Re-investigation of fossil Lemmini specimens from the early and Middle Pleistocene of Western and Central Europe: Evolutionary and paleoenvironmental implications
By Louis, A., Tereza, H., Aurélien, R., Sophie, M., Oldřich, F., & Ivan, H.
Summarized by Luka Del Canto. Luka is a senior at Binghamton University studying biological sciences. He plans to finish his bachelor’s degree and pursue a master’s in genetics, with the ultimate goal of becoming a genetic counselor. Luka is also an avid sports fan who spends most of his Sundays watching football with his friends. Additionally, he loves to go on hikes when the weather is nicer and has a passion for cooking as well.
What was the hypothesis being tested? Researchers from this study hypothesized that a genus of the lemming species known as Myopus was wrongly disregarded during the European Pleistocene fossil record (2.6 Ma-11.7Ka), and that it actually coexisted with other lemming genera like Lemmus during this period. These small arctic rodents have very similar dental morphologies, which is what made them so hard to distinguish from one another, and the reason they were previously regarded as just Lemmus. This study will also test the validity of the earliest species of the Lemmini tribe, L. kowalskii, and whether or not it can be considered the ancestral form of the Lemmus genera.
What data were used? The study included lemming specimens from fossil and living species, including well-preserved skulls, mandibles, and isolated teeth obtained from museums and fossil siles. A total of 65 M1 and 41 M3 fossil teeth were gathered from eight locations in the Czech Republic, France, and Germany, representing early to middle Pleistocene populations. Another 295 third upper molars (M3) and 324 first lower molars (M1) were collected from museums as living species referential data. To analyze taxonomic differences between the genera Lemmus and Myopus, detailed measurements of dental features were taken with focus on tooth size and shape. Then, these measurements were used to perform statistical analyses to identify morphological variations and potential evolutionary relationships.
Methodology: Researchers outlined specific points on the teeth called landmarks and mapped them out digitally. Since the teeth of fossil and living specimens differed in size and orientation, those differences needed to be removed to prepare them for further analysis. A Generalized Procrustes Analysis (GPA) aligned the teeth landmarks, and principal component analysis (PCA) was then used to analyze all the dental features by summarizing the data into a few principal components. Researchers then identified variations and similarities between these features in fossil and living specimens. Finally, linear discriminant analysis (LDA) was used to take the variations and separate them into different genera like Lemmus or Myopus by finding the features that best distinguished each group. Modern teeth were used as a model to train the LDA into identifying major differences in the genera. Once the model was trained and validated, it was applied to fossil specimens and predicted whether each fossil belonged to Lemmus or Myopus based on its distinguishing features.
Results: Initial results were promising, as many Myopus and Lemmus fossils were both identified at multiple sites, providing proof that they lived in the same time period as they were in the same rock layer. Analysis showed significant shape variation in both M1 and M3 molars, with the first three principal components accounting for nearly 50% of total shape variation (Fig.1). More compact teeth and a less pronounced rear portion of the molar were among the morphological changes observed on the PC graphs. In terms of overall tooth size, fossil populations had teeth similar to Myopus, slightly larger but much smaller than Lemmus, which had the largest teeth for both M1 and M3. Lemmus also had a larger convex hull volume than Myopus, meaning a greater variety in shape and size. However, fossil populations had variation scores comparable to or exceeding those of modern genera, even with fewer specimens, suggesting that the fossils may include multiple genera. The LDAs also showed clear differences between Lemmus and Myopus and were able to assign different fossils to each genus, showing that these genera indeed coexisted during the European Pleistocene fossil record. PCA analysis showed that fossil teeth assigned to L. Kowalskii clustered with modern Lemmus teeth, ultimately validating its taxonomic status as a species within the Lemmus genus.
Why is this study important? Misassignment of different genera can lead to a misunderstanding of the evolutionary interactions between them. Due to their similar morphologies, Lemmus and Myopus were misclassified as the same genus. Our knowledge of the taxonomic relationships between Myopus and Lemmus in the Pleistocene fossil record were redefined, representing a reliable method for future fossil identification studies. Through the methods used, the study enabled researchers to quantify shape variations that may be missed by more conventional classification approaches that were used in the past like visual inspection and simpler measurements. Modern techniques like GPA, PCA and LDA are particularly useful for differentiating between species that have similar morphologies.
Broader implications beyond this study: The results of this study have wider ramifications for our comprehension of species classification and divergence in the fossil record. The history of the Lemmini tribe has changed as we now know that Myopus and Lemmus coexisted. The capacity to distinguish between closely related species by dental morphology aids in the reconstruction of an evolutionary history with greater accuracy, not only for lemmings but possibly for other mammals as well. It also calls into question the validity of earlier taxonomic hypotheses and the necessity of reexamining other ancient groups that have morphological similarities, like voles and muskrats.
Citation: Louis, A., Tereza, H., Aurélien, R., Sophie, M., Oldřich, F., & Ivan, H. (2024). Re-investigation of fossil Lemmini specimens from the early and Middle Pleistocene of Western and Central Europe: Evolutionary and paleoenvironmental implications. Palaeogeography, Palaeoclimatology, Palaeoecology, 641, 112128. https://doi.org/10.1016/j.palaeo.2024.112128