Pneumodesmus

Pneumodesmus newmani is a species of myriapod. It is originally considered that it lived during the late Wenlock epoch of the Silurian period around. However, a 2017 study dates its occurrence based on zircon data analysis as the Early Devonian (Lochkovian). Although the 2023 study confirmed the age identification of the 2004 study through palynological, palaeobotanical and zircon analyses incorporating newly discovered additional data, this is based on adjacent structurally separated block with different stratigraphy and sedimentology to the block with fossil site it was discovered, and it is confirmed as unsustainable. It is one of the first myriapods, and among the oldest creatures to have lived on land. It was discovered in 2004, and is known from a single specimen from Stonehaven, Aberdeenshire, Scotland.

Discovery and naming
The fossil of P. newmani was found by Mike Newman, a bus driver and amateur palaeontologist from Aberdeen, in a layer of sandstone rocks on the foreshore of Cowie, near Stonehaven (Cowie Formation). The species was later given the specific epithet "newmani" in honour of Newman. The holotype is kept in National Museum of Scotland, Edinburgh. The genus name is said to be derived from the Greek pneumato, meaning "air" or "breath", in reference to the inferred air-breathing habit. The proper word in ancient Greek for "air" or "breath" is however pneuma (πνεῦμα).

Description
The single, 1 cm-long fragment of P. newmani depicts small paranota (keels) high on the body, long, slender legs. There are six body segments preserved, and the dorsal portion of each segment is ornamented with a horizontal bar and three rows of roughly hexagonal bosses (bumps). Myriapods are the group that include millipedes and centipedes, and Pneumodesmus newmani would have resembled a millipede in appearance. However it did not belong to the same branch of myriapods as modern millipedes.

Significance
The fossil is important because its cuticle contains openings which are interpreted as spiracles, part of a gas exchange system that would only work in air. This makes P. newmani the earliest documented arthropod with a tracheal system, and among the first known oxygen-breathing animal on land.

Trace fossils of myriapods are known dating back to the late Ordovician (the geologic period preceding the Silurian), but P. newmani may be the earliest body fossil of a myriapod, if it had been dated at (Silurian, late Wenlock epoch to early Ludlow epoch). However, if based on (Early Devonian (Lochkovian)) estimated from Zircon age estimate, it cannot be called as the oldest myriapod, or the oldest of air-breathing terrestrial arthropods, because records from Kerrera (425 millions years ago) and Ludlow (420 millions years ago) become older than that. In spite of the recent competing arguments, the 2023 study suggests that this taxon is still most likely the earliest body fossil of a myriapod, with its age reconfirmed as the late Wenlock epoch (around ) through various analyses. 2024 study doubts that conclusion however, because this analyses are not based on fossil-bearing sediment itself.

During the Silurian, the rocks that would later be part of Scotland were being laid down on the continent of Laurentia, in a tropical part of the Southern Hemisphere.