Bangiomorpha


When I was putting together my post about Rafatazmia, the 1.6 billion-year-old fossil tentatively interpreted as a red alga, I searched Fierce Roller to see what I had written about Bangiomorpha, the previous record-holder for the oldest red algal fossil. I was surprised to find that I never have published anything about Bangiomorpha. This is a serious oversight!

Bangiomorpha was described by Nick Butterfield back in 1990, from a series of fossils collected on Somerset Island in Nunavut, the northernmost territory in Canada:

Must have been a fun field site. The fossils don’t look all that impressive to the untrained eye (like mine):

Figure 5 from Butterfield 1990. Bangiomorpha pubescens.

They do tell us several important things, though. First, Bangiomorpha was filamentous, with individual filaments including uniseriate (one row of cells) and multiseriate (multiple rows of cells) portions.

Second, Bangiomorpha was probably a red alga. This conclusion seems to be accepted by most everyone in the field. In fact, I don’t know of any dissenters, and that kind of consensus is rare for fossils this old. The fossils are essentially identical to the modern red alga Bangia:

Bangia fuscopurpurea by Fabio Rindi (image from Algaebase).

Red algae are among the Archaeplastida, the direct descendants of the primary endosymbiotic event that led to the establishment of chloroplasts. The chloroplasts of all other photosynthetic organisms result from secondary or higher order endosymbiotic events, that is, the photosynthetic partner that became the chloroplast in these lineages was itself a eukaryote, usually a red or green alga. Until the discovery of Rafatazmia, Bangiomorpha was the oldest known fossil of a ‘complex’ multicellular organism, that is, one with cellular differentiation. The cellular differentiation in this case is pretty modest, just a multicellular ‘holdfast’ to keep the algae attached to the substrate:

Figure 6 from Butterfield 1990. Bangiomorpha pubescens n. gen. n. sp. showing range of differentiated holdfast structures.

We also know that Bangiomorpha developed from a single cell, just like most modern multicellular organisms:

Figure 4A-D from Butterfield 1990. Early ontogeny of Bangiomorpha pubescens n. gen. n. sp. A, single cell with characteristic dark inner cell wall and translucent outer wall. B, two cells. C, four cells. D, eight cells.

We know this because there are lots and lots of Bangiomorpha fossils, and some of them represent early developmental stages. Of course, the specimens in Figure 4 above are different individuals, but they were preserved at different stages and can be arranged as a developmental series, starting with one cell, then two, then four, and so on.

Finally, by comparison with modern red algae, different filament morphologies are interpreted as asexual, male, or female, indicating that Bangiomorpha had a life cycle with alternating sexual and asexual phases, as in many modern algae:

Given the compelling case for identifying Bangiomorpha as a bangiacean red alga, it is worth considering the marked variation in (multiseriate) form as reflecting different reproductive types, as it does in modern Bangia. It is the mature, spore-bearing filaments (i.e., those bearing differentiated spheroidal cells) that provide the most convincing evidence: whereas Type 2 filaments (Fig. 5A,D) appear to have derived just a single spore from each of four vegetative, wedge-shaped cells, Type 3 filaments (Fig. 5H) clearly underwent significant tertiary division such that each wedge-shaped cell produced many spores. By comparison with Bangia, the former would appear to represent asexual monospores or unfertilized carpogonia, and the latter fertilized carpospores or spermatia (compare with Fig. 7) (Garbary et al. 1980). By the same token, Type 1 filaments are identified as vegetative plants that have yet to differentiate identifiable spore or gamete types. Although imperfect preservation frustrates exact identification of particular reproductive types in Bangiomorpha, the marked differences in ontogenetic pattern make a convincing case for the presence of differing reproductive types and therefore sexual reproduction.

Dr. Butterfield goes a step further (and a step further than I’d have gone), suggesting that the origin of eukaryotic sex must have led in short order to the evolution of multicellularity:

Sex was critical for the subsequent success of eukaryotes, not so much for the advantages of genetic recombination, but because it allowed for complex multicellularity. The selective advantages of complex multicellularity are considered sufficient for it to have arisen immediately following the appearance of sexual reproduction. As such, the most reliable proxy for the first appearance of sex will be the first stratigraphic occurrence of complex multicellularity.

Simon Conway Morris is thanked in the acknowledgements, and I suspect his influence on the deterministic theme running through that quote.

Regardless, Bangiomorpha is a crucial find, giving us a rare, detailed glimpse of multicellular life over a billion years ago. It’s no longer the oldest multicellular eukaryote, nor even the oldest red alga (if the interpretation of Rafatazmia is right), but the variety of developmental and life-cycle stages represented gives an amazing look at a truly ancient alga.

 

Stable links:

Bengtson S, Sallstedt T, Belivanova V, Whitehouse M (2017) Three-dimensional preservation of cellular and subcellular structures suggests 1.6 billion-year-old crown-group red algae. PLOS Biology, 15, e2000735.

Butterfield NJ (2000) Bangiomorpha pubescens n. gen., n. sp.: implications for the evolution of sex, multicellularity, and the Mesoproterozoic/Neoproterozoic radiation of eukaryotes. Paleobiology, 26, 386–404.

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