Research

 

The following topics are currently being investigated in the lab:

 

Early eureptiles and the evolution of Diapsida

Basal eureptiles include the oldest-known amniote, Hylonomus from the Upper Carboniferous of Joggins, Nova Scotia , the so-called “protorothyridid” reptiles from the Permo-Carboniferous of North America and Europe , the cosmopolitan, Late Paleozoic Captorhinidae, and the earliest members of Diapsida, the clade that includes all modern reptiles (and birds). Together with Robert Reisz, University of Toronto , we have been investigating the early evolution and phylogeny of the group during various recent research projects, which also included the application of novel approaches to phylogenetic studies using morphological data. Among other things, we showed that diapsid reptiles group within the paraphyletic “protorothyridids”, their main evolutionary novelties being a lengthening of the extremities and a reduced ossification of the skull. Also, we have been working on the evolution and biogeography of captorhinid reptiles including the description of new taxa, our main emphasis being the number of independent migrations into Gondwana, the southern part of the supercontinent Pangaea, and its possible correlation with climate change. In addition, we are also interested in the evolution of marine reptiles (thalattosaurs) and the phylogeny and fossil history of squamates.

 

The evolution of parareptiles

Parareptilia are the smallest of the three main clades of basal Amniota, the other two being Synapsida and Eureptilia. For a long time parareptiles were considered to include turtles, but this hypothesis is no longer accepted by the majority of phylogeneticists. Parareptiles therefore seem to be the only major clade of Amniota that went completely extinct. Nonetheless, Parareptilia are highly interesting because they independently evolved many features that are also found in eureptiles and synapsids, such as herbivory and bipedality. Furthermore, we recently showed that they also include the oldest-known amniotes in possession of an impedance-matching middle ear. Our current research in this area focuses on the phylogeny of a fairly popular, but scientifically poorly known group of parareptiles, the large-sized, plant-eating pareiasaurs. This clade originated in the Middle Permian and quickly became widespread all over Pangaea, with the most species discovered in Russia and South Africa . In addition to this rather derived clade, we are currently working on new basal parareptiles from the famous Dolese quarry ( Fort Sill , Lower Permian), Oklahoma , which is done in close collaboration with Robert Reisz, University of Toronto .

 

Patterns of early amniote diversification

Our knowledge of early amniote evolution has recently increased significantly due to new discoveries and advanced methods of phylogenetic analysis. This now allows us to investigate large-scale patterns of basal amniote evolution, particularly the biogeography and modes of diversification during the Late Paleozoic and Early Mesozoic. In particular, we are interested in how and when early amniotes achieved a cosmopolitan distribution, if there is a correlation between diversification patterns and the end of the Permo-Carboniferous ice age and the following global warming of the Permo-Triassic, and how the P/T mass extinction affected amniote evolution. For our investigations we use the Paleobiology Database, which means that we are currently feeding the PBDB with data from early amniotes, which have so far been widely neglected. Patience is required for this type of study, and we look forward to many hours of exciting data entry…

 

Molecular approaches to amniote paleobiology

Recently developed techniques from molecular biology now allow for alternative and novel ways to investigate the paleobiology of fossil organisms. In close collaboration with the lab of Belinda Chang, University of Toronto , we study the evolution of visual pigments, or opsins. Visual pigments are key components of the visual transduction process, forming the first step in the biochemical cascade within the photoreceptors of the eye, eventually resulting in a neural signal to the brain that light has been perceived. Because the visual adaptations of an animal are closely tied to its ecology, we use insights from opsin evolution for paleobiological inferences. For this purpose, we sample the opsin genes of previously neglected amniote taxa and use them for studying patterns of selection and reconstructing ancestral states, along with the eventual recreation of the ancestral opsins in the laboratory. We have two major research projects in which this novel approach is used: a) investigating the ecological circumstances under which snakes initially evolved (aquatic, terrestrial, or fossorial); b) testing the hypothesis that mammals had originally been nocturnal, which would have implications for the origin of endothermy.

 

Evolutionary morphology of the amniote skull

Ontogenetic variation seems to be major factor in the evolution of novel morphologies, and this is also true for the amniote skull. However, while heterochrony and associated patterns of ontogenetic variation have been fairly well investigated in fossil and extant amphibians, the amniote skull is poorly known aside from a few model organisms. We study heterochrony and its potential (extrinsic and intrinsic) factors on a somewhat larger scale, using the African radiation of collared lizards (Lacertidae) as a model group. Within this clade, there are several independent but poorly understood radiations into arid environments, which interestingly seem to be associated with a mosaic pattern of paedomorphic and peramorphic features in the skull. We use a combination of phylogenetic, molecular, morphological, and ecological techniques to evaluate how often these patterns had evolved in the clade, if there is a correlation in the development of paedomorphic and peramorphic features, and how this relates to other evolutionary aspects such as life history and adaptations to desert environments. Also, we try to decipher the genetic basis for some of the typical features that can often be seen in the amniote skull.

 

Molecular clocks and the fossil record

Molecular clock studies have become increasingly crucial for evolutionary divergence estimates and the analysis of diversification patterns. However, despite the recent statistical advances in bioinformatics information from the fossil record is still needed for the calibration of the clock. Together with Robert Reisz we have been involved in several theoretical discussions about how fossil calibration dates should be applied to molecular clock studies, and we also came up with a list of potentially suitable calibration points from the fossil record. Nevertheless, there are still cases in which neither the knowledge about the phylogenetic relationships nor a good number of fossil calibration dates is sufficiently known, but in which divergence estimates would be desirable. For that reason, we are currently experimenting with novel Bayesian techniques and the application of statistical distributions for accommodating the maximum date of a fossil split, which is done in close collaboration with (and couldn't be done without) Lin Himmelmann, University of Frankfurt. In this context, we also investigate different models of rate variation and their specific usefulness for divergence estimates.