#fractofusus

My 25 years of palaeoart chronology...

Unsolved Paleo Mysteries Month #17 -- Enigmatic Ediacarans

Although Precambrian fossils have been known since the mid-1800s, the overwhelming belief among 19th and early 20th century scientists that complex life couldn’t have originated that early meant such discoveries either weren’t taken seriously or were forcibly assigned to a Cambrian age. It wasn’t until the discovery of Charnia in the 1950s that views began to change.

(Or, rather, the second discovery of Charnia, since the schoolgirl who first found it wasn’t taken seriously either.)

Since then, a wide variety of strange soft-bodied fossils have been identified from over 30 different localities around the world, on every continent except Antarctica, dating to ages from over 600 to 542 million years ago. They’re now known as the Ediacaran biota, after the Ediacara Hills in Australia where some of the most famous examples have been found.

A few show possible similarities to known groups, but we still don’t know what sort of lifeforms most of them they actually were. Animals, fungi, algae, foraminifera, microbial colonies, or lichens have all been proposed -- but they might also belong to a completely unique kingdom or phylum, a “failed experiment” in multicellular life with no living descendants.

And they’re gradually turning out to be not nearly as “simple” as once thought, showing evidence of their own thriving ecosystems and evolutionary specializations -- which makes their sudden disappearance at the end of the Ediacaran Period all the more mysterious.

Tribrachidium has been found in Australia, Ukraine, and Russia (558-555 mya), grew up to 5cm in diameter (2″), and shows unusual tri-radial symmetry. Affinities to both cnidarians and echinoderms have been suggested, but no classification has really stuck. Recent 3D modelling and fluid dynamic studies reveal its shape was adapted to direct water currents into the nooks between its “arms”, allowing it to feed on suspended organic particles.

Yorgia is known from Australia and Russia (~555 mya), and appears to be a transitional form between two other ediacarans, Dickinsonia and Spriggina. It has what appears to be a “head” end with an asymmetrical lobe, and a segmented body in a glide reflection pattern, growing up to 25cm long (10″). Trace fossils from its feeding strategy have also been found -- chains of imprints over seafloor microbial mats, where it moved from spot to spot and “grazed” with the entire underside of its body.

My 25 years of palaeoart chronology…

Fractal models of F. andersoni (top) and F. misrai (bottom) from Cuthill and Conway Morris 2014

Species: F. andersoni, F. misrai

Etymology: “Fractal spindle,” after its shape

Age and Location: Ediacaran of Newfoundland

Classification: ?Eukarya: incertae sedis: Rangeomorpha

Fractofusus is a genus of Ediacaran organism even stranger than yesterday’s Ernietta; like all rangeomorphs, its body plan was essentially defined by a fractal branching pattern--a pattern followed so closely that the genus and species within it can be diagnosed mathematically. Both known species essentially were a fractally-branching network of cells that formed a biconvex spindle-like shape. F. misrai, the longer (up to 40 cm) and more spindle-shaped species, is the younger of the two; it is possible that it is a direct descendant of F. andersoni.

Like most Ediacaran organisms, Fractofusus was an immobile osmotroph. . Due to its fractal branching (and therefore high surface area-to-volume ratio), Fractofusus were well-suited to this lifestyle and probably could obtain all of its nutrients trough direct diffusion, without any distribution of nutrients within the body. In life, it laid flat on top of the microbial mat that covered the seafloor in Ediacaran times, deep underwater. Fractofusus, like many Ediacaran animals, appears to have had no distinct "breeding season.” The young of Fractofusus, whatever they may have been like, were probably planktonic. Once a population was established in a given site, individuals could possibly have reproduce asexually by sending out runners, like some plants do today.

Sources:

Darroch S a. F., Laflamme M., Clapham ME. 2013. Population structure of the oldest known macroscopic communities from Mistaken Point, Newfoundland. Paleobiology 39:591–608.

Hoyal Cuthill JF., Conway Morris S. 2014. Fractal branching organizations of Ediacaran rangeomorph fronds reveal a lost Proterozoic body plan. Proceedings of the National Academy of Sciences of the United States of America.

Laflamme M., Xiao S., Kowalewski M. 2009. Osmotrophy in modular Ediacara organisms. Proceedings of the National Academy of Sciences 106.

*Emily G. Mitchell1, Alex G. Liu1, Charlotte G. Kenchington1 and Nicholas J. Butterfield1

1University of Cambridge

Fractofusus is one of the most distinctive members of the Rangeomorpha, a clade of fractally branching Ediacaran macroscopic fossils that has defied palaeobiological resolution.  Fractofusus positions were mapped out on three bedding planes (‘D’ and ‘E’ Surfaces, Mistaken Point and H14 surface, Bonavista Peninsula) in Newfoundland, Canada (565-570 Ma), allowing spatial distributions to be analysed and shedding light on their reproductive biology.   For each bedding plane, the spatial patterns of fossils were described using pair correlation functions and the patterns were compared with different types of clustering models. All three of the Fractofusus populations were found to exhibit recurrent, species-specific spatial clustering indicative of reproductive processes.  On ‘E’ and H14 we identified three generations ofFractofusus, but only two on ‘D’.  Where body-size was recorded (H14), Fractofusus specimens in the smallest size class preferentially cluster around those in the medium size class, which preferentially cluster around those in the largest size class.   Comparison of pair correlation functions, cluster directionality and mean cluster radius suggest that Fractofusus grew via a process of stolon-like reproduction, similar to that seen in plants that propagate via asexual runners. The identification of this reproduction strategy represents a fundamental new level of palaeobiological understanding of rangeomorphs.