* This is the latest installment of “Problematica.” It is written by Max Dresow. It is a response to an older post written by Scott Lidgard and Alan Love (Max’s PhD supervisor) called “Re-thinking Living Fossils.” The original post attempts a rehabilitation of the living fossil concept. The response puts pressure on this, drawing on some recent work by Beckett Sterner…
I intend to discuss the living fossil concept, standards of evidence, and “eliminativism” (an approach to conceptual fragmentation that consists in eliminating the fragmented term from our lexicon). But these are abstract matters, and I want to begin this post closer to the ground.
The tuatara is often called a “living fossil,” which is odd, because the group to which it belongs has a pittance of a fossil record. There is no master definition of “living fossil,” but one way an animal can earn this designation is by impersonating a fossilized member of its lineage. Spot a horseshoe crab in the surf and permit yourself the thought that it crawled out of a slab of limestone quarried from the Jurassic Solnhofen. The living animal does a passable impression of its extinct counterpart. Hence the term “living fossil” is an apt one.
A living horseshoe crab and an extinct relative from the Solnhofen Limestone in Germany
The tuatara, by contrast, has no Solnhofen. Extinct members of its lineage are known mostly from jaws or teeth with nary a postcranium in sight.* This puts claims of living fossil status on a rather different footing than those of horseshoe crabs. Indeed, as a recent paper put it, “despite the longevity of sphenodontians and the relevance of the extant tuatara to broadscale evolutionary and conservation studies… we have surprisingly little understanding of the evolutionary changes between early sphenodontians and Sphenodon [tuatara]” (Simoes et al. 2022, 1).
[* Sphenodontia is a clade that includes Sphenodon as its only living member. Sphenodontians make up most of the Linnean order Rhynchocephalia, which also has the tuatara as its sole survivor.]
The tuatara (Sphenodon punctatus)
This has not prevented all kinds of folks from calling the tuatara a “living fossil” for all kinds of reasons. An obvious one is that these animals seem to resemble their Mesozoic counterparts—at least approximately, and as far as we can tell. The judgment has been reinforced by the suspicion that tuatara retain primitive features like a fused lower temporal bar (the bone forming the lower margin of the lateral temporal fenestra). Alas, this fusion is now viewed as a secondary acquisition, and therefore as derived rather than primitive. Moving beyond morphology, the broader sphenodontian clade was once thought to contain few members, especially when compared to its speciose sister clade, Squamata (the “scaled reptiles”). So, assuming that living fossils are evolutionarily sluggish, tuatara seem to fit the bill. But here too the reason is not as compelling as it used to be, owing to the many fossil sphenodontians that have been described in recent decades, which together have greatly filled out the base of Sphenodon’s family tree.
A final reason for regarding Sphenodon as a living fossil is that this once florid tree has been winnowed to a single branch. This means that the tuatara is the lone living representative of a clade populated almost entirely by extinct forms; ergo, “living fossil.” Sphenodontians once ranged widely across Pangea, achieving a near global distribution during the peak of their diversity in the Jurassic. Yet today they are confined to thirty-two island sanctuaries fringing the North Island of New Zealand. In total, fewer than 100,000 of their kind weigh upon the Earth: an ominous figure that will have to grow if Sphenodon is not to join the ranks of regular old fossils.
So is the tuatara a living fossil or not? Based on extinction patterns in Sphenodontia, as well as its modest geographical distribution, the designation seems apt. Morphologically too, the case is pretty strong. That was the conclusion of a recent study in Palaeontology, which used a phylogenetic analysis of Rhynchocephalia to estimate rates of morphological evolution and to document patterns in morphospace occupation through time (Herrera-Flores et al. 2017). What the analysis showed was that Sphenodon exhibits statistically slow rates of morphological evolution, consistent with its designation as a living fossil. It also showed that its morphology is conservative in the context of the broader rhynchocephalian clade, occupying a central position in a morphospace whose outer limits were defined in the Mesozoic. While not mentioned in the study, a newly described fossil sphenodontian, remarkably complete, suggests that “fundamental patterns of mandibular ontogeny and skeletal architecture in [tuatara] may have originated at least ~190 Mya” (Simoes et al. 2022, 1). This further strengthens the case for morphological conservatism in this unusual reptile.
