JAC: Both Matthew and I had forgotten about Matthew’s 2013 post here on mites, describing a phenomenon that might explain the the “Kite Runner fossil that received a lot of attention. At that time the Kite Runner wasn’t known, but biologist/author Ross Piper submitted a post (he’s a friend of Matthew) arguing that the objects tethered to the Kite Runner arthropod might not be progeny, but phoretic symbionts (“phoretic” animals are ones that used members of other species to transport them). Pity that Derek Briggs and his colleagues didn’t read WEIT before they published their paper!
By Ross Piper
Last week Jerry posted a discussion of the ‘Kite Runner’ fossil, an intriguing 1 cm-long fossil arthropod that was found in 430 myr old volcanic ash. Aquilonifer spinosus is intriguing because of the small objects that are tethered to the animal, which Derek Briggs and his colleagues interpret as the animal’s offspring. They conclude that the fossil represents a unique form of brood care unknown in the animal world, something that Jerry agreed with. Here it is again in all its glory:
I am a little sceptical, as there is another possibility that the authors did not fully consider. I think that the smaller organisms may be using A. spinosus to hitch a ride, rather than being babies. This kind of behaviour is known as phoresis.
Briggs et al. discussed the possibility that the attached structures are phoronts/epizoans/parasites, and dismissed this idea (again, Jerry found their arguments convincing). However, in looking for potential phoretic candidates they only looked at crustaceans. But if we look at mites, specifically a stage in the life-cycle of Uropodina mites known as the deutonymph, we can find a very good match. Furthermore, these mites and their weird life-style have been discussed here about three years ago!
These mites are fond of habitats that are very patchily distributed in space and time, such as mounds of dung, carcasses, dead wood and other similarly attractive places. Mites are small and wingless, so to reach new habitats they enlist the help of animals they share these habitats with, in particular various beetles (Aphodiidae, Geotrupidae, Scarabaeidae).
To attach themselves to the shiny exoskeleton of a beetle that can fly very swiftly Uropodina mites have evolved the ability to tethering themselves to a beetle using a long anal pedicel (Figure 1 and 2).
Like the silk of a spider, the pedicel is secreted by glands in the rear part of the mite’s body and extruded from its anus. The mite rubs its anus against the beetle before extending its hind legs or walking away from the anchor point to extend the tether.3
Both the glands from which the pedicel is produced, and the way it hardens, suggest it the pedicel is made of some form of silk. As an entomologist I have seen these deutonymphs atop their pedicels on numerous occasions, often a profusion of them on a single beetle (Figure 2).
I think something like these mites is a better interpretation of the Aquilonifer fossil ‘babies’.
Briggs et al argued that the relatively large number of small individuals associated with the fossil was evidence against these being hitchhikers: “[Aquilonifer] is unlikely to have tolerated the presence of so many drag-inducing epizoans”.
But the Deutonymphs shown in the photos travel in groups and are often found in profusion on their host. Frequently, one deutonymph is attached next to the other, even if other beetle body parts are free of mites.2 In fact, these phoretic deutonymphs atually prefer places already infested by deutonymphs.3 [iii] The impact of these passengers on the flying ability of a beetle is unknown, but it must be at least as significant as the potential impact on an aquatic host.
Something else that points to a phoretic interpretation for the A. spinosus fossil 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 (this is what we can see in the modern crayfish with attached embryos, which Jerry included in his post). Instead the tethered individuals are scattered across the body of Aquilonifer, which is very similar to mite deutonymphs (Figure 2).
I was also struck by this the description of the attached individuals in the Briggs et al. paper:
‘The very small size and consequent lack of detail revealed by the grinding technique make the individuals attached to Aquilonifer difficult to interpret….The outer covering of the capsules resembles a carapace that encloses the body and opens at one extremity.’
The deutonymphs of some other mites with their extended carapace are good fit for this description, as per this image from the excellent macromite blog:
Although there are no known mites of the same age as this Aquilonifer fossil, mites are known from the early Devonian4 and there are marine mites today. Having hitch-hikers rather than babies would still be pretty exciting, and a consideration of modern mites would have enriched the paper. That having been said, it is hard to see how we could test between the baby and the hitchhiker hypotheses.
Cutting edge technology and the ability to visualise small specimens in three dimensions has revolutionised palaeontology, but in the clamour to interpret how these long dead animals lived we sometimes run the risk of overlooking the insights offered by the remarkable adaptations of living organisms.
1. Bajerlein D, Witaliński W (2014). Localization and density of phoretic deutonymphs of the mite Uropoda orbicularis (Parasitiformes: Mesostigmata) on Aphodius beetles (Aphodiidae) affect pedicel length. Naturwissenschaften 101:265–272.
2. Bajerlein D, Witaliński W, Adamski Z (2013). Morphological diversity of pedicels in phoretic deutonymphs of Uropodina mites (Acari: Mesostigmata). Arthropod Struct Dev 42(3):185-96.
3. Faasch H (1967). Beitrag zur Biologie der einheimischen Uropodiden Uroobovella marginata (C. L. Koch 1839) und Uropoda orbicularis (O. F. Müller 1776) und experimentelle Analyse ihres Phoresieverhaltens. Zool Jahrb Abt Syst 94: 521–608.
4. Hirst S (1923). On some arachnid remains from the Old Red Sandstone (Rhynie chert Bed, Aberdeenshire). Annals and Magazine of Natural History (Series 9), 12: 455-474.