Utatsusaurus hataii

Primitive Features that Retain Traces of a “Lizard”

Compared to ichthyosaurs that appeared in later eras, Utatsusaurus had an “intermediate body” that strongly retained the characteristics of its terrestrial ancestors.
It reached a length of 2 meters up to a maximum of 3 meters, combining features of land animals and evolved ichthyosaurs.

Bone and Limb Characteristics: A Transitional Evolutionary Stage

Primitive Flippers

It had a long, slender body like a lizard, and its rear flippers were almost the same size as its front flippers.
This is in contrast to later ichthyosaurs, where the forelimbs became noticeably longer.

Pelvic Traces

Because it represents the earliest stage of evolution, it retained faint traces of a pelvis (pelvic girdle) that would be lost in later ichthyosaurs.
Although the connection between the pelvic girdle and the spine was weak, it showed slight characteristics of a land animal.

Limb Homogeneity

Its humerus and femur were almost the same length, showing characteristics of a transitional stage between the two extremes: later ichthyosaurs (with long humeri) and terrestrial amniotes (with long femurs).

Head and Swimming Style

Skull Morphology

Its skull was slenderer than the rounded skulls of later ichthyosaurs, and it had a long snout and large eyes.
Its mouth was lined with sharp teeth well-suited for catching prey.

Swimming Method

It did not yet have the dorsal fin or sufficient caudal fin that evolved ichthyosaurs possessed. It is presumed to have swum by undulating its long body like an eel.

Adaptation to Homeothermy and the Debate on Classification

Despite its primitive characteristics, studies of Utatsusaurus’s bone tissue and simulations suggest that it had already acquired a homeothermic (warm-blooded) metabolism and was actively swimming in the open ocean.
This fact is important evidence showing that the Ichthyosauria group underwent dramatic aquatic adaptation in a very short period during the Early Triassic.

Evolutionary Debate

In 1998, a research team led by Ryosuke Motani of UC Berkeley proposed a phylogenetic analysis suggesting that Utatsusaurus was closely related to lizard-like diapsids, and that Ichthyopterygia were distant relatives of lizards, snakes, and crocodiles.
However, another research team reported in 2013 that Ichthyopterygia are a monophyletic group, with Utatsusaurus representing its basal (most primitive) classification.

Historical Significance and Discovery Location

Nearly complete skeletons of Utatsusaurus have been discovered in Minamisanriku Town (formerly Utatsu Town) and Ogatsu Town (now Ishinomaki City), Miyagi Prefecture.
Minamisanriku Town features many geological layers from the Permian to the Jurassic periods, and Utatsusaurus, as the oldest ichthyosaur species that inhabited the waters near Japan, is key to unraveling the subsequent evolutionary history of ichthyosaurs.

While the holotype specimen is housed at the Tohoku University Museum, a portion of the fossil was stolen in a grave robbing incident at the excavation site in Tatezaki, Utatsu Town, in 1990.
The Tatezaki excavation site in Minamisanriku Town is currently designated as a National Natural Monument, along with the fossil itself.