Animals - Early

Many springtails probably stayed in the sandy surface layers where they could continue grazing roots and spring around to make sure they stayed in touch with water. Others adapted to the new darker deeper conditions which provide more stable living environments. We can clearly see that the conditions and key characters to make micro-aggregates in the sand beeches  around then. Add to the above mix the creatures, like springtails and other primitive ones like symphylans, proturans and diplurans, running around moving fungal spores and bacteria and we can see how this crucial aggregate development happened around this time 360-300 ma

We saw in this period that springtails carry out all sorts of functions, mainly accidentally, like helping fungal spores reach the roots.As the water became ever more present in soils, so the springtails could stay in the soils longer, that led to their spending more time in tunnels in the dark. There are three main sorts of springtails - globular, elongate and pods. As time went on, each group have they lost their colouring and their eyes too – and the organ that gives these creatures their common name – the springing organ, called a ‘furca’.

We ave seen previously that only the AM fungi produce glomalin, and that springtails consume a lot of fungal hyphae, so predict that the springtails transform the glomalin into glomalin-related-soil-proteins. Those GRSPs may now be helping make aggregates. We saw in the earlier trial that AMF helped aggregation, but it also showed that 1) Collembola contribute to soil aggregation, 2) Collembola and AM fungi interact in the process of soil aggregation and 3) Collembola and AM fungi effects on soil aggregation have a similar magnitude. Treatment applications were successful for both AMF and collembola.

They used a typical springtail – an Isotomid, which has eyes and furcula when adult but lacks these during nymphal stages. Both measures for soil aggregates - ‘Water stable aggregates’ and ‘mean weight diameters’ - doubled in both test plants when both AMF and Collembola were added. The number of collembola doubled too! “Our findings show that collembola can play a crucial role in maintaining ecological sustainability through promoting soil aggregation, and point to the importance of considering organism interactions in understanding formation of soil structure." (Siddicky et al 2012)

It would have been fascinating to have found out which of the stages of the springtails life cycle – as clearly they did multiply- had the most effect. However, it does open up the window for how soil micro-aggregation could have started on the beeches and near the ponds. Presumably it had been going on for a few million years under the surface litter, but there was the potential to now go deeper. The adult stages could remain in the sand layer and the clastic substrates. Nymphs could go deeper, especially if the roots are going deeper. They would then leave their poo deeper, providing an excellent building block for early soil evolution. 

Some of the springtails may well have completed their life cycle deep down, evolving into other creatures, like these.

Diplura (2-pronged bristletails) are another order alongside collembola, both being 'non-insect hexapods. They would have been running round in the litter layer, but there is also evidence of burrowing.

The fossil record of Diplura is sparse, owing to their soft-bodied nature. The oldest fossil commonly referred to this order lived in this Period. Presently, the order contains approximately 800 species grouped into three (two main ones) suborders and nine families.

Diplurans feed on live prey, dead soil animals, fungi, living plants, and decaying vegetation. Most species are omnivorous, combining several food sources. Some diplurans locate prey with their antennae; they stalk to attain striking distance, and rush to capture it. Prey items include small, soft-bodied mites. Other diplurans (Japygids) burrow into soft, moist soil so that only the abdominal pincers are exposed. If a small creature passes near the pincers, it is captured and dragged into the burrow to be consumed. Other diplurans feed on decaying vegetation.

"Diplura" refers to the characteristic pair of caudal appendages or filaments (cerci) at the terminal end of the body. This is another group once considered to be an insect, but that has been ruled out because they have internal mouthparts. Yet they may well give indicators of where insects come from.

Similar to springtails diplurans have long antennae with 10 or more bead-like segments projecting forward from the head. The abdomens of diplurans bear eversible vesicles, which seem to absorb moisture from the environment and help with the animal's water balance. The body segments themselves may display several types of scales and setae.

