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How to render species comparable taxonomic units through deep time: A case study on intraspecific osteological variability in extant and extinct Lacertid lizards


Tschopp, Emanuel et al. (2021), How to render species comparable taxonomic units through deep time: A case study on intraspecific osteological variability in extant and extinct Lacertid lizards, Dryad, Dataset,


Generally, the species is considered to be the only naturally occurring taxon. However, species recognised and defined using different species delimitation criteria cannot readily be compared, impacting studies of biodiversity through Deep Time. This comparability issue is particularly marked when comparing extant with extinct species, because the only available data for species delimitation in fossils is derived from their preserved morphology, which is generally restricted to osteology in vertebrates. Here, we quantify intraspecific, intrageneric, and intergeneric osteological variability in extant species of lacertid lizards using pairwise dissimilarity scores based on a dataset of 253 discrete osteological characters for 99 specimens referred to 24 species. Variability is always significantly lower intraspecifically than between individuals belonging to distinct species of a single genus, which is in turn significantly lower than intergeneric variability. Average values of intraspecific variability and associated standard deviations are consistent (with few exceptions), with an overall average within a species of 0.208 changes per character scored. Application of the same methods to six extinct lacertid species (represented by 40 fossil specimens) revealed that intraspecific osteological variability is inconsistent, which can at least in part be attributed to different researchers having unequal expectations of the skeletal dissimilarity within species units. Such a divergent interpretation of intraspecific and interspecific variability among extant and extinct species reinforces the incomparability of the species unit. Lacertidae is an example where extant species recognised and defined based on a number of delimitation criteria show comparable and consistent intraspecific osteological variability. Here, as well as in equivalent cases, application of those skeletal dissimilarity values to palaeontological species delimitation potentially provides a way to ameliorate inconsistencies created by the use of morphology to define species.


Personal visits to scientific collections by Emanuel Tschopp and Lukardis Wencker.

Some specimens were scored based on figures in published literature.

Python and R scripts were developed by Lukardis Wencker and James Napoli, respectively.

Usage Notes

The complete character matrix is also available on MorphoBank (, where sources for characters and character scores are marked within the matrix as comments.


Seventh Framework Programme, Award: Marie Skłodowska–Curie grant agreement No. 609402-2020 researchers: Train to Move (T2M)

FP7 Research infrastructures, Award: FR-TAF-5839

FP7 Research infrastructures, Award: AT-TAF-5725

Erasmus+, Award: 2 Traineeships for LCM Wencker

Richard Gilder Graduate School, Award: PhD fellowship for JG Napoli

American Museum of Natural History, Award: Newt and Callista Gingrich endowment

Università degli Studi di Torino, Award: PhD fellowship for LCM Wencker

Università degli Studi di Torino, Award: Fondi di Ricerca Locale UNITO 2017-2019