Friday 29 April 2016

A Scorpion Mating Couple From The Permian

The fighting dinosaurs is one of the most remarkable and evocative fossils ever discovered. Consisting of a small theropod locked in combat with a Protoceratops mother desperately guarding a clutch of eggs, the pair were buried by the collapse of a nearby sand dune in their desert home. Rarely does in situ preservation capture such poignant and momentary glimpses of past worlds, particularly when it comes to actual body fossils. Trace fossils can be similarly evocative, but lack the tangible connection that a body fossil provides. As such, fossils like the fighting dinosaurs are priceless for the insight they give into the ecology and ethology of prehistoric creatures.

Recently, however, such a specimen has been uncovered from the Permian of Germany. As deserts spread across Pangaea, the lush Carboniferous forests began to fragment into isolated pockets and oases in the midst of a much harsher landscape. These green islands were points of rich biodiversity. The remains of one such island can be found under the city of Chemnitz, Germany. The ecosystem contained a wide variety of plants and arthropods which were buried 291 million years ago by ash from a nearby volcano, leading to the remarkable in situ preservation of its flora and fauna - a geological equivalent of Pompeii. Everything from tree trunks to leaf litter are in the same positions now as 291 million years ago, along with evidence of seasonality, articulated skeletons of amphibians and the impressions of reptile skin.

Birgit preserved within her burrow and surrounded
by fragments of moulted exoskeleton
In addition to the plant and vertebrate fossils, a small, hand dug excavation within the city itself uncovered two specimens of a new scorpion species, Opsieobuthus tungeri. The specimens were well preserved enough not just for species level identification, but their sex.

Scorpions possess a comb-like structure on the underside of their bodies which is used for prey and mate detection and general olfactory orientation. In males, these combs have more and longer teeth. The Chemnitz specimens were male and female, nicknamed Jogi and Birgit respectively. As well as being preserved within just two metres of one another, Birgit was found within a burrow and surrounded by fragments of a moulted exoskeleton.

An artist's impression of Jogi guarding the entrance to Birgit's burrow
The distance between the two is typical of a mating pair of scorpions of their size (12 centimetres) and the differing locations suggests that Jogi may have been guarding Birgit, a behaviour observable in modern day scorpions. The fragments of exoskeleton may also help explain why Birgit was in the burrow.

Scorpions, like all other arthropods, moult their exoskeletons in order to grow. This leaves them temporarily vulnerable to attack as their newly exposed cuticle takes time to mineralise and harden. Moulting within a burrow would have proffered some defence, along with Jogi. The nearby volcano, however, had other ideas.

Jogi and Birgit belonged to a primitive and now extinct group of scorpions. Other fossil evidence, however, shows that their temporal range overlapped with early representatives of the extant clade of scorpions as well as sharing similar behaviours in terms of mate guarding and moulting within burrows. This mating pair is a wonderful glimpse into life in the Permian period, along with the wider palaeontological setting of the Chemnitz lagerstatten.