The gap has an arithmetic worth doing. Undisputed hexapod body fossils appear in the Early Devonian Rhynie Chert at about 405 million years ago, and clearly recognisable insects are not abundant until the Late Carboniferous, leaving an interval of roughly 80 million years [16]. Chosha sits at 324 million years [1], which is 81 million years after Rhynie, at the closing edge of that interval rather than in the middle of it [18]. Erik Tihelka, who with Chenyang Cai of the Nanjing Institute of Geology and Palaeontology led the work published in Nature on August 26 [2][3], told Science News the discovery straddles a big gap in the fossil record [25]. On the timeline it leans against the far wall, and the older end is addressed by the other half of the paper, a revision of the Devonian Leverhulmia and two undescribed Mazon Creek fossils [14].
So the reconciliation on offer is morphological rather than chronological. Molecular clocks put the hexapod split from marine crustacean relatives as far back as the Cambrian-Ordovician [17], and the authors write that these stem-group fossils are the earliest uncontested insects and partly reconcile that mismatch with the rock record [19]. Partly, because nothing here shortens the stretch between a Cambrian-Ordovician split and a Devonian first appearance. What it supplies is an account of how the stretch was spent, with early forms keeping and reworking features inherited from aquatic ancestors [31].
Why the limbs were legible in 2026 and not in 1985 comes down to what the animal was thought to be. Collected from an outcrop north of Big Bend National Park, in calcareous claystone concretions of the Marathon Uplift [4], it was identified as a juvenile of Tesnusocaris goldichi, a crustacean from the same shale, and Science News reports it sat in a San Diego drawer with little significance attached [22]. On a crustacean larva, abdominal appendages are expected. The ovipositor is what breaks that reading, making the specimen an adult female insect [5][6], and an adult insect is not supposed to carry nine pairs of limbs behind its three pairs of walking legs [7][8][11]. Twenty-four legs in all [9].
What the material does not settle is what those limbs did. The Nature abstract calls them gill-like and keeps the inference to at least some stem-insects [20], while Science News reports the flattened, grooved paddles as possible aids to swimming or scuttling through muddy shallow seas [21]. Those are different jobs, and a compression fossil yields no measurement of gas exchange. The build helps a little: streamlined and fusiform, with terminal filament and cerci adding 17.57 millimetres, about 55 per cent of the trunk length again, behind it [10][12]. Suggestive of a swimmer, and no stronger than suggestive.
Placement decides how far any of this generalises. The phylogeny puts Chosha within Hexapoda but outside the crown group, with crustaceans a distant branch away [13][15], so this is a cousin that kept the ancestral kit, not the lineage that shed it. Dany Azar, a palaeoentomologist at Lebanese University and the Nanjing institute, told Science News the fossil is a milestone in a period from which almost no fossil evidence exists [29]. That is the fair size of the claim: one carefully re-read specimen, from a window that had close to nothing, carrying a body plan no living insect would have led anyone to predict. Its name joins the Navajo word for insect to the Latin for predecessor [26].
