Antetonitrus | Early Sauropod Relative “Before the Thunder”

In the Early Jurassic, amidst South Africa’s vast floodplains, a remarkable dinosaur walked the Earth. Known as Antetonitrus, its name translates to “Before the Thunder,” referencing its position as an early relative of the famous Brontosaurus, or “Thunder Lizard.” As a basal sauropodiform, this herbivorous giant bridges the evolutionary gap between smaller, earlier sauropodomorphs and the colossal sauropods that would come to dominate the Mesozoic landscape.

Antetonitrus is a key taxon in the study of sauropodomorph diversity at the outset of the Jurassic. Revealing a fascinating mix of primitive and advanced features, its fossils offer invaluable insights into the range of adaptive strategies these burgeoning giants employed to navigate their environments. With its blend of robust limbs, transitional anatomy, and substantial size, this dinosaur provides a vivid glimpse into a critical chapter of dinosaur evolution.

Antetonitrus Key Facts

Antetonitrus Origins, Taxonomy, and Timeline

The name Antetonitrus combines Latin words that mean “before the thunder.” At the time of its naming, this title was intended to draw a direct link between the new species and later titans like Brontosaurus (often called “Thunder Lizard”), of which it was hypothesised to be an early ancestor. As is so often the way, later studies have complicated this narrative a little bit – but more on that below. The species name, ingenipes, translates to “massive foot,” a fitting nod to the dinosaur’s powerful, weight-bearing limbs. This poetic name reflects both its evolutionary position and the awe its remains inspire, capturing the essence of a creature that stood at the dawn of sauropod gigantism.

Although there is some minor controversy as to where precisely the sauropod clade “begins”, with several competing phylogenetic definitions floating around, Antetonitrus is typically classified (by me, anyway) as a basal sauropodiform – placing it very close (but not within) the origin of the sauropod lineage. Within Sauropodiformes it belongs to the lessemsaurid family, a group that showcases transitional features bridging the early, mostly bipedal sauropodomorphs and the fully-developed quadrupedalism of later sauropods. This highlights its evolutionary significance, demonstrating the adaptive experimentation that appears to have taken place within Sauropodomorpha near the Triassic-Jurassic transition.   

Antetonitrus lived during the Early Jurassic, spanning the Hettangian to Sinemurian Epochs, approximately 201.3 to 190.8 million years ago. This period followed the Triassic-Jurassic extinction event, a time when ecosystems were recovering and diversifying. Antetonitrus emerged as one of the largest herbivores of its era, thriving in an environment teeming with vegetation and dynamic competition.

Discovery & Fossil Evidence

The first fossils of Antetonitrus were discovered in 1981 by paleontologist James Kitching in the red mudstones of the upper Elliot Formation, within the Ladybrand District of South Africa’s Free State Province. These remains lay unstudied for over two decades until Adam Yates reexamined them, leading to the formal description of the species in 2003. Originally given only a brief, “abstract” description, the species was later given a comprehensive description by myself and colleagues in 2014. 

The holotype assemblage, BP/1/4952, includes a remarkable set of bones from at least three subadult-to-adult specimens. This includes parts of the neck, such as a cervical vertebra, backbone (dorsal vertebrae) and tail bones (caudal vertebrae). Other finds include ribs, the shoulder blade (scapulae), the upper (humerus) and lower (ulna, radius) arm bones, as well as the upper (femur) and lower (tibia, fibula) leg bones. Additionally, the assemblage has several hand (metacarpals) and foot (metatarsals) bones, including at least two unguals (claw bones).  

This collection gives a detailed glimpse into the anatomy of this Early Jurassic dinosaur.

Antetonitrus Size and Description

Short Description of Antetonitrus

Antetonitrus was a large herbivorous dinosaur characterized by its sturdy frame and transitional anatomy. Its body displayed a mix of traits, with a long tail, strong limbs, and a moderately elongated neck. Unlike true sauropods, its forelimbs retained some flexibility, allowing it to adapt its movement and feeding strategies.

The dinosaur’s robust vertebrae and thick pelvic bones supported its considerable weight. Its feet, particularly the massive first toe, were adapted for stability and weight distribution. Signaling its ability to transition between a (mostly) quadrupedal and (sometimes) bipedal locomotory stance as circumstances required. 

Size and Weight of Type Species

Estimates place Antetonitrus at a length of 26.0 to 33.0 feet. With a height of 4.9 to 6.6 feet at the hips. It likely weighed around 6.2 tons, making it one of the largest dinosaurs of its time. These dimensions highlight its significance as an evolutionary precursor to the giant sauropods that followed.

Despite its size, Antetonitrus was smaller and less specialized than later sauropods. Its proportions reflect its transitional nature, bridging earlier, smaller sauropodomorphs and the enormous sauropods that would dominate the Jurassic and Cretaceous.

