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
