• Precambrian;
  • biomarker;
  • algae;
  • sponge;
  • Porifera;
  • origin of animals;
  • Oman;
  • Cryogenian


  1. Top of page
  2. Abstract
  3. Geological context and dating
  4. The biological arguments
  5. Discussion
  6. References

Elevated concentrations of an organic compound, 24-isopropylcholestane, found in the Precambrian Huqf Supergroup of Oman may provide the oldest known sponge ‘fossil’. This evidence is of critical importance for a properly balanced understanding of the origin of animals. Several different pelagophyte (Class Pelagophyceae part of the Stramenopiles within the Chromaveolata) algae are also capable of producing these exact compounds, and may similarly have done so in deep time. Modern marine algae are also reported to produce structural isomers that are compositionally identical to the sponge marker; they do this in copious quantities. Further, 24-isopropylcholestane can be produced by diagenetic alteration of compounds produced in large quantities by algae. It is also possible that contamination by petroleum derived lubricating oil used when coring while extracting these compounds from subsurface layers, has affected the data. All extinct organisms that may have produced this compounds are unavailable for analysis by the modern organic chemist and cannot be eliminated from the list of possible producers of the sponge marker. There are also significant uncertainties regarding the dating of the strata from which these ancient compounds are found. Although the compounds are widely reported as c. 751 Ma, they are younger than 645 Ma. It seems more likely that these compounds represent algal biochemical evolution at a time when algal burial occurred in great quantity with well known coeval algal fossils but no sponge fossils. The macroalgal biomass may have declined during the agronomic revolution at the base of the Cambrian Period owing to processing by metazoans, accounting for the comparative scarcity of these sponge markers in Phanerozoic sediments, after which time sponge spicules and body fossils become evident.

The Ediacaran Period is famous for fossils which some suggest represent the earliest animal ancestors (Jenkins 1984; Gehling 1991, though it is equally famed for the vigorous debates about the veracity of such claims (Seilacher 1992; Budd and Jensen 2000, Antcliffe and Brasier 2007a, b, 2008; Brasier and Antcliffe 2008). Detailed analyses of Precambrian fossils can reveal much about their interrelations and ecology (Narbonne 2005; Antcliffe and McCloughlin 2008; Brasier and Antcliffe 2009; Liu et al. 2011; Brasier et al. 2012) and are helping to form a coherent narrative of the early evolution of complex organisms and their ecosystems (Narbonne 2005; Brasier et al. 2011). High concentrations of molecular 24-isopropylcholestane in Proterozoic rocks of the Huqf Supergroup, Oman (McCaffrey et al. 1994; Hod et al. 1999; Love et al. 2009) are another recent claim for animals in deep time and have received much attention (Maloof et al. 2010; Sperling et al. 2010; Erwin et al. 2011). Interpretation of these organic compounds as the earliest fossil evidence for demosponges is highly significant for framing the narrative concerning the origin of animal life. They fit neatly with results from recent molecular clock analyses (Peterson et al. 2008; Benton et al. 2009; Sperling et al. 2010; Erwin et al. 2011) that predict deep time Precambrian diversifications for the animal phyla. As a result, the Cambrian radiation is now considered less significant than previously (Erwin et al. 2011), meaning that the origin of animals is decoupled from this primary evidence of their diversification in the fossil record. That being so, it has become essential to examine the veracity of the evidence for sponge biomarkers in deep time. Two potential sources of error are explored below. First to be examined is the geological context and the way in which it relates to the reported dates of c. 750–713 Ma now being used in molecular clock analyses for calibration points (Peterson et al. 2008; Benton et al. 2009; Sperling et al. 2010; Erwin et al. 2011). The second point concerns the veracity of biological arguments concerning the use of these organic compounds as diagnostic criteria for sponges in deep time.

