the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 08 Nov 2024
Phylogeochemistry: exploring evolutionary constraints on belemnite rostrum element composition
Abstract. The biogenic carbonate hardparts of a large range of marine organisms are the most important geochemical archives of Earth’s climate dynamics through time and the evolution of life. That said, biomineralisation pathways, i.e., the secretion of mineral phases by organisms, are complex and may differ significantly between different taxa. In light of this, it is critically important to evaluate if related taxa might display similar hard parts geochemistry. If so, this relation might bear information on evolutionary relationships and has great significance in carbonate archive research. Here, we test the evolutionary constraints on main and trace element ratios of belemnite rostra using Bayesian phylogenetic tools. For this purpose, we assembled a large dataset on element ratios from 2241 published samples of belemnite rostra and used comparative Bayesian phylogenetic tools to reconstruct ancestral states and evolutionary rates. While Mn/Ca and Fe/Ca appear to be taxon-independent and probably mainly reflect environmental and diagenetic effects, Mg/Ca and Sr/Ca display stronger taxon-specific patterns, even though their interpretation remains complex. The evolutionary rates are high, with average estimated changes in element ratios of 12.4 % (Mg/Ca) and 12.3 % (Sr/Ca) of the overall mean element ratio per 1 million years. While the distribution of Sr/Ca ratios is relatively homogeneous across the tree, Mg concentrations are divided among two distinct groups (< 5.5 and >7.5 mmol/mol, respectively), with at least five evolutionary transitions between them. Beyond carbonate archive research, our phylogenetic analysis provides insights into the evolution of belemnites. This study highlights the complex interplay between evolutionary, ontogenetic, environmental and diagenetic effects and calls for caution when using belemnite element ratios as proxies for palaeoclimatic studies. We propose the term ’phylogeochemistry’ for the investigation of geochemical data using phylogenetic modelling techniques.
- Preprint
(1521 KB) - Metadata XML
- BibTeX
- EndNote
Status: open (until 20 Dec 2024)
-
RC1: 'Comment on egusphere-2024-3383', Clemens Vinzenz Ullmann, 02 Dec 2024
reply
In their study “Phylochemistry: exploring evolutionary constraints on belemnite rostrum element composition” Alexander Pohle and co-authors propose the use of chemical parameter to assess evolutionary links between extinct fossils. The main message is that of the four studied element/Ca ratios, Mg/Ca appears most variable, but also Sr/Ca shows some distinctions amongst taxa that are of potential taxonomic value. The authors conclude that there may be value in utilising fossil chemistry for the study of evolution in extinct taxa.
The chemical composition of macrofossils has received comparatively scant attention beyond the use for identifying the preservation state of materials taken for geochemical studies. For belemnites, there have been attempts to link element/Ca ratios to palaeotemperature, but also to extract from such data information about changes in ocean chemistry and details on biomineral formation. A dedicated assessment of the taxonomic and phylogenetic value of geochemical indices, however, has so far not been done for rostra, even though the existence of species-specific chemical signatures has received some mention in various contributions.
I find the idea of “phylogeochemistry” quite exciting and think there is merit in pursuing this line of investigation further. If species-specific chemical signatures of belemnite rostra can be determined and linked to inform evolutionary relationships, there would be an added layer of robustness to the designation and identification of taxa, and an improved understanding of how such extinct taxa link together. This stated, I feel that there are a few points that may be worth considering in relation to the current contribution and how it may add to the establishment of such assessments.
The nature of the data:
1) Given that no primary chemical data was obtained for this work, the available base of data is driven by the published literature and studies that were not designed to feed into phylogenetic analysis. However, it is I think important to keep in mind that the data are heavily skewed towards a small number of individuals (c. 1/7 of the dataset come from one specimen). For a number of the two dozen or so genera, therefore, information will be quite limited, and potentially skewed and biased. Can it be shown confidently that the raw data are suitable and sufficient to carry out the analysis both for taxonomic delineation and supposed evolutionary rates of change in element/Ca ratios?
2) It seems to me that the study only utilises two chemical indices – Mg/Ca and Sr/Ca, given that Mn/Ca and Fe/Ca yield little taxonomic value. I agree that the latter are not meaningful for the understanding of the biomineralisation process, but rather indicate post-depositional processes in belemnite rostra. However, are two geochemical indices enough to allow for an independent assessment of phylogenetic relationships? It feels to me that rather than being interpreted in isolation such chemical features should be used as additional characters alongside morphological and structural features.
