The definition of what constitutes a species has been an area of contention in biology since before the time of Darwin. Here, we discuss concepts of species in regards to the Araneae and particularly focus on diagnosing fossils. Spiders are primarily diagnosed by their copulatory organs, which may be difficult to observe in fossils due to a number of confounding factors, thus potentially hindering identification and systematic classification. However, despite potential difficulties, fossils should and must be studied alongside extant Araneae in order to garner a full understanding of the evolutionary history of this megadiverse group.
La définition de ce qui constitue une espèce a été un domaine de controverse en biologie, depuis l’époque de Darwin. Dans cet article, est discuté le concept d’espèce dans le cas des Araneae, en se focalisant plus particulièrement sur les fossiles caractéristiques. La diagnose des araignées se fait essentiellement par les organes copulatoires, difficiles à observer chez les araignées fossiles, du fait du nombre de facteurs de confusion, ce qui gêne potentiellement l’identification et la classification systématique. Cependant, en dépit de ces difficultés potentielles, les fossiles devraient être étudiés en fonction des Araneae existants pour parvenir à une compréhension totale de l’histoire évolutive de ce groupe très diversifié.
To date, arachnologists have identified more than 41,000 extant spider species and over 1100 fossil spider species (
Myriad species definitions exist, some of which focus on the nature of species or what they are (ontology) and others that focus on how we recognize them (epistemology). One of the more common ontological species definitions is the biological species concept (BSC), whereby species are defined as actually or potentially interbreeding groups of organisms (
Many volumes have been dedicated to the discussion of species concepts (e.g.
The practice of using copulatory organs for species identification within the Araneae harkens to the late 1800s (
Recently, some researchers have questioned the general assumption that genitalia are species-specific (
If, however, genital specificity is the overwhelming pattern observed in nature (e.g.
In contrast to species, higher-level groupings within spiders are traditionally based on somatic characters rather than genitalia (
Because of their fragility, the fossil record of spiders is controlled by the occurrence of Konservat-Lagerstätten (i.e., exceptionally well-preserved fossil deposits,
Although the term Lagerstätte conjures up images of exceptionally preserved fossils, fine-scale anatomical details are often not visible. Identifying spider species is therefore frequently difficult due to preservational constraints. Even in amber, which preserves specimens with the highest fidelity, the genitalia may be distorted, obscured or absent, and this has interesting implications for species definition. Genitalia are even less likely to be preserved in sedimentary deposits because the three-dimensional palps and epigynes are reduced to two-dimensions. Preservational infidelity is potentially more troublesome when dealing with members of the monophyletic Entelegynae, characterized by more complex reproductive systems, compared to those specimens belonging to the Haplogynae (
Other anatomical features, including trichobothria, leg spination, eye pattern, and coloration may also be tricky, if not impossible, to observe in fossil specimens, meaning that species identification must proceed in a different manner than when dealing with extant taxa. Although confined to available morphological characters, we do not imply that palaeo-species differ from extant species; as mentioned previously, palaeontologists simply must employ (potentially) different epistemological means of species identification. For instance, molecular methods cannot be drawn on to delimit fossil species, although this does not differ from the majority of modern systematic studies still firmly rooted in morphology. Using molecules to parse out inter- and intraspecific relationships has been utilized more frequently within spiders (e.g.
