Two fossilized enantiornithine eggs/Gareth Dyke
From ScienceDaily:
Now in the collections of the Transylvanian Museum Society in Cluj Napoca, Romania, the samples date from the late-Cretaceous period (approx. 70 million years ago) and were discovered near the city of Sebeş in Transylvania by local palaeontologist Mátyás Vremir about nine years ago.
Led by Centro Regional Universitario Bariloche in Argentina, the scientists examined sophisticated electron microscope images of the unique, fossilised material from the site. They established it contains four different types of egg shell, indicating that four types of animals all shared the same nesting site; extinct birds within a group known as enantiornithes, birds of undetermined classification, gecko-like lizards and smaller predecessors of today’s crocodiles.
Christian Laurent, Tizard Scholar and member of the Aerodynamics and Flight Mechanics Group at the University of Southampton, comments: “We know very little about the parental behaviour of Mesozic birds, We know they had nests, laid eggs and hatched young which were relatively mature and able to move around after hatching — but evidence is scant beyond this. This research suggests they were tolerant of creating their nests, not only alongside other birds, but also reptiles.”
You are probably wondering aboout the same thing we are, right? Well, here’s the answer:
Christian Laurent adds: “Evidence supporting our theory can still be seen today in Argentina, where lizards (Salvator merianae) co-habit and lay eggs inside the nests of the caiman crocodile — safe in the knowledge that the female doesn’t feed during the incubation of her eggs and poses no threat to the hatchling lizards.” Paper.(open access) – Mariela Soledad Fernández, Xia Wang, Mátyás Vremir, Chris Laurent, Darren Naish, Gary Kaiser, Gareth Dyke. A mixed vertebrate eggshell assemblage from the Transylvanian Late Cretaceous. Scientific Reports, 2019; 9 (1) DOI: 10.1038/s41598-018-36305-3 More.
The ability to share nesting sites that way increased the overall biomass then and now.
See also: Did prehistoric reptiles care for their offspring?
and
Animal parenting at 508 mya