A drowned Mesozoic bird breeding colony from the Late Cretaceous of Transylvania

Abstract

Despite a rapidly improving fossil record, the reproductive biology of Mesozoic birds remains poorly known: only a handful of undisputed, isolated Cretaceous eggs (some containing embryonic remains) are known. We report here the first fossil evidence for a breeding colony of Mesozoic birds, preserved at the Late Cretaceous (Maastrichtian) Oarda de Jos (Od) site in the Sebeş area of Transylvania, Romania. A lens of calcareous mudstone with minimum dimensions of 80 cm length, 50 cm width and 20 cm depth contains thousands of tightly packed, morphologically homogenous eggshell fragments, seven near-complete eggs and neonatal and adult avialan skeletal elements. Eggshell forms 70-80 % of the matrix, and other fossils are entirely absent. The bones exhibit clear characters of the Cretaceous avialan clade Enantiornithes, and the eggshell morphology is also consistent with this identification. Both taphonomy and lithology show that the components of this lens were deposited in a single flood event, and we conclude that it represents the drowned remains of a larger enantiornithine breeding colony, swamped by rising water, washed a short distance and deposited in a shallow, low-energy pond. The same fate often befalls modern bird colonies. Such a large concentration of breeding birds suggests aquatic feeding in this species, augments our understanding of enantiornithine biology and shows that colonial nesting was not unique to crown birds.

Taphonomical and palaeoecological investigation of the Late Cretaceous (Maastrichtian) Tuştea vertebrate assemblage (Romania; Haţeg Basin) - insights into a unique dinosaur nesting locality

“…Such inferences were also used to genetically link enantiornithin skeletal remains and ratite-type eggshells in a coquina-like accumulation from the uppermost Cretaceous of the Tansylvanian Basin that was interpreted as a reworked bird nesting colony by Dyke et al (2012).…”

Sedimentological Analyses of Eggshell Transport and Deposition: Implication and Application to Eggshell Taphonomy

“…Oser and Jackson (2014) further supported this hypothesis by Eberth (1990) through observations of abrasion present on these eggshells. On the other hand, fossil eggshells associated with fine sediment (Maxwell and Horner 1994;Kennedy 1997;Dyke et al 2012;Moreno-Azanza et al 2014) (Table 1) may have been transported by nonhydraulic mechanisms, such as mud flow.…”

“…Knowing the hydraulic equivalence of particles permits the reevaluation of eggshell assemblages and the size of associated sedimentary particles in previous studies (e.g., Erben et al 1979;Maxwell and Horner 1994;Kennedy 1997;Liang et al 2009;Dyke et al 2012) ( Table 1). Presence of hydraulically equivalent particles in the associated sediment would suggest that a flow transported and then deposited both the eggshells and the particles.…”

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

“…Experiments with domestic chicken (Gallus gallus) eggs demonstrated that both hydrated (simulating fresh eggs) and partially dehydrated eggs (simulating incubated eggs) placed on a beach were readily recruited by rising tides; however, only partially dehydrated eggs floated (Hayward et al 1997). These examples provide modern analogs of fossilized eggs that underwent transport prior to preservation (e.g., Dyke et al 2012).…”