Geological context and dating

  1. Top of page
  2. Abstract
  3. Geological context and dating
  4. The biological arguments
  5. Discussion
  6. References

Long chain organic compounds have been recovered (Love et al. 2009) from sediments correlated to levels lying above Sturtian glacial sediments (723 +16/−10 Ma U–Pb from Brasier et al. 2000) and beneath the Marinoan cap carbonate (635.5 ± 0.6 Ma U–Pb from Condon et al. 2005). By definition, Marinoan glacial rocks are correlative with levels just beneath the global stratotype for the base of the Ediacaran System (Knoll et al. 2004), for which absolute dates have been given as 580 ± 10 Ma (Gradstein et al. 2004), 635–599 Ma (U–Pb and Pb–Pb dates repectively from Knoll et al. 2004), and 635.5 ± 0.6 Ma (Condon et al. 2005). The application of precise dates clearly requires regional stratigraphic comparisons between the glacial deposits in Oman and other dated sequences. Such correlations are difficult because numerous glacial deposits in the Neoproterozoic show geological sequences that may be compared with those in the Huqf Supergroup (Brasier et al. 2000). The direct dating evidence from the Ghubrah Member comes from detrital zircons dated using U–Pb to 751 Ma (Love et al. 2009), 723 Ma (+16/−10; Brasier et al. 2000), or 713 Ma (Hoffmann et al. 2004; Bowring et al. 2007). The overlying Fiq Member has an absolute maximum date for this member of 645 Ma (Pb–Pb CA-TIMS from Bowring et al. 2007), also based on detrital zircons (Fig. 1). These dates from the lower Ghubrah Member of the Ghadir Manqil Formation imply an as yet undiscovered intra-formational unconformity to explain the potential (minimum) 50 million year gap between it and the Fiq member which lies conformably above, though Bowring et al. (2007) state that this gap may indeed occur in the volcanic Saqlah Member. Glacial events can of course be highly erosive so such unconformities are expected. Since the dates of 751, 723, and 713 Ma in the Ghubrah Member depend on detrital zircons, they provide a date that is known to be older (by an unknown amount) than the actual date of the strata in which they are found. Indeed, Bowring et al. (2007, p. 1109) also state that ‘nevertheless, even though this unit is a distinctive, uniform layer within the diamictite, we stress that it is a clastic rock and could contain a still younger population of zircons’. When working with detrital zircons, the youngest date is always chosen, for the obvious reason that it is not possible to incorporate a younger grain into an older rock. Therefore the current youngest date of 713 Ma (Bowring et al. 2007) is the one we must consider to be the most reliable. The alternative and arguably more likely interpretation is that the oldest chosen date of 751 Ma (from Love et al. 2009) has been inherited from the proximal basement. If so, it should be regarded as substantially older than the real age of the formation. A Re-Os date of 643.0 ± 2.4 has been reported from the Tindelpina Shale Member, which immediately overlies the Sturtian Glaciation in Australia (Kendall et al. 2006). This date is strikingly similar to those reported by Bowring et al. (2007) for the maximum age of the lowermost Fiq member in Oman (the first post-Sturtian date in Oman). Thus the Sturtian glaciation could terminate globally at around 645 Ma and be over 50 Ma in duration, or it could be diachronous, or it could represent multiple glacial events taking place over tens of millions of years.


Figure 1. Stratigraphic and geochronological framework for the Huqf Supergroup, Oman. Adapted from Brasier et al. (2000), Bowring et al. (2007) and Love et al. (2009). Snowflakes mark major glacial formations which are used for global correlations, see text for discussion. Clocks show positions of seemingly reliable dates from these sections. Test tubes mark the position of the strata from which the oldest biomarkers are drawn.

