More than one way to make a giant dino
Argentinian fossils reveal an alternative evolutionary route to large body size. Andrew Masterson reports.
Newly described fossils challenge the conventional view regarding the biomechanics necessary for the evolution of well-known gigantic animals such as Brachiosaurus and Diplodocus.
In a paper published in the journal Nature Ecology and Evolution, palaeontologists led by Cecilia Apaldetti from the Universidad Nacional de San Juan in Argentina reveal a new species, Ingentia prima, and new remains of a previously discovered species, Lessemsaurus sauropoides.
Both dinosaurs inhabited at least the southeast corner of the supercontinent Pangaea – a landmass that is now Argentina.
Both, too, were giants, weighing as much as 10 tonnes. That’s less than the weight of the truly huge and better known species, which topped out at 70 tonnes, but two aspects make Apaldetti’s dinos, known collectively as lessemsaurids, special. First, they predate Brachiosaurus and Diplodocus by at least 47 million years, and, second, they grew large using significantly different anatomical adaptations.
The ancestors of Brachiosaurus and Diplodocus were rather small, and known as sauropodomorphs. The earliest member of the lineage so far discovered, dubbed Panphagia protos, was formally described in 2009 and was only about the size of a turkey.
The mini-dino, however, already possessed the anatomical features thought essential for the gigantism that millions of years later its descendants would display: straight legs and a continuous rate of growth.
The lessemsaurids, it turns out, took a different route. Apaldetti and her colleagues show that the species walked on legs that were bent rather than straight, and that their bones grew in punctuated bursts.
The anatomy, the scientists report show “the presence of a novel growth strategy that allowed them to attain large body sizes without having the anatomical traits previously regarded as [essential] adaptations to gigantism”.
The researchers conclude by suggesting that the fossil evidence shows that straight legs and the pelvic and spinal changes they require “were not strictly necessary for the acquisition of gigantic body size”.