Two major lineages within Diplura are distinguished by their cerci – Japygidae possess forceps-like cerci and are very aggressive predatory diplurans, using their pincer-like cerci to capture prey, including springtails. They would also have attacked isopods, small myriapods and even other diplurans.The second family, Campodeidae, possess elongate, flexible cerci that may be as long as the antennae and have many segments. They were and are omnivores which feed on soil fungi and detritus but also springtails and other small soil invertebrates

There are many methods of burrowing are found within the order. The Campodeids prefer loose soil, burrowing with wormlike movements of their streamlined bodies, while also being capable runners on the surface. The Japygids push into pre-existing soil cavities with their strong legs, which are useless for running. Their mouthparts help burrowing and they can draw their antennae in.

Diplurans burrow in several ways. The Campodeids prefer loose soil, burrowing with wormlike movements of their streamlined bodies, and are also capable runners on the surface. Japygids push into pre-existing soil cavities with their strong legs, which are useless for running. They burrow using their mouthparts, and by telescoping their antennae.

We have already met giant millipede Arthropleura (above) a millipede the size of a dog.

A diverse fauna, including millipedes has been recovered from this ‘Romers Gap’ time in the Scottish Borders, by the late Stan Wood. Scotland seems to have been a good stomping ground for millipedes, with many examples between 400-350mya. Wood found four millipede specimens a rock called Willie’s Hole. Although there were abundant plant remains, the biggest concentration of plants was in a black shale above the millipede horizon. They get bigger

Six different forms are present and five of them are based on only one specimen, which the authors believe means further finds could belong to yet more different forms. But they don’t help with the phylogeny(family tree). Here they would have lived in a changeable floodplain environment with seasonal wet and dry.

“Most of the millipedes are from slightly finer grained lithologies at other horizons and their articulated nature suggests they were probably deposited in less dynamic conditions. (translate)Given that tetrapods had only just crawled out onto land at that time, millipedes could have been a ready food source." (Ross et al 2018)

“Along with numerous amphibian, plant and other invertebrate remains have filled ‘Romer’s Gap’ and demonstrate that this perceived gap was due to a lack of suitable non-marine deposits of that age rather than a dearth of terrestrial life due to low oxygen level“ (Ross et al 2018)

It looks like these millipedes were surface / mud dwellers, whereas other related forms were going down. Here are two new sorts of myriapods – Symphylans and Pauropods.

It was seeing onychiurids feeding along fruit tree roots, over 50 years ago, that got me thinking about what they were doing. I'm still learning.

If we were to compare Onychiurid with other springtail families – let’s say Isotomids used in the experiment above - how would we tell whether the Onychiurids were primitive – few developed significant organs - to Isotoma? Or was it the other way round – that the distinguishing features have become redundant? I predict the latter.

Onychiurids live in the deeper soil levels – the mineral layers. Onychiurids, along with Isotomids and  Hypogastrurids, go through a period of reduced activity, wherein they develop a unique morphology and are capable of 'anhydrobiosis. They can become completely dry without dying. These animals form small ball-like capsules around themselves before entering this state. If wetted, the animal resumes normal activity in an hour or two. This unusual feature of these Collembola gives them the ability to live very long periods without food. The longest survival was a specimen of Onychiurus, which lived over a year without food and was then accidentally killed. Clearly this is an important survival mechanism in places where such conditions can prevail for long periods of time – deep down in soil. It may also explain why they are hard to extract from soils using tradition Tullgren funnels which rely on heating and drying (Edwards 1991).  I knew that ‘Edwards’ – Clive - quite well, often drinking with him in Harpenden, and he acted as my unofficial supervisor.

While the phylogeny of collembola is continually debated, the Poduromorpha probably followed the entomobryomorphs in earth. They are the ‘jelly fruits’ of the Collembola, as they have short stubby legs and a usually plump, often flattened body and they have a much smaller furca. They also have a strange, fully aquatic life cycle, laying eggs that drop to the bottom of the water. When hatched, the young have a hydrophobic cuticle, so are immediately thrust upward and out onto the water surface, where they spend the rest of their lives. All this points to a life lower down.