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The Shuram Formation of the overlying Nafun Group possesses a remarkably large negative carbon isotope excursion which has been used for global correlations for this interval (Fike et al. 2006), which is dated to 551 ± 0.7 Ma (U–Pb from Condon et al. 2005) based on correlations of the excursion to one at the top of the Doushantuo Formation in China. This correlation is contended by Halverson et al. (2005) who correlate the Shuram excursion with similar excursions found in the Wonoka Formation, Australia, and the Johnnie Formation, USA which they correlate with the Gaskiers Glaciation of c. 580 ± 1 Ma (Bowring et al. 2003; U–Pb from Hebert et al. 2010). The Ara Group contains the Precambrian–Cambrian boundary geochemical signals (Brasier et al. 2000) as well as a large suite of dates from U–Pb and Pb–Pb dating of zircons (Bowring et al. 2007) that indicate that this correlation is appropriate, giving boundary dates of 543 Ma. Using large isotope excursions alone as a means for global correlation at this time has just been brought in to serious question by Verdel et al. (2011, p. 1539), who report that ‘the negative excursion associated with the base of the Cambrian is not a unique post-Shuram event, and that post-Shuram, pre-Cambrian [there were]… repeated large-magnitude fluctuations in the carbon isotopic composition of the global ocean’ Thus global correlations are problematic but there are reasonable constraints that we can apply using the best dated sections and the most reliable lithostratigraphic correlations, used in concert with the carbon and strontium isotope curves.

Next we need to look at the cores from which the actual biomarker samples were extracted. Late Ediacaran markers such as Cloudina were found in the subsurface, allowing correlation to the Ara Group. For strata below these, the age and placement within the stratigraphy is a calculation based on assumed rates of sedimentation per million years. Such calculations contain large uncertainties and have been the source of considerable debate in the literature regarding these particular cores (Bowring et al. 2007). In the Miqrat 1 core for example (from Bowring et al. 2007; usually abbreviated to MQR1 in Love et al. 2009) which yields biomarkers, the diamictites must be from the Ghrubah Member of the Ghadir Manqil Formation if the biomarkers are to date from the 713 to 751 Ma dates given by Love et al. (2009). This is critical because the deepest source of biomarkers is said to come from within this formation in this core. This formation contains two diamictites; the older from the Gubrah Member (dated to 713 Ma, as outlined above) and the younger from the Fiq Member (dated to younger than 645 Ma; Fig. 1). Curiously, it seems that it is the oldest date that has been given for these biomarkers, even though they seemingly come from the Fiq Member. Indeed, it seems that the biomarkers come from above the Saqlah volcanics (which are not shown to be present in this core and mark the interval between the two glacial members, see Love et al. 2009 SI page 11) as well as above the Lower Cap Carbonate that marks the top of the Ghrubah Member. If these biomarkers have indeed been extracted from the upper Fiq Member rather than the lower Ghrubah Member, then the appropriate maximum date becomes 645 Ma and not 751 Ma as given in Love et al. (2009). These lithostratigraphic difficulties mean that there is considerable doubt in using these Proterozoic dates as calibration points for molecular clocks. For example, Erwin et al. (2011) used the date of 713 Ma for their biomarker calibration point. That date is substantially older than the actual maximum age of strata that yielded the biomarkers. Other authors have started to use this data uncritically, in support of their searches for very early animals in the fossil record (Maloof et al. 2010; Brain et al. 2012).

When critically examining calibration points for major evolutionary events, it is essential to put forward well-established dates provided with well-constrained error bars, and to describe all uncertainties openly when reporting fossil discoveries. Currently, the date range for the oldest of the biomarkers is constrained between 645 and c. 580 Ma if we use the Shuram Formation excursion correlation as a minimum constraint or 645–c. 635 Ma if we correlate the Fiq Glaciation with the Marinoan Glaciation. Even this careful approach requires that these glacial units have been appropriately correlated to better dated glacial sequences elsewhere in the world. Using a calibration date of 751 Ma, as given in Love et al. (2009), will skew molecular clock results by giving a false confidence in deep time divergences for the Metazoa. Even when divergence estimates are not substantially altered by the calibration point, the date of 751 Ma forms the basis of a confirmation bias. A minimum constraint such as 580 Ma would lessen such errors. Using geochronological minimum constraints is best practice when constructing molecular clock analyses using body fossils (Benton et al. 2009). At a date of 580 Ma the discovery of sponge fossils is interesting but not nearly as remarkable as those same fossils at 751 Ma. From around 550 Ma there is clear transition zone from the Precambrian world to that of the Phanerozoic where there are undoubted and abundant animal remains.