We have discussed how genitalia are used as species-specific characters and can potentially reveal SMRSs within the Araneae, but does this imply that somatic characters cannot and should not be used in species identification? In other words, are these characters epistemologically useless for species delimitation? A review of the spider systematic papers from the three most recent issues of the
The proceeding discussion on preservational media hints at the difficulty of comparing the rock and amber records of spiders, both to each other and to living species, because each recovers a potentially different (sub)set of characters. The question is then raised of how to deal with similar looking fossils found under different preservational conditions. We posit that if there are no obvious differences between the fossils, the specimens should be placed at the appropriate taxonomic level for which the available diagnostic characters allow. No designation should be made for depauperate specimens lacking synapomorphies and/or distinguishing morphological data. The possibility exists that specimens that appear different due to preservation will be named as different species, when in reality they are synonymous (NB: due to the scarce nature of the fossil spider record, this is unlikely to be a major problem). Again, incorrectly diagnosing species is not the exclusive problem of palaeontology, and if there is no
Occasionally, specimens are discovered that do not easily fit into extant classification systems (i.e., family, genera), but which are clearly related to modern lineages (i.e., so-called plesion or stem lineages). When this occurs, how should these specimens be treated so that classifications are in accord with hypotheses of evolutionary history and poly- and/or paraphyletic groups are avoided? Should modern genera be diagnosed more broadly to accommodate closely related fossil forms, or should the fossil (plesion) taxa be maintained separately? We advocate the methodology of
Since spiders are relatively scarce in the fossil record, species descriptions are primarily based on singletons or only a few specimens (rather than a series of similar specimens), and only one specimen may represent the fossil record of entire families/lineages. Of course, this makes it impossible to assess levels of within-species variation. Reliance on Lagerstätten also means that range extensions and ghost lineages are quite common within the Araneae. These terms refer to the history of taxa with no direct record, as elucidated within a phylogenetic framework (
Unfortunately, an increasing number of Mesozoic specimens have been assigned to extant genera (e.g.,
Typically, the amount of accessible morphological data, particularly with respect to copulatory organs, differs for male and female specimens in the fossil record. For instance, many of the female genital structures used in species identification, such as coiled connecting ducts, seminal receptacles and fertilization ducts of the epigyne, are internal and usually not visible in specimens preserved in amber or rock (for an exception see
Fossil juvenile spiders are also hard (usually impossible) to identify to the species or even family level, and distinguishing between males and females in the absence of mature sex organs is often difficult. Again, these problems are not restricted to the fossil record but, in the modern, context clues and large sample sizes may aid in identification. Fossil juveniles are generally not studied, which leaves much diversity unexamined. Immature specimens are not entirely uninformative, however, as indicated by the Cretaceous fossil spider family Lagonomegopidae, which is described entirely from juvenile (and/or female) specimens (
We do not attempt to address and provide solutions for every issue involved in the study of fossil spider species, nor is this likely possible. We endeavoured to provide an overview of the most relevant concepts and epistemological problems encountered. Although our preceding discussion does recognize that there are some difficulties involved with identifying and delineating fossil spider species, it is undoubtedly true that the fossil record of spiders has contributed in an important way to the understanding of the group's evolution (
Studies comparing modern and fossil forms are becoming increasingly common (for example,
New technological developments are opening further avenues for the study of fossil spider species in ways that allow even broader comparison with extant taxa. Very-High-Resolution X-Ray Computed Tomography (VHR-CT) and propagation phase contrast based X-ray synchrotron imaging techniques offer non-destructive methods for capturing fine-scale morphological details, including internal morphology (
In the past, fossil species were sometimes designated without complete knowledge of the copulatory organs or other morphological features, without a complete understanding of phylogenetics, and/or without regard for the extant literature and knowledgebase. This is now changing, and X-ray imaging is contributing to this renaissance because it allows for full characterization of a specimen, perhaps revealing the SMRS so that an epistemology and ontology of spiders would coincide.
When we study species in evolutionary biology, we hope the units correspond to real evolutionary units in nature. The primary manner that this is done within the Araneae is by examining copulatory organs. As discussed, this is potentially problematic when studying fossils because these structures may not be preserved or visible. However, recent technological advances may overcome some of these barriers, especially with the advent of propagation phase contrast X-ray synchrotron imaging.
Still, the study of fossil spiders, regardless of how difficult or troublesome, is extremely important for understanding evolutionary history and for calibrating the tree of life. For example, fossils often provide evidence for character transformations obscured when only recent taxa are examined (
We thank Curtis Congreve, Bruce Lieberman, Corinne Myers, David Penney, and Vincent Perrichot for helpful discussions, reviews, and editorial comments. An anonymous reviewer improved the manuscript immeasurably. We are grateful to Didier Néraudeau for inviting us to contribute to this issue.
Number of spider species described per time bin. Notice the spike in the Palaeogene and Neogene, which is primarily controlled by the Baltic and Dominican amber Lagerstätten. Data derived from
Nombre d’espèces d’araignée décrites par époque. À noter le pic au Paléogène et au Néogène, qui est essentiellement contrôlé par les « Lagerstätten » d’ambre baltiques et dominicains. Données issues de