Pterosaurs: dragons of the air
The Devonian: insects take flight
The Devonian is best known for the expansion and diversification of terrestrial plants which spread around the globe beginning as small, moisture-loving, moss-like plants, but becoming gigantic tree-like forms by the end of the period. It is also known as the time when an obscure group of freshwater fishes with fleshy fins evolved limbs and took the first steps on land as something that today we informally call ‘amphibian’. But behind these evolutionary scenes a quiet revolution among tiny six-legged arthropods was taking place: the invention of the insect wing. Details are sketchy, but a lucky discovery in the Rhynie Chert in Scotland strongly suggests that insects had evolved wings by about 410 million years ago.
Global reconstruction courtesy of Professor Ron Blakey, Northern Arizona University, Geology.
Photograph courtesy of Robert Loveridge
The Rhynie Chert
A small field close to the village of Rhynie in north east Scotland is one of the most important sites of special scientific interest (SSSI) in the United Kingdom. About 1m under the lush grass is a fossil peat bog formed where silica-rich waters bubbled up from the ground, perhaps like a Yellowstone Park geyser. As the waters flowed through the peat, microscopic crystals of quartz precipitated to form the flint-like rock called chert. The crystals grew on the plants in the peat, perfectly preserving their remains. Chert is almost insoluble once it has formed, so the mineralised peat has survived for over 400 million years. Usually peat deposits get compressed under the weight of overburden, and slowly they turn into coal, a process that destroys most of the fine detail of the plants that formed the peat.
Seen under the microscope, it reveals many types of plants and just occasionally the tiny invertebrates that lived among them. This is a window into one of the very first terrestrial ecosystems on planet Earth.
A lucky discovery
The Rhynie Chert is full of beautifully preserved plant remains. They are so well preserved that they show cellular details with no squashing during burial: an indication that they were fossilised very quickly. The plants are studied by slicing the rock into thin slabs that are glued to glass slides. They are then ground down further until they are just 30 microns thick. The included plant fossils are then studied using a binocular microscope.
Just occasionally, hiding among the remains of the plants’ stems are the remains of early terrestrial animals, including trigonotarbids and the spring tail Rhyniella praecursor. One specimen, thought to represent the mouthparts of a spring tail was first described in 1926 and named Rhyniognatha hirsti in 1928. The name Rhyniognatha simply means the Rhynie jaws. The specimen then remained unstudied until 2004 when Michael Engel and Dave Grimaldi realised that these mouthparts were from an animal much more closely related to mayflies than to the wingless insects. Although the specimen does not preserve any wings (they are probably still in the slab from which the thin section was cut), the jaws are so similar to those of modern mayflies that Rhyniognatha was almost certainly a winged insect.
Photograph courtesy of Robert Loveridge