What is so surprising about the fossil of the Propanoplosaurus baby?

Painting of baby Propanoplosaurs, showing armor.

The artist's reconstruction of the baby nodosaru includes armor typical of adults. Click to zoom.

The fossil of the baby Propanoplosaurus has bones, but no evidence of armor.

Baby Propanoplosaurus fossil with a sketch of the probable position in death. The fossil shows no evidence of armor. Click to zoom.

All known nodosaurs were armored, but the impression fossil of the Propanoplosaurus baby doesn't appear to have any armor. What's going on? The hard, bony armor of nodosaurs was readily preserved and has been found associated with the bones of adults of all species, so the lack of armor on the baby's skeleton is surprising. Scientists have two hypotheses. It could be that the skeleton of the dead baby nodosaur slipped out of its armored skin before the skeletal impression formed. It is also possible that nodosaurs were born without armor and developed it as they grew. We know that other types of dinosaurs were born without some of the features that they had as adults. For example, Pachycephalosaurus, famous for its domed skull with knobby protuberances, lacked the dome when it was young. Likewise, the horns and neck frill of Triceratops species were different in young, mature, and old individuals.