Rhyniognatha

Rhyniella, the collembolan in Early Devonian Rhynie chert, has long been heralded as the oldest hexapod, while fragmentary remains from the same chert had been mostly forgotten. In the original paper announcing the discovery of Rhyniella, Hirst and Maulik (1926) also reported a pair of mandibles preserved with largely unidentifiable tissues surrounding them (Figure 5.8). Later, Tillyard (1928b) formally described the mandibular elements and named them, Rhyniognatha hirsti. Tillyard was the first to note that they were insect-like, but he did not place them formally into any group. Indeed, subsequent authors relied on Tillyard's illustration and followed his intrepretation that the mandibles were "suggestive" of an insect (e.g., Hennig, 1981). Most authors, however, considered Rhyniognatha as too fragmentary to make any determination, and Carpenter (1992) even excluded it from his monumental treatise on insect paleontology.

Rhyniognatha Hirsti
5.8. The oldest insect, Rhyniognatha hirsti, from the Early Devonian chert near Rhyme, Scotland. Only portions of the head are preserved, but the dicondylic mandibles indicate it was an insect; their triangular shapes indicate it may even have been a winged insect. NHML In. 38234.

In a recent study of the unique holotype of Rhyniognatha (Engel and Grimaldi, 2004a), the presence of an anterior acetabulum and posterior condyle on the mandibles was confirmed, conclusively demonstrating that the mandibles are dicondylic (Figures 5.8, 5.9). Furthermore, the mandibles are short and triangular, a morphology characteristic of a subset group among the pterygote insects, the Metapterygota, implying that Rhyniognatha possessed wings. This would place the origin of wings at least 80 million years earlier than previous fossil evidence allowed, and interestingly agrees with a recent molecular study that estimated insects originated in the Early Silurian and neopterous insects in the

Marellomorpha Phylogeny

5.9. Phylogeny of basal insect lineages indicating the position of Rhyniognatha, the oldest insect, as based on the structure of mandibles. Fossils (numbers): 1. Rhyniella praecursor; 2,3. undescribed; 4. Rhyniognatha hirsti. Characters (letters): A. insectan (see text), B. dicondylic mandibles, C. wings, D. entognathous mouthparts.

5.9. Phylogeny of basal insect lineages indicating the position of Rhyniognatha, the oldest insect, as based on the structure of mandibles. Fossils (numbers): 1. Rhyniella praecursor; 2,3. undescribed; 4. Rhyniognatha hirsti. Characters (letters): A. insectan (see text), B. dicondylic mandibles, C. wings, D. entognathous mouthparts.

mid-Devonian (Gaunt and Miles, 2002) (see also the Origin of Wings in Chapter 6). A Devonian origin of wings could only be conclusively proven with fossilized wings from that period.

It is impossible to say to what order Rhyniognatha might have belonged, or if it belonged to an unknown, extinct lineage of primitive insects. All than can be said is that Rhyniognatha is the oldest insect, and that it was more derived than bristletails and silverfish, and probably more than Ephemeroptera. Regardless, Rhyniognatha's occurrence in the Early Devonian indicates that insects likely originated in the latest Silurian and were among the earliest of terrestrial faunas (Engel and Grimaldi, 2004a). Rhyniognatha also reflects the serious need for intensive exploration of insects from the Devonian and Early Carboniferous.

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