3) The placement in time of the data seems to align with the first appearance of the taxon and there does not appear an allowance for the evolution of geochemical data within the taxon as a function of changes in environmental parameters, such as temperature (if it indeed has an effect) or ocean chemistry. A considerable body of research exists that attempts to utilise Mg/Ca and Sr/Ca in belemnites for temperature reconstruction (with varying conclusions), and some studies have made use of belemnite data to attempt to reconstruct seawater Sr/Ca ratios (see Ullmann et al., 2013, Geology). The authors implicitly dismiss the latter as an issue (L350-356) but remain vague about data backing up this interpretation.
4) The authors appear to misunderstand some of the more significant controls on element incorporation into the rostrum, noting a tendency to enrich Mg and Sr during early ontogeny (L383). There is strong evidence to suggest that enrichments of Mg and Sr close to the apical line are not related to ontogenetic trends. The defining feature rather than ontogeny is proximity to the apical line, and also growth increments formed late in ontogeny will show these enrichments where they are meeting the apical line (Ullmann et al., 2015). All other published datasets showing serial measurements from apical line to margin of the rostrum that I am aware of align with this statement, but do not allow to disentangle spatial controls from ontogenetic controls, because only one transect was analysed for a specimen, rather than multiple ones. Overall this point has consequences for the interpretation stated in L385, though: Ultimately, these Sr and Mg enrichments can be avoided by sampling calcite in intermediate growth bands away from apical line and margin of the rostrum, and this substantially reduces observed data variability within individuals (Ullmann and Pogge von Strandmann, 2017). This is something which is likely to continue to skew datasets assembled by researchers that are unaware of the spatial controls on Sr and Mg distribution in the rostrum.
5) The authors state in L80 that data identified as compromised were excluded from consideration, but plots in figure 2 do not support this statement. For instance, according to published data (Sørensen et al., 2015), Sr/Ca ratios lower than 1.5 mmol/mol are not observed in well-preserved Bellemnellocamax and this is the only cited reference for this genus. However, data as low as 0.5 mmol/mol appear to be included in the analysis for this genus given the representation in panel B of Figure 2. Also, Fe/Ca ratios are not reported for Bellemnellocamax, but rather Mn/Ca ratios, so I assume this was erroneously attributed in this plot.
Accessibility and focus of the text
The text, where it relates to phylogenetic work, is overall quite rich in technical terminology and dense methodological detail that for a non-specialist in these techniques makes it relatively inaccessible. This stands in opposition to the description of geochemical data, which appears somewhat superficial. It is understandable that technical detail is required and terminology, where established, should not be compromised by oversimplification, but I think more could be done to write the text in a way that can be understood by non-specialist readers.
In my opinion, part of this challenge also relates to understanding exactly what aspects of the chemical data led to the construction of the phylogenetic trees. From my point of view, a good starting point would be an attempt to use Figure 2, and plot ranges of Mg/Ca and Sr/Ca for taxa (amended by number of analyses per box plot) against time such as in Figure 7, but without any inferred phylogenetic links. Personally, I am not convinced how much added value for the analysed dataset derives from sophisticated probability-driven modelling. An appraisal of chemical data in the framework of existing phylogenetic schemes at this stage may be equally if not more powerful, until a broader and more reliable database (in terms of number of chemical proxies and number of analyses for all taxa) for belemnite rostra becomes available.
Detailed comments:
Title: Is there a reason why one should call this line of study “phylogeochemistry” rather than “phylochemistry”?
L1: It may be questionable whether biogenic carbonate hard parts are the most important geochemical archives. Certainly they are very important, but also organic geochemistry has yielded important insights regarding carbon cycle and climate dynamics, and significant work has been done also on detrital silicates and other substrates.
L6: Might it be better to speak about element/Ca ratios in general here? Sr may be a minor or trace element in belemnite rostra and Mg is also not usually a major element in these materials depending on exact definitions.
L10: Personally I strongly doubt this inference of evolutionary rates and the reliability of this estimate.
L44: I think this statement needs a bit more quantitative detail. What is meant by significant geochemical variation? How is it determined that it is significant? What kind of proxies and/or fossils are compared here?
L47: There seems to be a wrongly formatted reference here. From a chemical point of view quite a bit has been published on brachiopods in this relation and there is a reasonable amount of information compiled also for bivalves and barnacles – maybe it would be worth mentioning some of this research given the focus of the paper?