The biological arguments

  1. Top of page
  2. Abstract
  3. Geological context and dating
  4. The biological arguments
  5. Discussion
  6. References

There are numerous problems with the biological arguments for using these particular organic compounds as evidence for the earliest sponges. The argument, as it is widely understood, requires the demonstration of an exclusive relationship between a certain biomolecular group and a given biological group, which is a challenge, given the vastness of the modern biosphere and the non-availability of extinct forms. For instance, Sperling et al. (2010, p. 25) say ‘24-isopropylcholestane are absent from all other potential groups’. This is a fallacy that has been perpetuated throughout the literature. While one might hope for exclusivity for the chosen compounds and clear demonstration of that exclusivity, Love et al. (2009) never actually claimed that their compound is unique to sponges. In fact, Love et al. (2009) document occurrence of this compound in a variety of pelagophyte algae (Class Pelagophyceae, part of the Stramenopiles ‘heterokont algae’ within the Chromaveolata, see Adl et al. 2005 for a comprehensive review of eukaryotic taxonomy). The biomarker that Love et al. (2009) use to argue for the presence of sponges in deep time is the ratio between 24-isopropylcholestane and its structural isomer 24-n-propylcholestane, both of which can be produced by algae. This second compound is copiously produced by modern marine algae (Moldowan et al. 1990) while the first is produced in lower abundance. Love et al. (2009) used the relative abundances of these two compounds, both of which are produced by modern Demosponges and algae, to argue as follows. Because Demosponges produce 24-isopropylcholestane in greater relative abundance, then they are more likely to have been the producers responsible for it within Proterozoic strata. This seems an extraordinary claim. First, it assumes that all possible producers are now known, and it ignores the possibility of production by organisms not present, or not sampled in the modern biota (as argued by Brocks and Butterfield 2009). But it is surely more rational to reason that algal biochemistry is likely to have evolved sufficiently over the last 700 million years to alter the relative abundance between structurally isometric organic compounds now produced in modern forms.

Love et al. (2009) note that the choanoflagellate Monosiga brevicollis does not appear to produce these compounds or their precursors (Kodner et al. 2008), acknowledging the need to constrain the exclusivity of these compounds to sponges. Additionally, other modern sponge groups, such as the Hexactinellida do not seem to produce these particular organic compounds (Thiel et al. 2002). Yet since these compounds are also produced by algae it becomes clear that, contra to the phylogenetic models of Sperling et al. (2010), these compounds could be derived within at least one algal group and also for the demosponges. This means that these compounds are not distinctive for the demosponges or any other sponge group. It is arguably misleading, therefore, to consider when the sponges acquired these biomolecular compounds in comparison to morphologically diagnostic characters, such as distinctive siliceous spicules, because the biomarker itself is not a diagnostic character. This convergent evolution of an organic compound provides a valid alternative hypothesis. It also shows a false dichotomy within the two models put forward in Sperling et al. (2010), where the known algal source for such compounds was never discussed. It is also worth noting that the interpretation of Sperling et al. (2010) requires the paraphyly of the major sponge groups (as suggested by analyses of Sperling et al. 2007) but recent work suggests that this may not be the case and that sponges may be monophyletic with the paraphyly hypothesis being the result of highly restricted data sets (Philippe et al. 2009; Pick et al. 2010; Srivastava et al. 2010). It is also interesting to raise the point that if these compounds are so critical to sponge biochemistry (Love et al. 2009) then it is necessary to argue the Hexactinellida have managed to lose them secondarily, as Sperling et al. (2010) infer. But if such compounds can be gained and lost so readily, one could argue that it is unsafe to make uniformitarian inferences about their biomolecular evolution over the last 700 Ma particularly when this uniformitarian inference is based on a contentious phylogenetic arrangement. Recent work has examined the ecology of bacteria associated symbiotically with marine sponges (Siegl et al. 2011). This work found that a group of bacteria, the Poribacteria which seem to be exclusively symbiotic with sponges, contain the same genes thought to be responsible for the production of the biomarker compounds in sponges (Siegl et al. 2011). It is possible that the Poribacteria are producing the biomarker compound, in addition to, or instead of the demosponge host. This needs to be validated by future work to demonstrate that the bacteria are actually capable of producing the compounds. If indeed the Poribacteria can produce the biomarker then no reliability can be placed in the Oman sponge biomarkers. Certainly this case means we must look much more carefully at the bacterial ecosystem associated with these biomarker claims.

