SILURIAN

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Name: Imidten Formation

Creator: AlDodo

Time period: 425-420 mya

Location: Baltica (Northwestern Eurasia), Scandinavia, (Metropolitan) Denmark, North Jutland, Læsø island

The Silurian, following the end of the Ordovician around 443 mya, and after the Ice Age period defining the latest part of the latter period, which resulted in the Ordovician–Silurian extinction events, causing up to 60% of marine life being wiped out, might be superficially seen as a period of relative stability compared to most other previous and future periods. However, we often observe different important climatic switches within its differents parts, being more unstable and changing at a higher degree.

As a whole, this period is predominantly notable for being comparatively short in geologic terms, being the third and shortest period of the Paleozoic Era, but still a very significant and important one, nonetheless, for Earth’s life as a whole.

It acts as a period of recovery for life and of transition from the Ordovician to the Devonian, both climatically and geology.

For the most part, the Silurian was marked by Earth entering a warm greenhouse period, particulary defined and supported by high CO2 levels of 4500 ppm as well as warm shallow seas which covered much of the equatorial land masses. The climate was dominated by violent storms generated by warm sea surfaces.

With the global temperatures increasing again since the late Ordovican, in which they only slowly continued to fall step by step, the glaciers created by the Ordovician’s ice ige as an effect retreated back into the south polar areas and continued to decrease to even almost disappearing by the middle of the Silurian.

While life does thrive usually better in warm contexts, it still must be said however that the warming climate, which ended the massive ice age period which itself ended the Ordovician and started the Silurian, initially also prolongued the mass extinction and cementing the boundary between the two periods, as the organisms which initially survived the increasing intense cold climate and temperatures, surviving under these conditions, suddenly have to deal with warmer temperatures.

But the warming period of the Silurian was nonetheless quickly favorable for life as a whole, and with rising sea levels, especially during the first half of the period, the shallow waters which mostly disappeared in the Ordovician, due to the cooling climate of the ice age, reappeared and allowed opportunities for many clades. Which radiated and recovered their diversity and population from the losses at the end of the Ordovician.

And while the global climate underwent many drastic fluctuations in temperatures throughout the era, going back and forth between warm and cold, it was never as low as the extreme cold of the late Ordovician nor as high as the extreme heat of the following Devonian.

This would result on minor extinctions occuring within the period, such as around 434 mya, with the start of a global cooling period event making a huge drop in deep ocean oxygen levels, particulary affecting deep ocean life (conodonts, trilobites and graptolites).

At about 428 mya, the start of another oxygen drop led to the extinction of 95% of graptolites.

One, at 424 mya, occured seemingly due to a global drop of sea level as a result of glacier formation. These extinctions were however affecting life more at a local level than at a global level for the most part.

It is only near the end of the Silurian that the climate, which would have remained relatively "stable" for the last almost 25 my, would swift from decently warm to hot temperatures. Creating a new boundary starting the Devonian.

The Silurian is usually viewed by several palaeontologists as an extended recovery interval following the late Ordovician mass extinction (LOME), which interrupted the cascading increase in biodiversity that had continuously been going on throughout the Cambrian and most of the Ordovician.

While the Silurian itself was a rather short period, it still, like the other periods, witnessed other important grades in the evolution of life, not just one but two.

While the Precambrian was marked by the appearance of life, the Cambrian the appearance of derived and recognizable trophic levels and food chains, with the earliest recognizable modern predator and prey dynamics, and the Ordovician the complexification of the ecosystems, the Silurian sees the appearance of the first fully jawed vertebrates, aka the first jawed fish, which would mark the appearance of what is recognizeable as a typical fish by people, and starting the vertebrate line which would lead to one of the most successfull organism lineages that woud dominate Earth for most of the rest of its history.

The clade in question, in which the major evolutionary innovator of the jaws' trait is the Gnathostomata (familiarily refered as "jawed fish"), sitting next to members of the paraphyletic clade Agnatha (refered as "jawless fish", and represented today only by the extant lampreys and kin, and divided into two major morphs, the armored jawless fish and the non-armored jawless fish) from which it is derived (having seemingly evolved from representatives of the armored jawless fish morphs), appeared at the very start of the Silurian, over 439 mya.

The revolution of the movable jaws is as a consensus believed to have evolved from, as surprising as it may sound, from one of the gills that fish have, with the said jaws evolving from the first and closer to the mouth gill arch.

Subsequently it became connected to the skull, giving rise to the typical jaws of most vertebrates, with the following teeth and jaw muscles coming along with it in addition.

