Systematics, biomechanics and ecology of mammals of the Kilmaluag Formation (Jurassic) of Scotland

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

New partial dentaries of amphitheriid mammal Palaeoxonodon ooliticus from Scotland, and posterior dentary morphology in early cladotherians

We describe two partial dentaries of mammals from the Middle Jurassic of Scotland. They belong to the early cladotherian Palaeoxonodon ooliticus. These dentaries comprise the first specimen of P. ooliticus ever found—although its significance was initially unrecognised so it remained undescribed until now—and the most recently discovered specimen, found during fieldwork in 2017. The new specimen preserves part of the coronoid process of the dentary, previously unknown for P. ooliticus, demonstrating the presence of a deep masseteric fossa, with a prominent crest enclosing the fossa anteriorly, and a masseteric foramen, located in the masseteric fossa on the buccal surface of the dentary. On the lingual surface, the mandibular foramen is offset from the Meckel’s sulcus, and positioned below the alveolar plane. These morphologies allow an updated analysis of the phylogenetic position of P. ooliticus, confirming a sister-taxa relationship between Palaeoxonodon and Amphitherium. The position of the mandibular foramen, and the slight extension of the masseteric fossa into the body of the dentary are new autapomorphies for Palaeoxonodon

A reassessment of the postcanine dentition and systematics of the tritylodontid Stereognathus (Cynodontia, Tritylodontidae, Mammaliamorpha), from the Middle Jurassic of the United Kingdom

Tritylodontidae was a successful advanced cynodont clade with a close relationship to mammals, but falling outside the clade Mammaliaformes. Stereognathus ooliticus was the first tritylodontid to be named and described in 1854, but since then no comprehensive description for this species has been produced. A second species, S. hebridicus, was named in 1972 and diagnosed based solely on size difference, being larger than the S. ooliticus holotype. We reexamined all postcanine tooth material attributed to the genus Stereognathus to test the species diagnosis and identify diagnostic morphological characters for this genus. We find no statistical difference in size distribution between S. ooliticus and S. hebridicus postcanine specimens. Specimens previously attributed to the different species fall along an ontogenetic spectrum of size, with no clear clustering. Morphologically, we affirm many previously described features for Stereognathus and identify new morphological features in upper and lower postcanines. We find no morphological features to distinguish these two species, and therefore we synonymize S. hebridicus under S. ooliticus. We reevaluate the scoring of S. ooliticus in previous phylogenetic analyses, generating a new tree using rescored Stereognathus characters. Finally, we suggest that similar reevaluations and redescriptions of other poorly described tritylodontid material are necessary to further clarify relationships among Tritylodontidae and the evolution of characters in derived genera such as Stereognathus.Peer reviewe

Middle Jurassic fossils document an early stage in salamander evolution

Salamanders are an important group of living amphibians and model organisms for understanding locomotion, development, regeneration, feeding, and toxicity in tetrapods. However, their origin and early radiation remain poorly understood, with early fossil stem-salamanders so far represented by larval or incompletely known taxa. This poor record also limits understanding of the origin of Lissamphibia (i.e., frogs, salamanders, and caecilians). We report fossils from the Middle Jurassic of Scotland representing almost the entire skeleton of the enigmatic stem-salamander Marmorerpeton. We use computed tomography to visualize high-resolution three-dimensional anatomy, describing morphologies that were poorly characterized in early salamanders, including the braincase, scapulocoracoid, and lower jaw. We use these data in the context of a phylogenetic analysis intended to resolve the relationships of early and stem-salamanders, including representation of important outgroups alongside data from high-resolution imaging of extant species. Marmorerpeton is united with Karaurus, Kokartus, and others from the Middle Jurassic–Lower Cretaceous of Asia, providing evidence for an early radiation of robustly built neotenous stem-salamanders. These taxa display morphological specializations similar to the extant cryptobranchid “giant” salamanders. Our analysis also demonstrates stem-group affinities for a larger sample of Jurassic species than previously recognized, highlighting an unappreciated diversity of stem-salamanders and cautioning against the use of single species (e.g., Karaurus) as exemplars for stem-salamander anatomy. These phylogenetic findings, combined with knowledge of the near-complete skeletal anatomy of Mamorerpeton, advance our understanding of evolutionary changes on the salamander stem-lineage and provide important data on early salamanders and the origins of Batrachia and Lissamphibia

Combined visual and biochemical analyses confirm depositor and diet for Neolithic coprolites from Skara Brae

Coprolites (fossilized faeces) can provide valuable insights into species’ diet and related habits. In archaeozoological contexts, they are a potential source of information on human-animal interactions as well as human and animal subsistence. However, despite a broad discussion on coprolites in archaeology, such finds are rarely subject to detailed examination by researchers, perhaps due to the destructive nature of traditional analytical methods. Here, we have examined coprolitic remains from the Neolithic (third millennium BCE) settlement at Skara Brae, Orkney, using a range of modern methods: X-ray computed tomography, scanning electron microscopy, lipid and protein analysis (shotgun proteomics of the coprolite matrix as well as collagen peptide mass fingerprinting of isolated bone fragments). This combined approach minimised destructiveness of sampling, leaving sufficient material for subsequent study, while providing more information than traditional morphological examination alone. Based on gross visual examination, coprolites were predominantly attributed to domestic dogs (Canis familiaris), with morphologically identified bone inclusions derived from domestic sheep (Ovis aries) and common voles (Microtus arvalis). Partial dissection of a coprolite provided bone samples containing protein markers akin to those of domestic sheep. Considering the predominance of vertebral and distal limb bone fragments, Skara Brae dogs were probably consuming human butchery or meal refuse, either routinely fed to them or scavenged. The presumably opportunistic consumption of rodents may also have played a role in pest control