Rhynchocephalian morphospace occupation based on a morphometric study of the dentary (lower jaw). Here temporal groupings are identified, along with the outer limits of the region of morphospace occupied by a temporal group. Sphenodon (the tuatara) is marked with a orange diamond. (From Herrera-Flores et al. 2017)
There is one sense, however, in which the tuatara is not very living fossil-like. Back in 2008, researchers announced that Sphenodon exhibits the highest rate of molecular evolution known among vertebrates. This was unexpected in light of its low growth rate and morphological conservatism. One does not expect an evolutionary tortoise to have the motor of a hare. But perhaps we should not be surprised that this reptilian anomaly was living a double life. You don’t become the lone survivor of a once mighty clade without breaking a few rules. (One more example, too good not to mention. A few years ago, researchers found that tuatara have the fastest sperm of any reptile put through the paces. Presumably this is because tuatara lack an intromittent organ (a penis or whatever), and so rely on motile gametes to achieve fertilization.)
* * *
Talking about Sphenodon is fun, but we have serious business to attend to. This concerns the status of the living fossil concept, which has become an object of controversy in paleontology over the past decade or so (Vanschoenwinkel et al. 2012, Casane and Laurenti 2013, Mathers et al. 2013; Carnal 2016).
There have been many critiques of the living fossil concept, but most boil down to the complaint that there is no master definition of “living fossil”—just a list of membership criteria lacking clear rules of application. In the absence of such rules, it is difficult to know how to handle cases that satisfy some of the conditions but not others. Is Sphenodon a living fossil or not? That depends on what matters for living fossil-dom, and whether high rates of molecular evolution disqualify a taxon from claiming this title. (There is also a political worry, which is evident to anyone who has typed “living fossil” into a search engine. Creationists love living fossils. According to Answers in Genesis, these creatures show that even if two species are not found together in the fossil record, they may still have inhabited the earth at the same time. So, because modern coelacanths look a bit like Jurassic ones, we should take seriously the possibility that humans lived alongside non-avian dinosaurs.)
Worries about classification track a real phenomenon. As Scott Lidgard and Emma Kitchen (2023) document, the number and variety of criteria used to identify living fossils has greatly expanded over the past century or so. These authors put a positive spin on this, highlighting the role the concept has played in stimulating research across a range of biological disciplines. But others see the same growth as evidence that the concept has become overextended, and too pliable to do any real scientific work. In philosophy-talk, these critics complain that “living fossil” isn’t a natural kind. Instead, it is a wastebasket category that gathers together a group of superficially similar but in fact quite disparate objects: an un-natural kind.
A graph showing relative changes in criteria for living fossil status among 853 unique living fossil entity names from 1860 to 2020. (From Lidgard and Kitchen 2023)
But maybe this isn't such a big deal. In another recent paper, Scott Lidgard and Alan Love argue that worries about definitional criteria have been taken too far. Sure, if the only role scientific concepts play is to classify, then we should be worried about a concept that gathers together apples and oranges. (Unless that concept is “fruit.”) But in fact scientific concepts play a variety of roles in research practice. An especially important one is representing broad investigative domains: that is, “mark[ing] out what requires explanation in a given instance for a particular entity in order to account for [its salient properties].” Approaching things from this perspective,
[the] role of the living fossil concept can be understood as setting an integrated agenda for research—interrelated suites of questions about patterns in need of explanation and processes relevant to specific character constellations and wholes—that advances our understanding of evolutionary stasis across hierarchical levels of organization. (Lidgard and Love 2018, 766)
Basically, when we shift our focus from the semantics of the living fossil concept to its role in coordinating a suite of research projects, we begin to see its real value and “the legitimacy of divergent criteria used to isolate answers to [research] questions.” (Here is the link to Lidgard and Love’s paper, and the corresponding Extinct post.)