The Gnathostomata, perhaps incorrectly refered as the "true fish" ("true fish" evolved by the early-mid Cambrian) initially started with the famous "placoderms" (Placodermi), aka the armored jawed fish, before evolving overtime from this stock into 2 major groups leading to:

•  the cartilaginous fish (or Chondrichthyes), whose skeleton is made of flexible cartilage (that is often poorly fossilized and preserved), which are thought to have derived from a paraphyletic clade named the acanthodians (Acanthodii) and to which belong the modern extant chimaeras, rays and sharks

• and the bony fish (or Osteichthyes), whose skeleton is usually made of bone and in which belong all other modern jawed vertebrates (all modern non-agnathan and non-cartilaginous fish, and includes all tetrapods).

And it is also during this period that the bony fish divided into two groups that would eventually lead to: the modern ray-finned fish and the lobe-finned fish. The latter line would be the one from which later tetrapods will descend from.

And while it was an age of fish radiation and diversification, such as Guiyu oneiros, one of the earliest known fossil member of the lobe-finned fish lineage known, being around 30 centimeters long, or Megamastax at around 1 meter long (among the largest vertebrates on Earth during that time), fish would  still remain relatively small, and only reach larger sizes and their peak in diversity during the Devonian.

But the Silurian is even more remembered for another, more important, aspect regarding life’s evolution on Earth, which took place during this period.

This being none others than appearance of the earliest major land animal communities, out of the water.

This major establishment of terrestrial life, no doubt one of the major steps in life’s history, is known as the Silurian-Devonian Terrestrial Revolution.

With plants and fungi further expanding themselves and diversifying, at least three major groups of arthropods (myriapods, arachnids and hexapods) were independently full terrestrial influences, near the end of the period.

Already forming decently complex ecosystems (being the earliest major verifiable soil ecoystem blueprints, persisting for quite a long time), with predatory invertebrates and non-predatory prey animals showing simple food webs in place, also including what seemed to already be detritivores and grazers as well.

The herbivores would be feeding on the simple plant cast composed mostly of representatives of the moss lineage, as well as a few other "tall" plants, and fungi.

Plants and fungi are the initial macrorganisms to have established out of the water and settle on land, with them starting to, overtime, make the terrestrial atmosphere even more breathable, helping create oxygen from CO₂, and as an effect allowing animals to live on land. Arthropods came much before vertebrates, thanks to their lungs which, while more primitve and less sophisticated, were still able to catch the minimal air oxygen level present in the atmosphere at that time.

Despite this remarkable step forward for life, most of its diversity was still predominantly by a extreme margin still living underwater and thriving at a higher level in the seas. Organisms were becoming more diverse and complex than before.

New types of corals arrived (including lineages of tabulate corals, from orders of the Tabulata class) and new crinoids, exploding in diversity.

Brachiopods became abundant and diverse, with the taxonomic composition, ecology, and biodiversity of Silurian brachiopods mirroring Ordovician ones, in good part due to developing, again, high endemism in various parts of the world, and after the LOME event developed novel adaptations for environmental stress. Some of twhe most abundant groups were the atrypidans and pentameridans.

Other successful groups witnessed recovery and major degrees of diversity included the bryozoans, exhibiting particulary significant degrees of endemism to particular shelfs, as well as also developing symbiotic relationships with cnidarians and stromatolites, and the bivalves, including deep sea lineages, while other groups of marine animals such as members of the chiton lineage, hederelloids and tentaculites saw, at that time, a peak of diversity.

Trilobites, in their own side, continued to enjoy success in the Silurian, as they had in the Ordovician, after ecovering from the LOME at the end of the last period, but still, they're now more reduced in clade diversity due to that event.

Some groups, however, faced a decrease in biodiversity and number around that time, such as a sharp decline of jawless fish such as conodonts and "ostracoderms", that could correlate with the diversification and increase of jawed fish. Jawed fish potentially were outcompeting their jawless counterparts due to their novel jaws, allowing them to better consume resources in their environnement, being able to explore more niches.

Members of the cephalopod lineage, the "orthocones", which were, for the most part, dominant invertebrate clades in the Ordovician, were hit hard by the end of said period, but survived and recovered in the Silurian, but would still be on a lesser diversity and would overtime, by the late Silurian, and within the Devonian, decline, step by step, while the jawed fishe would increase in size, number, diversity and population. The fish would be able to use their jaws to break the shells of these cephalopod lineage members.

All to say that this period of Earth’s history was equally, if not more, important than the other periods about the evolution of life on the planet, even so, however, it is relatively less studied, despite putting the very bases and seeds which would one day lead to the land cast of organisms that would end up dominating the planet in the following periods.

