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Animals
Animals
A plethora of soil creatures appeared in this period. A particularly rich soil fauna assembly has been found in Givetian Gilbao rocks in New York dating back to about 375 mya. The fauna “is strongly predator-dominated, both in terms of individuals and species (below). The few non-predator species present were probably detritivores, and there is no evidence for direct consumption of live plant material. The predator populations must have supported themselves on abundant detritivore species, as in today's litter faunas, but there is no obvious taphonomic reason why the remains the detritivores would not have been preserved...in functional terms the individual species are fully modern, and in a few cases it would be no surprise to find them among the extant litter fauna”(Shear 1991]
This period of the explosion of land animals produced a lot of legged creatures moving about.“Most arthropod diversity is now found on land, with hexapods, arachnids, and myriapods, The predicted emergence times for these is now agreed with both molecular and fossil dating (Edgecombe et al., 2020)
There were three main legged groups emerging - 6-legged, 8-legged, and multi-legged The newly emerging 6-legged included flightless hexapods (6-legged) like protura and diplura, but most especially collembola (springtails). The other main group of animals now running around are the 8 - legged ones - arachnids. The myriapods – with many legs like millipedes, centipedes, pauropods can also be found in this period. Other myriapods - symphylans who live deeper - are not found till later.
"Refinement of the phylogenetic pathways of evolution of terrestrial arthropods, the allocation within the type of different levels of adaptation to life outside water (in the soil and other wet substrates and in the open atmosphere, volume of humidity) allows you to find out the patterns of evolution of organisms during habitat changes. Phylogenetic development is a natural process that has a certain direction. Finally, phylogenetically young and ecologically very complete series of transitions illustrate crustaceans such as isopods (woodlice, found under stones). The secondary transitions to life in the water of insect larvae (and the secondary transitions of secondary-water larvae to life on land!) make it possible to isolate more plastic adaptation features when changing habitats. “The origin of the most diverse terrestrial animal group, Atelocerata (myriapods and hexapods), is obscured by an incomplete fossil record. Early (Silurian and Devonian- 440-400mya) body fossils of terrestrial arthropods have been found only in Laurussia (to be Euamerica), with key sites in Britain and eastern North America. Here authors describe the first atelocerates from the Devonian stages of Gondwana; these are perhaps the earliest known remains of Australian land animals. They illustrate the widespread distribution of parts of the Devonian terrestrial fauna." (Edgecome 1998)” This paper raises the issue I’d like to see explored more - where some these various key characters came from – eg Gondwana or Laurasia before mixing in Pangea.
3-way symbiosis
3-way symbiosis
I suggest that the biofilms and microbial mats that covered much of the vast tidal flats, have now got a covering of plant debris from mosses that provided some protection for animals to grow and run around underneath. Piles of moss may have provided the nourishment (first bacteria, then nemtaodes, and now springtails and mites) that lured other arthropods, like centipedes and spiders, into this space trying to catch the numerous prey. The plant debris provided cover for a community of creatures running around., comprising varied and mixed associations of bacteria, arthropods, fungi, green algae, and bryophytes. New plants arriving by spore started to put out feelers, called roots.
The famous symbiotic relationship between roots and fungi has a third component - small arthropods (legged creatures). The fungi provide food - nitrates and phosphates to the plants and energy compounds to the small creatures in the soil. Their exudate - glomalin - is eaten by springtails and proturans who accidentally spread the fungal spores, helping the fungi colonise other roots - so starting to create a rhizosphere, a more complex environment. These animals provide food for others, but they also provide – in their poo - the breakdown products of glomalin - we call GRSP - which are the important building block of soil, a process I am now calling glomalisation. This is part of the method and mode of energy transfer from the skies that feeds soil animals. Remember energy has to go into soils to grow and maintain their complex structures. This 3-way relationship was probably vital to life, both in and on earth.
“A variety of animals whose close ancestors may have made up part of the Devonian fauna have been implicated in fungus transmission e.g. nematodes, molluscs, worms, woodlice, centipedes and millipedes, proturans, collembola, and mites”(Kevan et al 1975).
We have seen that the mites millipedes and springtails have been around for tens of millions of years, but I'm not sure woodlice were around. Their first fossils are only about 100my old (Broly et al 2012), who say that they don't make good fossils. I think they would and that they did not come to ground till the soil was well formed.
Otherwise all the other creatures must have been around the fungi. Why? Surely the glomalin is an attractive food source.
Proturans are sometimes nicknamed coneheads are very small (0.6-1.5mm long), litter dwelling animals, so inconspicuous they were not noticed until the 20th century. There are no fossil records, but here presumed to have appeared around the same time as their allies the springtails. Densities of over 90,000 individuals per square metre have been measured.
