The Furongian (late Cambrian) Biodiversity Gap: Real or apparent?
Abstract: Two major, extended diversifications punctuated the evolution of marine life during the Early Palaeozoic. The interregnum, however, between the Cambrian Explosion and the Great Ordovician Biodiversification Event, is exemplified by the Furongian Gap when there was a marked drop in biodiversity. It is unclear whether the gap is apparent, due to sampling failure or lack of rock, or real — associated with unique and fluctuating environments, a distinctive palaeogeography and extreme climates during the late Cambrian. Indications suggest that there has been little attention paid to this interval compared with those below and above, while some of the classical areas for Cambrian research, such as Bohemia, have poor coverage through the Furongian. Moreover, based on information available in databases and the literature, together with the ghost ranges of many higher taxa through the Furongian, data suggest that biodiversity in this stage has been significantly underestimated. Emphasis, to date, has been placed on widespread, deeper-water dark shale facies of the interval, with generally low diversity faunas, whereas shallow-water communities have often been neglected.
Summary (2 min read)
- Palaeontologists have long accepted that the fossil record is incomplete but nevertheless adequate to describe and understand the history of life on their planet.
- Some hundred years after publication of the 1st edition of Darwin’s influential work, interest intensified on the adequacy and quality of the fossil record as more complex and sophisticated analyses of the evolution of fossil organisms and their diversity were developed through deep time.
- Raup, in a succession of key papers, developed the concept of time-dependent and timeindependent biases (Raup, 1972, 1976a, 1976b).
- These key factors may provide some explanation for the current dearth of data from this critical interval.
4. Fact or artefact?
- Furongian rocks are known from all major Cambrian palaeocontinents and widely distributed in many regions, such as in Laurentia, South China, Siberia and Baltica.
- This is true for a number of classic areas of Cambrian research from western Gondwana, for example the Barrandian area of Bohemia, Spain and Morocco, together with parts of the Baltic (e.g., Estonia) where the Furongian is poorly represented or consists of shallow-water deposits that are poorly fossiliferous.
- In fact, total and SIB diversity follow comparable trajectories, which seems to fit to occurrence signal, while BC diversity reflects an independent pattern (Fig. 1).
- To avoid inconsistences generated by false positives, the authors ran a two-time data analysis from raw and generalized-differenced data for comparisons (see http://www.graemetlloyd.com/methgd.html for implementation).
- Evidence seems to suggest that, the overall observed diversity (in particular total diversity) may be driven by sampling, sampling does not account for the entire diversity signal; a biological signal is still legible in the fossil record.
5. Natural causes
- Diversity curves based on the Sepkoski Database indicate a high frequency of extinctions during the late Cambrian (Fig. 3).
- The frequency and magnitude of these events, especially when displayed as proportions of extinct genera, are impressive (see e.g., Melott and Bambach, 2012; Erlykin et al., 2018).
- Two globally significant carbon isotope excursions are recognized in the Furongian, the Steptoean Positive carbon Isotope Excursion in the Paibian Stage and the HEllnmaria — Red Tops Boundary Event (HERB) or Top of Cambrian Excursion (TOCE) in provisional Stage 10 (Zhu et al., 2006; Fig. 3 herein).
- The magnitude of the SPICE and the HERB Event in shale successions is, however, subdued compared to the δ13Ccarb excursions recorded in carbonate successions, and the δ13Corg signal is commonly half, or less than half, of the magnitude the δ13Ccarb signal (see Ahlberg et al., in press and references therein).
- The interpretation of the SPICE as a global anoxic event has, however, been questioned, because the presence of benthic faunal elements and bioturbation in almost all SPICE-related sections excludes widespread and persistent anoxia or euxinia, but rather suggest oxic or dysoxic sea floor conditions during most of the SPICE interval (Egenhoff et al., 2015; Wotte and Strauss, 2015).
- Currently there is marked interregnum in biodiversity between the high-profile, exceptionally-preserved biotas of the Cambrian Explosion, preserved across a number of Lagerstätten, and the four-fold increase in numbers of families, genera and species during the Great Ordovician Biodiversification Event.
- There are relatively few fossil collections, compared with older and younger strata, through this interval coupled with a lack of taxonomic work on its biotas.
- Extreme fluctuations are present in Furongian environments, providing a barrier to the expansion of the marine ecosystem and its biodiversity.
- That had to wait until the Early Ordovician.
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Cites background from "The Furongian (late Cambrian) Biodi..."
...Second, the interval is also a time of abnormally high rates of turnover in marine fauna (Bambach et al., 2004; Harper et al., 2019)....
...2 in Harper et al. (2019) for a generalized facies map for this time)....
...It should be noted that Harper et al. (2019) highlights the paucity of fossiliferous deposits and a lack of detailed taxonomic studies of Furongian strata as potential biases that contribute to the biodiversity patterns during this interval....
...…feature of this time period is that it represents an evolutionary “plateau” between the rapid increases in biological diversity and complexity of the Cambrian Explosion that precedes it and the subsequent evolutionary radiation of the GOBE that followed (Bambach et al., 2004; Harper et al., 2019)....
...The middle Cambrian to Early Ordovician is characterized by abnormally high rates of turnover in marine fauna (Bambach et al., 2004; Harper et al., 2019)....
Cites background from "The Furongian (late Cambrian) Biodi..."
...The Furongian Biodiversity Gap has been recently noted in a number of papers (e.g. Servais & Harper, 2018) and examined in detail (Harper et al. 2019b)....
...Some of the subsequent benchmark studies during the twentieth and twenty-first centuries have been recently charted by Harper et al. (2019b) in connection with unravelling the Furongian Biodiversity Gap; the studies of Newell (1959), Raup (1972) and Valentine (1973) have been particularly…...
...Harper et al. (2019b) have discussed this gap in detail, concluding that diversity has been significantly underestimated by a paucity of examined rock, compounded by a distinctive palaeogeography, extreme climates and fluctuating environments....
...…with exceptional preservation, such as the Burgess Shale in the Canadian Rocky Mountains (e.g. Briggs et al. 1994; Erwin & Valentine, 2013; Briggs, 2015), Chengjiang in South China (e.g. Hou et al. 2017; Yang et al. 2018), Sirius Passet in North Greenland (e.g. Harper et al. 2019a) and others....
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Frequently Asked Questions (2)
Q1. What are the contributions in this paper?
Indications suggest that there has been little attention paid to this interval compared with those below and above, while some of the classical areas for Cambrian research, such as Bohemia, have poor coverage through the Furongian. Moreover, based on information available in databases and the literature, together with the ghost ranges of many higher taxa through the Furongian, data suggest that biodiversity in this stage has been significantly underestimated.
Q2. What are the future works in this paper?
The latter presenting the intriguing possibility that the diversification of marine ecosystems was on a single trajectory that peaked in the Devonian.