New evidence for mammaliaform ear evolution and feeding adaptation in a Jurassic ecosystem

Abstract

Stem mammaliaforms are forerunners to modern mammals, and they achieved considerable ecomorphological diversity in their own right. Recent discoveries suggest that eleutherodontids, a subclade of Haramiyida, were more species-rich during the Jurassic period in Asia than previously recognized. Here we report a new Jurassic eleutherodontid mammaliaform with an unusual mosaic of highly specialized characteristics, and the results of phylogenetic analyses that support the hypothesis that haramiyidans are stem mammaliaforms. The new fossil shows fossilized skin membranes that are interpreted to be for gliding and a mandibular middle ear with a unique character combination previously unknown in mammaliaforms. Incisor replacement is prolonged until well after molars are fully erupted, a timing pattern unique to most other mammaliaforms. In situ molar occlusion and a functional analysis reveal a new mode of dental occlusion: dual mortar-pestle occlusion of opposing upper and lower molars, probably for dual crushing and grinding. This suggests that eleutherodontids are herbivorous, and probably specialized for granivory or feeding on soft plant tissues. The inferred dietary adaptation of eleutherodontid gliders represents a remarkable evolutionary convergence with herbivorous gliders in Theria. These Jurassic fossils represent volant, herbivorous stem mammaliaforms associated with pre-angiosperm plants that appear long before the later, iterative associations between angiosperm plants and volant herbivores in various therian clades.

A comparative study on auditory and hyoid bones of Jurassic euharamiyidans and contrasting evidence for mammalian middle ear evolution

“…The anterior process of the malleus in A. allinhopsoni is short and the medial process robust; the latter is unique to A. allinhopsoni , and is unknown in any mammals. The manubrium of the malleus in A. allinhopsoni is a slim bony prong, similar to that of extant mammals but different from that interpreted for Vilevolodon (Luo et al ; see below). Because the robust medial process in A. allinhopsoni was interpreted as homologous to the retroarticular process of the articular, the manubrium in A. allinhopsoni was interpreted as evidence supporting it as a neomorphic structure (Allin & Hopson, ; Clack & Allin, ; Meng et al ), which was also echoed by developmental studies (Fleischer, ; Presley, ; Mallo, ; Sánchez‐Villagra et al ; Takechi & Kuratani, ).…”

“…Vilevolodon was dentally similar to other euharamiyidans from the Yanliao Biota (Zheng et al ; Bi et al ; Mao & Meng, 2019a; Mao et al 2019), but its reconstructed auditory apparatus is distinctive from that of Arboroharamiya (Luo et al ; Fig. ).…”

“…(C–D) Posterodorsal view of the posterior half of the mandible, showing the mandibular foramen, the groove extending from it, and the possible location of the ‘ectotympanic’ as interpreted by Luo et al. (). agp, angular process; cop, coronoid process; gf, glenoid fossa; gmf, groove leading from the mandibular foramen; mc, mandibular condyle; mdf, mandibular foramen; ptf, pterygoid fossa; pm, promontorium; vrpf, ventral ridge of the pterygoid fossa; za, zygomatic arch.…”

“…(C) and (D) are modified from Luo et al. (). Some figures have been flipped for convenience of comparison.…”

“…The discoveries of the auditory apparatus in the ‘haramiyidan’ Vilevolodon (Luo et al ) and Arboroharamiya (Meng et al. ; Han et al ) have extended our knowledge of the mammalian middle ear to ‘haramiyidans’, an ancient but poorly known group of mammaliaforms, and revealed novel morphological structures that have enriched as well as complicated our understanding on the evolution of the mammalian middle ear.…”