One reason Lidgard and Love advocate this perspective is that it helps us think about a ubiquitous problem in the living fossil literature: part-whole ambiguity. Think again of the tuatara, with its conservative morphology and rapidly evolving genome. Now, there is nothing contradictory about a taxon that exhibits morphological stability while evolving furiously at the molecular level. But the fact that phenotypic and genotypic evolution come apart is a problem for studies that would use molecular characters as proxies for morphological ones. A similar problem arises for studies that would use one morphological character as a proxy for the rest. In the latter case, an important consideration is that different characters can exhibit different modes of evolution at one and the same time, with some characters exhibiting stasis while others evolve directionally or in the manner of a “random walk.” So depending on what character is studied, one might receive a very different picture of the overall stability of the phenotype.*
[* Simple, you say: just study more characters! But remember that in fossil specimens, the number of characters available for study is often severely limited. And studies of fossil time series have a central role to play in attempts to empirically measure stasis in groups of organisms.]
Evolutionary modes of different characters in a hypothetical fossil fish lineage (from Lidgard and Love 2018). In Lidgard and Love’s description, “(a) Population samples are taken at successive intervals from sedimentary layers that contain fossils. (b) Characters are measured for each sample. Different evolutionary modes are seen in character trajectories plotted against stratigraphic positions for eye width (c), tail fin length (d), and pectoral fin length (e).” (Illustration by Monica Jurik)
How does the Lidgard/Love picture help us grapple with this problem? In a sense, it doesn’t. It simply reveals that the problem is not as bad as some have made it out to be, at least once we zoom out to survey the entire living fossil research agenda. This agenda has an internal structure, which can be rendered (more) transparent by attending to the adequacy criteria used to characterize living fossils and evaluate candidate explanations. Anyway, to do this is to appreciate that biologists have resources for operationalizing notions like “stasis” and for measuring empirical parameters like evolutionary rate. So critics can stop worrying. “Living fossils” are complicated, sure, but biologists have the tools for navigating this complexity, and for negotiating part-whole ambiguities in a sensible way.
* * *
The living fossil concept is starting to look a lot better. Despite part-whole ambiguities, the research program has a coherent structure owing in part to the very “cross-cutting membership criteria that make the [concept] contentious.” The absence of a master definition is apparently not an impediment to research on the relative stability of different biological entities. Perhaps it is even a good thing. Calls to drop the concept from our lexicon are therefore misguided, and maybe even harmful.
Or maybe not. As Beckett Sterner has recently argued, “[an] important concern for [the Lidgard/Love account] is that controversies over the classification of … living fossils may threaten to overwhelm the perception of a shared problem cluster” (Sterner 2023, 2). In particular, differences in evidential standards threaten to undermine “the concept’s explanatory interest” whenever these differences prevent researchers from making "like-to-like comparisons of data, analyses, and conclusions across cases and [disciplinary traditions]” (5). Sterner has two sets of standards in mind. The first has to do with the trait and taxon sampling strategies used to provide an “evidential basis for classifying living fossils (or evolutionary patterns more broadly).” The second has to do with the analytical methods “taken to be most appropriate for determining the evidence supporting a lineage’s classification.” Sterner’s worry is that in the absence of agreement on these norms and methods, research on living fossils is liable “to get stuck in [a] swamp of endless classificatory disputes.” Pace Lidgard and Love, shared aims and adequacy criteria do not defray the costs of a semantically muddled concept all by themselves.
