In Franz Kafka's famous short story "The Metamorphosis", the main character Gregor Samsa wakes up one morning just to realize he's been unaccountably turned overnight into a human-sized, unidentified arthropod (the original German text has the word Ungeziefer, which indicates parasitic insects, spiders and similar animals).
And while the reader can be confident that such a creepy transformation is extremely unlikely in the real world, there was indeed a time in the history of Earth when similarly sized multi-legged invertebrates actually used to roam the planet.
Arthropleura is a genus of millipedes that could grow up to 2,5 metres long, with a maximum estimated weight of about 50 kg.
Different species of this genus are known from various parts of Europe and North America, and lived between the Early Carboniferous and the Early Permian, 345 to 290 million years ago.
Although its remains are relatively common, all Arthropleura known fossils represent molting shells (exuviae) and are often fragmentary; the fossil trackways known as Diplichnites consisting in double, parallel rows of densely arrayed small footprints are as well attributed to this genus.
While millipeds aren't the most noticeable creatures on earth, they represent one of the oldest groups of land creatures, with their earliest representatives dating back to the Late Silurian or Early Devonian, around 420 million years ago.
They form the class Diplopoda, which forms in turn the sub-phylum Myriapoda together with centipedes (Chilopoda) and the lesser known groups Pauropoda and Symphyla, mostly including very small creatures.
The name "millipede" comes from Latin and literally means one thousand feet, although the real number of their walking appendages may vary greatly, with some species of the short pill millipedes (Glomeridae) sporting little more than 30 legs, while the recently described Australian species Eumillipes persephone has about 1,360 of them.
Differently from other arthropod groups, in millipedes most body segments bear two pairs of legs, hence the Greek-derived scientific name Diplopoda, meaning "double feet".
Unlike centipedes which are carnivorous and in many cases are able to deliver painful venomous bites, millipedes generally feed on decaying plant matter and are harmless to humans; despite their defenseless appearance, they represent a very successful group, having lived across three geologic eras and still thriving today with about 12,000 described species divided into 16 orders.
The Palaeozoic genus Arthropleura includes five currently recognized species: Arthropleura armata, A. cristata, A. fayoli, A. maillieuxi and A. zeillieri.
The picture shows a real life reconstruction of both the upper and lower side of a female individual of the type species, Arhtopleura armata; a human is shown for size scale.
Monochromatic details show three larval stage of a generic millipede, illustrating the segments and legs number increase at every moult (top left to center left); a partial underside view of a male paradoxosomatid millipede, showing the pair of modified legs used in copulation and called gonopods (bottom left); an adult Arthropleura walking and leaving its typical double track (center right) and a fossil partial exuvia (bottom right).
MAIN REFERENCES:
Rosa Fernández, Gregory D. Edgecombe, Gonzalo Giribet – Phylogenomics illuminates the backbone of the Myriapoda Tree of Life and reconciles morphological and molecular phylogenies – in Nature – 08 January 2018;
Alessandro Minelli, Sergei I. Golovatch – Myriapods – in Encyclopaedia of Biodiversity – 2013 – pages 421-432.
Aucun commentaire:
Enregistrer un commentaire