A partial skeleton of a new species of Tynskya Mayr, 2000 (Aves, Messelasturidae) from the London Clay highlights the osteological distinctness of a poorly known early Eocene \u201cowl/parrot mosaic\u201d

Gerald Mayr

doi:10.1007/s12542-020-00541-8

Abstract

Tynskya eocaena is an early Eocene bird with a raptor-like skull and semi-zygodactyl feet, whose description is based on a skeleton from the North American Green River Formation. In the present study, three-dimensionally preserved bones of a new species of Tynskya, T. waltonensis, are reported from the London Clay of Walton-on-the-Naze (Essex, UK). The fossils belong to a single individual and provide new insights into the skeletal morphology of messelasturids. In particular, they reveal unusual vertebral specializations, with the cervical vertebrae having concave rather than saddle-shaped caudal articulation facets and the caudalmost thoracic vertebra being platycoelous (flat articular surfaces). The very deep mandible and a derived morphology of the ungual phalanges support a sister group relationship between Tynskya and the taxon Messelastur (Messelasturidae). Phylogenetic analyses of an emended data matrix did not conclusively resolve the higher-level affinities of messelasturids and the closely related halcyornithids, with both taxa sharing derived characters with only distantly related extant taxa (Accipitriformes, Strigiformes, Falconiformes, and Psittaciformes). An analysis that was constrained to a molecular scaffold, however, recovered messelasturids as the sister taxon of a clade including psittaciform and passeriform birds. The derived morphologies of the mandible and cervical vertebrae suggest specialized feeding adaptations of Tynskya, and messelasturids may have exploited a feeding niche, which is no longer available to extant birds.

Highlights

Cenozoic fossils often exhibit mosaic character distributions, which impede a straightforward phylogenetic assignment
Deep mandibular rami were listed in previous diagnoses of the Messelasturidae (Mayr 2005, 2011), the mandible has so far only been described for Messelastur, and the holotype of T. waltonensis for the first time confirms the presence of deep rami for the taxon Tynskya
Other derived features shared by Tynskya and Messelastur include a characteristic tarsometatarsus morphology with a single hypotarsal sulcus and the ungual phalanx of the third toe

Summary

Introduction

Cenozoic fossils often exhibit mosaic character distributions, which impede a straightforward phylogenetic assignment Such is true for the Messelasturidae and Halcyornithidae, which occur in the early Eocene of Europe and North America and combine derived characteristics of owls (Strigiformes), diurnal birds of prey (Accipitriformes and Falconiformes), and parrots (Psittaciformes). Senckenberg Research Institute acquired a partial skeleton of a London Clay messelasturid from the collection of the late Paul Bergdahl (Kirby-le-Soken, UK), which is described in the present study This fossil includes several skeletal elements, whose morphology cannot be assessed in the holotype of T. eocaena, and thereby adds to an improved knowledge of the osteology of the taxon Tynskya and messelaturids in general. Many parts of this specimen are fabricated, it provides new data on some skeletal elements that are absent or poorly preserved in the main slab of the T. eocaena holotype

Material and methods

Discussion