Begin with trait and taxon sampling strategies. Here the worry is that different strategies may produce divergent assessments of living fossil status even among researchers who are otherwise in agreement about norms and definitions. As Sterner writes, “the multi-variate and multi-level nature of living fossils” has important implications for data collection (Sterner 2023, 5). Studies of fossil time series have shown that many taxa exhibit conflicting patterns of evolutionary change in their traits, for example. One study, which considered 635 traits across 153 lineages, found that evolutionary mode varied among traits in a majority of lineages, and also that “the likelihood of conflicting within-lineage patterns increases with the number of traits analyzed” (Hopkins and Lidgard 2012). This suggests that determinations of living fossil status will be sensitive to which traits and taxa are selected for analysis. Again, Sphenodon provides an example. But here it bears mentioning that studies of morphological evolution in Sphenodontia are typically based on measurements of the skull, in particular, the lower jaw bone or dentary. From this a conclusion is drawn about the evolution of the “net phenotype” or “[total] morphology”: but results of this sort will be misleading whenever there are mismatches between patterns at different levels of organization. Sterner discusses a second example, in crocodiles:
Biologists have historically used 2-dimensional lateral (side) or dorsal (top-down) profiles of crocodilian skulls to measure morphological evolution, partly to increase taxonomic sampling because more complete fossils are rare. Based on these profile views, biologists have found that crocodilian morphology is highly conserved, reflecting their shared semi-aquatic, predatory lifestyle, with most variation occurring in the length of the snout. A recent paper, however, applies new 3D imaging techniques to incorporate other potential sources of variation, such as in the shapes of the front and back of the skulls and internal chambers and bone structures (Felice et al. 2021). In addition to the known elongation of the snout, they find that “the cross-sectional shape of the snout is also a key feature separating aquatic and piscivorous taxa from terrestrial and omnivorous/herbivorous taxa” (Felice et al. 2021, 6). Crucially, incorporating a new dimension of morphological variation brought into focus a new ecological source for divergence among lineages that contrasts with the convergent pattern of elongation that responds to the clade-wide shift to a semi-aquatic lifestyle. This illustrates how adding more traits may change estimates of evolutionary rates simply because the additional traits may reflect new historical events or relationships. (Sterner 2023, 4)
As before, we should resist the notion that studying more characters will provide a clearer picture of the evolutionary rate of the lineage. For one thing, in fossil taxa, complete specimens are rarely available, so opportunities for character sampling are inherently limited. For another, it isn’t clear how to even conceptualize a “representative sample” of traits “given the variety of compositional levels and types of properties one could study” (Sterner 2023, 3). More traits provide more information; but is more information better here, or just different? That is, does the information contained in a broader sample get us closer to the real (comprehensive or composite) rate for the lineage, or is it wrong to think that such a rate even exists?
Variation in disparity (a)-(c) and evolutionary rate (d)-(f) for cranial elements, together illustrating mosaic evolution in the crocodyliform skull (from Felice et al. 2021)
There is another worry. Namely, we can imagine situations in which one’s definition of “living fossil” matters for data collection, either because it influences which traits are selected for analysis or which lineages are included in a sample:
If one conceives of living fossils as showing morphological and genetic stability because they occupy stable ecological niches, then there is no reason to expect slow rates in ecologically irrelevant traits… As we saw with the crocodilian [example], sampling skull morphology from dorsal vs. lateral perspectives can change which ecologically or phylogenetically important traits appear in the dataset. Such an ecological view of living fossils would therefore treat some traits as having high evidential relevance for classifying lineages and others as low relevance. On the taxon sampling side, one could classify lineages as living fossils by comparing their evolutionary rates to other lineages (e.g., in the tuatara genome example) or by evolutionary mode, regardless of rate. Early studies of the tuatara exaggerated its morphological stability by not incorporating extinct fossil lineages that would have provided phylogenetic context for the evolution of ancestral states (Meloro and Jones, 2012). (Sterner 2023, 6)
This goes to show that norms regulating trait and taxon sampling strategies matter, especially when researchers disagree about what a living fossil is. But even when researchers agree, these norms can still produce disagreement, most notably when evolutionary patterns differ across levels of organization or morphological modules, such that different sampling strategies uncover different patterns of evolution.
