Description and Classification

Planicoxa was a medium-sized herbivorous dinosaur, estimated to be about 6 meters (20 feet) long and weighing around 1 ton. Like other iguanodontians, it likely walked on its two strong hind legs most of the time but could also move on all fours. It had a long, stiff tail to help with balance and a head with a beak for cropping plants, along with teeth further back in its jaws for chewing.

Planicoxa belongs to the group Ornithopoda, which includes other beaked plant-eaters like Camptosaurus and the later duck-billed dinosaurs (hadrosaurs). More specifically, it is classified as an iguanodontian, placing it as a more advanced relative of Iguanodon but not as specialized as the hadrosaurs. It is considered a type of styracosternan, a group known for their robust builds and features that were leading towards the hadrosaur lineage. Its discovery provided more insight into the diversity of ornithopods in North America during the Early Cretaceous.

Distinguishing Features

Planicoxa can be identified by several specific characteristics of its skeleton, especially its hip bones:

• The most notable feature is the flat, horizontal surface on the part of the ilium (the largest hip bone) that is behind the hip socket. This is the “flat hip” that gives Planicoxa its name.
• The ilium also has a relatively short and stout projection pointing forward.
• Compared to some other iguanodontians, its metatarsals (foot bones leading to the toes) were somewhat shorter and more robust.
• It likely possessed a thumb spike, a feature characteristic of many iguanodontians, which would have been used for defense or foraging.

Paleoenvironment and Diet

Planicoxa lived in what is now Utah during the Early Cretaceous period. The environment of the Cedar Mountain Formation at that time included river systems, floodplains, and open woodlands. The climate was likely warm with distinct wet and dry seasons. This ecosystem supported a diverse range of dinosaurs.

As an herbivore, Planicoxa would have fed on various plants available in its habitat. Its beak would have been useful for snipping off leaves, ferns, cycads, and possibly young conifers. It lived alongside other dinosaurs such as the armored dinosaur Gastonia, sauropods like Cedarosaurus and Venenosaurus, and predatory theropods like Utahraptor. These animals formed a complex food web within the Early Cretaceous landscape of North America.

Significance and Ongoing Research

The discovery of Planicoxa has been important for understanding the evolution and diversity of iguanodontian dinosaurs, particularly in North America. It helps fill a gap in the fossil record between earlier ornithopods and the later, highly successful hadrosaurs (“duck-billed dinosaurs”). The unique features of its hip bones provide valuable information for paleontologists studying how different ornithopods moved and were related to each other.

Ongoing research related to Planicoxa and its relatives includes further analysis of its anatomy to refine its position within the ornithopod family tree. New fossil discoveries in the Cedar Mountain Formation continue to shed light on the animals that lived alongside Planicoxa and the environment they inhabited. Continued study helps scientists piece together the story of dinosaur evolution during the Cretaceous period.