Althought the earliest records of animal life in true first major ecosystems on land are recorded from this period, actually very little is known regarding life on land at this point in time, as the number of good and informative formations of such biota are few and far between.

Some of the available ones are of dubious, undertermined dates, such as the Cowie Formation in Scotland, United Kingdom, which dates from the middle Silurian at 430–428 mya to the early Devonian.

However, one novel Silurian formation, not found in our timeline, seemingly offers an additional source of data that may help to better understand furthermore some aspects of how life was on land and filling some gaps in our understanding of Earth’s life in this period.

Present in a very spot on and inhabited area in the middle of human civilization, it remained undiscovered for a very long time to the nearby inhabitants, oblivious to it, while in front of them, being the Imidten Formation.

Found on the small and very isolated island of Læsø, in the territory of Denmark, in North Jutland area, and named in the local tongue from the words i midten, meaning at center in Danish (as the formation is at the very center of the island, inland).

The Imidten Formation is a formation which, compared to the other ones found in our timeline, was difficult to access or hard to spot.

The island of Læsø is not very highly populated, with only a demographic population less than 1,800-1,900 inhabitants (around our timeline's 2020’s), being a very small island and a place that is not particularly heard of (despite a pretty decent level of tourism), but also that just no major paleontological surveys and studies for potential formations are usually conducted. Only a few acheological patrols are made regarding local human populations and related settlements, tools and objects of antiquity, in the last century, for the most part.

Data of the formation is relatively limited, being rather small, only spanning some kilometers (the size of 2-3 small fields), but still offers a very surprising condensed and compacted number of well-preserved fossils and data from the geological rocks.

The rock elements allow the formation's date to be precisely determined to the late middle Silurian and almost the entire late Silurian, around 425-420, spanning the second half of the Ludlow epoch (Ludfordian) and most of the late Silurian (Přídolí). It ends roughly one million year before the end of the Silurian. Some data may suggest the formation potentially may have had a longer lifespan, existing for some additional million years and through the first 2 to 3 million years of the Devonian, but as of now, this idea is conjectural.

Back then, the formation was located somewhere near northern areas of what was once the continent of Baltica, which represent today most of northwestern Eurasia, including Scandinavia and subsequently Denmark.

For the most part, the Imidten Formation was revealed to have been relatively similar, if not identical in broad strokes, to the landscapes on land of other Silurian formations, giving a glance of life out of water already at display.

With the formation itself determined as composed of both land and underwater settings, relatively close to the sea, it’s isn’t known for certain if the exact location was set directly on the coasts or inland, with the sea out of view, instead with important water systems composed of saltwater going pretty far in the interior of the landmass.

The overall landscape of the formation is typical for terrestrial biota at that time, being mostly a flat region (mountains in thr lands on almost all continents were low in number back then), not very high above sea level, and composed of various brackish inland estuarine systems of shallow deltas and rivers of variable sizes scattered within the region. This created many island areas of equally variable sizes.

Most plants remained close to freshwater and brackish ponds, and areas in which both types meet each other and mix.

Almost the entirety of the landscape was otherwise recovered as just a "rug" of members of the moss lineage and several fungi, here and there, forming an entire green ground spanning to infinity and beyond on all the landmass.

These representatives of the moss lineage, and few other plants, with stems directed to the sky by only a few centimeters high, were, as everywhere else back then, part of the earliest major "plains" and "forests" to exist, with various very small arthropods inhabiting this ecosystem.

Most of these found so far, by a wide margin, are millipedes (recognizable typical millipedes, accompanied also by centipedes) and trigonotarbid arachnids. But other arthropods have been found, such as members of the true scorpion lineage, with some representatives of the family Dolichophoniidae, including what seem to be the genus Dilochophonus itself or a similarily closely related genus.

Surprisingly, remains of several thylacocephalan and pycnogonidan (aka sea spiders) have also been found, but it is uncertain, especially for the second group, if these findings represent freshwater species or specimens lost or rejected from the nearby sea into the inland ecosystem of the formation (much more likely for the sea spiders). What appears to be potentially some early hexapods of uncertain affinities have also been recovered from there too.

Additionnaly, what appear to represent an arthropod of an yet undescribed species closely related to the egnimatic genus Parioscorpio of the late early Silurian of the Waukesha Biota (438-433 mya), part of the larger Brandon Bridge Formation, has been uncovered there. If indeed a related animal to it, would not only reveal a new lineage of arthropod in the region but also considerably extend the lifespan of the lineage to almost the entire Silurian period itself. A family suggested for these animals would be named Parioscorpionidae.

