New Research Suggests That A So-Called 'Walking Cactus' Is The Ancestor Of All Arthropods

A fossil of the enigmatic Diania cactiformis

Diania cactiformis is a bizarre creature known from just one fossil found at the 525 million year old Maotianshan Shales in China. They have been described as the Burgess Shale of the Far East due the incredible quality and completeness of the specimens discovered there. Despite this, when the first fossils of Diania were recovered from the rocks, palaeontologists did not know whether to class it as a strange animal or a simple plant.

Eventually it was placed with the lobopodians, a poorly understood group of arthropods which have often been described as worms with legs. A recent study of the three or so complete specimens alongside 30 odd fragments by Chinese and German palaeontologists, led by Jiani Liu from the Northwest University in Xi'an, believe that its relationships to other lobopods, such as tardigrades or velvet worms, as well as the rest of the armoured invertebrate world, may be rather different to their current definitions.

A reconstruction of the 'walking cactus' Diania cactiformis

An impartial observer might think that Diania was indeed a plant due to its multi-branched, cactus-like appearance. However analysis by the researchers revealed that the 'branches' were in fact segmented limbs and that the creature underwent sclerosis. The process of excreting minerals to the surface of the body and allowing them to harden into a solid, protective shell, which is still used by invertebrate animals today. All these characteristics are used by arthropods.

This led the palaeontologists to the theory that the group Lobopodia is not actually a separate group of arthropods, but instead a name given to a collection of organisms who happen to share a similar body plan rather than a close evolutionary relationship. As Diania is one of the most primitive lobopods, lobopods are still arthropods and also some of the most primitive, the team logically concluded that this strange, cactus-like creature is one of the oldest arthropods on Earth, and perhaps even, the first ever.

As the Cambrian biota evolved from the mysterious Ediacara biota, organisms whose body plans are neither truly plants, animals or fungi, it seems likely that Diania, due to its bizarre mix of plant and animal characteristics, could represent the missing link between Ediacarans such as Spriggina or Dickinsonia and the rest of the arthropod kingdom. New and better preserved fossils of these strange intermediate forms will either confirm or dispel these theories.