Jurassic handprint shows link to birds

Marks left in the mud next to an ancient Jurassic lake by a carnivorous dinosaur and preserved as fossils; reveal that dinosaurs had hands that were not suitable for walking on very early in their evolution. In a report published in the online paleontological journal PLos ONE, a team of US scientists concluded that fossil handprints, perhaps the best theropod handprints discovered to date, indicate that these dinosaurs abandoned the use of their forelimbs. as legs at the beginning of their evolutionary development. .

The link between dinosaurs and birds

The term Theropod (actually means “Beast Foot”), or to be more scientifically precise and use the formal classification – Theropoda; refers to a group of bipedal dinosaurs with lizard-hipped dinosaurs. Most of the Theropoda suborder are carnivorous, dinosaurs such as Tyrannosaurus rex, Spinosaurus, and Megalosaurus are theropods. The main classification of dinosaurs took place in the second half of the 19th century, when our knowledge of these animals and their diversity was just beginning to develop. The term Theropod, for example, was first attributed to the American paleontologist Charles Othniel Marsh. He used the classification of Theropoda to help determine the relationship of Allosauridae fossils from the western US to other types of dinosaurs. The term replaced the earlier classification for large “angle-footed” bipedal Goniopoda dinosaurs that had been proposed by Marsh’s great rival, Edward Drinker Cope. Ironically, the term Goniopoda is more appropriate, as scientists are now certain that it is the theropods or “beast feet” that are the ancestors of birds.

Paleontologists comment on scientific evidence

Commenting on the scientific paper, lead author Andrew RC Milner, of the St George Dinosaur Discovery Site at Johnson Farm in Utah, stated that because of the disproportionately small front limbs of most theropods, the fossil remains of them while they rested in the ground they are extremely rare. . Only a few other examples of Theropod handprints are known, but the discovery of a beautifully preserved set of prints in 2004 has allowed scientists to shed more light on the range of motion of dinosaur hands and arms.

Vertebrate paleontologist Milner and colleagues describe in the paper a clear set of 2-inch-deep impressions preserved among hundreds of other trace fossils in the sediment that have been dated to the early Jurassic (Sinemurian faunal stage), approximately 198 million years ago. of years. The rock preserves fossil traces of worm holes, footprints made by crabs, as well as fish body fossils. The sediment represents a part of the shoreline adjacent to an ancient lake. The water level appears to have been disturbed, and as a result, the sediment shows signs of falling water levels and the muddy sand cracked and dried in the sun.

Milner commented that the hardened rocks preserve many of the details of the lakeside topography from that time period.

Marks left in the mud by a theropod dinosaur

Theropod’s handprints are part of a fossilized trail that appears to have been made when the dinosaur climbed a slight incline and then crouched down to rest. Although the actual genera of dinosaurs cannot be identified from the tracks and no theropod dinosaur bones have been associated with the track, the Utah-based team calculated that the marks were made by a meat-eating dinosaur that would have measured about 4, 5 meters long. Little is known about the dinosaurs of this early Jurassic era, the fossil record being particularly poor. However, Dilophosaurus is known to have lived in this part of the world at the time the handprints were made, and scientists have speculated that a Dilophosaurus or some similar animal might have left the prints.

dilophosaurus models

In scientifically created Dilophosaurus models, the sculpting often attempts to show the Dilophosaurus’s hands turned slightly toward the body, away from the classic “bunny” position of most theropod dinosaur models and, indeed, the hands of the Dilophosaurus. museum exhibits. In this, sculptors have attempted to demonstrate the limited range of movement of Saurischian forelimbs, Theropods.

dinosaur footprints

Each imprint was made with the edge of the hand, not the palm, and indicates that the fingers of each forelimb curled inward, Milner says. This configuration supports anatomical studies of later theropods, suggesting that these creatures could not turn their palms to face downwards; in fact, the range of motion of the forelimbs of theropods was very different from the range of motion of the arm in humans.

Discussing the findings, University of Maryland vertebrate paleontologist Thomas R. Holtz Jr. stated that in this situation the dinosaur would be expected to plant its hand as naturally as possible. The fact that the hand is not palm down is strong evidence that this type of dinosaur could not do this.

The fossilized handprints are presumed to have been made by a resting theropod dinosaur. Tracks left in the sediment indicate that this dinosaur could not turn the palm of its hand to look down.

Changing the way dinosaurs are represented

In contrast to the way most theropods are depicted in books, artwork, and even museum exhibits, Theropod’s arm range of motion was limited compared to ours. Theropods and other bipedal lizard-hipped dinosaurs could not rotate their forearms so that their palms faced the ground or back toward their hind legs. This ability to rotate the hands is called pronation. It is accomplished by moving the two forearm bones, the radius and ulna relative to each other. From studies of dinosaur bones, it appears that in most theropods, the radius and ulna were less mobile relative to each other, so movement was limited. Freedom of movement around the shoulder joint and wrist bones was also greatly reduced compared to human anatomy. However, studies of the structure of some theropod forelimbs, such as the early Cretaceous carnivorous Acrocanthosaurus, show that the fingers could flex backwards against the wrist, perhaps an adaptation to allow sharp claws on all three fingers. of this dinosaur dig into the fighting prey. .

Limited range of motion in the arms

Scientists have speculated that the range of arm movement in many theropods reflects their relationship to Aves (birds). In theropods, the only way for the palm of the hand to face the ground would perhaps have been by spreading the entire forelimb open, as in a bird raising its wing. The possible range of motion of the forelimbs in posterior theropods such as troodonts reflects their close kinship with birds.

It appears that from fossil remains found at the site of an ancient lake in Utah, the forelimbs of certain dinosaurs had become much more specialized and no longer useful for walking much earlier in their evolution than previously thought.