microdictyon

3

Microdictyon.
Found in the shale deposits in China, this was an odd worm-like organism from the Early Cambrian epoch. It possessed pairs of sclerotic structures along its sides, which may have been early eye-like structures. These were matched by pairs of tentacles which appear to have been used for locomotion.

The armored worm, Microdictyon (1989)

Phylum : Lobopodia
Class : Xenusia
Order : Scleronychophora
Family : Eoconchariidae
Genus : Microdictyon
Species : M. sinicum

  • Early/middle Cambrian (540 - 507 Ma)
  • 25 mm long (size)
  • China (map)

Microdictyon is an extinct “armored worm” coated with dot-like scleritic scales, known from the Early Cambrian Maotianshan shale of Yunnan China. Microdictyon is sometimes included in a somewhat ill-defined Phylum – Lobopodia – that includes several other odd worm-like and segmented free-swimming animals that do not appear to be arthropods or worms. The phylum includes Microdictyon, Onychodictyon, Cardiodictyon, Luolishania, Paucipodia, as well as the Anomalocarids. The isolated sclerites of Microdictyon are known from other Lower Cambrian deposits. Microdictyon sclerites appear to have moulted; one sclerite seems to have been preserved during ecdysis.

Microdictyon sinicum (Chen, Hou and Lu, 1989) is typical. The wormlike animal has ten pairs of sclerites (suggestions that these may be eyes or eye-like structures have no weight) on the sides, matched to a pair of tentacle-like feet below. The head and posterior are tubular and featureless.

Microdictyon fossil

From one of the runners up entries of the forum’s 63rd Contest of the Month, by Flisch:

We often consider gyrobrates to be the dominant fauna of our time, but what few people realize is that arthrognaths make up almost 80% of all animal species on our planet. Arthrognaths have colonized every little nook of ecosystem and have produced such a vast variety of forms that it would take entire libraries to describe. And yet, they are always overlooked, possibly only outmatched in terms of understatement in palaeontology books by lichens.

The success story of the arthrognaths begins in the cambrian. Hallucigenia is possibly the most well-known candidate for an ancestor, mostly because it was the first identified as such. Since then, Microdictyon has largely been replaced as the closest relative in expert circles. Both animals already showed features that clearly marked them as members of the stem group, such as multiple pairs of legs, likely outfitted with claws as well as a long proboscis-like mouth. The most important feature however was the existence of sclerites at the sides of each segment. Although all modern arthrognaths possess three rows of sclerites, with one row running along the back of the animal, the connection is unmistakable.

The idea that trilobites are related to the arthognaths has been proposed based on the premise that trilobites possess three lobes, which could be homologous to the three rows of sclerites of arthognaths. However, trilobites possess compound eyes that have a completely different structure than the non-compound eyes of arthrognaths. Furthermore, trilobites don’t seem to possess a segmented proboscis characteristic of all arthrognaths. Trilobites were once thought to be a relatively short-lived group of animals that perished at the end of the cambrian, though their range has been extended significantly by the recently discovered Hinderschannes of the early devonian. Their origin is even more nebulous, seemingly coming from nowhere without any obvious relatives both extinct or alive. Should trilobites indeed be related to arthrognaths, then the split must have happened very early, possibly at the very beginning of the cambrian, though for now trilobites are considered ‘incertae sedis’ on the phylum level.

Meanwhile the first crown group arthrognaths have been found in the early ordovician, quickly radiating into a variety of various forms. During the devonian, arthrognaths were already starting to colonize the land, long before gyrobrates had evolved bony skeletons. It is interesting to note that there was not one colonisation wave of terrestrial arthrognaths but several. The last wave would give rise to the entosects, the largest group of arthrognaths by far and infact the majority of all animals species-wise.

The carboniferous has often been dubbed the 'age of arthrognaths’ due to the fact that during that time they reached sizes that they would never reach again. This name however serves as a reminder of how dismissive we tend to be towards arthrognaths as a whole, as if they have earned the right to be recognized only due to their size. If we’d go by sheer numbers, we have never left the “age of entosects” ever since they appeared on the evolutionary stage.

Entosects have produced a downright dizzying number of forms and lifestyles. They are the only group of animals to have evolved true eusociality. Furthermore it is notable that entosects are also the only ingyrobrate group to evolve flight. Strangely enough, despite their success, there are next to no marine entosects. It is quite possible that if they ever evolve a successful marine group, their already impressive numbers would swell and overshadow all other groups of animals into obscurity.

The pictured animal is a Flutterby, a group of entosects that is popular for their vivid colours. Flutterbies are a good anatomy example for arthrognaths and entosects in particular. They possess three pairs of wings, that are in truth highly specialized lateral sclerites. They also have five pairs of legs, like all entosects, though their last two pairs have been greatly reduced in size. Their legs are divided into two parts, that can be likened to an upper arm and a forearm. The upper arm contains only muscles and is relatively flexible. The forearm however is held in shape by a thin rod that ends in a curved claw, which is used to hold onto rough surfaces. On each side of the claw is a small pad that acts as a suction cup so the animal can even cling to smooth surfaces. The end of the rod ends in a little bulb for muscles in the upper arm to attach to. These muscles can bend the arm of the animal by moving the rod.

The segmented proboscis of the Flutterby is simple in anatomy and lacks the biramous segments that are present in other members of arthognaths. Flutterbies live in symbiosis with flowering lichen and drink nectar, while spreading pollen around as they dip their heads into the calyx. Due to their mode of feeding, biramous segments are not only useless, they would even hinder the feeding process as many flowers have evolved a narrow shape to specialize on a few symbionts in order to maximize fertilization.

Flutterby eyes are highly sophisticated and can detect four different colours, making them tetrachromatic. The eyes of flutterbies, as well as all arthrognaths are not compound eyes like those of gyrobrates, but instead form a single spherical eye with a lens. This eye can even be moved around in their socket. The black pupil spot that can be seen on their eye, around the coloured 'halo’ is in fact a hole in their eye, that lets the light in, to be processed by the light sensitive cells at the back of the eye. This means that when halo with the pupil spot appears to be centered on their eye, then they are currently directly looking at the observer.