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​Parenthood in the Age of Dinosaurs

Your momma’s so old, she’s a Philydrosaur.

On Monday, paleontologists announced the discovery of a fossilized family of philydrosaurs, smallsemi-aquatic reptiles that lived in the Early Cretaceous period (100-146 million years ago). Found in Liaoning Province, China, the family included a clutch of six juveniles accompanied by an adult parent, making it among the oldest evidence of post-natal child-rearing in the fossil record.

"That Philydrosaurus shows parental care of the young after hatching suggests protection by the adult, presumably against predators," wrote the authors of a study of the fossils, recently published in Geosciences Journal.

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"Their relatively small size would have meant that choristoderes [the larger order of which Philydrosaurus is part] were probably exposed to high predation pressure and strategies, such as live birth, and post-natal parental care may have improved survival of the offspring," the researchers added. Though this particular family lived in the Early Cretaceous, the protective maternal instinct among choristoderes may have evolved as early as the Middle Jurassic.

Concept drawing of the Philydrosaurus family. Image: Chung Zhao

It's very rare for paleontologists to find such a clear smoking gun of parental investment in the fossil record. There are, for instance, some ancient examples of animals that incubated their eggs, including the 450-million-year-old crustacean Luprisca incuba. But concrete signs of dedicated child-rearing after the young are hatched or born is harder to come by.

"Making the link between juveniles and adults in the fossil record that would possibly indicate parental care is very difficult," paleontologist Charles Denning, a co-author of the Geosciences Journal, told me over email. "It may have been quite common but the fossil record is rather a blunt tool to use to see behaviour of extinct species."

Some dinosaurs left behind tantalizing circumstantial evidence, such as this exquisite embryo of an early sauropod (or "long-neck" in Disney taxonomy), which dates back 190 million years. The fact that this adorable baby dinosaur was so tiny and ungainly strongly suggests that adult sauropods were needed to fend off predators looking to take a bite out of Little Foot.

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Similar analysis of dinosaur nesting habits, supported by observations of their avian descendents, also points to the possibility that both male and female carnivorous dinosaurs may have been involved in protecting their nests. But again, this is more of an inference based on extant behaviors, and it is limited to the incubation period. How much time non-avian theropods, like tyrannosaurs and raptors, invested in their young once they were hatched remains an open question.

The fossils as found, with the parent (1) and children (2-7). Image: Lü et. al

Indeed, even though the Philydrosaurus fossils provide compelling evidence of post-natal involvement on the part of at least one parent, it is difficult to paint a larger picture of that relationship. When I asked Denning if it would be possible to estimate the age at which the six juveniles would be independent, he told me "not with any certainty."

"You would need a robust growth series to make such an estimate and this is not available as yet," he said.

As with so many paleontological mysteries, reconstructing the details of prehistoric parenthood simply requires a bigger set. Luckily, the 21st century is shaping up to be a fruitful time for the field.

"One of the great things about paleontology is that we're learning new things at an astronomical rate," Steve Brusatte, a prolific paleontologist based out of the University of Edinburgh, told me last year. "People are finding a new dinosaur species somewhere around the world once a week now."

As the analysis of the Philydrosaurus family has proven, every new discovery adds a new piece to the evolving puzzle of the deep past. Considering the fascinating sliding scale of parental involvement that has been observed across the extant world—from "superprecocial" animals requiring no parental guidance whatsoever, to our own species's laughably defenseless babies—these long lost ecosystems must have some exciting stories to tell.