Looks like it's dino season, y'all!

Last December, a 66 million-year-old dinosaur embryo was found inside a fossilized egg in China. And last week, an Egyptian-American team of researchers announced that they had discovered a new kind of predatory dinosaur from a famous fossil site in the Sahara Desert.

Now, a team of researchers from Delhi University, India, has discovered an abnormal 'egg-in-egg' dinosaur egg, a condition known as ovum-in-ovo or multi-shelled eggs, in the state of Madhya Pradesh's Dhar district, for the first time in fossil history.

According to the researchers, the discovery is a "rare and important find" as no 'ovum-in-ovo' egg has been found in dinosaurs and reptiles like lizards and crocodiles hitherto.

The new finding, which indicates that the ovum-in-ovo pathology is not unique to birds and that dinosaurs could have a similar reproductive function, has been published in the journal Scientific Reports.

The eggs that were discovered in the Dinosaur Fossil National Park belong to a species of titanosaurs, a group of sauropod dinosaurs. The microstructure of the pathological egg and that of an adjacent egg in the same nest identified it with that of titanosaurid sauropod dinosaurs, according to a release by the University.

A reproductive system reminiscent of the ovum-in-ovo pathology of birds

During their dig, scientists found 52 nests of titanosaur sauropods near Padlya village close to Bagh town. Among them, they found a sauropod dinosaur nest consisting of 10 eggs, including the abnormal egg, which had two continuous and circular eggshell layers separated by a wide gap reminiscent of ovum-in-ovo pathology of birds.

In the past, it was suggested that dinosaurs had a reproductive function that was similar to that of turtles and other reptiles. According to the release, this was in contrast to the segmented reproductive tract of crocodiles and birds with separate regions of membrane and shell deposition.

Although crocodiles have separate regions of shell membrane and mineralized shell deposition, like turtles and other reptiles, they ovulate and release all the eggs simultaneously, as opposed to the sequential ovulation of birds, which lay one egg at a time. 

“The finding of an ovum-in-ovo egg from a titanosaurid nest opens up the possibility that sauropod dinosaurs had an oviduct morphology similar to those of crocodiles or birds, and they may have adapted to a mode of the egg-laying characteristic of birds,” said DU researcher Dr. Harsha Dhiman, lead author of the paper.

108 titanosaur nests were identified

According to paleontologists, birds-like behavior has been observed in the fossil record of dinosaurs in the form of nest construction and evidence of parental care. 

The Upper Cretaceous Lameta Formation of Central India has been long known for the discovery of dinosaur fossils (both skeletal and egg remains).

The region of Dhar, in fact, made news in 2007 when the first evidence of titanosaur species roaming the area was found. According to Wion, conservation work has progressed in the area since 2007.

The eggs were found in the lower Narmada valley's sandy limestone/calcareous sandstone. During multiple field works conducted by the authors in the Bagh-Kukshi areas of the Dhar district, 108 titanosaur nests were identified, including intact clutches, isolated eggs, and several eggshell fragments.

Abstract: Pathologic eggs have been documented in the amniote eggs of birds, turtles, and dinosaurs. These eggs occur either in the form of one egg within another egg, a condition known as ovum-in-ovo, or multi-shelled eggs showing additional pathological eggshell layer/s besides the primary shell layer. Though multi-shelled eggs and eggshells were previously recorded only in reptiles and ovum-in-ovo eggs in birds, now it has been shown that multi-shelled egg pathology occurs in birds as well. However, no ovum-in-ovo egg has been reported in dinosaurs or for that matter in other reptiles. Here we describe an ovum-in-ovo pathological egg from a titanosaurid dinosaur nest from the Upper Cretaceous Lameta Formation of western Central India which makes it the first report of this pathology in dinosaurs. Birds possess a specialized uterus while other amniotes have a generalized uterus. However, alligators and crocodiles retain a specialized uterus like birds along with a reptilian mode of egg-laying. The discovery of ovum-in-ovo egg from a titanosaurid dinosaur nest suggests that their oviduct morphology was similar to that of birds opening up the possibility for sequential laying of eggs in this group of sauropod dinosaurs. This new find underscores that the ovum-in-ovo pathology is not unique to birds and sauropods share a reproductive behavior very similar to that of other archosaurs.