Systematics, biomechanics and ecology of mammals of the Kilmaluag Formation (Jurassic) of Scotland
Creators: name: Panciroli, Elsa Louise
Publisher: name: The University of Edinburgh
Year: 2020
Subjects: evolution
mammals
biomechanics
palaeontology
anatomy
Kilmaluag Formation
Borealestes serendipitus
trans-cochlear canals
niche partitioning

description: The Middle Jurassic was a pivotal time in the macroevolution of mammals. The earliest mammaliaform branches flourished alongside non-mammalian cynodonts (tritylodontids) and the first crown group mammals. Recent fossil material from China suggests not only that mammaliaforms were unexpectedly ecologically diverse, but that Docodonta had exceptionally high ecomorphological diversity for such a geologically early-diverging clade. Understanding these macroevolutionary patterns is hindered by the paucity of Middle Jurassic material globally. The Kilmaluag Formation of the Isle of Skye, Scotland, provides a rare, exceptionally well-preserved fossil mammaliaform assemblage. The analysis of this material provides new data to answer phylogenetic and ecological questions about Middle Jurassic mammaliaforms. I utilise synchroton tomographic data of the first skeleton of a Middle Jurassic mammaliaform from the British Isles, Borealestes serendipitus. As a basal docodontan, Borealestes provides key anatomical information for resolving docodontan phylogenetic relationships. Using these data I clarify the diagnosis of Borealestes and identify two new petrosal structures: the trans-cochlear canals anterior and posterior. I perform two phylogenetic analyses: 1) using a large docodontan dataset of dentomandibular characters; 2) an analysis with few docodontans but multiple mammaliaform lineages using dentomandibular, cranial, postcranial and soft tissue characters. These provide conflicting results that suggest the lack of skeletal characters for most docodontan genera impacts the usefulness of an expanded character dataset for resolving docodontan relationships. In the dentomandibular-based phylogenetic analysis I find Borealestes belongs to a previously proposed basal docodontan clade along with Docodon, Docofossor and Haldanodon. Additionally, using conventional micro computed tomography of multiple teeth and dentaries I describe the anatomy and test the systematic position of Stereognathus, Wareolestes, and Palaeoxonodon material. These data permit the clarification of the diagnosis of Stereognathus ooliticus (=hebridicus), and outline previously unknown anatomy, and the identity of disputed lower molars in the holotype, of Wareolestes. By combining new Palaeoxonodon material with previously collected material from Skye, I find additional characters of the posterior dentary including a deep, anteriorly enclosed masseteric fossa, and mandibular foramen offset from the Meckel’s sulcus and positioned below the alveolar plane. Finally, I explore the ecology and ecomorphology of the mammaliaforms of the Kilmaluag Formation using body mass estimation and biomechanical and morphometric analyses. These analyses support niche partitioning among the Kilmaluag mammaliaform assemblage. They also suggest early mammaliaform biomechanics cannot be easily assessed using morphological datasets of extant, phylogenetically distant eutherian, monotreme and metatherian relatives due to the conserved morphology of many skeletal structures—particularly the calcaneus and astragalus—in early mammaliaform taxa.