A Large Accumulation of Avian Eggs from the Late Cretaceous of Patagonia (Argentina) Reveals a Novel Nesting Strategy in Mesozoic Birds

Abstract

We report the first evidence for a nesting colony of Mesozoic birds on Gondwana: a fossil accumulation in Late Cretaceous rocks mapped and collected from within the campus of the National University of Comahue, Neuquén City, Patagonia (Argentina). Here, Cretaceous ornithothoracine birds, almost certainly Enanthiornithes, nested in an arid, shallow basinal environment among sand dunes close to an ephemeral water-course. We mapped and collected 65 complete, near-complete, and broken eggs across an area of more than 55 m2. These eggs were laid either singly, or occasionally in pairs, onto a sandy substrate. All eggs were found apparently in, or close to, their original nest site; they all occur within the same bedding plane and may represent the product of a single nesting season or a short series of nesting attempts. Although there is no evidence for nesting structures, all but one of the Comahue eggs were half-buried upright in the sand with their pointed end downwards, a position that would have exposed the pole containing the air cell and precluded egg turning. This egg position is not seen in living birds, with the exception of the basal galliform megapodes who place their eggs within mounds of vegetation or burrows. This accumulation reveals a novel nesting behaviour in Mesozoic Aves that was perhaps shared with the non-avian and phylogenetically more basal troodontid theropods.

Revisiting Sabath's "Larger Avian Eggs" from the Gobi Cretaceous

“…The occurrence of possibly clutch-associated adults with the "larger avian eggs" differentiates them from all these examples. Additionally, their distinct clutch configuration differs from that of both Gobioolithus minor, the Soviet collected examples of G. major, and the Neuquén eggs (Fernández et al 2013), suggesting potentially important reproductive differences. Consequently, we here revisit these larger avian eggs of Sabath (1991), providing additional description, photo documentation, and discussion of their taxonomic assignment and implications for reproductive behaviors.…”

“…Styloolithus sabathi also lacks the small tubercles sometimes present on the surface of S. microtuberculatus eggshell (Mikhailov 1991). S. sabathi shares the presence of a possible external zone with Parvoolithus tortuosus and unnamed eggs containing enantiornithine embryos from the Late Cretaceous of Mongolia (Zelenitsky 2004;Balanoff et al 2008;Varricchio et al in press) and Argentina (Schweitzer et al 2002;Fernández et al 2013), as well as a small unnamed egg from the Campanian Bayn Dzak locality, Mongolia (Grellet-Tinner and Norell 2002). S. sabathi differs from all of these eggs by having a larger CL: ML ratio, greater eggshell thickness (except for the Argentine eggshell, which has a nearly equal thickness), and larger egg size.…”

“…Eggs with embryonic remains include Gobioolithus minor with Gobipipus reshetovi (Kurochkin et al 2013), as well as unnamed eggs from the Late Cretaceous of Neuquén, Argentina (Schweitzer et al 2002;Fernández et al 2013), the Sebeş Formation of Romania (Dyke et al 2012), and an isolated egg from the Khugenetslavkant locality, eastern Gobi Desert, Mongolia (Varricchio et al in press). Additionally, the type specimen of Nanantius valifanovi occurred in association with in situ eggshell of Subtiliolithus microtuberculatus (Kurochkin 1996).…”

Aqueous Transport of Gull Nests and Eggs: Taphonomic Implications for Fossil Amniote Nests

“…A nest structure composed of mud or sand, as constructed by some Mesozoic dinosaurs and birds (Mikhailov et al 1994;Chiappe et al 2004;Zelenitsky and Therrien 2008;Fernandez et al 2013), would unlikely be subject to intact transport; however, nests composed of hollow grass stems or twigs could float if inundated by water. For example, Bongiorno (1970) reported rising waters from a spring storm were responsible for transporting intact laughing gull (Larus atricilla) nests in a New Jersey tidal marsh.…”

“…Undoubtedly, each of these structures represented an example of autochthonous preservation, given that transport of intact nests formed entirely of sediment is not possible. In addition to these finds, apparently intact clutches and vertically oriented eggs of Cretaceous dinosaurs and birds have been described but without evidence of associated nest structures (e.g., Sabath 1991; Moratalla and Powell 1994;Fernandez et al 2013;Varricchio et al 2013).…”