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The order of springtails called Symphypleona can jump by means of furcula or spring attached to caudal extremities. Symphypleona are found in damp situations; possess striking coloration; and the antennae are distinctly segmented We know Springtails extend back nearly 400 mil yrs, but have ‘a meagre record as fossils’ until this period, where many have been found in amber. Symphypleona is reported since the early Cretaceous with genera distributed on every continent (Robin et al 2019). Is this their first appearance, and why so many in amber? They are generally ‘superficial’ in soils and now common in grass, but then may have evolved to make most of newly emerging leaves and any associated fungi. The largest family in the order are smintuhrids.
Amber fossils
Amber fossils
Did they not fossilise until caught in amber the resin from trees.?
“The fauna includes representatives of all of the principal suborders and infraorders, and most superfamilies, of the Symphypleona. This revision of the fauna includes the discovery and description of five new genera and species scattered across the phylogenetic diversity of the clade" (Sanchez-Garcia & Engel 2017):
“Previously only one specimen of springtail (Collembola) has been described worldwide from this period The examination of seventy-eight collembolan specimens from Canadian Upper Cretaceous amber. Sixty-three specimens have been identified at the generic level, none of which belong to present day genera. All are placed within eight newly erected genera. Most of these specimens belong to a single new genus, Protoisotoma, of the family Isotomidae" (Christiansen & Pike 2002)
Another work deals with 109 specimens from amber slightly later of Myanmar, 37 of which have been identified to the generic level and 14 new species are described. Two species of the family Isotomidae are placed in previously described genera and two placed Protoisotoma - again (Christiansen & Nascimbene 2006) That sounds pretty similar, and there is a similar story from Spain, again emphasising how similar the creatures are to existing species. (Sanchez-Garcia & Engel 2016)
Sminthurids
Sminthurids
We saw the arrival of springtails (collembola) as early as 350-400mya with the emergence of endomobryomorpha. We could see that some there were ‘living fossils’, ie they look very similar to today’s springtails. Others - like Rhyniella, look so much like insects that their classification has gone back and forth, implying that something like this evolved into insects. Other springtails would have evolved according to new soil circumstances, like Onychurids (Poduromorphs 360-300mya) going deep and loosing their furca, while others maintained their furca for springing in different habitats.
You may have thought that for a group comprising the largest and most diverse group amongst early hexapods, with over 7000 species worldwide and inhabiting almost every terrestrial habitat on Earth, from ice-free areas of the polar regions to tropical deserts, that we may have got relations sorted out better. The time frame crosses four hundreds of millions of years.
There is still a raging dispute about the relations and time-frame between the four main springtail groups the Neelipleona, the Poduromorphs, the Entomobryomorphs and the Symphypleona (inc Sminthurids), as to which were first (or ‘basal’ in taxonomic terms.) I put the arrival of Sminthurids around this period.
Even as late as 2021, “Despite a range of studies and the application of both morphological and genetic approaches (singly or in combination) to assess the evolutionary relationships of major lineages in the group, no consensus has been reached." (Cucini et al 2020)
Morphological analyses changed around the millennium, and “at least five competing hypotheses proposed (left), and nuclear and/or mitochondrial DNA markers increasingly becoming the main source of data.”
Basically, pick your phylogenetic marker, and each presents different relations, with the morphological and molecular methods again predicting different outcomes. Globular-shaped Collembola (i.e., Neelipleona and Symphypleona) have been generally considered the recently most derived springtail groups based on morphology, while molecular analyses repeatedly suggested an early branching for them.
As you can see from the diagram, Neelipleona and Entomobryomorphs were around earliest, Poduromorphs appearing round 250mya, while the globular Symphypleona (Sminthurids) appeared after 200mya increasing around 150mya.
Mitochondrial markers
Mitochondrial markers
So let’s choose mitochondrial markers. In one study “two new mitochondrial genomes were sequenced and all known Collembola mt-genomes reanalysed, including selected metagenomic data, to produce an improved phylogenetic hypothesis for the group, develop a tentative time frame for their differentiation, and provide a comprehensive overview of gene order diversity" (Leo et al 2019).
Litter or Shelter?
Litter or Shelter?
