Non-technical Summary

Most species exhibit little change (stasis) in form following speciation, and this is a key feature of the concept of punctuated equilibria. Stasis results in species often having durations that span millions of years. Durational data are fundamental to many types of paleobiological analyses and are ideally based on occurrence data represented by specimens in museum collections. Often, however, durational data are presented without supporting information about voucher specimens that document stratigraphic ranges, including first and last appearances. We use the iconic Devonian trilobite Eldredgeops rana to demonstrate that durational data can be challenging to determine at multiple taxonomic levels. Further, we show that different datasets derived from the published literature and museum collections give different results concerning the first and last appearances of species in the fossil record. We argue that paleontologists should adopt two general best practices to help address these problems. First, paleontologists should clearly identify voucher specimens that represent stratigraphic occurrences of species. Second, we recommend that high-quality photographs of occurrence vouchers be placed in open access websites and be assigned public domain licensing before being paywalled by journals. Such voucher images also have a role to play in training artificial intelligence (AI) systems that will be applied to future paleobiological questions.

What Every Paleontologist Knows, Revisited

The radical insight of Eldredge and Gould’s (Reference Eldredge, Gould and Schopf1972) concept of punctuated equilibria is not that morphological change is rapid on geological timescales, but rather that the forms of most species tend not to change much following speciation (see also Gould and Eldredge Reference Gould and Eldredge1977; Gould Reference Gould2002). That is, speciation does not tend to be characterized by a series of intermediate forms that link an ancestral species to its descendants (phyletic gradualism). Instead, morphological change typically occurs in a geological instant at the time of speciation (consistent with the allopatric model of Mayr Reference Mayr1963; see also Eldredge Reference Eldredge1971) and this is often followed by subsequent morphological stability or stasis. This stability in form is one reason we can consider species as being analogous to organisms and having finite individuality (Hull Reference Hull1980): they have a birth (speciation), a life span (duration), and a death (extinction). These features collectively make species the fundamental units of the study of macroevolution (Lieberman and Eldredge Reference Lieberman and Eldredge2014).

The general stability of species over their life spans has practical utility and is the foundation of the science of biostratigraphy. Every biostratigraphic chart is an argument for the utility of species-level stasis for assigning ages to rocks. Eldredge and Gould (Reference Eldredge, Gould, Kauffman, Hazel and Heffernan1977) recognized this and noted that “by the mere recognition of any nontrivial stratigraphic range of any morphologically defined taxon at near specific rank, we are necessarily implying a stability or stasis in species-specific differentia” (p. 29, italics in original). Gould (Reference Gould2002) characterized this as “What Every Paleontologist Knows” (p. 745) in a heading at the beginning of chapter 9 of his opus, explaining that “paleontologists have always recognized the longterm stability of most species, but we had become more than a bit ashamed by this strong and literal signal” (p. 749).

Our purpose here is not to review the strong evidence and general support for stasis, which has been provided elsewhere (e.g., Gould [Reference Gould2002] and Eldredge et al. [Reference Eldredge, Thompson, Brakefield, Gavrilets, Jablonski, Jackson, Lenski, Lieberman, McPeek and Miller2005] and papers cited therein). Instead, we focus on the underlying data from specimens that support all accounts of species durations and examples of stasis. We are fundamentally interested in how durational data are generated in many modern analyses and present some suggestions for best practices in the future. If embraced, we think that these changes will facilitate prospecting for more examples of stasis in the age of “big data.”