Now consider norms pertaining to analytical methods. Here the issue is that one can analyze data in several ways, with different methods suggesting different classifications of living fossils. Again, Sterner’s muse is the crocodile. He begins by observing that in studies of time-series data, “one can quantify rates in terms of the dynamics of [either] population means and variances” (Sterner 2023, 4). In the Felice et al. (2021) study, for instance, the authors conclude that crocodilians are not aptly called living fossils, because “despite having low overall disparity, modern crocodyloids are not experiencing evolutionary stasis. Instead, they are rapidly and repeatedly exploring a limited range of phenotypes” (Felice et al., 2021, 6). Yet this distinction is “fuzzier than it appears,” Sterner observes, “when we shift from qualitative theorizing to statistical modeling.”
The standard model for a trait exhibiting stasis in fossil lineages describes the population average as fluctuating according to a Gaussian distribution around a fixed mean. While the model is stationary with respect to time, evolution does occur as the population average moves up and down. If the variance of the Gaussian distribution is large (relative to other traits, let us assume), then the lineage actually evolves quite rapidly as it travels above and below the mean, but the trait fails to accumulate any net divergence. Alternatively, the qualitative idea of rapidly exploring a limited range of phenotypes is also consistent with a constrained random walk model, where the population average can accumulate net displacement from its original starting position but cannot go above or below certain boundaries. Both models can show rapid change in population averages within a fixed phenotypic space, but only the latter model can be interpreted as having a rate of cumulative divergence over time. (Sterner 2023, 4)
Here again, the concern is that different norms can lead to different categorization choices even when researchers are otherwise agreed about what living fossils are. This suggests that, despite Lidgard and Love’s optimism, existing evidential practices face serious problems that may threaten the stability of the research program.
Where does this leave us? As Sterner writes, “the living fossil concept is [presently] inadequate for the purposes of classifying and explaining the evolutionary behaviors of lineages” (Sterner 2023, 5). An important reason is that evidential standards are disputed or unclear, providing ample scope for ongoing disagreement about living fossil classification. Sterner argues that attention to these standards is needed to sustain the explanatory interest of the concept and the coherence of the associated research program. In the absence of improved standards, classificatory disputes are likely to persist even where there is consensus about the living fossil concept, and especially where there isn't. This threatens to undermine the research program by nurturing disagreement independent of any agreement (or agreement to disagree) about explanatory aims and criteria of adequacy.
* * *
A stem reptile from the Triassic (Taytalura alcoberi), whose skull “suggests that the strongly evolutionarily conserved skull architecture of sphenodontians represents the [ancestral] condition for all lepidosaurs [squamates + sphenodontians]” (from Martínez et al. 2021)
But does this mean that living fossil research is bound to get stuck in a quagmire of mixed evidential standards? Not necessarily. According to Sterner, “the best path forward is an iterative process of theoretical and empirical investigation [aiming] to align the contents and boundaries of the concept with the epistemic roles it plays in associated research problems” (Sterner 2023, 5). The desired result would be a shared evidential framework capable of guiding research into living fossils and facilitating communication between researchers with different aims and assumptions. (In Sterner’s words, “the purpose of having a shared evidential framework… [is to guide] productive, intersubjectively reliable research on living fossils when so little about their defining features and proper methods of study are settled as common knowledge.”) Here is how Sterner explains the utility of such a framework:
A shared evidential framework can help researchers meet several prerequisites for dialectical investigation of the living fossil concept. One prerequisite is enabling comparative analysis: there has to be a basis for making like-to-like comparisons of data, analyses, and conclusions across cases and researchers. Even if it proves impossible for researchers to eliminate sources of empirical disagreement due to divergent background assumptions (Sterner and Lidgard, 2021), progress is still possible if researchers can readily translate results between alternative viewpoints (Rescher, 2000; Sterner et al., 2022). Closely related is the need to clarify theoretical assumptions or principles. Methodology rests on theory in order to justify the correctness of a particular way of collecting and analyzing information to answer a question. Comparative analysis can be positively misleading in this respect if the cases we are comparing are based on faulty analyses. A third requirement is to clarify the shared or divergent aims that researchers bring to evaluating definitions of the living fossil category. This is critical for seeing which categories need to be kept because they are epistemically (or otherwise) valuable, even if terminology needs to expand or be revised. (Sterner 2023, 7)