The Dinosaur in Detail

Antetonitrus’ unique blend of features reveals both its adaptability and evolutionary significance. Primitively, the dinosaurian forelimb was oriented “elbows out”, that is to say, it retained the strong lateral flexibility we still see in lizards and amphibians. As bipeds, this lateral flexibility – in conjunction with a mobile grasping hand – was likely useful in manipulating objects closer to the body, be it prey or foliage. In contrast, true sauropods had what we call the “parasagittal” condition. Like most modern mammals today (well, the larger, quadrupedal ones), this means that the limbs are oriented directly below the body as a kind of vertical support strut with limited lateral flexion. The advantage of the parasagittal condition is that weight is distributed much more efficiently and safely at larger body sizes.   

retention of a mobile grasping forelimb

Curiously, although it was similar in weight to the largest of today’s elephants, Antetonitrus retained a laterally flexed “prosauropod”-type forelimb, suggesting that it still had use of a mobile, grasping appendage out front. It has long been a pet theory of mine that the retention of a primitive forelimb in lessemsaurids is suggestive of their “unwillingness” to forgo the behaviour that made sauropodomorphs such successful herbivores in the first place: the ability to rear-up on the hindlegs to reach higher vegetation. In addition to helping lean against the trunks of trees, returning your muti-ton mass to earth is a heck of a lot safer with a flexed elbow than a rigid one! That enlarged first toe of Antetonitrus may have also developed as part of this mixed feeding strategy. As it would have allowed more weight to be transferred along the inside of the foot during bipedal feeding phases. 

Nonetheless, the retention of a mobile grasping forelimb had its own disadvantages – namely in placing an upper limit on how big these animals were truly allowed to get. A much more efficient form of quadrupedal gigantism came on the scene towards the end of the Early Jurassic with the parasagittal sauropods, likely contributing to the doom of Antetonitrus and its ilk.

Contemporary Dinosaurs

Among its contemporaries was Ledumahadi, another lessemsaurid dinosaur. Larger and more robust than Antetonitrus (a whopping 12 tons!), Ledumahadi likely represented the upper size threshold of the “lessemsaurid” condition. Ledumahadi likely made use of its size to reach food inaccessible to smaller herbivores, including possibly Antetonitrus. This difference in feeding heights may have allowed these two dinosaurs to coexist without directly competing for the same resources.

In contrast, the basal ornithischian Lesothosaurus represented the small, agile herbivores of the Early Jurassic. This fleet-footed dinosaur relied on speed and nimbleness to evade predators. While Antetonitrus lumbered through the landscape, Lesothosaurus thrived in areas where it could forage on low-lying plants. Their vastly different sizes and behaviors highlight the diversity of survival strategies in their shared environment.

Another contemporary, Pulanesaura, shared similarities with Antetonitrus but possibly focused its feeding on ground-level vegetation. As a fully obligate quadruped, Pulanesaura likely grazed on ferns and other plants closer to the ground. Its specialized feeding approach – possibly pointing towards the strategy that would introduce true sauropods to the world – complemented Antetonitrus’ more generalized approach. Showing how different herbivores could occupy the same habitats by exploiting distinct food sources.

Adding predatory pressure to this ecosystem was Dracovenator, a carnivorous theropod. This predator likely posed little threat to fully grown Antetonitrus due to its size and strength. However, it would have targeted juveniles or smaller dinosaurs like Lesothosaurus. The presence of such a predator shaped the social behaviors of herbivores. Encouraging group living and vigilance to protect vulnerable individuals.

Interesting Points about Antetonitrus

Antetonitrus in its Natural Habitat

The landscape that Antetonitrus inhabited was predominantly arid floodplain criss-crossed by a series of braided rivers. Vegetation was concentrated most heavily around these river channels and, while likely fluctuating during successive flood and drought periods, was nonetheless abundant enough to support a large community of varied herbivores. The dinosaur’s diet consisted of cycads, ferns, and other Jurassic flora, forming the foundation of its sustenance.

Its quadrupedal stance allowed it to navigate its environment, while the retention of an efficient rearing posture, in conjunction with its long neck, enabled it to reach vegetation at varying heights. Antetonitrus likely traveled in small groups, providing mutual protection against predators like Dracovenator. This social structure may have been crucial for juvenile survival.

As a herbivore, Antetonitrus played a vital role in its ecosystem, influencing vegetation patterns and nutrient cycling. Its feeding habits helped maintain the balance of its environment. Paving the way for the ecosystems that supported the even larger sauropods of later periods.

Frequently Asked Questions

Sources

The information in this article is based on various sources, drawing on scientific research, fossil evidence, and expert analysis. The aim is to provide a comprehensive and accurate overview of Antetonitrus.

Article last fact checked: Joey Arboleda, 12-14-2024

Featured Image Credit: PaleoEquii, CC BY-SA 4.0, via Wikimedia Commons