The rivers and deltas themselves were more typical underwater ecosystems, forming pretty regular brackish swamps, with mostly animals adapted to saline conditions and many coming from nearby sea being found. Found would be either annual coastal residents or occasional seasonal marine migrants.

"Agnathan" jawless fish are well-represented, with several species of anaspids, few armored osteostracans and 3 traquairaspidiforms, but most fish are represented by acanthodiians, with around up to 16 species known so far.

Besides these, not a lot is known about this formation.

What follows are several well-known organisms from the formation.

Name: Prototaxites laeso

Size: Around 8 meters tall on average

Clade: Fungi, Eumycota, Amastigomycota, Symbiomycota, Dikarya, Ascomycota, Prototaxitaceae, Prototaxites

Prototaxites laeso ("Laeso’s first conifer") is a titanic fungi found in very high abundance in the Imidten Formation and the largest and tallest organism recorded from there. The genus itself is known from a very wide distribution, especially in North America while this specific species so far is only known from this formation in Europe. Its named from the island on which the location is present.

A terrestrial fungi of impressive sizes, it belongs to the Ascomycota, as it is strongly thought as an early member of the Prototaxitaceae family, making it a species of ascomycetes or, more commonly known, a sac fungi. Ascomycetes includes none other than the biggest and most diverse group of the mushroom-bearing lineage of fungi on Earth today.

While several ecologies (also uncertain) have been proposed, there is some consensus that this mushroom fed on a range of substrates, instead of performing photosynthesis, feeding on the remains of whatever other organisms were nearby.

More precisely, it was a fungal rhizomorph, possessing mycelial cords as root-like structures, living by being saprotrophic like many modern fungi.

This is supported and consistent with the fact that Prototaxistes, for all its species, including P. laeso, needed, obviously, a high level of biomass to sustain its body.

At a size of already around 8 meters tall, like for most of the other species of the genus recorded, (variations in sizes exist depending on specimens and populations), this organism was none other than the first (recorded from the fossils record) large land lifeform, largest, longest and tallest (non-animal) organism that existed on Earth’s history, since the appearance of life itself and of its own time. It dwarves all others known Silurian organisms, be them other fungi, plants and animals of both land and sea.

Plants, such as the contemporary Cooksonia (present at the Idmiten Formation), were among the terrestrial lifeforms to arrive in second position in term of size after the giant mushroom, being only around 15-20 centimeters (6-8 inches) tall, to 30 centimeters (12 inches) tall maximum, in comparison.

This makes Prototaxistes laeso, like the rest of the genus as a whole, the ruler and dominant species within its ecosystem. They stood as the "moss-like forests" grew beneath it, in its shadow, while invertebrates were the only other major moving land-dwelling multi-cellular life.

Name: Apuprimiumcutio terrestris

Size: Around 12 centimeters long on average

Clade: Arthropoda, Deuteropoda, Euarthropoda, Antennulata, Mandibulata, Pan-Myriapoda, Euthycarcinoidea, Myriapoda, Progoneata, Diplopoda

Apuprimiumcutio ("among first millipede") is a small archaic millipede of uncertain taxonomic placements living on land, above water, and found so far strictly only in the Imidten Formation and known from around a dozen specimens found at 3 locations within the formation. Its among the earliest and largest "megafaunal" creatures of both this formation and of the world back at the age of the latest Silurian.

A diplopod myriapod, it is a species of multi-legged arthropod which seems to be a moderate presence within the ecosystem’s cast of the terretrial parts of the formation, but still among the largest creatures found on land at the time and seemingly was within the dominant non-predatory creatures within the trophic level. As such, it played an important role in maintaining the local ecosystem, much like the other present arthropods, them too forming what was one of the earliest complex terrestrial food chains to have evolved.

Apuprimiumcutio is a large land animal by the standards of this era, with a lenght of around 12 centimeters and 11 segments both composed of a small paranota (keels) high on the body alogn with long and slender legs. It had 2 pairs of legs for each segments, for a total of 22 legs. The dorsal portion of each segment is ornamented with a horizontal bar and some rows of roughly hexagonal bosses.

The species have its cuticle containing openings which are spiracles, and so part of a gas exchange system that would only work in air, providing evidences that this animal was indeed well a fully terrestrial creature.

Because Apuprimiumcutio is recorded from undoubtable dates earlier than the other millipede Pneumodesmus of uncertain occurrence (due to the formation from which it was recovered, the Cowie Formation, being of debatable geological date, either the Silurian or Devonian), this makes Apuprimiumcutio the earliest documented arthropod with a tracheal system, and among the earliest known oxygen-breathing animals on land. This is, at least compared to the ones we do have definitive dates of occurrence.

