Phylogeny, evolution, and anatomy of Taeniodonta (Mammalia: Eutheria) and implications for the mammalian evolution after the Cretaceous-Palaeogene mass extinction

Abstract

Mammals originated in the Mesozoic and were among the survivors of the Cretaceous-Palaeogene (K-Pg) mass extinction. However, the mode and the tempo of evolution for the eutherian mammals, the placental mammals and their close relatives remain unclear. Right after the K-Pg extinction, there were many groups of “archaic” mammals in the Palaeogene. Among these mammals are the Taeniodonta, a rather enigmatic group, known for their worn teeth.

Taeniodonts are believed to have crossed the K-Pg boundary and started their diversification early in the Paleocene. Fossils of taeniodonts have been found only in North America and their worn teeth and robust bodies are distinctive among other animals of their time. There are nine genera of taeniodonts, traditionally arranged into two families: the smaller Conoryctidae, which are believed to have had a more generalised body plan, and the robust Stylinodontidae, adapted for digging.

The main aim of this thesis is to shed light on the species-level inner relationship of Taeniodonta and to understand the morphological adaptations and functional morphology of the early members of this group. Central to this study are the numerous new fossils from the San Juan Basin in New Mexico, USA. These include new elements of dentition assigned to various members of Taeniodonta, as well as postcranial elements belonging to the genus Conoryctes comma.

Chapter one reviews previous work that was carried out for taeniodonts and provides a background on the anatomy and evolution of the group. Well-known palaeontologists such as Cope, Wortman, Matthew and others have studied taeniodonts and were among the first to assign many new materials to new genera.

More recent studies have focused on the anatomy of taeniodonts, their functional morphology and the structure of their unique teeth. There have been very few attempts to analyse their phylogeny.

In Chapter two, new dental fossils from the Puercan and Torrejonian of the San Juan Basin are described and used to understand the unique dentition of taeniodonts. Using these specimens and various others that were studied in many museums, a detailed species-level cladistic analysis is presented. It includes 133 taxa, scored for 630 dental, cranial, and postcranial characters. Among these are fourteen species of taeniodonts and other taxa that are potentially related to the group. All the previous phylogenetic studies of taeniodonts included only dental characters. The present study captures the dental, cranial and postcranial morphology. The dataset was then analysed under maximum parsimony to create a strict consensus topology. Using the first and last appearance of the taxa, a time-calibrated tree was created.

The results of this analysis show that Taeniodonta is a monophyletic group and excludes the Cretaceous taxon Schowalteria which has been a dubious member of the group. The interrelationship of Taeniodonta generally conforms to previous studies as the group is divided into two clades, corresponding to the Conoryctidae, and the Stylinodontidae. Interestingly, Huerfanodon polecatensis is resolved as a sister taxon to a clade of Huerfanodon torrejonius and Conoryctes comma. This topology of the phylogenetic tree and other anatomical features led to questions about the validity of the genus Huerfanodon. The results also propose that, among the numerous Palaeogene mammals in the dataset, Taeniodonta are cimolestids and potentially related to other groups including the leptictids and the palaeanodonts. These relationships need to be further tested with a broader spectrum of phylogenetic characters in a matrix that includes more taxa.

Chapter three presents a detailed anatomical description of the new postcranial elements of the genus Conoryctes comma. Previously, the postcranial skeleton of this genus was represented only by partial humerus and radius, leading to unanswered questions about the postcranial anatomy and locomotion of this animal. This chapter includes detailed descriptions and photographs of nine new specimens from the San Juan Basin. These specimens include elements of the vertebrae, forelimb and hindlimb of Conoryctes, and were compared with the skeletons of other taeniodonts and Palaeogene mammals. These comparisons and the functional anatomy of the postcranial fossils suggest a digging mode of life for Conoryctes.

The final chapter concludes the findings of this thesis. Based on the cladistic analysis and the fossil record, taeniodonts share common dental characteristics such as the progressively smaller molars in the tooth row and the absence of ectoflexus in the upper molars, while there are various similarities in the postcranial anatomy too. Unlike previous studies that consider only Stylinodontidae as highly specialised animals, this study finds that even early Paleocene Conoryctes had a specialised skeleton, with robust forelimbs and hindlimbs and sharp claws. This points to a high degree of fossoriality in both clades of Taeniodonta.

Therefore, digging adaptations is a common trait in all taeniodonts, possibly shared with their closest relatives too. This mode of life might have impacted the survivability of their ancestors during the Cretaceous-Palaeogene extinction and led taeniodonts to radiate in the Paleocene.