Their name comes from the Greek proto meaning "first" and ura, meaning "tail".The microcephalic state of the head seems to be correlated with a sucking mode of food uptake. Although details on feedings habits are still unknown, the pointy head suggests they are fluid feeders; some species suck out the contents of fungal hyphae. They go for scratched-off particles or fluid food (Strullu-Derrien et al., 2018).
They are generally restricted to the uppermost 0.1 m (3.9 in) but can go deeper. Perhaps like springtails they evolved under a litter layer and pretty much stayed there. Protura predominantly feed on mycorrhizal hyphae via sucking up hyphal cytoplasm.
What is the point of proturans?
Classification
Classification
Some do not consider them to be hexapods – because two of the six legs are not working as legs. That throws up the classification again. These are creatures with protected mouthparts, where their first pair of legs thrown forward to help suck fungi.
These have a curious relation with creatures now classified as insects. They used to be but the taxonomists decided that their structures did not relate with insects. They all have heads, thorax, abdomens and antennae and ‘six’ legs – like insects. But these have internal mouthparts rather than ‘external‘ ones like insects. This may be suited to life underground, where protected mouthparts may be more significant than above ground.
Insects
Insects
The largest group of 6-legged creatures are insects. “Particularly vexing is the paucity of insect remains in the Devonian. The primitive bristletails..are the only potential insects known from his period. Remarkably, insects do not appear in the fossil record again for another 35my” (Edgecombe, 2011) The author, Shear, was the most prominent scientist at the time examining early land life. He, like me and I suspect many others, think insects would have been around earlier. But they were not; that is not because they did not fossilise but because they were not there. There is no evidence that they were at this time. Some - like dragonflies - emerged a little later, and stayed close to aquatic environments. However, most insects appeared when the soil had itself formed more fully. There are monumental numbers of papers produced about the classification of early insects, and how they relate with other creatures, particularly other hexapods, myriapods, and crustacea, These may be helped by seeing what the environment was like for their evolution.
Firebrats
Firebrats
Firebrats also have a bristle tail – 3 ‘bristle-like appendages at the rear – and are wingless. They thrive in warm, humid environments like kitchens and boiler rooms. Dating back nearly 400 million years, these wingless creatures are adept scavengers, feeding on starchy materials. With a preference for high temperatures, they're resilient survivors, adapted to withstand harsh conditions.
Bristletails
Bristletails
Among existing insect taxa, bristletails are some of the most evolutionary primitive. They appeared around 400 mya. As well as running, the bristletails also have the ability to jump several centimetres into the air, probably to avoid predators. Although sometimes found in leaf litter and detritus, the members of this group often live under or among stones and the genus Petrobius lives on the seashore, probably feeding on algae and lichens. Bristletails have external mouthparts (ectognathy), which is a characteristic of all insects. Bristle tails (Archaeognatha) consist of two main families, one silverfish, and the other firebrats.
Silverfish
Silverfish
The predecessors of silverfish are considered the earliest, most primitive insects and evolved around 400mya (mid-Devonian), although it could have been earlier.
Silverfish are small, primitive, wingless insects in the order Zygentoma (formerly Thysanura). Its common name derives from the animal's silvery light grey colour, combined with the fish-like appearance of its movements. The scientific name (L. saccharinum) indicates that the silverfish's diet consists of carbohydrates such as sugar or starches. They could be eating glomalin. The silverfish is a nocturnal insect typically 13–25 mm (0.5–1.0 in). The silverfish is completely wingless and typically live for up to three years. It seems they have always lived near the surface – they can come into houses – implying that they have always lived in the litter layers, rather than digging deeper. These early wingless insects ran around for a long time, with little evolution for around 50 million years, when we will catch up with a diverse range of insects around 325mya.
Mites
Mites
"Today, we can find in dry, sandy, and nutrient-poor soils a fauna of primitive acariform mites that show little morphological change from their Devonian fossil ancestors. This entire community may consist of such living fossils, with this type of nutrient-poor soil"
Oribatid mites first appeared, later to become one of the most important group of soil animals - the 'denizens of the dirt. "We conclude that this group, central to the trophic structure of the soil food web, diversified in the early Paleozoic and resulted in functionally complex food webs by the late Devonian. The evolution of body size, form, and an astonishing trophic diversity demonstrates that the soil food web was as structured as current food webs already in the Devonian, facilitating the establishment of higher plants in the late Paleozoic "(Schaefer & Caruso 2019)
Cosmochthonius - related to early oribatid mite
Multi-legged
Multi-legged
Myriapods
Myriapods
Apart from the six-legged lot, there was another group with lots of legs running round – the multi-legged lot – Myriapods. I This subphylum of creatures havefour classes. We have seen one class - herbivorous millipedes - had been around since 430mya, while we now see carnivorous centipedes (Chilopda) and two groups which are hard to spot, symphylans and pauropods.
