A fossilized bone unearthed two decades ago in northeastern Japan belongs to one of the smallest iguanodontian dinosaurs ever found in the country’s Cretaceous marine rocks, according to new research led by the University of Tsukuba.
Reconstruction of the iguanodontian dinosaur from the Ashizawa Formation in Japan; the red bone is the right sternal. Image credit: Masato Hattori.
“Iguanodontia is a major group of small- to large-bodied ornithopod dinosaurs — approximately 2 m (6.6 feet) to greater than 10 m (33 feet) in length — that existed from the Late Jurassic through the latest Cretaceous,” said University of Tsukuba paleontologist Masami Kondo and colleagues from Japan and Canada.
“Iguanodontians attained a worldwide distribution during this time, and their fossils are known from every continent.”
“Asia was an important region for their diversification across Laurasia, as dispersals occurred both between Asia and North America and between Asia and Europe.”
“A high diversity of iguanodontian species has been recovered from the Cretaceous of eastern and central Asia.”
“The increased number of reports of iguanodontian fossils from Japan over the past two decades is significant, as this region represents an easternmost margin of Cretaceous Laurasia.”
In the study, Dr. Kondo and co-authors re-examined an isolated right sternal plate excavated in 1999 near Iwaki in Japan’s Fukushima prefecture.
The fossil comes from the Ashizawa Formation, part of the Futaba Group, a roughly 90-million-year-old layer of sandstone and mudstone deposited on an ancient seabed.
The specimen had sat unstudied for over two decades after being tentatively labeled a hadrosauroid (duck-billed) dinosaur bone.
The paleontologists determined the hatchet-shaped bone belongs to an indeterminate styracosternan, a broad group within Iguanodontia that includes hadrosauroids and their close relatives.
Its features closely resemble those seen in more derived iguanodontian species rather than in primitive ones.
Based on comparisons with the sternal bones of related species, the researchers estimate this iguanodontian dinosaur measured only about 3 m (10 feet) in body length, making it small by the standard classification scheme for these dinosaurs.
“This is significantly smaller than that of other iguanodontian fossils previously unearthed from the same formation,” they said.
“Because larger iguanodontians, including hadrosauroids, are known to have existed in Japan during the Late Cretaceous, the discovery suggests that iguanodontians of varying body sizes inhabited the coastal environments of East Asia.”
“The finding provides new insights that further corroborate the diversity of ancient coastal ecosystems.”
The team’s results were published on July 6 in the journal Paleontological Research.
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Masami Kondo et al. 2026. An iguanodontian sternal plate from the Upper Cretaceous Ashizawa Formation (Futaba Group) of Fukushima Prefecture, Japan. Paleontological Research 30: 122-134; doi: 10.2517/prpsj.250042
