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Around 400-366mya the distinct litter layer was developing and changing the cover of the land. Some creatures have lived in and stayed the same in that environment for hundreds of millions of years. But others would adapt to many new conditions that arose. It is not clear which of the main arachnids - spiders, scorpions and mites, found the new environment first.
Arachnids
Arachnids
There are 3 main sorts: Spiders or Araneae, Mites or Acari/Acarina and Scorpions. The key character of arachnids is the appendage before their mouth – the chelicerae. Most members of the subphylum Chelicerata and all those on land, belong to the class Arachnida. In most sub-groups, the chelicerae are modest pincers used to feed. However, spiders' chelicerae form fangs that most species use to inject venom into prey.
Some mites have specialised respiratory organs, and present-day scorpions have only book lungs, while most spiders use both lungs and trachea. This may seem a useful way of looking at the evolution of this group as they developed in the soil, needing to breathe air more.
Origins
Origins
As usual, the origins of this ancient group are ‘murky’. Said to be among the first land dwellers’. “There are very few rocks laid down on land around 400 mya so little of arachnids’ early history is preserved. And figuring out arachnid evolutionary relationships from DNA is likewise difficult because arachnids diversified so early, leaving few traceable evolutionary changes in their genes.”
“The question of whether Arachnida conquered terrestrial habitats only once or several times is controversial”[Sholtz & Kamenz 2006). It seems that their ‘book lungs’ – one of their distinct characters - are much the same in all arachnids, implying ‘only once’.
The oldest true fossil spider is a mere 305 my old, but moleculists can go earlier. Based on the 200 most slowly evolving genes, the “difficulty of phylogenetic resolution is compounded by accelerated evolution in certain arachnid orders. Specifically, mitochondrial genes of Pseudoscorpiones, Acariformes, and Parasitiformes (or at least Mesostigmata) have demonstrably higher rates of evolution.” (Sharma et al 2014). Mesostigmata are an important group of predatory soil mites, but not sure whether they had appeared by this time.
If we originally thought molecular analysis would provide simple universal clocks, we have learned that the rate of molecular evolution also varies. “We consistently observed short internodes in basal relationships attributable to ancient rapid radiation” (Sharma et al 2014). This says that arachnids evolved into a wide range of forms rapidly – and this would seem to be the time it happened.
Trigonotarbids
Trigonotarbids
Among the oldest known land arthropods are Trigonotarbids, members of an extinct order of spider-like arachnids.Trigonotarbids look a bit like spiders, including respiration through book lungs, and walking on eight legs, with a pair of leg-like pedipalps near the mouth and mouth parts. However, they lack the ability to spin silk, so are not true spiders, and the trigonotarbids have no living descendants.But when did they live? Another extinct sort of spider that could produce silk but without proper spinnerets has been put at 380 mya (Seldon et al 2008)
Click to see trigonotarbid come back to life
Spiders
Spiders
There is little evidence of true spiders – ones that can spin silk - around at this time. It is also noticeable that both the early groups of spiders are now extinct.
The first definite spiders, thin-waisted arachnids with abdominal segmentation and silk-producing spinnerets, are known from fossils like Attercopus fimbriungus. This spider lived 380 million years ago during this Devonian Period.
Mesothelae
Mesothelae
Most of the early segmented fossil spiders belonged to the Mesothelae, "a group of primitive spiders with the spinnerets placed underneath the middle of the abdomen (rather than at the end as in 'modern' spiders). They were probably ground-dwelling predators, living in the giant clubmoss and fern forests of the mid-late Palaeozoic, where they were presumably predators of other primitive arthropods (like cockroaches, giant silverfish, slaters and millipedes). Silk may have been used simply as a protective covering for the eggs, a lining for a retreat hole, and later perhaps for simple ground sheet web and trapdoor construction".
Amazingly, segmented mesothelid spiders have survived in eastern Asia (China to Indonesia) in burrows providing another 'living fossil'
Mites
Mites
It appears that a suborder of mites (Acari/Acarina) called Endostigs was the first to appear just over 400mya. "The ancient origin of Acariformes and the early diversification of major extant lineages linked to the soil are consistent with a pioneering role for mites in building the earliest terrestrial ecosystems"(Arribas et al 2019).
Oribatids
Oribatids
"We conducted a phylogenetic trait analysis of a major soil animal group (Oribatida) to reveal the deep time story of the soil food web. 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." (Schaefer and Caruso 2019)
These early mites probably gave rise to a variety of different mite branches, which we’ll pick up n as we go along. There are two major suborders, one Aracirformes which include soil forms Asigmata and Oribatids, and the other Parastiforms, which are present-day parasites and predators, the most prevalent in soil today being Mesostigmata. There are billions of mites in soils these days, living in a litter layer as they need space to move quickly and cover to protect them.
Wastebasket taxon
Wastebasket taxon
Nanorchestidae may well be a ‘wastebasket taxon’, a term first coined by Stephen Jay Gould. (Plotnick & Wagner 2016) The term has the sole purpose of classifying organisms that do not fit anywhere else. It is not a proper taxonomic term. Wastebasket taxa usually consist of several orgins (polypyletic) and thus not considered true taxa by strict cladistic rules, which can be quite difficult. They may have a ‘ragbag’ of about 10 families with no single ancesto. But they may have given rise to many mites over millions of years. Nanorchestidae is a family of endeostigs with at least two genera and two described species. They live in soil throughout world and may live in both hot and very cold deserts. While mites of the genus Speleorchestes are able to live in the hottest deserts, those of the genus Nanorchestes live in the coldest areas on earth. Clearly they can survive all sorts of conditions that have beset them for the last 400 my.
