Overlap Indices: Tools to quantify the amount of anatomical overlap among groups of incomplete terminal taxa in phylogenetic analyses

Phylogenetic value of jaw elements of lacertid lizards (Squamata: Lacertoidea): a case study with Oligocene material from France

Several extinct species are known from the family Lacertidae, but due to poor preservation, many of them are based on single bones. Here, we compare phylogenetic signals of disarticulated premaxillae, maxillae and dentaries of lacertids from four French Oligocene localities (Coderet, La Colombière, Roqueprune 2, Mas de Got B). We identified five morphotypes among the premaxillae, six among the maxillae, and ten among the dentaries. These morphotypes were scored as individual taxa per locality into three separate character matrices with the same 246 characters, one matrix for each jaw element. Subsequently, the phylogenetic position of the morphotypes was tested using maximum parsimony. The consensus trees with the dentaries and the maxillae found a large polytomy including all taxa except the outgroup taxon Gekko gecko. The consensus tree with the premaxillae showed a considerably more resolved topology but found all morphotype taxa outside Lacertidae. In a second step, we compared the constitution of our three datasets and the morphotype taxa. Our results suggest that a combination of convergent characters and missing data led to the outgroup position of the premaxilla morphotype taxa. The poor resolution of the maxillae strict consensus is likely a consequence of their fragmentary preservation. For the dentaries, a high amount of missing data due to the high number of morphotype taxa most likely caused the poor tree resolution. Indeed, tests with fewer morphotypes found tree resolutions comparable to the premaxilla data. When linking the morphotypes, five possible lacertid "species" were found. Comparison with already known French Oligocene lacertid species points to a slightly higher species richness of Lacertidae at that time than known before. Reliable species classification based on phylogeny only seems possible when combining the jaw elements or in association with other cranial and postcranial material, putting some doubt on species identifications based on single bones.

How to Render Species Comparable Taxonomic Units Through Deep Time: a Case Study on Intraspecific Osteological Variability in Extant and Extinct Lacertid Lizards

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.

Reappraisal of the morphology and phylogenetic relationships of the alligatoroid crocodylian Diplocynodon hantoniensis from the late Eocene of the United Kingdom

Diplocynodon is a genus of basal alligatoroid comprising nine species, which spanned the late Palaeocene to middle Miocene of Europe. Despite recent revisions of most Diplocynodon species, one of the earliest named and most complete, Diplocynodon hantoniensis, has not been re-described for over 150 years. This species is known from the remains of numerous individuals from the Priabonian (late Eocene) Headon Hill Formation, which crops out at Hordle (Hordwell) Cliff in Hampshire, United Kingdom. Here we re-describe and diagnose Diplocynodon hantoniensis, providing the first detailed description of postcranial anatomy in Diplocynodon, and indeed any basal alligatoroid. Diplocynodon hantoniensis is diagnosed by four autapomorphies, including retention of the ectopterygoid–pterygoid flexure through ontogeny and a unique anterior process of the ectopterygoid adjacent to the posteriormost maxillary alveoli. A critical review of previously referred remains from elsewhere in Europe and the USA restricts Diplocynodon hantoniensis to the late Eocene of the UK. Through comparisons with extant crocodylians, the well-preserved postcranial skeleton enables the interpretation of numerous muscle attachments in the forelimbs and hindlimbs, providing a potentially rich source of character data for future phylogenetic analyses. Based on a comparison of humeral morphology between a large sample of crocodylian species, we outline two new morphological characters in the humerus. We include D. hantoniensis in a phylogenetic analysis, including all putative Diplocynodon species (103 taxa scored for 187 characters). We use four different character-weighting schemes: equal weighting, implied weighting (k value = 8) and extended implied weighting with k-values of 4 and 8. In general, these weighted analyses produce congruent results with the equal-weights analysis, and increase the resolution within Diplocynodon. We recover a monophyletic Diplocynodon in three of the four analyses. However, the fourth analysis, with the strongest downweighting of homoplastic characters and missing data (extended implied weighting with k = 4), recovers the Palaeocene Diplocynodon remensis outside Diplocynodon. Our comprehensive revision of one of the most completely known Diplocynodon species facilitates comparisons in the genus, as well as between other basal alligatoroids, and forms the basis for comparing postcranial anatomy in other fossil crocodylians.