Smallest fossilized non-avian dinosaur egg

A fossilized egg from a theropod dinosaur, designated Himeoolithus murakamii, has dimensions of approximately 45 x 20 millimetres (1.77 x 0.79 inches) and an estimated mass of just 9.9 grams (0.35 ounces), as described in the journal Cretaceous Research published online on 23 May 2020. The nearly complete egg, along with several partial eggs and hundreds of egg fragments, was discovered in a mudstone layer at the Kamitaki Egg Quarry near Tamba City in Hyogo, Japan. The quarry is located within the upper part of the Ohyamashimo Formation, which dates to between 113 and 100 million years old in the Early Cretaceous period.

There are even smaller examples of avian dinosaur eggs but this record relates specifically to non-avian varieties. Technically, modern-day birds are avian dinosaurs, among which the smallest-known eggs are produced by the vervain hummingbird (Mellisuga minima) of Jamaica and nearby islets; two egg specimens from this species have been documented measuring 10 mm (0.39 in) long with masses of 0.365 g (0.0128 oz) and 0.375 g (0.0132 oz). Historically, even smaller still bird eggs may have existed; a large fragment recovered from amber in the Dominican Republic (aged between 15 and 45 million years old) was extrapolated in its entirety to have been 9 x 6.2 mm (0.35 x 0.24 in) by Poinar et al. (2007).

There have been some scientific reports of avian dinosaur fossilized eggs from the Mesozoic era (c. 250–65 MYA, aka the “age of the dinosaurs”) of similar proportions, such as those of Jinfengopteryx elegans and Compsognathus longipes, however interpretation of these fossils is debated. It’s difficult to ascertain whether the eggs were fully mature or even eggs at all, with some suggesting they may instead be dermal ossifications (bony deposits in the skin).

The mean average thickness of the Himeoolithus egg was 0.15 mm (0.006 in). For such a small egg, it bears an unusually elongated shape, with a length:width ratio of 2.25.

By contrast, the largest known dinosaur eggs unearthed to date are those of oviraptosaurs of the genus Macroelongatoolithus. Fossilized remains of its eggs have measured up to 61 x 17.9 cm (2 ft 7 in) for M. xixiaensis in Asia and 39.8 x 10.2 (1 ft 3.6 in x 4 in) for M. carlylei found in North America. Among extant avian dinosaurs (i.e., birds), the largest eggs are produced by the largest bird, the ostrich (Struthio camelus), with average examples measuring 15–20 cm (6–8 in) long and 10–15 cm (4–6 in) in diameter.

The research was a collaboration between the University of Tsukuba, University of Hyogo and Museum of Nature and Human Activities, Hyogo (all Japan) and the University of Calgary and the Royal Tyrrell Museum of Palaeontology (both Canada) – led by Assistant Professor Kohei Tanaka (Japan). The eggs are now curated at the Museum of Nature and Human Activities, Hyogo.