Postcrania of Borealestes (Mammaliformes, Docodonta) and the emergence of ecomorphological diversity in early mammals

Abstract

The Middle Jurassic witnessed the early diversification of mammal groups, including the stem-mammalian clade, Docodonta. Recent discoveries in China indicate docodontans exhibited ecomorphological diversity akin to small-bodied mammals living >100 million years later, in the Cenozoic. Our understanding of the emergence of this ecological diversity is hindered by a lack of Middle Jurassic fossil material from other parts of the world. The two partial postcranial skeletons of Borealestes described here come from the Kilmaluag Formation, Scotland. These are the most complete Mesozoic mammaliaform skeletons currently known from the UK, and among the best preserved in Europe. As an early member of Docodonta, Borealestes provides key anatomical information for understanding the clade's evolution, and the emergence of mammaliaform ecomorphological diversity. Using digital reconstructions from micro-CT and synchrotron scans, we describe the postcranial anatomy of Borealestes and provide an updated phylogenetic analysis incorporating cranial and postcranial characters. We find Borealestes species form a sister group to a clade comprising Agilodocodon and Microdocodon. To complement observational analyses of the skeleton, we carry out principal components analyses using 3D landmarks on a comparative dataset of 42 extant mammal taxa. Our results indicate Borealestes lacked specializations for derived locomotor behaviour. We detect some similarity in the humerus between Borealestes and Ornithorhynchus. Borealestes is morphologically intermediate between the robust morphology of fossorial and semi-fossorial/semi-aquatic Haldanodon and Docofossor, and the gracile morphology for scansorial Agilodocodon and Microdocodon. We suggest ecological diversity in Docodonta may arise from an unspecialized basal bauplan, of which Borealestes may be representative.

Adaptive landscapes unveil the complex evolutionary path to mammalian forelimb function and posture

“…Basal cynodonts generally occupy the central region of morphospace, as do the two more derived eucynodont subclades, cynognathians and probainognathians, but in both subclades some species independently move into the therian region of morphospace (see Supplementary Figure 3). Some mammaliaforms - Eozostrodon and Megazostrodon 27 - fall within therian morphospace, but others - Borealestes 28 - do not (Figure 1C, see Supplementary Figure 3). The two stem therians included here fall well within the range of extant, crown-therian morphologies (Figure 1A, see Supplementary Figure 3).…”

The fossil record of appendicular muscle evolution in Synapsida on the line to mammals: Part II—Hindlimb

“… Anterolateral fibula and dorsal fibulare Lateral aspects of calcaneum and base of metatarsal V [process(es)] Lateral aspect of distal metatarsal V Plantar aspect of calcaneal [tuber] and lateral aspect of metatarsal V Plantar aspect of metatarsal I, and possibly also entocuneiform Remarks —The calcaneum of early amniotes forms a thin plate of bone laterally, a basic structure that persisted (albeit reduced in size) on the line to mammals to form the “peroneal shelf” (Luo & Wible, 2005; Panciroli et al, 2021). The margin of this plate in large, well‐preserved “pelycosaur” calcanei often preserve fine rugosities or a striated surface texture indicative of soft tissue attachment (Figure 28a,b), possibly at least in part the insertion of the PL (state 1).…”

“…Remarks-The calcaneum of early amniotes forms a thin plate of bone laterally, a basic structure that persisted (albeit reduced in size) on the line to mammals to form the "peroneal shelf" (Luo & Wible, 2005;Panciroli et al, 2021). The margin of this plate in large, well-preserved "pelycosaur" calcanei often preserve fine rugosities or a striated surface texture indicative of soft tissue attachment (Figure 28a,b), possibly at least in part the insertion of the PL (state 1).…”

Synchrotron tomography of a stem lizard elucidates early squamate anatomy

“…Identification of Bellairsia as a stem-squamate provides support for the transitional nature of the Middle Jurassic tetrapod assemblages, which retained components of archaic lineages alongside early members of the living crown groups, not only among squamates and other lepidosaurs, but also in mammals and amphibians. For example, Bellairsia co-occurs at both Kirtlington and the Elgol SSSI alongside the stem-lepidosaur Marmoretta, stemmammals such as docodonts, and stem-salamanders such as Marmorerpeton 18,23,[36][37][38][39] . Our phylogenetic analysis indicates that some of these stem-squamate elements persisted longer still, and with greater diversity than previously recognised.…”