As Ghilrarov would have said, let’s look at their environment to see how they may have evolved. Were some of these new species developed in the new leaf litter? The new environment would have provided shelter, feed in the form of fungi, and more open pores, that may have enabled their spring to work better.
The new ecological factor would be driving the local distribution of springtail species evolution. There is the vertical stratification of the environment. In woodland, a continuous change in species assemblages can be observed from tree canopies to ground vegetation then to plant litter down to deeper soil horizons.
Collembola made up a greater proportion of mesofauna communities in deciduous woodlands. (George et al 2017) The Collembola was found in the O and the upper soil layer. Under adverse conditions in summer (low soil humidity) a vertical migration to deeper layers was observed. No similar response was observed in winter.
Feeding
Feeding
Sminths consume fungi, moulds, and decaying organic matter, playing a role in the decomposition process indirectly by breaking down fungal material.
They are more specialized in feeding on fungal hyphae, spores, and other microorganisms that colonize decomposing organic matter rather than directly decomposing the litter themselves.
Sex
Sex
Early springtails - Entomobryomorphs (top) - left spermatophores for females to find, Poduromorphs (left) left that less to chance with males giving a nudge, while closer connection among Sminthurids (bottom right)
(From MS Ghilarov Regularities in Adaptations of arthropods to the terrestrial life NAUKA Moscow 1970 p182)
"Oldest evidence of aggregative behaviour - 105 mya" (Sanchez-Garcia et al 2018) Aggregative behaviour is thought to reduce interspecific competition
Deciduous v Conifer litter
Deciduous v Conifer litter
Springtails are the most abundant arthropods in terrestrial ecosystems and, are considered as key indicators of organic matter turnover and soil functioning. So we can use what is happening now to see what may have been happening then. The newly formig broad-leaved litter, offered many opportunities for the springtails. There are 4 functional groups (atmobiotic living in water plants, and those in litter, euedaphic, hemiedaphic, epedaphic).
One study Korboulewsky et al 2021) looked the abundance and species richness of species of the last three. Epedaphic species, with high metabolic activity inhabit upper litter layers and fallen logs and about 5-8mm.with less pronounced pigments, poorly developed limbs and ocelli. Hemiedaphic species inhabit the lower litter layers of decomposing organic material. They are 1-2 millimeters in length, have dispersed pigmentation, shortened limbs, and a reduced number of ocelli. Euedaphic species have lower metabolism and inhabit upper mineral layers known as the humus horizon. They are smaller than hemiedaphic species; have soft, elongated bodies; lack pigmentation and ocelli; and have reduced or absent furca. They found that oak or beech stands host a higher species richness than fir and pine stands. That would fit with th increased opportunities of broadleaved litter that would have ben occurring in this period.
In a lowland site, they found 13 species were present only in one type of stand, 6 of those in deciduous and only 3 in conifer stands. Similar but not so striking results were found in mountain stands,
Deciduous:,
Smithurides schoetti
Pygmarrhopalites pygmaeus
Isotoma riparia
Protaphorura armata
Pseudosinella alba
Pseudachorutes parvulus
Coniferous:
Ceratophysella denticulata
Pseudisotoma sensibilis
Sminthurinus aureus ;
Hitching a ride
Hitching a ride
But how would they have overcome that problem of distribution in soil? Their springing organs wont work in soil.
“A winged termite and ant display not some, but 25 springtails, attached or in close proximity to the body. The collembola exhibit rare features for fossils, reflecting their courtship and riding on back of others (phoretic) behaviours. By observing the modes of attachment of springtails on different arthropods…we infer a likely mechanism for dispersal in Symphypleona." Robin et al 2019) This would make sense - that Symphyleons would be distributed like this in the Cretaceous soils, as there were ants and termites there are that time.
Springtails
Was there a springtail ‘explosion’ bought on by the new leaf litter and creating ‘nutrient rich patches through faecal pellets deposition’ -- their poo? I think we can presume that those springtails, particularly Symphyleons and Isotomidae, in the broad-leaved litter, deciduous or evergreen, may well have evolved during this period, although it is hard to see any particular pattern. However, if we try to imagine the new environment they were living in, we could make a better analysis of their possible evolution.
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