L52: I fully support this, but as stated above – why do geochemical data need to be seen in isolation from other characters valuable for taxonomic work? If one accepts the identification of the rostra taken for chemical work to be correct, why cannot morphological features such as grooves, striations, cross-sectional shape, placement of the apical line, bluntness of the apex etc. be used in conjunction with this data?
L62: See above for definitions of main and trace elements.
L63: Why must isotope data be the focus of upcoming work? These data could already now be included, but it might not be a surprise that objectively disentangling taxon-specific effects from environmental effects would be very challenging? This issue is for instance at the core of the debate about the utility of Mg/Ca in rostra as a thermometer.
L92: Equations 1-3 seem quite cumbersome to me and are also not correct in their current form in the case of equation 2. I assume the authors meant for the concentration of Sr to be in µg/mol rather than g/mol? A simpler way to get to this result would be to calculate Sr/Ca in µg/wt% and multiply by m(Ca)/(10*m(Sr)).
L116-87: As opposed to the assessment and debate about chemical data – which is comparatively simplistic and avoids dealing with some of the complexity of some of these data – the phylogenetic analysis is extremely densely written, with little explanation of the rationale of parametrisation and adopted approach. I am not sure why this bi-modal approach to complexity was adopted, but feel that there is scope to make the study much more accessible to the audience by laying out the rationale and consequences of the chosen methodology in some more detail.
L124: I assume what is meant here are “100,000” generations rather than “100.000” generations? See also L179
L166: What is the rationale to adopt the mean of these ratios as the most meaningful? My feeling is that the median will be more reliable.
L258: In the figure caption, should it read “within a genus”?
L310: I am not sure how these values are meaningful? As far as I can tell, the observed rates of change in long-term records are smaller than postulated (Ullmann et al., 2013, Geology). Over short periods of time, a well-documented case of a succession of taxa is at the Toarcian OAE, where Passaloteuthis is followed by Acrocoelites. Acrocoelites already at its first appearance has a high Mg/Ca ratio which may reduce somewhat towards the middle Toarcian (Ullmann et al., 2014, PNAS) as opposed to a hypothetical ramping up to a stable value that one may read off Figure 8.
L342: I am surprised about the way that this is presented. Most authors use Mn and Fe as the basis for their quality control, and it is well established that enrichments of Mn and Fe (qualitatively) link to diagenetic modification of calcite in rostra. The levels of Fe and Mn that are observed in the samples that have not been excluded from further interpretation are therefore expected to be erratic and too small to correlate strongly with other proxies than other redox-sensitive metals. However, they are still likely to be dominated by postdepositional enrichment rather than original incorporation, only that they otherwise signify (hopefully) negligible degrees of chemical overprint on other proxies.
L349: The authors are quite vague here in what “seawater properties” may be. Is this to mean that some physical properties of seawater influence the way in which belemnites would have incorporated elements into their rostra, or is this rather envisioned to be the chemical signature of the water itself? It would be useful to explain in some more detail the envisioned mechanisms of this, especially as both physical and chemical parameters of seawater have been proposed to have had an influence on chemical signatures of belemnite rostra.
L351: I am not sure if I understand fully what the authors want to express here (what exactly is meant by non-systematic in terms of duration/spatial context), but it appears to me that this is the scenario that has been previously put forward regarding the Toarcian OAE.
L354: However, one finds in sedimentary successions without major changes in palaeoenvironment different taxa of belemnites through time with quite variable Mg/Ca and Sr/Ca?
L376: Should “Hibolithes” here be in italics? I think this element of research is the most promising regarding the chemical signatures and should be explored in some more detail.
L383: This is an incorrect reflection on the published literature (see above in main comments).
L391: One may also take a inorganic geochemical approach to this problem and stipulate that a belemnite taxon has a distinct element partition coefficient for the studied elements, and therefore a change in the element/Ca ratio may simply be driven by a change in this ratio in seawater through time, which, given the typical residence time of Ca, Mg and Sr in seawater in the 10^6 year range, is at least conceptually feasible.
L396: EDS analysis is quite problematic for Mg and Sr content determination – if it is SEM-based then it would be nearly impossible to get reasonable data from this; if it is EPMA-based then still, error bars are likely to be significant. Most of the published data, however are very likely from wet chemical analysis, for which the described factors are irrelevant.
L399: This also would have an effect if an electron beam technique was employed. This topic has been published on a fair amount and studies could be referenced here that illustrate this point.
