by Matthew Cobb
Regular readers may recall that a few weeks back we had a guest post from Ross Piper about the spectacular ‘kite runner’ fossil Aquilonifer spinosus, which Jerry posted about. Ross argued that the tiny organisms attached to the main fossil may not have been offspring, as Derek Briggs and colleagues, but instead Deutonymph mites that attach to organisms in order to disperse (this is called ‘phoresis’ so they are ‘phoretic mites’), and which we have previously described here.
Ross submitted a letter to the Proceedings of the National Academy of Sciences, where the fossil originally appeared, and this has now appeared, along with a brief reply from Briggs et al. Both the letter and the reply are behind a paywall, so I’ll give some quotes here:
Ross writes:
The relatively large number of small individuals associated with the Silurian fossil is one reason why Briggs et al. (1) reject them as epizoans. The authors state that “[Aquilonifer] is unlikely to have tolerated the presence of so many drag-inducing epizoans” (1). Deutonymphs are known to travel in groups and they are often found in profusion on a suitably vagile host. Frequently, one deutonymph is attached next to the other, even if other beetle body parts are free of mites (3). Indeed it has even been shown that phoretic deutonymphs prefer places already infested by deutonymphs (4). The impact of these passengers on the flying ability of a beetle is unknown, but it must be at least as significant as the impact of tethered phoronts on the swimming ability of an aquatic host.
One other feature of the Aquilonifer fossil that points to a phoretic interpretation is the location of the tethered individuals. If they were genuinely offspring, you would expect them to be clustered in one area to limit their impact on the parent’s swimming/foraging abilities. Instead, the tethered individuals are scattered across the body of Aquilonifer, which is very similar to mite deutonymphs.
Briggs et al reply:
Clearly these two examples are profoundly separated by time (∼430 Mya) and ecology (the one fully marine, the other terrestrial), but it is worth considering the possibility that the adherence of tiny arthropods to Aquilonifer represents the behavior of some sort of marine mite ancestor. (…)
We considered the possibility that the arthropods attached to Aquilonifer represent epizoans or parasites and concluded that this is less likely than their being juveniles (2). We focused on behavioral comparisons with crustaceans because they represent almost the entire diversity of modern aquatic arthropods; marine chelicerates, in contrast, are very rare (e.g., horseshoe crabs). Aquatic mites (Hydrachnidia) invaded water secondarily from land, probably in the Mesozoic, and most are freshwater (7, 9). Aquatic mites include examples that apparently attach their eggs to their limbs (10). Some aquatic mites (members of the Halacarida) are marine and occupy habitats from subtidal to abyssal (9). At least some freshwater mites are dispersed from one water body to another by a parasitic association with flying insects (9). Phoresy is practiced by mites that live on the strandline but is an unlikely strategy for fully marine (subaquatic) mites and we can find no reports that it occurs.
The evidence indicates that any similarity between the attachment of mites to hosts today, and that of the tiny individuals to Aquilonifer, is convergent. The individuals attached to Aquilonifer had at least six pairs of appendages confined to one portion of the body (2), whereas mites have fewer extended limbs that are usually more uniformly distributed. Furthermore, Aquilonifer does not appear to have been primarily a swimmer, and therefore was not an ideal dispersal agent, and whereas it could have adjusted its molting cycle to avoid casting off juveniles, it is unlikely to have done so to favor epizoans.
And that’s more or less it. We aren’t much further on, and I personally didn’t find Briggs et al’s response particularly convincing – certainly not enough to justify the rather peremptory title to their reply: ‘Aquilonifer’s kites are not mites’. However, because the fossil was destroyed in the scanning process, unless we find something similar, it isn’t likely this will be resolved one way or the other…
PNAS is uaually open access I thought…
It seems a pity to destroy such a fossil to uncover it. These days archaeologists are often reluctant to dig except in circumstances where there is rescue of a site going on before building work, as they consider new ground imaging techniques may still improve what we can discover without shovelling soil.
If I recall, the fossil had to be revealed by grinding down the rock and photographing and digitally reconstructing each section. This is a good way of revealing very fine detail that cannot be recovered by dental drills.
Correct. It’s a technique called “serial sectioning”.
Specifically, the fossils are found in very hard concretions in the rock, so that the concretion material is as hard, if not harder, than the fossil. That makes conventional excavation very difficult – they tried on dozens of previous specimens in the past before settling on the “serial sectioning” technique. They’ve been working this Lagerstatte (exceptional preservational deposit) since the late 90s.
There’s a good amount of Open Access on PNAS, but there’s a lot more on PLOS. Yeah, I get them confused too.
Phoresis was suspected by analogy with mites, and this gave rise to the hypothesis of dispersion. If Aquilonifer is benthic (i.e., not a swimmer), this would give support to the idea that the mystery organism may have belonged to a now-extinct group of ectoparasites. Ectoparasitism is especially likely to arise (evolve) in species that live in intimate contact with sediments (facilitate contagion). I agree with Matthew that the question is far from being resolved.
It is important to have a spirited debate between the concerned parties. But what is now needed to help resolve this is more specimens.
Which they’re looking for. The problem is that the concretion nodules are undistinguished from each other, and most are empty. So they need to chip a fragment off the concretion,then try to work out if there’s anything visible in the broken facet. Nothing there? Chip another bit off. Lather, rinse and repeat. Supply of specimens is probably not the problem, but the supply of microscope-eyeball-hours is more of a limitation.
Whether kite or mite,
evidence was lite.