Tuesday, May 7, 2013

Strashilidae Make Paleontologists Look Stupid (Not That That's Anything New)



File:Ornithopsis.jpg
Lectotype of Ornithopsis hulkei (Brachiosauridae?)
Science is a matter of trial and error—and in the case of paleontology, more error than its counterpart. Correctly interpreting fossil remains is a tricky business, especially since they are often incomplete: consider this drawing of a chunk of bone dating from the Barremian Epoch (130-125 m.y.a.) of England. It takes an expert's eye to determine that what we're looking is not only petrified bone (as opposed to an inorganic rock), but, to be more exact, a largish vertebra, belonging to an organism called Ornithopsis; so named because it was thought to belong to a pterosaur (Seeley, 1870), or some sort of more bird-like flying tetrapod. Nowadays, we know beyond a doubt that Ornithopsis was a sauropod (Lyddeker, 1889), and therefore a distinctly land-bound creature.

Файл:GRIMALDI 2005 Strashila incredibili.jpg
Reconstruction of S. incredibilis (bottom) with holotype (male) above (Grimaldi & Engel, 2005)
Excusable, you say? Indubitably. This specimen could be a fossilized Barremian "meadow muffin", for all I can see (which is why I didn't go into paleontology); but even the best-preserved fossils can be misinterpreted. A case in point would be the excellent remains dating from the Jurassic of Asia that are referred to the family Strashilidae: yet their fidelity to life hardly aided paleontologists in making sense of what these insects actually were. The first known strashilid (Strashila incredibilis) was described from a single Siberian specimen. Its hind legs were burly and hooked; its body was, louse-like, laterally flattened; its abdomen looked as though it was easily distended; and it appeared to have suctorial mouthparts (although subsequent damage to the holotype has prevented later paleoentomologists from corroborating this supposition): all these anatomical traits were taken to suggest that Strashila had an ectoparasitic lifestyle, probably on pterosaurs (Rasnitsyn, 1992). The other known strashilids (S. daohugouensis and Vosila sinensis, both of them Inner Mongolian and somewhat earlier than S. incredibilis) both conform to this habitus. But the pleural rows of fleshy protuberances running the length of the abdomen remained an enigma.

Mrs. and Mr. Pseudopulex wangi (Pseudopulicidae)
Examination of strashilid genitalia showed them to belong to the superorder Antliophora (consisting of the orders Mecoptera, Siphonaptera, and Diptera), often being specifically likened to fleas (Siphonaptera: Grimaldi & Engel, 2005). Fossil fleas are rare (since they are obligate ectoparasites): the majority originate in Cenozoic amber, all of these belonging to the extant families Ctenophthalmidae, Rhopalopsyllidae, and Pulicidae (Lewis & Grimaldi, 1997; Beaucornu & Wunderlich, 2001). Only two species from prior to the K-T mass extinction (the one that did in the dinosaurs, y'know) have been consistently referred to the Siphonaptera: Saurophthirus longipes (Ponomarenko, 1976), dating to 145-140 m.y.a. (Rasnitsyn, 1975); and the younger Australian Tarwinia australis (Jell & Duncan, 1986) from 125-112 m.y.a. The three genera of the notorious 4-in.-long (in females), probably dinosaur-biting Pseudopulicidae were doubtlessly akin to fleas (if not fleas themselves; Huang et al., 2013a), but may not belong strictly within the Siphonaptera (Gao et al., 2012); they lived from 165-125 m.y.a.