If you like, you can think of a shared evidential framework as a tool for mitigating incommensurability: in particular, incommensurability arising from differences in classificatory practice and research aims. It works by rendering transparent evidential standards for classification, including those standards that inform which data are analyzed (in relation to which models) and how statistical evidence is calculated.*
[* I am not going to discuss Sterner’s comparison of potential evidential frameworks for living fossil research. Interested readers should consult his article, which is available through open access. The two frameworks he considers are the “zero-force evolutionary law (ZFEL) framework” and the “statistical model selection (SMS) framework.” Sterner seems to favor the latter, but the point of the exercise is not to pick a winner. It is rather to show how each framework draws on biological and statistical theory to justify classificatory practices, and to consider how this might inform research on living fossils.]
I want to close with a more general reflection. In recent years, philosophers of science have made a cottage industry out of analyzing “polysemy” (the coexistence of several meanings for a word, or in this case, a scientific term). Species, gene, homology, evolutionary novelty—these are just some of the terms given the polysemic treatment in the last decade. (I have even pitched in, with an analysis of uniformitarianism.) As we’ve seen, living fossil has received the treatment as well, with Lidgard and Love encouraging us to embrace the mess: to accept that “living fossil” has diverse meanings adapted to its diverse uses in biological research. The claim seems to be that conceptual complexity is not mere fuzz—it is more like a coat of fur that adapts an animal to a complex and demanding environment. But as Rose Novick points out in a recent paper, this sort of claim is harder to establish than philosophers have tended to suppose (Novick 2023).
The argument is basically this. To say that conceptual complexity is like a biological adaptation (that is, something that evolved to perform its current function), it is not enough to show that this complexity provides epistemic benefits. The reason is that complexity will often incur serious costs. Consider that polysemous terms are frequently confusing, and may encourage dubious comparisons or the illicit transfer of reasoning patterns across domains of inquiry. To really establish that polysemous terms are adaptive, something like a global cost-benefit analysis is needed. But as Novick remarks, this is no mean task. Perhaps it is not even a feasible one. How could one realistically show that the cross-cutting definitions of “living fossil” bring epistemic benefits to outweigh the costs of confusion and sloppy reasoning?
A visual summary of the differences between the “Selection theory” (for our purposes, “adaptationism”), “Neutral theory,” and “Nearly neutral theory” in molecular evolution. What is depicted here are the assumptions each theory makes about the prevalence of different kinds of genetic variation
Anyway, Novick’s critique of “adaptationism” needs to be heeded. Without explicitly weighing the costs of conceptual complexity against the benefits, adaptationist hypotheses remain open to objection. There will always be the possibility that some cost has been overlooked, some difficulty suppressed, in the service of the adaptationist narrative. Indeed, just this seems to have happened in the case of living fossils. Here what was overlooked was the mischief caused by the absence of shared evidential standards for classifying living fossils. Depending on how mischievous this absence is, the conceptual complexity may not count as adaptive. It is even possible that the complexity evolved by a kind of drift, which enabled meanings to accrue not because they were useful, but because they weren’t actively harmful. (This alternative, though not the former one, is opposed to the Lidgard-Love hypothesis.)
Novick suggests in her paper that most concepts evolve in this neutralist fashion. Or at least that we should take seriously the possibility that they do. In her words, “Conceptual complexity is probably net neutral or, more precisely, nearly neutral” (Novick 2022, 7). This means that it is not adaptive in the sense indicated above; but just as significantly it is not deleterious. “Using complex concepts may be slightly beneficial or slightly deleterious [depending on the concept]… but these (dis)advantages are not so large as to occasion much worry.”