Apuprimiumcutio is believed to be an herbivorous animal, and among the main large herbivores of its ecosystems, as indicated by its notably large and robust mandibles. These are thought to serve to cut, process and help to ingest particulary tough and robust vegetal matters.

Thus, the inclination of the head towards the ground seems to suggest this species only eats what is directly growing on and from the ground, meaning its diet includes predominantly members of the moss lineage found in extreme abundance. This would be instead of stems of others plants or the growing layers of mushrooms. This would make Apuprimiumcutio, if true, among the earliest terrestrial "grazing" species to have evolved on Earth.

It is not known if Apuprimiumcutio had a good vision or if it is rather poor or blind, nor if it was diurnal or nocturnal, but its antennae seem to have served an equal role in helping the arthropod move around in its habitat and sense obstacles, predators and congeners.

Name: Dupliciculmus tamisium

Size: Around 15 centimeters long on average

Clade: Arthropoda, Deuteropoda, Euarthropoda, Antennulata, Artiopoda, Trilobitomorpha, Trilobita, Phacopida, Phacopina, Dalmanitoidea, Dalmanitidae, Dalmanitinae, Dalmanitini, Dupliciculmisina

Dupliciculmus ("double stalk") is an unusual species of trilobite recovered from the Imidten Formation, known from plenty of specimens primarily found in what would be areas of mostly freshwater rivers rather than the more saline, brackish parts thought to have logically been closer to the sea.

A dalmanitoid phacopidan, and belonging to the suborder Phacopina, this animal is among the oddest creatures reported from the formation, as it is an unexpected species of this famous group of arthropods in such a context, as trilobites aren’t known to have frequented, even occassionally, freshwater biota in inland regions. This makes it a very remarkable and unique genus within this whole class of extinct Paleozoic creatures. This makes it the first ever undoubted freshwater trilobite ever discovered to date.

Dupliciculmus has been found to be a close relative of the genus Dalmanites, a well-known genus which lived during all the Silurian period, and had a very similar bodyplan.

As a whole, it had a moderately vaulted, slightly convex exoskeleton with a broddly inverted egg-shaped outline, a head shield (or cephalon) which is semicircular, and its limbs are very analogous to the ones of Dalmanites.

However, it differed from this genus in some key aspects. Notably by having the frontal margin of the cephalon diagnosed with several protrusion structures extending the cephalon forward, with 4 pairs in total, and with the first pair being considerably larger than the 3 others.

Meanwhile, the front margin of the cephalon of Dalmanites is semicircular to parabolic, and it may have a simple and short anterior extension.

Given this trait has been found in both in males and females in Dupliciculmus, it is unlikely this would have been a sexual display structure, used for sexual purposes, such as fighting other male opponents over females; such have ben recorded in several families of Phacopidans such as the Acastidae (with Walliserops) in the Acastoidea. In these, males had specialized extension structures in their head to fight each other, so to reverse their opponents on their back.

These would not be used for intimidating other males nor impress females.

Instead, scientists currently think these structures may have served likely for digging and shift sand, soil and substrate underwater, when foraging for food. This makes it a convergent feature but used for entirely different purposes.

Because of these different divergent traits, its concluded, as a consensus, that the Imidten’s trilobite was different enough and having enough diagnostics traits from other related trilobites to be safely assigned to a different genus from Dalmanites and kin, of the Dalmanitidae, but perhaps also within an entirely different family within the Dalmanitoidea group.

This leads to the proposed erection of the family Dupliciculmisidae, to which Dupliciculmus is so far the sole member.

This would make it the fourth family known of this group, after the type family of the Dalmanitoidea, the Diaphanometopidae and the Prosopiscidae.

Name: Aliusalesnumus primus

Size: Around 75-80 centimeters long on average

Clade: Arthropoda, Deuteropoda, Euarthropoda, Arachnomorpha, Chelicerata, Euchelicerata, Prosomapoda, Planaterga, Dekatriata, Sclerophorata, Eurypterida, Eurypterina, Diploperculata, Pterygotioidea, Pterygotidae

Aliusalesnumus ("another winged one") is a decently large predatory aquatic arthropod inhabiting the rivers and shallow lakes and deltas scattered within the Imidten Formation, and whose remains are found in an appreciable number, yet not in a high population. An entire agglomeration of around 30 or so specimens is known, thought to represent a regrouping of female individuals while spawning eventually going deceased, condensed and superimposed on each other. Some other specimens of varying sizes, representing several stages of growth, are found independently.

