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Not to be confused with Microdictyon (genus of green algae).

Prehistorica CM/sandbox Temporal range:
Diagrammatic reconstruction of Microdictyon sinicum
Microdictyon sinicum Fossil
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
(unranked):	Panarthropoda
Phylum:	†"Lobopodia"
Class:	†Xenusia
Order:	†Archonychophora
Family:	†Eoconchariidae
Genus:	†Microdictyon Bengtson, Matthews & Missarzhevsky, 1986
Type species
Microdictyon effusum Bengtson, Matthews & Missarzhevsky, 1986
Species
See text

Microdictyon is an extinct genus of lobopodian worm characterized by its net-like sclerite armour plates, known from Cambrian deposits around the world. Soft-bodied fossils which preserve more than the sclerites are only known from the Chengjiang Lagerstätte of Yunnan, China.

History

Microdictyon sclerite plates have been recovered from around the globe, recovered from rock via acid dissolution which eats away at the rock but leaves behind compositionally distinct microfossils. The name Microdictyon was erected by Stefan Bengston, Vladimir Missarzhevsky, and S. C. Matthews in 1981, as an enigmatic microfossil. It was unknown at the time what animal could have produced it - suggestions for its producer included echinoderms, sponges, and radiolarians. A number of Microdictyon species were named in the following years. The genus Eoconcharium was named in 1987 based on fossils from China, and a family was erected to contain it, the Eoconchariidae. Although it was later recognized as a junior synonym of the earlier named Microdictyon, the name of the family group Eoconchariidae, which today contains Microdictyon, Fusuconcharium, and Quadratapora, kept priority. The first complete specimens of Microdictyon were found in the Chengjiang Biota of Yunnan, China. The two soft bodied fossils, which showed the characteristic Microdictyon plates armouring the body of a caterpillar-like worm, were discovered in 1989 and given the name Microdictyon sinicum. A more complete descripton based on over 70 newly discovered fossils, also from Chengjiang, was published in 1995.

Description

Excluding their sclerites, Microdictyon had a relatively simple, conserved body plan among lobopodians. The trunk was elongate, up to 77 millimetres (3.0 in) in length, with 10 pairs of tube-like limbs and 9 pairs of netted sclerite plates. Each limb pair was associated with a single pair of plates, except for the posterior-most set of limbs which shared a sclerite pair with the penultimate leg pair. The legs are undifferentiated, tipped with a pair of small, curved claws. The legs were finely annulated, and ornamented further with small papillae. The trunk itself was also annulated and dotted with small papillae between the limb pairs. At each sclerite plate pair, the body swells slighlty to accomodate it, starting at a negligible height and becoming larger posteriorly as the plates increase in diameter. These plate-bearing mounds, and the area where the limbs attach below them, are devoid of annulation. The sclerite plates are subcircular and convex outwardly. They are composed of a fine, roughly hexagonal mesh of rounded hollow pits and specialized nodes at the mesh intersections. The nodes vary in morphology between species, but are typically mushroom-shaped. The walls of the sclerite were thin, and the sclerotized bars between nodes may have further bumps on their surface, and the entire mesh is surrounded by a thin marginal rim. Rarely, the plates may develop large spines randomly on their surface. The head and neck were smooth or only finely annulated, with a small mouth set slightly ventrally at the anterior of the head. No evidence for eyes, cephalic appendages, or any structure resembling teeth or jaws has been reported. The gut was simple and straight, with the anus found at the end of a small posterior tail-like projection.

