Researchers don’t know if they’re plants or animals but they assume that rangeomorphs were “some of the earliest large organisms on Earth,” as Science Daily puts it.
“These organisms were ocean dwellers that lived during the Ediacaran period, between 635 and 541 million years ago. Their soft bodies were made up of branches, each with many smaller side branches, forming a geometric shape known as a fractal, which can be seen today in things like lungs, ferns and snowflakes.”
Some rangeomorphs were tiny, only a few centimetres tall, but others could reach a height to two metres.
They could grow so big as “they extracted nutrients from their surrounding environment.”
Dr Jennifer Hoyal Cuthill (University of Cambridge) and Professor Professor Simon Conway, whose recent paper was published in the journal Nature Ecology and Evolution, suggest that the answer might be found in (assumed) “changes in ocean chemistry.”
Scince Daily quotes Dr, Hoyal Cuthill as saying:
"During the Ediacaran, there seem to have been major changes in Earth's oceans, which may have triggered growth, so that life on Earth suddenly starts getting much bigger. It's probably too early to conclude exactly which geochemical changes in the Ediacaran oceans were responsible for the shift to large body sizes, but there are strong contenders, especially increased oxygen, which animals need for respiration."
However, to make big organisms one needs a lot more than just oxygen. Intelligence would be a more viable explanation.
University of Cambridge. 2017. Big, shape-shifting animals from the dawn of time. Science Daily. (10 July).