Dinosaurs dominated the terrestrial ecosystem for more than 140 Myr during the Mesozoic Era (which started 250 Myr and ended 66 Myr ago). The sauropodomorphs group, a huge family of herbivore dinosaurs, comprises the largest land species ever recorded. So far, the first giant dinosaurs, which weighted more than 10 tones, appeared during the Jurassic period -180 Myr- and then they evolved and reached 70 tones, like the ones from most recent times we know about in Patagonia.
After the discovery of a new species called Ingentia prima in the district of Balde de Leyes, in the province of San Juan, Argentina, CONICET scientists conducted a study that revealed that the first giant dinosaurs actually appeared during the Triassic period (210 Myr ago) about 30 Myr before the time experts believed. According to the results published in Nature Ecology & Evolution, the dinosaurs of that time weighted 10 tones, like the first giants of the Jurassic period.
“The name of this new species ‘Ingentia’ means immense and refers to its enormous size for the time in which it lived, and ‘prima’ indicates that it is the first giant known so far”, Cecilia Apaldetti, principal author of the study, explains. She is a CONICET assistant researcher at the Centro de Investigaciones de la Geósfera y la Biósfera (CIGEOBIO, CONICET-UNSJ) and the Institute and Museum of Natural Sciences.
“The research also shows that the new species is closely related to one species already known of the Triassic period in Argentina and other from South Africa, what led to the definition of a new family of dinosaurs: Lessemsauridae”, Diego Pol stated. He is a CONICET principal researcher at the Museo Paleontológico Egidio Feruglio (MEF) and one of the authors of the study. “This confirmed the connection between both continents and the evidence of the great radiation and the evolutionary success of these herbivore dinosaurs during almost all the Mesozoic era”, the scientist adds.
The difference of the species of this new group is one novel strategy of growth that distinguishes them from the giant sauropods of the Jurassic period, as well as from the most primitive species that coexisted in the Triassic.
“While the giant dinosaurs of the Jurassic grew quickly and continually until they became adults, the primitive dinosaurs of the Triassic did it seasonally, like the way trees grow. What distinguishes these first giants of the Lessemsauridae family is that they grew cyclically but in an extremely accelerated way. The most surprising fact is that during the season of accelerated growth they did it at an even higher rate than those of the giants that grew continually”, explains Ignacio Cerda who is a CONICET associate researcher at the Instituto de Investigación en Paleobiología y Geología (IIPG, CONICET-UNRN) and co-author of the study.
This information was obtained thanks to the analysis of internal microstructure of bones, what enabled us to know, among other aspects, the way in which the dinosaurs and other extinct animals grew.
Furthermore, the research showed the great size dinosaurs could reach as regard other anatomic changes, which are considered a crucial condition for gigantism. “In general, to become giants, different anatomical changes that accompany the increase of body mass were required. In the Triassic, the sauropodomorphs dinosaurs were quadruped or facultative bipeds, meaning that the arms were used not only to walk but also to fulfill other functions. In the case of giant-sized species, the arms adopt an exclusive walking function –they are columnar, lose flexibility and the fingers’ phalanxes tend to disappear–. Other changes remarked were the lengthening of the neck and the reduction of the skull” Apaldetti describes.
However, the study reports that the first giants, like Ingentia and other Lessemsauridae species, reached this size but lack an extremely long neck, and despite being quadrupeds, their arms are not completely columnar and had well developed phalanxes and claws. This leads to the conclusion that certain anatomic features were not actually vital for gigantism as it used to be considered so far. They were, in fact, the product of a later evolution. Nevertheless, the researchers acknowledge that without the mediation of some anatomic changes, it would not have been possible for dinosaurs to reach those extraordinary sizes of up to 70 tones like they did it most recent times.
Apart from the similar body mass, there is a very particular characteristic that distinguishes this new dinosaur family from the rest of the sauropodomorphs of the Triassic and places them closer to the giants of the Jurassic: traces of an improved respiratory system similar to current birds.
“There’s evidence in the skeleton of deep cavities that indicates the presence of extensions of the respiratory system invading the bones like the air sacs of current birds do. This is highly developed in the cervical area –like in current birds- and suggests the appearance of a very efficient respiratory system that coincides with the time of the origin of gigantism”, Pol explains.
This multidisciplinary research included the participation of Ricardo Néstor Martínez and Oscar Alcober, both scientists at the Instituto and Museo de Ciencias Naturales-UNSJ –of which the second is the director–and also the participation of the CIGEOBIO, who designed and led the project.
“The Triassic rocks of Balde de Leyes complement the history of the main groups of vertebrates that emerged some Myr before and now we see them preserved in Ischigualasto, for instance. This new find also contains rocks of Jurassic age with a high fossil content, what provides continuity of information about the evolutionary history of this animals that originated, diversified and dominated almost all the terrestrial ecosystems during the rest of Mesozoic era”, Martínez states.
By Miguel Faigón.