The Sauropodomorph Dinosaurs include the largest land Animals every known to have lived on Earth, with some species producing individuals which exceeded 90 tons in mass. However, they were not always this large, with the earliest known Sauropodomorphs being bipedal omnivores which lived in the Carnian (Earliest Triassic, 233-231 million years ago), and had body masses of less than 15 kg. By the Early Jurassic, the group had achieved a global distribution, and included both bipedal and quadrupedal forms, with many species exceeding 10 tons in mass. This increase in body mass is remarkable, and therefore has attracted considerable interest among scientists. The favoured explanation is that ecological circumstances created a cascade of evolutionary pressures which favoured larger Sauropodomorph species. By the Early Jurassic small (less than one ton) Sauropodomorphs are rare, although they do persist across most of the globe until the Pliensbachian (192.9-184.2 million years ago). It is possible that smaller Sauropodomorphs were at a disadvantage due to the large number of competing groups occupying this niche in the Triassic and Early Jurassic, which included Gomphodont Cynodonts, Ornithischian Dinosaurs, herbivorous Crocodyliforms, and secondarily herbivorous Theropods. However, modern Mammal-dominated ecosystems commonly host a wide range of herbivore species, which range in size from about 10 g to about 7 tons, with the largest number of different species in the smaller and middle parts of this range, which suggests that the driving pressure which created large Sauropodomorphs is unlikely to have been as simple as this model suggests.
In a paper published in the journal Royal Society Open Science on 14 June 2023, Kimberley Chapelle of the Division of Paleontology at the American Museum of Natural History, and the Evolutionary Studies Institute at the University of the Witwatersrand, Jennifer Botha, also of the Evolutionary Studies Institute, and of the Centre of Excellence in Palaeosciences at the University of the Witwatersrand, and Jonah Choiniere, again of the Evolutionary Studies Institute at the University of the Witwatersrand, describe the left humerus of an exceptionally small Sauropodomorph Dinosaur from the Early Jurassic Elliot Formation of Free State in South Africa.
The specimen, BP/1/4732, described by Chapelle et al. was collected by palaeontogist James Kitching in 1978, from an outcrop in what was then Bethlehem District in Orange Free State, and is now Thabo Mofutsanyana District in Free State Province. Its form suggests that it came from a bipedal Sauropodomorph, and sufficiently from any other described Sauropodomorph humerus to establish that it comes from a previously unknown species within the Massopoda, although the humeri of Sauropodomorphs are insufficiently distinctive to classify it further.
A section taken through the cortex revealed large medullary cavity is surrounded by a relatively narrow, compact cortex. The cortex shows a number of lines of arrested growth, seasonal pauses in growth associated with a season which did not favour growing, such as a winter or dry season, similar to the rings seen in trees. It is not possible to estimate the age of the specimen, as the innermost portion of the cortex has been secondarily remodelled, but there are eleven tightly-packed lines in the outer part of the cortex, which is a strong indicator that growth had stopped by the time the specimen died; i.e. it was an adult.
The midshaft circumference of the humerus has previously been shown to be strongly tied to overall bodymass in bipedal Sauropodomorph Dinosaurs. BP/1/4732 has a humeral midshaft circumference of 66 mm, smaller than the same measurement for any other Early Jurassic bipedal Sauropodomorph, and which Chapelle et al. calculate would indicate a living Animal with a mass of about 73.35 kg.
This makes BP/1/4732 the smallest-known post-Triassic Sauropodomorph, with the second smallest being Ngwevu intloko, another Early Jurassic Massospondan from South Africa, with a predicted mass of 107.91 kg, based upon a single specimen which, importantly, isn't considered to have been fully grown at the time of death. BP/1/4732 therefore significantly increases the known size range of Early Jurassic Sauropodomorphs, with sizes now ranging from tens of kilograms to tens of tons. Notably, this is less than the size range of modern Mammals, a group which includes many species with masses below 10 kg, although some non-Sauropodomorph Dinosaurs from the period, such as the Ornithischians Heterodontosaurus tucki and Lesothosaurus diagnosticus did weigh less than 10 kg.
See also...
Online courses in Palaeontology.
Follow Sciency Thoughts on Facebook.
Follow Sciency Thoughts on Twitter.