A pterygotid pterygotioidean eurypterid, this animal, from its type and sole known species, A. primus, named in reference to be among the first organisms known from the Imidten Formation, is a member of the famous arthropod group popularily known as sea scorpions. Its a rather typical member of this group regarding its general bodyplan, being the one people think of when having in mind these animals of this clade.

If Aliusalesnumus, as a sea scorpion, isn’t notable bodyplan-wise, it is, however, on the other hand, sufficiently remarkable for being found in a non-marine setting, which was composed of a decent percentage of freshwater constitution. Various areas of the Imidten Formation weren’t brackish nor were mixed together with freshwater all the time.

This might be seen as unusual for eurypterines, where many prefered a marginal marine and saline setting.

The reason for the presence of such a member of the group isn’t known, so it might be due to potential intense competition in the oceanic setting, given sea scorpions were, at the time, at the peak of their golden age.

But what is sure is that this genus was a year-round resident to the formation, not composed of migrating specimens that momentarily came to a more shallow, shore-like biome to lay their eggs, in which young would live their early years, which is what the find of numerous deceased female specimens found together would have initially made one believe. If that were the case, only females and young with a few years old, would have been recovered, but many males and specimens of various stages of growth have been found as well.

In the ecosystem of the formation, Aliusalesnumus is strongly believed to be an ambush predator, instead of an active pursuit predator, much like most other sea scorpions.

This sea scorpion is even justly extremely remarkable for a trait supporting such lifestyle, not seen anywhere else in the entire group, and something to do with its specialized chelicerae converted into claws similar to the ones of scorpions or crustacean, in which they are unique in that they are built specifically for retaining and keeping prey rather than grasping.

The "teeth" of the claws act as spikes on both the "hand" and "finger", being disposed in a way that the spikes of one side fit in the holes between two of the spikes of the other side. The spikes of the "hand" and the spikes of the "finger" interlock and fit perfectly when the pliers are completely closed.

Ending with an efficient tool to retain prey, with the spikes of both sides sinking into the flesh of the target, in two different areas, the victim is unable to easily free itself, as attempting to swim in either direction leads to one row of spikes sinking deeper into the flesh even more by doing so.

Because of this, it is believed that the genus had a diet of predominantly small soft-bodied prey, which are thought to have been mainly unarmored jawless and jawed fish, as its claws aren’t strong enough to deal with hard or armored species, and its spike-like structures served to retain prey would break under a too high stress or effort.

Name: Asadrepanopterus longumfinus

Size: Around 60 centimeters long on average

Clade: Arthropoda, Deuteropoda, Euarthropoda, Arachnomorpha, Chelicerata, Euchelicerata, Prosomapoda, Planaterga, Dekatriata, Sclerophorata, Eurypterida, Stylonurina, Mycteropoidea, Drepanopteridae

Asadrepanopterus ("like-Drepanopterus") is another, relatively common sea scorpion compared to Aliusalesnumus, and of similar sizes, albeit slightly smaller in average than the latter, found in various parts of the Imidten Formation, in deposits offering better conditions for fossilization processes. A female specimen is preserved with a clutch of eggs found inside her body and thought to be about to be layed.

A drepanopterid mycteropoid, this genus, composed by its only known species, A. longumfinus, is one of the 3 stylonurine sea scorpions found within the formation, making a total of 4 sea scorpions as a whole, with Aliusalesnumus if eurypterines are included, and the one which is the most abundant with all the members of both groups recorded from the formation.

It possesses a broad, rounded head carapace, with its abdomen rather surprisingly thin, with the transition from the broad form to the thin, slender rear being rather drastic at a given point in the body. The telson at the end of the abdomen is also notably elongated compared to most other stylonurines, forming a sort of spear-like structure in some perspectives. The type species of this genus has been named based on this trait with longumfinus meaning "long fin".

This animal also possesses lateral compound eyes with parallel axes on a subrectangular to subovate prosoma (head). With the first three pairs of legs, appendages I (not visible in a view from above such as in the picture provided here), II and III being short and powerful, with spines. The following three pairs are moderately long, with each ending in a strongly curved terminal claw. The last leg of the three reaches as far as the penultimate abdominal segment (not including the telson).

Because of the disposition and size of its legs, as well as the fact its a rather heavily built animal, Asadrepanopterus isn’t a fast nor agile arthropod, and instead a pretty slow-moving creature.

Its locomotion is well-known thanks to numerous fossilized trackways found in the formation, attributed to this genus.

These fossilized eurypterid trackways tend to be large and heteropodous, often having the associated telson dragging mark along the mid-line and leavign a clear line between two rows of rounded spots on the ground. The trackways of this species are indeed formed as such.