Durational Data in Paleontology

Because of stasis, species often have long durations, sometimes spanning millions of years. Estimates vary considerably across different taxonomic groups and tend to be extrapolations from higher-level taxonomic data (Lamkin and Miller Reference Lamkin and Miller2016). For example, Cambrian trilobites (1.5 Myr; Foote Reference Foote1988), Mesozoic ammonoids (1–2 Myr; Kennedy Reference Kennedy and Hallam1977), and Cenozoic terrestrial mammals (1–2 Myr [Vrba Reference Vrba1985]; 2.4 Myr [Prothero and Heaton Reference Prothero and Heaton1996]) have relatively shorter durations on geological timescales, although still tremendously long with respect to the predictions of phyletic gradualism. Animal groups with relatively longer durations on geological timescales include Devonian invertebrates from the Appalachian Basin (3–7 Myr; Brett et al. Reference Brett, Ivany and Schopf1996) and Cenozoic bivalves (10 Myr; Raup and Stanley Reference Raup and Stanley1978: p. 323). Reported durations of some microfossil groups are sometimes much greater. For example, Buzas and Culver (Reference Buzas and Culver1984) reported benthic foraminifera durations of 16–26 Myr (see also Strotz and Allen Reference Strotz and Allen2013). For additional summaries of typical species durations across other taxonomic groups, see Stanley (Reference Stanley1979), Raup (Reference Raup1991), and May (Reference May2002). A general rule of thumb, however, is that “the average life span of a species in the fossil record … is typically a few million years” (May Reference May2002: p. 1328).

Such species durations are fundamental data in many paleobiological studies. This includes analyses conducted at the genus level (or above), because the geological duration of a genus reflects the combined durations of all its constituent species (Hendricks et al. Reference Hendricks, Saupe, Myers, Hermsen and Allmon2014). A duration is determined by the first and last appearance of a species in the fossil record, and net stasis is demonstrated by minimal overall morphological change between those two end points, even if there are some oscillations in form in between. Quantitative examples of this were demonstrated in two Devonian brachiopods—Athyris spiriferoides (Eaton, Reference Eaton1831) and Mediospirifer audaculus (Conrad, Reference Conrad1842)—by Lieberman et al. (Reference Lieberman, Brett and Eldredge1995).

Durational data are fundamental to much of analytical paleobiology. For example, there is a general interest in the typical life spans of species from varied clades (see earlier examples), as knowing this is key to calculating extinction rates. Durational data are also essential to the development of diversity curves across geological time, for example, the iconic depiction of Phanerozoic marine diversity presented by Sepkoski (Reference Sepkoski1981: fig. 5). Such studies of past biodiversity have allowed paleontologists to quantify the scale of ancient extinction events and provide context for understanding modern biodiversity loss (e.g., Barnosky et al. Reference Barnosky, Matzke, Tomiya, Wogan, Swartz, Quental and Marshall2011; Kiessling et al. Reference Kiessling, Raja, Roden, Turvey and Saupe2019). Among other uses, durational data have also been used to investigate whether taxon attributes like geographic range confer resistance to extinction, resulting in longer durations (Jablonski and Hunt Reference Jablonski and Hunt2006; Payne and Finnegan Reference Payne and Finnegan2007).

Ultimately, durational data are derived from fossil specimens, ideally housed in museum drawers that are accessible to paleontologists. In practice, however, durational data are usually presented without reference to voucher specimens and instead rely on earlier tabulations and summaries that may or may not be tied to actual vouchers. We illustrate this below with the example of the phacopid (Bault et al. Reference Bault, Crônier, Monnet, Balseiro, Serra, Waisfeld, Bignon and Rustán2023) trilobite Eldredgeops rana (Green, Reference Green1832) (formerly Phacops rana; Fig. 1), which featured prominently in Eldredge and Gould’s (Reference Eldredge, Gould and Schopf1972) initial example of punctuated equilibria. Focusing mostly on purported first and last occurrences, we demonstrate that durational data can be challenging to pin down at multiple taxonomic levels, even for this classic example of morphological stasis.

Figure 1.

A–F, Specimens of Frasnian (likely Ithaca Formation) Eldredgeops rana from Tompkins County, New York (PRI 57222). A, Original sample from Kindle (Reference Kindle1896) (originally catalogued as Cornell University 11796); scale bar, 1 cm; sample card is lost; image captured in 2006 provided by J. Zambito. B, Magnified view of label in A. C, D, Cephalon and magnified view of eye of one (specimen to left of label in A); scale bar below C pertains to that image and equals 5 mm. E, Pygidium of specimen on third row of card, second from the left. F, Thorax of specimen on second row of card, third from the left. Scale to left of D pertains to images D–F and equals 1 cm. G, Specimen of E. rana from the Pecksport Mbr. of the Oatka Creek Fm. (lower Givetian) Madison County, New York (KUMIP 419279); scale bar, 1 cm.