The Huqf sediments are also found to contain 24-n-propylcholestane, which is a compound copiously found within modern marine algae and an essential constituent of the ratio argued to be exclusive to sponges (Moldowan et al. 1990; Love et al. 2009). The only difference between 24-n-propylcholestane and 24-isopropylcholestane is the attachment site of the propyl~ substituent to the cholestane. There are only two options in the three carbon-chain propyl: either as an end carbon (in which case it is n-propyl~); or as the central carbon (in which case it is isopropyl~). In modern terminology, they are referred to as 24-propan-1-ylcholestane and 24-propan-2-ylcholestane respectively. The biomarker literature commonly refers to these compounds by their antiquated names that obfuscates this isometry. These molecules have exactly the same chemical composition and there is only one structural difference, the attachment of carbon in the propyl~ substituent. The similarity of these compounds means that it is possible that they and similar compounds (there are many similar organic long chain carbon compounds) could be affected by the processes or diagenesis and taphonomy, meaning that their respective ratios can alter. When the diagnostic criteria for the recognition of ancient demosponges is given as a ratio between these compounds, this raises a serious concern. Indeed, Love et al. (2009, in SI, p. 27) state that 24-n-propylidenecholesterols, which are common in marine pelagophyte algae, can be transformed by diagenesis into 24-isopropylcholestane. Thus elevated levels of 24-isopropylcholestane in deep time can result from the diagenetic transformation of compounds produced by marine algae. Love et al. (2009) go on to argue that this was not the case for sediments of the Huqf supergroup because they show reasonably low alteration rates of hopanes, which also transform during diagenesis. This requires, however, that the hopanes are original (as argued by Love et al. 2009) and not a later influx from petroleum based lubricant. It also requires that the diagenetic processes that affect hopanes and 24-n-propylidencholesterols are always linked and can never occur separately, otherwise it is not possible to infer from one to the other. Love et al. (2009) reject the idea that there is significant influx of 24-isopropylcholestane, from the lubricating oil because Phanerozoic oils do not have elevated levels of this compound (Love et al. 2009, in SI, p. 13). However, it also seems that the main record of elevated levels of 24-isopropylcholestane come from ‘a temporal pulse from the late Neoproterozoic to the Ordovician’ (Love et al. 2009, in SI, p. 29). The problem here is that the Cambrian and Ordovician Periods are in the Phanerozoic. Even if lubrication oils are drawn predominantly from post-Ordovician strata, this is no longer an exclusive rejection of contamination. It also assumes that the biomolecular signal of all post-Ordovician oils are known and that the statement that sponge biomarkers are absent from them is accurate. This justification also seems to require knowledge of the biomarker composition of all the lubricating oils used during the study, but none of these data have been provided, meaning that there are no control data for these experiments.

