Preservation and Classification of Macroalgae
Classification and Preservation
Preservation of Fossil Macroalgae
Because noncalcified macroalgae lack hard parts, they generally are not preserved as fossils. Strata that contain their remains, therefore, reflect deposition under a special set of conditions. Two factors are of paramount importance in this regard: low oxygen levels and rapid influxes of sediment, ideally composed of carbonate mud1. Both factors must act in tandem, and even here the odds of preservation are low. In modern shallow marine settings, this combination of conditions can be found in poorly circulated areas between reefs, and in tropical bays and lagoons largely cut off from the open ocean. Although normally calm and effectively stagnant, the waters in these settings are occasionally violently disrupted by tropical storms, resulting in rapid burial of the seafloor and all upon it by a thick layer of mud. It was within settings of this nature on the ancient Earth that the rare examples of fossil seaweeds highlighted in this website were preserved. Notably, because of the very particular conditions necessary for preservation, the fossil record does not provide a complete picture of ancient seaweeds, but rather just a narrow “slice.” The setting that is today occupied by giant kelp, for example, lies beyond the afforded field of view.
Fossil deposits that preserve the remains of organisms without hard parts, including seaweeds, are collectively known as Lagerstätten. A number of Lagerstätten containing the remains of seaweeds have been discovered in the last 40 years, and most of these are from the timeframe covered by this website, 700 to 400 million years ago. In some of these, such as the Burgess Shale Biota of western Canada, seaweeds are preserved alongside a diverse array of soft-bodied animals; in others, however, seaweeds are the main, and in some cases sole, nonbiomineralized elements. The latter comprise a distinct subset of Lagerstätten known as algal-Lagerstätten2. In total, some 45 seaweed-bearing Lagerstätten of early Paleozoic age have been described since 1973, nearly double the number known prior to that time, and a sizable portion of these qualify as algal-Lagerstätten.
Chemical analyses indicate that specimens of fossil seaweed consist of thin films of carbon, these appearing dark brown to black against the rock3. Occasionally, discontinuous patches of mineral material, such as pyrite, locally overprint the carbon films4. Thalli in most cases are pressed flat, like herbarium specimens, but in rare instances the branches retain their original circular cross-section3, an indication that the burial muds hardened and lithified very soon after deposition.
Macroalgae that produce a hard “coat” of calcium carbonate, known as calcareous macroalgae, are common is some parts of the modern-day ocean and these, too, were preserved as fossils. In some cases, their calcareous remains form entire rock units, a reflection of the fact that this material has substantially higher fossilization potential than the “soft” material of their noncalcified relatives. Definitive examples of calcareous macroalgae, however, are not known from rocks older than Ordovician. As a consequence, they cannot provide information about the biosphere during the early diversification of macroscopic multicellular animals.
Classification of Fossil Macroalgae
The first step in classifying a specimen suspected to be a fossil seaweed is to determine that it really is an alga, as opposed to having something to do with an animal. Such a task is not as easy as it might seem! Indeed, most of the specimens described as fossil seaweeds during the early history of paleontology were later found to be burrows made by animals, including Arthrophycus, Palaeophycus, and Chondrites5,6. Although simple tubular burrows, with or without branching, can still prove difficult to distinguish from seaweeds, many early Paleozoic fossils currently regarded as seaweeds have far more complex morphologies. Those that do not often can be distinguished from burrows by their flattened form, carbonaceous composition, well-defined lateral borders and jagged endings, tapered ends or terminations, non-disturbance of associated sediment layers, and, in some cases, internal details such as filaments4,7,8.
Difficulties can also arise when attempting to distinguish between fossil seaweeds and the remains of certain kinds of animals that produced an organic skeleton, especially graptolites. Chemical analyses, in terms of molecular composition, generally are not helpful here, despite the materials differing somewhat in life, because any distinctiveness in this regard typically is lost as a consequence of alteration after burial9. Nonetheless, a variety of other tools have proven to be useful in this regard. Chief among these is scanning electron microscopy (SEM), especially in backscattered mode (BSE), which can reveal small-scale morphological features not otherwise evident10,11. For example, detection of fusellae (microstructures of graptolite tubaria) in specimens assigned to Yuknessia from the Burgess Shale and the Cambrian of Utah indicates that this taxon, long regarded as a seaweed, is actually an animal with a hemichordate affinity12,13. Another approach shown to have merit in this regard involves stable carbon isotopic composition. For example, Medusaegraptus, a taxon originally described as a graptolite on the basis of material from the Silurian of New York but later redescribed as a dasycladalean alga on morphological grounds14, shows δ13C values comparable to those of associated seaweeds but distinctly lower than those of associated graptolite taxa15, a finding consistent with a lack of fusellae in BSE images.
Microbial mats can also resemble seaweeds when fragmented. Here, an affinity with the former is indicated by a combination of (1) lack of consistent form, (2) poorly defined margins, and (3) absence of a differentiated holdfast. On this basis, the various Cambrian species of Morania can be confidently regarded as microbial in origin, in keeping with the original interpretation proposed by Walcott16.
Once a seaweed identity has been established for a specimen, classifying it within the algae presents an additional set of challenges because fossil specimens do not provide biochemical or cytological data, both of which weigh heavily in classification and phylogenetic analyses of modern macroalgae, particularly since macroalgae commonly display convergent evolution of thallus form. Thus, in most cases, assigning fossil macroalgae to higher-level taxa (i.e., red, green, or brown algae) can be done only with a large degree of uncertainty. Fossil and molecular data indicate that both red and green macroalgae, including Bangiophyceae and Florideophyceae within the former and Siphonocladales within the latter, had originated before the Cambrian17,18,19,20,21,22,23. Molecular clock studies, however, indicate a post-Paleozoic origin for brown algae24,25, thereby pointing to the possibility that Proterozoic and Paleozoic taxa described as brown algae, such as Miaohephyton from the Ediacaran of China26 and Thalassocystis from the Silurian of Michigan27, only superficially resemble such forms as a result of convergence. Some early Paleozoic macroalgae do seem resolvable to the ordinal level. Among these are forms with monopodial thalli consisting of an upright central axis surrounded in radial fashion by lateral appendages, an architecture found primarily in the green algal order Dasycladales among extant marine algae, and thalli composed of bundles of intertwined tubes, a morphology unique to members of bryopsidalean green algae in the modern marine biosphere.
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