The Example of Eldredgeops rana (Trilobita: Phacopidae)

Specimens of the “frog-eyed” phacopid trilobite Eldredgeops rana are very common in the Devonian Hamilton Group of New York State (Bartholomew and Ver Straeten Reference Bartholomew, Ver Straeten, Ver Straeten, Over and Woodrow2023; Brett et al. Reference Brett, Baird, Zambito, Bartholomew, Ver Straeten, Over and Woodrow2023) and are much sought after by collectors, resulting in substantial representation of the species in museum collections, and undoubtedly even more in avocational collections. The most important systematic treatment of E. rana remains the monograph of Eldredge (Reference Eldredge1972), which provided the fundamental data in support of stasis in this species that was published by Eldredge and Gould (Reference Eldredge, Gould and Schopf1972) the same year. The species is thus an apt subject for evaluating how durational data are underpinned in paleontological research. We consider this support at the family, genus, and species levels using data from the literature and online databases, with a focus on several large datasets (Sepkoski, Reference Sepkoski1982, Reference Sepkoski2002; Paleobiology Database [PBDB], https://paleobiodb.org) and museum collections (Integrated Digitized Biocollections [iDigBio] https://www.idigbio.org) that have provided key insights into the evolution of life. Given that most published and museum records of E. rana are attributed to P. rana, we included both genera in our investigation. Our attention is on how first and last appearances are underpinned and represented in these databases, as they determine total fossil durations in analyses.

Summary

Our purpose here is not to provide a definitive accounting of the first and last appearances of E. rana and its parent genus and family ranks. The data we present are based solely on the literature- and specimen-based datasets that we analyzed and do not include unpublished or undigitized museum records. We anticipate that additional records may become available that will impact the durations that we have presented. Instead, our goal here is to demonstrate that our understanding of the duration of the species-, genus-, and family-level taxonomic ranks associated with E. rana is little connected to tangible specimen-based support and that different datasets give inconsistent results.

Toward a Systematic Paleontology

Linnaean taxonomy is underpinned by the concept of voucher types: species are tied to type specimens, genera to type species, and families to type genera. Species may shift between higher taxa according to the whims of systematists, but concepts and definitions of higher taxa—as well as their properties—are ultimately circumscribed by the features of real specimens (see also Hendricks et al. Reference Hendricks, Saupe, Myers, Hermsen and Allmon2014). However, the properties of ancient taxa that are of greatest interest in studies of macroevolution—especially duration over geological time, but also including geographic range and morphological trait data—tend not to be explicitly tied to voucher specimens. This makes verification impossible and contributes to the inconsistency of results, including for taxa as well sampled and documented as Eldredgeops rana.

The PBDB has facilitated important contributions to our understanding of ancient life and has made it simple for anyone to attain durational data for species and higher taxa, democratizing a process that previously required years of research in the library (e.g., Sepkoski Reference Sepkoski1993). The records in the PBDB, which is largely built from the literature, are often estranged from specimens in museum collections. Verifying stratigraphic occurrences for individual taxa often leads one down a proverbial rabbit hole, sometimes with no cataloged specimen at the end of the tunnel. When durational data are not tied to corresponding specimen data, users of these data have limited means to verify taxonomic assignments or independently evaluate reported durations or stasis. The iDigBio database provides the opposite: all records are inherently tied to cataloged museum specimens, but taxa may be misidentified, or there may be data entry errors that distort temporal and geographic occurrences. Obvious errors are simple to recognize and discard from downloaded datasets (e.g., Pleistocene records of E. rana). But what about records that are not altogether unreasonable? Would someone who does not have expertise on phacopid trilobites immediately discard Lower Devonian or Lower Mississippian records of E. rana? In general, as is almost certainly the case for E. rana, establishment of voucher specimens will have the likely effect of reducing durations of taxa relative to reports or data compilations that have not been critically evaluated.

