2017-06-08 09:00:34
About 160 million years ago, at the end of the Jurassic, the global climate was warm. The Shishugou Formation, located on the western edge of the Gobi Desert in northwest China, was then a forested region with dry winters and rainy summers due to the influence of the monsoons. Large monkey-puzzle trees shaded an understory of conifers, horsetails, ferns, and tree ferns. Shishugou means “the valley of the stone trees”, because of the petrified trunks that have been found there. Dinosaurs, pterosaurs, mammals, crocodiles, turtles and amphibians inhabit the area. Next to a small mountain range with active volcanoes is a swampy area. When a volcano erupts, the ash rain forms a viscous sludge that accumulates in the pits created by the stomping and splashing of the great dinosaurs. Other smaller animals become trapped in the mud and sink due to the passage of new dinosaurs and the attempts to escape by new victims. Some predators also fall for it by trying to take advantage of easy food. This is how vertical accumulations of skeletons between one and two meters deep were formed in this site.
The most common species in these wells is a small dinosaur discovered in 2009, Limusaurus inextricabilis, “the lizard trapped in the mud.” In one of the pits, three Limusaurus were found under two predators of the Guanlong wucaii species. After the fall of the three Limusaurus, the first predator, a young specimen, tried to eat them; It was trapped in turn, and the second predator, an adult, tried to eat the first one, which managed to break its neck before dying.
Limusaurus is a theropod, a bipedal dinosaur like the tyrannosaurus and the velociraptor. Within the theropods, it is classified in the group of ceratosaurs. It is a relatively small, bipedal and slender dinosaur, measuring one meter seventy in length and about fifteen kilos in weight, with a long neck, hind legs and tail, elongated feet and very short front legs. The hips are raised seventy centimeters above the ground. The skull is short, with large eyes. The abundance of Limusaurus specimens at the sites suggests that it was a gregarious animal, and the structure of its hind legs indicates that it was a fast runner. It is estimated that he could exceed fifty kilometers per hour, with strides of one meter eighty. Adult and young specimens have been found; the smallest calf is half a meter in length and must have weighed about three hundred and forty grams.
Limusaurus is very similar to the ornithomimids, the ostrich dinosaurs of the Cretaceous; one more example of convergent evolution, which causes two living things that are not closely related to evolve into similar forms because they share a way of life.
Limusaurus anatomy, like that of many dinosaurs, changes radically with age. In the case of Limusaurus, the head is flattened, the hands are elongated… Thus, up to seventy-eight anatomical differences in the skeleton. The most striking difference is that of the teeth. The young have forty-two teeth, but with age they lose them, and are replaced by a beak. In an intermediate phase, eight teeth are lost, of which only two, in the lower jaw, are replaced by smaller ones. At this time, the absence of wear marks on the teeth indicates that their use is decreasing. And by the age of one year, all the teeth have been lost. And also, the alveoli where the teeth are inserted, or disappear, or merge into a canal that runs through the lower jaw. At six years old, Limusaurus reach maturity, and their growth slows down.
Total loss of teeth during development is only known in a handful of species: the platypus and certain mullets and catfish. Isotopic analysis of the fossils indicates that toothed juveniles were omnivorous, while toothless adults were herbivorous. Also, only adults swallowed stones, called gastroliths, to aid digestion, as many herbivorous dinosaurs did. Limusaurus is the only known herbivorous ceratosaurus; all the rest were carnivorous. Or so we believed until now: perhaps, after the discovery of Limusaurus, other ceratosaurs known from incomplete remains, and which were supposed to be carnivorous simply because they belonged to the ceratosaur group, could be reinterpreted as herbivores.
The hands of Limusaurus are different from those of any other dinosaur. It only has four fingers: the first is the smallest, and lacks phalanges; the second is very robust, almost as strong as the first finger of other dinosaurs, and has three phalanges; the third only has three phalanges, instead of the four that other theropods of the time have; and the fourth has only one phalanx. The claws are short, broad and strong.
For almost two centuries, paleontologists and biologists have disagreed about the evolution of fingers from dinosaurs to birds, which is called the problem of finger homology. The first theropod dinosaurs had five fingers on each hand. Throughout the evolution that led to birds, the number of fingers was reduced to three. Due to the structure of these three fingers in the most advanced theropods, paleontologists believe that the missing fingers are the last two, the fourth, and the fifth. But certain studies on the embryonic development of birds indicate that the fingers that develop in these are the intermediate, the second, the third and the fourth. In 1982, Australian paleontologists Tony Thulborn and Tim Hamley proposed that in the evolution of the theropod hand, the missing fingers were the first and last, as in birds, and the remaining second, third, and fourth fingers were lost. they had modified to resemble the first, second, and third respectively. The discovery of Limusaurus was initially considered as proof of this model. However, ceratosaurs like Limosaurus are not in the evolutionary line that leads to birds, these are the tetanuran group. Also, the hands of ceratosaurs are quite different from those of tetanurans; it is thought that the latter used them to grab their prey, while the former did not. So the structure of the Limusaurus hand is now considered irrelevant to the issue of bird finger evolution. There are many proposals that try to explain it, but the problem is still not satisfactorily resolved.
(German Fernandez, 06/2017)
CONSTRUCTION OF GERMAN FERNÁNDEZ:
reticular infiltrate
Reticular Infiltrator is the first novel in the trilogy The Saga of the Borelians. Do you want to see how it starts? Here you can read the first two chapters.
The Karnak file. Ed.Rubeo
The hanged man and other fantastic tales. Ed.Rubeo
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