Early tiny ancestors of huge sauropods weighed less than average humans

"Until now, we didn’t know that early sauropodomorphs could get this small in the Jurassic."
Mrigakshi Dixit
Representational image.
Representational image.


In 1978, a tiny bone fossil was discovered in South Africa. Reanalysis of this bone fragment has now created a whole new story on evolution about the diversity and size of huge sauropod dinosaurs, a press release said.

The bone fragment originated from the Lower Jurassic geological time period(from 201 to 174 million years ago). Previously, it was thought that the bone belonged to a juvenile dinosaur who perished young. 

The latest analysis indicated that the fossil belongs to a fully grown unknown dinosaur species, which weighed less than most humans — roughly 165 pounds(75 kilograms).  

An early relative of the Sauropod clan

For decades, experts assumed that the small bone was an arm of a young Massospondylus carinatus. Mainly because it was discovered in the Massospondylus Assemblage Zone, named after the dinosaurs that previously ruled this region of south-central Africa.

According to the new study, this tiny dinosaur species might be one of the earliest ancestors of the large titanosaurs. And its evolution with time eventually led to the emergence of the world's biggest terrestrial creatures. Mamenchisaurus, Diplodocus, and Titanosaur are among the gigantic sauropods.

“Until now, we didn’t know that early sauropodomorphs could get this small in the Jurassic, so the smallest skeletons were assumed to be babies. We can now reassess these skeletons discovered in southern Africa and hopefully find a more complete individual from which we can name a new species,” said Dr. Kimberley Chapelle, the lead author of a new study. 

Early tiny ancestors of huge sauropods weighed less than average humans
The bone's structure suggests the dinosaur was fully grown at the time of its death.

Analysis of fine bone rings

The scientists reexamined the BP/1/4732 fossil humerus bone., and analyzed tiny portions of the bone from various dinosaur fossils to better understand the differences and identify the species. 

They arrived at the new conclusion by studying the tiny line structures that emerge in the bones when growth halts. In the bone material of this unknown species, significant lines of arrested growth(LAGs) were noticed. This suggests that the bone belongs to an adult dinosaur, not a juvenile, as previously thought. 

After careful examination, the scientists concluded that the fossil belonged to the "smallest sauropodomorph." The adult was probably around a meter long and stood on two legs like its bigger cousins.

At the moment, there isn't enough fossil evidence to designate this species just yet. Nonetheless, the bone fossil suggests that the sauropods' ancestors were more diversified than previously assumed. 

The findings have been published in the journal Royal Society Open Science.

Study Abstract:

The earliest sauropodomorphs were small omnivores (less than 10 kg) that first appeared in the Carnian. By the Hettangian, early branching sauropodomorphs (EBSMs) were globally distributed, had variable postures, and some attained large body masses (greater than 10 tonnes). Small-bodied EBSMs like Massospondylus carinatus (less than 550 kg) persist at least until the Pliensbachian at nearly all dinosaur-bearing localities worldwide but are comparatively low in alpha diversity. One potential reason for this is competition with other similarly sized contemporary amniotes, including Triassic gomphodont cynodonts, Jurassic early branching ornithischians, herbivorous theropods and potentially early crocodylomorphs. Today's herbivorous mammals show a range of body size classes (less than 10 g to 7 tonnes), with multiple species of small herbivorous mammals (less than 100 kg) frequently co-occurring. Comparatively, our understanding of the phylogenetic distribution of body mass in Early Jurassic strata, and its explanatory power for the lower thresholds of body mass in EBSMs, needs more data. We osteohistologically sectioned a small humerus, BP/1/4732, from the upper Elliot Formation of South Africa. Its comparative morphology and osteohistology show that it represents a skeletally mature individual of a new sauropodomorph taxon with a body mass of approx. 75.35 kg. This makes it one of the smallest known sauropodomorph taxa, and the smallest ever reported from a Jurassic stratum.

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