Hypochthonioidea'lower' oribatid - with hairs.
Oribatids
Oribatids
Oribatid mites are now classed in the order of Acariformes, and often called ‘true’ or ‘chewing’ soil mites' and 'moss mites'. They are small – 0.2 – 1mm and are fungivorous or detritivorous. All the stages of their growth feed on a wide variety of materials including living and dead plant and fungal material, lichens, and carrion. Oribatids - Lower (Carboniferous)
Success
Success
Three factors make oribatids stand out – they are very numerous, their nymphal stages can bear little resemblance to adults and they breed relatively slowly They have changing and complex sex relations, often parthenogenic, which is said to date back to this time. This and their slow growth indicate difficult living conditions, but ones they are well adapted to. Their thick cuticles when rolled up can withstand temperatures.
There is not much competition either, presumed to be because of the state of food sources, loads that not many others will use. They move much slower than springtails. They live longer than the other arthropods down there, surviving 2 years – that is 8 seasons of different conditions. They have been surviving well for 300 million years, and dominate the world.
They are by far and away the most numerous arthropods in soil, differing from other micrarthropods by having a sclerotised often calcareous exoskeleton. During this time, these 'moss' mites were pale and hairy, and called 'lower' oribatids. It would be another 200 million years before they evolved into the dark brown 'higher' oribatids, also known as 'armadillo' mites. Some parts of the world, like India, have only 'lower' oribatids.
Decomposition
Decomposition
Oribatid “Mites, for their part, are a dominant group of soil invertebrates that play an active role in litter decomposition vertical translocation of organic matter for deeper soil, humus formation and dispersion of decomposer taxa”(Banerjee et al 2010).
“Trophic interactions between (oribatid) mites and fungi are diverse and complex, affecting core ecosystem processes such as nutrient dynamics. Fungal grazing by mites has been associated with increased decomposition rates, nutrient cycling, and plant growth. Furthermore, changes in microbial respiration, fungal biomass production, interactions, and distribution have also been attributed to mite grazing (Peer J 2018).
This certainly applies to 'higher' oribatids, but it is another 200 million years before they arrive. Did the lower - earlier - oribatids have same impact? The answer to this could be to compare oribatid life in Europe (predominantly Higher) with that in India, where there are predominantly lower. Higher oribatids must have evolved after Pangea split up.
They date back to 350-400mya when they appear as lower oribatids, and increased noticeable at this time. They could go down the tunnels and pores, survive poor conditions. There may be a hundred thousand species, as each species have particular niches. They are found in all terrestrial ecosystems, from arctic, to tropics and from deciduous forest to desert.
Continuing Oribatid evolution
330-300mya (Late Carb)
300 -250 mya (Permian)
Higher (200-145 mya Jurassic)
Mites (145-66 mya (Cretaceous)
Prostigmata (order Trombidiformes), includes the many families of spider mites, velvet mites, gall mites and snout-nosed mites that now terrorise plants and other invertebrates.
Prostigmata
This sub order of Acariformes Have a broad diversity of feeding habits. Many are predators, but some fungal eaters which can be numerous. There is evidence of their origins around 400-350 mya (Coddington & Colwell 2001) Fungal feeds can reproduce rapidly to changing circumstances.
They can become numerous in temperate areas taking over disturbed conditions, indicating they can take over new areas easily. Many feed not only on fungi but also nematodes depending on conditions – generally the larger ones are predacious on arthropods and smaller on nematodes..
Pseudoscorpions
Pseudoscorpions
Pseudoscorpions ar a group closely related with mites (Acari) and are named because of their distinctive mandibles (Harms & Dunlop 2017) The first fossils date to 385mya (Shear & Bonamo 1989
A powerful predator
A specimen was being found at the Gilboa site (image b) in New York. It looks like a modern pseudoscorpion, but is more primitive than any living pseudoscorpion. Some of the over 3,000 species have a very elaborate mating dance. They are found under tree bark, in leaf and pine litter, in soil, in tree hollows, under stones, and in caves as well as the intertidal zone.,
Scorpions
Scorpions
The key group in terms of evolution on earth of the arachnids may be the Scorpions, although the arguments still rage. The arachnid orders with the old fossil record are scorpions. “It seems that due to their unusual glossy (called hyaline) cuticle scorpions are better preserved than other arthropods”,(Selden 1989). They show a fairly continuous record from fully aquatic forms over 400mya to both aquatic and terrestrial faunas by 300mya. Most arthropods have a cuticle of chitin, which is a good water repellent but not well preserved in fossils. The scorpion fossils from this time possess but still maintain gills – not ‘book-lungs’. This implies the scorpions at this time were aquatic.
Arachnids made a big breakthrough at this time, with the arrival of mites, spiders and scorpions. While making their presence felt, as many were predators, they were all to evolve into other forms over the next 350my
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