Ecology

Microdictyon was epibenthic, walking along the seafloor using is lobopodous limbs. The anterior portion of the body was the most flexible - fossils showing Microdictyon strongly curled up indicate that the cause of death was anoxia, inferred from studies on modern arthropods and annelids. The claws of Microdictyon are not particularly suited for walking on muddy substrate, but rather for climbing or grabbing — it was once proposed that, based on frequent association with fossils of Eldonia, and a presumed pelagic lifestyle for the jellyfish-like Eldonia, that Microdictyon would use its claws to grab onto the large disc-shaped animal as it swam in order to feed on it. However, Eldonia is no longer thought to be pelagic, and probably lived gregariously on the seafloor. The frequent association with Microdictyon still likely indicates an ecological link, with Microdictyon possibly using its claws to feed on Eldonia or other carcasses — a similar relationship has been proposed for Paucipodia. Paucipodia and Microdictyon have also been found preserved alongside mass concentrations of living, dead, and decaying Cricocosmia worms.

The purpose and development of Microdictyon's sclerotized plates remains a subject of debate. Some authors, beginning with Jerzy Dzik in 2003, propose that based on a similar proposed method of sclerite growth and a similar morphology, these sclerites could be interpreted as large compound eyes, similar to phacopid trilobites — this idea has been widely rejected. The other proposals, that being defense and or muscle attachment, are viewed as much more plausible. Evidence from a small number of specimens, including both complete fossils from the chengjiang as well as isolated microfossil plates, indicate that Microdictyon and other eoconchariids moulted their plates asynchronously, with the new, larger plates forming beneath the old ones, starting with the plates in the middle of the body and spreading to the anterior and posterior plates later. The new plates were not formed all at once, and the old plates remained overtop of the new ones for some time before the old ones were finally dropped and discarded. This is similar to the biphasic moulting of modern isopods (the posterior half of the body moults first, followed later by the anterior half), as opposed to most crustaceans, which moult their entire exoskeleton in a single step.

Species composition

• Type species. Microdictyon effusum Bengtson, Matthews et Missarzhevsky, 1981; Lower Cambrian, Atdabanian Stage, Kazakhstan; Atdabanian and Botomian Stages, Russia (Siberian Platform) and England; Lower Cambrian, Sweden.

In addition to the type species, 13 species:

• M. anus Tong, 1989, Lower Cambrian, upper Meishucunian Stage (= Atdabanian Stage), China (Shaanxi).
• M. chinense (Hao et Shu, 1987), Lower Cambrian, Qiongzhusi Stage (= upper Atdabanian-lowermost Botomian Stages), China (Shaanxi); Atdabanian through Botomian stages, Siberian Platform.
• M. cuneum Wotte et Sundberg, 2017, Lower Cambrian, Montezuman Stage, the United States.
• M. depressum Bengtson, 1990, Lower Cambrian, Atdabanian through Botomian Stages, South Australia.
• M. fuchengense Li et Zhu, 2001, Lower Cambrian, upper Meishucunian Stage (Atdabanian Stage), China (Shaanxi).
• M. jinshaense Zhang et Aldridge, 2007, Lower Cambrian, Qiongzhusi Stage (= upper Atdabanian Stage-lowermost Botomian), China(Shaanxi).
• M. montezumaensis Wotte et Sundberg, 2017, Lower Cambrian, Montezuman Stage, the United States.
• M. rhomboidale Bengtson, Matthews et Missarzhevsky, 1986, Lower Cambrian, upper parts of the Atdabanian Stage, Kazakhstan; Atdabanian Stage, Canada, the United States (M. cf. rhomboidale).
• M. robisoni Bengtson, Matthews et Missarzhevsky, 1986, Middle Cambrian, Amgan Stage, the United States;
• M. rozanovi Demidenko, 2006, Lower Cambrian, Toyonian Stage, Siberian Platform.
• M. sinicum Chen, Hou et Lu, 1989, Lower Cambrian, upper Meishucunian Stage (= Atdabanian Stage) Stage, China (Yunnan.
• M. sphaeroides Hinz, 1987, Lower Cambrian, Atdabanian Stage, Great Britain.
• M. tenuiporatum Bengtson, Matthews et Missarzhevsky, 1986, Lower Cambrian, Atdabanian Stage, Siberian Platform.