L410: I find this statement to be a bit vague. Mg/Ca ratios are taxon-specific, but one should be careful using them as taxon-specific markers?
L416: If Mg/Ca ratios correlate with Sr/Ca ratios, and Mg/Ca has a link to phylogeny, why then should Sr/Ca ratios not?
L420: Again, I find this statement quite vague – how are these ionic radii linked to element incorporation into belemnite rostra? Why should one wonder about aragonite if the rostra are calcite (which I think is well established)? How is Sr then linked with Mg? Why would Mg and Sr change if it is always calcite that is being formed?
L479: This may be true for some studies, but it is not entirely fair as a generalised statement. What is correct is that thresholds vary, but they also ought to do so for good reason: The levels of Fe and Mn present in pore fluids that are agents of chemical alteration are specific to local context, and therefore spatially (and temporally) quite variable, very likely even within individual sedimentary successions. It thus has to first be evaluated if Fe and Mn are powerful in delineating altered data at all, and – if so – at which level of these metals other tracers are seen to be substantially different from original values. The consequence is that no uniform limit is given in the literature.
L484: It might be correct that some studies were too optimistic with their threshold values, but it is also worth pointing out that a threshold is a compromise that assumes the fraction of altered material in a sample to be uniform in chemical character, which is most certainly not the case. Diagenetic fluids will evolve and this is reflected in the nature of chemical overprint, and hence will cause knock-on effects for the validity of any given preservation threshold. It is therefore important not to be too dogmatic about such values, but to also use common sense in the interpretation of other chemical and structural observations to identify materials that may inadvertently have slipped through a yes/no exclusion routine based only on set element concentration limits. I do disagree with the concluding statement that chemical assessments can only ever be supplementary to optical and CL screening. Researchers have their preferred modes of preservation screening, and all of these techniques have their benefits and drawbacks. There is – in my opinion – no reason to treat chemical analyses as second-class diagenetic screening; they relate to the exact material that is tested for target geochemical proxies, so in this sense much more appropriate than looking at correlative fragments or sections on an SEM or optical microscope, and they are quantitative in nature rather than qualitative. It is also worth keeping in mind that a CL image is typically akin to a 2D Mn map in a calcite fossil, so in the end does tell the same thing as a Mn/Ca determination, but trades quantitative character for spatial resolution. SEM and microstructural imagery can be very powerful to illuminate what kinds of structural overprint may occur in a fossil, but they do not show exactly where it occurs in the entire specimen, and what the geochemical consequences of this alteration are. So, altogether optical and chemical techniques all have their place, and I do not think that there is reason to use value statements such as the ones in L491.
L496: I am not sure how this approach allows to differentiate between environmental and evolutionary effects? Going through the text, this particular element seemed to still have scope to be looked at in more detail.
L504: C and O isotope data were available for incorporation into the study. What was the rationale for not including them but to here point out that they should be? My feeling is that environmental controls are even more important on these isotopic systems than on element/Ca ratios, and that would be a challenge that would need to be addressed.
Citation: https://doi.org/10.5194/egusphere-2024-3383-RC1 -
RC2: 'Comment on egusphere-2024-3383', Madeleine Vickers, 03 Dec 2024
reply
This study uses a novel approach, bringing in a powerful palaeontological method to examine geochemical data from belemnites. It looks like there are potentially some very interesting applications of this approach, however, I do not think this comes accross well in the discussion and conclusions of the manuscript. It is not clear what the significance of the phylogenetics really is in terms of using the geochemical data to reconstruct paleoenvironmental (oceanographic/temperature) trends. From the current way the manuscript is presented, it appears that there are significant differences between belemnite genera in terms of Mg and Sr incorporation, but that it is not necessary to apply phylogenetics to look at them; identifying the samples to genus level and not comparing different families/distantly related groups will be sufficient to avoid bias introduced by variable minor element incorporation. I think this manuscript would benefit from presenting more of a case for what the phylogenetic part of the methods can reveal; perhaps it is necessary to look beyond the potential use of using minor/trace elements to reconstruct environmental conditions. What else might the phylogeochemistry tell us? What questions can it answer?
Viewed
| HTML | XML | Total | BibTeX | EndNote | |
|---|---|---|---|---|---|
| 176 | 35 | 11 | 222 | 1 | 2 |
- HTML: 176
- PDF: 35
- XML: 11
- Total: 222
- BibTeX: 1
- EndNote: 2
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1