Strudiella

Strudiella devonica is a species of extinct arthropod from the Devonian. It was recovered in the Strud (Gesves, Belgium) environment from the Bois des Mouches Formation, Upper Famennian. It was originally described as the first complete Late Devonian terrestrial insect, but due to its poor state of preservation, its affinity is discussed.

Description
Strudiella is known from single specimen. It is a small arthropod with length about 8 mm. Structure like antennae and numbers of pairs of legs can be seen. Due to poorly preserved nature of this fossil, its interpretation and classification is depending on authors.

Garrouste et al. (2012)
In first description by Garrouste et al. (2012), it was described as the first complete Devonian insect. Median abdominal structures are filled with guts, which excludes possibility that is molting shell. It is interpreted to have 3 pairs of legs from thorax, tibiae and femora are long and thin. Antennae are uniramous, scape and pedicel are wider than 10-segmented flagellum. Triangular mandible have continuous series of sharp, small irregular molar and incisor cusps. Its small head had large eyes. Large thorax had rounded structure covering its head, corresponding to an expanded pronotum. Abdomen had 10 segments (while drawing in the paper shows 11 segments ), and authors considered that lacked any lateral leglets, gills or other appendicular structures. There is rounded apical structure on the tip of abdomen. Lack of wings and small size would indicate that it is a nymph. It is interpreted to be a terrestrial insect, and mandible morphology suggests its diet would be fungivore and/or saprophagy.

Hörnschemeyer et al. (2013)
However, Hörnschemeyer et al. (2013) questioned its interpretation as insect. Mandible and its teeth cannot be confirmed, mandibular teeth would be caused by the idiosyncratic way the rock parted. Eyes are vague, indication of eye rims by Garrouste et al. is considered as arbitrary. Scape, pedicel and flagellum on its antennae cannot be confirmed, and whole antennae is wider than legs which is highly unusual for insects. Alleged subdivision of the trunk into thorax and abdomen is also questionable. Importantly, over 3 pairs of legs can be observed. This poor state of preservation allows numbers of alternate interpretations like decayed crustacean.

Garrouste et al. (2013)
In the same volume of Nature, Garrouste et al. replied to Hörnschemeyer et al. (2013). They supported mandibular teeth structure again. They argued that visible maxillary palps are abnormal under crustacean hypothesis. They considered that scape, pedicel and flagellum can be confirmed from width of antennae, and antennae being wider than legs would not deny insect affinity. For extra legs that Hörnschemeyer et al. confirmed, they considered as internal organs extruded during compression and decay.

Other studies
Multiple later studies about Strud fossils call Strudiella as "putative insect". Haug and Haug (2017) listed presumed Devonian insect fossils, and commented that "Its very incomplete preservation makes its interpretation problematic".

Importance of discovery
Discovery of Strudiella as Devonian insect reduces a previous gap of 45 million years in the evolutionary history of insects, part of the arthropod gap (the 'gap' still occurs in the early Carboniferous, coinciding and extending past the Romer's gap for tetrapods, which may have been caused by low oxygen levels in the atmosphere). However, Hörnschemeyer et al. (2013) which denied its interpretation as insect commented that it is crucial to prevent this fossil from entering entomology textbooks. Most of fossil records of Devonian insects like Rhyniognatha or Leverhulmia are questionable, and the early fossil records of insects still remains scarce and problematic.