedia.org/wiki/Alcyonacea

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Order: Lichida
Reference: https://en.wikipedia.org/wiki/Lichida

If these are present in the IDU’s waters at all then, as mentioned earlier, it is probably only in waters that members of the Neotrilobita have been unable to reach… although there might be some situations where the extra protection provided by their spines does allows them to colonise areas that are already inhabited by Neotrilobites without that feature.
If they have “leaked” in from another Earth that is still in its Paleozoic Era, as I suggested earlier, then they have probably not had as long as the Neotrilobita in which to evolve different adaptations and lifestyles. Therefore they are probably all equivalent to ‘Type One’ Neotrilobita in general lifestyle, and the only possible modifications that they are likely to have (apart from ‘D’, defensive spines, which seems to be the default setting for this Order but could possibly have been lost by one or more lineages of them here) are ‘A’, ‘B’, & ‘C’, all of which are known to have occurred in RL Trilobites; ‘E’, ditto; ‘L’, which wouldn’t have shown on fossils if any RL species did have it; ‘N’, ditto, but only for brackish water rather than fresh; and ‘O’, probably only if they also have N.

[Bears Armed]

(Reserved for the summary that I will make of people’s wishes about Trilobites present in their own nations’ waters, or discovered by their explorers in international waters.)

[Bears Armed]

(Reserved for the summary that I will make of people’s wishes about Trilobites present in their own nations’ waters, or discovered by their explorers in international waters.)

[Bears Armed]

(Reserved for the summary that I will make of people’s wishes about Trilobites present in their own nations’ waters, or discovered by their explorers in international waters.)

[Bears Armed]

(Reserved for the summary that I will make of people’s wishes about Trilobites present in their own nations’ waters, or discovered by their explorers in international waters.)