Secondly, many long chain organic compounds are differentially taken up by kerogen due to differences in the way that they form bonds (Pancost et al. 2008). Thus some compounds will be relatively enriched in kerogen and others relatively depleted. In some cases, the compounds may have a finite capacity to dissolve within kerogen because of a limited number of bonding sites to which the organic macromolecule can attach itself (Pancost et al. 2008). As a result, the use of ratios to diagnose specific organic inputs through deep time for a given biomarker having multiple organismal sources is unreliable. The ratio could be controlled by the chemical properties of the solvent rather than the nature of the organism producing the solute. This is similar to trying to tie a particular isotopic fractionation ratio to a single specific geological cause in deep time. Though one may suspect, for good reason, that δO18 values broadly relate to changes in global temperature, that is not the same as arguing that a particular oxygen isotope excursion from the Jurassic for instance, must exclusively be evidence for enhanced solar flare activity at that time.

Thirdly, we have no concept of how to distinguish plesiomorphic and apomorphic character states for these organic compounds when they are found in the fossil record. This is because, unlike for morphological characters (see for example homology criteria defined by Patterson 1982) or for genes (see determination of orthology for instance in Page and Holmes 2001, and for more recent discussions Jenner 2006) there are no set criteria for understanding the homology of organic compounds and their evolution in deep time. Organic compounds should be thought of as fossils like any other, with original form that decays and then reacts to the taphonomic processes. Like all characters to be used for unequivocal diagnosis of the presence of a given phylogenetic group, they should be unique and distinctive. Palaeontologists will always seek to avoid basing their classifications on convergent characters. There is no reason why the case should be different for the character of organic molecular fossils. The state of the art concerning biomarkers is currently like that of trying to diagnose a sponge fossil based on the length to width ratios of the body and arguing that such a ratio is unique to sponges or most probably formed by sponges, rather than demonstrating diagnostic characters such as siliceous spicules with layered secretion and axial canals.


  1. Top of page
  2. Abstract
  3. Geological context and dating
  4. The biological arguments
  5. Discussion
  6. References

