10 March 2017

Fossil eggs from Portugal show a very slow evolution of the crocodile-like eggs since the Late Jurassic 


New research published in PLoS ONE this week shows that 152 million-year-old fossil eggs from Portugal were laid by close relatives of the crocodile, and that their basic egg types have changed very little ever since. The eggs were found in the Late Jurassic rocks of the coastal cliffs of Western Portugal, in Lourinhã (~70 Km North of Lisbon).

Study made by an international team

The study is made by an international team that includes Marco Marzola, a double degree PhD candidate in vertebrate paleontology from the Department of Geosciences and Natural Resource Management at the University of Copenhagen and NOVA University of Lisbon. Marco Marzola is the coauthor of the study lead by João Russo, MSc in Paleontology and paleontologists Octávio Mateus both from NOVA University of Lisbon and Ausenda Balbino from University of Évora.

The oldest crocodile eggs ever found

The eggs were found in rocks about 152 million years old, making these the oldest crocodylomorph fossil eggs known so far. Crocodylomorphs are the ancestor group to all modern crocodiles. The oldest crocodile-like egg known was from Texas and is dated to the Early Cretaceous (~112 million years old). So the new discovery from Portugal extends back the knowledge of this type of eggs by approximately 40 million years.

Very slow evolution of the crocodile-like eggs

One of the more remarkable aspects of these eggs is how similar they are to modern crocodile eggs, indicating that the evolution of egg morphology has been very slow and changed very little in the last 152 million years of the history of crocodiles and their ancestors. Obviously, there is a degree of variation between different eggs, for instance, on egg size, or microscopic changes in the eggshell, but the fundamental morphological characteristics are the same. The eggs were considered slightly different from any other eggs previously described, therefore considered as two new “oospecies", which are equivalent to new species, but only applied to eggs.

In fact, the fossil record tells us that crocodiles and their relatives (forming the larger group of crocodylomorphs) were much more diverse in the past, with different feeding habits, ecological niche distribution, and morphology.

“This research shows that the eggs of modern crocodiles and their 150-million-year-old ancestors are fairly similar and remained pretty much unchanged throughout the evolution of this group during at least the last 150 million years”, says Marco Marzola.

Furthermore, one of the specimens described is one of the best preserved examples of a crocodilian unhatched clutch, and one of the smallest too, with each egg the size of a thumb. Some eggs had embryo remains, but too fragmentary to identify with further detail. The specimens belong to the fossil collection of the Museu da Lourinhã, in Portugal, already renowned by the extensive and unique record of Late Jurassic dinosaurs bones and eggs, including the presence of nests and embryos, but crocodylomorphs eggs were unknown this far back until now.