The earliest evidence for a supraorbital salt gland in dinosaurs in new Early Cretaceous ornithurines

Xia Wang,Julia A Clarke,Xiaoyu Liu,Yuanchao Hu,Jennifer Peteya,Jiandong Huang

doi:10.1038/s41598-018-22412-8

Abstract

Supraorbital fossae occur when salt glands are well developed, a condition most pronounced in marine and desert-dwelling taxa in which salt regulation is key. Here, we report the first specimens from lacustrine environments of the Jehol Biota that preserve a distinct fossa above the orbit, where the salt gland fossa is positioned in living birds. The Early Cretaceous ornithurine bird specimens reported here are about 40 million years older than previously reported Late Cretaceous marine birds and represent the earliest described occurrence of the fossa. We find no evidence of avian salt gland fossae in phylogenetically earlier stem birds or non-avialan dinosaurs, even in those argued to be predominantly marine or desert dwelling. The apparent absence of this feature in more basal dinosaurs may indicate that it is only after miniaturization close to the origin of flight that excretory mechanisms were favored over exclusively renal mechanisms of salt regulation resulting in an increase in gland size leaving a bony trace. The ecology of ornithurine birds is more diverse than in other stem birds and may have included seasonal shifts in foraging range, or, the environments of some of the Jehol lakes may have included more pronounced periods of high salinity.

Highlights

Supraorbital fossae occur when salt glands are well developed, a condition most pronounced in marine and desert-dwelling taxa in which salt regulation is key
Supraorbital salt gland fossae have only been reported in Late Cretaceous marine birds Ichthyornis dispar and the Hesperornithes (Hesperornis regalis and Parahesperornis alexi14–16) proposed to have a more tern-like and cormorant-like ecologies, respectively
It is similar to Yanornis martini but differs from Gansus yumenensis in possessing a more elongated rostrum, a metatarsal III longer than IV, the length of digit III is subequal to IV, a reduced fibula and the claws of pedal digits III and IV lacking a prominent pendant flexor tubercle

Summary

Introduction

Supraorbital fossae occur when salt glands are well developed, a condition most pronounced in marine and desert-dwelling taxa in which salt regulation is key. We find no evidence of avian salt gland fossae in phylogenetically earlier stem birds or non-avialan dinosaurs, even in those argued to be predominantly marine or desert dwelling. Salt glands have been identified in at least 40 families, representing most traditional orders of birds except the Passeriformes[6] They are most common in marine birds (e.g. gulls, petrels, albatrosses, auks, and penguins), and seen in some more freshwater species (e.g. dabbling ducks, mallards, rails7), desert dwelling taxa (e.g. Ostrich, North African partridge6,8) and in some carnivorous birds with high protein diet (e.g. Tawny Eagles[9]). We propose the new specimens (Figs 1 and 2) and the Gansus zheni holotype specimen are referable to the earlier named species, Iteravis huchzermeyeri[21]

Methods

Findings

Discussion

Conclusion