There are interesting temporal restrictions to the unusual ratios of organic compounds ‘elevated ratios of 24-iso/n-propylcholestanes (>0.5, and frequently >1.0) have only been recorded in Neoproterozoic to Ordovician age sediments (with the main pulse recorded from Ediacaran-Early Cambrian)’ (Love et al. 2009, in SI, p. 6). This is deeply peculiar, as noted by Brocks and Butterfield (2009), that soon after sponges make an unequivocal appearance in the fossil record during the Cambrian, they rather quickly stop leaving a biomarker record. This biomarker record is one that sponges are thought to have left for around 200 million years whilst simultaneous evading fossilisation of the body during a time of prodigious preservation of soft tissues (see for instance Brasier et al. 2011 for a discussion of the high quality of soft tissue preservation in the Ediacaran Period). The hypothesis of Sperling et al. (2010) that siliceous sponges spicules could not be preserved in the Proterozoic does not match with what we know from the geological record. There are numerous chemical cherts of Proterozoic age (Barghoorn and Tyler 1965; Schopf and Klein 1992; Hoffman and Schrag 2002), as well as numerous deposits with chemically precipitated silicate cements. These precipitates likely formed because the Proterozoic ocean was supersaturated in silica in comparison to modern oceans (Maliva et al. 1989, 2005; Tiwari and Knoll 1994; Xiao 2004) meaning that there were many places where silica was so abundant the ocean was precipitating not dissolving. It therefore seems dubious to argue that siliceous sponge spicules or body fossils could have evaded all of these habitats for hundreds of millions of years. Many of these cherts have yielded abundant microfossils (Barghoorn and Tyler 1965; Schopf 1968; Tiwari and Knoll 1994), but never any convincing sponge fossils (Sperling et al. 2010). This abundance of chemical cherts in the Precambrian can be explained by the lack of biomineral sinks that deplete oceanic silica content in the Phanerozoic, because these had not yet evolved (Maliva et al. 1989, 2005; Brasier et al. 2011). The silica content of the ocean as well as the appearance of chemical cherts declines rapidly during the Cambrian also in response to the effects of bioturbation (Brasier et al. 2011). Soft tissue preservation also occurs globally in the Proterozoic (Narbonne 2005; Brasier et al. 2011), and yet many still acknowledge the lack of convincing sponge body fossils (including proponents of deep time divergences Sperling et al. 2010). The temporal correlation of these organic compounds with very early predictions for sponge radiations from molecular clock calculations should not be given any weight. This is because these clocks use sponge biomarkers as a calibration point (Peterson et al. 2008; Benton et al. 2009; Erwin et al. 2011) and so do not provide an independent estimation of origins, just an example of correlative confirmation bias. Indeed the analyses of Erwin et al. (2011) predict the divergence of Demosponges at 713 Ma, which is exactly the date they use as a calibration point for the earliest sponges. The Erwin et al. (2011) calibration point given for the earliest sponges is also 148 Ma older than the next oldest calibration point, given for earliest bilaterian traces at 565 Ma, which seems a very large gap with a paucity of evidence, despite an abundance of exquisitely preserved fossils from this time. The inclusion of the biomarker calibration point increases the time range of the calibration points by 26.2 per cent. In other words the interpretation of 24-isopropylcholestane as a demosponge add over a quarter to the time duration predicted for the origin of animals. Further, the interpretation of the next oldest calibration point (in Erwin et al. 2011) ‘bilaterians traces’ are highly suspect, with Liu et al. (2010b, p. e224) stating ‘We emphasize here that our paper’ (Liu et al. 2010a) ‘only claimed to report the earliest evidence for locomotion in the Ediacara biota, and not the first evidence for animal locomotion’. Finally, it cannot be overstated the weight that has been placed on the interpretation of these complex organic compounds as the earliest fossil representatives of Demosponges with Erwin et al. (2011, p. 1093) stating that: ‘Our divergence estimates suggest that crown group demosponges and crown-group cnidarians have deep origins, both at nearly 700 Ma. These could represent artifacts, although the former is corroborated by Cryogenian-age fossil molecules (biomarkers) of demosponges’.

Algal fossils are well documented from the Proterozoic (Butterfield 2009). It was a world where microbialites dominated the fossil record for hundreds of millions of years. The pelagophyte algae that produce these complex organic compounds are capable of producing brown tides (Giner et al. 2009). The brown tides produced by the pelagophyte alga Aureococcus anophagefferens have been documented with elevated organic nutrient ratios (Lomas et al. 2000). This ties well with what we know of Proterozoic ecology, ocean ventilation, and nutrient cycles (Butterfield 2009). Demosponges are complex animals, with many recognisable features and good fossilisation potential, and the implied absence of their body fossils, for hundreds of millions is surprising. Further, once sponges appear convincingly in the fossil record from the Cambrian onwards, all the sponge biomarkers more or less vanish. A more parsimonious hypothesis sees the temporal window of biomarkers as due to the rise, and then the demise, of an algal-dominated world. During the Proterozoic algae made these complex carbon chains, as they do now. However at this time, they dominated the ecology and more easily entered the sedimentary record because of the lack of animal processing. With the rise of animals, at around the Precambrian–Cambrian boundary (Brasier et al. 1994), algae lose their dominance, they are consumed and reprocessed (Bottjer et al. 2000; Orr et al. 2003). As a result there was less algal input to the fossil record, and their diversity and character was altered by their interaction with animals. The alternative view, therefore, is not a cryptic record of sponge evolution. It is a completely understandable record of the rise and then the gradual decline of marine algae as animals took to the stage in the very latest Neoproterozoic to early Cambrian and changed the world and its biochemistry forever.


  1. Top of page
  2. Abstract
  3. Geological context and dating
  4. The biological arguments
  5. Discussion
  6. References
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