Jakapil

MEANING: Shield bearer

PERIOD: Late Cretaceous

CONTINENT: South America

Jakapil is a basal thyreophoran dinosaur with large low osteoderms and a bipedal stance. These unique characteristics put its phylogenetic placement into question, and it is under some debate. Jakapil was about 1.5 m long and 4.5-7 kg in weight. It likely processed tough plant material through chewing, as evidenced by visible wear on the teeth.

Jakapil is from the Late Cretaceous. The Cretaceous is the third and final geological period of the Mesozoic Era, with the Late Cretaceous making up roughly the second half of it, lasting from about 100 to 66 million years ago. It was a time of significant evolutionary change, with dinosaurs reaching their greatest diversity before the mass extinction at the end of the Cretaceous.

The Cretaceous was a period with a relatively warm climate, though the Late Cretaceous experienced a global cooling trend, caused by falling levels of carbon dioxide in the atmosphere. The continents were nearing their present positions, but high sea levels flooded low-lying regions, turning Europe into an archipelago, and forming the Western Interior Seaway in North America. These seas were home to a variety of marine reptiles, including mosasaurs and plesiosaurs, while pterosaurs and birds shared the skies.

On land, dinosaurs continued to thrive and diversify during the Late Cretaceous, producing many of the most well-known groups, including tyrannosaurs, hadrosaurs, and pachycephalosaurs. Established Cretaceous dinosaur clades like the ceratopsians, ankylosaurs, and dromaeosaurs continued to flourish. Sauropod species consisted almost exclusively of titanosaurs, which seemed to be confined to the Southern Hemisphere for much of the Late Cretaceous. Flowering plants and grasses diversified and spread, becoming the dominant flora similar to what we see today.

The Cretaceous (along with the Mesozoic) ended with the Cretaceous-Paleogene (K-Pg) extinction event, a large mass extinction in which many groups, including non-avian dinosaurs, pterosaurs, and large marine reptiles, died out. This event, likely triggered by an asteroid impact, is marked by the abrupt K-Pg boundary, a distinct geologic layer separating the Mesozoic and Cenozoic Eras. In its aftermath, mammals and avian dinosaurs rapidly diversified, becoming the dominant land animals of the Cenozoic Era.

Jakapil is a basal thyreophoran. Thyreophora is a group of ornithischian dinosaurs best known for their body armor, a feature that defines the clade and eventually led to the evolution of two derived subgroups: stegosaurs and ankylosaurs. While these later forms became large, heavily built quadrupeds covered in elaborate defensive structures, the earliest thyreophorans were quite different. Basal members were small, likely bipedal or facultatively quadrupedal animals. These early species possessed simple rows of osteoderms - bony plates embedded in the skin - along their backs and flanks, offering protection from predators without the extreme specialization seen in their later relatives.

Fossils of basal thyreophorans have been found in a wide range of locations, suggesting that the lineage diversified quickly during the Early Jurassic and possibly even earlier. Their anatomy suggests a combination of primitive and derived traits, giving researchers insight into how armor and body plan changes evolved over time. Unlike the robust ankylosaurs or the spike-bearing stegosaurs, these early thyreophorans were light and mobile, representing the foundation of what would become one of the most visually distinctive lineages of dinosaurs.