A 324-million-year-old stem insect still carried paddles on nine abdominal segments

The fossils of C. praecursor were found in calcareous claystone concretions in the Tesnus Formation of the Marathon Uplift in western Texas, and preserve unusually fine anatomical detail.

Cross-polarized light imaging allowed the researchers to identify features showing the fossils had been misclassified for years: rather than crustacean larvae, the specimens were adult female insects.

The unusual abdominal gill-like appendages of Chosha suggest a semiaquatic mode of life in at least some stem-insects and revise understanding of the assembly of insect morphological organization.

The paddles may have helped the insects swim or scuttle through muddy shallow seas.

The phylogenetic reconstruction recovers C. praecursor as a hexapod, strongly favouring its placement as an early-diverging insect.

A revision of three other enigmatic hexapod fossils, the Devonian Leverhulmia and two undescribed Carboniferous fossils from the Mazon Creek Lagerstaette, demonstrates diverse body organization in Palaeozoic wingless insects.

The compared fossils included a 405-million-year-old specimen from Scotland and an undescribed pair of multi-legged, insectlike fossils from the Mazon Creek beds of Illinois; all sat just outside the group containing every insect alive today, with crustaceans a distant branch away.

Undisputed hexapod body fossils first appear in the Early Devonian Rhynie Chert around 405 million years ago, and clearly recognizable insect fossils do not become abundant until the Late Carboniferous, leaving an approximately 80 million year gap in the fossil record.

Molecular clock studies have suggested that hexapods split from their marine crustacean relatives and began adapting to terrestrial environments as early as the Cambrian-Ordovician interval.

The findings suggest insects did not become fully terrestrial immediately after beginning to colonize land; early forms retained and modified features inherited from aquatic ancestors and spent an extended period in a semi-aquatic, amphibious way of life.

Together these stem-group fossils represent the earliest uncontested insects and partly reconcile the incongruence between molecular clock estimates and the fossil record.

Molecular clock analyses, trace fossils, exceptionally preserved amphibious stem-myriapods and sea scorpions have recently challenged classical assumptions about the timeline of the animal colonization of land, suggesting terrestrial ecosystems may date back as far as the Cambrian.

The bodies measured 32.09 millimetres long; including the median caudal filament and two cerci, total length reached 49.66 millimetres, and the bodies were streamlined and fusiform.

Under polarized light filters the specimen showed a three-segmented thorax, an abdomen with 11 sections, whiplike antennae, a long central tail flanked by two shorter ones, a stiff tube for laying eggs, and three pairs of walking legs.

The paper's reference list includes a 2020 Current Biology report on air breathing in an exceptionally preserved 340-million-year-old sea scorpion, and a 2020 PNAS report on aquatic stem group myriapods closing a gap between molecular divergence dates and the terrestrial fossil record.

Chosha praecursor postdates the oldest undisputed hexapod fossils by 81 million years, placing it at the young end of the roughly 80-million-year gap rather than in its middle.

Collected from a rocky outcrop north of Big Bend National Park in 1985, the specimen was initially mistaken for a juvenile of a crustacean called Tesnusocaris goldichi found in the same dark shale; researchers saw little significance in the find and it languished in a museum drawer in San Diego for years.

Tihelka spotted a line drawing of the fossil in a secondhand Russian paleontology textbook, tracked down the original papers, and travelled to the San Diego Museum of Natural History to see the specimen.

Tihelka recalls that at first glance the fossil resembled little more than a black smudge, but polarized light filters, which cut glare and sharpened contrast, revealed anatomical details pointing toward insects.

Tihelka says the discovery straddles a big gap in the fossil record and shows that there were transitional forms along the way before insects shed their extra legs and settled on six.

The name Chosha praecursor combines the Navajo word for insect, ch'osh, with the Latin for predecessor, and is inspired by the Navajo creation myth of a primordial world inhabited by insectlike people.

All data analysed in the paper, including the morphological character matrix and full analysis files, are available at Zenodo, and the paper is registered at ZooBank.

Dany Azar, a paleoentomologist at Lebanese University in Fanar and the Nanjing Institute of Geology and Palaeontology, says the fossil is a major milestone in a puzzling period when everything was changing from mostly aquatic to terrestrial and there was almost no fossil evidence from that time.

William Shear, a retired paleobiologist from Hampden-Sydney College in Virginia, says the find opens a lot of doors to the study of insect evolution and hopes more fossils of this organism turn up upon further searching.

Counting both sets of limbs, the animal carried 24 legs.

The caudal filament and cerci account for 17.57 millimetres, about 55 per cent of the trunk length.