Paratype of Coonilla longictena (Ischnopsyllidae)
Tarwinia is most probably a true flea, as it has many distinctive siphonapteran traits: laterally flattened body, helmet-like head, shortened thorax, and saltatorial hind legs (Krasnov, 2008). Saurophthirus bears elongated and ctenidium*-bearing extremities and shortened antennae, suggesting an ecology analogous to the modern bat bugs (Polyctenidae) or bat flies (Streblidae sensu lato*) and no leaping capability; a lifestyle of clambering on a downy integument, such as pterosaurs are known to have borne (Ponomarenko, 1976), thus drawing comparison to the Strashilidae: but comparison and phylogenetic relation are two different things. The idea that strashilids were "pre-fleas" (Rasnitsyn, 2002) never really took hold by consequence; and the resultant hypothesis that basal fleas were initially pterosaur parasites (later transferring to Mammalia) also failed under scrutiny, since living bat-specialist fleas (Ischnopsyllidae and Hectopsylla) aren't at all similar to Saurophthirus (Whiting et al., 2008). And if S. longipes isn't a genuine flea, than it must be admitted that strashilids bear no resemblance to fleas whatsoever. Instead, a new order was proposed to contain the "nasty-looking creature" (Rasnitsyn, 1992) and its kin: Nakridletia, the "paragliders"; an antliophoran lineage that developed ectoparasitism independently from the Siphonaptera, and was restricted to pterosaurian hosts (Vršanský et al., 2010).

A parasitic taxon uniquely occurring on pterosaurs has interesting ramifications for vertebrate paleontology, since it implies that these volant reptiles were endothermic and social (Vršanský et al., 2010). Which would be pretty darn coolif it were true.

http://www.dinoastur.com/wp-content/uploads/2013/02/Huang_etal_2013_fig4.jpg
Reconstruction of S. daohugouensis ' natural history
Up until a few months ago, there was a gap in our knowledge of the Strashilidae: all known specimens were male. Females turned out to be critical to our understanding of the family, for they bore a single pair of broad, seta-fringed wings, utterly lacking in venation. Not only that, but some male specimens were likewise equipped; and a few females were apterous. Additionally piquing paleoentomologists' interest are several traits exhibited in these newly described remains that reveal sexual dimorphism: females' hind legs were not muscular and bristly, as were those belonging to the opposite gender; and the male abdominal processes, hitherto so inexplicable, turned out to be feathery gills evidently retained from the larval stage—an atavism unknown among holometabolous insects aside from the Strashilidae (Huang et al., 2013b). 

Palaeodipteron walkeri, wing
Line drawing of N. walkeri's wing (Kevan & Cutten, 1981)
These discoveries drew attention to a previously ignored fact: strashilid genitals were quite similar to those of certain living insects; namely, nematocerous flies, such as gnats (Chironomidae) and mosquitoes (Culicidae). Thus, it can be safely declared that strashilids were true flies (Diptera), apparently shedding their wings after eclosion (probably during copulation); most likely in order to submerge themselves for oviposition. (This hypothesis is summarized in the artist's reconstruction shown in the previous paragraph.) How can we guess all this regarding their behavior? Because the Strashilidae closely parallel an extant fly taxon in their phenology (Huang et al., 2013b): the Nymphomyiidae, tiny (1.5-2.5) insects restricted to cold montane streams in eastern Asia and North America (Marshall, 2006) and classified in the single genus Nymphomyia (Oliver, 1981). Their maggots inhabit aquatic mosses, grasping the substrate with 8 pairs of prolegs; the non-feeding adults' wings are reduced to rachides flanked with long setae (see above) shed by both sexes before they descend into the waters and oviposit there, subsequently dying in flagrante delicto (Courtney, 1994). Because of their extreme specialization, the Nymphomyiidae have been frequently regarded as the earliest-diverging living lineage of Diptera (Rohdendorf, 1964; Griffiths, 1990), although this phylogeny is presently out of favor (Wiegmann et al., 2011).

In summation, the Strashilidae were not at all ectoparasitic, despite the universal consensus for the past 21 years that they were. The lesson we can derive from of all this is that (as the Raptorex fiasco amply displayed) in paleontology, "the experts" can (and will) screw up. Repeatedly. It's not dissimilar to other professional fields—say, medicine—but paleontologists will own up to their mistakes without the prodding of litigation.            



*"In the broad sense"—including the Nycteribiidae (see "Mormotomyiids' Terrible Hairiness").
Not belonging to the suborder Brachycera: that is, their antennae have more than 3 segments and lack an arista.
Muscular extensions of the abdomens of some insect larvae, which act as legs. The flies whose maggots possess prolegs (Nymphomyiidae, Blephariceridae, and Deuterophlebiidae) were at one time classified together in the infraorder Blephariceromorpha on the basis of that characteristic (Wood & Borkent, 1989).   
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