It is accepted, as a consensus, that this animal is a slow predator and detritivorous scavenger, and only eats very small to near microscopic prey (mainly fauna and maybe alongside a small part of plant matter, accidentably, reminding one of the bonnethead shark, Sphyrna tiburo), fitting a niche akin to horseshoes crabs, and being more precisely a sweep-feeder (raking through the substrate in search of prey).

Name: Sawdonia abundans

Size: Around 8-9 centimeters long on average

Clade: Plantae, Charophyta, Phragmoplastophyta, Embryophytes, Polysporangiophytes, Tracheophyta, Eutracheophytes, Lycophytina, Zosterophyta, Zosterophyllophytina, Zosterophyllopsida, Zosterophylls, Sawdoniales, Sawdoniaceae, Sawdonia

Sawdonia abundans ("abundant Sawdonia") is a widespread and dominant species of early vascular plants known from the Imidten Formation, with most fossils being recovered from areas which were near water ponds, and from which its density of population is the most abundant. Hence its species’ name.

A member of the zosterophylls in the order Sawdoniales, to which its genus is the type from which this group is named, belongs to Sawdonia, which has an almost cosmopolitan distribution across the globe. It is found in an extremely broad range of countries, from Canada in North America (at the Battery Point Formation), Venezuela (at the Campo Chico Formation) in South America, as well as in England and Scotland in Europe (at the Strathmore Group). Some possible still undefined records exist from China, in Asia.

The species found in the Imidten Formation, S. abundans, represents one of the easrliest occurrences of this genus.

Sawdonia abundans is overall pretty analogous to the other known species of this plant, best recognized by the large number of spikes/spines (enations) covering it, clearly visible along the leafless green photosynthetic stem they grow on. But despite being a vascular plant, there aren't vascular systems in said enations. These are tapered and pointed lower on the plant but form loose spikes at end of the plant.

The stems of the plant are monopodially branched, being unridged, spinous and circinately tipped, bearing sporangia which are round in an abaxial view and oval in a lateral view. These same sporangia are formed laterally and singular on short stalks.

Another traits of the plant being the xylem being simple as displayed as one solid strand, while the epidermal cells have cuticular papillae and stomata being present on the stem but not on spine surfaces.

In this specific species, S. abundans, the stems grow at the ends by unrolling themselves, a process known as circinate vernation.

To be added

• Milne Bay Formation / Papua New Guinea (430 mya) - Not all of the Cambrian fauna died out after the period’s end. In fact many are still around as of the Silurian. Some have even managed to reach the fringes of the land, just a few steps away from becoming terrestrial. However, almost none will achieve this, whether because of diet, breathing methods or competition. Nevertheless, the Milne Bay Formation gives us an insight as to what intertidal life looked like in the shallows of Silurian Gondwana, alongside some interesting precursors to the life of the Devonian. (IC1101)