For the sake of verification, quality of analysis, and reproducibility of results, paleontology needs a better way to document occurrences and tie such records to voucher specimens, just as it needs a better system for recognizing the scientists who make these associations (Smith et al. Reference Smith, Raja, Clements, Dimitrijević, Dowding, Dunne and Gee2023). We have two general recommendations to address this issue. First, as a matter of best practice, systematists should publish museum catalog numbers for voucher specimens that represent stratigraphic occurrences when they summarize the durations of species, whether newly described, revised, or comprehensively monographed. First and last appearances are especially important to document, because they set the boundaries for studies of morphological evolution (or stasis) in the fossil record, as well as provide core data for biostratigraphic analysis. It is also useful to directly document with vouchers all of the stratigraphic units (formations or members) from which a species has been found. This is simple for a taxonomist to do in the systematics portions of a manuscript. For example, where Hendricks (Reference Hendricks2009) reported the occurrence of Conus marylandicus Green, Reference Green1830 (now Conasprella (Ximeniconus) marylandica (Green), n. comb.) as “Virginia (Yorktown Formation), North Carolina (Duplin Formation), and Florida (Tamiami and Jackson Bluff formations)” (p. 23), it would have been better practice to present this as “Virginia (Yorktown Formation; PRI 52915), North Carolina (Duplin Formation; PRI 82912), and Florida (Tamiami [PRI 53183] and Jackson Bluff formations [UF 78488]).” Although our focus is on documentation of stratigraphic occurrences, vouchers are also useful for formally documenting specimens found at extremes of a geographic range or representing morphological end-members. In addition to being formally recognized in the literature, established voucher specimens should also be incorporated into online databases like the PBDB. Occurrences that are supported by taxonomist-approved vouchers should be clearly identified as such. It is also critical that—just like type specimens—occurrence voucher specimens be reposited in museum collections where they can be accessed and evaluated by the research community.

Our second recommendation is that high-quality photographs of occurrence voucher specimens be placed in established open access websites (e.g., GBIF, MorphoSource, or FigShare) and be assigned public domain licensing. This should be done before publication so that images of significant specimens (including future types, but also occurrence vouchers) cannot become “paywalled” later by journal copyright restrictions. To our knowledge, no study has investigated the number of fossil species whose type specimens (as well as descriptions) are only represented by single images locked behind journal paywalls, but we anticipate that it is a significant percentage; the same likely applies to extant taxa. Organizations such as the Biodiversity Heritage Library have commendably liberated such data from articles whose copyrights have expired, as well as through partnerships with society journals that have made the decision to make available some or most of their holdings. But, much literature—including from the Paleontological Society’s own flagship systematics journal—remains “paywalled” and this is ultimately a detriment to the advancement of our understanding of ancient life, especially for researchers who do not have access to large academic research libraries. (Note that the Treatise on Invertebrate Paleontology, which contains numerous images of invertebrate fossil type specimens, is now open access.) Fortunately, authors now have much more control over the scientific content that they generate and how it is shared. Releasing images of important specimens into the public domain before copyright is transferred to a journal removes barriers for future workers. If such images are assigned their own stable web addresses (e.g., a digital object identifier, or DOI), it also becomes possible to link them to other online records, for example, in the PBDB. Depending on journal requirements, an alternative approach is to place images of stratigraphic voucher specimens in supplementary material associated with a paper, although such repositories are not always easy to find or can be overlooked.

Novel methodological approaches, for example, ecological niche modeling in combination with newly digitized specimens, have renewed the importance of museum collections for addressing questions that are broader than systematic studies of individual species or clades (e.g., Lieberman and Kimmig Reference Lieberman, Kimmig, Rosenberg and Clary2018). Further, it is likely that the millions of specimens that reside in museum collections—once digitized—will provide the fuel for paleontology’s big data future (e.g., Allmon et al. Reference Allmon, Dietl, Hendricks, Ross, Rosenberg and Clary2018). The success of these approaches rests upon the underlying data being sound, and we have argued here that the establishment of expert-vetted voucher specimens is important for both modern and future workers. We have no doubt that artificial intelligence (AI) will soon play a role in taxonomic work in ways that have yet to be determined (or perhaps even imagined). Such AI systems will need to be trained, however, and voucher specimens will and should play an important part in this process. Paleontologists should not fear or delay this future, for it will allow us to prospect for stasis and other features of macroevolutionary history in new ways, as well as spur the research questions that will occupy our field for the next 50 years.