Cranium of an Eocene/Oligocene pheasant-sized galliform bird from western North America, with the description of a vascular autapomorphy of the Galliformes

Gerald Mayr,James L Goedert,Renate Rabenstein

doi:10.1007/s10336-021-01935-4

Abstract

We describe the fossil cranium of a pheasant-sized galliform land bird from latest Eocene or earliest Oligocene marine rocks of the Jansen Creek Member, Makah Formation (Washington State, USA), which is the only three-dimensionally preserved cranium of a Paleogene representative of the Galliformes. The specimen was freed from a hard calcareous nodule with dilute formic acid. Micro-computed tomography provided further osteological details and a virtual cranial endocast. The fossil exhibits a plesiomorphic temporal morphology, lacking an ossified aponeurosis zygomatica, a feature characterizing some extant Cracidae and most Odontophoridae and Phasianidae. Overall, the fossil is most similar to the skull of the Asian phasianid taxon Arborophila, but this resemblance may well be plesiomorphic for a more inclusive clade. Still, we consider it possible that the fossil represents an archaic member of the Phasianoidea, in which case it would be the earliest record of this taxon from the New World. The fossil exhibits a previously unnoticed cranial autapomorphy of galliforms, a foramen in the temporal region that enables the vena profunda to enter the braincase, for which the name foramen temporale venosum is here introduced. Consistently present in all studied extant galliform taxa and absent in all other extant birds, this foramen enables a vascular connection between the brain and the ophthalmic rete, the latter playing an important role in thermoregulation of the avian brain.

Highlights

Extant Galliformes include the Australasian Megapodiidae, the Neotropic Cracidae, the African Numididae, the New World Odontophoridae (New World quail), and the globally distributed Phasianidae
Cross sectional images generated by micro-CT scans show that the bone walls of the dorsal portion of the neurocranium are very thick in the area of the parietals, as they are in at least some crown group Galliformes, whereas the skull roof of the fossil has thinner bone walls than G. gallus in the area of the frontals (Fig. 3a‒d)
SMF. Olaf Vogel (SMF) Av 666 exhibits sessile basipterygoid processes, which are an apomorphy of galloanserines

Summary

Introduction

Extant Galliformes (landfowl) include the Australasian Megapodiidae (megapodes or mound-builders), the Neotropic Cracidae (chachalacas, guans, and currasows), the African Numididae (guineafowl), the New World Odontophoridae (New World quail), and the globally distributed Phasianidae (quail, pheasants, and allies). Two further similar-sized species, Archaealectrornis sibleyi and Palaeonossax senectus from the early Oligocene Brule Formation of Nebraska and South Dakota, respectively, are only represented by isolated humeri (Wetmore 1956; Crowe and Short 1992). The affinities of these species are poorly resolved. They were assigned to the Cracidae (Procrax and Palaeonossax) and Gallinuloididae (Archaealectrornis), but neither classification is well based and detailed comparisons with Paleogene stem group Galliformes from Europe have not yet been performed (Mayr 2009). Report a previously unrecognized vascular character, which represents a cranial autapomorphy of Galliformes and is likely to be physiological significance in the living species

Materials and methods

Discussion