• Imidten Formation / Baltica (Northwestern Eurasia), Scandinavia, (Metropolitan) Denmark, North Jutland, Læsø island (425-420 mya) - One novel Silurian formation, not found in our timeline, seemingly offers an additional source of data that may help to better understand furthermore some aspects of how life was on land and filling some gaps in our understanding of Earth’s life in this period. Present in a very spot on and inhabited area in the middle of human civilization, it remained undiscovered for a very long time to the nearby inhabitants, oblivious to it, while in front of them, being the Imidten Formation. Found on the small and very isolated island of Læsø, in the territory of Denmark, in North Jutland area, and named in the local tongue from the words i midten, meaning at center in Danish (as the formation is at the very center of the island, inland). The Imidten Formation is a formation which, compared to the other ones found in our timeline, was difficult to access or hard to spot. The island of Læsø is not very highly populated, with only a demographic population less than 1,800-1,900 inhabitants (around our timeline's 2020’s), being a very small island and a place that is not particularly heard of (despite a pretty decent level of tourism), but also that just no major paleontological surveys and studies for potential formations are usually conducted. Only a few acheological patrols are made regarding local human populations and related settlements, tools and objects of antiquity, in the last century, for the most part. Data of the formation is relatively limited, being rather small, only spanning some kilometers (the size of 2-3 small fields), but still offers a very surprising condensed and compacted number of well-preserved fossils and data from the geological rocks. The rock elements allow the formation's date to be precisely determined to the late middle Silurian and almost the entire late Silurian, around 425-420, spanning the second half of the Ludlow epoch (Ludfordian) and most of the late Silurian (Přídolí). It ends roughly one million year before the end of the Silurian. Some data may suggest the formation potentially may have had a longer lifespan, existing for some additional million years and through the first 2 to 3 million years of the Devonian, but as of now, this idea is conjectural. Back then, the formation was located somewhere near northern areas of what was once the continent of Baltica, which represent today most of northwestern Eurasia, including Scandinavia and subsequently Denmark. For the most part, the Imidten Formation was revealed to have been relatively similar, if not identical in broad strokes, to the landscapes on land of other Silurian formations, giving a glance of life out of water already at display. With the formation itself determined as composed of both land and underwater settings, relatively close to the sea, it’s isn’t known for certain if the exact location was set directly on the coasts or inland, with the sea out of view, instead with important water systems composed of saltwater going pretty far in the interior of the landmass. The overall landscape of the formation is typical for terrestrial biota at that time, being mostly a flat region (mountains in thr lands on almost all continents were low in number back then), not very high above sea level, and composed of various brackish inland estuarine systems of shallow deltas and rivers of variable sizes scattered within the region. This created many island areas of equally variable sizes. Most plants remained close to freshwater and brackish ponds, and areas in which both types meet each other and mix. Almost the entirety of the landscape was otherwise recovered as just a "rug" of members of the moss lineage and several fungi, here and there, forming an entire green ground spanning to infinity and beyond on all the landmass. These representatives of the moss lineage, and few other plants, with stems directed to the sky by only a few centimeters high, were, as everywhere else back then, part of the earliest major "plains" and "forests" to exist, with various very small arthropods inhabiting this ecosystem. Most of these found so far, by a wide margin, are millipedes (recognizable typical millipedes, accompanied also by centipedes) and trigonotarbid arachnids. But other arthropods have been found, such as members of the true scorpion lineage, with some representatives of the family Dolichophoniidae, including what seem to be the genus Dilochophonus itself or a similarily closely related genus. Surprisingly, remains of several thylacocephalan and pycnogonidan (aka sea spiders) have also been found, but it is uncertain, especially for the second group, if these findings represent freshwater species or specimens lost or rejected from the nearby sea into the inland ecosystem of the formation (much more likely for the sea spiders). What appears to be potentially some early hexapods of uncertain affinities have also been recovered from there too. Additionnaly, what appear to represent an arthropod of an yet undescribed species closely related to the egnimatic genus Parioscorpio of the late early Silurian of the Waukesha Biota (438-433 mya), part of the larger Brandon Bridge Formation, has been uncovered there. If indeed a related animal to it, would not only reveal a new lineage of arthropod in the region but also considerably extend the lifespan of the lineage to almost the entire Silurian period itself. A family suggested for these animals would be named Parioscorpionidae. The rivers and deltas themselves were more typical underwater ecosystems, forming pretty regular brackish swamps, with mostly animals adapted to saline conditions and many coming from nearby sea being found. Found would be either annual coastal residents or occasional seasonal marine migrants. "Agnathan" jawless fish are well-represented, with several species of anaspids, few armored osteostracans and 3 traquairaspidiforms, but most fish are represented by acanthodiians, with around up to 16 species known so far. Besides these, not a lot is known about this formation. (AlDodo)

• Friendship Formation / Off the coast of Mauritania (421 mya) - Terrestrial life has been establishing across the world in the Silurian, to a level far more complex than in previous periods. While notable fossil formations in Laurentia and Baltica showcase a diverse early terrestrial environment, little is known on how it was manifested in the larger Gondwana continent. Scant evidence exists about the terrestrial life that existed there, with records of a diverse array of plant life, as well as a few signs that arthropods, like myriapods and others, were present. Notably, in Australia, some of the oldest terrestrial arthropod fossil tracks are known, noting how life was already establishing inland perhaps even before the iconic Ludlow and Přídolí terrestrial communities of other continents, even in harsh environments like deserts. However, terrestrial life in Gondwana, during the Silurian, is still vastly unknown. The Friendship Formation is a peculiar ecosystem, dated to the Přídolí epoch, some 421 million years, roughly contemporaneous with the Cowie Formation of Scotland. Unlike the latter, which depicts a nice and warm tropical environment, the Friendship Formation was located on what is today the seaways west to Mauritania, back then part of an ancient landstrip in the Antarctic Circle. After the Lau event, at the end of the Ludlow, global temperatures increased, and old land bridges started to fragment into smaller landways surrounded by sea, increasing endemism in various landmasses. The warmer temperatures also promoted higher biotic diversity and allowed life to thrive even in the polar regions, free of glaciation, during the Přídolí. The Friendship Formation is notable because it shows a variety of incredibly tiny organisms that would never or hardly preserve in any ideal fossil condition, giving us a glimpse on the diversity of Silurian terrestrial ecosystems. (YellowPanda2001)