“Evidence about myriapods suggests the group must have arisen in the Early Cambrian (500+mya), with a major period of cladogenesis in the Late Ordovician (450mya) and early Silurian. Large terrestrial myriapods were on land at least by mid-Silurian (430mya)” (Nelson et al., 2019) There is some molecular evidence, based on RNA sequence divergence, that supports this (Rosenstiel et al., 2012), but little fossil evidence. "The water-to-land transition in the myriapod lineage is mostly a mystery"
Myriapods confound paleobiologists’ said one major review a few years back, adding that ‘the dearth of well-founded comparisons is a reflection… of a lack of appropriately aged soil and litter fauna’ (Rosenstiel et al. 2012) That is hardly surprising as many agree with McGhee, who wrote in 1996“Prior to the Devonian (the time we are in now), there was no terrestrial ecosystem to speak of”. However the discovery of myriapod trackways around 450mya in Llandeilo in Wales and the Lake District in England throws some doubt (Cooper, Henwood and Brown, 2012). But perhaps they give a clue to what happened all those years ago. Were the tracks of myriapods in silt...yet to become soil?
In the debate about who myriapods are most closely related to, the morphologists tend to go for a closer relationship with the hexapods, like springtails - in a clade called Atelocerata - while the moleculists prefer a closer relationship with crustacea – in a newish clade with hexapods called Pancrustacea
Centipedes
Centipedes
The lightly sclerotised cuticle of centipedes (chilopods), coupled with their litter and soil-dwelling habits, make it difficult to find fossils. Yet an extinct order dates the divergence of 2 of the 5 main existing orders to about 385mya. Two of the existing orders are known only in New Zealand and Tasmania, implying they evolved much later (Retallack, 2019). Most species of centipede are solitary and can mate without any direct contact. Males spin webs and deposit their sperm in them. Females find the web take the sperm and fertilize their ova. There are species of centipede that do have courtship rituals.
Centipedes, unlike their millipede relatives, are carnivorous. Centipedes have poor vision, so are predominantly nocturnal but can find prey by using their antennae. They make burrows to wait for their prey. Once a suitable prey passes, they quickly capture, bite, and inject their prey with paralyzing venom. The venom comes from a pair of claws on the centipede's first body segment, from where it is passed to the centipede's mouth whose mandibles chew the prey.
While they can move fast, most centipedes are also adapted to burrowing - as a way of laying their eggs and protection against the sun. It may be that their burrows play a part in opening up tunnels in litter and surface soil.
Prey
Prey
I am making the presumption that centipedes feed on the many springtails, thus perhaps later evolved than herbivorous millipede sisters. However, when I came to try to provide evidence that centipedes eat springtails, it was not that straightforward. There are many predators of springtails, as they are so much in abundance, their soft bodies make easy pickings and during their many moult stages they cannot spring away.
However, some researchers used some modern-day techniques. By combining three approaches (gut chemistry, fatty acids & isotopes):
“We investigated trophic niches of two centipede species (Lithobiidae, Chilopoda). Molecular gut content analysis suggested that feeding on collembolan prey increases with litter mass in L. crassipes whereas the opposite was true for L. mutabilis confirming that dense needle litter restricts prey accessibility in L. mutabilis. Natural variations in stable isotope signatures suggest that compared to L. mutabilis the smaller L. crassipes relies more on root derived carbon.” (Shaw et al., 2010). Are they feeding on glomalin too? If so the breakdown products of glomalin are now running round in centipedes as well as springtails, and perhaps going further afield.
Venom
Venom
“Centipedes are thought to have evolved venom fairly early into their evolution, around 400 million years ago. This venom, and the proteins it contains have continued to evolve and change for hundreds of millions of years” Analysis of the venom suggests that ancestral centipede venoms remained simple cocktails for roughly the first 50 million years, after which each of the five main centipede orders split from each other, independently evolving more complex venoms, each comprising a unique cocktail of toxins. The most complex were in two Australasian species, again implying late evolution of some centipedes from the others. These venoms are based on proteins and then concentrated to use as a venom, A researcher at the Natural History Museum says:” The interesting thing is that it is not just random proteins that the centipedes have borrowed from fungi and bacteria. They have used bacterial weapons that already kill cells. It's already an appropriate bullet for a venomous animal to use.”
The complex world of legged creatures came together in this time, with new predators preying on springtails and mites, with some herbivores and detrivores, like millipedes, but with a whole new environment to adapt to.
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