We have seen the slim coloured springtails near the surface but now we find them living further underground, but they are smaller (under 1mm) with short antennae and legs, and the jumping structure greatly reduced or absent. The main family of springtails matching this description is the Onychiurids, one of the poduromorphs. Their eyes are reduced or absent and their bodies are pale without pigment. They run round a lot, probably distributing bacteria and passing on fungal spores, like other springtails.

Sympylans are another class of myriapods and are translucent rapid runners. They are primarily herbivores and detritus feeders living deep in the soil, under stones, in decaying wood, and in other moist places. At least one sort of symphylan is predatory. The symphylan fossil record is poorly known, with only five species recorded, all placed in living genera. The oldest records are found in amber around 100mya

Symphylans are very small, non-venomous and move rapidly through the pores between soil particles, and are typically found from the surface down to a depth of about 50 centimetres (20 in). They consume decaying vegetation, but can do also be a pest in gardens and plantations consuming seeds, roots, and root hairs in cultivated soil. Population densities of several thousands specimens per square meter are not unusual. One estimate predicted 80m/acre. 

Sympyhlans can move down dark tunnels to feed on springtails. Creatures like symphylans could move across the surface as well as move down through soils. Video of 2 sympylans moving where they can go 50cm deep now that aggregates are building. Nothing is known about other types of social behaviour and communication. Their display and territoriality are unknown. Vertical and horizontal migrations occur when soil conditions change.

It looks like centipedes and millipedes run over the surface – originally of rocks with lichen, and then when the sandy soil of the levees got deeper, some evolved to bury into the ground  - where they lost their colour - as there would have been a massive food source of springtails. They could have evolved during this aggregate building period, making use of the newly created pores. 

It doesn’t look as if there are two sorts of symphylans as they have a remarkably uniform anatomy and outer morphology. Only two families have been distinguished: Scutigerellidae, with five genera and about 125 swift-moving species, 4–8 mm long; and Scolopendrellidae, with eight genera and about 75 generally slow-moving species, 2–4 mm

Wiki says: “They look rather like centipedes, but are probably the sister group to millipedes”. The former are carnivores the latter herbivores.

They seem to be an ‘old group’ closely related to the millipedes (Diplopoda). Their head capsules show great similarities to millipedes: both have three pairs of mouthparts and the genital openings occur in the anterior part of the body. Moreover, both groups have a pupal phase at the end of the embryonic development. The two groups probably have a common origin. But again, there is an uncertainty based on morphology and molecules.

But millipedes are disputed. They certainly share some common attributes, but their differences are also large. In 1873, John A Ryder, an American naturalist, described the difficulties even initially trying to fit the Pauropoda into a one single group.

'In the form of the body and legs the creature recalls the large carnivorous myriapods or centipedes, whilst in the possession of a pulvillus or pad, and a claw on the feet, they resemble in a measure true insect; in their branched antenna they resemble crustaceans and in their herbivorous habitats they resemble the herbivorous, and in the distribution of the legs they combine characters of both the herbivorous and the carnivorous myriapods.'

Others suggest they may be related with those other multilegged creatures - symphylids. Their body segments have spiracular pouches similar to those in millipedes and Symphylids, but without the trachea usually connected to these structures.

Throughout the creation of soils, different gas exchangers, like tracheae, were essential.[16] Creatures moved from being surrounded by water, to face the barriers and opportunities of both water and air. Terrestrial arthropods including diplopods, chilopods, Collembola, scorpions, Acari, pseudoscorpions, harvestmen, as well as some myriapod groups, all breathe with a tracheal system.  Living in the soil gives them to time to evolve these new breathing structures. But not pauropods.

Pauropods are shy of light, and will attempt to distance themselves from light sources. They migrate upwards or downwards throughout the soil based on moisture levels. They have two sexes, male pauropods place small packets of sperm on the ground, which the females use to impregnate themselves with, leaving the egg on the substrate. Pauropods eat fungal hyphae, and the root hairs of plants, so it easy to see how they would have followed the roots and fungi down.

