Rise of placental mammals: the anatomy, palaeobiology and phylogeny of Periptychus and the Periptychidae

Abstract

The diversification of eutherian mammals following the end-Cretaceous mass extinction was a critical period in evolutionary history. The Palaeocene is marked by the proliferation of archaic mammals which exhibit a mosaic of primitive and derived anatomies and whose phylogenetic affinities with extant mammals remain contentious. Consequently, macroevolutionary studies assessing the timing and recovery of eutherian mammals following the end-Cretaceous mass extinction are inhibited by our relatively poor knowledge of the mammals which thrived during the Palaeocene. One group of Palaeocene mammals in particular, the ‘Condylarthra’ have proven especially enigmatic and, as historically conceived, includes families of ungulate-grade mammals some of which are considered the ancestral stock from which modern perissodactyls and artiodactyls arose. The Periptychidae are a distinctive ‘condylarth’ family and were among the first mammals to appear after the extinction. As such they constitute an excellent empirical case study towards resolving the evolutionary relationships and understanding the palaeobiology of Palaeocene mammals. The overarching aim of this thesis has been to generate a comprehensive higher-level phylogenetic hypothesis of Periptychidae and shed light on the species-level interrelationships of taxa historically identified as periptychids and other ‘condylarth’ exemplars. This aim has been achieved by the undertaking a comprehensive anatomical re-description of the archetypal periptychid, Periptychus carinidens, based on a wealth of new fossils recovered from the San Juan Basin in New Mexico, USA. The anatomical information described in this thesis has also facilitated a greater understanding of ecology and functional morphology of Periptychus and its kin. Periptychus carinidens was a medium-sized, robust, stout-limbed animal that was mediportal and adopted a plantigrade mode of locomotion. The cranial and dental anatomy of Periptychus is broadly concurrent with the inferred plesiomorphic eutherian condition albeit more robust in its overall construction. The broad facial region, tall sagittal and nuchal crests and distinctive dentition with strong enamel crenulations and compressive wear are likely indicative of durophagous diet made up of dense, fibrous, plant-based food stuffs. The postcranial skeleton of Periptychus is a miscellany of morphologies with often paradoxical functional implications. Despite its robustness, Periptychus retained a moderately high degree of multiaxial movement and dexterity in its limbs with prominent muscle attachment sites indicative of powerful, non-rapid limb movements. Well-developed manual and digital flexors and extensors are further indicative of some scansorial and fossorial capability. Periptychus and other Palaeocene mammals are characterised by their robust anatomy and tend to lack any obvious extant analogues impeding our understanding of eutherian ecological diversity during the Palaeocene and the roles of many so-called ‘archaic’ mammals. Multivariate analyses on a dataset of functionally significant limb measurements show that Palaeocene mammals exhibit a distinct and more constrained range of locomotor ability defined by their prevalent robust morphology. However, there are subtle distinctions between archaic taxa indicating ecomorphical diversity possibly due to niche partitioning, that are not easily comparable to extant mammals. This suggests that, far from being generalized ancestral stock, Palaeocene taxa were experimenting with their own unique locomotor styles. The extinction of many archaic groups at the end of the Palaeogene is associated with a trend towards increasingly open habitats, which was less conducive to the survivorship of robust, ambulatory mammals. The anatomy of Periptychus combines a basic early placental body plan with numerous unique specialisations in its dental, cranial and postcranial anatomy that not only exemplify the ability of mammals to adapt and evolve following catastrophic environmental upheaval but provide a prime exemplar by which to tackle the taxonomic and systematic conundrum that is ‘Condylarthra’. A cladistic analysis was conducted to determine the phylogenetic affinities of Periptychidae within Placentalia. 141 taxa were scored for 503 characters including 40 periptychid species and 63 novel characters. The dataset was analysed under parsimony optimality criteria and the resulting phylogeny shows a well resolved strict consensus topology with numerous well-supported relationships which help elucidate periptychid phylogeny. The analysis presented here finds Periptychidae as a monophyletic group to the exclusion of several purported periptychid taxa which are recovered with the ‘arctocyonid’, Baioconodon nordicum. The in-group relationships of Periptychidae are resolved to broadly support the subfamilial arrangement proposed by previous workers. Alticonus is recovered the most basal, unambiguous periptychid taxon. Ampliconus forms a paraphyletic stem from Alticonus to all other unequivocal periptychid taxa. Conacodontinae forms a clade which includes Auraria as the most basal taxon relative to Oxyacodon, which forms a paraphyletic stem to Conacodon. The hypsodont periptychids, Haploconus + Goleroconus form a clade, separate from both ‘Anisonchinae’ and Conacodontinae, both of which they have previously been affiliated to. ‘Anisonchinae’ forms a paraphyletic stem relative to Periptychinae. Mithrandir oligustus is the most basal ‘anisonchine’. Gillisonchus is generically distinct from both Mithrandir and ‘Anisonchus’ due to morphological similarities with Hemithlaeus and the Periptychinae. Periptychinae forms a well-supported clade with Hemithlaeus and Tinuviel resolved to be more closely related to Ectoconus than Periptychus + Carsioptychus. Periptychus is a member of Periptychinae and most closely related to Carsioptychus within Periptychini. The phylogeny reported here indicates that Periptychidae were an incredibly successful family during much of their early history and were particularly prolific during the middle Puercan. Most species were small to medium sized animals; however, members of Periptychinae attained large body sizes within less than half a million years of the end-Cretaceous mass extinction. Periptychids were prolific during early Puercan, but spent the majority of their evolutionary history exhibiting high turnover, with many short-lived species, with the notable exception of three genera: Anisonchus, Haploconus and Periptychus, which prevailed through the Torrejonian. These periptychids are among the most enduring Palaeocene taxa known and reiterate the importance of the Periptychidae in understanding the recovering and radiation of Placentalia following the end-Cretaceous mass extinction.