I find myself wondering whether this where discussions of living fossils are headed. Toward the recognition that the benefits provided by conceptual complexity roughly balance out the costs? If so, we will have to say that the polysemous nature of “living fossil” isn’t exactly helpful, but it isn’t a scientific disaster, either. Or perhaps the situation is graver than this, and the absence of a shared evidential framework really does threaten the long-term viability of the concept. If a suitable framework can be constructed, perhaps the concept will dodge the sickle of purifying selection. If not it will probably vanish, like so many sphenodontians in the forests and waterways of prehistory.
References
Carnal, M. 2016. Let’s make living fossils extinct. The Guardian. July 6, 2016. https://www.theguardian.com/science/2016/jul/06/why-its-time-to-make-living-fossils-extinct.
Casane, D. and Laurenti, P. 2013. Why coelacanths are not “living fossils.” BioEssays 35: 332–338.
Felice, R.N., Pol, D., and Goswami, A. 2021. Complex macroevolutionary dynamics underlie the evolution of the crocodyliform skull. Proceedings of the Royal Society B 20210919: doi.org/10.1098/rspb.2021.0919.
Herrera-Flores, J.A., Stubbs, T.L. and Benton, M.J. 2017. Macroevolutionary patterns in Rhynchocephalia: is the tuatara (Sphenodon punctatus) a living fossil? Palaeontology 60:319–328.
Hopkins, M.J. and Lidgard, S. 2012. Evolutionary mode routinely varies among morphological traits within fossil species lineages. Proceedings of the National Academy of Sciences, U.S.A. 109:20520–20525.
Lidgard, S. and Kitchen, E. 2023. Revealing the rise of a living fossil menagerie. Frontiers 11: doi.org/10.3389/fevo.2023.1112764.
Lidgard, S. and Love, A.C. 2018. Rethinking living fossils. BioScience 68:760–770. https://academic.oup.com/bioscience/article/68/10/760/5065827.
Martínez, R.N., Simoes, T.R., Sobral, G. and Apesteguía, S. 2021. A Triassic stem lepidosaur illuminates the origin of lizard-like reptiles. Nature 597: doi: 10.1038/s41586-021-03834-3.
Mathers, T.C., Hammond, R.L., Jenner, R.A., Hanfling, B. and Gomez, A. 2013. Multiple global radiations in tadpole shrimps challenge the concept of “living fossils.” PeerJ 1 (art. e62). (17 January 2018; https://peerj.com/ articles/62).
Novick, R. 2023. The neutral theory of conceptual complexity. Philosophy of Science https://doi.org/10.1017/psa.2023.25.
Rescher, N. 2000. Pluralism: against the demand for consensus. Oxford: Clarendon Press.
Simoes, T.R., Kinney-Broderick, G., and Pierce, S.E. 2022. An exceptionally preserved Sphenodon-like sphenodontian reveals deep time conservation of the tuatara skeleton and ontogeny. Communications Biology 5:1–19.
Sterner, B. 2022. Explaining ambiguity in scientific language. Synthese 200:354. doi: 10.1007/s11229-022-03792-x.
Sterner, B. 2023. Norms of evidence in the classification of living fossils. Frontiers 11: https://doi.org/10.3389/fevo.2023.1198224.
Sterner, B. and Lidgard, S. 2021. Objectivity and underdetermination in statistical model selection. British Journal for Philosophy of Science. doi: 10.1086/716243.
Vanschoenwinkel, B., Pinceel, T., Vanhove, M.P.M., Denis, C., Jocque, M., Timms, B.V. and Brendonck, L. 2012. Toward a global phylogeny of the “living fossil” crustacean order of the Notostraca. PLOS ONE 7 (art. e34998).