Stating the obvious: Charnia is a planula extended family

Figure 1. Illustration of Charnia showing growth stages.

Figure 1. Illustration of Charnia showing growth stages. Each segment is a planula colony or family.

Summary for those in a hurry:
Charnia (Figs. 1, 2) can be nothing else than a family of interconnected planulae (Fig. 4). There were no other choices back in the Ediacaran. Taxonomically, it’s that simple.

Enigmatic Charnia
(Figs. 1, 2) is a frond-like animal near the base of all macroscopic animals. It lived in the Ediacaran Period prior to the Cambrian Period. The Cambrian lasted for 55 million years (so, no explosion). That’s when a wide variety of animal taxa appeared for the first time and many disappeared. By contrast, the Ediacaran was much less diverse and has a reputation for being more mysterious.

The extant planula
is a tiny blastula full of cells without a mouth or anus (Fig. 4). Several more derived animals go through a planula stage during their development (Figs. 5, 6). Others, develop more directly from an invaginated blastula.

Taxonomically
a planula also nests at the base of all macroscopic animals. Hmmm. Happy coincidence?

Given no other taxonomic choices, 
Charnia can only be a family of interconnected tiny planula, convergent with the extant demosponge Callyspongia (Fig. 3), but without the central invagination (Fig. 5).

Dunn, Liu and Donoghue 2017 reported,
“Extant members of Porifera do not show a serially repetitive body plan in the same way as certain cnidarians, and do not display the same level of colonial integration (i.e. the division of labour). However, certain sponges (e.g. the demosponge Callyspongia vaginalis) are constructed of serially repeated units.”

Figure 2. Charnia in situ with holdfast.

Figure 2. Charnia in situ with holdfast.

Dunn et al. 2018 presented
a wide-ranging study of Charnia (Fig. 1), a frond-like sessile animal known from tiny 1.7cm juvenile specimens to 2m tall adults all confined to the Ediacaran era.

From the abstract:
“The Ediacaran macrofossil Charnia masoni Ford is perhaps the most iconic member of the Rangeomorpha: a group of seemingly sessile, frondose organisms that dominates late Ediacaran benthic, deep-marine fossil assemblages. We evaluate specimens from the UK, Canada and Russia, representing the largest morphological study of this taxon to date. 

Figure 3. Callyspongia is an extant sponge that grows in a manner similar to Charnia according to Dunn, Liu and Donoghue 2017.

Figure 3. Callyspongia is an extant sponge that grows in a manner similar to Charnia according to Dunn, Liu and Donoghue 2017.

Earlier Antcliffe and Brasier 2008
reviewed the history of this taxon. They reported, “We show that Charnia cannot be related to the modern cnidarian group, the sea pens, with which it has for so long been compared, because they have opposite growth polarities.”

“Next to sponges, Cnidaria comprise the most primitive animal phylum and their presence in Precambrian strata before the emergence of the other animal phyla is a reasonable expectation.”

Figure 4. Planula evolution from a microscopic blastula. It simply fills with cells that have to be fed from external cells.

Figure 4. Planula evolution from a microscopic blastula. It simply fills with cells that have to be nourished from external cells. This is the level of animal development at the time of Charnia. Illustration from Peters 1991.

The term ‘planula’
is not found in these works. Seems the planula was forgotten or overlooked.

Figure 5. Planula can develop into sponges, hydras, medusas or flatworms (see figure 6) simply by invaginating, something Charnia parts never did.

Figure 5. Planula can develop into sponges, hydras, medusas or flatworms (see figure 6) simply by invaginating, something Charnia parts never did. Illustration from Peters 1991. In Charnia the planula does not invaginate.

Figure 6. When a planula evolves a ventral opening the internal cells can spill out to cover prey items, the retreat again to continue digestion. This is a stage that Charnia never reached.

Figure 6. When a planula evolves a ventral opening the internal cells can spill out to cover prey items, the retreat again to continue digestion. This is also a stage that Charnia never reached. No mouth/anus is found on any Charnia frond.

Given the time (the Ediacaran) 
and the primitive development of animal tissues at the time, Charnia can be nothing else than a collection of planulae, none with an invagination, mouth or anus. Reproductive divisions did not separate siblings and that’s how the colony grew serially. The shape and size was variable (Fig. 7).

Figure 7. Ediacaran biota. These are all variations on a single note, the planula.

Figure 7. Ediacaran biota. Many of these are all variations on a single note, the planula. The mobile ones are more derived segmented flatworms. All we had in the Ediacaran were planulae, sponges, hydras, medusae and flat worms.

Evidently defenseless sessile planula families
did not repeat this temporarily very successful experiment when the Cambrian became a thing. By then predaceous relatives, like hungry flatworms developed. Some large, swimming flatworms became hyper-predators in the Cambrian as discussed earlier here.

What about the other Ediacaran taxa?
Same answer for many of them. Planula colonies experimented with several immobile shapes that ultimately fell prey to the one mobile shape that preyed upon them, the flatworm, which, in turn, gave rise to a wide variety of swimming, crawling and burrowing taxa… ultimately humans. All we had in the Ediacaran were planulae, sponges, hydras, medusae and flatworms to work with. Everything else arose in the Cambrian from these basal animals at the genesis of all animals.


References
Antcliffe JB and Brasier MD 2008. Charnia at 50: Developmental models for Ediacaran fronds. Palaentology https://doi.org/10.1111/j.1475-4983.2007.00738.x
Dunn FS, Liu AG and Donoghue PCJ 2017. Eidacaran developmental biology. Biological Reviews doi: 10.1111/brv.12379
Dunn FS et al. (6 co-authors) 2018. Anatomy of the Ediacaran rangeomorph Charnia masoni. Papers in Palaeontology 2018:1–20.
Ford TD 1958. Pre-Cambrian fossils from Charnwood Forest. Proceedings of the Yorkshire Geological & Polytechnic Society, 31, 211–217.
Ford TD 1962. The oldest fossils. New Scientist, 15, 191–194.
Laflamme M, Narbonne GM, Greetree C and Anderson MM 2006. Morphology and taphonomy of an Ediacaran frond: Charnia from the Avalon Peninsula of Newfoundland. Geological Society London Special Publications 286(1):237-257.
Peters D 1991. From the Beginning – The story of human evolution. Wm Morrow.

wiki/Charnia