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Let’s Talk About Sex—At the Dawn of Life on Earth

Whether sexual or asexual, reproduction is a necessity for all organisms that want to ensure their genetic material survives after they’ve bitten the dust (or in this case, the wet sand).

Half-a-billion-year-old animals are no different. Although in many ways it’s difficult to study how these ancient organisms reproduced, doing so is a critical aspect of our research. Here are some of the things we know about reproduction in the Ediacaran Period, and some of the ways we know them.

Rock All Night

Important to the study of reproduction in these organisms is the way they are preserved. We unearth the fossils in situ and on large planar rippled beds, which can have hundreds of fossils on one surface. This, as opposed to studying individual pieces of rock with one or two fossils on them at the most, is good for examining reproductive strategies and habits. We look at these fossils in relation to other animals that lived at the same time and in the same immediate area, so we have much more context than we might otherwise. We can observe and evaluate the size range and frequency of individual species, among other things, and look for clues that point us to how they reproduced.

This method allowed us to examine and describe the reproductive strategies of two interesting organisms: Funisia dorthea and Tribrachidium. Funisia are animals that grow via the addition serial tubular units to an anchor attached to the seafloor, while “tribes” are disk-shaped organisms, ranging in diameter from a few dozen millimeters to about an inch. Tribes also have triradial symmetry, which essentially means they have three equal parts that are all symmetrical, like the Mercedes logo (or, if you prefer, a peace sign without the line bisecting one of the thirds). There aren’t any modern triradial organisms, so tribes are pretty cool and unique.

Three “tribes,” all the same size, are just visible in the upper portion of this Australia-shaped rock. Photo by Emily Hughes

Signs of the Mating Season

The way we interpret the fossils, Tribrachidium and Funisia both appear to have reproduced in cohorts, meaning that we see groups of individuals that are the same size and in the same immediate area as being the same age. This process of growth is called synchronous aggregate growth, and it’s an indication of sexual reproduction. So, for example, if we’re looking at a bed of rock and see a swath of organisms that are closely packed together and the same size, we can interpret that to mean they were produced as a cohort.

Cohorts, as aforementioned, are indications of sexual reproduction. Funisia is actually the oldest known sexually reproducing animal, which is evidenced by the cohorts we see preserved in rock. As my mother likes to say, we’re not catching these animals in the act of sex (though that would be a fun fossil find), but rather seeing the evidence of their sexual reproductive strategy in the offspring.

Tightly packed and similar sized, these Funisia attachment structures, which we call “buds,” are evidence of sexual reproduction. Photo by Mary Droser

We see this in modern animals too, such as sexually reproducing corals. Corals will send eggs and sperm out into the water column, which will then join, forming larvae called planulae. After a time, planulae will settle on the ocean floor, metamorphose into polyps, and then continue to grow in size in a synchronized fashion. There weren’t corals in the Ediacaran Period that we study, but Funisia and tribes did demonstrate advanced reproductive strategies, like the ones corals use, and they might even be in the Phylum Cnidaria, which includes corals.

Like corals, tribes and Funisia also reproduced during specified periods, rather than continually. The periods during which Funisia and Tribes reproduced may have been based on environmental cues, as is often true for animals today with mating seasons.

Always at It

Unlike tribes and Funisia, there are Ediacaran animals that we know reproduced continually, rather than at a specific time of year. Dickinsonia, one of the most famous Ediacaran animals, is soft-bodied and modular, roughly the shape of an oval. It also reproduced throughout the year. Charniodiscus, a frond similar to a sea pen that is attached to the seafloor by a holdfast that leaves a fossil imprint called Aspidella, also reproduced continually. We often find beds of rock dominated by one or the other, but without evenness in terms of size or distribution across a surface.

A good look at a Dickinsonia: an iconic Ediacaran fossil. Photo by Mary Droser

 

In addition to these, we have found beds that we call “Nursery Beds,” because they’re full of teeny organisms that we’ve identified as juveniles. These specimens can be as small as the head of a pin, and are often fairly difficult to find and identify. We’ll find baby Parvancorinas, which are organisms shaped like anchors, Dickinsonias, and Sprigginas living among algae. We also find baby “parvs” on beds that have much larger organisms.

There is still much to learn about reproduction in the Ediacaran Period, as the reproductive strategies of many animals remains unknown. However, as we continue to unearth beds and study the fossils at this site, we are learning more and more about the ways these animals reproduced. And they keep getting sexier and sexier.