While this group will be part of the chewin n pooin army, they don’t seem to do much – as they are sparsely distributed. Yet they are present all over the world, including New Zealand, implying that they were around by the time of Pangea.

Here is on the those chicken and egg questions. Did centipedes give rise to symphylans, or did symphylans evolve into centipedes? Both are carnivores, feeding particularly on springtails. They are now put together as Trignatha, a grouping united by similarities of mouthparts.  I predict that centipedes evolved into symphylans, as they descended deeper into the newly created soils, still running fast but becoming ever paler.

Ghilarov in his introduction to Regularities in Adaptations of arthropods to the terrestrial life NAUKA Moscow 1970  said “Rows of emerging transitions of release from constant communication with a wet environment can be traced in different classes of centipedes. Among helixes, such series are outlined in general in a complex of different orders of arachnids and even within smaller taxa (for example, in ticks). Spiders’ jaws allow us to analyse the whole range of transition from life in the water to the terrestrial existence.

If the currently distinguishing selection of scorpions from the arachnid class to the horseshoe crabs, then scorpions give another parallel branch of evolution from aquatic forms (fossil Palaeophonus– ancient scorpion) to terrestrial.

Nematodes, roundworms or eelworms, are everywhere. 23 000 species have been formally described, estimates of true species-level abundance range from 0.5 million to over 10 million About 90% of nematodes reside in the top 15 cm of soil. They do not decompose organic matter, but, instead, are parasitic and free-living organisms that feed on living material. Nematodes can play an important role in the nitrogen cycle by way of nitrogen mineralization.

Nematodes demonstrate a distinct dialectic among multicellular animals; they combine endless variation with a deceptively simple underlying anatomical pattern. They are just a gut with a sheath (cuticle) around. And that gut has only one opening. The common characteristic between round worms is the body cavity – to house the gut. They are hard to find as fossils as they have little structure, are not segmented and v small. Most biologists believe the nematodes we see today are much like those hundreds of million years ago.

"The oldest known nematodes are from about 400 million years ago, but I believe they probably date back to around 1 billion years,"Poinar said. "That would mean they were one of the very oldest of all life forms, coming along before almost all other animals and just after bacteria, protozoa and fungi. They literally emerged from the primordial ooze."

They probably did. But it is likely that the first nematodes, and second third and fourth were marine. Yet they probably increased dramatically in this period as the ideal conditions – wet and stable - for extending their existence increased as the number of moist surfaces in the aggregates developed. Nematodes are usually very small, and the smallest are microscopic. But they have nervous and digestive systems, longitudinal muscle but not circular ones, good mobility, and are capable of rapid reproduction and learned behaviour.

Nematodes need a constant water coating. They have mechanism to withstand dry spells, but when adults they must have high humidity and water coating. When near the surface this would be difficult, as conditions change more rapidly. The further down we go with a plethora of protected pores, there would be many more living spaces. Temperatures will be buffered from the air and the water holding capacity provide more stable conditions.

“Several of its constituent clades cover a large subset of the ecological spectrum, but interestingly none of them appears by itself capable of covering the full ecological range of the phylum. This suggests that the evolution of ecological adaptations within each nematode taxon was constrained by limitations on the rates of change in genes and ecophysiology, or by competitive exclusion from habitats previously colonized by other taxa, or both."

Poinar believes nematodes evolved in the sea and one of their first roles was as a parasite to marine invertebrates. They make great parasites. One species causes heartworms in dogs, another is the most infectious agent in sheep around the world. You may have bought packets of nematodes to kill off slugs. As plant parasites they can cause severe crop losses, often feeding on roots. But at the same time, they are an important "secondary decomposer" in soil biology.

It is not surprising that there is little fossil evidence of nematodes from hundreds of millions of years ago as they have no real exoskeleton and just a liquid endoskeleton, with a one-way gut inside. They are the first organisms found to have an internal body cavity, or coelom, which contributes to more efficient mobility. A coelum is found in just about every creature which evolved after nematodes.

