You don’t have to be an expert on the fossil birds of the Mesozoic Era to know that our knowledge of these animals has increased exponentially in recent years. An extraordinary number of new species have been described, we’ve learnt a great deal about their anatomy thanks to spectacular new fossils – some of which are even preserved in amber – and we’ve gained insights into their behaviour and ecology thanks to discoveries made about their teeth, feathers, stomach contents, phylogeny and the environments in which they lived.
Among the most important and species-rich of Mesozoic bird groups are the enantiornithines, or ‘opposite birds’, so named because a few aspects of their skeletal anatomy (the way their shoulder and chest bones fit together in particular) are unusual relative to those of modern birds. Enantiornithines are known from rocks worldwide and were present from the start of the Cretaceous until its close 66 million years ago. They might even have evolved in the Jurassic, in which case older members of the group await discovery.
Among the many interesting things discovered recently about enantiornithines is that at least some of them were colonial nesters. In a Naturwissenschaften paper published in 2012, Gareth Dyke, Mátyás Vremir, Gary Kaiser and myself reported a remarkable fossil assemblage: a big, lens-shaped mass of calcareous mudstone (about 80 cm long, 50 cm wide and 20 cm deep) packed with literally thousands of enantiornithine eggshell fragments. A few enantiornithine bones were present as well, but eggshell fragments form 70-80% of the entire mass (Dyke et al. 2012).
This eggshell mass was found at Oarda de Jos near Sebeş, Transylvania, western Romania and comes from the latest Cretaceous Sebeş Formation (Dyke et al. 2012). Pleurodire turtles, azhdarchid pterosaurs and such dinosaurs as ornithopods and the peculiar Balaur bondoc (which is probably a flightless member of the bird lineage, not a dromaeosaur as originally proposed: see Cau et al. 2015) also come from the Sebeş Formation. I wrote about the discovery and initial interpretation of the eggshell assemblage at TetZoo ver 3, though (sigh) it now appears without any of its images.
As described in our 2012 paper, the assemblage is a fairly big deal, since it means that colonial nesting was practised by at least some enantiornithine species, is not unique to crown-birds, and evolved in birds more stem-ward than other colonially nesting birds (Naish 2014).
But as revealed in our new paper, published in Scientific Communications this past week and led by Mariela Soledad Fernández, things turn out to be a bit more complex than we originally thought. Rather than consisting of enantiornithine eggshell fragments and bones alone, the assemblage actually contains eggshell fragments (and probably bones) of several different, additional animal groups. Namely, gekkotan lizards, crocodylomorphs, and a bird different from the enantiornithine otherwise so well represented in the assemblage (Fernández et al. 2019).
It should be noted to start with that enantiornithine remains dominate the assemblage by far, around 70% of the sampled eggshell fragments belonging to that group (and presumably to the same one species). Sadly, the gekkotan and crocodylomorph eggshell fragments aren’t informative enough to tell us anything particularly interesting about the species concerned, other than that their remains are present. The Oarda de Jos crocodylomorph eggshell is different in thickness and microscopic surface texture from crocodylomorph eggshell fragments reported from the Upper Cretaceous of the USA and Brazil and is most similar to fossil eggs from the Eocene of Colorado, called Krokolites wilsoni (Hirsch 1985). But we don’t have a good handle on what sort of crocodylomorph we’re talking about. After the enantiornithine, its eggs are the most abundant in the sample, forming about 28% of the assemblage. Those artistic reconstructions previously created for the location are thus in error: they really should have at least a few crocodylomorphs in view.
The gekkotan eggshell pieces – they have an eggshell morphology termed ‘geckoid’ – have features in common with the eggs of modern geckos, and hence were presumably produced by crown-geckos (Fernández et al. 2019). Beyond that, we can say no more. Less than 1% of the eggshell in the assemblage comes from this lizard, so we’re not saying that its eggs are abundant in the sample.
Similarly, the second bird in the assemblage appears (from microscopic details of eggshell anatomy) similar to crown-birds – more so than to enantiornithines – but cannot be matched with any specific bird group and might represent something new (Fernández et al. 2019). So, we don’t know exactly what sort of bird we’re dealing with. Again, less than 1% of the eggshell in the assemblage is of this type, so it’s rare in our sample.
Piecing all of this together, what does it all mean? If our enantiornithine eggshell assemblage doesn’t involve enantiornithines alone, but also includes the remains of a second bird, a lizard and a crocodylomorph… do we have an example here of a multi-species nesting colony, perhaps one involving so-called parasitic nesting or even communal or cooperative nesting?
Several examples of this sort of thing are known for the modern world. They involve turtles laying their eggs at the edges of crocodylian nests, South American geckos which lay their eggs within the nests of cormorants and gulls, and seemingly harmonious nesting associations involving rheas, tinamous and penguins. Maybe the behaviours listed here occurred in Maastrichtian Romania. Perhaps the gekkotan lizard concerned was cheekily laying its (hard-shelled, perhaps sticky-shelled) eggs at the edges of enantiornithine nests, and maybe the mystery bird and crocodylomorph were non-threatening enough to be tolerated, their nests perhaps being close to those of the abundant enantiornithines (Fernández et al. 2019).
It may also be that the association we report is not quite as interesting as just described, but more to do with geological and hydrodynamic processes. That is, that the eggshell fragments and bones concerned became associated due to their presence in the same general floodplain area, their remains becoming mixed together by the actions of floodwater and not being all that informative as goes behaviour and ecology. Even if this is true, however, we can at least say that these animals were nesting in the same area and environment, and seemingly in proximity. That alone is interesting, and there are indications from elsewhere in the Cretaceous fossil record that it might have been a fairly regular occurrence.
Our paper is open access (OA) and available here; it’s one of several technical papers I hope to see published in the year. On that note, here’s a reminder that I’m not a salaried academic researcher and that any contribution I make to the technical literature is done in my own time. Thanks to those who support me via pledges at patreon, and please consider doing so if you don’t already.
For previous TetZoo articles relevant to Mesozoic birds, the Late Cretaceous animals of Romania and other relevant issues, see…
A drowned nesting colony of Late Cretaceous birds, May 2012 (now missing all illustrations)
A new azhdarchid pterosaur: the view from Europe becomes ever more interesting, January 2013 (now missing all illustrations)
Bird behaviour, the ‘deep time’ perspective, January 2014
While compiling this list I’ve discovered that essentially ALL of the TetZoo articles on enantiornithines are hosted at sites (ScienceBlogs and SciAm) that have stripped them of their original images. I must therefore make some effort to republish them here, with their pictures. Great, more stuff to do.
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Cau, A., Brougham, T. & Naish, D. 2015. The phylogenetic affinities of the bizarre Late Cretaceous Romanian theropod Balaur bondoc (Dinosauria, Maniraptora): dromaeosaurid or flightless bird? PeerJ 3: e1032.
D’Ami, R. D., Invernici, F. & Quochi, G. 1969. Animals of Lake and Marsh. Casa Editrice AMZ, Milan.
Hirsch, K. F. 1985. Fossil crocodilian eggs from the Eocene of Colorado. Journal of Paleontology 59, 531-542.