An early trend towards gigantism in Triassic sauropodomorph dinosaurs

Abstract

Dinosaurs dominated the terrestrial ecosystems for more than 140 Myr during the Mesozoic era, and among them were sauropodomorphs, the largest land animals recorded in the history of life. Early sauropodomorphs were small bipeds, and it was long believed that acquisition of giant body size in this clade (over 10 tonnes) occurred during the Jurassic and was linked to numerous skeletal modifications present in Eusauropoda. Although the origin of gigantism in sauropodomorphs was a pivotal stage in the history of dinosaurs, an incomplete fossil record obscures details of this crucial evolutionary change. Here, we describe a new sauropodomorph from the Late Triassic of Argentina nested within a clade of other non-eusauropods from southwest Pangaea. Members of this clade attained large body size while maintaining a plesiomorphic cyclical growth pattern, displaying many features of the body plan of basal sauropodomorphs and lacking most anatomical traits previously regarded as adaptations to gigantism. This novel strategy highlights a highly accelerated growth rate, an improved avian-style respiratory system, and modifications of the vertebral epaxial musculature and hindlimbs as critical to the evolution of gigantism. This reveals that the first pulse towards gigantism in dinosaurs occurred over 30 Myr before the appearance of the first eusauropods.

Dinosaurs: Four Legs Good, Two Legs Bad

“…The Greenland trackway shows evolutionary innovations of sauropods, such as the laterally deflected foot claws, and is practically indistinguishable from sauropod trackways from the later Mesozoic. The Greenland trackway thus suggests that sauropods were around in the Late Triassic, which is consistent with the phylogenetic evidence [6,7] and shows that sauropods must have coexisted with bipedal as well as quadrupedal prosauropods such as the lessemsaurids. The new Argentinian giant now corroborates this view because it is of the same age of the tracks, Late Triassic, and could have left those Tetrasauropus tracks.…”

“…About 209 million years old, this animal comes from the Late Triassic of Argentina and was about 70% of the linear size of Ledumahadi. New material of an older but closely related form, Lessemsaurus, also indicates that these animals could reach 7 to 10 tons [7]. Together with Ledumahadi and Ingentia, Lessemsaurus forms the newly recognized group Lessemsauridae.…”

“…This new species, christened Ledumahadi mafube (''dawn giant thunderclap''), reached the enormous (by non-sauropod standards) body mass of 12 tons and walked on four legs. Another new species, Ingentia pirma (''first giant''), is described by Apaldetti et al [7]. About 209 million years old, this animal comes from the Late Triassic of Argentina and was about 70% of the linear size of Ledumahadi.…”

“…Weighing in at 80 or more metric tons [3], sauropods required four pillar-like limbs to distribute this weight [4,5]. Two new papers [6,7] -one by Blair McPhee, Jonah Choiniere and colleagues [6] in this issue of Current Biology -now shed light on the early evolution of these giants, describing remarkably similar dinosaurs from very different places (South Africa and Argentina) and times (before and after the major extinction event at the end of the Triassic, 201 million years ago). Both teams reached very similar conclusions, culminating in the christening of a new group of dinosaurs -in a remarkable case of independent scientific convergence both teams named this group the Lessemsauridae.…”

Cranium and Vertebral Column of Xingxiulong chengi (Dinosauria: Sauropodomorpha) from the Early Jurassic of China

“…It is the earliest known group of dinosaurs (Carnian Stage of Late Triassic) and presents a high diversification of at least seven genera in South America (Müller et al, ; Pretto et al, ). During the Norian Stage of the Late Triassic, it achieved a global distribution and approached large body sizes of 10 tons (Apaldetti et al, ). The end‐Triassic extinction, although removing many previously dominant groups, gave rise to the true age of dinosaurs with a great radiation of sauropodomorphs (Padian, ; McPhee et al, ).…”