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Dinosaur Handprints Reveal Link to Birds
Jurassic handprint demonstrates affinities with birds
Marks left in the mud near an ancient Jurassic lake by a carnivorous dinosaur and preserved as fossils; find that dinosaurs had hands that were not suitable for walking very early in their evolution. In a report published in the online paleontology journal PLos ONE, a team of American scientists have concluded that fossil handprints, perhaps the best theropod handprints discovered to date, show that these dinosaurs abandoned the use of their limbs. forelegs at the beginning of their evolutionary development. .
The Connection Between Dinosaurs and Birds
The term Theropod (actually means “Leg of the beast”), or to be more scientifically accurate and use the formal classification – Theropoda; refers to a group of bipedal dinosaurs with a lizard. Most of the suborder Theropoda are carnivores, dinosaurs such as Tyrannosaurus rex, Spinosaurus and Megalosaurus are Theropods. The primary classification of dinosaurs was developed in the second half of the 19th century, when our knowledge of these animals and their diversity was just beginning to develop. For example, the term Theropod was originally attributed to the American paleontologist Charles Othniel Marsh. He used the classification Theropoda to help determine the relationship of Allosauridae fossils from the western United States to other types of dinosaurs. The term replaced the earlier classification for the large, bipedal dinosaurs of the “angle-footed” Goniopoda, which had been proposed by Marsh’s arch-rival Edward Drinker Cope. Ironically, the term Goniopoda is more appropriate, as scientists are now certain that it is Theropods or “Beast Legs” that are the ancestors of birds.
Paleontologists comment on the scientific evidence
Commenting on the scientific paper, lead author Andrew RC Milner of the St George Dinosaur Discovery Area at Johnson Farm in Utah stated that due to the disproportionately small forelimbs of most theropods, their fossil footprints while stay on the ground are extremely rare. . Only a few other examples of theropod handprints are known, but the discovery of a set of beautifully preserved impressions in 2004 has enabled scientists to shed more light on the range of movement of the dinosaurs’ hands and arms.
Vertebrate paleontologist Milner and his colleagues describe in the paper, a clear set of 5 cm deep impressions preserved among hundreds of other trace fossils in sediment that has been dated to the Early Jurassic (the Sinemurian phase of the fauna), approximately 198 million years ago. seen. The rock preserves fossilized traces of worm burrows, traces made by crabs as well as fossilized fish bodies. The sediment represents part of a coast adjacent to an ancient lake. The water level appears to have changed and as a result the sediment shows signs of falling water levels and the silty sand has cracked and dried in the sun.
Milner commented that the petrified rocks preserve many of the details of the lakeside topography of that time period.
Marks left in the mud by a Theropod Dinosaur
The theropod’s handprints are part of a fossilized track that appears to have been made when the dinosaur walked up a slight incline and then curled up to rest. Although the actual genus of dinosaur cannot be identified from the footprints and no theropod dinosaur bones have been associated with the track, the Utah-based team has calculated that the marks were made by a meat-eating dinosaur that would have been approximately 4.5 meters long. Little is known about dinosaurs from this time in the early Jurassic, the fossil record being particularly poor. However, dilophosaurs are 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 could have made the tracks.
Models of Dilophosaurus
In models of Dilophosaurus created by scientists, the sculpt often tries to show the hands of Dilophosaurus slightly turned towards the body, a departure from the classic “bunny” position of most Theropod dinosaur models and indeed museum exhibits. In this the sculptors have tried to demonstrate the limited range of movement of the forelimbs of Saurischian, Theropods.
Each handprint was made from the edge of the hand, not her palm, and shows that the fingers on each forelimb were curled inward, Milner says. This configuration supports anatomical studies of later theropods, suggesting that those creatures could not rotate their palms to turn downward, indeed the range of motion of the theropod’s forelimbs was very different from the range of motion of arm to the people.
In discussing the findings, Thomas R. Holtz Jr. (vertebrate paleontologist at the University of Maryland) stated that in this situation you would expect the dinosaur to plant its hand in the most natural way 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 were assumed to have been made by a resting Theropod dinosaur. Traces left in the sediment show that this dinosaur could not rotate its palm straight down.
Changing the way dinosaurs are portrayed
Unlike the way most Theropods are depicted in books, pieces of art, and even museum exhibits, the range of motion of Theropod’s arms was limited compared to ours. Theropods and other lizard-hipped bipedal dinosaurs could not rotate their forearms so that their palms face off the ground or back toward their hind legs. This ability to rotate the hands is called pronation. It is achieved by moving the two bones in the forearm, the radius and the ulna, in relation 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 carnivorous Acrocanthosaurus of the early Cretaceous, show that the digits could be bent back against the wrist, perhaps an adaptation to enable the sharp claws on this dinosaur’s three fingers to dig into struggling 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 to face the ground would probably have been to open the entire forelimb, like a bird raising its wing. The range of motion possible for the forelimbs in later theropods such as troodonts reflects their close relationship to birds.
It appears from fossil footprints found at the site of an ancient lake in Utah that the forelimbs of some dinosaurs became much more specialized and less useful for walking much earlier in their evolution than previously thought.
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