There is a sheath, which moults till adult, that holds liquids in the body cavity which acts a hydrostatic skeleton, used to move themselves in a whiplash way. So there are no tracks nor burrows left behind, making fossil finding even harder.

Within Chromadoria, there are various groups, some predominantly marine, some well diversified in freshwater sediments, and some in moist soils (e.g., Plectida). The Chromadoria includes a hugely successful radiation of predominantly terrestrial nematodes. (Blaxter 2004).

It would seem that nematodes were in great abundance at this time, and looking very much like they do today. They probably came from freshwater, perhaps as parasites on other creatures. While not direct debris feeders they fed on other living matter and in so doing provided food sources to others and maintained an environment, including nitrogen capture, fit for the other walking creatures. 

They are very numerous in soils and 100 cc of soil may contain several thousand of them. Most nematodes look similar apart from their mouthpart, which determine what they eat. The Herbivores suck plant tissues, many of the order Tylenchida and they have mouthpart of a needle like stylet which is used to puncture cells during feeding. There are two sorts - ectoparasites remain in the soil and feed at the root surface, while endoparasites enter roots and can live and feed within the root.

Many kinds of free-living nematodes feed only on bacteria. Luckily these are very abundant in soils as they  can consume at 5000/min. In these nematodes, the "mouth", or stoma, is a hollow tube for ingestion of bacteria. This group includes many members of the order Rhabditida and are beneficial in the decomposition of organic matter.

“Bacterial feeding nematodes excrete N assimilated in excess of that required for growth... Cumulative N, as NH+4 or NO-3, leached from columns (of organic matter & bacteria) containing nematodes was consistently greater than from columns without nematodes.

The Fungivores feed on fungi and uses a stylet to puncture fungal hyphae. Members of the order Aphelenchida in this group are again very important in decomposition. In return there are several sorts of fungi which eat nematodes – some by lassoeing them, others using super glue. Predatory nematodes feed on other soil nematodes and on other animals of comparable size. They are not common, but some of them can be found in most soils. While most nematodes are specific eaters – perhaps because of their distinctive mouthparts, omnivores may feed on more than one type of food material, and may ingest fungal spores as well as bacteria. The food habits of some of are unknown, as it is difficult to see if they are feeding on dead cells or fungi growing on them.

“Three major lineages exist within the phylum: Chromadoria, Enoplia and Dorylaimia. The exact order of appearance of these lineages is not yet resolved, which also leaves room for uncertainty about the biology and morphology of the exclusive common ancestor of nematodes. Enoplia and Dorylaimia differ considerably in many respects from C. elegans, which is a member of Chromadoria, a widely diverse group."

It was hoped the moleculists could come to the rescue.

“While there was to a large extent consistency between the new molecular trees and morphological interpretation of nematode phylogeny(family tree), placements of many smaller and larger clades were far from expected. As a result, a revised trichotomy-based classification system was proposed by that partially reflected the 18S rDNA phylogeny and partially remained morphology-based for those taxa that had not been sequenced or for which placement in the phylogeny was not unequivocal." (Ahmed et al 2022)

One nematode, Caenorhabditis elegans, has been well studied and is becoming a respectable model for evolutionary studies. “Despite remarkable progress toward a molecular understanding for C. elegans, an ecological context for this model system has lagged behind. Although these features must somehow be important for the organism's life and its interaction with the environment, the actual ecological context and evolutionary "pressures" to which such features are adaptations have remained quite unclear."

While the molecular data confirms the presence of those three early nematode lineages, the exact order of appearance is not yet resolved. It seems likely that Enoplia appeared first but on the other hand, SSU data also allow for the possibility that Dorylaimia diverged first.  This is an intriguing possibility because all known Dorylaimia are absent from marine habitats. A “Dorylaimia first” topology would therefore imply that the ancestor of all nematodes was perhaps a freshwater organism, and not a marine animal as more commonly assumed. Freshwater to soil sounds possible.