The mandible and dentition of Borealestes serendipitus (Docodonta) from the Middle Jurassic of Skye, Scotland

The Middle Jurassic docodont Borealestes serendipitus was the first Mesozoic mammal found in Scotland over 40 years ago. Its affinities and morphology have remained poorly understood. Although multiple dentary fragments and isolated teeth have been recovered from Scotland and England, they have not yet been described in sufficient detail. We report new, more complete specimens collected during recent field work on Skye, Scotland, combined with previously collected material. This includes upper and lower dentition and an almost complete right dentary. We present an updated description and diagnosis of the genus Borealestes, based on high-resolution micro-computed tomography (micro-CT) and synchrotron scans. We identify seven key features that distinguish Borealestes from other docodonts, including a pronounced a–c crest, absence of the a–g crest on cusp a, an anterior fovea at the buccolingual midpoint of the upper molar, and the convergence of the Meckel’s groove with the ventral margin of the mandible. We also present a revised diagnosis for the second species, B. mussettae. Our phylogenetic analysis supports a clade formed by Borealestes, Haldanodon, Docofossor, and Docodon. Ontogenetic variation in the mandibular morphology of Borealestes is similar to that seen in Docodon and Haldanodon, with the delayed emergence of the ultimate lower molar, the shift of the last molar to the front of the coronoid process, and a posterior shift of the Meckel’s sulcus in successively older individuals. This supports a distinctive growth pattern in the clade including Borealestes and Docodon, one that may be present in Docodonta as a whole

First dinosaur from the Isle of Eigg (Valtos Sandstone Formation, Middle Jurassic) Scotland

Dinosaur body fossil material is rare in Scotland, previously known almost exclusively from the Great Estuarine Group on the Isle of Skye. We report the first unequivocal dinosaur fossil from the Isle of Eigg, belonging to a Bathonian (Middle Jurassic) taxon of uncertain affinity. The limb bone NMS G.2020.10.1 is incomplete, but through a combination of anatomical comparison and osteohistology, we determine it most likely represents a stegosaur fibula. The overall proportions and cross-sectional geometry are similar to the fibulae of thyreophorans. Examination of the bone microstructure reveals a high degree of remodelling and randomly distributed longitudinal canals in the remaining primary cortical bone. This contrasts with the histological signal expected of theropod or sauropod limb bones, but is consistent with previous studies of thyreophorans, specifically stegosaurs. Previous dinosaur material from Skye and broadly contemporaneous sites in England belongs to this group, including <jats:italic>Loricatosaurus</jats:italic> and <jats:italic>Sarcolestes</jats:italic> and a number of indeterminate stegosaur specimens. Theropods such as <jats:italic>Megalosaurus</jats:italic> and sauropods such as <jats:italic>Cetiosaurus</jats:italic> are also known from these localities. Although we find strong evidence for a stegosaur affinity, diagnostic features are not observed on NMS G.2020.10.1, preventing us from referring it to any known genera. The presence of this large-bodied stegosaur on Eigg adds a significant new datapoint for dinosaur distribution in the Middle Jurassic of Scotland

The origins of mammal growth patterns during the Jurassic mammalian radiation

We use synchrotron x-ray tomography of annual growth increments in the dental cementum of mammaliaforms (stem and crown fossil mammals) from three faunas across the Jurassic to map the origin of patterns of mammalian growth patterns, which are intrinsically related to mammalian endothermy. Although all fossils studied exhibited slower growth rates, longer life spans, and delayed sexual maturity relative to comparably sized extant mammals, the earliest crown mammals developed significantly faster growth rates in early life that reduced at sexual maturity, compared to stem mammaliaforms. Estimation of basal metabolic rates (BMRs) suggests that some fossil crown mammals had BMRs approaching the lowest rates of extant mammals. We suggest that mammalian growth patterns first evolved during their mid-Jurassic adaptive radiation, although growth remained slower than in extant mammals

The mandible and dentition of Borealestes serendipitus (Docodonta) from the Middle Jurassic of Skye, Scotland

The Middle Jurassic docodont Borealestes serendipitus was the first Mesozoic mammal found in Scotland over 40 years ago. Its affinities and morphology have remained poorly understood. Although multiple dentary fragments and isolated teeth have been recovered from Scotland and England, they have not yet been described in sufficient detail. We report new, more complete specimens collected during recent field work on Skye, Scotland, combined with previously collected material. This includes upper and lower dentition and an almost complete right dentary. We present an updated description and diagnosis of the genus Borealestes, based on high-resolution micro-computed tomography (micro-CT) and synchrotron scans. We identify seven key features that distinguish Borealestes from other docodonts, including a pronounced a–c crest, absence of the a–g crest on cusp a, an anterior fovea at the buccolingual midpoint of the upper molar, and the convergence of the Meckel’s groove with the ventral margin of the mandible. We also present a revised diagnosis for the second species, B. mussettae. Our phylogenetic analysis supports a clade formed by Borealestes, Haldanodon, Docofossor, and Docodon. Ontogenetic variation in the mandibular morphology of Borealestes is similar to that seen in Docodon and Haldanodon, with the delayed emergence of the ultimate lower molar, the shift of the last molar to the front of the coronoid process, and a posterior shift of the Meckel’s sulcus in successively older individuals. This supports a distinctive growth pattern in the clade including Borealestes and Docodon, one that may be present in Docodonta as a whole